U.S. patent application number 10/750904 was filed with the patent office on 2004-07-29 for method and apparatus for a satellite payload and radiodetermination.
This patent application is currently assigned to Inmarsat, Ltd.. Invention is credited to Kinal, George Vladimir, Nagle, James Robert II, Ryan, Fintan Richard, Soddu, Claudio.
Application Number | 20040145517 10/750904 |
Document ID | / |
Family ID | 26307994 |
Filed Date | 2004-07-29 |
United States Patent
Application |
20040145517 |
Kind Code |
A1 |
Kinal, George Vladimir ; et
al. |
July 29, 2004 |
Method and apparatus for a satellite payload and
radiodetermination
Abstract
A satellite radiodetermination system comprises global
navigation service (GNSS) satellites 2 such as GPS satellites,
which generate GNSS ranging signals R.sub.n, geostationary
satellites 6 which retransmit ranging signals R.sub.g generated at
a navigation land earth station (NLES) 8, including augmentation
data A, and medium earth orbit (MEO) satellites 10 which generate
ranging signals R.sub.m including regional augmentation data RA
transmitted from a satellite access node (SAN) 14. The regional
augmentation data RA is supplied by regional augmentation systems
21a, 21b. A navigation receiver 11 receives the ranging signals
R.sub.g, R.sub.m, R.sub.n and calculates ionospheric delay values
for those ranging signals which are provided on dual frequencies.
Using these ionospheric delay values, and optionally the regional
augmentation data RA and the augmentation data A, the navigation
receiver estimates ionospheric delay values for those ranging
signals which are provided on single frequencies. The navigation
receiver uses the ranging signals, corrected for ionospheric delay
and errors indicated by the augmentation data A and regional
augmentation data RA, to calculate position and time
accurately.
Inventors: |
Kinal, George Vladimir;
(London, GB) ; Nagle, James Robert II; (Northwood,
GB) ; Soddu, Claudio; (Harrow, GB) ; Ryan,
Fintan Richard; (Waybridge, GB) |
Correspondence
Address: |
BANNER & WITCOFF
1001 G STREET N W
SUITE 1100
WASHINGTON
DC
20001
US
|
Assignee: |
Inmarsat, Ltd.
London
GB
EC1Y 1AX
|
Family ID: |
26307994 |
Appl. No.: |
10/750904 |
Filed: |
January 5, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10750904 |
Jan 5, 2004 |
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09490154 |
Jan 24, 2000 |
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09490154 |
Jan 24, 2000 |
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08730208 |
Oct 15, 1996 |
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6040798 |
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Current U.S.
Class: |
342/357.31 ;
342/357.41; 342/357.44; 342/357.48 |
Current CPC
Class: |
G01S 5/0009 20130101;
G01S 19/072 20190801; G01S 19/41 20130101; G01S 19/071 20190801;
G01S 19/235 20130101; G01S 5/009 20130101; G01S 19/02 20130101;
G01S 19/40 20130101 |
Class at
Publication: |
342/357.02 |
International
Class: |
G01S 001/00; H04B
007/185 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 24, 1995 |
GB |
9521777.4 |
Claims
1. Satellite radiodetermination apparatus, comprising: a radio
receiver for receiving a multiple frequency ranging signal from a
first satellite and a further ranging signal from a second
satellite; deriving means for deriving first ionospheric delay data
from said multiple frequency ranging signal; and estimating means
for estimating second ionospheric delay data for the further
ranging signal on the basis of said first ionospheric delay
data.
2. Apparatus as claimed in claim 1, further comprising data
receiving means for receiving ionospheric data, said estimating
means being operable to estimate the second ionospheric delay data
additionally on the basis of said ionospheric data.
3. Apparatus as claimed in claim 2, wherein said data receiving
means is arranged to receive said ionospheric data from a
satellite.
4. Apparatus as claimed in claim 2 or 3, wherein said ionospheric
data represents a plurality of ionospheric delay values
corresponding to a plurality of spatially separated points.
5. Apparatus as claimed in any preceding claim, wherein said
estimating means includes modelling means for generating a model of
spatial variation of ionospheric delay on the basis of said first
ionospheric delay data, said estimating means being operable to
estimate said second ionospheric delay data on the basis of said
model.
6. Apparatus as claimed in claim 5 when dependent on claim 2,
wherein said modelling means is operable to generate said model on
the basis of said ionospheric data.
7. Apparatus as claimed in claim 5 or claim 6, wherein said model
comprises a function which is variable in accordance with one or
more parameters, said modelling means being arranged to calculate
said one or more parameters such that said function is fitted to
said first ionospheric delay data.
8. Apparatus as claimed in claim 7 when dependent on claim 6,
wherein said modelling means is arranged to calculate said one or
more parameters such that said function is additionally fitted to
said ionospheric data.
9. A method of satellite radiodetermination, comprising receiving a
multiple frequency ranging signal from a first satellite; receiving
a further ranging signal from a second satellite, deriving first
ionospheric delay data from said multiple frequency ranging signal;
and estimating second ionospheric delay data for the further
ranging signal on the basis of said first ionospheric delay
data.
10. A satellite payload adapted for a satellite designed for a
non-geostationary orbit, comprising: a clock for generating a time
signal; a ranging signal generator for generating a ranging signal
including timing data derived from said time signal; relaying means
for retransmitting data received by the satellite from a ground
station; and means for selectively activating and deactivating said
relaying means independently of the activation of the ranging
signal generator.
11. A satellite payload adapted for a satellite designed for a
non-geostationary orbit, comprising: a clock for generating a time
signal; a ranging signal generator for generating a ranging signal
including timing data derived from said time signal; relaying means
for retransmitting received data received by the satellite from a
ground station; means for detecting an absence of said received
data; and means for generating dummy data for transmission in
response to detection of said absence.
12. Apparatus for providing augmentation data for transmission by a
satellite, comprising: means for receiving the augmentation data;
means for providing position data relating to the position of the
satellite; determining means for determining whether the position
data satisfies a predetermined criterion; and means for selectively
enabling output of said augmentation data for transmission to said
satellite in response to the determining means.
13. A method of providing augmentation data for transmission by a
satellite, comprising: providing position data relating to the
position of the satellite; determining whether the position data
satisfies a predetermined criterion; and selectively enabling
output of said augmentation data for transmission to a satellite in
response to the result of said determining step.
14. Satellite radiodetermination apparatus, comprising: means for
receiving a plurality of ranging signals from a corresponding
plurality of satellites; and means for receiving ionospheric delay
data, the apparatus being arranged to perform radiodetermination on
the basis of said plurality of ranging signals and selectively on
the basis of said ionospheric delay data in response to
authorization data provided at said apparatus.
15. Apparatus as claimed in claim 14, further including means for
receiving differential correction data which is substantially
independent of ionospheric delay, wherein said apparatus is
arranged to perform radiodetermination additionally on the basis of
said differential correction data.
16. Apparatus as claimed in claim 14 or claim is, wherein said
ionospheric delay data is encrypted, and said apparatus includes
decryption means for decrypting said ionospheric delay data in
response to said authorization data.
17. Apparatus as claimed in any one of claims 14 to 16, including
input means for inputting said authorization data.
18. Apparatus for providing augmentation data for transmission via
a satellite, comprising: means for receiving said augmentation data
which includes unencrypted ionospheric delay data and unencrypted
differential correction data which is substantially independent of
ionospheric delay; means for encrypting said ionospheric delay
data; and means for outputting said encrypted ionospheric delay
data and said unencrypted differential correction data for
transmission via said satellite.
19. A method of providing ionospheric delay data and differential
correction data which is substantially independent of ionospheric
delay for transmission via a satellite, comprising: receiving said
ionospheric delay data and said differential correction data in an
unencrypted form; encrypting said ionospheric delay data; and
outputting said encrypted ionospheric delay data and said
unencrypted differential correction data for transmission via said
satellite.
20. Satellite radiodetermination apparatus, comprising: means for
receiving a plurality of ranging signals from a corresponding
plurality of satellites; means for receiving ionospheric delay
data; and means for receiving residual error data relating to
residual errors in said ionospheric delay data, said satellite
radiodetermination apparatus being arranged to perform
radiodetermination on the basis of said plurality of ranging
signals, said ionospheric delay data and said residual error
data.
21. Apparatus as claimed in claim 20, wherein said residual error
information includes error bound information relating to the error
bounds of said ionospheric delay information.
25. Apparatus for determining residual errors in a satellite
radiodetermination system, comprising: means for receiving a
plurality of ranging signals from a plurality of satellites; means
for receiving ionospheric delay data and differential correction
data relating to errors in said ranging signals which are
independent of ionospheric delay; calculating means for calculating
a position or time on the basis of said ranging signals corrected
on the basis of said differential correction data and said
ionospheric delay data; and error calculating means for calculating
errors in said ionospheric delay data on the basis of the
difference between said calculated position or time and a
predetermined reference position or time.
23. Apparatus as claimed in claim 22, wherein said means for
receiving said ranging signals comprises a dispersed plurality of
receiving stations, said calculating means being arranged to
calculate a plurality of positions corresponding respectively to
said receiving stations, and said error calculating means being
arranged to calculate said errors on the basis of the respective
differences between said calculated positions and predetermined
positions corresponding to said receiving stations.
24. A satellite radiodetermination receiver including apparatus as
claimed in any one of claims 1 to 8, 14 to 17, 20 and 21.
25. A terrestrial station including apparatus as claimed in any one
of claims 12, 18, 22 or 23.
26. A satellite including a satellite payload as claimed in claim
10 or 11.
27. A satellite radiodetermination system including a plurality of
satellite radiodetermination receivers as claimed in claim 24 and
at least one terrestrial station as claimed in claim 25.
Description
[0001] This invention relates to methods and apparatus for
radiodetermination. Radiodetermination comprises the determination
of position and/or time by the use of ranging signals between a
terminal and a plurality of beacons. In satellite
radiodetermination the beacons are satellites in orbit.
[0002] At the present time, two global radiodetermination systems
exist. The GPS/NAVSTAR system comprises a constellation of
satellites in twelve hour orbits, operated by and for the US
Department of Defense. The GLONASS positioning system provides
similar facilities under the control of the Russian government (and
will not be discussed further herein).
[0003] In the GPS/NAVSTAR system, each satellite carries a highly
accurate atomic clock and the clocks of all the satellites are
synchronised. The orbits of all the satellites are well
characterised, and each satellite is therefore able to derive its
instantaneous position. The satellites periodically receive
information on variations in their orbits from a terrestrial
station.
[0004] The satellites broadcast regular messages which carry:
[0005] 1. the time, as indicated by the on-board atomic clock,
[0006] 2. the position of the satellite, and
[0007] 3. status messages.
[0008] Details of the GPS signal format may be found in the "Global
Positioning System Standard Position Service Signal Specification",
2nd edition, 2 Jun. 1995, incorporated herein by reference.
[0009] A GPS receiver on earth is able to acquire signals from
several satellites. The constellation is designed so that, for
almost every point on earth at almost every time of day, at least
four satellites are simultaneously in view. By noting the different
times of arrival of signals from different satellites, using the
received clock data, and with knowledge of the satellite positions
(transmitted with the signals), the GPS receiver is able to
calculate the relative range from each satellite and, from these
four relative ranges thus calculated, to calculate its position in
three dimensions and calibrate its clock.
[0010] Changes in the delay caused by variations in the ionosphere
can degrade the accuracy of radiodetermination measurements, and
accordingly, to enable some compensation for this, each satellite
broadcasts on two frequencies (termed L1 and L2). A military GPS
receiver is able, by measuring on two frequencies, to estimate and
correct the ionospheric delay, since the ionospheric delay varies
as a function of frequency.
[0011] Although GPS/Navstar is primarily for military use,
receivers are widely available to civilians. However, the civilian
receivers cannot decode the "P-code" by which information signals
are encrypted on the first and second frequencies and hence cannot
perform a two-frequency ionospheric compensation. Also, to limit
the accuracy of the service to non military users, the so called
"selective availability" feature introduces deliberate minor errors
of timing and/or position into the information signals encoded by
the "C/A code" and transmitted only on the L1 frequency from
various satellites. Military receivers are able to decode signals
without these deliberate errors.
[0012] It is possible for a ground station of accurately known
position to determine which satellites are in error and by how
much, and it is known to broadcast a signal which indicates which
satellites are in error, and the amount of correction to apply for
reception by GPS receivers, to enable them to compensate the errors
from a single frequency measurement and thus derive a reliable
position signal. Broadcasting such signals via a geostationary
satellite is taught in, for example, U.S. Pat. No. 4,445,110.
[0013] However, whilst the correction to be applied can be
calculated exactly by the reference ground station of known
position, this correction becomes progressively less accurate
further away from the reference ground station, because of
differences in the ionosphere (and other layers of the atmosphere
such as the troposphere). Accordingly, the usefulness of such
"differential GPS" techniques is limited.
[0014] Rather than providing a GPS receiver which operates to
receive two frequencies in order to compensate for ionospheric
variations, or one which receives a differential GPS correction, it
is possible to broadcast a signal which includes some data about
ionospheric conditions. In recent years, the possibility of a wide
area differential system has been discussed. One example is the
Wide Area Augmentation System (WAAS) proposed by the US Federal
Aviation Authority to provide differential correction information
over the US. In such a wide area system, ionosphere correction data
for a grid of spaced apart points in an area (e.g. Europe or the
US) is broadcast via a geostationary satellite serving that area,
and at the receiver, an interpolation is performed between grid
points to derive a value for the ionosphere delay correction to be
applied to a single frequency signal from each GPS satellite in
view. Also broadcast is correction data for compensating for
"selective availability" errors. U.S. Pat. No. 5,323,322 describes
a satellite radiodetermination system in which ionospheric data is
broadcast.
[0015] It has been proposed in the papers "Evolution to civil GNSS
taking advantage of geostationary satellites", ION 49th Annual
Meeting, June 1993, "Implementation of the GNSS integrity channel
and future GNSS growth considerations", INA 18th Annual Meeting,
October 1993, and "Global Navigation Satellite System (GNSS)
Alternatives for Future Civil Requirements", PLANS '94 Technical
Program, April 1994, all by J. R. Nagle, G. V. Kinal and A. J. Van
Dierendonck, to supplement the GPS/NAVSTAR system by additional
civil satellites in low earth, intermediate or geostationary
orbits.
[0016] According to one aspect of the present invention, there is
provided a satellite radiodetermination receiver which receives a
multiple frequency ranging signal and a single frequency ranging
signal, derives an ionospheric delay value from the multiple
frequency ranging signal by measuring the relative delay between
the different frequencies at which the multiple frequency ranging
signal is transmitted and estimates a delay value for the further
ranging signal on the basis of the measured ionospheric delay value
for the multiple frequency ranging signal.
[0017] According to another aspect of the present invention, there
is provided a satellite payload which is operable in either one of
two modes. In the first mode, the satellite payload generates
autonomous ranging signals which do not include augmentation data
received from a ground station. In the second mode, the satellite
additionally relays augmentation data received from the ground
station.
[0018] In another aspect of the present invention, there is
provided a satellite payload which is able to generate an
autonomous ranging signal and to broadcast data received from a
ground station. If no data is received from the ground station, the
satellite payload generates dummy data for broadcast, so as to keep
the broadcast channel open.
[0019] According to another aspect of the present invention, there
is provided apparatus for connection to a satellite access node,
which is arranged to receive augmentation data, to determine the
position of a satellite accessible by the satellite access node, to
determine whether the position of the satellite falls within a
predetermined range dependent on the source of the augmentation
data and to output augmentation data to the satellite access node
if the position of the satellite falls within the predetermined
range.
[0020] According to another aspect of the present invention, there
is provided a satellite navigation receiver which is able to
receive ionospheric delay data but is only able to use the
ionospheric delay data when performing radiodetermination if access
to the ionospheric delay data is enabled, for example by means of a
code for decrypting the ionospheric delay data.
[0021] According to a further aspect of the present invention,
there is provided an apparatus for providing augmentation data to a
satellite access node which is arranged to encrypt ionospheric
delay data without encrypting differential correction data which
does not relate to ionospheric delay and to output the encrypted
ionospheric delay data and the unencrypted differential correction
data to the satellite access node.
[0022] According to another aspect of the present invention, there
is provided a satellite navigation receiver which is arranged to
receive ionospheric delay data and residual error data and to
correct the ionospheric delay data using the residual error
data.
[0023] According to another aspect of the present invention, there
is provided a differential correction network which receives
ionospheric delay data from another differential correction
network, receives ranging signals from navigation satellites and
thereby calculates errors in the ionospheric delay data.
[0024] Embodiments of the invention will now be illustrated, by way
of example only, with reference to the accompanying drawings in
which:
[0025] FIG. 1 is a schematic diagram of a satellite
radiodetermination system;
[0026] FIG. 2 is a schematic diagram of a portion of the satellite
radiodetermination system serving North and South America;
[0027] FIG. 3 is a block diagram of the navigation payload of a MEO
satellite;
[0028] FIG. 4 is a block diagram of a navigation receiver;
[0029] FIG. 5 is a diagram of a grid used to represent ionospheric
information; and
[0030] FIG. 6 is a diagram to illustrate calculation of ionospheric
pierce points.
[0031] Navigation System
[0032] FIG. 1 shows schematically the elements of a satellite
radiodetermination system and the relationship between them.
Autonomous ranging signals R.sub.n are provided by one or more GNSS
(Global Navigation Satellite Service) satellites 2, such as GPS
Navstar satellites and/or GLONASS satellites. A plurality of
geostationary satellites 6, such as the proposed Inmarsat-3
communications satellites or dedicated geostationary navigation
satellites, each carry a navigation transponder for relaying
differential correction and other augmentation data A from a
navigation land earth station (NLES) 8 to navigation receivers 11,
the augmentation data A providing integrity, error and ionosphere
information relating to the GNSS satellites 2 and their ranging
signals R.sub.n.
[0033] One or more medium earth orbit (MEO) satellites 10, such as
the proposed satellites for the ICO.TM. global satellite
communications system, relay regional augmentation data RA
transmitted by a terrestrial satellite access node (SAN) 14 to the
navigation receivers 11 incorporated in autonomous ranging signals
R.sub.m synchronised with the ranging signals R.sub.n from the GNSS
satellites 2. The proposed satellites for the ICO.TM. system are a
constellation of ten satellites in 6 hour orbits in two orbital
planes, each carrying a communications and navigation payload.
[0034] A network of monitoring stations 16a, 16b and 16c, of
accurately known location, receive the ranging signals R.sub.n from
the GNSS satellites 2, and the ranging signals R.sub.m from the MEO
satellites 10 and calculate errors in the position and time
information contained in these ranging signals from the difference
between the positions calculated from the ranging signals R and the
actual positions of the monitoring stations 16. Differential
correction data is transmitted from the monitoring stations 16a,
16b and 16c to a regional control station 18 which derives the
augmentation data A, including errors in the reported positions and
time signals of the MEO satellites 10, and of the GNSS satellites
2. The monitoring stations 16 may alternatively be simple receivers
with the calculation of differential correction being performed at
the regional control station 18.
[0035] The position and timing errors in the ranging signals R do
not vary between the monitoring stations 16a, 16b and 16c. However,
the differential correction data received from the monitoring
stations 16a, 16b and 16c will differ because of the difference in
ionospheric delay in the signals received by each of the monitoring
stations 16, dependent on the quantity of free electrons in the
parts of the ionosphere through which the signals travel, together
with other delays such as tropospheric delays caused by
tropospheric refraction.
[0036] Therefore, the regional control station 18 is able to derive
separately data for errors in the ranging signals R.sub.m, R.sub.n
and for values of ionospheric delay in the region of the ionosphere
through which the ranging signals travel to reach each of the
monitoring stations 16a, 16b and 16c. This data is transmitted to
the NLES 8 for transmission as the augmentation data A via the
geostationary satellites 6 to the navigation receivers 11.
[0037] Additionally, the augmentation data is transmitted to a
service network 20 accessible by providers of regional augmentation
systems 21a, 21b. Such regional augmentation systems 21a, 21b may
include local monitoring stations for calculating differential
correction data for specific regions. Regional augmentation data
RA, which may for example include more accurate ionospheric data
and corrections to the augmentation data A relevant to the specific
regions, is input by the service providers at the service network
20. The regional augmentation data RA may include some or all of
the augmentation data A. The regional augmentation data RA is
transmitted to the SAN 14 for transmission via the MEO satellites
10 and selective reception by the navigation receivers 11.
[0038] The satellite radiodetermination system described above
provides, in addition to existing satellite radiodetermination
services such as GPS and GLONASS, the additional ranging signals
R.sub.m from the MEO satellites 10. In this embodiment, the ranging
signals R.sub.m are dual-frequency ranging signals similar to the
signals available to military users in the GPS system, but are
unencrypted and therefore available to any user. The above
radiodetermination system also broadcasts augmentation data A over
a wide area via the geostationary satellites 6, which is
supplemented by additional regional augmentation data RA broadcast
by the MEO satellites 10.
[0039] The augmentation data A is encoded in ranging signals
R.sub.g generated by the NLES 8 and broadcast via the geostationary
satellite 6 to the navigation receivers 11. The location of the
geostationary satellite 6 is determined at the NLES 8, which also
includes an accurate time reference, such as an atomic clock or a
dual frequency satellite radiodetermination apparatus, synchronised
to those on board the MEO satellites 10 and the GNSS satellites 2.
The delay involved in transmitting the ranging signal R.sub.g from
the NLES 8 to the geostationary satellite 6 is determined and the
ranging signal R.sub.g includes position and time data calculated
so as, when retransmitted by the geostationary satellite 6,
accurately to represent the time of retransmission and the position
of the geostationary satellite 6.
[0040] The signal retransmitted to the NLES 8 by the geostationary
satellite 6 provides a timing loop which allows the delay from the
NLES 8 to the geostationary satellite 6 to be determined and also
allows ionospheric effects to be measured. In this way, the ranging
signal R.sub.g is sufficiently precise to be processed as if it
were autonomously generated by the geostationary satellite 6.
[0041] The satellite radiodetermination system described above may
be implemented in stages to provide a progressively enhanced
service relative to that provided by the GNSS satellites 2 above,
as follows.
[0042] Stage 1--Existing or previously planned geostationary
communication satellites such as the Inmarsat-3 satellites are used
as the geostationary satellites 6 to relay ranging signals and
augmentation data R.sub.g,A.
[0043] Stage 2--Additional dedicated navigation satellites are put
into geostationary orbit as additional geostationary satellites 6.
These dedicated navigation satellites are able to generate
autonomous ranging signals R.sub.g.
[0044] Stage 3--The MEO satellites 10 are launched, providing
additional ranging signals R.sub.m and relaying regional
augmentation data RA.
[0045] Stage 1 provides wide area augmentation, for example in
accordance with the WAAS specification. States 1 to 3 provide more
ranging signals, to reduce the reliance on the GNSS satellites 2,
which have selective availability.
[0046] System Operation Example
[0047] FIG. 2 shows an example of the satellite radiodetermination
system of FIG. 1 providing a navigation service over North and
South America. One of the geostationary satellites 6 broadcasts
augmentation data A derived by a first regional control station 18a
over both North and South America. In a first service area 24a
which covers the US, a first service network 20a provides regional
augmentation data RAa which is valid only over the first service
area 24a, such as more accurate ionospheric data concerning the
ionosphere above the US. The regional augmentation data RAa is
transmitted to a first satellite access node 14a and broadcast via
a first MEO satellite 10a over the first service area 24a. Ranging
signals R.sub.g are available in the first service area 24a from
GNSS satellites 2a and 2b. The number and identity of the GNSS
satellites 2 which are visible above a predetermined minimum
elevation angle in the first service area 24a will change with time
as these satellites 2 progress in their orbits.
[0048] A first MEO satellite 10a will also move relative to the
first service area 24a until its elevation angle falls below a
threshold which is suitable for broadcast reception. The first SAN
14a then selects another MEO satellite 10 having an elevation angle
above the threshold for broadcast reception, preferably one that is
approaching the first service area 24a. The SAN 14a ceases
transmission to the first MEO satellite 10a and begins transmission
to the selected MEO satellite 10 instead. To avoid interruption in
broadcast of the regional augmentation data RA, the SAN 14a may
transmit to both the first MEO satellite 10a and the selected MEO
satellite 10 during handover.
[0049] A second service area 24b in South America contains a second
regional control station 18b which receives information from a
monitoring network which monitors ranging signals received in the
second service area 24b. Information from the second regional
control station 18b is sent to the first regional control station
18a so that wide area differential correction information is
gathered from a monitoring network extending through both North and
South America. In this way, the timing and position errors of
ranging signals may be determined more accurately. A second service
network 20b receives information from the second regional control
station 18b and additionally derives more accurate ionospheric
information within the area 24b. This information is relayed to a
second SAN 14b which transmits the information via a second MEO
satellite 10b for broadcast over the second service area 24b.
[0050] Hence, augmentation information which is valid over a wide
area is broadcast by the geostationary satellite 6, which has a
direct line of sight to a wide area. More detailed information of
narrower geographic validity is broadcast by the MEO satellites 10
which are able to cover a smaller area of the earth's surface. In
this way, the information broadcast by geostationary and MEO
satellites is matched with the coverage areas of these
satellites.
[0051] Navigation Satellite
[0052] FIG. 3 shows the navigation payload of one of the MEO
satellites 10.
[0053] The payload includes a frequency standard 30, such as an
atomic clock. A highly precise frequency signal f is supplied from
the frequency standard 30 to oscillators 32, which provide a time
signal t which is referenced to a standard time such as UTC.
[0054] The time signal t is supplied to a navigation signal
generator 34. The payload also includes a telemetry tracking and
control (TT&C) interface 42 which receives encrypted TT&C
data from a TT&C ground station (not shown). The TT&C
interface 42 supplies tracking information to a processor 44 which
generates data containing information on the ephemerides of the MEO
satellite 10. The ephemerides are written into a memory 46, which
applies error correction to avoid data corruption from external
radiation and has a double buffer so that previous ephemerides are
not immediately overwritten by new ephemerides until the new
ephemerides are verified. The ephemerides are read from the memory
46 by the navigation signal generator 34, which encodes the time
signal t and ephemerides using a Gold code of the same family as is
used by GPS/Navstar satellites as described in the GPS
specification. The Gold code is a pseudo-random noise (PRN) code
having low auto-correlation and low cross-correlation with other
Gold codes.
[0055] Intermediate frequency signals IF containing the encoded
ephemerides and time data are supplied to an up-converter 36 which
converts the intermediate frequency signal IF to different
frequencies F1 and F2 which are respectively transmitted through
broadcast antennas 40 and 38.
[0056] The frequencies F1 and F2 may be substantially the same as
the GPS L1 and L2 frequencies, to maintain compatibility with
existing GPS receivers, or they may be offset from the L1 and L2
frequencies so that signals from the MEO satellites 10 may only be
received by modified navigation receivers 11. In one embodiment, F1
is 1576 MHz and F2 is 1228 MHz.
[0057] The operation of the navigation signal generator 34 is
controlled by the processor 44, and status information is supplied
by the navigation signal generator 34 to the processor 44.
[0058] The payload also includes a feeder link channel interface 48
which receives regional augmentation information RA from the SAN
14. The processor 44 selectively supplies the regional augmentation
information RA to the memory 46 for inclusion in the signal output
by the navigation signal generator 34.
[0059] The processor 44 is operable in both an autonomous
navigation mode and a regional augmentation mode. In the autonomous
navigation mode, data supplied by the feeder link channel interface
48 is not sent to the memory 46 and therefore the MEO satellite 10
broadcasts only ranging and status information, at a data rate of
50 bits per second. In the regional augmentation mode, the regional
augmentation data RA received from the feeder link channel
interface 48 is supplied to the memory 46 for inclusion in the
navigation signal. In this mode, the MEO satellite 10 additionally
broadcasts information on the integrity and status of the GNSS
satellites 2, differential correction information supplied by the
service network 20 and alert messages to indicate when satellite
radiodetermination may not be possible to a predetermined level of
accuracy, at a data rate of 250 bits per second. The processor 44
is switched between these two modes by a command received by the
TT&C interface 42. The processor 44 may be switched into
autonomous navigation mode when no regional augmentation data is
available, for example because no SAN 14 is in view or the SAN 14
is faulty.
[0060] Alternatively, the processor 44 may generate dummy data for
transmission in the ranging signal R.sub.m, the dummy data
indicating to the navigation receivers 11 that no regional
augmentation data is available.
[0061] Satellite Radiodetermination Terminal
[0062] FIG. 4 shows a simplified block diagram of a navigation
receiver 11 for receiving ranging and augmentation signals in the
satellite radiodetermination system described above. The user
terminal 11 includes an antenna 50 for receiving the ranging
signals R.sub.g, R.sub.m and R.sub.n, containing the augmentation
information A and the regional augmentation information RA. A PRN
decoder 56 decodes each ranging signal R and outputs decoded
ranging signals R' and timing data t.sub.a relating to the time of
arrival of each ranging signal. A data decoder 58 extracts from the
decoded ranging signals data D, including the augmentation data A,
regional augmentation data RA, the transmission time t of each
ranging signal and the ephemerides of the satellites. A
radiodetermination section 54 receives the data D and timing data
t.sub.a, calculates therefrom the values of ionospheric delays
incurred by the dual frequency ranging signals R.sub.m and the
approximate position of the navigation receiver 11 and outputs this
data to an ionospheric modelling section 60, together with
ionospheric data included in the regional augmentation data RA and
the augmentation data A.
[0063] From this data, the ionospheric modelling section 60
calculates ionospheric pierce points at which each of the ranging
signals R passed through the ionosphere and estimates the
ionospheric delay thereby incurred by single-frequency ranging
signals R, such as the L1 GPS signals, for which the ionospheric
delay cannot be measured directly. The ionospheric modelling
section 60 outputs an estimated ionospheric delay for each of the
single frequency ranging signals R together with error bounds for
the estimated delay.
[0064] The radiodetermination section 54 receives the estimated
ionospheric delays and subtracts them from the time of arrival
t.sub.a of the single-frequency signals. The directly measured
delays incurred by the dual frequency ranging signals are
subtracted from the times of arrival t.sub.a of these signals. The
augmentation data A and regional augmentation data RA include
information on errors in the ranging signals, which is used to
correct the position and time information in each of the ranging
signals. The augmentation data A and regional augmentation data RA
also include integrity information which indicates whether any of
the satellites has failed or is operating incorrectly; the ranging
signals R from such satellites are not used for
radiodetermination.
[0065] The radiodetermination section 54 then calculates an
accurate position P and time T from the corrected ranging signals R
and an estimate of the error in the position P and time T from the
error bounds in ionospheric delay indicated by the ionospheric
modelling section 60 and from error bounds for the ranging signals
indicated by the augmentation data RA,A. If the likely error in the
position P and time T exceeds a predetermined value, the
radiodetermination section 54 may indicate a visual or audible
warning, so that users know that the output should not be relied
upon for certain applications.
[0066] An explanation of the operation of the ionospheric modelling
section 60 will now be given with reference to FIGS. 5 and 6.
[0067] The augmentation data A broadcast by the geostationary
satellites 6 and the regional augmentation data RA broadcast by MEO
satellites 10 includes ionospheric data comprising a set of values
for calculating ionospheric delay for points on a grid G mapped
onto the earth's surface. The grid is centred on the nadir N of a
geostationary satellite position and the ionospheric delay value at
each grid point g.sub.i represents the vertical ionospheric delay
at that grid point.
[0068] Information is seldom available for all the grid points
g.sub.i and the ionospheric data therefore comprises a list of
addresses i of grid points g.sub.i for which ionospheric data is
available, together with the associated vertical ionospheric delay
and delay error for each of these points. The ionospheric data also
includes the position of the nadir N on which the grid of points is
centred.
[0069] The format for ionospheric data described above is designed
for broadcast from a geostationary satellite, but is also used for
ionospheric information broadcast by the MEO satellites 10. The SAN
14 calculates a hypothetical geostationary position so that the
coverage area of the MEO satellite 10, within which the satellite
is visible above 5.degree. elevation, falls within the grid of
points g.sub.i based on that position. Ionospheric data is
broadcast for some or all of the grid points which fall within the
coverage area of the MEO satellite 10. Thus, the ionospheric data
broadcast by the geostationary satellite 6 and the MEO satellites
10 have compatible formats.
[0070] In order to estimate accurately the ionospheric delay for
each ranging signal R, the ionospheric modelling section 60 of the
navigation receiver 11 must calculate a pierce point PP at which
the ranging signal R passes through the ionosphere on its way to
the user and apply the appropriate ionospheric delay value for that
pierce point.
[0071] As shown in FIG. 6, a ranging signal R is transmitted by a
satellite, such as one of the MEO satellites 10, at an elevation
angle E with respect to the navigation receiver 11. The ranging
signal R passes the level of maximum electron density I.sub.0 of
the ionosphere at the pierce point PP. The height H of the level
I.sub.0 is assumed to be 400 km above the earth's surface. The
navigation receiver 11 is able to derive its approximate position
from the uncorrected ranging signal R, which also contains
information on the position of the satellite 10. From this
information, and from the radius r of the earth, the latitude and
longitude of the pierce point PP is calculated in a known
manner.
[0072] The calculated pierce point PP does not usually coincide
with one of the grid points g.sub.i for which ionospheric
information is available. The value for ionospheric delay must
therefore be interpolated between grid points g.sub.i, g.sub.i+1
for which ionospheric information is available. The ionospheric
modelling section 60 generates a modelling function which may be
varied by one or more parameters so as to fit the ionospheric
information at the surrounding grid points g.sub.i and which is
used to interpolate the ionospheric delay value at the pierce point
PP.
[0073] A suitable model for interpolating ionospheric delay values
has been specified for the WAAS. In this model, the ionospheric
delay values for the four grid points at the corners of a cell
containing the pierce point PP are used to interpolate an
ionospheric delay value at the pierce point by means of a weighting
function which provides a continuous surface as a function of
longitude and latitude. Alternatively, a linear interpolation may
be taken between pairs of grid points in both the longitudinal and
latitudinal directions.
[0074] In addition, the ionospheric delay modelling section 60 may
fit the modelling function to the directly measured ionospheric
delay values obtained from the dual frequency ranging signals
R.sub.m, by calculating the pierce points PP for these signals and
adjusting the parameters of the modelling function to fit the
measured ionospheric delay values for these pierce points PP.
[0075] The modelling function need not fit the ionospheric
information and measured delay values exactly; instead, an
approximate fit such as a least squares fit may be calculated.
[0076] The vertical ionospheric delay value at the pierce point is
thereby calculated. However, the ranging signal R travels a
distance 1 through the ionosphere which is greater than the
vertical height is h of the ionosphere, as a result of the slant
angle .alpha. which the path of the ranging signal R makes with the
vertical. The vertical ionospheric delay value is therefore
multiplied by an obliquity factor, to take into account the greater
length of ionosphere traversed.
[0077] User Operation Example
[0078] A specific example will now be described of the operation of
an alternative embodiment of a satellite radiodetermination system
which provides a navigation service for Africa in which a
geostationary satellite 6 provides a ranging signal R.sub.g and
augmentation information A comprising correction messages for the
GNSS satellite ranging signals R.sub.n, but ionospheric correction
information is not provided through the geostationary satellites 6
in this embodiment, since insufficient information is available for
Africa as a whole.
[0079] However, regional monitoring stations are provided by local
service providers in Kenya. The monitoring stations monitor the
ranging signals from the MEO satellites 10 and the GNSS satellites
2 and additionally receive the augmentation information A broadcast
by the geostationary satellites 6. From this information a vertical
ionospheric delay value and ionospheric delay error bounds are
estimated which are applicable for all users in Kenyan territory
and airspace. This information is relayed to a SAN 14 for broadcast
through an MEO satellite 10 which covers Kenya at that time.
[0080] The navigation satellites visible by a navigation receiver
11 approaching Nairobi are, for example, those shown in Table 1
below.
1 TABLE 1 Satellite Azimuth Elevation MEO 2 184 27 MEO 9 310 12 MEO
10 40 11.5 AOR-E GEO 269 30 IOR GEO 91 58 GPS 2 250 73 GPS 4 355 24
GPS 13 150 27
[0081] The satellite AOR-E is an Inmarsat.TM. satellite serving the
eastern Atlantic Ocean region, while the satellite IOR serves the
Indian Ocean region.
[0082] The navigation receiver is able to determine the approximate
positions of all the above satellites from ranging signals
generated or relayed by the satellites, without ionospheric
correction. Estimates of the ionospheric delay in the ranging
signals from each of the satellites are then obtained as
follows.
[0083] The ionospheric delay is calculated directly for the dual
frequency ranging signals transmitted by the satellites MEO.sub.2,
MEO.sub.9 and MEO.sub.10. The AOR-E geostationary satellite is
identified as being in the same quadrant as the MEO.sub.9
satellite. The ionospheric delay value in the MEO.sub.9 ranging
signal is used to estimate the ionospheric delay in the AOR-E GEO
ranging signal, by compensating for the difference in elevation
angles between the MEO.sub.9 satellite and the AOR-E geostationary
satellite. The ionospheric delays for the GPS.sub.4 and GPS.sub.13
satellites are estimated in the same way, using the ionospheric
delay value measured for the MEO.sub.9.
[0084] The GPS.sub.2 satellite is approximately overhead and
therefore the ionospheric delay information provided by the Kenyan
monitoring stations is used, with a small obliquity factor
correction for the difference between the actual elevation angle of
72.degree. and 90.degree.. The ionospheric delay error bound data
are applied to each of the estimated ionospheric delays which are
not measured directly from a dual frequency ranging signal.
[0085] In this example, the navigation terminal 11 receives ranging
signals from eight different satellites and is able to calculate or
estimate the ionospheric delay for each ranging signal without
ionospheric data being provided in the augmentation data A. In
addition, integrity information received from the geostationary
satellites is used to determine whether any of the ranging signals
should not be used for satellite radiodetermination. Thus, the
result of the radiodetermination is accurate and reliable.
[0086] Regional Augmentation Information
[0087] In the above example, ionospheric delay information relevant
to Kenya is relayed through one or more of the MEO satellites 10.
Each MEO satellite is able to broadcast over an area much wider
than Kenya, for example, and the SAN 14 receives both the data
provided by the Kenyan service provider and data provided by other
networks. In the transmission from the SAN 14, the data from each
network is assigned a different time slot in a repeating time
frame, so that the information from the different monitoring
networks is time division multiplexed. Table 2 below shows an
example of the allocation of time slots to each of the satellites
MEO.sub.2, MEO.sub.9 and MEO.sub.10 in the above example.
2TABLE 2 Satellite Slot 1 Slot 2 Slot 3 Slot 4 Slot 5 Slot 6 MEO 2
Kenya Africa Other Other Africa Other MEO 9 Kenya Other Europe
Other Other Europe MEO 10 Kenya Other Other Other Other Other
[0088] The satellite MEO.sub.2 transmits, in slots 2 and 5,
regional augmentation data which is valid over the whole of Africa
and this information may additionally be used by the navigation
receiver 11 in Kenya.
[0089] Each slot may for example be a 1 second slot in a 6 second
time frame and carry 250 data bits.
[0090] Slots may be allocated by the SAN 14 to service providers
during a period in which the nadir of the MEO satellite 10 carrying
the regional augmentation information passes over a predetermined
region, allocated to the service provider, defined for example by
longitude and latitude boundaries. For example, each region may
comprise a 5.degree. longitudinal strip of the Northern or Southern
Hemisphere.
[0091] Each slot may be shared by several service providers, which
individually do not supply enough data to occupy a whole slot, so
that the full capacity of each slot is used. The data from each
service provider within the slot is identified by a code allocated
to that service provider. More than one slot may be allocated to
service providers which require more capacity than can by provided
by one slot. Charging data for calculating a charge to the service
provider is generated according to the duration for which capacity
on the MEO satellite 10 is allocated to the service provider and
according to the proportion of the capacity used during that
time.
[0092] Alternatively, slots may be allocated to service providers
during a period in which the MEO satellite 10 is visible above a
minimum elevation angle, such as 5.degree., from the service area
for which the service provider provides regional augmentation data
RA.
[0093] The regional augmentation data in each time slot is
preferably encrypted to ensure that it can only be decoded by
licensed navigation receivers 11. All licensed receivers may use
the same algorithms for performing radiodetermination using the
ionospheric delay data, so that radiodetermination is performed to
a common standard.
[0094] Additionally, users may be required to purchase a smart card
for insertion in the navigation receiver 11 to allow access to some
or all of the regional augmentation information RA broadcast by the
MEO satellites. In this way, revenue may be collected by the
service providers. Alternatively, the user terminals may have a
keyboard for entering a code which enables access to one or more of
the regional augmentation information slots.
[0095] In this way, different types of information may be made
available to different users, depending on the area for which they
need ionospheric information or the level of accuracy which they
require, and the users may only be charged for the information
which they need.
[0096] In the above example, the different types of augmentation
information are time division multiplexed. However, the different
information channels may be multiplexed together in other ways well
known in the art, such as by code division multiplexing or
frequency division multiplexing.
[0097] The information in each regional augmentation information
channel may include data, such as country codes, for identifying
the area for which the ionospheric data is valid, data indicating
the reliability of ionospheric data and data indicating for what
period the ionospheric data is valid.
[0098] Since the ionospheric delay modelling section 60 of the
navigation receiver 11 is able to combine ionospheric delay
information with direct ionospheric delay measurements, a more
accurate model of the ionosphere is used to compensate for
ionospheric delay. Furthermore, regional augmentation data RA is
received which is relevant to the local area in which the
navigation terminal 11 is located. Integrity data and error bound
data is received both in the regional augmentation data RA and the
augmentation data A. In this way, highly accurate position readings
P and time readings T may be calculated, together with estimates of
the level of accuracy of these readings and warnings if the level
of accuracy falls below a predetermined threshold.
[0099] Such accurate and reliable radiodetermination greatly
extends the potential applications of satellite
radiodetermination.
[0100] For example, the above described satellite
radiodetermination system may be used where safety is critical,
such as landing aircraft in low visibility conditions. The system
also has maritime applications in that it provides sufficient
accuracy for harbour approaches in restricted visibility, and may
also find applications in train control where sufficient accuracy
is needed to determine on which track a train is running. The
system may provide accurate time readings for use by laboratories
or communication systems which require precise synchronisation.
[0101] Since all of the ranging and augmentation information is
provided by satellite, the need to install terrestrial differential
correction systems may be overcome.
[0102] Although the above embodiments use MEO satellites such as
ICO.TM. satellites, other satellite constellations could be used
such as those proposed for the ODYSSEY.TM., IRIDIUM.TM.,
GLOBALSTAR.TM. and TELEDESIC.TM. satellite communications systems.
Satellites in low earth orbits (LEO) produce greater Doppler shift
in their signals, but this may be overcome by appropriate
compensation in the receivers. The satellite configurations
described in the embodiments are particularly advantageous, but
alternative configurations may be used. For example, autonomous
navigation signal generation equipment may be replaced by
transponders for retransmitting navigation signals in the
non-geostationary satellites. Regional augmentation data may be
broadcast by geostationary or non-geostationary satellites having
multiple spot beams, with the regional validity of the data
broadcast in each spot beam being matched to the coverage area of
each that spot beam.
[0103] Navigation receivers may determine their altitude from map
data giving altitude as a function of longitude and latitude, or
from barometric pressure, so that only three ranging signals are
required to determine longitude, latitude and time.
* * * * *