Ad Astra Observatory

http://www.psychologie.uni-regensburg.de/Zimmer/trafficresearch/NMEA0183/index.html

http://home.mira.net/~gnb/gps/nmea.html#gprmc

According to the Gemini manual, pp.85, “Gemini requires only the
“Recommended Minimum” (RMC) NMEA 0183 sentence to be sent,”

This is:

$GPRMC,225446,A,4916.45,N,12311.12,W,000.5,054.7,191194,020.3,E*68

225446 Time of fix 22:54:46 UTC
A Navigation receiver warning A = Valid position, V
= Warning
4916.45,N Latitude 49 deg. 16.45 min. North
12311.12,W Longitude 123 deg. 11.12 min. West

000.5 Speed over ground, Knots
054.7 Course Made Good, degrees true
191194 UTC Date of fix, 19 November 1994
020.3,E Magnetic variation, 20.3 deg. East
*68 mandatory checksum

$GPRMC,154232,A,2758.612,N,08210.515,W,085.4,084.4,230394,003.1,W*43[CR][LF]

The first 5 characters following the ‘$’ are called the address field. The
rest of the line consists of the comma-delimited data fields. The first 2
characters of the address are the so called Talker-ID, in our example the
sender identifies as a GPS device (GP = GPS device). <talkers.htm>This is a
list of Talker-IDs. The Talker-ID is followed by 3 characters describing
the type of the sentence (type RMC = Minimum Navigation Information in our

example). <types.htm>This is a list of sentence types and description of
sentence syntax. Data fields which are undefined at send time are left
empty (two commas with nothing in between).

An optional checksum can be added to the sentence. This checksum must be
preceeded by a ‘*’. It is the XOR-combination (in hexadecimal notation) of
all characters between ‘$’ and ‘*’.

The Gemini will query at startup with a query string:

Anatomy of a query is:

$ttllQ,sss[CR][LF]

This maybe followed by optional checksum, as explained above. Let tt be the
talker ID of the querying device (e.g. CC=Computer). Let ll be the talker
ID of the queried device (e.g. GP=GPS device). Now think of sss being the
signature of the requested sentence. Congratulations, you got it: this is

the syntax of a query sentence.

So a very dumb program could basically:
– open the COM port at 4800,8,N,1, no handshake
– wait for $??GPQ,RMC (I assume it asks for RMC)
– send back $GPRMC,time,A,lat,N,lon,Q,,,date,,,*checksum

and that’s it. This would slam the Gemini’s clock to the computer’s clock

every bootup.