Printer FriendlyEmail Article Link

Positioning Application: What method do Spirent use to generate the Pseudo Y code in their GPS-capable simulators?

Environment/Versions
  • PosApp (SimGEN)
  • STR4730 / 4760 / 4790
  • GSS7700 / 7735 / 7790 / 7900
  • GSS8000 with GPS L1 and/or L2 capability
  • GSS9000
  • GSS7000
Answer
  • Pseudo-Y allows so-called “codeless” and “semi-codeless” receivers to be stimulated with a realistic signal by the simulator.
  • The simulator implementation is based entirely on Public Domain information.
  • Two sources of public information have been referred to:-
        i) "ASHAJEE & LORENZ Precision GPS Surveying After Y-Code. Proc. ION-GPS-92"
       ii) "LENNEN, G.R. US Patent 5,621,416 Optimised processing of signals for enhanced crosscorrelation in a satellite positioning system receiver"
     
  • The first source describes the operation of the Z12 receiver in it’s “Z Tracking” mode and the assumptions made therein about the Y-code structure.
  • The second source describes an unclassified technique that consists of replacing the P-code with a Pseudo Y-code which is generated without recourse to any classified process or information.
     
  • Spirent's Pseudo-Y implementation consists of
    • “the modulo two sum of the P-code and a substantially lower rate encryption code”
    • where the “encryption code" is simply a binary bitstream which has an appropriate bit rate.
  • This Pseudo-encryption code is simply a Pseudo-Random Binary Sequence, chosen at random, with no intentional correlation with real encryption code.
  • It has a chip rate of 465kHz (1/22nd of p-code rate) and its transitions are aligned with the X1A epoch.
  • As the real encryption code is classified and therefore unknown to the user, it will appear to the user to be a random binary sequence.
  • It is not known if the real encryption code repeats, but in order to minimise false cross-correlation a long sequence is chosen for the Pseudo-encryption code.
  • In practice, for ease of implementation, the Pseudo-encryption code is obtained by sampling the SV’s X1 code every 22nd bit.

Product : GSS7000,GSS9000 series,Positioning,SimGEN,PosApp,Pseudo-Y,GPS,STR4790,STR4760,STR4730