Next to the unencrypted L1 band Coarse/Acquisition Pseudo-Random Noise code (C/A PRN code), the Precise (P), but encrypted Pseudo Random Noise code is used to modulate both the L1, and the L2 carriers.
While GSG cannot replicate the encryption, it can emulate, and thus represent the P(Y) code, so as to allow for commercial GPS surveying receivers to be tested for their ability to derive the carrier in a codeless fashion.
Note that this technology does NOT use controlled encryption. Instead, it mimics the encryption so as to provide an RF signal in the L1/L2 P(Y) location.
Note: GPS receivers that use genuine encryption methods will NOT be able to use the L1/L2 P with Pseudo P(Y) code enabled because the encryption used is not as expected and they cannot decode it.
To turn P(Y) ON/OFF:
- Navigate to: Select [Select a Scenario] > Configure Scenario: View 3/3.
- Next to the GPS, enter a number of satellites greater than "0", or Auto, then press Enter to open this Configuration view:
- Navigate to the P(Y) entry at the bottom of the view, and select On, or Off.
Turning pseudo encryption ON/OFF
For most L1/L2 GPS receivers, there are two valid configuration modes:
- Enable L1 C/A, L1P, and L2P only:
- The L1P and L2P will be transmitted without encryption.
- Enable L1 C/A, L1P, and L2P, and Pseudo P(Y):
- The P code will be scrambled to mimic a realistic P(Y) signal for use in receivers that can make use of L1/L2 P(Y) signals for codeless applications, or to provide a signal in the band to better emulate the real world.
- In the GSG-6 series, the NAV message transmitted by the GPS satellites is updated to reflect if (pseudo-) encryption is active or not. This is specified by bit 19 in the second word of subframe one. This bit represents the anti-spoof (A-S) flag, where “1” indicates that the A-S mode is on in that satellite. It is recommended to enable Pseudo P(Y) when the GSG-unit supports it. This will set the A-S flag to ON which is required in some receivers. GPS receivers may reject L1CA code if the A-S flag is off.
- In GSG-5x units, where it is not possible to transmit Pseudo P(Y), the A-S bit is always set to ON to indicate that encryption is on (although the actual RF signal is not transmitted on such units).
- The NAV message also holds information on the type of L2 signal being transmitted (bits 11 and 12 of word three in subframe one). These bits are always set to indicate that the P code is active on L2.