Compass-M1 Broadcast Codes and Their Application to Acquisition and Tracking

With the launch of the Compass-M1 satellite on 14 April 2007, China is set to become the latest entrant into global navigation satellite systems (GNSS). The satellite, sometimes referred to as Compass-2 or Beidou-2, is the first of the Compass navigation satellite system (CNSS) that will provide global satellite navigation coverage. While China has launched several other navigation satellites, these previous satellites, also termed Compass or Beidou (in Chinese), provided only regional coverage. The Compass-M1 differs significantly from these previous satellites in terms of signal structure, frequency, and coverage. Most significantly, unlike previous satellites, it has similar frequencies and signal structure to other GNSS, making the prospect of interoperation a tantalizing one. Understanding the interoperability and integration of CNSS with GPS, Galileo and GLONASS, requires knowing and understanding its signal structure, specifically its codes and code structure. The knowledge of the code is necessary for designing receivers capable of acquiring and tracking the satellite. These receivers are necessary for evaluating the performance and benefits of CNSS. Just as important is determining if the signal may degrade performance of GPS/Galileo in the form of interference. Interference with and degradation of GPS/Galileo performance are possibilities if interoperability was not a driving concern in the signal design. This is of concern to military users as well since Compass overlays GPS M-code and Galileo Public Regulated Service (PRS) on E1/E2. So our preliminary step in studying Compass is a determination and analysis of the Compass-M1 codes. Additionally, we will implement these codes within our software GNSS receiver to verify and validate our analysis. In this paper, we decode the PRN codes of the E2, E5b and E6 signals broadcast by the Compass-M1 satellite. The E2 and E5 codes are identical. They are 2046 bits long and are 11-stage Gold codes. The E6 PRN code is a 10230-bit concatenated Gold code. The head and tail parts are both 13-stage Gold codes. We then apply the codes for acquisition and tracking. By using our own software receiver, we are able to successfully acquire and track the Compass-M1 satellite. This is useful for evaluating the performance of the selected codes.