A method of processing signals in a Global Positioning System (GPS) receiver has been invented to enable the receiver to recover some of the information that is otherwise lost when GPS signals are encrypted at the transmitters. The need for this method arises because, at the option of the military, precision GPS code (P-code) is sometimes encrypted by a secret binary code, denoted the A code. Authorized users can recover the full signal with knowledge of the A-code. However, even in the absence of knowledge of the A-code, one can track the encrypted signal by use of an estimate of the A-code. The present invention is a method of making and using such an estimate. In comparison with prior such methods, this method makes it possible to recover more of the lost information and obtain greater accuracy.

The limitation on space available for this article precludes a description of the prior methods. However, a description of pertinent generally applicable aspects of GPS signals and signal processing is presented in the next three paragraphs because it is prerequisite to a meaningful summary of the present method.

Each GPS satellite transmits two L-band signals, denoted L1 (at a carrier frequency of 1.57542 GHz) and L2 (at a carrier frequency of 1.2276 GHz). The L1 carrier is phase-modulated with two binary pseudorandom-noise codes that contain GPS information: (1) the coarse-acquisition (C/A) code, characterized by a chip rate of 1.023 MHz and (2) the precise (P) code, characterized by a chip rate of 10.23 MHz and modulated in quadrature with the C/A code. The L2 carrier is modulated with the P code only. The signals from different satellites are distinguishable from each other because each satellite transmits a unique C/A and a unique P code. Although the limitation on space also precludes a detailed description of the C/A and P codes, it can be said here that names of these codes convey an approximate idea of the roles played by these codes and of the relationship between them. The C/A and P codes of all the satellites are further modulated with a common binary code that conveys information about the satellites, their orbits, their clock offsets, and their operational statuses.

The basic principle of GPS receiver signal processing is to determine the time and the position of the GPS receiver from times of arrival of signals transmitted from several different GPS satellites. This basic principle is implemented, in practice, by use of correlations between (1) the received GPS signals and (2) model signals in the receiver constructed from model carriers modulated by model C/A, and P (and, when applicable, A) codes.

Processing is said to be done in a code mode when the receiver "knows" the code in question. Because the C/A code is not encrypted, C/A modulation is usually processed in the code mode, using the published C/A code. Processing is said to be done in an encryption mode when the receiver does not "know" the code in question. More specifically, processing is said to be done in an encryption mode when the receiver does not "know" the A code with which the P code is modulated. Hence, the present invention is characterized as a method of encryption-mode processing.

P-Code-Enhanced Encryption-Mode Processing of GPS signals according the present invention involves this sequence of steps.
The figure is a block diagram of signal processing according to the invention. The received GPS signal is down-converted from radio frequency (RF) to baseband to obtain two pairs of quadrature components — one pair for L1, the other for L2. Unlike in some prior methods, there is no cross-processing of signals between the L1 and L2 P channels. Instead, each of the two quadrature components obtained from each RF signal, independently of the other components, is counter rotated with its respective model phase, correlated with its respective model P code, and then successively summed and dumped over pre-sum intervals substantially coincident with chips of the respective encryption code. In the encryption mode, the effect of the unknown A-code sign flips is reduced, for each quadrature component of each RF signal, by combining selected pre-sums. The resulting combined pre-sums are then summed and dumped over longer intervals and further processed to extract the amplitude, phase, and delay for each RF signal. The precision of the resulting phase and delay values is approximately four times better than that obtained from conventional cross-correlation of the L1 and L2 P signals.

In comparison with prior encryption-mode receivers, a receiver according to this invention offers greater signal-to-noise ratios for the L1 and L2 P signals, and greater precision in the phases and delays of these signals. Unlike the prior receivers, this receiver offers the capability for separate and independent tracking of the L1 and L2 P signals to eliminate fading crossover, separate and independent measurement of the L1 and L2 P amplitudes, the option of dual-band measurements without a separate L1 P channel, removal of a half-cycle ambiguity in the L2 P phase, and the option of operation in either the code mode or the encryption mode with maximum commonality of hardware and software between modes. Finally, this processing method would still work even if the L1 and L2 P codes were to be encrypted with different A codes.

This work was done by Lawrence Young, Thomas Meehan, and Jess B. Thomas of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Information Sciences category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Intellectual Assets Office

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Refer to NPO-30367, volume and number of this NASA Tech Briefs issue, and the page number.

Topics:
Electronic equipment Global positioning systems (GPS) Information Technology Navigation and guidance systems Radio equipment Radio frequency
This Brief includes a Technical Support Package (TSP).
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P-Code-Enhanced Encryption-Mode Processing of GPS Signals

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This article first appeared in the March, 2003 issue of NASA Tech Briefs Magazine (Vol. 27 No. 3).

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Overview

The document is a technical support package prepared under the sponsorship of NASA, detailing advancements in GPS signal processing through a P-Code-Enhanced Encryption-Mode method. It is associated with NASA Contract No. NAS 7-918 and is part of the Jet Propulsion Laboratory's (JPL) New Technology Report NPO-30367.

The primary focus of the document is on a novel method for processing encrypted GPS signals, specifically the L1-P and L2-P channels. The invention addresses the limitations of prior art in GPS signal processing, which often required knowledge of the encryption code and produced less accurate results. The new method allows for the independent extraction of amplitude, phase, and delay for each encrypted channel without needing to know the encryption code, thereby enhancing the accuracy of GPS applications.

The document outlines the motivation behind this development, which was the need for improved processing techniques that could provide more precise and separate analysis of encrypted GPS signals. The solution presented involves a P-code enhanced method that cross-processes the quadrature components of the GPS channels, resulting in a more accurate extraction of the necessary signal parameters. This method overcomes several shortcomings of existing techniques, such as the inability to extract full-cycle phase information and the inadequate extraction of signal amplitudes.

The inventors of this technology—Thomas K. Meehan, Jess Brooks Thomas Jr., and Lawrence E. Young—are credited with this significant advancement, which is encapsulated in Patent No. 6,061,390, issued on May 9, 2000. The document emphasizes the potential applications of this technology in areas such as ionospheric scintillations and atmospheric occultations, where accurate GPS signal processing is crucial.

Additionally, the document includes a disclaimer regarding the use of specific commercial products and the lack of endorsement by the U.S. Government or JPL. It serves as a comprehensive overview of the innovative approach to GPS signal processing, highlighting its novelty, technical details, and the problem it aims to solve, ultimately contributing to the field of satellite navigation and positioning technologies.