Specifications

VP ONCORE COMMAND REFERENCE
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SATELLITE BROADCAST DATA MESSAGE
(@@Bl)
The Global Positioning System satellites each broadcast a 50 bits per second
(BPS) data message called the Navigation data. The structure of each satellite
data message consists of 1500 bits (transmission time is 30 seconds) distributed
across 5 sub-frames (6 seconds each). Each subframe is further subdivided into
ten 30 bit words, where each word consists of 24 bits of data and 6 bits of parity.
Subframes 4 and 5 are commutated over 25 pages spread over 12.5 minutes.
The data transmitted by the satellites consists of satellite Keplerian orbital
elements (ephemerides), almanac data, telemetry word, hand-over word,
ionospheric correction data, satellite health data, UTC time correction data,
special message fields, etc. More information on the content of the message is
provided in the Navstar GPS Space Segment/Navigation User Interface Control
Document ICD-GPS-200.
Only the 24 data bits of each word are output by the receiver. The 6 parity bits
from each word are not output but are used to verify the contents of the data field
internal to the receiver. The receiver collects each subframe (6 seconds of data)
and then outputs the data on the serial port after successful receipt of all 10
words of a particular subframe for each satellite tracked.
No format conversion is applied to the data; it is output in its raw binary form for
subsequent processing outside the receiver.
If the user selects the satellite broadcast data in the continuous output mode, the
receiver outputs a single satellite broadcast data message record for all channels
for the current subframe after word ten of that subframe has been collected.
Consequently, in continuous mode, the receiver outputs one message per
satellite approximately every six seconds (the subframe transmission rate). Data
being output for any channel will output data for all channels (active or inactive).
If the user selects satellite broadcast data in the polled output mode, the receiver
outputs a single satellite broadcast data record for all satellites tracked for the
current subframe after word ten of that subframe has been collected. No other
data is output after that time unless the user selects the data to be output again
in the polled or continuous modes.
For either case, the GPS receiver outputs the broadcast data through a series of
output messages, each of which is identified by the particular channel, satellite ID
(PRN number), subframe, and when appropriate, the page number (subframes 4
and 5 only.
The entire broadcast data output message consists of N output response
messages corresponding to the N receiver channels.
If the user requests the satellite broadcast data message and the receiver does
not properly receive the next subframe of data, only the valid words will be output
with the remaining fields set to zero.
99 Synergy Systems, LLC 05OCT01