Using the Wisconsin Network - Part 34

by Andy Nemec, KB9ALN

Over the course of the last few months, we have been discussing the integration of packet radio into your ARES/RACES operations. This time out, we will go a little further with a discussion of APRS.

Just what is APRS?

It is an acronym meaning "Automated Position Reporting System". The APRS software allows a station to broadcast it's position periodically, to receive position reports from others, and mark it all on a map appearing on your computer screen. It is all done via packet radio's AX.25 protocol, and most operations take place on 144.930 Mhz. In addition to the mapped position appearing on your screen, APRS software can also broadcast data from some selected home weather stations. You still can conduct conversations with APRS, though the process is slightly different and prone to errors.

How does it work?

The operator can enter the position coordinates manually, or a Global Positioning Receiver (GPS) unit can be added for truly automatic operation. The GPS receiver listens to signals from several satellites and is able to calculate it's position relative to the satellites. This data is displayed, as well as coded and sent to a data output port. Data from the GPS unit is sent to the TNC, where it is formatted and broadcasted as a "UI" (connectionless) packets.

Digipeaters are used to relay the signal as required. Some digis are devoted to localized areas for weak-signal stations, other digis provide wide-area digipeating, and others act as HF-to-VHF gateways. They all have specific designations in the APRS system, and specific functions.

The software used to interact with the GPS unit and TNC is specialized and responsible for much of the functionality of the system. You cannot use a standard terminal program to operate APRS, it just won't work. The good news is that the APRS software for DOS, Windows, and MAC formats is widely available free of charge. You will be pleased to know that the software does not require a powerhouse of a computer to run, an 8088 or better is sufficient. The software is also designed to work with all models of TNC's equipped with APRS.

What to do with it?

Well, the possibilities are plentiful if you are in the right situation. Think about these for a moment: - Precise position reporting for weather spotting - Following a parade route - Radio Direction Finding - Shadowing Emergency Government personnel Any or all of these can help you to do the job better and faster, not to mention doing things you have never done before. In short, anytime you need to know where someone or something is, you can use APRS to report it's position via packet radio.

Perhaps by now you have concluded that you can use APRS and wish to incorporate it into your system. Now you have to consider what to do in the way of equipment. There are two approaches here, a traditional and a new method that looks to be very cost-effective. First, the traditional approach.

Naturally, you will need all of the normal elements that comprise a packet station. The appropriate radio, antenna, and feedline all have to be there just as in a "normal" packet station. The TNC must be APRS compatible, a lot of TNC's of recent manufacture have APRS support. It is not economically feasible to "retrofit" an old TNC with current APRS support unless a manufacturer offers a low-cost kit for a specific model.

Now you need to think of the GPS unit you will attach to it. Handheld units are popular for roaming, but suffer from a little inflexibility. The antennas are built into these units, and they must have a direct view of the satellites. Permanent locations can take advantage of a remote antenna installations, which removes that problem. Most operators select a hand-held unit for it's portability.

Once you have decided on the style, you have to do a little bit of checking before you can determine if it will work with your TNC. The GPS unit has to send it's data to the TNC, of course. And it has to have the proper port and data format for your TNC to understand it. Luckily, this seems to be somewhat standardized. Most GPS units have the required NEMA data port with the proper format. If there is any doubt in your mind as to compatibility, refer to your TNC's owner's manuals or call the TNC manufacturer. Best to spend the time to check on this very important matter rather than buy a unit you can't use!

This traditional APRS approach requires a radio, antenna, TNC, and GPS receiver dedicated to the task. There is another method of utilizing APRS that may be especially appealing to Emergency Coordinators for more than one reason. This method comes to us in the form of a soon-to-be- released "semi-kit" from the Tuscon Amateur Packet Radio Association (TAPR). It is called the "MIC-E" and it allows you to operate conventionally, yet relay APRS data with your existing radio. It takes the GPS receiver data, formats it into a packet, and tacks it onto the end of your voice transmission. This is a transmit-only system, and does not require a TNC. It is intended to plug into the microphone jack of the radio, with the regular mike plugging into it. This is quite an advantage in a number of situations. There is a much lower cost of equipment, a less complex system, and the operators have fewer pieces of equipment to fiddle with. Imagine getting an updated position report from a weather spotter every time he unkeys his mike - and you can still utilize existing voice repeaters.

Those are two ways to implement APRS in your emergency communications system. If you would like more information on APRS, try this URL:
It is especially helpful as this is an ARES organization and use APRS effectively. They also have the current APRS software for you to download, if you wish.

There is also the Tuscon Amateur Packet Radio Association's web site. This is where you go for information on the "MIC-E", as well as general information on APRS.

On to Part 35 - Internet Gateways - Part 1

Back to Part 33  - Setting up a Packet Radio station for Emergency Operations

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