Receiver for determining a position on the basis of satellite ne

Details Receiver for determining a position on the basis of satellite ne Receiver for determining a position on the basis of satellite ne

1998-04-14

2000-06-27

Bost, Dwayne D.

Telecommunications

Carrier wave repeater or relay system

Portable or mobile repeater

455553, 455552, 701214, H04B 7185, H04B 138

Patent

active

060816910

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The invention relates to systems for determining the position of a point with respect to the earth, based on receiving radio-relay signals transmitted by a constellation of satellites orbiting the earth.
2. Discussion of the Background
There are currently two global positioning systems for carrying out this position determination almost anywhere on the earth; these being respectively the GPS system ("Global Positionning System") and the GLONASS system ("Global Orbiting Navigation Satellite System").
Both use a network of satellites circling the earth, each satellite regularly transmitting coded radio signals which a receiver can receive in order to compute its exact position in terms of longitude, latitude and altitude, and in addition its velocity and the exact time. A complementary system termed RGIC ("Random Geostationary Integrity Channel") is also being put in place; it uses geostationary satellites which transmit signals towards regions of the globe over which they are located.
The GPS system transmits spread-spectrum radio signals on a carrier frequency L1 equal to 1575.42 MHz, as well as signals on an auxiliary carrier frequency L2 of 1227.6 MHz. Here, only the frequency L1 will be of interest, but the principles set out below can be carried over, should the need be felt, to the frequency L2.
The signal spectrum is spread by pseudo-random codes, that is to say the signal on the carrier frequency L1 is modulated by a repetitive pseudo-random binary sequence also referred to as a PRN code (standing for "Pseudo-Random Noise"); in practice, the system uses two categories of PRN codes, namely; 1.023 MHz; they are 1023 bits long and the duration of a complete sequence is 1 millisecond; the C/A codes allow approximate position determination, 10.23 MHz and allowing a more accurate fix.
The logic transitions of the code are synchronized with the phase of the carrier L1, and the modulation is a phase modulation of a well defined type (BPSK).
The C/A codes are accessible to the public and must be generated locally within the receivers for three reasons: firstly, they make it possible to detect and demodulate the signals received, these signals being embedded in a very high level of noise (20 to 30 dB above the signal); detection is carried out by correlation between the code received from the satellite and an identical code produced locally; additionally, they make it possible to identify the transmitting satellite (in the GPS system a particular C/A code is assigned to each satellite); and lastly, they make it possible to measure time lags which are the indispensable basis for accurate computation of position.
The P codes are not accessible to the public; they are reserved for essentially military uses, and may moreover be encrypted on transmission.
The signals transmitted on the carrier frequency L1 are moreover coded using slow rate (50 baud) binary data which represent satellite navigation information, that is to say data serving in the position computations performed in the receiver. These navigation data are ephemerides which make it possible on the one hand to compute the exact position (accurate to within a meter) occupied by a satellite at any instant, and on the other hand to furnish all the receivers using the system with a common time reference. Here again, the modulation of the carrier L1 by the data is a phase modulation.
The distance between a satellite and the receiver is determined by measuring the duration of propagation, at the speed of light, of the radio signal between a satellite and the earth. It is therefore determined in particular by measuring the time discrepancy between the instant of transmission, by the satellite, of a characteristic bit (the "epoch" bit) of the pseudo-random code and the instant of reception by the receiver, of this characteristic bit.
The distances from the receiver to three different satellites make it possible to determine the position of the receiver in a fixed terrestrial frame once the receiver knows t

REFERENCES:
patent: 5535432 (1996-07-01), Dent
patent: 5812539 (1998-09-01), Dent
patent: 5822376 (1999-03-01), Renard
patent: 5850420 (1998-12-01), Guillard et al.
patent: 5884214 (1999-03-01), Krasner

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