Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
1999-04-13
2001-05-08
Hellner, Mark (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C701S215000
Reexamination Certificate
active
06229479
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a method and system for determining the relative positions between two or more locations, such as survey marks, using radio signals broadcast by the Global Positioning System (“GPS”) satellites of the United States and the Global Satellite Navigation System (“GLONASS”) satellites of the former USSR, and more particularly to improved methods of signal processing and instrumentation for determining these positions.
There are many GPS and GLONASS applications being implemented, both military and commercial. An appropriate receiver in an aircraft, ship, tractor, automobile, or the like, allows a user to determine position and/or velocity. Another application is surveying to accurately determine the location of a point or a distance between two or more points. It is this last application to which the present invention most closely pertains.
As is well known, GPS was established by the United States government, and employs a constellation of satellites in orbit around the earth at an altitude of approximately 26500 km. Currently, the GPS constellation consists of 24 satellites, arranged with 4 satellites in each of 6 orbital planes. Each orbital plane is inclined to the earth's equator by an angle of approximately 55 degrees.
Each GPS satellite transmits microwave L-band radio signals continuously in two frequency bands, centered at 1575.42 MHz and 1227.6 MHz., denoted as L
1
and L
2
respectively. The GPS L
1
signal is quadri-phase modulated by a coarse/acquisition code (“C/A code”) and a precision ranging code (“P-code”). The L
2
signal is binary phase shift key (“BPSK”) modulated by the P-code. The GPS C/A code is a gold code that is specific to each satellite, and has a symbol rate of 1.023 MHz. The unique content of each satellite's C/A code is used to identify the source of a received signal. The P-code is also specific to each satellite and has a symbol rate of 10.23 MHz. The GPS satellite transmission standards are set forth in detail by the Interface Control Document GPS (200), dated 1993, a revised version of a document first published in 1983.
GLONASS was established by the former Soviet Union and operated by the Russian Space Forces. The GLONASS constellation consists of 24 satellites arranged with 8 satellites in each of 3 orbital planes. Each orbital plane is inclined to the earth's equator by an angle of approximately 64.8 degrees. The altitude of the GLONASS satellites is approximately 19100 km.
The satellites of the GLONASS radio navigation system transmit signals in the frequency band near 1602 MHz, and signals in a secondary band near 1246 MHz, denoted as L
1
and L
2
respectively. The GLONASS L
1
signal is quadri-phase modulated by a C/A code and a P-code. The L
2
signal is BPSK modulated by the P-code. Unlike GPS, in which all of the satellites transmit on the same nominal frequency, the GLONASS satellites each transmit at a unique frequency in order to differentiate between the satellites. The GLONASS L
1
carrier frequency is equal to 1602 MHz+k * 0.5625 MHz, where k is a number related to the satellite number. The GLONASS L
2
carrier frequency is equal to 1246 MHz+k * 0.5625 MHz. The GLONASS C/A code consists of a length 511 linear maximal sequence. Details of the GLONASS signals may be found in the Global Satellite Navigation System GLONASS-Interface Control Document of the RTCA Paper No. 518-91/SC159-317, approved by the Glavkosmos Institute of Space Device Engineering, the official former USSR GLONASS responsible organization.
Although no carriers are present in the transmitted GPS and GLONASS signals, the carriers may be said to be implicit therein. The term “carrier” refers to a component having an effectively constant amplitude and phase. Because the GPS and GLONASS signals are modulated by pseudorandom codes, on average, the band center frequency carrier is suppressed. The term “carrier” is used herein refers to the dominant spectral component after the modulation is removed.
The carrier is reconstructed in a GPS or a GLONASS receiver by one of a number of techniques. The most straight forward technique is to multiply the received signal with a locally generated estimate of the modulation contained on the received signal. The P-code of GPS satellites, however, is usually encrypted, the encrypted precision ranging code being termed a “Y-code”. The purpose of encrypting the P-code is to prevent “spoofing,” which is the possibility of a hostile force emulating a GPS satellite signal to cause military airplanes, ships and the like to be misdirected and calculate incorrect values of velocity, position, time and the like. There are, however, methods utilized to recover the carrier from the encrypted signals and provide an estimate the P-code without decryption being necessary, such as the methods described in U.S. Pat. No. 5,134,407, Lorenz et al., that are useful for surveying and other commercial applications.
None of the GLONASS signals are encrypted but the P-code has not been officially published. However, the GLONASS P-code has been determined to be derived from a length 2**25-1 linear maximal sequence. [Lennen, Gary R., “The USSR's GLONASS P-Code Determination and Initial Results”, ION GPS-89, Colorado Springs, Sep. 27-29, 1989]. The inventors have verified the results of Lennen and have found them to be correct. There has been no assurance that this code will not be changed in the future.
There are many factors that affect the accuracy of measurements made through either GPS or GLONASS. The path of travel, or ephemeris, of each satellite is elliptical and subject to being altered by solar winds and other natural causes. Since the accuracy of any measurement is dependent upon knowledge of the position of the involved satellites at any given time, an estimate of the path of travel is calculated on earth for each of the satellites and periodically uploaded into it. The estimated position of each satellite is then part of the data that is transmitted as part of its signals that are used by a receiver on earth. Other causes of inaccuracies include variable effects of the ionosphere and troposphere on propagation of signals from the satellites to the receivers.
The accuracy of the GPS system is also intentionally degraded in order to limit its usefulness to users not authorized by the United States military. The intentional degradation is introduced through controlled variation of the satellite clocks and ephemeris data. The relative phases of signals transmitted by all the GPS satellites are periodically shifted by simultaneously dithering the internal clocks of the satellites. The resulting condition is generally referred to as Selective Availability (“SA”). The accuracy of GPS for the United States military is not affected, since military users are provided with cryptographic methods to remove the introduced satellite clock and ephemeris data errors. GLONASS has no such intentional accuracy degradation. When using GPS to determine the relative position between two stations, the SA induced errors are common to both stations and approximately cancel.
The use of GPS in commercial applications, such as surveying, is quite valuable and rapidly increasing. Since the GPS satellite positions are known, a point on earth may be geometrically determined by simultaneously measuring the range from that point to three satellites. This is, basically, a standard trilateration technique. The range to each of the three satellites can be viewed as the radius of a spherical surface having the satellite at its center. The point of intersection of all three spherical surfaces is the point whose position is being determined. In GPS, the positions of the satellites and locations on earth are expressed as vectors in a coordinate system with an (x,y,z) position of (0,0,0) located at the center of the earth.
The range from a receiver to a satellite is determined by measuring, in effect, the time that it takes for the signal to traverse from the satellite to the r
Kozlov Dmitry
Povaliaev Alexander
Rapoport Lev
Sila-Novitsky Stanislav
Yefriemov Vladimir
Coudert Brothers
Hellner Mark
Magellan Corporation
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