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Spirent KB Article
Doc ID: FAQ13998
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Positioning Application: What method do Spirent use to generate the Pseudo Y code in their GPS-capable simulators?
STR4730 / 4760 / 4790
GSS7700 / 7735 / 7790 / 7900
GSS8000 with GPS L1 and/or L2 capability
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 : Positioning,SimGEN,PosApp,Pseudo-Y,GPS,STR4790,STR4760,STR4730,GSS7790,GSS7735