Talking to the GPS inside a DPH153-AT Femtocell

• NMEA sentence descriptions

In the photo above, note the 4 pins labelled "GCON1" -- these are on 0.1 inch centers, so I soldered the usual 4 pin header into this spot on the PCB. If we call the pin next to the white corner on the silkscreen pin 1, the pins are:

• 1 - 3.3 volts
• 2 - Tx
• 3 - Rx
• 4 - Ground

Note that between "lines" (prior to the "$") there are various unprintable characters.

$GPGGA,000741.587,,,,,0,00,,,M,0.0,M,,0000*5E
$GPGSV,3,1,11,08,78,087,,02,76,332,,07,56,038,,17,36,195,*73
$GPGSV,3,2,11,28,26,124,,03,23,165,,10,21,070,,13,12,253,*71
$GPGSV,3,3,11,25,12,033,,16,10,210,,09,04,213,*45
$GPRMC,000741.587,V,,,,,,,300617,,,N*46

After power cycling the board, I discover that some interesting things go on before it settles down to the above, including:

SiRFLocClient3.5.0Cisco-Test2_3.5.00.00-C17P2.00 SiRFLocClient3.5.0

$GPGGA,002044.530,,,,,0,00,,,M,0.0,M,,0000*52
$GPGSV,1,1,01,00,00,000,*48
$GPRMC,002044.530,V,,,,,,,300617,,,N*4A

Once the GPS status light indicates a lock, the messages continue every 5 seconds and look like this:

$GPGGA,003101.042,3415.7713,N,11102.9081,W,1,04,3.3,747.3,M,-27.7,M,,0000*63
$GPGSV,3,1,12,24,63,326,,29,60,339,37,05,53,051,38,21,49,040,*7A
$GPGSV,3,2,12,18,39,048,,04,35,053,,19,34,315,,02,28,060,32*77
$GPGSV,3,3,12,12,26,178,23,13,17,120,29,15,06,047,,14,05,154,*7F
$GPRMC,003101.042,A,3415.7713,N,11102.9081,W,0.16,36.29,300617,,,A*47

The value 003101.042 is the UT time 00:31:01.042 (i.e about 5:31 PM in Tucson).

The elevation is given as 747.3 meters (which is 2451.8 feet), but using only 4 satellites. This updates later to 731.0 meters (2398 feet) with 7 satellites.

The topographic map indicates 2400 feet. The GPS in my cell phone indicates either 2409 or 2310 depending on which application I use (I tend to trust the 2410 number more).

Now watch the Rx line

Nothing was observed watching at 4800 baud. Watching at 38400 baud, we see a burst of activity, doubtlessly to initialize the GPS chip. We do not see the chip being polled every 5 seconds. Apparently it gets configured to send information on some schedule. And indeed this is done by the PSRF103 commands.