RTCM SC-104 Recommended Pseudolite Signal Specification

Abstract

Anyone involved in the design of GPS user equipment should be concerned with the potential impact of pseudolite signals. It is quite possible that pseudolite transmitters will be located near the majority of all airports and heliports, both rural and urban, in the United States and around the world. Therefore, even if a receiver is not intended for aircraft navigation or will not use the pseudolite signals, it may be affected by these signals. Differential GPS can improve the reliability, integrity, and accuracy of GPS navigation, but it cannot overcome the inherent geometric limitations of an 18 satellite constellation, especially when one of the satellites is out of service. Pseudolites transmit the differential correction message at the GPS L1 frequency, thus also providing an additional pseudorange measurement which overcomes poor satellite geometry. The key problem was to develop a GPS-like pseudolite signal structure which does not interfere with reception of GPS satellite signals even when a receiver is very close to a pseudolite transmitter. A simple and effective signal has been defined, but receiver manufacturers must assure that their designs are immune to pseudolite interference. This paper defines the proposed signal and describes the appropriate receiver design considerations.