U.S. patent application number 10/001077 was filed with the patent office on 2003-05-01 for skipping filter for inertially augmented landing system.
Invention is credited to Anderson, Leonard R., Krogh, Steven B., McIntyre, Melville D., Murphy, Thimothy.
Application Number | 20030083792 10/001077 |
Document ID | / |
Family ID | 21694262 |
Filed Date | 2003-05-01 |
United States Patent
Application |
20030083792 |
Kind Code |
A1 |
Anderson, Leonard R. ; et
al. |
May 1, 2003 |
SKIPPING FILTER FOR INERTIALLY AUGMENTED LANDING SYSTEM
Abstract
The present method and apparatus consists of storing past values
of estimated IRU error and using these past values to update the
coasting filter when switching from GPS to inertial mode. Through
the storage of past IRU error estimates, it is possible to avoid
misdirected guidance from an erroneous GPS signal. The MMR and
ground station can require up to 6 seconds to identify a failed GLS
signal.
Inventors: |
Anderson, Leonard R.;
(Lynnwood, WA) ; Krogh, Steven B.; (Issaquah,
WA) ; McIntyre, Melville D.; (Bellevue, WA) ;
Murphy, Thimothy; (Lynnwood, WA) |
Correspondence
Address: |
Conrad O. Gardner
M/C 13-08
P.O. Box 3707
Seattle
WA
98124-2207
US
|
Family ID: |
21694262 |
Appl. No.: |
10/001077 |
Filed: |
October 31, 2001 |
Current U.S.
Class: |
701/16 ; 342/358;
701/469 |
Current CPC
Class: |
G01S 19/15 20130101;
G01S 19/20 20130101; G01S 19/52 20130101; G01S 19/41 20130101; G01S
19/49 20130101 |
Class at
Publication: |
701/16 ; 701/213;
342/358 |
International
Class: |
G06F 019/00 |
Claims
What is claimed is:
1. In combination in an inertially augmented GPS landing system:
means for continuously storing a moving time period window of data
representative of an aircraft GPS and inertial state for the last
time period; and means for reconstructing the estimate of the
inertial state errors in the event of a GPS signal failure.
2. The invention according to claim 1 wherein said moving time
period window is 6 seconds.
3. A method for correcting the corruption of an inertial guidance
signal caused by a faulty GPS signal comprising the steps of:
recognizing a GPS signal failure; and, reconstructing in non-real
time the prior 6 seconds of blended GPS inertial filtering; and
replacing the 6 seconds of corrupted filter output data with an
uncorrupted set of data representing said 6 seconds.
4. In combination in an inertially augmented landing system: a
coasting filter; and a skipping filter; and, said skipping filter
comprising a complementary filter including two integrators for
separating an inertial guidance signal from a GPS guidance signal
thereby avoiding corruption for the delayed detection of a GLS
signal loss.
5. A method for utilizing a skipping filter comprising the steps
of: storing past values of estimated IRU error; and, then utilizing
said stored past values of estimated IRU error to update a coasting
filter when switching from GPS to inertial mode.
Description
TECHNICAL FIELD
[0001] This invention relates to inertially augmented landing
systems and more particularly methods and apparatus for overcoming
delays in detection of GLS input signal errors essential to safe
guidance in landing and rollout of an aircraft.
BACKGROUND OF THE INVENTION
[0002] Since 1993, the industry has been working to develop
automatic landing capability using differential GPS. This
capability is known as the GNSS Landing System GLS). GLS
developments to support CAT 1 operations are nearly complete. The
industry is now working on standards and performance requirements
for GLS to support CAT II/III operations. A key issue associated
with GLS CAT II/III operations is the expected failure modes and
effects of the GLS guidance system. It is anticipated that the most
common failure mode for GLS will be a total loss of the signal for
hundreds of seconds. U.S. Pat. No. 6,178,363B1 shows a GPS/Inertial
filtering scheme to enable the airplane to continue to land and
roll out after a total loss of GLS guidance below the alert
height.
[0003] Key to this concept is the ability of the GLS groundstation
to provide the aircraft systems with the information required to
determine with certainty when the GLS guidance signals are
unusable. The airborne multi-mode receiver (MMR) must respond
rapidly to switch away from the faulty GLS signals to updated
inertial guidance in order to prevent the inertial signals from
becoming corrupted by the errors in the GLS signals. Unfortunately,
the GLS groundstation cannot communicate the status of the guidance
signals instantaneously, and therefore the likelihood of corruption
exists. Accordingly, the present method and apparatus as
hereinafter described address this problem.
SUMMARY OF THE INVENTION
[0004] During a failure condition, it is possible for a
differential GPS ground station to provide corrupted data for up to
3 seconds before raising an alarm. Furthermore, the airplane is
allowed to continue to use the last data provided by the ground
station for up to 3.5 seconds after the ground station stops
transmitting data. Consequently, there could be a 3-6 second delay
between GPS signal corruption and detection of the corruption by
the airborne receiver. The present invention provides a means for
correcting the integrated GPS/INS solution and protecting the
airplane landing performance from any effects due to this potential
for data corruption. A skipping filter in combination with a
coasting filter shown in U.S. Pat. No. 6,178,363B1 enables recovery
from up to 6 seconds of corrupted GPS signal, thereby avoiding
subsequent miss-guidance from the anomalous GPS signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] The foregoing aspects and many of the attendant advantages
of this invention will become more readily appreciated as the same
becomes better understood by reference to the following detailed
description, when taken in conjunction with the accompanying
drawings, wherein:
[0006] FIG. 1 is a block diagram of a coasting filter as shown in
U.S. Pat. No. 6,178,363B1 issued Jan. 23, 2001 to McIntyre et al.
and assigned to The Boeing Company;
[0007] FIG. 2 is a block diagram of a preferred embodiment of the
present combination coasting filter and skipping filter;
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0008] Preferred Embodiment
[0009] Due to the requirements on the Ground Based Augmentation
System (i.e., the differential GPS ground station supporting the
GLS function) the detection of an anomalous GPS position or GPS
velocity signal could require from 3 to 6 seconds. Consequently,
the coasting filter is potentially exposed to up to 6 seconds of an
error in progress before the guidance signal is flagged. Once the
GPS anomaly is detected, the coasting filter switches to pure
inertial guidance mode. However, due to this 3-6 second lag time to
alarm, the landing guidance signal may be corrupted. It is
necessary to find a means of avoiding guidance corruption due to
the delay in GPS fault detection.
[0010] The solution to this problem is the hereinafter described
signal skipping filter which separates the inertial guidance signal
from the GPS guidance signal, and therefore avoids corruption
caused by the delayed detection of the GLS signal loss.
[0011] The coasting filter of FIG. 1 is shown in U.S. Pat. No.
6,178,363B1. The coasting filter of FIG. 1 receives GLS
(differential GPS) position and velocity signals as shown. These
high accuracy signals are used to identify the bias and bias-rate
in the IRU (Inertial Reference Unit) velocity signal, shown. At the
time of loss of GPS signal, the two switches go to open or "coast"
position and the aircraft landing guidance is completed with IRU
guidance alone. The IRU velocity and acceleration biases are
estimated by integrator outputs x1 and x2. The IRU position offset
is estimated by the output of integrator x3. The bias values for
IRU velocity and acceleration are slowly varying quantities. They
can be estimated with high accuracy (limited by the accuracy of GLS
signals) during two or more minutes of landing approach. The IRU
velocity and acceleration bias values are thus estimated for the
subsequent coasting interval which can be up to one minute in
duration.
[0012] Due to the time delay to alarm of the ground station, the
coasting filter may be exposed to several seconds of corrupted GPS
input before detection. In this case, after the switch to inertial
mode, the complementary velocity and complementary position would
have been corrupted and would therefore yield reduced accuracy
guidance. The skipping filter of FIG. 2 is directed to a solution
for overcoming this problem.
[0013] The general concept of the present skipping filter is as
follows: The states of the filters (along with any filter inputs if
necessary;) are stored in a time buffer for 6 seconds. When the GLS
guidance signal is lost and the switches are set to the "coast"
position, the filter states are reset to the states from N seconds
prior. Then the filter states are propagated forward in time by
applying the filter information recorded over the last N seconds.
In this manner, any corruption of the filter state due to GLS
guidance failures in progress will be removed. The time period N
depends on the exact conditions causing the coast mode to be
entered.
[0014] A specific embodiment of the present skipping filter
comprises a complementary filter. The skipping filter may be
applied to any linear state space filter (including a Kalman
Filter) implementation.
[0015] With the addition of two integrators it is possible to avoid
corruption of the IRU guidance signal. Also, in order to avoid any
unwanted effects of the anomalous 3-6 second GPS signal (either
position or velocity), at the time coasting filter switches to
inertial mode, the integrators x1, x2, x3, x4 are reset with stored
values as follows: Assume the delayed signal detection interval is
.DELTA.t seconds, and the values of the integrators just before the
corrupted GLS signal are x1old, x2old, x3old and x4old. These old
values of integrator outputs would be stored in MMR memory (up to 6
seconds). The current integrator values at the time of failure
detection are x1now, x2now, x3now. When a GPS anomaly is detected
the delay, .DELTA.t, will be estimated based on the failure mode.
The velocity bias rate estimator x1 is replaced with the
before-corruption value x1old. The velocity bias estimator x2 is
replaced with the before-corruption value x2old+.DELTA.t x1 old,
and the "position-effect-of-velocity-bias" integrator x4 is
replaced with the value x4old+.DELTA.t (x2old+.DELTA.t x 1old/2).
The position bias estimator x3 is replaced with the
before-corruption value x3old.
[0016] The IRU velocity/position information at integrator x5 is
uncorrupted by the GLS signal fault and can be used without
change.
[0017] It can thus be seen that the present skipping filter method
allows the guidance error to recover immediately to its low
pre-fault value with minor increase in filter complexity. This
improvement in guidance accuracy is provided through utilization of
two additional integrators and storage of a few values for
integrators x1, x2 and x3.
[0018] In addition to recovering best estimates of IRU bias values
when the coasting filter switches to coast mode, limiters are
placed on the difference between GLS and IRU position/velocity to
minimize temporary misguidance during delayed error detection. The
error limiting is performed by the two limiters shown in FIG. 2.
The error limit values will be chosen so that the required MMR
guidance accuracy is achieved without interfering with normal mode
filter operation.
[0019] The present skipping filter has been tested in simulations
with the result that whereas a 6-second uncorrected delay in
detecting GLS signal faults can increase final lateral position
error on the runway from 23 ft rms to 42 ft rms, the hereinbefore
described skipping filter allows the recovery of guidance accuracy
to a 24 ft rms level.
[0020] The present improvement to the MMR (Multi-Mode Receiver)
with GLS (differential GPS) will improve the availability of the
aircraft landing system in the event of loss of GPS signal.
[0021] While the preferred embodiment of the invention has been
illustrated and described, it will be appreciated that various
changes can be made therein without departing from the spirit and
scope of the invention.
* * * * *