Cochannel and adjacent channel interference to GPS use

Abstract

The growing international user base for the Global Positioning System, coupled with the increasing number of critical applications, is fostering the development of ground and satellite based augmentations. There is a concern that those augmentations that share the registered GPS band, especially using code division multiple access in a fashion similar to GPS, may cause harmful interference. In this paper, the impact of both cochannel and adjacent channel interference from these proposed services to the signals in the GPS LI band is analyzed. Laboratory test results are also presented to support the results obtained through analysis. The carrier-to-noise spectral density ratio (C/N0) degradation and excess tracking error due to cochannel interference are used as performance metrics. The analysis and test results indicate that under certain conditions, when the Interference-toSignal Ratio (ISR) is greater than 10 dB, the C/N0 degradation could be as high as 7 to 9 dB and the excess tracking error as high as 2 to 3 meters rms. The loss of C/N0 could mean the loss of significant interference margin for safety-of-life applications. The resultant degradation in tracking error will exceed the error budget of the GPS receiver specification for aviation applications. To be certain that this level of degradation is not experienced by the GPS receiver in critical applications, it is suggested that the proposed services not operate in the GPS LI band. For the services in adjacent bands fo LI, it is recommended to use appropriate emission limits obtained on the basis of total interference, from all the proposed services. Introduction Proposals have been made in the past by INMARSAT to provide augmentation signals for GPS navigation through their Intermediate Circular Orbit (ICO) satellites as well as Inmarsat-3 geostationary satellites at LI frequency. Recently Japan has filed paperwork with the ITU expressing its intent to operate an LI frequency payload on three geostationary satellites called MTSATs, the purpose of which is not clear. Mobile Satellite Services (MSS) has been considering frequencies 1559-1567 MHz and 1610-1625 MHz band, which are on either side of the GPS LI frequency. Analyses have been performed by some proponents of these new services, considering interference from their system alone to GPS LI C/A code operation only and shown the viability of their system's coexistence with the GPS. No attempt has been made to analyze the impact of interference from all the proposed sources collectively on LI C/A code operation. Also, none of them has considered the interference impact on the LI P(Y)-code (wideband service) operation. In this paper we consider interference from all the proposed services on both GPS LI C/A code and GPS P(Y)code operations. Theory The analysis of the interference to the GPS receiver considers the impact of the total signal on a channel of the receiver tracking a single satellite. The total signal received by the GPS receiver channel is the sum of the signals received from the GPS satellite being tracked, from other GPS satellites (L) in view of the receiver, cochannel interference from other proposed services (M) at LI frequency and interference from near-band services (N) such as pseudolites and the proposed MSS band and can be written as Copyright © 1997 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. 564 American Institute of Aeronautics and Astronautics Copyright© 1998, American Institute of Aeronautics and Astronautics, Inc.