Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2000-05-25
2001-11-13
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490, C701S213000
Reexamination Certificate
active
06317078
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to position determination with the aid of the GPS satellites of the NAVSTAR system. It relates more specifically to the reception processing of a GPS satellite signal L2 modulated by an encrypted code.
DICUSSION OF THE BACKGROUND
The NAVSTAR system is a system for positioning and for navigation around the globe by means of a cluster of flyby satellites. The satellites, 24 in number, are distributed over six fixed 12-hour orbital planes in such a way as to ensure the most regular possible terrestrial coverage at a rate of four satellites per orbital plane. Their positions are known accurately at any instant. They are provided with mutually synchronized clocks and emit signals which make it possible, when they are in direct line of sight of a receiver, to determine the distances between them and the receiver and consequently, knowing their positions, to deduce therefrom that of the receiver by triangulation.
A source of inaccuracy is due to the crossing of the ionosphere by the waves originating from the satellites orbiting at an altitude of greater than 20,000 kilometres. This is because, on crossing this medium charged with electrons, the waves undergo refractions which decrease their apparent speed. It is known practice to measure the effect of the ionosphere on the propagation of the waves originating from the satellites on the basis of the delay appearing on reception between two waves of different frequencies emitted coherently since the propagation delay due to the ionosphere which depends on the concentration of electrons, varies, to a first approximation, as an inverse function of the square of the frequency. This measure of the effect of the ionosphere is one of the justifications of the fact that each GPS satellite emits on two different frequencies in the L band.
More specifically, each GPS satellite emits on two carriers in the L band: a carrier Li at 1575.42 MHZ and a carrier L2 at 1227.60 MHZ. These carriers are phase modulated according to the BPSK (Binary Phase Shift Keying) technique by a pseudo-random binary sequence called the spectrum spreading code or PRN (Pseudo Random Noise), and by a satellite navigation signal which is referred to as data. More specifically, the carrier L1 is doubly modulated, in phase by a Coarse Acquisition C/A spreading code and the data, and in quadrature by a Precise P or an encripted P (Y) spreading code if it is jammed, and the data whereas the carrier L2 is simply modulated by the spreading code P(Y) and the data.
The modulation of the carriers L1 and L2 by pseudo-random binary sequences causes a spreading of the frequency spectrum of the signals emitted which are then less sensitive to jamming and to interference. In a satellite, all the codes are synchronous. The collection of signals: carriers, spreading codes and data, are coherent, the transitions of the navigation messages corresponding exactly to any possible transitions of the pseudo-random codes which themselves have a very accurate phase relationship with the carriers which themselves derive from a very stable unique clock.
The C/A and P(Y) spreading codes are individualized and different for each satellite so as to make it possible to distinguish the satellites from one another. The C/A spreading code has a length of 1,023 bits with a rating of 1.023 MHz, thereby giving it a duration of 1 millisecond and an occupied frequency bandwidth of about 2 MHz. The spreading code P(Y) has a duration of greater than 7 days with a rating of 10.23 MHz, thereby giving it an occupied frequency bandwidth of around 20 MHz. The C/A and P spreading codes are known. The jammed version Y of the spreading code P, which is that emitted most of the time, in fact more than 99.9% of the time, is on the other hand unknown to the civil user so as to avoid the possibility that one is able by a countermeasure to imitate the signal from a GPS satellite and falsify the positional pinpointing.
The distance from a receiver to a GPS satellite is measured by the time span which elapses between the instance of emission by the satellite, of a start of a characteristic pattern of a C/A or P(Y) pseudo-random spreading code and the instant of reception at the receiver of this same start of pattern. This time span is not directly accessible since the discrepancy between the clock of the satellites and that of the receiver is not known at reception. One only has access to a pseudodistance measured with respect to the clock of the receiver, thereby making it necessary, in order to remove the uncertainty with regard to the clock of the receiver, to resort to an additional satellite in the triangulation.
The accuracy of the position measurement depends on the accuracy with which one is capable of pinpointing the start of a pattern. It is better with the spreading code P(Y) of which the wavelength of one binary element is of the order of 30 metres than with the C/A spreading code of which the wavelength of one binary element is of the order of 300 metres. This is why it is usual to carry out a first location finding with the aid of the C/A (standing for “Coarse Acquisition”) spreading code and then to refine this first location finding with the aid of the P(Y) (P standing for “Precise”) spreading code. However, for civil use, where the key of the code Y (encrypted P) is not available, the positional pinpointing is performed on the basis of the C/A spreading code alone. One therefore has inferior accuracy. One may nevertheless hope for a location-finding accuracy of the order of a few metres since it is possible to pinpoint a start of pattern with the accuracy of one hundredth of the length of a binary element of the C/A spreading code. However, to achieve this accuracy, the ionospheric effect must be taken into account. For a user who has access to the key for enciphering the spreading code Y (encrypted P), this is not a problem since it is easy for him to measure the relative propagation delay existing between the patterns of the carriers Li and L2, and to deduce therefrom the delays affecting the carriers L1 and L2 due to the ionosphere. After correcting the satellite group delays, it is sufficient for this user to carry out the demodulations of the two carriers L1 and L2 by translation into lower band and correlations with spreading codes Y (encrypted P) produced locally in reception and phase-adjusted so as to be in synchronism with the spreading codes Y (encrypted P) modulating the two signals received on the carriers L1 and L2, then to measure the relative delay between the two locally produced spreading codes Y. The problem is entirely otherwise for a user who does not have access to the code Y (encrypted P) since he can no longer demodulate the carrier L2.
To solve this problem, various approaches have already been proposed:
A first approach consists in performing a cross correlation between the signals L1 and L2 since the signals L1 and L2 emitted by one in the same satellite are modulated coherently by the same spreading code Y (encrypted P). This cross correlation has the drawback of considerably increasing the noise power while the signal-to-noise ratio of the signals received is already very low. Furthermore, it does not make it possible to separate the various GPS satellites, nor to obtain a measurement on the carrier (speed).
A second approach of superior performance takes into consideration the fact that a spreading code Y (encrypted P) results from the product of the non-encrypted spreading-code P occupying a frequency band of the order of 20 MHz times a binary encryption code W occupying a frequency band which is forty times smaller, of the order of 500 kHz. It consists in demodulating the carrier L2 modulated by a spreading code Y (encrypted P) by means of the code P itself non-encrypted produced locally in reception by means of a spreading code generator phase-locked in such a way as to obtain maximum power for the demodulated signal obtained limited to a frequency band of 500 kHz and to square the signal emanating from the demodulation
Renard Alain
Revol Marc
Blum Theodore M.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Thomson-CSF Sextant
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