POTSwap User Guide JA16-EVDO-UM
Page 16 Rev: P07 Date: 08/01/16
© Copyright 2016 Janus Remote Communications
Specifications subject to change without notice
All Rights Reserved See website for latest revision. Not intended for life support applications.
7. GPS
7.1 GPS Operation
POTSwap models HSPA910PS V1.00 TAGFNG and EVDO910PS V3.00 TAGFNG have a GPS antenna connector
installed on the back plate and offer the ability to provide enhanced location information to emergency (911) operators.
A suitable GPS antenna must be used and the antenna must be properly located to receive the GPS signals.
The GPS operation is automatic, and is independent of the cellular radio operation. When the unit has acquired a
GPS location fix, the bottom LED on the RSSI indicator will blink several times for 1 second at 10 second intervals.
This indication is independent of the cellular signal strength indication. See section 5.1 – Front Panel. A GPS
location fix should be available within a few minutes of operation under normal conditions.
7.2 GPS Antenna
For best results, it is recommended that an active GPS antenna be used with the POTSwap. The GPS antenna
connector provides a 2.85V bias voltage to power an active antenna, and should easily be able to supply the 5 to
25mA of current that a typical active GPS antenna requires. Any standard off-the-shelf active GPS antenna that will
operate at the 2.85V bias voltage should suffice. See section 11 – Accessories.
7.3 GPS Antenna location
The ideal GPS antenna position would be an outdoors location with a clear view of the sky to the horizon in all
directions, with no obstructing structures or foliage. A higher antenna placement usually gives a better view of the
sky over surrounding obstructions. In the northern hemisphere, a southern sky view is preferable over a northern sky
view. If the antenna must be indoors, placement as near as possible to a window is preferable. Receiving a GPS
location fix deep inside buildings or in dense urban environments (“street canyons”) is normally not possible.
7.4 GPS Troubleshooting
With a terminal attached to the SERIAL port, debug messages are available that can be used to monitor the GPS
operation. See section 4 – Configuration, for information on attaching a terminal to the SERIAL port.
There are several types of debug messages output during normal operation. The message with the GPS information
includes “$GPSACP:” near the beginning of the output line, and is output about once per minute. The $GPSACP
message conveys 11 terms, separated by commas:
UTC
UTC time (hhmmss.sss)
latitude
format is ddmm.mmmm N/S
longitude
format is ddmm.mmmm E/W
hdop
x.x - Horizontal Diluition of Precision
altitude
x.x Altitude - mean-sea-level (geoid) in meters
fix
0 or 1 - Invalid Fix; 2 - 2D fix; 3 - 3D fix
cog
ddd.mm - Course over Ground (degrees, True)
spkm
x.x Speed over ground (Km/hr)
spkn
x.x- Speed over ground (knots)
date
ddmmyy Date of Fix.
nsat
nn - Total number of satellites in use, 00..12
The final term can be used to provide some useful information about the quality of the GPS antenna system being
used. Generally, the better the antenna system, the greater the number of satellites will appear in the ‘nsat’ term.
Though it varies over time, the typical maximum number of satellites available is 10. A minimum of 4 is required to
get a location fix, and numbers of 8 or more indicate an excellent antenna system.
With no location fix, the $GPSACP message will appear as follows:
[$GPSACP: ,,,,,1,,,,,]
This would be expected after the unit has powered up but before a location fix is aquired. It would also be indicative
of a poor antenna system if it was still being reported after several minutes of operation.
A typical fix might appear similar to the following:
[$GPSACP: 183117.000,4147.6251N,08815.5097W,1.25,191.0,3,191.69,0.12,0.06,290716,07]
In this example the ‘nsat’ term indicates that 7 satellites are being used.
