Metro Skywarn - Automatic Packet Reporting System

Automatic Packet Reporting System

APRS is a shareware program that takes advantage of the availability of inexpensive GPS receivers to display the locations of moving stations on your PC. Locations of automobiles, buildings, weather data, marathon runners, balloons, bicyclists, emergency vehicles, anything, can be displayed. By connecting your PC computer to a radio network via an inexpensive (~$130) radio data modem, then all stations can see the movements of all other stations. APRS permits any number of stations to exchange data just like voice users but without the complexity of maintaining separate point-to-point links. The primary APRS display screen is the MAP display. To help mobile units find each other, there are radio nets standardizing on several radio channels.

Let's say you're going on vacation, but you'd like to keep the folks at home informed of you're progress. Using APRS, along with a Global Positioning System (GPS) receiver, and a common 30m frequency (10.151 MHz LSB), you can transmit your location as you travel. Back home, a GATEWAY into the local APRS net on 2m (145.79 MHz) picks up your position and transmits it to everybody here.

Weather-station equipped APRS stations report local wind, speed and direction, temperature, and perhaps barometric pressure, relative humidity, and present rainfall amounts every five minutes. This provides a current picture of local weather conditions to Emergency Operations personnel for fire, chemical spills, or severe weather.

Many public events, such as the 100 mile bike race covering most of the county, are a natural for APRS. The leaders can be tracked, locations of "sag wagons" plotted, and the organizers kept informed minute by minute of the progress of the event.

WHAT IS APRS ?

Packet radio has great potential but so far has been best used for passing large volumes of message traffic from point to point or into the national distribution system. It has been difficult to apply packet to real time events where information has a very short life time. APRS avoids the complexity and limitations of a connected network. It permits any number of stations to exchange data just like voice users would on a voice net. Any station that has information to contribute simply sends it, and all stations receive it and log it.

APRS is an UNCONNECTED packet protocol. It uses {unconnected information frames} that your TNC {terminal node controller} sends to the address you specify in the UNPROTO command of your TNC. APRS uses several generic callsigns called "aliases" to allow mobiles and other stations to report without having to have specific knowledge of who is in the network, or alter their UNPROTO command as they travel. {Many mobiles consist of a TNC, RADIO, and GPS {Global Positioning System} receiver; no computer.}

These generic aliases are listed below with an explanation of their normal usage. (Note that these should not be used for normal APRS operations.)

WIDE: This alias is used as a link between widely separated areas. If there are more than two digis with the alias of in a given area they will digi in a circle and generate numerous unnecessary packets. (PLEASE don't set your MYALIAS or MYNODE callsign to without asking an experienced APRS operator if it is appropriate.)

GATE: This alias is used to allow mobiles on HF to report their location through a station and into local VHF networks. GATE should (almost) NEVER appear in a VHF stations outgoing path. (VHF networks are operating at 1200 baud while HF is only running at 300 baud. It only takes a few VHF beacons to totally saturate the HF network NATIONWIDE.) On occasion, special events may be gated VHF>HF but this is only done with prior planning. PLEASE don't set your UNPROTO to .

RELAY: This alias is used to allow low power mobiles that are in range of any APRS station to digi into the net through an APRS home operators station. It is OK to set your TNC MYALIAS to . However you should not set your UNPROTO to for a home APRS station. This path is correct for low power mobiles.

Now you no doubt are asking what should I use in order to be seen both locally and in other parts of the state. My suggestion as a place to start is to set the following in your TNC:

MYCALL =
MYALIAS = RELAY
MYNODE = OR Leave it blank
UNPROTO = APRS VIA CALLSIGN, WIDE
"Callsign" in the above example should use the callsign of the wide area digi or other APRS station that you hear the best. The WIDE will cause your packet to be repeated by the next wide area digi beyond the first callsign. The alias of RELAY in your MYALIAS will allow a low power mobile to digi through you to the network. {APRS software adds the APRS VIA when it initializes the TNC. You don't have to enter it.} (Courtesy of KC4ZGQ)

WHAT DO I NEED TO GET STARTED ?

If you're already on 2m Packet, the only additional item you should need is the APRS software itself.

APRS software is shareware; and available from a number of sources. (See "Where can I get APRS ?," below.) Anyone can use APRS for receiving and displaying data without charge. Registration for use of GPS locators, or WX stations, or to conveniently save configurations, is nominal. Registration instructions come with the software. APRS versions come for DOS and Macintosh systems; a Windows95-based version will be available in the Spring of 1996. (The beta version is available now.)

If you're not already on packet, you need an FM transceiver that tunes to the local APRS frequency (145.79 nationally, and in the Minnesota Metro area: some other areas of the country use different frequencies), a 1200 baud TNC, a computer, and of course, the APRS software. (If your license gives you privileges on 30m, you can also run APRS on 10.151 MHz, LSB providing national coverage for mobiles.)

Some newer TNCs have implemented special instructions and operating modes for APRS, and are worth the investment if you'll want to operate mobile, or set up a WX station; however, most any TNC will work with APRS. Of course, you'll have to make the cable that connects the TNC with your radio.

Almost any "PC-compatible" computer can be used to run APRS. Even old "8088" PCs with 640k of RAM will work! (This may be a good place to use that old boat anchor!) A '386 or '486-based PC with 2 MB of RAM, a modest Hard Disk, and EGA/VGA color display makes a very good machine for APRS operations. Laptops or Notebooks, even without color displays, work well for mobile or portable applications.

WHAT ABOUT APRS IN THE MINNESOTA METRO AREA ?

APRS is still in its beginning stages in the Metro area. There are currently some 8 - 12 stations on the air on a regular basis. The "hotbed" of APRS activity is currently centered in Faribault, about 50 miles south of the Twin Cities, including a WX station, and a well sited WIDE digipeater that provides excellent coverage across the southern metro and along I-35 south. Two additional stations in the Metro area provide digipeater coverage with WIDE or RELAY aliases as well as WX station data, and an HF GATE from 10.151 MHz LSB for national coverage. All of these stations are available 24 hrs a day. A number of other stations run APRS from their home QTHs on a more casual basis.

Other areas of APRS activity nearby include the Grand Forks, ND area, and Cedar Rapids, IA.

HOW DO I REPORT MY POSITION ?

Reporting your position from a mobile unit can be as simple as running APRS on a laptop and entering your position as you go. A better way is to use a GPS receiver.

GPS receivers are relatively inexpensive, starting at about $250 for ones that will be compatible with APRS. Any GPS receiver that you select must provide a "NMEA output," and preferably with the NMEA "RMC sentence." (NMEA is the "National Marine Electronics Association, and this organization has defined a protocol for navigational information. The RMC sentence provides both course and speed information.) The easiest way of providing position reports is to use a "stand-alone" tracker, consisting of just a GPS receiver, a TNC with "GPS firmware", and a radio on the local APRS frequency. No computer is needed. All of the major TNC manufacturers offer models with the necessary firmware. If you plan on using a stand-alone tracker, your GPS receiver MUST have the RMC sentence.

Of course, you can use a portable computer, such as a laptop and run APRS on it. GPS receivers that provide NMEA outputs will then provide position reports automatically. The RMC sentence is not needed in this case.

WHAT ABOUT WX STATIONS?

Currently, APRS supports two weather stations, the Peet Brothers Company Ultimeter II and Ultimeter 2000. Both provide temperature, wind speed and wind direction, with an optional rain gauge available. The U-2000 also provides barometric pressure, and has an optional relative humidity sensor. Call Peet Brothers Co., at 1-800-USA-PEET for further information and pricing. Numerous ham dealers also carry the Ultimeter II and the Ultimeter 2000. (NOTE that the Davis Instruments WX station, carried by some ham dealers, is NOT supported by APRS. Also note that currently there are still some bugs in APRS support of the U-2000. Many computers work just fine with it, but some, especially some of the newest models, "hang up" when the U-2000 is connected.)

In addition to using WX with APRS software, stand-alone WX stations can be configured with the WX station, a TNC with "GPS firmware," and a radio. A computer is not required.

WHERE CAN I GET APRS ?

EMAIL: Look for an APRS point of contact for your state. Some states have APRS phone line BBS's for distributing local APRS maps. The latest APRS releases are posted on the Annapolis BBS at 410 280-2503 and from there it goes everywhere. Some of the Internet/WEB sites w/APRS are:

http://www.tapr.org/tapr.html/sigf.html
ftp://aprs.rutgers.edu/pub/hamradio/aprs/
http://www.ccnet.com/~rwilkins/aprs.html
http://www.bsrg.org/aprs/aprs1.html
http://aprs.rutgers.edu/

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