Present and Future Global Navigation Satellite Systems (GNSS)

Anti-Spoofing (A-S)
• On block II satellites P-code is replaced by a secret Y-code (actually the
P-code is encrypted)
• To prevent military receivers from tracking "false" GPS satellites and to
prevent jamming
• Only (military) receivers with the appropriate "key" can use the Y-code
• Two frequency GPS receivers cope with A-S in the following way
Signal squaring on L2: L1 code & phase, L2 half wavelength phase
Cross-correlation: L1 code & phase, Y2-Y1 code & L2-L1 phase
P/W tracking: L1 code & phase, L2 code & phase
Anti-Spoofing will always result in some loss of information, even if
they can cope with A-S, the signal-to-noise ratio is worse than w/o A-S!
• A-S was enabled on Monday 31 January 1994 0h00m,
occasionally off for testing and other purposes
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GEO2TECDI – Bangkok, May, 2009 – GNSS Lecture 2