Two of the most exciting applications of APRS are the automatic reporting of weather conditions and vehicle positions. In this month's edition we'll touch on the basics of setting up each application.
To review, APRS can display weather information graphically over a basemap. Weather data can be transmitted manually by any packet station using a terminal program (if the proper format is known!), or it can be entered by a station running APRS using the W (weather) E (enter) commands and following the prompts. The method we will discuss today involves attaching a home weather station to another COM port on the computer running APRS to automate the data transmission.
Several different brands of weather stations can be used: the Peet Brothers' Ultimeter series, Davis, Radio Shack, and Heath. WJ9H runs the Ultimeter-II, and I will describe my setup. The Ultimeter-II comes standard with temperature probe, wind vane and anemometer. I also purchased the optional rain gauge with 0.01" resolution. The unit has an RJ-11 jack on its display box which is used to output data. Using a standard phone cord, I cut off one RJ-11 plug and wired it to a DB-9 connector, the green wire to pin 2 and black to ground, pin 5. In addition, a 5k pull-down resistor is connected between pins 2 and 3, since the Ultimeter-II does not output true RS-232 signal levels. (Note: the weather interface software does not run under the "freeware" version of APRS. The program has to be registered with the author, WB4APR, for $29, plus the $9 weather option.)
When APRS is started and the V (validate) routine is run, the program will prompt you for information about the attached weather instruments. Make sure you save the setup by pressing ALT-S then S. Now APRS will poll the weather instruments at an interval you set and transmit a properly formatted weather packet. In addition, your computer will display a blue circle with a wind vector over your location.
There is a way to set up a remote weather station using only a TNC and radio. The TNC needs to be GPS-compatible and the weather station needs to be one of the newer Ultimeters. Since the Ultimeter outputs data continuously, the TNC can be set to look for the beginning of the data string and transmit at regular intervals. To make the weather information APRS-compatible, the TNC has to be set to transmit a beacon text which includes the station's latitude and longitude. When the receiving APRS station has received both the position beacon and the weather packet, it will plot the weather station on the screen in the correct location.
How about attaching a GPS receiver to a GPS-compatible TNC? You get an APRS tracker! A typical system consists of a global positioning system receiver with a data output jack, a Kantronics KPC-3 TNC, and a 2-meter transceiver.
I use a Garmin 38 GPS receiver in my vehicle. Since Garmin wanted an exorbitant amount of money for their data interface cable, I made my own poor man's connectors using some junk-box female Molex pins crimped around the male pins on the GPS. The GPS data connects to the 25-pin connector on the TNC, and the TNC is set for 4800 baud, the GPS data rate.
There are several settings on the TNC which have to be set for proper operation. These are detailed in the F1-F help files in APRS. But basically, you are telling the TNC to watch the continuous GPS data stream and store in one of four memory slots the proper GPS "sentence." Then at predetermined intervals, the TNC transmits the sentence as position information. Typical desired sentences are $GPGGA, which gives latitude, longitude, and altitude; or $GPRMC, which gives latitude, longitude, course, and speed. I have mine programmed to alternate between the two and transmit every two minutes using different UNPROTO paths.
How your tracker is displayed on APRS depends on your callsign and your UNPROTO setting. The SSID of your callsign (the number added after your call) will cause different icons to be displayed. For instance:
WJ9H-9 displays a car
WJ9H-4 displays a bicycle
WJ9H-14 displays a truck
You can also set the UNPROTO parameter for more display options. Normally UNPROTO is set to CQ, BEACON, or APRS. If you set it to one of the settings specified in the SYMBOLS.TXT table, you can override the SSID setting. For instance, my tracker IDs as WJ9H-9 but the UNPROTO parameter is:
...which forces my icon display to a van ("LV").
Here are my settings in the KPC-3 TNC:
ABAUD 4800 (sets the TNC to the GPS's baud rate)
GPSHEAD 1 $GPGGA (TNC monitors for the GPGGA sentence
in memory slot 1)
GPSHEAD 2 $GPRMC (memory slot 2 gets the GPRMC sentence)
GPSHEAD 3 $GPGGA (repeat GPGGA in slot 3)
GPSHEAD 4 $GPRMC (repeat GPRMC in slot 4)
LTP 1 GPSLV VIA WIDE,WIDE (data in slot 1 is assigned
the LV icon and uses the unproto path WIDE,WIDE)
LTP 2 GPSLV VIA RELAY,WIDE (slot 2 also has the LV icon, but uses the path RELAY,WIDE)
LTP 3 GPSLV VIA WIDE,TRACE,WIDE (now the path uses a new alias, TRACE, whose digipeater can substitute its callsign for its alias)
LTP 4 GPSLV VIA N9UDO,WIDE (slot 4 data goes out via N9UDO then any WIDE after it)
BLT 1 EVERY 00:08:00 S 00:00:00
BLT 2 EVERY 00:08:00 S 00:02:00
BLT 3 EVERY 00:08:00 S 00:04:00
BLT 4 EVERY 00:08:00 S 00:06:00
(Timing for the four memory slots rotates through an 8-minute cycle, offset by 2 minutes each, so that there is a position packet every 2 minutes.)
BT WJ9H mobile tracker, firstname.lastname@example.org (my "generic" beacon)
B E 30 (send beacon infrequently, every 30 minutes)
For the Madison Marathon on May 30, we used a homebrew tracker (GPS, TNC and HT mounted inside a plastic toolbox) and a few of the new Kenwood TH-D7 handi-talkies with built-in TNCs. With the addition of GPS receivers to the Kenwoods, we then had trackers for three of the Marathon's shuttle buses and one in the vehicle preceding the lead runner. Back in the communications tent, we could see the position plots of all those vehicles over a map of Madison. For dispatching buses, it saved time not having to query all the bus operators what their positions were on the voice net. This is just one useful application combining GPS and APRS over amateur radio.
Finally, if the tracker is within range of an APRS internet gateway, web browsers can also display the position of the vehicle. Look for me at http://map.findu.com/wj9h-9. Or insert the desired callsign after the slant bar.
If you're joining this series on APRS in progress, you can find previous
articles at www.qsl.net/wj9h.
See you on 144.39!
This page originally appeared in June 1999 Badger
State Smoke Signals but was updated 2000 June 14.
Copyright 1999, 2000, Thomas C. Weeden, WJ9H