Introduction - Inside the Military GPS


Message posted by Ron Milione on February 19, 2010 at 9:46:40 PST:

The military’s GPS capability is known as the Precise Positioning Service (PPS). It relies on a much longer code called the P-code (for precise or precision) which is transmitted on both the L1 frequency and the L2 frequency at 1227.60 MHz. The P-code is encrypted (and it’s then called the Y-code) so that it cannot be accessed by unauthorized users. Encryption also prevents a military GPS receiver from being fooled or spoofed by a fake GPS signal transmitted by an enemy. The encryption process is known as Anti-Spoofing. Military GPS receivers have decryption capabilities which permit them to recover the Pcode. Each satellite’s unique P-code segment is one week long. In order to determine the distance to a satellite using the P-code, the receiver must align a replica of the code it generates with the arriving code. With such a long code, it was formerly difficult for the signal processors in P-code receivers to quickly find the correct alignment point in the code without help. It got this help from the C/A-code. So even though military GPS receivers determine their position (and velocity and time) from the P(Y)-code, they generally have acquired the C/A-code first and then using information from that signal have zeroed in on the P-code. Most of the military-grade GPS receivers now in existence work on this principle.


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