High-Accuracy Ranging using Spread-Spectrum Technology

Small satellite formation flying is an important new technology. Of prime importance in formation flying is the need to determine high-accuracy real-time satellite-to-satellite range and velocity information. Spread spectrum technology is usually used for this purpose, but there are limitations to this approach. Traditional spread-spectrum ranging involves either one-way ranging (e.g. the GPS constellation) or round-trip ranging. Both of these are useful, but each has limitations. One-way ranging requires highly accurate synchronized clocks on each satellite. Time synchronization, as well as frequency stability, is required and time synchronization is difficult to achieve between satellites. If the ranging accuracy requirements are stringent these requirements are especially difficult to meet. Two-way ranging eliminates the need for high-accuracy synchronicity between satellites, but when using traditional spread-spectrum ranging techniques the achievable chipping rate limits the resolution. This paper shows how other techniques can be used to generate much more accurate information than traditional spread-spectrum allows. The paper describes a step-by-step approach to extracting integer code range, sub-chip code phase and carrier phase information from a coded waveform. The method demonstrated in the paper explains qualitatively at each step how this is done, and then explains quantitatively at each step how to determine the expected system performance. The paper addresses in detail the quantitative limits on achievable performance. From these limits the requirements for system frequency accuracy and the tradeoffs between signal-to-noise ratio (SNR) and range updating are developed.