Tracking small satellites using translated GPS

This paper discusses using translated GPS for tracking small satellites, the technical trade-offs involved, and the position and timing accuracies which are achievable using translated GPS. The Global Positioning System (GPS) uses the relative times-of-arrival of multiple spread-spectrum signals at an antenna to determine the position of the antenna. The system can also determine the time the antenna was at that position. The direct sequence spread spectrum signals are transmitted from GPS satellites whose orbital position and timing (the ephemeris data) are accurately known. Once the GPS signals are received at the antenna, their relative timings are fixed, and are not changed in subsequent amplifications and signal processing. This fact can be used to track small satellites to a high degree of accuracy without placing a GPS receiver on the satellite. A GPS antenna is placed on the satellite, and the received signals are amplified, converted to a new frequency, and retransmitted to a ground-based GPS receiver. In the most cost-effective arrangement, the ground GPS receiver consists of a low-noise amplifier, a frequency converter to the original GPS frequencies, and an off-the shelf GPS receiver. The off-the-shelf receiver may be coupled to a computer for extensive post-processing. Such post-processing is currently available for IBM PC's. The advantages of this translated-GPS tracking are that high order GPS receivers with large amounts of post-processing can be used for tracking spacecraft, while leaving the spacecraft electronics very simple. This allows highly precise, inexpensive, and power-efficient GPS systems to be used on small satellites without: the risk of non space-qualified GPS receivers: the expense of space-qualified receivers: or the power-consumption of high-order receivers.