Adaptive threshold logic implementation for RAIM fault...

Details Adaptive threshold logic implementation for RAIM fault... Adaptive threshold logic implementation for RAIM fault...

2003-05-31

2004-09-28

Issing, Gregory C. (Department: 3662)

Communications: directive radio wave systems and devices (e.g.,

Directive

Including a satellite

C342S357490

Reexamination Certificate

active

06798377

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is in the field of satellite navigation. More specifically, the present invention relates to the field receiver autonomous integrity monitoring (RAIM) fault detection and exclusion implementation.
2. Discussion of the Prior Art
The Global Positioning System (GPS) is a system of satellite signal transmitters that transmits information from which an observer's present location and/or the time of observation can be determined. Another satellite-based navigation system is called the Global Orbiting Navigational System (GLONASS), which can operate as an alternative or supplemental system.
The GPS was developed by the United States Department of Defense (DOD) under its NAVSTAR satellite program. A fully operational GPS includes more than 24 Earth orbiting satellites approximately uniformly dispersed around six circular orbits with four satellites each, the orbits being inclined at an angle of 55° relative to the equator and being separated from each other by multiples of 60° longitude. The orbits have radii of 26,560 kilometers and are approximately circular. The orbits are non-geosynchronous, with 0.5 sidereal day (11.967 hours) orbital time intervals, so that the satellites move with time relative to the Earth below. Generally, four or more GPS satellites will be visible from most points on the Earth's surface, which can be used to determine an observer's position anywhere on the Earth's surface. Each satellite carries a cesium or rubidium atomic clock to ;provide timing information for the signals transmitted by the satellites. An internal clock correction is provided for each satellite clock.
Each GPS satellite continuously transmits two spread spectrum, L-band carrier signals: an L1 signal having a frequency f
1
=1575.42 MHz (approximately nineteen centimeter carrier wavelength) and an L2 signal having a frequency f
2
=1227.6 MHz (approximately twenty-four centimeter carrier wavelength). These two frequencies are integral multiplies f
1
=1,540 f
0
and f
2
=1,200 f
0
of a base frequency f
0
=1.023 MHz. The L1 signal from each satellite is binary phase shift key (BPSK) modulated by two pseudo-random noise (PRN) codes in phase quadrature, designated as the C/A-code and P-code. The L2 signal from each satellite is BPSK modulated by only the P-code. The nature of these PRN codes and accepted methods for generating the C/A-code and P-code are set forth in the document ICD-GPS-200: GPS Interface Control Document, ARINC Research, 1997, GPS Joint Program Office, which is incorporated by reference herein.
The GPS satellite bit stream includes navigational information on the ephemeris of the transmitting GPS satellite (which includes orbital information about the transmitting satellite within next several hours of transmission) and an almanac for all GPS satellites (which includes a less detailed orbital information about all satellites). The transmitted satellite information also includes parameters providing corrections for ionospheric signal propagation delays (suitable for single frequency receivers) and for an offset time between satellite clock time and true GPS time. The navigational information is transmitted at a rate of 50 Baud.
A second satellite-based navigation system is the Global Orbiting Navigation Satellite System (GLONASS), placed in orbit by the former Soviet Union and now maintained by the Russian Republic. GLONASS uses 24 satellites, distributed approximately uniformly in three orbital planes of eight satellites each. Each orbital plane has a nominal inclination of 64.8° relative to the equator, and the three orbital planes are separated from each other by multiples of 120° longitude. The GLONASS satellites have circular orbits with a radii of about 25,510 kilometers and a satellite period of revolution of 8/17 of a sidereal day (11.26 hours). A GLONASS satellite and a GPS satellite will thus complete 17 and 16 revolutions, respectively, around the Earth every 8 days. The GLONASS system uses two carrier signals L1 and L2 with frequencies of f
1
=(1.602+9 k/16) GHz and f
2
=(1.246+7 k/16) GHz, where k=(1, 2, . . . 24) is the channel or satellite number. These frequencies lie in two bands at 1.597-1.617 GHz (L1) and 1,240-1,260 GHz (L2). The L1 signal is modulated by a C/A-code (chip rate=0.511 MHz) and by a P-code (chip rate=5.11 MHz). The L2 signal is presently modulated only by the P-code. The GLONASS satellites also transmit navigational data at a rate of 50 Baud. Because the channel frequencies are distinguishable from each other, the P-code is the same, and the C/A-code is the same, for each satellite. The methods for receiving and demodulating the GLONASS signals are similar to the methods used for the GPS signals. Reference to a Satellite Positioning System or SATPS herein refers to a Global Positioning System, to a Global Orbiting Navigation System, and to any other compatible satellite-based system that provides information by which an observer's position and the time of observation can be determined, all of which meet the requirements of the present invention.
A Satellite Positioning System (SATPS), such as the Global Positioning System (GPS) or the Global Orbiting Navigation Satellite System (GLONASS), uses transmission of coded radio signals, with the structure described above, from a plurality of Earth-orbiting satellites. An SATPS antenna receives SATPS signals from a plurality (preferably four or more) of SATPS satellites and passes these signals to an SATPS signal receiver/processor, which (1) identifies the SATPS satellite source for each SATPS signal, (2) determines the time at which each identified SATPS signal arrives at the antenna, and (3) determines the present location of the SATPS satellites.
The range (r
i
) between the location of the i-th SATPS satellite and the SATPS receiver is equal to the speed of light c times (&Dgr;t
i
), wherein (&Dgr;t
i
) is the time difference between the SATPS receiver's clock and the time indicated by the satellite when it transmitted the relevant phase. However, the SATPS receiver has an inexpensive quartz clock which is not synchronized with respect to the much more stable and precise atomic clocks carried on board the satellites. Consequently, the SATPS receiver estimates a pseudo-range (pr
i
) (not a true range) to each satellite.
After the SATPS receiver determines the coordinates of the i-th SATPS satellite by demodulating the transmitted ephemeris parameters, the SATPS receiver can obtain the solution of the set of the simultaneous equations for its unknown coordinates (x
0
, y
0
, z
0
) and for unknown time bias error (cb). The SATPS receiver can also determine velocity of a moving platform.
The given above discussion assumes that a satellite navigational system used for the navigation purposes is functioning properly. GPS satellites are designed to provide users with warnings of satellite malfunctions. However, sometimes, the warnings may take more than 5 minutes before the user is aware of a malfunction. Aviation applications require a more timely notification of a satellite failure condition. Thus, a means to independently monitor satellite integrity is required.
One of these means is receiver autonomous integrity monitoring (RAIM). The RAIM method is based on a self-consistency check among the available measurements. The RAIM consistency check uses redundant measurements as a means to determine GPS integrity. The discussion given below can be found in “Global Positioning System: Theory and Applications”, Volume II, chapter 5, by Bradford W. Parkinson and James J. Spilker Jr., published by the American Institute of Aeronautics and Astronautics, Inc. in 1996.
There are two main approaches to RAIM (not considering several hybrid approaches). In the first main method, the snapshot scheme, only current redundant measurements are used for the self-consistency check. In the second main method, the averaging or filtering sc

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