Brainerd Area Amateur Radio Club  

Lyle Kohler KØLR

The BAARC will held a regular membership meeting Thursday, March 25th.  Twenty-four hams showed up even though there was extremely dense fog making driving difficult.  

A presentation on APRS (Automatic Position Reporting System) was given by Lyle Kohler KØLR.  Lyle has spoken to the club and other ham organizations before.  He is a good speaker and the topic was very interesting.

Among the many uses for APRS are emergencies and public service events, weather station reporting, Skywarn, radio direction finding, keeping track of traveling friends, and just plain fun. APRS station types include fixed weather stations, mobile trackers, digipeaters, and Internet gateways.

A mobile tracker station requires a GPS receiver with a serial data output, a packet terminal node controller (TNC), a 2 meter radio and an antenna. The TNC does not need to be a full-featured controller -- it doesn't even have to be able to receive. Low-cost TNCs designed specifically for this application include the Byonics TinyTrak3 ($36 kit, $60 wired), and the Tigertronics TigerTrak TM-1 ($89.95). Byonics has just announced a "PocketTracker", combining a TNC and a 2m QRP transmitter on a circuit board that fits into an Altoids tin. The price (not including Altoids tin and mints) is $89. Somewhat higher on the price scale, Kenwood offers the TH-D7AG dual-band HT with built-in TNC for about $325, and the TM-D700A dual-band mobile with internal TNC for about $465.  

Mobile trackers can operate in several modes, including a timed beacon mode or a "smart" beacon mode, in which a transmission is made whenever there is a significant change in speed or direction. There is also a "MIC-E" mode that adds a compressed data burst at the end of a voice transmission. As with other packet TNCs, mode information, station call sign, beacon message, etc. are programmed by the user via a computer's serial port.

An APRS base station requires a computer, a TNC (more about this later) and a 2 meter receiver. It takes only a few minutes to set up a field-portable APRS base station for emergencies or community service events. APRS software is available for all popular operating systems including DOS, Windows, Mac, Linux and Palm. Actually a TNC is optional. A packet engine program called "AWGPE", developed by George Rossopoulos, SV2AGW, uses the computer's sound card to perform the TNC functions, and will work just as well as a "hardware" TNC. For Windows, the most popular APRS programs include WinAPRS, UI-View, APRS + SA (Street Atlas), and a new one from SV2AGW called AGWTracker. I have been using UI-View, which is available in a freeware version or in a registered version for about $19 US. Some computer mapping programs are directly supported by your APRS software so that station positions will be automatically displayed on a detailed map. However, you can also load your own maps from other sources and create custom maps, if you are able to specify the exact geographic coordinates of two points on the map. 

In the US, the "official" packet frequency is 144.390 MHz. There is also some APRS activity using 300 baud packet on HF, primarily on 30 meters. By tuning your HF receiver to 10,151 kHz in LSB mode (10,151.51 kHz for some TNCs including the AWG sound card packet engine), you may be able to copy APRS stations from all over the country.

If you have Internet access, you don't need a receiver at all. There are several ways of accessing the www.findu.com database via your Web browser, allowing you to locate a specific station or to keep track of all APRS stations in a given region of the world. Using WinAPRS or UI-View, you can also connect to an APRS server directly, so that you can view "real-time" activity and even exchange text messages with other connected stations.

At present there is not much APRS activity in this part of Minnesota, and to my knowledge there are no digipeaters that will  pick up a mobile tracker reliably in the Brainerd area. That should change in the near future, as Terry, WØTVD and John, AF9T are planning to install a 144.39 digipeater at the Aitkin DX Cluster site northeast of Crosby. This will provide coverage for portions of Crow Wing and Aitkin Counties.

Packet radio is not very tolerant of noise or dropouts on the channel, and normally relies on multiple transmissions until an "ack" is returned from the receiving station. However, APRS transmissions are one-way -- the sending station simply hopes that all receiving stations get the message. Because of that, the range between a mobile tracker and a digipeater will be considerably less than the range of a voice repeater with the same antenna and power level. Fortunately, if one position report is missed, it is usually not critical to the "mission". And as with other amateur packet applications, any station with a full-featured TNC can become a digipeater to provide more complete coverage or to replace a digipeater that is out of service.

Packet radio requires that you specify the path that you want your transmissions to follow. With APRS, it's easy because the network uses identically named digipeaters. Every digi is a "Relay", and "Wide" is used for digipeaters higher than 100 feet. So if you set up your TNC with the path "Relay, Wide, Wide" it will get you through the first three hops. There are other more advanced methods of specifying the path, and you should use the minimum number of hops to keep from flooding the network with packets. Detailed information on digipeater operation and TNC setup can be found on the Internet.

In summary, APRS brings position tracking capability within the reach of every ham, and can provide a powerful tool for public service and emergency communications. If you already have a radio and a GPS, APRS tracking capability can be added for as little as fifty dollars. A wealth of information on APRS is available on-line. Here are a few links to get you started:

http://web.usna.navy.mil/~bruninga/aprs.html            Bob Bruninga's page

http://www.findu.com              APRS database

http://www.wulfden.org/APRSQuery.shtml    Search for APRS stations

http://www.byonics.com/        Byonics web site

http://www.gpstracker.com/   Tigertronics tracker products

http://www.winaprs.org           WinAPRS software

http://www.ui-view.com          UI-View software

http://www.qsl.net/soundcardpacket  How to use the sound card packet engine

http://www.raag.org/sv2agw/  AGW Packet Engine and AGWTracker

http://nwaprs.org/digipeating.htm       Notes on APRS Digipeaters

73  Lyle, KØLR

Jack Miller, BAARC V.P. and Program Chairman.