Section 4; Software Interface; Nmea Received Sentences; Almanac Information (Alm) - Garmin GPS 25 LP Technical Specification

Section 4

Software Interface

The GPS 25LP interface protocol design on TXD1/RXD1 is based on the National
Marine Electronics Association's NMEA 0183 ASCII interface specification, which is
fully defined in "NMEA 0183, Version 2.0" (copies may be obtained from NMEA, P.O.
Box 50040, Mobile, AL, 36605, U.S.A.) and the Radio Technical Commission for
Maritime Services' "RTCM Recommended Standards For Differential Navstar GPS
Service, Version 2.1, RTCM Special Committee No. 104" (copies may be obtained from
RTCM, P.O. Box 19087, Washington, D.C., 20036, U.S.A.). The GPS 25LP interface
protocol, in addition to transmitting navigation information as defined by NMEA 0183,
transmits additional information using the convention of GARMIN proprietary
sentences.
Binary phase data information is output on TXD2, see Appendix D for details.
The following sections describe the NMEA data format of each sentence transmitted
and received by the GPS 25LP sensor board. The baud rate selection, one-pulse-per-
second output interfaces and RTCM differential GPS input are also described.

4.1 NMEA Received sentences

The subsequent paragraphs define the sentences which can be received on RXD1 by
the GPS 25LP sensor boards. Null fields in the configuration sentence indicate no
change in the particular configuration parameter.
All sentences received by the GPS 25LP must be terminated with <CR><LF>, but do
not require the checksum *hh. The checksum is used for parity checking data and it is
recommended that the checksum be used in environments containing high
electromagnetic noise. It is generally not required in normal PC environments.
Sentences may be truncated by <CR><LF> after any data field and valid fields up to
that point will be acted on by the GPS 25LP.
4.1.1

Almanac Information (ALM)

$GPALM,<1>,<2>,<3>,<4>,<5>,<6>,<7>,<8>,<9>,<10>,<11>,<12>,<13>,<14>,<15>
*hh<CR><LF>
The $GPALM sentence can be used to initialize the sensor board's stored almanac
information if battery back-up has failed.
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