Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
1999-01-23
2001-05-22
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490, C342S357490, C342S357490
Reexamination Certificate
active
06236355
ABSTRACT:
BACKGROUND
The invention relates to any GPS receiver, however, the primary application would be a centimeter accurate global positioning system (GPS) receiver for real-time kinematic (RTK) measurement. The satellite positioning system (SATPS) can include different satellite systems or combinations of the satellite systems. One of those systems is a global positioning system (GPS).
The GPS is a system of satellite signal transmitters, with receivers located on the Earth's surface or adjacent to the Earth's surface, that transmit information from which an observer's present location and/or the time of observation can be determined. There is also the Global Navigational System (GLONASS), which can operate as an alternative GPS system.
The GPS is part of a satellite-based navigation system developed by the United States Defense Department under its NAVSTAR satellite program. A fully operational GPS includes up to 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. Theoretically, three or more GPS satellites will be visible from most points on the Earth's surface, and visual access to three or more such satellites can be used to determine an observer's position anywhere on the Earth's surface, 24 hours per day. Each satellite carries a cesium or rubidium atomic clock to provide timing information for the signals transmitted by the satellites. 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 f1=1575.42 MHz (nineteen centimeter carrier wavelength) and an L2 signal having a frequency f2=1227.6 MHz (twenty-four centimeter carrier wavelength). These two frequencies are integral multiplies f1=1,540 f0 and f2=1,200 f0 of a base frequency f0=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 (or Y-code). The L2 signal from each satellite is BPSK modulated by only the P-code (or Y-code). The nature of these PRN codes is described below.
Use of the PRN codes allows use of a plurality of GPS satellite signals for determining an observer's position and for providing the navigation information. A signal transmitted by a particular GPS satellite is selected by generating and matching, or correlating, the PRN code for that particular satellite. Some of the PRN codes are known and are generated or stored in GPS satellite signal receivers carried by ground observers. Some of the PRN codes are unknown.
A first known PRN code for each GPS satellite, sometimes referred to as a precision code or P-code, is a relatively long, fine-grained code having an associated clock or chip rate of 10 f0=10.23 MHz. A second known PRN code for each GPS satellite, sometimes referred to as a clear/acquisition code or C/A-code, is intended to facilitate rapid satellite signal acquisition and hand-over to the P-code and is a relatively short, coarser-grained code having a clock or chip rate of f0=1.023 MHz. The C/A -code for any GPS satellite has a length of 1023 chips or time increments before this code repeats. The full P-code has a length of 259 days, with each satellite transmitting a unique portion of the full P-code. The portion of P-code used for a given GPS satellite has a length of precisely one week (7.000 days) before this code portion repeats.
Accepted methods for generating the C/A-code and P-code are set forth in the document GPS Interface Control Document ICD-GPS-200: GPS Interface Control Document, ARINC Research, 1998, GPS Joint Program Office, which is incorporated by reference herein.
A first unknown PRN code is Y-code. To prevent jamming signals from being accepted as actual satellite signals, the satellites are provided with a secret Y-code, which replaces the known P code when the “anti-spoofing” (AS) is ON. When the AS is OFF, the Y-code is turned OFF, and the known P-code (see above cited ICD-GPS-200) is used. Thus, the secret Y-code can be turned ON or OFF at will by the U. S. Government. The AS feature allows the GPS system to be used for the military or other classified United Government projects.
The C/A code is transmitted on L1 under all conditions as it is generally required to provide timing access to L1 and L2 P(Y) code. When AS is OFF, the known P code is transmitted on both L1 and L2, allowing authorized and unauthorized users alike access to full coded receiver operation on both L1 and L2 frequencies. As it is indicated above, when AS is ON, the known P code is replaced with a secret Y code on both L1 and L2.
The GPS satellite bit stream includes navigational information on the ephemeris of the transmitting GPS satellite (which includes a complete information about the transmitting satellite within next several hours of transmission) and an almanac for all GPS satellites (which includes a less detailed information about all other satellites). The satellite information transmitted by the transmitting GPS has the 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 useful discussion of the GPS and techniques for obtaining position information from the satellite signals is found in
The NAVSTAR Global Positioning System
, Tom Logsdon, Van Nostrand Reinhold, New York, 1992, pp. 17-90.
A second alternative configuration for global positioning is the Global Navigation Satellite System (GLONASS), placed in orbit by the former Soviet Union and now maintained by the Russian Republic. GLONASS also 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 circular orbits have smaller radii, about 25,510 kilometers, and a satellite period of revolution of {fraction (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 f1=(1.602+9k/16) GHz and f2=(1.246+7k/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 code is modulated by a C/A- code (chip rate=0.511 MHz) and by a P-code (chip rate=5.11 MHz). The L2 code 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 analyzing the GLONASS signals are similar to the methods used for the GPS signals.
Reference to a Satellite Positioning System or SPS herein refers to a Global Positioning System, to a Global Orbiting Navigation System, to any other compatible satellite-based system, or combination of satellite systems that can provide 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), the Global N
Blum Theodore M.
Tankhilevich Boris G.
Trimble Navigation LTD
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