U.S. patent application number 12/709657 was filed with the patent office on 2010-06-17 for gps receiver raim with slaved precision clock.
This patent application is currently assigned to HONEYWELL INTERNATIONAL INC.. Invention is credited to David W. Meyers, Kelly Muldoon, Brian Schipper, Lawrence C. Vallot.
Application Number | 20100149025 12/709657 |
Document ID | / |
Family ID | 40231344 |
Filed Date | 2010-06-17 |
United States Patent
Application |
20100149025 |
Kind Code |
A1 |
Meyers; David W. ; et
al. |
June 17, 2010 |
GPS RECEIVER RAIM WITH SLAVED PRECISION CLOCK
Abstract
A method and a system for providing a substituted timing signal
for a missing satellite ephemeris in execution of a RAIM algorithm
includes deriving a plurality of position, velocity, and time
solutions from a GPS navigation system. The position, velocity and
time solutions are derived from a plurality of satellite
ephemerides. An atomic clock provides an atomic clock signal. The
atomic clock signal is compared to the derived time solutions to
arrive at a correction factor. The atomic clock signal is adjusted
according to the correction factor to develop an adjusted atomic
clock signal. The adjusted atomic clock signal is substituted for a
missing satellite ephemeris to execute the RAIM algorithm.
Inventors: |
Meyers; David W.; (Brooklyn
Park, MN) ; Vallot; Lawrence C.; (Shoreview, MN)
; Schipper; Brian; (Brooklyn Park, MN) ; Muldoon;
Kelly; (Minneapolis, MN) |
Correspondence
Address: |
HONEYWELL/FOGG;Patent Services
101 Columbia Road, P.O Box 2245
Morristown
NJ
07962-2245
US
|
Assignee: |
HONEYWELL INTERNATIONAL
INC.
Morristown
NJ
|
Family ID: |
40231344 |
Appl. No.: |
12/709657 |
Filed: |
February 22, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11869614 |
Oct 9, 2007 |
7667644 |
|
|
12709657 |
|
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Current U.S.
Class: |
342/357.29 |
Current CPC
Class: |
G01S 19/20 20130101;
G01S 19/23 20130101; G01S 19/235 20130101 |
Class at
Publication: |
342/357.02 ;
342/357.12; 342/357.14 |
International
Class: |
G01S 19/24 20100101
G01S019/24; G01S 19/49 20100101 G01S019/49 |
Claims
1. A method for providing a timing signal to the execution of a GPS
RAIM algorithm to eliminate the time unknown of the GPS solution
and thus reducing the number of required satellite measurement sets
by one, the method comprising: deriving position, velocity, and
time solutions from a GPS navigation system derived from a
plurality of satellite ephemerides and measurements; receiving an
atomic clock signal from one or more atomic clocks located in the
navigation system; comparing the atomic clock signal to the derived
time solutions to derive a correction factor; adjusting the atomic
clock signal according to the correction factor to develop an
adjusted atomic clock signal; and using the adjusted atomic clock
signal to remove the clock unknown of the GPS RAIM solution and
thus reducing the number of required satellite measurement sets by
one.
2. The method of claim 1, wherein the navigation system is an
inertial GPS system.
3. The method of claim 1, wherein the atomic clock is a chip scale
atomic clock.
4. A GPS navigation system including a RAIM processor, the GPS
navigation system comprising: a GPS receiver for receiving
satellite ephemerides from a plurality of GPS satellites, the
receiver configured to derive position, velocity, and time
solutions along with range measurements from all tracked
satellites; an atomic clock producing a clock signal; a clock
follower to compare time solutions from the GPS receiver to the
clock signals of the navigation system and deriving a correction
factor to synthesize a corrected clock signal; and a RAIM algorithm
processor to receive the satellite measurements and the time
solutions from the GPS receiver and the corrected clock signal to
test the integrity of each of the satellite ephemeris in the
satellite ephemerides.
5. The system of claim 4, wherein the navigation system includes an
inertial measurement unit.
6. The system of claim 4, wherein the ephemerides include at least
one military GPS satellite ephemeris.
7. The system of claim 4, wherein the atomic clock is a chip scale
atomic clock.
8. An apparatus for providing a timing signal to reduce the number
of required satellite measurements by one in execution of a RAIM
algorithm, the method comprising: a GPS receiver for deriving
position, velocity, and time solutions from a GPS navigation system
along with range measurements from each satellite tracked; an
atomic clock for generating an atomic clock signal; a clock
follower for: receiving an atomic clock signal; comparing the
atomic clock signal to the time signal of the navigation system to
derive a correction factor; adjusting the atomic clock signal
according to the correction factor to develop an adjusted atomic
clock signal; and a processor for executing the RAIM algorithm
based upon the adjusted atomic clock signal to eliminate the time
unknown of the GPS solution and this reduce the number of required
satellites by one.
9. The system of claim 8, wherein the navigation system is an
inertial GPS system.
10. The system of claim 8, wherein the atomic clock is a chip scale
atomic clock.
Description
RELATED APPLICATION(S)
[0001] The present application is a continuation application of
U.S. application Ser. No. 11/869,614 (the '614 application), filed
Oct. 9, 2007 (pending). The '614 application is incorporated herein
by reference.
BACKGROUND OF THE INVENTION
[0002] RAIM is the abbreviation for Receiver Autonomous Integrity
Monitoring, a technology developed to assess the integrity of
Global Positioning System (GPS) signals in a GPS receiver system.
It is of special importance in safety-critical GPS applications,
such as in aviation or marine navigation.
[0003] RAIM detects faults by utilizing redundant GPS pseudorange
measurements. That is, when more satellites are available than
needed to produce a position fix, the extra pseudoranges should all
be consistent with the computed position. A pseudorange that
differs significantly from the expected value (i.e., an outlier)
may indicate a fault of the associated satellite or another signal
integrity problem (e.g., ionospheric dispersion). Traditional RAIM
uses fault detection only (FD); however, newer GPS receivers
incorporate Fault Detection and Exclusion (FDE) which enables them
to continue to operate in the presence of a GPS failure.
[0004] Because RAIM operates autonomously, that is, without the
assistance of external signals, it requires redundant pseudorange
measurements. To obtain a 3-dimensional position solution, at least
4 measurements are required. To enable RAIM FD (Fault detection in
RAIM), at least 5 measurements are required, and to enable RAIM FDE
(Fault detection in RAIM with the ability to exclude faulty data),
at least 6 measurements are required. However, more measurements
are often needed depending on the satellite geometry. Typically,
there are 7 to 12 satellites in view.
[0005] Conventional RAIM availability thus requires 6 or more
satellite measurements with good satellite geometry. This is two or
more satellites than is required for the basic navigation solution.
However, if time can be eliminated from the list of unknowns, and
thus drop the required number of satellites from 4 to 3, then RAIM
FDE can be achieved with only 5 satellites. Time can be eliminated
by proving the GPS receiver with a precise time reference such as
that available from an atomic clock.
[0006] Since GPS requires "line of sight" reception to receive the
GPS navigational signal, terrain surrounding runways can occlude
one or more of the satellites at critical times. Removal of one or
more of the several satellites compromises or prevents the
availability of RAIM. Aircraft GPS precision approaches are
frequently interrupted by RAIM outages. Certain flight operations,
such as precision approach, can no longer be executed without RAIM
availability.
[0007] There is an unmet need in the art for improving the
availability of RAIM by using the aid a precise and accurate time
signal.
SUMMARY OF THE INVENTION
[0008] A method and a system for providing a substituted timing
signal for a missing satellite ephemeris in execution of a RAIM
algorithm includes deriving a plurality of position, velocity, and
time ("PVT") solutions from a GPS navigation system. The position,
velocity and time solutions are derived from a plurality of
satellite pseudorange measurements and ephemerides. An atomic clock
provides an atomic clock signal. The atomic clock signal is
compared to the derived time solutions to arrive at a correction
factor. The atomic clock signal is adjusted according to the
correction factor to develop an adjusted atomic clock signal. The
adjusted atomic clock signal can then be substituted for a missing
satellite measurement to execute the RAIM algorithm.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] Preferred and alternative embodiments of the present
invention are described in detail below with reference to the
following drawings:
[0010] FIG. 1 is a block diagram of an exemplary GPS navigation
system with an atomic clock and clock follower; and
[0011] FIG. 2 is a system of executing a RAIM algorithm based upon
clock coasting with an atomic clock.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0012] Receiver Autonomous Integrity Monitoring (RAIM) refers to a
class of self-contained GPS integrity monitoring methods based on a
consistency check among redundant ranging signals to detect an
unacceptably large satellite range error due to either erroneous
satellite clock or erroneous satellite ephemeris data.
[0013] RAIM involves two types of functions. The first function is
to detect whether a malfunction that results in a large range error
has occurred on any satellite, that is, to detect the presence or
absence of such a malfunction. The second function is to identify
the faulty ephemeris from a satellite from among the several
ephemerides. Detection requires at least 5 satellites be visible.
Identification requires at least 6 by conventional means.
[0014] FIG. 1 illustrates a navigation system 10, which includes a
first atomic clock 18, a second atomic clock 21, a GPS slaving unit
27, GPS receiver 30 and a system processor 33. In a non-limiting
alternate embodiment, the navigation system includes a third atomic
clock 24. A clock processor 20 includes the GPS slaving unit 27 and
is used to slave the clocks 18, 21, 24 and to provide a frequency
stable time standard to the remainder of the navigation system
10.
[0015] The GPS receiver 30 is configured to receive navigation
signals from GPS satellites. The system processor 33 implements
modified RAIM algorithms or functions enhanced by the output from
the GPS slaving unit 27. In this embodiment, the input from GPS
slaving unit 27 is used, not only to refine position, velocity, and
time solutions, but also to detect faults in the individual output
of the plurality of atomic clocks 18, 21, 24 as may become
prominent over a short time interval.
[0016] As in some prior art systems, in the navigation system 10
illustrated in FIG. 1, the GPS satellite measurement provides a
beginning time reference generally in a pulse per second (PPS)
interval signal. A PPS signal is an electrical signal that very
precisely indicates the start of a second. PPS signals are output
by various types of precision clock, including some models of GPS
receivers. Depending on the source, properly operating PPS signals
have an accuracy ranging from a few nanoseconds to a few
milliseconds.
[0017] PPS signals are used for precise timekeeping and time
measurement. One increasingly common use is in navigation system
timekeeping, including the NTP protocol, which is used to link the
several subsystems in aircraft avionics. It should be noted that
because the PPS signal does not specify the time but merely the
start of a second, one must combine the PPS functionality with
another time source that provides the full date and time in order
to ascertain the time both accurately and precisely. Nonetheless,
PPS signals can be extremely useful in slaving a plurality of
clocks; in the case of this invention, the atomic clocks 18, 21,
24.
[0018] The basic physics of atomic clocks 18, 21, 24 have been
fairly well understood for some time, along with the
macro-engineering challenges in creating a clock 30 with frequency
stability of one part in 10 billion-equivalent to gaining or losing
just one second every 300 years. Exploiting
micro-electro-mechanical systems (MEMS) chip fabrication
technology, the atomic clocks 18, 21, 24 have a volume of less than
0.1 cm3 and consume only a few tenths of milliwatts of power,
enabling the atomic clocks 18, 21, 24 to be used in solid state
packages having a suitably small form factor.
[0019] A slave clock is a clock that is coordinated with a master
clock, and the GPS slaving unit 27 is used to slave at least one of
the plurality of atomic clocks 18, 21, 24 to the GPS receiver 30 to
achieve what is known as "clock coasting". Clock coasting is a
free-running operational timing mode in which continuous or
periodic measurement of clock error, i.e., of timing error, is not
made, in contrast to tracking mode. Operation in the coasting mode
may be extended for a period of time by using clock-error data or
clock-correction data (obtained during a prior period of operation
in the tracking mode occurring at the clock processor 20) to
estimate clock corrections for the no-satellite situation.
[0020] Slave clock coordination is usually achieved by
phase-locking the slave clock signal to a signal received from the
master clock, in this non-limiting example, a PPS signal. The GPS
slaving unit 27 is used for the phase-locking by noting the phase
relative to the master clock. To adjust for the transit time of the
signal from the master clock to the slave clock, the phase of the
slave clock may be adjusted with respect to the signal from the
master clock so that both clocks are in phase. Thus, the time
markers of both clocks, at the output of the clocks, occur
simultaneously.
[0021] Atomic clocks generally produce great short term precision
but may suffer over long periods with stability deficiencies. GPS
clocks, on the other hand, have short term stability deficiencies
but are stable over longer periods. The distinct and complementary
natures of time derived by the atomic clock 30 and the time
solution derived from data received at the GPS receiver 12 assure
greater accuracy of solutions of the RAIM algorithms at the
processor 27.
[0022] The GPS receiver 30 has a clock bias from GPS time as
received. If a highly stable clock reference is used, however, the
GPS receiver 30 time could be based on the highly stable clock
without solving for a bias. "Clock coasting" requires an atomic
clock with superior long term stability, thereby combining the
strengths of each time discerning system to get a far more accurate
and precise determination of system time.
[0023] The clock processor 20 facilitates coasting by accumulating
errors for calculating RAIM availability within a measurement
period. The subsequent estimation for the next calculations predict
and constrain time values based on higher precision atomic clocks
18, 21, 24. RAIM can be synthesized to provide ephemeris for a
missing satellite. Synthesis is based upon an assumption that the
user clock error "dynamics" are milder than the vehicle dynamics,
thus a clock processor 20 may be allowed to quit tracking the clock
error for a short period and determine integrity with only four
satellites. The length of time for which this may reasonably be
done depends, of course, on the user clock frequency stability.
[0024] The on board precise frequency standard is slave-locked to
the GPS receiver 30 exploiting a one PPS signal based on UTC.
Within the clock processor 20, the on board time received time
stamps and the on board precision atomic clock form a closed loop
system that has the historical inaccuracies of the precision clock
contained within the loop parameters, allowing the instability of
the clock to be effectively zeroed prior to a RAIM outage. As a
result, the clock processor 20 derives a signal through clock
coasting that during periods of RAIM outage is well determined and
predictable due to the closed-loop hardware. Where, as in one
non-limiting embodiment one atomic clock 18 is used, the operation
of the clock 18 is compared to the time signal derived at the GPS
receiver 30 to determine the operable status of the clock 18.
[0025] In another non-limiting option, at least two calibrated and
slaved atomic clocks 18 and 21 are exploited. The time outputs from
the clocks 18 and 21 can be compared during periods of coasting.
Any inconsistency between two clocks triggers a failure detection
capability; thereby, to assure integrity of the clock signal, if
one of the clocks fails (and the failure is not a "common mode"
failure, i.e. a failure that kills off the accuracy of both clocks
in the same way) a fault is indicated such that the navigation
solution is not trustworthy.
[0026] In an embodiment exploiting three or more clocks 18, 21, 24,
an exclusion exploits the output of two of the clocks that
generally agree; for example, the second and third atomic clocks 21
and 24. The remaining clock 18 is determined an outlier and the
output of the first atomic clock 18 is disregarded in developing a
clock solution at the clock processor 20. In the embodiment, an
enunciator (not shown) might, optionally, be used to signal the
need for examining the trio of clocks 18, 21, 24 to determine and
correct the source of the fault.
[0027] Continued RAIM availability is facilitated by the
availability of a precise frequency standard within the
navigational system 10. When the time solution derived from the GPS
receiver 30 is in error, the system processor 33 can isolate the
offending satellite. As a result, the system processor 33 will set
an internal satellite health indicator to "unhealthy" which causes
the GPS receiver 30 to remove the satellite from the tracked list.
Furthermore, upon isolation, the system processor 33 recalculates
the receiver time and controls without the offending satellite. The
system processor 33 will continue to monitor the removed satellite
and compare its derived time signal to the onboard signal to
determine when to set the internal satellite health indicator to
healthy again and include it in the tracked list.
[0028] In both embodiments, the time stamping is optionally based
on Coordinated Universal Time ("UTC") although any internally
consistent time stamping convention will suitably serve the ends of
the invention. Ongoing time stamping of GPS and IMU data allows the
clock processor 20 to implement a closed loop system that measures
the historical inaccuracies of the atomic clocks 18, 21, 24. In
operation, the clock processor 20 compares the atomic clocks' 18,
21, 24 time with the time solution from such GPS signals as are
received at the GPS receiver 30. With designated loop parameters,
any instability of the atomic clocks 18, 21, 24 is effectively
zeroed prior to the RAIM outage. The resulting clock coasting
during the RAIM outage is well-determined and predictable due to
the closed-loop hardware.
[0029] A closed-loop multiple clock 30 system to reduce clock 30
errors in frequency, drift and second order rate of change for RAIM
calculation in the absence of over determination (less than five
satellites) of PVT and clock 30 errors and drift.
[0030] Referring to FIG. 2, a method 50 for executing a RAIM is
illustrated. The RAIM algorithm is based upon an atomic clock
signal received from at least one atomic clock. In some
embodiments, a plurality of clocks is disciplined with a process of
clock coasting.
[0031] At a block 51, GPS ephemerides are received and identified
as emanating from distinct satellites. The number of distinct
satellite ephemerides are identified. Conventional RAIM algorithms
rely upon ephemerides from six or more satellites. To obtain a
3-dimensional position solution, at least 4 measurements are
required. To detect a fault, at least 5 measurements are required,
and to isolate and exclude a fault, at least 6 measurements are
required; however, more measurements are often needed depending on
the satellite geometry. Typically, there are 7 to 12 satellites in
view.
[0032] If, at the block 51, six or more satellites were visible,
then at a block 54, ephemerides from the visible satellites are
received to calculate RAIM based upon conventional methods. With
the RAIM solution, appropriate ephemerides are identified to derive
a time solution. With that time solution, at a block 60, the
availability of GPS time allows the slaving of one or more atomic
clocks to a derived portion of a PVT solution derived at a GPS
receiver.
[0033] If, at the block 51, fewer than six satellites had been
available, at a block 72, the method 50 progresses to execute the
RAIM algorithm using the adjusted or conditioned atomic clock
output as a substitute for the missing sixth satellite ephemeris.
Upon the execution of the RAIM algorithm using the conditioned
atomic clock output, the system responds by known means. By virtue
of the closed-loop multiple clock system to reduce clock errors in
frequency, drift and second order rate of change for RAIM
calculation in the absence of over determination (less than five
satellites) of PVT and clock errors and drift, the RAIM solution
will be accurate with one less satellite.
[0034] While the preferred embodiment of the invention has been
illustrated and described, as noted above, many changes can be made
without departing from the spirit and scope of the invention.
Accordingly, the scope of the invention is not limited by the
disclosure of the preferred embodiment. Instead, the invention
should be determined entirely by reference to the claims that
follow.
* * * * *