RNAV Training Manual - Keilir

How does GPS work?
3. The structure of the Navigation Message
The Navigation Message is transmitted as a stream
of digital data organised into a sequence of Frames
Each satellite begins sending a Frame exactly on the
minute and half-minute, according to its internal clock
Each Frame is made up of 5 Subframes
Subframes 1,2 and 3 are repeated in consecutive
Frames and updated every 1-2 hours, on the hour
The almanac data in Subframes 4 and 5 is sub
commutated ; it takes a cycle of 25 Frames (with
different Subframe 4&5 data) ) for the receiver to
assemble the full almanac
The almanac is thus repeated every 25 Frames
and is updated approximately every 24hrs
EEach h SSubframe bf iis made d up of f 10 WWords d
Words 3-10 carry the data content of the frame as
described above
Word 1 is called the Telemetry word and
contains a sync pattern used by the receiver to
synchronize itself with the Navigation Message and
thus decode the data content
Word 2 is the Handover word, analogous to a
counter that increments by 1 in each Subframe
Each Word is made up of 30 Bits of data
Frame:1500 bits long; 30s @ 50bps
Frame
5 Subframes each 300 bits long: 6s @ 50bps
Subframe 1 Subframe 2 Subframe 3 Subframe 4
Subframe 5
Clock data
for this satellite
Ephemeris data
for this satellite
Data that allows the GPS An ephemeris is a table of values
receiver to calculate the small that gives the position of
offset between the satellite's astronomical objects in the sky. In a
internal clock time and GPS GPS context, it means the detailed
time defined by the ground
station atomic clocks
model of a satellite s orbit
Almanac data
common to all GPS satellites
The almanac consists of basic orbit and
status information for all satellites satellites, an
ionospheric model, and information to
relate GPS time to UTC
A Subframe is made up of 10 Words, each 30 bits long: 0.6s @ 50bps
Word 1 Word 2 Word 3 Word 4 Word 5 Word 6 Word 7 Word 8 Word 9 Word 10
30bits in a word
1 2 .. ..
.. .. 29 30
First 6 bits are
parity data
Final 24 bits carry the data content
The Navigation Message is the real-time reference manual for the GPS receiver,
which helps it calculate an accurate position based on the C/A Code ranging signals
Source: Article by Ed Weston, http://gpsinformation.net/gpssignal.htm.
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