The IIR-M satellites provide capability to transmit a new GPS L2C signal, an additional civil signal designed to meet commercial needs. Earlier this year, the Air Force made a commitment to the U.S. civil community to begin broadcasting a subset of the modernized data messages on the L2C signal to enable commercial GPS manufacturers the opportunity to start early receiver development prior to when the signal will reach full operational capability in the 2016 timeframe. The data portion of the L2C signal is different than the current “legacy” navigation structure used on the first civil signal’s coarse acquisition code (L1 C/A), which has subframes of data repeating in a fixed pattern. The new civil navigation signal structure, which will be used on L2C and the third civil signal (L5), has individual messages which can broadcast in a flexible order with variable repeat cycles. The new format will also provide for more accuracy in the data, since the parameters used to convey the orbit prediction will be better once CNAV reaches it full capability. All GPS IIR-M satellites carry the L2C signal, as will all GPS IIF and GPS III satellites.
The CNAV Message Type 0, as defined in IS-GPS-200D (available online at http://www.navcen.uscg.gov/gps/geninfo), is currently broadcasting from one GPS IIR-M satellite (SVN-49). Over the next several months, the CNAV Message Type 0 will broadcast from all 8 GPS IIR-M satellites. Further data modulation on L2C will be added in phases. The CNAV Message Types 10, 11, 30 and 33 will be added to L2C in the 2014 timeframe. The remaining CNAV messages will be added after further ground control system upgrades. The L2C signal is currently providing higher accuracy to civil users by allowing dual-frequency corrections to the ionospheric ranging errors.