U.S. patent application number 10/232322 was filed with the patent office on 2003-04-24 for methods and systems for navigating under water.
Invention is credited to Larsen, Mikael Bliksted.
Application Number | 20030078706 10/232322 |
Document ID | / |
Family ID | 8159274 |
Filed Date | 2003-04-24 |
United States Patent
Application |
20030078706 |
Kind Code |
A1 |
Larsen, Mikael Bliksted |
April 24, 2003 |
Methods and systems for navigating under water
Abstract
In a method for determining absolute position under water of a
submersible vessel (1) having a dead reckoning navigation system
and receiving acoustic signals from a reference station (19),
signals are received from one reference station in several
positions (15-18) of the vessel. Estimated absolute positions of
the vessel are calculated using range data and relative position
data. Range rate derived from the signals are preferable utilised.
In a method for scanning an underwater survey area, the absolute
position of a vessel (1) is intermittently being determined
according to said method. The reference station may be placed at a
fixed absolute position (19), or on the surface of the water,
preferably in a buoy or a vessel. A system for determining the
absolute position under water of a vessel comprises: acoustic
communication means in a reference station and on board the vessel;
a dead reckoning navigation system on board the vessel; and
computing means.
Inventors: |
Larsen, Mikael Bliksted;
(London, GB) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 Pennsylvania Avenue, N.W.
Washington
DC
20037-3213
US
|
Family ID: |
8159274 |
Appl. No.: |
10/232322 |
Filed: |
September 3, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10232322 |
Sep 3, 2002 |
|
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PCT/DK01/00141 |
Mar 2, 2001 |
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Current U.S.
Class: |
701/21 ;
701/494 |
Current CPC
Class: |
G01S 5/30 20130101; G01C
21/00 20130101; G05D 1/10 20130101; Y02A 90/30 20180101; G01S 5/18
20130101; G01S 15/874 20130101 |
Class at
Publication: |
701/21 ;
701/217 |
International
Class: |
G01C 021/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 3, 2000 |
DK |
PA200000351 |
Claims
1. A method for determining the absolute position under water of a
submersible vessel having a dead reckoning navigation system not
receiving position information from outside the vessel, where the
vessel receives acoustic signals from a reference station having a
known absolute position and calculates its range from the reference
station, wherein said acoustic signals are received from the same
reference station in several arbitrary positions of the vessel, and
that estimated absolute positions of the vessel are calculated
using sets of data, each set of data comprising said calculated
range and navigation data from the dead reckoning navigation
system, said navigation data being valid concurrently with said
calculated range.
2. A method according to claim 1, wherein data from each received
signal are processed immediately or shortly after reception,
providing for a substantially continuous estimation of absolute
position.
3. A method according to claim 1, wherein the position of the
reference station in a relative coordinate frame of said dead
reckoning navigation system is estimated.
4. A method according to claim 1, wherein the estimated absolute
position data are used for updating the dead reckoning system's
relative position data.
5. A method according to claim 1, wherein estimates are made of
parameters intrinsic to the nature of the dead reckoning navigation
system, such as sea currents, and relative position data from the
dead reckoning navigation system are compensated by the estimate of
said parameters.
6. A method according to claim 5, wherein a least-squares algorithm
is used to estimate absolute position and parameters intrinsic to
the nature of the dead reckoning navigation system.
7. A method according to claim 5, wherein a Kalman filter is used
to estimate absolute position and parameters intrinsic to the
nature of the dead reckoning navigation system.
8. A method according to claim 1, wherein said estimates are made
further utilizing information on the depth of the reference
station.
9. A method according to claim 1, wherein the reference station is
placed at a fixed absolute position.
10. A method according to claim 9, wherein the absolute position of
the reference station is determined by the submersible vessel at
the surface of the water collecting absolute position data in a
number of positions from a positioning system usable at the surface
of the water, and while surfaced receiving acoustic signals from
the reference station, and calculating range data from said
signals, position and range data preferably being processed on
board the vessel.
11. A method according to claim 1, wherein the reference station is
launched from the submersible vessel.
12. A method according to claim 1, wherein the reference station is
collected by the submersible vessel after estimating an absolute
position.
13. A method according to claim 1, wherein the reference station
comprises an acoustic transponder.
14. A method according to claim 1, wherein the reference station
comprises an acoustic beacon.
15. A method according to claim 1, wherein the reference station is
placed on the surface of the water, preferably in a buoy or a
vessel.
16. A method according to claim 15, wherein the reference station
receives absolute position data from a positioning system usable at
the surface of the water, and relays such data to the submersible
vessel.
17. A method according to claim 15, wherein the reference station
exchanges communication data with a communication system usable at
the surface of the water, and preferably as well exchanges such
data with the submersible vessel.
18. A method according to claim 15, wherein the reference station
is placed in a submersible vessel being surfaced during use of the
reference station.
19. A method for determining the absolute position under water of a
submersible vessel having a dead reckoning navigation system not
receiving position information from outside the vessel, where the
vessel receives acoustic signals from a reference station having a
known absolute position and calculates its range from the reference
station, wherein said acoustic signals are received from one
reference station in one or more positions of the vessel; wherein
data for rate of change of the vessel's range from the reference
station ("range rate data") are derived from said acoustic signals;
and wherein estimated absolute positions of the vessel are
calculated using said calculated range, said range rate data, and
navigation data from the dead reckoning navigation system.
20. A method according to claim 19, wherein said range rate data
are derived from recordings of Doppler shifts in frequencies of
said acoustic signals.
21. A method according to claim 19, wherein said range rate data
are derived from recordings of time discrepancies in the arrival
times of spread spectrum pulses embedded within said acoustic
signals.
22. A method according to claim 19, wherein data from each received
signal are processed immediately or shortly after reception,
providing for a substantially continuous estimation of absolute
position.
23. A method according to claim 19, wherein the position of the
reference station in a relative coordinate frame of said dead
reckoning navigation system is estimated.
24. A method according to claim 19, wherein the estimated absolute
position data are used for updating the dead reckoning system's
relative position data.
25. A method according to claim 19, wherein estimates are made of
parameters intrinsic to the nature of the dead reckoning navigation
system, such as sea currents, and relative position data from the
dead reckoning navigation system are compensated by the estimate of
said parameters.
26. A method according to claim 25, wherein a least-squares
algorithm is used to estimate absolute position and parameters
intrinsic to the nature of the dead reckoning navigation
system.
28. A method according to claim 25, wherein a Kalman filter is used
to estimate absolute position and parameters intrinsic to the
nature of the dead reckoning navigation system.
29. A method according to claim 19, wherein said estimates are made
further utilizing information on the depth of the reference
station.
30. A method according to claim 19, wherein the reference station
is placed at a fixed absolute position.
31. A method according to claim 30, wherein the absolute position
of the reference station is determined by the submersible vessel at
the surface of the water collecting absolute position data in a
number of positions from a positioning system usable at the surface
of the water, and while surfaced receiving acoustic signals from
the reference station, and calculating range data from said
signals, position and range data preferably being processed on
board the vessel.
32. A method according to claim 19, wherein the reference station
is launched from the submersible vessel.
33. A method according to claim 19, wherein the reference station
is collected by the submersible vessel after estimating an absolute
position.
34. A method according to claim 19, wherein the reference station
comprises an acoustic transponder.
35. A method according to claim 19, wherein the reference station
comprises an acoustic beacon.
36. A method according to claim 19, wherein the reference station
is placed on the surface of the water, preferably in a buoy or a
vessel.
37. A method according to claim 36, wherein the reference station
receives absolute position data from a positioning system usable at
the surface of the water, and relays such data to the submersible
vessel.
38. A method according to claim 36, wherein the reference station
exchanges communication data with a communication system usable at
the surface of the water, and preferably as well exchanges such
data with the submersible vessel.
39. A method according to any of claim 36, wherein the reference
station is placed in a submersible vessel being surfaced during use
of the reference station.
40. A method for scanning an underwater survey area by means of a
submersible vessel traveling a desired path, the vessel having a
dead reckoning navigation system not receiving position information
from outside the vessel, where the vessel receives acoustic signals
from a reference station having a known absolute position and
calculates its range from the reference station, wherein the
absolute position of the vessel is intermittently being
determined.
41. A method according to claim 40, wherein said area extends
beyond the operational reach of said reference station, and the
intended trajectory of the vessel is arranged to bring the vessel
within said operational reach at regular intervals of time.
42. A method according to claim 40, wherein the intended trajectory
of the vessel is arranged to bring the vessel within a minimum
distance of every point in said area.
43. A method according to claim 12, wherein said reference station
is placed at a fixed absolute position.
44. A method according to claim 40, wherein the absolute position
of said reference station is determined by said submersible vessel
at the surface of the water collecting absolute position data in a
number of positions from a positioning system usable at the surface
of the water, and while surfaced receiving acoustic signals from
said reference station, and calculating range data from said
signals, position and range data preferably being processed on
board said vessel.
45. A method according to claim 40, wherein said reference station
is launched from said submersible vessel.
46. A method according to claim 40, wherein said reference station
is collected by said submersible vessel after estimating an
absolute position.
47. A method according to claim 40, wherein said reference station
comprises an acoustic transponder.
48. A method according to claim 40, wherein said reference station
comprises an acoustic beacon.
49. A method according to claim 40, wherein said reference station
is placed on the surface of the water, preferably in a buoy or a
vessel.
50. A method according to claim 49, wherein said reference station
receives absolute position data from a positioning system usable at
the surface of the water, and relays such data to said submersible
vessel.
51. A method according to claim 49, wherein said reference station
exchanges communication data with a communication system usable at
the surface of the water, and preferably as well exchanges such
data with said submersible vessel.
52. A method according to claim 49, wherein said reference station
is placed in a submersible vessel being surfaced during use of the
reference station.
53. A system for determining the absolute position under water of a
submersible vessel by means of the method in claim 1, the system
comprising: a reference station having acoustic communication
means; acoustic communication means on board the vessel; a dead
reckoning navigation system on board the vessel; wherein the system
comprises computing means, preferably on board the vessel, adapted
to estimating absolute position data from consecutive receptions of
signals from one and the same reference station, together with
relative position data from the dead reckoning navigation
system.
54. A system according to claim 53, wherein the dead reckoning
system comprises an Inertial Navigation System.
55. A system according to claim 53, wherein the dead reckoning
system comprises: a number of gyros; a number of accelerometers; a
Doppler Ground Velocity Log; a direct or indirect speed of sound
measurement sensor; and a pressure sensor.
56. A system according to claim 53, wherein the submersible vessel
is adapted to carry a number of reference stations and to launch
the stations independently.
57. A system according to claim 53, wherein the submersible vessel
is adapted to collect a number of reference stations.
58. A system according to claim 53, wherein the reference stations
are acoustic transponders or beacons, resting on the sea floor or
suspended above an anchor resting at the sea floor.
59. A system according to claim 53, wherein the reference stations
are located on buoys or vessels floating at the surface of the
water.
60. A system for determining the absolute position under water of a
submersible vessel by means of the method in claim 19, the system
comprising: a reference station having acoustic communication
means; acoustic communication means on board the vessel; a dead
reckoning navigation system on board the vessel; wherein the system
further comprises computing means, preferably on board the vessel,
adapted to estimating absolute position data from one or more
receptions of signals from one and the same reference station,
together with relative position data from the dead reckoning
navigation system.
61. A system according to claim 60, wherein the dead reckoning
system comprises an Inertial Navigation System.
62. A system according to the claim 60, wherein the dead reckoning
system comprises: a number of gyros; a number of accelerometers; a
Doppler Ground Velocity Log; a direct or indirect speed of sound
measurement sensor; and a pressure sensor.
63. A system according to claim 60, wherein the submersible vessel
is adapted to carry a number of reference stations and to launch
the stations independently.
64. A system according to claim 60, wherein the submersible vessel
is adapted to collect a number of reference stations.
65. A system according to claim 60, wherein the reference stations
are acoustic transponders or beacons, resting on the sea floor or
suspended above an anchor resting at the sea floor.
66. A system according to claim 60, wherein the reference stations
are located on buoys or vessels floating at the surface of the
water.
Description
[0001] The invention relates to a method and a system for
determining the absolute position under water of a submersible
vessel, such as e.g. an unmanned, autonomously operating submarine,
as well as a method for scanning an underwater survey area.
[0002] The submersible vessel is of the kind having a dead
reckoning navigation system not receiving position information from
outside the vessel, and the vessel collects data by means of
acoustic signals from a reference station having a known absolute
position and calculates its distance from the reference station by
computing means, preferably an on-board computer.
[0003] Several methods and systems are known for unambiguously
determining the position of a vessel or vehicle on the surface of
the earth or surface of the sea. As examples, generally known
satellite navigation (GPS; NAVSTAR; GLONASS), and the previously
known DECCA and LORAN systems can be mentioned.
[0004] Such systems are almost universally based on short wave
radio signals and are thus not usable under water due to the very
poor propagation of such radio signals through water, especially
sea water.
[0005] Systems are known for communicating and/or navigating under
water by means of very long- wave radio signals, but such systems
do not offer features needed for precise commercial navigation,
such as availability, resolution and precision.
[0006] A principle and a system for acoustic underwater navigation
over limited distances is known as "Long Base Line Navigation"
(below designated as "LBL"); cf. e.g. Jerome Vaganay et al.:
"Outlier Rejection for Autonomous Acoustic Navigation", Proc. IEEE
Int'l. Conf. Robotics and Automation, Minneapolis (US) April 1996;
or, for a more exhaustive discussion P. H. Milne: "Underwater
Acoustic Positioning Systems", Gulf Publishing Company, Houston
(US) 1983, ISBN 0-87201-012-0.
[0007] Using LBL, a number of reference points are established by
placing e.g. transponders on the sea floor in a net or array. Such
transponders are adapted to each transmitting an individual
acoustic signal when they receive a common acoustic signal.
[0008] It is known as well to use for this purpose e.g. acoustic
beacons, which simply transmit acoustic signals in an autonomous
mode.
[0009] When a submersed vessel is to determine its position
relative to such a net of transponders, the vessel transmits an
acoustic interrogation signal on a common frequency. Upon receival
of the interrogation signal, each transponder transmits a response
signal on its own individual frequency, after a predetermined,
individual delay.
[0010] The response signals are picked up by hydrophones in the
vessel, and a system on board analyses the time delays of the
returning response signals and calculates the distance to each
transponder; based hereupon, the position of the vessel relative to
the positions of the transponders can be determined unambiguously,
provided certain conditions as to the number and location of the
transponders are met.
[0011] The position of each transponder may e.g. be determined as
described in Milne, paragraph 5.2, p. 55 et seg. Often, a unit with
similar acoustic equipment as the submersible vessel is suspended
from a surface vessel having absolute position determining means
such as GPS navigation. The surface vessel is positioned in
different positions, determined by means of the navigation system.
From each of these positions, the unit exchanges signals with the
transponders, as explained. Based upon sufficiently many sets of
time delay registrations from these different known positions, the
locations of each transponder may now be calculated with
satisfactory precision.
[0012] In some known LBL systems, the transponders are able to
determine the distance between each other. This eliminates the need
for determining the position of every transponder from the surface;
when the positions of a few transponders with sufficient spacing
are known, the positions of the rest of the transponders may be
determined by simple triangulation.
[0013] It is a disadvantage of LBL navigation that placing and
calibrating many transponders is necessary, the transponders often
being quite expensive and not always being recovered successfully
after a mission. A minimum of three transponders is necessary in
order to be able to determine any one position, cf. Milne chapter
5, in particular section 5.2.
[0014] In the conference paper A. Ph. Scherbatjuk: "The AUV
Positioning using Ranges from One Transponder LBL", OCEANS '95,
MTS/IEEE Proceedings of `Challenges of Our Changing Global
Environment`, 1995, ISBN 0933957149, vol. 3, pp. 1620-1623,
disclosure is made of navigating an underwater vehicle using range
data from only one LBL type transponder.
[0015] In this paper, serious restrictions are however placed on
the usable areas of survey and trajectories of the underwater
vehicle. It is thus a precondition for use of the method disclosed
that the vehicle operates at constant depth, and follows
trajectories shaped as regular meanders, made up entirely of
straight lines. It is explicitly stated in the summary of the paper
that the "application of the meander like trajectories is not a
serious restriction for use of the method . . . ". Thus, the author
admits that his method will only work if these restrictions are
being complied with.
[0016] Of course, the method disclosed in this paper will not be of
general use, contrary to the author's assertion, since most
underwater tasks will indeed imply the underwater vehicle following
many differently shaped trajectories.
[0017] In the conference paper Richard J. Babb: "Navigation of
Unmanned Underwater Vehicles for Scientific Surveys", AUV '90, IEEE
Proceedings of the Symposium on Autonomous Underwater Vehicle
Technology, 1990, pp. 194-198, it is in section 4 suggested to
combine LBL with Dead Reckoning (referred to below as "DR"). DR may
e.g. comprise use of an acoustic log measuring the speed relative
to the sea floor, and/or use of an Inertial Navigation System
(referred to below as "INS").
[0018] Hereby is it possible to obtain satisfactory results with
fewer transponders than in simple LBL, DR being used when
navigating between areas in which response can be had from as many
transponders as necessary for determining the position by the LBL
method with adequate accuracy.
[0019] It is briefly described in section 4 of this paper with
reference to FIG. 4 of same paper that "Since the DR system is
capable of determining the direction of the course made good over
ground to high accuracy (much better than 1 degree) it is possible
to obtain an unambiguous fix from a single transponder, by
combining the radial distance to the transponder with the true
course from the DR system" (a "fix" meaning a determination of
absolute position).
[0020] The skilled person will know that both of the terms "the
direction of the course made good" and "true course" has the same
meaning, that is the direction of the vehicle trajectory over
ground with respect to true (geographic) north.
[0021] The only way combination of "true course" with measurement
of range can provide an "unambiguous" fix is by having the vehicle
travel in a straight line (as clearly indicated in FIG. 4) while
receiving signals from the transponder. At the position where
minimum range is recorded (as clearly indicated by the dotted lines
in FIG. 4), the transponder is known to be located in a direction
orthogonal to the "direction of the course made good" at the
measured minimum distance (as indicated by the dotted lines in FIG.
4).
[0022] Since now both direction and range to the transponder is
known, an unambiguous fix is indeed provided. However it is a
significant drawback of the described method that the vehicle has
to follow a linear trajectory past the transponder. In particular,
any inability of the vehicle guidance system to exactly follow a
straight line will introduce additional errors into the position
fix.
[0023] In general, the method described to obtain an unambiguous
fix will provide two solutions for the position of the vessel
relative to the transponder, that is, one position to the starboard
and one position to the port side of the transponder (seen in the
direction of travel of the vessel). Some essential prior knowledge
of the position of the transponder and the trajectory of the vessel
will therefore be necessary in order to discriminate correctly
between such two solutions.
[0024] In the paper, the right solution seems to be chosen using
prior knowledge of the trajectory of the vessel relative to the
absolute position of the transponder. This is a severe disadvantage
of the method disclosed in the paper.
[0025] In the conference paper A. Ph. Scherbatjuk et al.:
"Integrated Positioning System for Underwater Autonomous Vehicle
MT-88", OCEANS 94, IEEE Proceedings of Oceans Engineering for
Today's Technology and To-morrow's Preservations, 1994, ISBN
0780320565, vol. 3, pp. III/384-388, a similar integrated
positioning system is disclosed.
[0026] This system is based upon joint processing on board the
underwater vehicle of data from an on-board autonomous navigation
system and data from a Long Base Line acoustic positioning
system.
[0027] The LBL data are provided by use of at least two and
preferably three transponders, but it seems not to be explained how
large a sea floor area may be surveyed by means of this system.
[0028] In the conference paper A. H. Carof: "Acoustic Differential
Delay and Doppler Tracking System for Long Range AUV Positioning
and Guidance", IEEE Proceedings of the 1994 Symposium on Autonomous
Underwater Vehicle Technology, 1994, ISBN 0780318080, pp. 370-375,
a system consisting of two dual frequency synchronised reference
beacons and a hydrophone is described.
[0029] The beacons concurrently transmit signals at individual,
fixed and predetermined frequencies. Using the hydrophone, the
underwater vehicle measures the differential delay and the
differential doppler shift of said signals transmitted from said
beacons. Assuming and using prior knowledge of the velocity of the
underwater vehicle the position is determined from said
measurements.
[0030] From U.S. Pat. No. 5,357,437 an underwater navigation system
is known where a submersed vessel drops one or more magnetic
markers on the sea floor and uses the magnetic field from these
markers for determining its current position. The markers may have
permanent magnets or electro-magnets fed by alternating
current.
[0031] Further, the vessel has a DR and/or INS navigation system
and will therefore be able to navigate for some time without
position signals from the markers.
[0032] It is a substantial disadvantage by this known system that
the absolute position of the markers is not determined.
[0033] Finally, from U.S. Pat. Nos. 5,119,341 and 5,579,285,
methods and apparatus are known for extending radio based
navigation systems such as the GPS for use under water.
[0034] A number of buoys are floating on the surface, and each buoy
is continuously recording its position by receival of signals from
the radio based navigation system. Each buoy transmits acoustic
signals into the water.
[0035] In the latter of the two U.S. Pat. No. 5,119,341, the
position of a submersed vessel may then be determined relative to
the buoys, e.g. by means of a strategy corresponding to the one
used in the GPS itself. Data is besides transmitted between the
submersed vessel and a land based station through the same acoustic
channels and through radio channels via communication
satellites.
[0036] Apparently, only in the former of the US Patents the vessel
is in fact determining its position in this way; in the latter
patent the position it determined at the surface or the land based
station.
[0037] U.S. Pat. No. 5,579,285 also mentions a concept for
determining the position of the underwater vehicle from using only
a single buoy, col. 4, l. 44 et seg. However, the mentioned
approach requires the buoy be carried by and released from the
underwater vehicle. Also, the buoy has to be pre-programmed with
the position at which it is released.
[0038] Of course, the underwater vehicle will not "know" where it
is, otherwise it would not need to release any buoy to get a
position "fix". Hence the position of releasing the buoy will in
any case be indefinite. Alternatively, a position estimate has to
be programmed into the buoy prior to its release from the
underwater vehicle, which requires the vehicle to have this
capability.
[0039] The buoy also needs to know the direction of the trajectory
which the underwater vehicle intends to follow after the release of
the buoy. Especially for military applications it may also be a
significant disadvantage that the underwater vehicle has to
transmit signals, eliminating the possibility of covert
operation.
[0040] Consequently, this disclosure seem not to provide any usable
solution either.
[0041] It is an object of the present invention to provide a method
and a system for determining absolute position under water, wherein
a precise position can be obtained from a minimum number of
reference stations, preferably only one reference station.
[0042] It is a further object of the invention to eliminate the
need for a surface vessel to deploy and calibrate transponders or
to generally assist or support the submersible vessel.
[0043] It is finally an object of the invention to provide a method
for scanning an underwater survey area with substantially reduced
needs for deployment of transponders or assistance or support from
surface vessels.
[0044] In a method for determining the absolute position under
water of a submersible vessel having a dead reckoning navigation
system not receiving position information from outside the vessel,
where the vessel receives acoustic signals from a reference station
having a known absolute position and calculates its range from the
reference station, said objects are met in that signals are
received from one reference station in several positions of the
vessel, and that estimated absolute positions of the vessel are
calculated using range data based on the received signals and using
relative position data from the dead reckoning navigation
system.
[0045] By receiving and processing said signals in several
positions of the vessel, a corresponding multiple-dimensional
measurement and/or redundance is being had as when receiving
signals in one position from several reference stations.
[0046] Preferably, signals are received and data processed at short
intervals of time, providing for a substantially continuous
estimation of absolute position data. Hereby, a determining and
updating of position is achieved, far superior to the prior
art.
[0047] Preferably, the position of the reference station in a
relative coordinate frame of said dead reckoning navigation system
is estimated. This provides for an advantageous mathematical
modelling of the absolute position on board the vessel.
[0048] In one embodiment, the estimated absolute position data are
preferably used for updating the dead reckoning system's relative
position data. The latter data will hereby constitute a continuous
source of reliable absolute position information.
[0049] It is generally preferred that estimates are made of
parameters intrinsic to the nature of the dead reckoning navigation
system; such as sea currents, and relative position data from the
dead reckoning navigation system are compensated by the estimate of
said parameters.
[0050] In this way, the impairing influence from such parameters
will be minimised in an efficient way.
[0051] Preferably, a least-squares algorithm is used in determining
the values of the estimates. This algorithm has proven to be
superior in supplying fast and reliable estimates.
[0052] According to a preferred embodiment of the invention, data
for rate of change of the vessel's range from the reference station
are derived from received signals together with range data.
[0053] Hereby, the requirements to the number and/or quality of the
measurements are vastly reduced, providing for increased accuracy
and reliability of the method of the invention.
[0054] Said "Range Rate" data are preferably derived from
recordings of doppler shifts of frequencies of the acoustic signals
from the reference station, or alternatively from recordings of
spread spectrum pulses in the acoustic signals from the reference
station. Both methods have shown to provide reliable and efficient
range rate data.
[0055] It is generally preferred to estimate the position of the
reference station in a relative coordinate frame of said dead
reckoning navigation system from processing of data comprising
range rate data as well as range data. As stated earlier, this
provides for an advantageous mathematical modelling of the absolute
position on board the vessel.
[0056] Preferably, said estimates are made further utilising
information on the depth of the reference station. Hereby, an
advantageous redundancy is introduced into the position estimating
data.
[0057] In a method for scanning an underwater survey area by means
of a submersible vessel travelling a desired path, the vessel
having a dead reckoning navigation system not receiving position
information from outside the vessel, where the vessel receives
acoustic signals from a reference station having a known absolute
position and calculates its range from the reference station, the
objects mentioned earlier are met in that the absolute position of
the vessel is intermittently being determined according to the
method of the invention.
[0058] Hereby, an underwater area may be surveyed using only a
single reference station, or at least a vastly reduced number of
reference stations, relative to the prior art.
[0059] Preferably, if said area extends beyond the operational
reach of the reference station, the intended trajectory of the
vessel is arranged to bring the vessel within said operational
reach at regular intervals of time.
[0060] Hereby, the ability of the DR system to navigate with a
satisfying precision for a limited period of time is utilised in an
optimal way, while at the same time a satisfying precision is
maintained for the duration of the entire mission.
[0061] It is preferred that the intended trajectory of the vessel
is arranged to bring the vessel within a minimum distance of every
point in the area, in order to ensure a complete coverage of the
area with a minimum of effort and expense.
[0062] In preferred embodiments of the methods of the invention,
the reference station is placed at a fixed absolute position.
[0063] This provides for a simple and effective configuration and
minimal costs.
[0064] In an especially preferred embodiment of the invention, the
absolute position of the reference station is determined by the
submersible vessel at the surface of the water collecting absolute
position data in a number of positions from a positioning system
usable at the surface of the water, and while surfaced receiving
acoustic signals from the reference station, and calculating range
data from said signals, position and range data preferably being
processed on board the vehicle.
[0065] Further, in this embodiment the reference station is
preferably launched from the submersible vessel, and especially
preferred as well collected by the submersible vessel after
estimating the absolute position.
[0066] In these very important embodiments, the need for a surface
vessel is reduced, and may in fact often dispensed with. Hereby
costs for an underwater survey mission may be further minimised in
a very efficient way.
[0067] The reference station may preferably comprise an acoustic
transponder, or alternatively an acoustic beacon. These features,
known per se, will provide efficient signal sources for various
types of mission.
[0068] In another preferred embodiment, the reference station is
placed on the surface of the water, preferably in a buoy or a
vessel.
[0069] According to the invention, this will provide for the very
important possibility of giving the reference station direct access
to reliable absolute position data via e.g. the GPS.
[0070] Preferably, such data are relayed to the submersed vessel,
providing for immediate updating of the DR system's absolute
position estimate.
[0071] Further in this embodiment, it is preferred that the
reference station exchanges communication data with a communication
system usable at the surface of the water, and exchanges such data
with the submersible vessel.
[0072] Hereby an efficient channel of communication to and from the
vessel may be established in a particularly simple and advantageous
way.
[0073] In a certain embodiment of the invention, the reference
station is placed in a submersible vessel being surfaced during use
of the reference station.
[0074] This provides for, say, a pair of submersible vessels
operating autonomously, or subject to remote control, in an
underwater mission for very extended periods of time.
[0075] In a system for determining the absolute position under
water of a submersible vessel, the system comprising:
[0076] a reference station having acoustic communication means;
[0077] acoustic communication means on board the vessel;
[0078] a dead reckoning navigation system on board the vessel;
[0079] the objects mentioned are met in the system further
comprising computing means, preferably on board the vessel for
estimating absolute position data from consecutive receivals of
signals from one and the same reference station.
[0080] These features will enable the system to operate according
to the methods of the invention.
[0081] The dead reckoning system is preferably an Inertial
Navigation System, and/or preferably comprising:
[0082] a number of Ring Laser Gyros;
[0083] a number of solid-state accelerometers;
[0084] a Doppler Ground Velocity Log;
[0085] a direct or indirect speed of sound measurement sensor;
and
[0086] a pressure sensor.
[0087] These features provide for reliable autonomous navigation of
the underwater vehicle for comparatively long periods of time
without having the DR system updated.
[0088] In a preferred embodiment of the system, the submersible
vessel is adapted to carry a number of reference stations and to
launch the stations independently.
[0089] It is further preferred that the submersible vessel is
adapted to collect a number of reference stations.
[0090] These very important features further provide for dispensing
with the need for an assisting surface vessel, as has been
explained above.
[0091] In the system, the reference stations are preferably
acoustic transponders or beacons, resting on the sea floor or
suspended above an anchor resting at the sea floor, or
alternatively being located on buoys or vessels floating at the
surface of the water.
[0092] By these measures, advantages are obtained corresponding to
those explained above with reference to the methods of the
invention.
[0093] The invention will be explained in more detail below, by
means of embodiment examples and with reference to the drawing, in
which same reference designations indicate similar objects in all
figures, and in which:
[0094] FIG. 1 shows determining of the absolute position of a
submersible vessel by means of an LBL navigating system of the
prior art, in schematic top view;
[0095] FIG. 2 shows determining of the absolute trajectory and
absolute positions of a submersible vessel by means of the method
according to the invention, in schematic top view;
[0096] FIG. 3 shows an unmanned submersible vessel determining the
absolute position of a transponder placed on the sea floor, and
afterwards determining its own position relative to the transponder
when submersed near the sea floor, in schematic perspective
view;
[0097] FIG. 4 shows a path to be followed by a submersible vessel
scanning an underwater survey area, in schematic top view;
[0098] FIG. 5 shows the same procedure as in FIG. 3, where the
submersible vessel itself drops a transponder and calibrates it as
the need for absolute position determining arises, in schematic
side view;
[0099] FIG. 6 shows an underwater vehicle determining its absolute
position from one or two processings of acoustic signals from a
single transponder, in schematic side view;
[0100] FIG. 7 shows a submersible vessel determining its absolute
position from one or two processings of passively received acoustic
signals from a single buoy, in schematic side view;
[0101] FIG. 8 shows an underwater vehicle determining its absolute
position from a number of measurements on passively received
acoustic signals from a single acoustic beacon or "pinger", in
schematic side view; and
[0102] FIG. 9 shows an underwater vehicle determining the absolute
position of a submersed pipe-line and later surveying the pipe-line
using previously collected position data.
[0103] FIG. 1 illustrates the basic principle of working of a
generally known LBL navigating system. FIG. 1 is a top view of an
area of the sea floor. A submersed vessel 1 is navigating near the
sea floor. On the floor, or suspended above it, four transponders
2-5 are placed.
[0104] The transponders are of any type known per se for use in LBL
systems; they are each equipped with a hydrophone and a speaker,
the positionings of which are indicated by reference numerals
11-14.
[0105] Each of the transponders 2-5 are adapted to transmit an
acoustic response signal via the speaker upon receival of an
acoustic interrogation signal via the hydrophone. Preferably, the
response signal is delayed by a predetermined time delay unique to
each transponder, relative to the receival of the interrogation
signal, and the response signal is transmitted on a frequency
unique to each transponder.
[0106] Thus, when a unit is transmitting an interrogation signal,
it will receive response signals from the transponders with
different delays and on different frequencies.
[0107] By determining the time delay from transmittal of the
interrogation signal to receival of a response signal on the
frequency of one particular transponder, the distance or "range"
from the unit to this transponder may then be calculated when the
sound velocity in the water and the time delay of the transponder
are known.
[0108] The absolute positions of the transponders are determined
beforehand, preferably by triangulation from a surface vessel
establishing its own positions from e.g. a satellite navigation
system such as the GPS.
[0109] The submersible vessel 1 is likewise equipped with a
hydrophone and a speaker, the positioning of which is indicated by
reference numeral 10.
[0110] In order to determine its position when needed, the
submersed vessel now transmits an interrogation signal from its
speaker 10, the signal being received by the hydrophones 11-14 of
the transponders 2-5. The transponders transmit response signals as
mentioned from their speakers 11-14, and these signals are received
by the hydrophone 10 of the vessel 1.
[0111] Now, the four individual delays are determined and processed
in a system inside the vessel 1, and the distances 6-9 to each of
the transponders are calculated as mentioned. Hereby, the position
of the vessel 1 relative to the transponders 2-5--and thus the
vessel's absolute position--may be calculated. This is preferably
done on board the vessel, which then will be able to utilise the
calculated absolute position when navigating.
[0112] Transponders of the type utilised may be equipped with
sensors such as pressure, temperature and salinity sensors for
determining speed of sound and depth of the transponder. This
information may then be relayed to the interrogator via telemetry
and used for aiding use of the transponder.
[0113] Further, transponders are known which are able to determine
the distance between each other, thus aiding in determining their
absolute positions.
[0114] It is a precondition for reliable and reproducible
determining of the position, however, that the "Baseline" i.e. the
distance 26 between any two transponders is sufficiently long
compared to the measured distances 6-9; hence the designation Long
Base Line (LBL). In other words, acute angles should be avoided
between lines from the vessel to any two transponders.
[0115] In FIG. 2, one embodiment of the method according to the
invention is illustrated, in the same view as in FIG. 1. A
submersed vessel 1 is navigating near the sea floor; on the floor,
or suspended above it, one transponder 19 is placed. The vessel 1
is to follow a desired trajectory 25.
[0116] According to the invention, the vessel 1 is equipped with a
Dead Reckoning (DR) navigation system, enabling the vessel to
navigate with a desired accuracy for shorter periods of time
without updating its absolute position. Such a navigation system is
known per se and will typically comprise a compass, a log and a
depth indicator. In high-grade systems of this kind, an Inertial
Navigation System (INS) will be included as well.
[0117] In the present embodiment, the DR navigation system is
preferably of very high grade, enabling the submersed vessel 1 to
navigate for rather long periods of time with only small deviations
from the absolute position. State of the Art in this respect will
enable the DR navigation system to estimate the absolute position
of the submersed vessel with a maximum deviation of 3 m over a
period of time of 1 hr; or about 0.03% to 0.1% of the distance
made, depending on the regularity of the trajectory.
[0118] A preferred DR navigation system for the described use will
preferably comprise the following systems: a so-called "Strap-Down"
Inertial Navigation System (INS) (i.e. an INS where the inertial
sensors are attached rigidly ("strapped down") to the body of the
vehicle, e.g. based on Ring Laser Gyros and solid-state
accelerometers), a Doppler Ground Velocity Log (DVL) or Correlation
Velocity Log, a CTD sensor (Conductivity, Temperature and Depth) or
a direct speed of sound measurement sensor, and a pressure sensor.
Such a system is able to determine the absolute heading by
"alignment", by sensing the gravity and earth rotation vectors and
measure absolute velocity by means of the log.
[0119] A suitable INS is the type KN-5053 from Kearfott G & N
Corporation of America, New Jersey, USA.
[0120] In the embodiment example in FIG. 2, the submersed vessel 1
follows the trajectory 25 crossing through the points 15-18. The
vessel 1 interrogates the transponder 19 in each of these points,
and from the response signals of the transponder, the distances
(ranges) 21-24 are calculated.
[0121] The vessel's 1 DR navigation system will be able to navigate
accurately through the whole of the path 15-16-17-18-15 relative to
the starting point 15, if the path be concluded in a sufficiently
short time. Assuming this, the mathematical problem of determining
the absolute positions 15-18 in the path 25 relative to the known
position of the transponder 19 reduces to a problem of the same
kind and complexity as the problem in FIG. 1 of determining the
position of the vessel 1 relative to the known positions of the
transponders 2-5.
[0122] The same geometrical requirements apply for the location of
the points 15-18 as for the transponders 2-5 in FIG. 1, i.e. the
baselines should be long and acute angles should be avoided in
order to achieve optimum accuracy.
[0123] In practice, the position of the transponder 19 will
preferably be determined in a relative coordinate system of the
vessel's 1 DR navigation system. The offset of the DR navigation
system is easily calculated as the difference between the known
absolute position of the transponder and the determined relative
position of the transponder, and hence the absolute position of the
path 25 is determined.
[0124] It is generally preferred that estimates of the position of
the vessel 1 relative to the single transponder 19 are derived by
means of the position output by the DR navigation system and the
distance measurements. Furthermore, parameters intrinsic to the
nature of the DR navigation system, such as e.g. the sea current,
may be estimated as part of the process. Each measurement of
distance is related to the position output by the DR navigation
system compensated by the estimate of parameters intrinsic to the
DR navigation system, and an estimate of the position of the
transponder in the relative coordinate frame of the DR navigation
system is made.
[0125] Preferably, a least-squares algorithm (cf. e.g. Lennart
Ljung: "System Identification, Theory for the User", Prentice-Hall,
1987, ISBN 0-13-881640-9) is used to determine values of the
parameters intrinsic to the DR navigation system and a position of
the transponder in the relative coordinate frame of the DR
navigation system that best fit the set of relations obtained from
the distance measurements, with respect to minimising of squares. A
recursive method such as e.g. the Kalman filter known per se may
also be used to implement the triangulation calculations.
[0126] The offset of the DR navigation system is now calculated as
the difference between the known absolute position of the
transponder, and the determined position in the relative coordinate
frame of the DR navigation system.
[0127] In general, a minimum of three or four range measurements
will provide an unambiguous solution, subject to whether or not the
depth of the transponder is known a priori (the depth of the
vehicle may be measured directly by pressure sensing, as
mentioned). A few more ranges will be required if the DR system's
log only measures displacement relative to the body of water. In
general, extra ranges would be measured as well in order to add
redundancy in case of spurious measurements, and to improve
accuracy.
[0128] It is an important advantage of the method according to the
invention that the vehicle is not required to follow any specific
path. It is also an advantage of the method that the position of
the vehicle relative to the transponder is determined without
requiring knowledge of the absolute position of the
transponder.
[0129] Regarding the LDL systems of the prior art, it is well known
that in order to achieve optimum accuracy, the baselines between
each transponder must be long and acute angles be avoided. The
present invention however, holds a very significant advantage over
LBL systems in that it is possible to make range measurements to
the one transponder from any number of positions, i.e. a much
higher number than the number of transponders in a LBL system. The
LBL equivalent of this would be deploying a correspondingly large
number of transponders, which is both very impractical and very
expensive.
[0130] If the DR system is not able to compensate for sea currents,
i.e. if only speed through water is measured, the sea current may
also be estimated from the range measurements and hence be
compensated for. In this case, however, the number of measurements
will have to be increased in order to obtain the same accuracy.
[0131] It is seen from the above that prerequisites for the
accurate navigation in FIG. 2 are i.a.: 1) a DR navigation system
on board the vessel 1; 2) that the vessel has to travel a
trajectory of certain length and dimensions before knowing its
updated absolute position; whereas 3) in return, only one
transponder 19 with a known absolute position is needed.
[0132] In addition to reliable position fixes in the points 15-18,
the method of the present invention provides accurate position
information along the entire path 15-16-17-18-15, and for some time
following the last fix as well, subject to the quality of the DR
navigation system. It is not possible to fully compensate a heading
error of the DR navigation system using only range measurements
from one fixed source. Thus, DR navigation system of considerably
higher quality than those commonly used in LBL systems will be
preferred for use with the system of the invention.
[0133] As the costs for such an "extra high quality" DR navigation
system is an initial investment, whereas the need for several
transponders in an LBL system entails considerable operational
expenditures as compared to only one transponder, the method of the
present invention will allow very considerable savings as compared
to the known LBL navigation system.
[0134] In particular, the rather heavy costs for a surface vessel
normally needed to deploy, calibrate and recover the many
transponders needed in the known LBL system (typically around US$
30,000/day) may be cut to a minimum, or entirely dispensed with as
will be explained below.
[0135] In FIG. 3, another embodiment example of the method of the
invention is illustrated. An unmanned underwater vehicle 1 has been
launched on the surface 41 of the sea, a lake or a river, and a
transponder 34 has been dropped on the floor 40 of the same body of
water.
[0136] The vehicle 1 has aerial means 30 and a suitable receiver
for receiving absolute position radio signals from a positioning
system such as GPS. In order to determine the absolute position of
the transponder 34, the vehicle 1 travels through a path 42
comprising a number of positions 51-53 and in each position
measures the range 35-37 to the transponder by means of acoustic
signals. The absolute position of the vessel 1 according to the
radio based positioning system is recorded for each of the
positions 51-53.
[0137] For reasons of clarity in the drawing, only three such
positions 51-53 with corresponding ranges 35-37 are shown. Even if
a minimum of three positions will be necessary to determine the
exact position of the transponder, a higher number will be
preferred in any case.
[0138] The position of the transponder in three dimensions is now
determined by triangulation, using the lines inter-connecting the
positions 51-53 as baselines together with the range measurements.
In order to provide some redundancy to the measurements, the
transponder may be provided with a depth indicator, giving an
priori depth information.
[0139] Also, the transponder may be equipped with additional
sensors for estimating the speed of sound to be used in converting
the time delay into a measurement of range.
[0140] The vehicle 1 then submerges (31) to near the sea floor 40
and travels through a path 32 comprising a number of positions
54-56. In each of these positions, measurements are made of the
range 57-59 from the vehicle to the transponder, in a similar way
as explained above with reference to FIG. 2.
[0141] As the absolute position of the transponder 34 has been
determined, the absolute positions 54-56 may now be calculated, and
the vehicle's 1 DR navigation system be updated accordingly. The
underwater vehicle 1 may now continue on its desired path 33, being
able to continuously record its estimated absolute position
according to data from its DR navigation system.
[0142] With the DR navigation system thus updated, the position
continuously given by this system is known to be correct within a
certain error which is increasing with time in a predictable
manner, subject to the intrinsic qualities of the DR navigation
system.
[0143] Future updatings of the DR system will be made as necessary
incorporating exchange of acoustic signals with the transponder 34,
as explained above with reference to FIG. 2.
[0144] In FIG. 4 is illustrated how an area extending far beyond
the acoustic range of one transponder may according to the
invention be surveyed.
[0145] According to the invention, the trajectory of the
submersible vessel is arranged in such a way that the vessel will
return to within the range of the transponder at regular intervals.
Thus, the DR system may be reset as soon as response signals from
the transponder can be had and analysed. As long as this objective
is met, the trajectory may be arranged in any way convenient for
the application in question.
[0146] It is a characteristic known per se of most DR navigation
systems that navigation in a confined area will cause some of the
inherent error sources to cancel out, improving DR performance in
terms of position error relative to distance travelled. It is
easily seen that the position error arising from e.g. a fixed
heading angle error in the DR system will be cancelled if the
vehicle travels along a straight line for a certain distance and
then returns along the same line to its starting point. In fact,
this will apply regardless of the trajectory made. It can be
demonstrated that similar conditions apply to several other forms
of error build-up in DR navigation systems.
[0147] In FIG. 4, an arrangement example of a suitable trajectory
for scanning an underwater survey area 68 is illustrated, which
utilises the characteristics mentioned.
[0148] An underwater vehicle 1 will start at the position 60, and
initially follow a path 61 such as a closed loop, updating its DR
navigation system as described above against the known absolute
position of the transponder 34 situated at or above the sea floor.
The acoustic reach or range 81 of the transponder is quite limited
as compared to the extension of the survey area 68, and is
illustrated by a circle 82 having a radius 81 equal to said
range.
[0149] With its DR navigation system thus updated, the vessel
proceeds surveying along a path 67 leading from the transponder,
and proceeding in directions 72 and back again in opposite
directions 73. The opposite direction portions 72, 73 of the path
67 are preferably offset from each other by a distance 74 in order
to scan the survey area 68 as regularly as possible. Along the path
67, the vehicle will collect desired samples or data, according to
the purpose of the mission in question, e.g. at positions
62-66.
[0150] According to the invention, the trajectory 67 is so arranged
that it leads back into relative vicinity of the transponder 34,
e.g. at 69. Here, the underwater vehicle 1 will exchange signals
with the transponder 34, collecting range data for determining its
absolute position, as explained with reference to FIG. 2. This is
done while the vehicle follows a suitable path 69, which may be a
closed loop, or e.g. a curve with a suitable radius as indicated in
FIG. 4, bringing the vehicle to the proper course for following the
next path 70.
[0151] As is seen from FIG. 4, the survey area is in the embodiment
illustrated being scanned in consecutive quadrants of the survey
area, the vehicle 1 following paths 67, 70, 71, etc., as indicated
in FIG. 4.
[0152] It is an important advantage of this scanning method that
the vessel 1 is brought back into relative vicinity of the
transponder 34, so that position error build-up in the DR system
can be cancelled by updating the DR system with the known absolute
position of the transponder 34.
[0153] It is an intrinsic advantage of this scanning method that
the position errors arising in the vehicle's DR navigating system
tend to balance out as the vehicle after following one path 72
turns around and travels back following an opposite course path 73,
returning to the vicinity of its starting point. Thus, the relative
position error of closely spaced points on lines in opposite
directions will be small.
[0154] It must be noted as a specific advantage that the path 67
may reach far beyond the distance 81 at which acoustic signal
contact may be had between the vessel 1 and the transponder 34.
[0155] The fundamental accuracy of the system is limited by the
fact that a fixed heading error is not observable from range
measurements to a fixed source. Thus, the expected maximum position
error is as a minimum the heading error in radians multiplied by
the distance to the transponder. Thus, a heading error of e.g. 0.5
milliradians and a maximum distance of 5 km will equal a position
error of 2-5 m.
[0156] The quadrants 70, 71, etc. are scanned in a way similar to
the scanning described of the first quadrant 67. Between the
scanning of two consecutive quadrants 67, 70, it is ensured that
the vehicle 1 travels a suitable path 69 within the range 81 of the
transponder 34, allowing a proper position fix to be had, and the
DR system of the vessel 1 to be updated accordingly.
[0157] A person skilled in the art will be able to devise suitable
trajectories following the principles explained above, for the
purpose of surveying areas that do not have a square or rectangular
configuration, or where certain specific conditions will have to be
considered.
[0158] Experiments have revealed that an area of 10.times.10 km
(100 km.sup.2) can be surveyed to an accuracy of less than 4 m with
the use of only one transponder in combination with a DR system
such as the make Kearfott G & N, type KN5053 "SeaNav"
doppler-inertial Navigation System.
[0159] This survey may be made with very considerable economical
savings indeed as compared to the known LBL system incorporating a
large number of transponders.
[0160] In FIG. 5, a particularly advantageous embodiment of the
method in FIG. 3 is illustrated. This embodiment is superior i.a.
in very elongate survey areas, such as when performing a "Line
Survey" i.e. surveying a quite narrow strip of the sea floor where
e.g. a cable or a pipe line is to be submerged.
[0161] In such a survey, a submersible vessel performing the survey
will typically travel along the siting only once, recording
characteristics of e.g. the sea floor.
[0162] As the vessel will thus never return or indeed travel back,
the method in FIG. 4 will not be of any use. Instead, the
embodiment shown in FIG. 5 of the method according to the invention
can be used.
[0163] The submersible vessel 1 carries a number of transponders 75
which can be dropped at command from the vessel's control system or
at command from e.g. a manned control centre. One transponder 34 is
dropped (43) before the vessel descends (31) to the operating
depth, and the position of the transponder is calibrated by the
vessel 1 travelling a path 42 while receiving position information
such as GPS via an aerial 30, as explained above with reference to
FIG. 3.
[0164] Descended to its operating depth, the vessel travels a path
32 and determines its absolute position as explained above with
reference to FIGS. 2-3. The DR navigation system of the vessel 1
now being updated, the vessel sets out on its mission, travelling a
desired path 33.
[0165] After a certain distance has been covered or a certain time
has lapsed, the predictable error in the absolute position as
estimated by the vessel's DR navigation system has reached a
predetermined maximum level, and a new position fix will be
necessary for updating the DR system.
[0166] Then, the submersible vessel 1 drops (44) one 45 of the
carried transponders 75. The position at which the DR navigation
system believes the transponder was dropped (the "relative
position" of the transponder), is recorded or alternatively, if
this does not provide sufficient accuracy, the "relative" position
of the transponder is determined by the vehicle travelling a path
as described above and recording range measurements.
[0167] The submersible vessel ascends (46) to the surface 41 and
determines the absolute position of the dropped transponder 45 by
travelling a path 47, receiving position signals and making range
measurements, in the same way as described above with reference to
the initially dropped transponder 34.
[0168] The vessel 1 then descends (48), determines its absolute
position by travelling a path 49, exchanging signals with the
dropped transponder 45, and then continues its mission (50).
[0169] The absolute position of the transponder now being known
allows for determining the position error prior to surfacing, by
comparing the transponder's relative position at the time (44) of
dropping to the transponder's relative position after the recent
updating of the DR system, the latter relative position now
referring to the absolute positioning system used at the
surface.
[0170] Since the position error is often an approximate linear
function of time and/or distance, it is possible to determine the
actual trajectory between the transponders (34, 45) with very high
accuracy by post-processing of data. Typically this post-processing
scheme will be able to compensate more than 90% of the position
error build-up between transponder fixes.
[0171] According to the invention, transponders can be carried and
dropped in any number necessary for carrying out the mission in
question with any desired position accuracy, subject to limitations
mostly in the submersible vessel's 1 payload and battery
capacity.
[0172] To compensate for the change in buoyancy from dropping a
transponder the underwater vehicle may be equipped with a variable
buoyancy system, known per se. Alternatively, an object having a
positive buoyancy may be released together with each
transponder.
[0173] The embodiment in FIG. 5 of an underwater survey has a very
distinct advantage in that it can be performed without use of a
surface vessel at all. The submersible vessel 1 may be launched by
means of a helicopter, the vessel carrying the required number of
transponders at launch. When the mission is finished, the vessel is
recovered by helicopter as well after surfacing. The costs for such
two helicopter missions are much lower than for a survey ship
mission, primarily owing to the much shorter durations of the
former.
[0174] The transponders may be pre-programmed to surface by
dropping a weight after a predetermined time or at a suitable
command. According to one preferred embodiment of the invention, it
is however preferred to let the vessel 1 collect the transponder
immediately after updating of its DR system; in this way only one
transponder may be needed for the whole mission.
[0175] In a preferred embodiment of the method according to the
invention, the underwater vehicle, in addition to the range,
calculates the rate of change of the range (the "Range Rate") from
the acoustic response signal sent from the transponder upon
interrogation from the vehicle.
[0176] In one approach it is utilised that the range rate is
proportional to the doppler frequency shift of the response signal
and may be calculated from said doppler frequency shift. The
Doppler frequency shift is measurable since the frequency of the
response signal from the transponder is known priori.
[0177] One alternative approach would be to use spread spectrum
signalling techniques, as described in e.g. the conference paper T.
C. Austin: "The Application of Spread Spectrum Signalling
Techniques to Underwater Acoustic Navigation", AUV '94, IEEE
Proceedings of the 1994 Symposium on Autonomous Underwater Vehicle
Technology, 1994, pp. 443-449. In that case the response signal of
the transponder 34 would include spread spectrum pulses, e.g. with
a particular coding such as Barker Code or Gold Codes, pulses being
separated by a fixed and known amount of time T e.g. 0.1 s, 1 s or
5 s.
[0178] The separation in arrival time Tm of said spread spectrum
pulses will be detected by the vehicle's hydrophone and associated
spread spectrum detector circuits. The discrepancy dT=Tm-T is then
derived and used for calculating the rate of change of distance or
"range rate" (RR): RR=dT/T.times.v.sub.s, where v.sub.s is the
speed of sound. Other ways of determining the range rate from the
acoustic response signal(s) could be devised, and/or will be
familiar to the person skilled in the art.
[0179] As it will now be explained with reference to FIG. 6, this
embodiment of the invention allows for very much improved
performance in determining the absolute position under water of the
vehicle 1. In principle, reception of only one or two signals from
the transponder will suffice in order to obtain a recording of the
position in three dimensions of the vehicle relative to the
transponder.
[0180] In particular the underwater vehicle will be able to
determine absolute position close to the range limit of the
acoustic signals without having to perform an extended trajectory
to allow for a long baseline. This is because the range rate
provides significant information on the direction to the
transponder, which is complemental to and independent of the range
information provided by the time delay.
[0181] Assuming the vehicle 1 has an absolute log, such as e.g. a
Doppler Velocity Log measuring the velocity vector over the sea
floor in the direction of the axis of the vehicle, or other means
of measuring said velocity vector, and an attitude sensor such as
e.g. a magnetic or gyro compass or an INS, the velocity vector V of
the vehicle 1 will be known in direction as well as magnitude.
[0182] Now, the speed v of the vehicle 1 towards the transponder 34
is equal to minus said determined range rate. Said speed v is also
equal to the magnitude of V multiplied by the cosine of the angle
.alpha. between the velocity vector V and the direction towards the
transponder 34, i.e. the "dot product" of V and a unit vector in
the direction from the vessel towards the transponder.
[0183] Consequently, the transponder 34 will be located on the
surface of a cone having its vertex in the vehicle's 1 hydrophone
10 and its axis coinciding with the velocity vector V. This conical
surface is schematically illustrated in FIG. 6 by the two lines L,
M.
[0184] Further, the transponder 34 will be located on the surface
of a sphere S having its centre in the hydrophone 10 and a radius r
equal to the computed range (distance) to the transponder.
[0185] These two criteria defines a circle C (the intersection of
said conical surface with said spherical surface) lying in the
plane P and intersecting the plane of the paper in FIG. 6 in two
points A, B. The circle C is illustrated schematically in FIG.
6.
[0186] As the depth of the transponder is known (e.g. from the
initial triangulation or from telemetry data from a built-in
pressure sensor), this will be located in a horizontal plane D,
which intersects the mentioned circle in two points, being the
possible positions of the transponder as seen from the underwater
vehicle 1.
[0187] Distinguishing between the true and the false of these two
points is believed to be quite easy, as the virtual, absolute
position (the absolute position as estimated by means of the method
of the invention) of the false point will change from one
measurement to another, as the velocity vector changes. Also, once
it is established which point is the true one, the absolute
position of this point will be on record, and distinguishing
between future true and false points will be quite easy.
[0188] Accordingly, in this embodiment of the invention, only very
few, say, two or three recordings will suffice to establish the
absolute position in three dimensions of the vehicle 1 (the
absolute position of the transponder being known priori in three
dimensions), and future updatings of the vehicle's DR navigation
system may each be made with only one or very few calls to the
transponder. This will bring about considerable operational savings
as well as a much increased precision of the underwater
navigation.
[0189] The drawing in FIG. 6 is only schematic; of course the
velocity vector V and the transponder 34 will not normally both lie
in the plane of the paper, and thus the plane P will not normally
be orthogonal to the plane of the paper. Also, the depth (D) of the
transponder is illustrated as being coincident with the lowest
point on the circle C, which will not normally be the case.
[0190] Another preferred embodiment of the method of the invention
is illustrated in FIG. 7. Here, the reference station is a buoy 76
floating on the surface 41 of the water body and having an aerial
77 for receiving absolute position data from e.g. the GPS, and a
speaker 78 for transmitting acoustic data into the water. The buoy
may be anchored, it may be drifting with a velocity vector U, or
its motion may be controlled by a propulsion device, according to
the character of the water body and of the mission in question. The
buoy may be powered by batteries; by wind or solar power, by a
generator set or any combination thereof.
[0191] According to the invention, the buoy 76 continuously
receives absolute position and velocity data and preferably also
precise time information via the aerial 77. It transmits these data
acoustically into the water through the speaker 78.
[0192] A submersible vessel 1 travels submersed with a velocity
vector V; it has a hydrophone 10 and through this receives the data
transmitted from the buoy. The vessel also records the doppler
shift of an acoustical signal from the buoy 76 in order to obtain
range rate information, preferably the doppler shift of a carrier
frequency included in the signal and having a known, certain
frequency. Alternatively, the vessel determines range rate by means
of spread spectrum signalling technique as described in the
previous embodiment example.
[0193] Further, the vessel 1 may utilise depth information provided
by a depth indicator or pressure sensor on board the vessel 1.
[0194] Finally, the vessel 1 is equipped with an accurate clock,
previously being synchronised with a time base of the buoy,
preferably the time base used by the satellite positioning
system.
[0195] According to the invention, the following information is
preferably being processed in the vessel's control system or
computer:
[0196] 1) The absolute position and velocity of the buoy, being
comprised in the received signal;
[0197] 2) Distance information being derived from the difference
between the time information in the received signal and the time of
receival of the signal;
[0198] 3) Range rate information, i.e the rate of change of
distance between the vehicle 1 and the buoy 76, which is equal to
the projection of (V-U) in the direction of the position of the
buoy at the time of transmission, U and said position being
comprised in the received signal;
[0199] 4) Depth information as provided by a depth indicator in the
vessel 1;
[0200] 5) The vessel's absolute velocity vector as provided by the
vessel's log and heading reference; and
[0201] 6) The position estimate from the DR navigation system.
[0202] The mathematics required to determine two possible absolute
positions of the vessel now correspond directly to those used with
reference to FIG. 6, and an unambiguous position is determined
nearly as easily as in that case, the motion of the buoy being
known to the vessel from the contents of the acoustic signals.
[0203] This embodiment of the invention has numerous advantages.
Firstly, a buoy is easier to deploy and to collect than a
transponder, and in particular it does not need to be calibrated,
having access to e.g. GPS data. Secondly, no signals are
transmitted from the submersed vessel. This will be important to
military applications, and it will as well serve to save energy on
board. Thirdly, if the buoy is drifting, it will in fact be
possible to determine the absolute position of a stationary,
submersed vessel, utilising the method just explained.
[0204] It is not an absolute requirement of this embodiment that
the buoy transmits velocity information. Instead, only range
measurements may be used to calculate the absolute position of the
underwater vehicle.
[0205] Further, the buoy may transmit additional motion data such
as e.g. acceleration which may be utilised by the vessel to
determine position. It may be noted that the buoy will be very
similar to the buoys mentioned in U.S. Pat. No. 5,119,341.
[0206] Furthermore, it is not an absolute requirement that the
absolute velocity vector of the vehicle is known. If only the
velocity vector relative to the body of water is known, the sea
current may be estimated as well, at the expense of decreased
accuracy or increased number of measurements.
[0207] An embodiment of the method of the invention illustrated in
FIG. 8 will now be explained: A reference station in the form of a
unit 79 autonomously transmitting short acoustic signals ("pings")
from a speaker 80 at fixed and known intervals and frequencies, a
so-called "Pinger" or acoustic beacon, is placed at or near the sea
floor 40. An underwater vehicle 1 having a hydrophone 10 is
navigating with a velocity vector V within audibility of the
pinger.
[0208] Assuming accurate clocks are available in both the pinger
and the submersible vessel, the ping reception times will
constitute pseudo range measurements. As the submersible vessel
travels on, more pseudo ranges are made. It is seen that the
mathematical problem of determining the position of the pinger in
the relative coordinate frame of the DR navigation system is
equivalent to the well known GPS pseudo range problem, and may e.g.
be solved by using a least-squares algorithm, a Kalman filter, or
the algebraic solution given in Stephen Bancroft: "An Algebraic
Solution of GPS Equations", IEEE Transactions on Aerospace and
Electronic Systems, Vol AES-21 No. 7, January 1985, pp. 56-59. As
described in the previous embodiment, the range rate may also be
calculated and used to determine the relative position of the
beacon.
[0209] Assuming that the absolute position of the pinger has been
determined from the surface, the offset of the DR navigation system
is now calculated by subtracting the positions of the transponder
in the DR frame and the absolute frame, and hence the absolute
position of the submersible vessel is determined.
[0210] As one main advantage of using a pinger is that it is much
cheaper than a transponder 34 or a buoy 76 of the types mentioned,
no high requirements should be made of it. Accordingly, the
stabilities of its signal frequency and intervals might not be
usable as basis for measurements.
[0211] Similar to the previous embodiments, unknown parameters such
as e.g. interval and frequency of the beacon may to some extent be
estimated as part of the process. However, estimating additional
parameters will typically require additional measurements and may
put some constraints on the motion pattern of the vehicle in order
to provide observability.
[0212] Thus, experimental simulations have shown that provided
adequate computing power is installed on board the underwater
vehicle, and provided short term frequency stability of the pinger
can be assumed, it will be possible to determine the vehicle's
absolute position from such a low quality pinger signal.
[0213] In the extreme situation that no information is available as
to the pinger's absolute position or depth, this will require
rather many recordings of sets of the available variables, which
comprise:
[0214] The vehicle's depth;
[0215] The vehicle's absolute velocity vector (three dimensions and
magnitude), or velocity vector relative to the body of water;
[0216] Continuous displacement information from the vehicle's DR
navigation system; and
[0217] Received pinger acoustical signal frequency and short term
frequency deviations.
[0218] When these variables are recorded while the vehicle travels
a path which is varied appropriately as to course, depth, ascend
and speed, it will according to the simulations mentioned be
possible to solve the mathematical problem of determining the
absolute position of the vehicle 1.
[0219] Of course, any supplementary information will tend to
letting the mathematical problem be solved more quickly and with
less elaborate travel of the vehicle; such information could be
that the pinger frequency is in fact stable, or maybe even known
beforehand. Needless to say, if the absolute position of the pinger
is known as well, the problem is easily solved.
[0220] In cases where a survey area is to be surveyed more than
once, it will hardly be advantageous to collect the transponders
after use, as described with reference to FIG. 5. In such cases,
e.g. when a pipe-line is to be surveyed regularly, say, once a
year, it will be preferable to let the dropped transponders stay in
place, ready to be used in future missions. As the position of such
transponders do not change, it is an evident advantage to be able
to re-use the same transponders in future missions.
[0221] An example of a preferred embodiment of the method of the
invention for surveying pipelines, power cables, telecommunication
cables or other underwater installations with an elongated
configuration will now be explained with reference to FIG. 9.
[0222] The trajectory 83 of an installation 84 is known a priori to
a certain accuracy, e.g. 10 m. Following the procedures previously
described and using transponders 85-88, the installation 84 is
initially surveyed (33) by a vehicle 1 using e.g. a side-scan or
swath bathymetry sonar installation 89 which has a sufficiently
large "footprint" 90 on the sea floor to ensure that the
installation 84 is covered despite said uncertainty of its absolute
position and the predictable navigation error of the underwater
vehicle 1.
[0223] In the present embodiment of the invention, said initial
survey may be carried out using pre-deployed and pre-calibrated
transponders 85-88, in which case position fixes 98 are obtained
via range measurements 91 whenever passing a transponder, following
the procedure explained above with reference to FIG. 2, in order to
update the DR navigation system.
[0224] In either case, the distances 93 between consecutive
transponders 86-88 should be such that the accuracy of the
real-time navigation of the vehicle 1 between the position fixes
provided by means of range measurements from points 98 in the
initial survey and in subsequent surveys is sufficient for the
survey sensor 89, e.g. a side-scan sonar to sense the installation
84, and could be e.g. 10 km if a very high accuracy DR navigation
system is being used. Post-processing of the position data as
described above will provide an excellent estimation of the actual
trajectory 33 of the underwater vehicle 1 during the initial
survey.
[0225] The absolute trajectory 83 of the installation 84 will now
be determined from analysing the sonar imagery obtained of the
installation 84 during said initial survey. The transponders 85-88
remain on the sea floor for use in subsequent surveys of the
installation.
[0226] In subsequent surveys, an underwater vehicle 100 is
commanded to follow a trajectory 94 which is calculated from the
previously determined trajectory 33 to more closely follow the
installation 84, and hence allow close-range survey sensors 95,
such as e.g. a video camera, an acoustic camera, a swath bathymetry
sonar or a laser scanner (range finder), to sense the installation
84 from a shorter range.
[0227] Position fixes are obtained via range measurements 96 from
points 97 whenever the underwater vehicle 1 passes within the
acoustic range of a transponder 85-88. Since the trajectories 33,
94 may be almost straight lines, a position fix will be ambiguous
as to lying to the port or starboard side of the transponder (as
seen in the direction of travel of the vehicle 1, 100). However,
this ambiguity is easily solved using prior knowledge of the
locations of the transponders 86-88.
[0228] Following the survey trajectory 94 with the required
accuracy, e.g. 1-2 m, is a very challenging task even for a very
high grade DR navigation system. However, since the position error
of such high grade DR navigation systems is almost a linear
function, i.e. very systematic, of time and distance, especially in
the case of an almost linear trajectory, it will be possible to
update said DR navigation system after position fixes has been
obtained from two transponders 85, 86 with known absolute
positions.
[0229] This updating will typically provide a tenfold improvement
in heading accuracy allowing extended distances, e.g. 10 km to be
navigated between position fixes with the required accuracy, e.g.
1-2 m. It may be advantageous to deploy two transponders 85, 86
with a reduced interval 92 at the beginning of the survey to allow
a first updating to be performed without having to travel the full
distance 93 between transponders 86-88. In addition, an initial
path of the type 32 (FIGS. 3, 5) may be travelled by the vessel as
needed.
[0230] According to a further embodiment of the invention, it will
be possible to navigate for extended periods of time and over
extended areas or distances by means of at least two co-operating
submersible vessels. This will permit autonomous survey of a very
large area or a very long line without the need for reference
transponders or a survey ship.
[0231] In this embodiment, each of the co-operating submersible
vessels have aerial means and a suitable receiver for receiving
absolute position data from a positioning system such as GPS. Each
submersible vessel also have a speaker for transmitting acoustic
data into the water.
[0232] At regular intervals one of the submersible vessels ascends
to the surface from where it transmits position, velocity and time
data into the water by means of its speaker, in the same way as
explained for the buoy 76 (FIG. 7). These data are received by the
other, submersed vessel as described with reference to FIG. 7, and
used to update the DR system of that vessel.
[0233] Each of the submersible vessels may further be equipped with
generator means capable of recharging the vessel's batteries as
long as the vessel is surfaced. Thus accurate absolute navigation
over very extended ranges, even e.g. a transatlantic survey, can be
achieved. Also, the vehicles may communicate with a ground station
or an operator via satellite or surface radio communication means
while surfaced.
[0234] Even if in the preceding description and the attached
claims, reference is being made to navigation under the surface of
the sea, nothing will prevent the invention from being used in
other media where the signals of the generally used, radio based
navigation systems do not propagate effectively.
[0235] Even if in the preceding description and the attached
claims, reference is only being made to the use of the invention in
a submersible vessel, nothing will prevent the invention from being
of use in other types of submersible units, the positions of which
are to be determined.
* * * * *