CNAV is the name for the civilian navigation message that will be carried by the modernized GPS system. And while the CNAV message will carry similar data to the existing NAV message, its structure will be completely different, with a packetised format that will increase message bandwidth to allow for greater information density and pave the way for future system expansion. To this end, the system is designed to support 63 satellites, compared with 32 for the L1 NAV message.
Each packet of the CNAV message is 300bit in length. Although only a small proportion of the available packet types have so far been defined, the basic structure has been set. Two out of every four packets will contain ephemeris data and at least one in four packets will include clock data. One of these clock packets will incorporate a GPS time offset to simplify timebase integration between GPS and other GNSSs, such as Galileo and GLONASS. A further packet is available for differential correction, which can be used to correct the L1 NAV clock data.
As part of the new message structure, the CNAV message uses forward error correction to effectively double its bandwidth. This means that while the 300bit packet would normally take 12 seconds to transmit, each packet is transferred in 6 seconds.
Each packet incorporates an error flag that can be tripped if the satellite data cannot be trusted. Therefore, users will be aware within 6 seconds if any satellite in the constellation is no longer usable, which is important for safety-of-life applications.
Developers of GNSS receivers designed to support modernised GPS and other systems such as GLONASS and Galileo can test their systems today using Spirent's range of multi-GNSS constellation simulators.