Global Positioning System Reference
In-Depth Information
2.6.2 Data in Navigation Message
In addition to the TLM and HOW words, each subframe contains eight words of
data. This will only be a cursory description of the data in the different words and
not a complete description of all bits.
Subframe 1 - Satellite Clock and Health Data
The first subframe contains first
of all clock information. That is information needed to compute at what
time the navigation message is transmitted from the satellite. Additionally,
subframe 1 contains health data indicating whether or not the data should
be trusted.
Subframes 2 and 3 - Satellite Ephemeris Data
Subframes 2 and 3 contain the
satellite ephemeris data. The ephemeris data relate to the satellite orbit and
are needed to compute a satellite position.
Subframes 4 and 5 - Support Data
As mentioned, the last two subframes repeat
every 12.5 minutes, giving a total of 50 subframes. Subframes 4 and 5 con-
tain almanac data. The almanac data are the ephemerides and clock data
with reduced precision. Additionally, each satellite transmits almanac data
for all GPS satellites while it only transmits ephemeris data for itself. The
remainder of subframes 4 and 5 contain various data, e.g., UTC parameters,
health indicators, and ionospheric parameters.
For a more in-depth description of the contents of the navigation data, see SPS
(1995).
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