Principals of Operation
6 -25
6.1.8.B
CNAV Navigation Message
The CNAV data is an upgraded version of the original NAV navigation message. It
contains higher precision representation and nominally more accurate data than the NAV
data. The same type of information (Time, Status, Ephemeris, and Almanac) is still
transmitted using the new CNAV format; however, instead of using a frame / subframe
architecture, it features a new pseudo-packetized format made up of 12-second 300-bit
message packets.
In CNAV, two out of every four packets are ephemeris data and at least one of every four
packets will include clock data, but the design allows for a wide variety of packets to be
transmitted.
With a 32-satellite constellation, and the current requirements of what needs to be sent,
less than 75% of the bandwidth is used. Only a small fraction of the available packet
types have been defined; this enables the system to grow and incorporate advances.
Important changes in the new CNAV message
CNAV message uses Forward Error Correction (FEC) in a rate 1/2 convolution code, so
while the navigation message is 25 bit/s, a 50 bit/s signal is transmitted.
The GPS week number is now represented as 13 bits, or 8192 weeks, and only repeats
every 157.0 years, meaning the next return to zero won't occur until the year 2137. This
is longer compared to the L1 NAV message's use of a 10-bit week number, which returns
to zero every 19.6 years.
There is a packet that contains a GPS-to-GNSS time offset. This allows for
interoperability with other global time-transfer systems, such as Galileo and GLONASS,
both of which are supported.
The extra bandwidth enables the inclusion of a packet for differential correction, to be
used in a similar manner to SBAS and which can be used to correct the L1 NAV clock
data.
Every packet contains an alert flag, to be set if the satellite data can not be trusted. This
means users will know within 6 seconds if a satellite is no longer usable. Such rapid
notification is important for safety-of-life applications, such as aviation.
Finally, the system is designed to support 63 satellites, compared with 32 in the L1 NAV
message.
L2CS Frequency information
An immediate effect of having two civilian frequencies being transmitted is the civilian
receivers can now directly measure the ionospheric error in the same way as dual
frequency P(Y)-code receivers. However, if a user is utilizing the L2C signal alone, they
can expect 65% more position uncertainty than with the L1 signal.
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