Hammerhead/Protocol

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m (→‎NMEA: add formatting)
m (add nmea log)
Line 137: Line 137:
 
256 write(NMEA, "\x24\x47\x50\x47\x53\x41\x2c\x41\x2c\x33\x2c\x30\x32\x2c\x30\x34\x2c\x30\x38\x2c\x31\x30\x2c\x31\x33\x2c\x32\x37\x2c\x2c\x2c\x2c\x2c\x2c\x2c\x36\x2e\x37\x2c\x33\x2e\x30\x2c\x36\x2e\x30\x2a\x33\x45\x0d\x0a", 51) = 51
 
256 write(NMEA, "\x24\x47\x50\x47\x53\x41\x2c\x41\x2c\x33\x2c\x30\x32\x2c\x30\x34\x2c\x30\x38\x2c\x31\x30\x2c\x31\x33\x2c\x32\x37\x2c\x2c\x2c\x2c\x2c\x2c\x2c\x36\x2e\x37\x2c\x33\x2e\x30\x2c\x36\x2e\x30\x2a\x33\x45\x0d\x0a", 51) = 51
 
256 write(NMEA, "$GPGSA,A,3,02,04,08,10,13,27,,,,,,,6.7,3.0,6.0*3E
 
256 write(NMEA, "$GPGSA,A,3,02,04,08,10,13,27,,,,,,,6.7,3.0,6.0*3E
 +
 +
 +
The whole log is
 +
<pre>
 +
$GPGGA,154035.21,,,,,00,00,5.0,,M,-0.586099,M,-0.1700199,*6A
 +
$GPRMC,154035.24,V,,,,,,,240407,,*1A
 +
$GPGSA,A,1,,,,,,,,,,,,,11.2,5.0,10.0*36
 +
$GPGGA,154035.61,,,,,00,00,5.0,,M,-0.586099,M,-0.1700199,*6E
 +
$GPRMC,154035.63,V,,,,,,,240407,,*19
 +
$GPGSA,A,1,,,,,,,,,,,,,11.2,5.0,10.0*36
 +
$GPGGA,154038.24,,,,,00,04,300.0,,M,-0.586099,M,0.0000199,*4B
 +
$GPRMC,154038.29,V,,,,,,,240407,,*1A
 +
$GPGSA,A,1,02,04,08,10,,,,,,,,,669.0,300.0,600.0*33
 +
$GPGGA,154038.54,,,,,00,04,300.0,,M,-0.586099,M,0.0000199,*4C
 +
$GPRMC,154038.56,V,,,,,,,240407,,*12
 +
$GPGSA,A,1,02,04,08,10,,,,,,,,,669.0,300.0,600.0*33
 +
$GPGGA,154042.72,,,,,00,06,300.0,,M,-0.586100,M,0.0000199,*46
 +
$GPRMC,154042.74,V,,,,,,,240407,,*1F
 +
$GPGSV,3,1,09,27,77,114,24,08,59,192,33,10,46,302,33,02,33,244,34*7E
 +
$GPGSV,3,2,09,04,18,206,33,13,42,076,25,29,11,271,29,24,243,352,*49
 +
$GPGSV,3,3,09,23,14,085,*49
 +
$GPGSA,A,1,02,04,08,10,27,,,,,,,,669.0,300.0,600.0*36
 +
$GPGGA,154043.67,,,,,00,06,300.0,,M,-0.586100,M,0.0000199,*43
 +
$GPRMC,154043.69,V,,,,,,,240407,,*12
 +
$GPGSA,A,1,02,04,08,10,27,,,,,,,,669.0,300.0,600.0*36
 +
$GPGGA,154046.89,,,,,00,06,300.0,,M,-0.587100,M,0.0000199,*47
 +
$GPRMC,154046.93,V,,,,,,,240407,,*12
 +
$GPGSA,A,1,02,04,08,10,,,,,,,,,669.0,300.0,600.0*33
 +
$GPGGA,154047.14,,,,,00,06,300.0,,M,-0.587100,M,0.0000199,*42
 +
$GPRMC,154047.19,V,,,,,,,240407,,*11
 +
$GPGSA,A,1,02,04,08,10,,,,,,,,,669.0,300.0,600.0*33
 +
$GPGGA,154050.07,,,,,00,06,300.0,,M,-0.586100,M,0.0000199,*47
 +
$GPRMC,154050.10,V,,,,,,,240407,,*1E
 +
$GPGSA,A,1,27,,,,,,,,,,,,669.0,300.0,600.0*39
 +
$GPGGA,154050.45,,,,,00,06,300.0,,M,-0.586100,M,0.0000199,*41
 +
$GPRMC,154050.47,V,,,,,,,240407,,*1C
 +
$GPGSV,3,1,09,27,77,114,24,08,59,192,26,10,46,302,25,13,42,076,16*78
 +
$GPGSV,3,2,09,02,33,244,26,04,18,206,25,29,11,271,25,24,243,352,*44
 +
$GPGSV,3,3,09,23,14,085,*49
 +
$GPGSA,A,1,27,,,,,,,,,,,,669.0,300.0,600.0*39
 +
$GPGGA,154053.35,,,,,00,06,300.0,,M,-0.586100,M,0.0000199,*45
 +
$GPRMC,154053.37,V,,,,,,,240407,,*18
 +
$GPGSA,A,1,27,,,,,,,,,,,,669.0,300.0,600.0*39
 +
$GPGGA,154058.70,,,,,00,03,300.0,,M,-0.581014,M,0.0000199,*49
 +
$GPRMC,154058.72,V,,,,,,,240407,,*12
 +
$GPGSA,A,1,02,,,,,,,,,,,,669.0,300.0,600.0*3E
 +
$GPGGA,154058.97,,,,,00,03,300.0,,M,-0.581014,M,0.0000199,*40
 +
$GPRMC,154058.98,V,,,,,,,240407,,*16
 +
$GPGSA,A,1,02,,,,,,,,,,,,669.0,300.0,600.0*3E
 +
$GPGGA,154059.38,,,,,00,03,300.0,,M,-0.581014,M,0.0000199,*44
 +
$GPRMC,154059.40,V,,,,,,,240407,,*12
 +
$GPGSA,A,1,02,13,,,,,,,,,,,669.0,300.0,600.0*3C
 +
$GPGGA,154104.95,4948.950339,N,00957.955979,E,1,06,5.0,220.0,M,-0.581014,M,-0.1900190,*40
 +
$GPRMC,154104.95,A,4948.950339,N,00957.955979,E,000.0,000.0,240407,,*3E
 +
$GPGSV,3,1,09,08,59,192,42,10,46,302,42,04,18,206,41,27,77,114,16*74
 +
$GPGSV,3,2,09,13,42,076,18,02,33,244,18,29,11,271,40,24,243,352,*48
 +
$GPGSV,3,3,09,23,14,085,*49
 +
$GPGSA,A,3,04,08,10,,,,,,,,,,11.2,5.0,10.0*39
 +
$GPGGA,154106.52,4948.936977,N,00957.930742,E,1,06,3.0,245.0,M,-0.581025,M,-0.1900190,*4A
 +
$GPRMC,154111.60,A,4948.936977,N,00957.930742,E,000.0,000.0,240407,,*35
 +
$GPGSA,A,3,02,04,08,10,27,,,,,,,,6.7,3.0,6.0*3C
 +
$GPGGA,154122.94,,,,,00,06,3.0,,M,-0.581010,M,-0.1900190,*62
 +
$GPRMC,154122.96,V,,,,,,,240407,,*14
 +
$GPGSA,A,1,13,,,,,,,,,,,,6.7,3.0,6.0*36
 +
$GPGGA,154119.10,4948.936977,N,00957.930742,E,1,05,3.0,245.0,M,-0.576004,M,-0.1870190,*4D
 +
$GPRMC,154119.10,A,4948.936977,N,00957.930742,E,000.0,000.0,240407,,*3A
 +
$GPGSA,A,3,04,08,10,27,,,,,,,,,6.7,3.0,6.0*3E
 +
$GPGGA,154128.15,4948.936977,N,00957.930742,E,1,06,5.0,245.0,M,-0.587003,M,-0.1860190,*47
 +
$GPRMC,154128.15,A,4948.936977,N,00957.930742,E,000.0,000.0,240407,,*3D
 +
$GPGSV,3,1,09,27,77,114,22,08,59,192,39,10,46,302,39,02,33,244,40*7B
 +
$GPGSV,3,2,09,04,18,206,38,13,42,076,11,24,243,352,,23,14,085,*48
 +
$GPGSV,3,3,09,29,11,271,*4F
 +
$GPGSA,A,3,02,04,08,10,27,,,,,,,,11.2,5.0,10.0*3E
 +
$GPGGA,154131.86,4948.936977,N,00957.930742,E,1,06,3.0,245.0,M,-0.578004,M,-0.1860190,*44
 +
$GPRMC,154131.86,A,4948.936977,N,00957.930742,E,000.0,000.0,240407,,*3F
 +
$GPGSA,A,3,02,04,08,10,13,27,,,,,,,6.7,3.0,6.0*3E
 +
$GPGGA,154138.55,4948.936977,N,00957.930742,E,1,06,3.0,245.0,M,-0.578000,M,-0.1860190,*47
 +
$GPRMC,154138.55,A,4948.936977,N,00957.930742,E,000.0,000.0,240407,,*38
 +
$GPGSA,A,3,02,04,08,10,13,27,,,,,,,6.7,3.0,6.0*3E
 +
$GPGGA,154141.69,4948.936977,N,00957.930742,E,1,06,3.0,245.0,M,-0.583002,M,-0.1840190,*42
 +
$GPRMC,154141.69,A,4948.936977,N,00957.930742,E,000.0,000.0,240407,,*39
 +
$GPGSA,A,3,02,04,08,10,27,,,,,,,,6.7,3.0,6.0*3C
 +
$GPGGA,154146.16,4948.936977,N,00957.930742,E,1,06,3.0,245.0,M,-0.595000,M,-0.1830190,*4F
 +
$GPRMC,154146.16,A,4948.936977,N,00957.930742,E,000.0,000.0,240407,,*36
 +
$GPGSA,A,3,02,04,08,10,27,,,,,,,,6.7,3.0,6.0*3C
 +
$GPGGA,154151.34,4948.936977,N,00957.930742,E,1,06,3.0,245.0,M,-0.587002,M,-0.1860190,*4D
 +
$GPRMC,154151.34,A,4948.936977,N,00957.930742,E,000.0,000.0,240407,,*30
 +
$GPGSV,3,1,09,27,77,114,26,08,59,192,42,10,46,302,41,13,42,076,25*7A
 +
$GPGSV,3,2,09,02,33,244,42,04,18,206,37,29,11,271,37,24,243,352,*46
 +
$GPGSV,3,3,09,23,14,085,*49
 +
$GPGSA,A,3,02,04,08,10,13,27,,,,,,,6.7,3.0,6.0*3E
 +
$GPGGA,154200.05,4948.936977,N,00957.930742,E,1,06,3.0,245.0,M,-0.587002,M,-0.1860190,*48
 +
$GPRMC,154200.05,A,4948.936977,N,00957.930742,E,000.0,000.0,240407,,*35
 +
$GPGSA,A,3,,,,,,,,,,,,,6.7,3.0,6.0*36
 +
 +
</pre>

Revision as of 10:40, 29 April 2007

Christian did some stracing on TomTom device, and result is great logs at http://www.maintech.de/download/hammerhead-strace.log .

GPS seems to communicate in packets.

Direction GPS -> machine

read(3, "\xfe\x00\xfd\x80\x16\x19\x0b\x00\x00\xfc", 2048) = 10

\xfe: begining of packet \x00: type of packet? seems to determine length. \xfd: follows .... \xfc: packet end

GPS signals at wikipedia seems to be required reading: http://en.wikipedia.org/wiki/GPS_signals

Analysis

Were there by any chance 8 satellites overhead at the time that log was made?

The protocol appears to be oriented around 32-bit words (the single-byte markers notwithstanding.) I'm not absolutely certain, but I strongly suspect the byte order is LSB-first.

The 80 80 80 stuff is probably just to force resynchronizing after an error, or wake the device up, or something like that.

It is - we have this information from another source. It's to train the UART in the hammerhead to the correct speed. --Speedevil 11:39, 28 April 2007 (CEST)

Packet format (same format in both directions):

  • The start of a packet is marked by FF or FE.
    • FF in packets that do not carry data, but rather explicitly request a response. (This isn't used very often. More often we receive data without explicitly requesting it.) The response will be an FE-packet with the same length and type as the FF-packet. The GPS does not send any FF packets, only gltt does.
    • FE in packets that carry data sections.
  • The first word (32 bits) following the start-of-packet marker is the header.
    • The first byte (if it's indeed little-endian, the least significant byte) gives the data length, measured as the number of data words minus 1. For FE-packets, this is the length of the data section of this packet. For FF-packets, it's the length of the data section in the expected response.
    • The second byte of the header is always FD.
    • MS nibble of the third byte might be flags for the packet.
    • LS nibble of the third byte might be an identifying number which is echoed in responses to this packet.
    • The fourth byte is the packet type.
  • In FE-packets only, the data section (n 32-bit words) follows.
  • Finally, FC is sent to mark the end of the packet.
  • After the FC in an FF-packet, gltt sends a bunch of zeroes. In some cases it sends a number of zeroes equal to the number of bytes in the response packet; in some cases it sends more. I would guess that these have no effect.

For example: 

ff 04fdc00c fc

(possibly followed by zeroes), is a request for a packet of type 0C, with length 20d ((4 + 1) * 4), and flags C0. 

fe 04fdc00c 0025102a 45dbdd4e 36030000 4b260000 16010000 fc

would be an appropriate response.

Packet types:

Frequency Type Max length Min length
9 16 73 11
86 08 19 11
8 e2 15 15
7 23 133 101
63 0a 15 15
6 24 71 39
52 0b 11 11
4 01 11 11
263 05 19 15
24 9d 315 19
2 06 25 19
16 e0 11 11
13 20 15 15
13 10 577 59
128 18 11 11
12 9e 303 15
117 00 65 39
11 0c 33 11
10 9f 477 61
10 0e 541 31
1 e5 11 11
1 80 17 17
1 13 11 11
1 04 23 23
1 02 11 11

NMEA

It is possible to translate NMEA messages into something readable using script like this: 

#!/bin/bash
grep -v gettimeofday | \
	sed 's/read.3/read(GPS/' | \
	sed 's/write.3/write(GPS/' | \
	sed 's/write.5/write(NMEA/' | \
	sed 's/\\/\\\\/g' | \
	while read LINE; do
		if echo $LINE | grep write.NMEA; then
			echo -e $LINE
		else
			echo "$LINE"
		fi
	done

NMEA description is available at http://www.gpsinformation.org/dale/nmea.htm .

At this point:

256 write(GPS, "\xfe\x00\xfd\x40\x08\x40\x60\x00\x00\xfc", 10) = 10 256 write(GPS, "\xfe\x00\xfd\x40\x08\x40\x60\x00\x00\xfc", 10) = 10 256 write(NMEA, "\x24\x47\x50\x47\x47\x41\x2c\x31\x35\x34\x31\x30\x34\x2e\x39\x35\x2c\x34\x39\x34\x38\x2e\x39\x35\x30\x33\x33\x39\x2c\x4e\x2c\x30\x30\x39\x35\x37\x2e\x39\x35\x35\x39\x37\x39\x2c\x45\x2c\x31\x2c\x30\x36\x2c\x35\x2e\x30\x2c\x32\x32\x30\x2e\x30\x2c\x4d\x2c\x2d\x30\x2e\x35\x38\x31\x30\x31\x34\x2c\x4d\x2c\x2d\x30\x2e\x31\x39\x30\x30\x31\x39\x30\x2c\x2a\x34\x30\x0d\x0a", 91) = 91 256 write(NMEA, "$GPGGA,154104.95,4948.950339,N,00957.955979,E,1,06,5.0,220.0,M,-0.581014,M,-0.1900190,*40 ", 91) = 91

...6 satelitte GPS fix was obtained. (And yes, there's big read few lines before that in the log). As far as I can tell, only 5-6 sattelite were _used_ till the end of log.

GSA sentence looks interesting, too. It tells us satellites #02, #04, #08, #10, #13 and #27 were used at this point.

256 write(NMEA, "\x24\x47\x50\x47\x53\x41\x2c\x41\x2c\x33\x2c\x30\x32\x2c\x30\x34\x2c\x30\x38\x2c\x31\x30\x2c\x31\x33\x2c\x32\x37\x2c\x2c\x2c\x2c\x2c\x2c\x2c\x36\x2e\x37\x2c\x33\x2e\x30\x2c\x36\x2e\x30\x2a\x33\x45\x0d\x0a", 51) = 51 256 write(NMEA, "$GPGSA,A,3,02,04,08,10,13,27,,,,,,,6.7,3.0,6.0*3E


The whole log is 

$GPGGA,154035.21,,,,,00,00,5.0,,M,-0.586099,M,-0.1700199,*6A
$GPRMC,154035.24,V,,,,,,,240407,,*1A
$GPGSA,A,1,,,,,,,,,,,,,11.2,5.0,10.0*36
$GPGGA,154035.61,,,,,00,00,5.0,,M,-0.586099,M,-0.1700199,*6E
$GPRMC,154035.63,V,,,,,,,240407,,*19
$GPGSA,A,1,,,,,,,,,,,,,11.2,5.0,10.0*36
$GPGGA,154038.24,,,,,00,04,300.0,,M,-0.586099,M,0.0000199,*4B
$GPRMC,154038.29,V,,,,,,,240407,,*1A
$GPGSA,A,1,02,04,08,10,,,,,,,,,669.0,300.0,600.0*33
$GPGGA,154038.54,,,,,00,04,300.0,,M,-0.586099,M,0.0000199,*4C
$GPRMC,154038.56,V,,,,,,,240407,,*12
$GPGSA,A,1,02,04,08,10,,,,,,,,,669.0,300.0,600.0*33
$GPGGA,154042.72,,,,,00,06,300.0,,M,-0.586100,M,0.0000199,*46
$GPRMC,154042.74,V,,,,,,,240407,,*1F
$GPGSV,3,1,09,27,77,114,24,08,59,192,33,10,46,302,33,02,33,244,34*7E
$GPGSV,3,2,09,04,18,206,33,13,42,076,25,29,11,271,29,24,243,352,*49
$GPGSV,3,3,09,23,14,085,*49
$GPGSA,A,1,02,04,08,10,27,,,,,,,,669.0,300.0,600.0*36
$GPGGA,154043.67,,,,,00,06,300.0,,M,-0.586100,M,0.0000199,*43
$GPRMC,154043.69,V,,,,,,,240407,,*12
$GPGSA,A,1,02,04,08,10,27,,,,,,,,669.0,300.0,600.0*36
$GPGGA,154046.89,,,,,00,06,300.0,,M,-0.587100,M,0.0000199,*47
$GPRMC,154046.93,V,,,,,,,240407,,*12
$GPGSA,A,1,02,04,08,10,,,,,,,,,669.0,300.0,600.0*33
$GPGGA,154047.14,,,,,00,06,300.0,,M,-0.587100,M,0.0000199,*42
$GPRMC,154047.19,V,,,,,,,240407,,*11
$GPGSA,A,1,02,04,08,10,,,,,,,,,669.0,300.0,600.0*33
$GPGGA,154050.07,,,,,00,06,300.0,,M,-0.586100,M,0.0000199,*47
$GPRMC,154050.10,V,,,,,,,240407,,*1E
$GPGSA,A,1,27,,,,,,,,,,,,669.0,300.0,600.0*39
$GPGGA,154050.45,,,,,00,06,300.0,,M,-0.586100,M,0.0000199,*41
$GPRMC,154050.47,V,,,,,,,240407,,*1C
$GPGSV,3,1,09,27,77,114,24,08,59,192,26,10,46,302,25,13,42,076,16*78
$GPGSV,3,2,09,02,33,244,26,04,18,206,25,29,11,271,25,24,243,352,*44
$GPGSV,3,3,09,23,14,085,*49
$GPGSA,A,1,27,,,,,,,,,,,,669.0,300.0,600.0*39
$GPGGA,154053.35,,,,,00,06,300.0,,M,-0.586100,M,0.0000199,*45
$GPRMC,154053.37,V,,,,,,,240407,,*18
$GPGSA,A,1,27,,,,,,,,,,,,669.0,300.0,600.0*39
$GPGGA,154058.70,,,,,00,03,300.0,,M,-0.581014,M,0.0000199,*49
$GPRMC,154058.72,V,,,,,,,240407,,*12
$GPGSA,A,1,02,,,,,,,,,,,,669.0,300.0,600.0*3E
$GPGGA,154058.97,,,,,00,03,300.0,,M,-0.581014,M,0.0000199,*40
$GPRMC,154058.98,V,,,,,,,240407,,*16
$GPGSA,A,1,02,,,,,,,,,,,,669.0,300.0,600.0*3E
$GPGGA,154059.38,,,,,00,03,300.0,,M,-0.581014,M,0.0000199,*44
$GPRMC,154059.40,V,,,,,,,240407,,*12
$GPGSA,A,1,02,13,,,,,,,,,,,669.0,300.0,600.0*3C
$GPGGA,154104.95,4948.950339,N,00957.955979,E,1,06,5.0,220.0,M,-0.581014,M,-0.1900190,*40
$GPRMC,154104.95,A,4948.950339,N,00957.955979,E,000.0,000.0,240407,,*3E
$GPGSV,3,1,09,08,59,192,42,10,46,302,42,04,18,206,41,27,77,114,16*74
$GPGSV,3,2,09,13,42,076,18,02,33,244,18,29,11,271,40,24,243,352,*48
$GPGSV,3,3,09,23,14,085,*49
$GPGSA,A,3,04,08,10,,,,,,,,,,11.2,5.0,10.0*39
$GPGGA,154106.52,4948.936977,N,00957.930742,E,1,06,3.0,245.0,M,-0.581025,M,-0.1900190,*4A
$GPRMC,154111.60,A,4948.936977,N,00957.930742,E,000.0,000.0,240407,,*35
$GPGSA,A,3,02,04,08,10,27,,,,,,,,6.7,3.0,6.0*3C
$GPGGA,154122.94,,,,,00,06,3.0,,M,-0.581010,M,-0.1900190,*62
$GPRMC,154122.96,V,,,,,,,240407,,*14
$GPGSA,A,1,13,,,,,,,,,,,,6.7,3.0,6.0*36
$GPGGA,154119.10,4948.936977,N,00957.930742,E,1,05,3.0,245.0,M,-0.576004,M,-0.1870190,*4D
$GPRMC,154119.10,A,4948.936977,N,00957.930742,E,000.0,000.0,240407,,*3A
$GPGSA,A,3,04,08,10,27,,,,,,,,,6.7,3.0,6.0*3E
$GPGGA,154128.15,4948.936977,N,00957.930742,E,1,06,5.0,245.0,M,-0.587003,M,-0.1860190,*47
$GPRMC,154128.15,A,4948.936977,N,00957.930742,E,000.0,000.0,240407,,*3D
$GPGSV,3,1,09,27,77,114,22,08,59,192,39,10,46,302,39,02,33,244,40*7B
$GPGSV,3,2,09,04,18,206,38,13,42,076,11,24,243,352,,23,14,085,*48
$GPGSV,3,3,09,29,11,271,*4F
$GPGSA,A,3,02,04,08,10,27,,,,,,,,11.2,5.0,10.0*3E
$GPGGA,154131.86,4948.936977,N,00957.930742,E,1,06,3.0,245.0,M,-0.578004,M,-0.1860190,*44
$GPRMC,154131.86,A,4948.936977,N,00957.930742,E,000.0,000.0,240407,,*3F
$GPGSA,A,3,02,04,08,10,13,27,,,,,,,6.7,3.0,6.0*3E
$GPGGA,154138.55,4948.936977,N,00957.930742,E,1,06,3.0,245.0,M,-0.578000,M,-0.1860190,*47
$GPRMC,154138.55,A,4948.936977,N,00957.930742,E,000.0,000.0,240407,,*38
$GPGSA,A,3,02,04,08,10,13,27,,,,,,,6.7,3.0,6.0*3E
$GPGGA,154141.69,4948.936977,N,00957.930742,E,1,06,3.0,245.0,M,-0.583002,M,-0.1840190,*42
$GPRMC,154141.69,A,4948.936977,N,00957.930742,E,000.0,000.0,240407,,*39
$GPGSA,A,3,02,04,08,10,27,,,,,,,,6.7,3.0,6.0*3C
$GPGGA,154146.16,4948.936977,N,00957.930742,E,1,06,3.0,245.0,M,-0.595000,M,-0.1830190,*4F
$GPRMC,154146.16,A,4948.936977,N,00957.930742,E,000.0,000.0,240407,,*36
$GPGSA,A,3,02,04,08,10,27,,,,,,,,6.7,3.0,6.0*3C
$GPGGA,154151.34,4948.936977,N,00957.930742,E,1,06,3.0,245.0,M,-0.587002,M,-0.1860190,*4D
$GPRMC,154151.34,A,4948.936977,N,00957.930742,E,000.0,000.0,240407,,*30
$GPGSV,3,1,09,27,77,114,26,08,59,192,42,10,46,302,41,13,42,076,25*7A
$GPGSV,3,2,09,02,33,244,42,04,18,206,37,29,11,271,37,24,243,352,*46
$GPGSV,3,3,09,23,14,085,*49
$GPGSA,A,3,02,04,08,10,13,27,,,,,,,6.7,3.0,6.0*3E
$GPGGA,154200.05,4948.936977,N,00957.930742,E,1,06,3.0,245.0,M,-0.587002,M,-0.1860190,*48
$GPRMC,154200.05,A,4948.936977,N,00957.930742,E,000.0,000.0,240407,,*35
$GPGSA,A,3,,,,,,,,,,,,,6.7,3.0,6.0*36
Retrieved from "http://wiki.openmoko.org/index.php?title=Hammerhead/Protocol&oldid=8113"
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Christian did some stracing on TomTom device, and result is great logs at http://www.maintech.de/download/hammerhead-strace.log .

GPS seems to communicate in packets.

Direction GPS -> machine

read(3, "\xfe\x00\xfd\x80\x16\x19\x0b\x00\x00\xfc", 2048) = 10

\xfe: begining of packet \x00: type of packet? seems to determine length. \xfd: follows .... \xfc: packet end

GPS signals at wikipedia seems to be required reading: http://en.wikipedia.org/wiki/GPS_signals

Analysis

Were there by any chance 8 satellites overhead at the time that log was made?

The protocol appears to be oriented around 32-bit words (the single-byte markers notwithstanding.) I'm not absolutely certain, but I strongly suspect the byte order is LSB-first.

The 80 80 80 stuff is probably just to force resynchronizing after an error, or wake the device up, or something like that.

It is - we have this information from another source. It's to train the UART in the hammerhead to the correct speed. --Speedevil 11:39, 28 April 2007 (CEST)

Packet format (same format in both directions):

  • The start of a packet is marked by FF or FE.
    • FF in packets that do not carry data, but rather explicitly request a response. (This isn't used very often. More often we receive data without explicitly requesting it.) The response will be an FE-packet with the same length and type as the FF-packet. The GPS does not send any FF packets, only gltt does.
    • FE in packets that carry data sections.
  • The first word (32 bits) following the start-of-packet marker is the header.
    • The first byte (if it's indeed little-endian, the least significant byte) gives the data length, measured as the number of data words minus 1. For FE-packets, this is the length of the data section of this packet. For FF-packets, it's the length of the data section in the expected response.
    • The second byte of the header is always FD.
    • MS nibble of the third byte might be flags for the packet.
    • LS nibble of the third byte might be an identifying number which is echoed in responses to this packet.
    • The fourth byte is the packet type.
  • In FE-packets only, the data section (n 32-bit words) follows.
  • Finally, FC is sent to mark the end of the packet.
  • After the FC in an FF-packet, gltt sends a bunch of zeroes. In some cases it sends a number of zeroes equal to the number of bytes in the response packet; in some cases it sends more. I would guess that these have no effect.

For example: 

ff 04fdc00c fc

(possibly followed by zeroes), is a request for a packet of type 0C, with length 20d ((4 + 1) * 4), and flags C0. 

fe 04fdc00c 0025102a 45dbdd4e 36030000 4b260000 16010000 fc

would be an appropriate response.

Packet types:

Frequency Type Max length Min length
9 16 73 11
86 08 19 11
8 e2 15 15
7 23 133 101
63 0a 15 15
6 24 71 39
52 0b 11 11
4 01 11 11
263 05 19 15
24 9d 315 19
2 06 25 19
16 e0 11 11
13 20 15 15
13 10 577 59
128 18 11 11
12 9e 303 15
117 00 65 39
11 0c 33 11
10 9f 477 61
10 0e 541 31
1 e5 11 11
1 80 17 17
1 13 11 11
1 04 23 23
1 02 11 11

NMEA

It is possible to translate NMEA messages into something readable using script like this: 

#!/bin/bash
grep -v gettimeofday | \
	sed 's/read.3/read(GPS/' | \
	sed 's/write.3/write(GPS/' | \
	sed 's/write.5/write(NMEA/' | \
	sed 's/\\/\\\\/g' | \
	while read LINE; do
		if echo $LINE | grep write.NMEA; then
			echo -e $LINE
		else
			echo "$LINE"
		fi
	done

NMEA description is available at http://www.gpsinformation.org/dale/nmea.htm .

At this point:

256 write(GPS, "\xfe\x00\xfd\x40\x08\x40\x60\x00\x00\xfc", 10) = 10 256 write(GPS, "\xfe\x00\xfd\x40\x08\x40\x60\x00\x00\xfc", 10) = 10 256 write(NMEA, "\x24\x47\x50\x47\x47\x41\x2c\x31\x35\x34\x31\x30\x34\x2e\x39\x35\x2c\x34\x39\x34\x38\x2e\x39\x35\x30\x33\x33\x39\x2c\x4e\x2c\x30\x30\x39\x35\x37\x2e\x39\x35\x35\x39\x37\x39\x2c\x45\x2c\x31\x2c\x30\x36\x2c\x35\x2e\x30\x2c\x32\x32\x30\x2e\x30\x2c\x4d\x2c\x2d\x30\x2e\x35\x38\x31\x30\x31\x34\x2c\x4d\x2c\x2d\x30\x2e\x31\x39\x30\x30\x31\x39\x30\x2c\x2a\x34\x30\x0d\x0a", 91) = 91 256 write(NMEA, "$GPGGA,154104.95,4948.950339,N,00957.955979,E,1,06,5.0,220.0,M,-0.581014,M,-0.1900190,*40 ", 91) = 91

...6 satelitte GPS fix was obtained. (And yes, there's big read few lines before that in the log). As far as I can tell, only 5-6 sattelite were _used_ till the end of log.

GSA sentence looks interesting, too. It tells us satellites #02, #04, #08, #10, #13 and #27 were used at this point.

256 write(NMEA, "\x24\x47\x50\x47\x53\x41\x2c\x41\x2c\x33\x2c\x30\x32\x2c\x30\x34\x2c\x30\x38\x2c\x31\x30\x2c\x31\x33\x2c\x32\x37\x2c\x2c\x2c\x2c\x2c\x2c\x2c\x36\x2e\x37\x2c\x33\x2e\x30\x2c\x36\x2e\x30\x2a\x33\x45\x0d\x0a", 51) = 51 256 write(NMEA, "$GPGSA,A,3,02,04,08,10,13,27,,,,,,,6.7,3.0,6.0*3E