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Karl Venneberg, KBØWBM
Electronic System Analyst
National Weather Service Office
Bismarck, North Dakota
701-250-4224
Introduction
The Automatic Position Reporting System (APRS) can be used in conjunction with weather equipment to form a stand-alone remote weather site. This information can then be relayed to the National Weather Service Office to provide real time wind speed, direction, temperature, and rainfall. Once the information arrives at a National Weather Service Office it is sent to their computer systems so that it can be displayed using the Advanced Weather Interactive Precessing System (AWIPS). This additional information would assist the forecasters in making quick decisions on fast moving storms. Having these systems installed around the County Warning Area (CWA) will help the National Weather Service by filling in the gaps where no equipment currently exist.
Basic Operation of the APRS
The APRS is a system which, unlike PBBS's, nodes and DX clusters, uses an un-connected protocol to transmit your exact position, a symbol denoting the type of station you're running and a brief comment about it. Parts of the APRS also uses direct keyboard-to-keyboard "chatting," has direction-finding capabilities and much more. Another use of the APRS is the ability to track your location with a Global Positioning System (GPS). How does it work? In its most simplistic form you transmit a packet which contains your callsign, exact latitude and longitude, as well as a brief comment of your choosing along with some symbols necessary to make the system work. With this information your station appears graphically on a map (actually, one in a series of many maps) on the monitor as would other stations that are on frequency. Since this is an UNCONNECTED protocol, on-air packets can be kept to a minimum.
When you connect to a local station using standard AX.25, you send a connect request to that station, they acknowledge that packet, then send you a connected packet which you must then acknowledge. The same thing happens with EVERY packet you, or the other station, sends. With APRS you only send ONE packet to convey your information. If it's not received on the first transmission, APRS retransmits this information using a decaying time delay (that is, the second packet is sent fifteen seconds after the first, the third thirty seconds later, the fourth a minute later, the fifth two minutes later etc. until it levels out at a period of twenty minutes!) This makes more efficient use of the frequency.
APRS uses four different kinds of digipeaters, which use the aliases RELAY, WIDE, ECHO and GATE. RELAY stations (the default setting) are base stations used to digipeat low-power portable and mobile stations. WIDE stations will digipeat packets addressed either to their specific callsign or the generic WIDE to other VHF stations and WIDEs. An ECHO performs a similar function on HF. A GATE will digipeat from HF to VHF. When setting up APRS for your location you'll set your digi- peater path based on the situation at that QTH and where you want your information to go. For keyboard-to-keyboard communications (which are the only comms in which "ACK's" are used) you can also set alternate digipeater paths. Not only does this direct your message via the short- est possible route, but it also reduces QRM.
APRS Networks in North America are all on 144.39 MHZ as of the end of 1998. This is a nationally-coordinated frequency in the United States.
Weather Station Reporting – APRS position reports can also include the wind speed and direction, as well as other important weather conditions. APRS supports a serial interface option to home weather stations. With the Internet taking its share of packet users away from the radio, APRS along with your home weather station is a practical use for AX.25 Packet Radio.
Weather Equipment
There are two major suppliers of weather equipment for the home enthusiast and they are Peet Brothers () and Davis Instruments (). I’m sure there is other equipment out there, but these are the most commonly used ones today. The pricing of these two systems are relatively the same. The main difference is that the Peet Brothers system has a serial output that is made for packet radio. This means that the output can be directly connected to a TNC. Where the Davis Instrument system requires a computer to interface to the TNC. The ‘basic’ systems have wind speed and direction, and temperature. To the basic system you can add pressure, tipping bucket, and relative humidity.
There are three different Peet Brothers systems the Ultimeter 100, 800, and 2000. Any of these systems will work fine with a TNC and Radio. The Davis Instruments systems includes the Weather Monitor II and the Weather Wizard III. Both of these systems require software and a computer to interface to your TNC.
TNC
A Terminal Node Controller (TNC) is similar to the modem you use when connecting to the internet. One difference is that the TNC is used to interface your terminal or computer into the "RF" or radio (wireless) medium. There is one other very significant difference. Inside the TNC they have added some internal firmware called a "PAD". The PAD or "Packet Assembler/Dissembler captures incoming and outgoing data and assembles it into ‘packets’ of data that can be sent to and from a data radio or transceiver.
In addition to the data stream conversion to and from packets, the PAD also enables the Push-To-Talk (PTT) circuits of the radio transceiver. When you press the enter key of your computer keyboard, the typed in data is sent out over the air to the target station or a nearby ‘store-and-forward’ device known as a ‘node’.
The system that I’m using has a Kantronics 3 Plus.
Data Radio (Transceiver)
You can use your normal Transceiver on 144.390 MHz, or you can get a Data Radio to connect to your TNC. The system I’m using has a MJF VHF Data Radio Model MFJ-8621. This radio has direct modulation and lets you use all data rates from 1200 to 9600 baud without modification. It has ultra-fast PIN diode switching to give you near instantaneous change-over between transmit and receive. It employs unsquelched audio feeds directly to your TNC. It’s a great tiny 5 x 5 x 1 ½ inches, it draws just 15 mA on receive and less than 1 AMP on transmit using 12 VDC.
Weather Equipment Setup
The first thing that you need to do is decide where you want to install your weather equipment. For accurate readings the wind equipment needs to be mounted level and a minimum 5 feet above any obstructions. Official weather station wind systems are required to be at 10 meters (I have my system mounted 20 feet above the ground.). If you have a tipping bucket for precipitation you need to make ensure that it is clear through a 45 degree angle from the center out, all the way around it.
TNC Setup
The following is for a Kantronics 3 Plus TNC, others TNC will have a different setup, but the basic information should help you in setting up your TNC.
The first thing I did was to reset the TNC to the default values. To do this with the Kantronics 3 Plus, you have to remove the cover and place the jumper across the RESET pins, then turn power on for about 5 seconds, then remove power, then remove the jumper. You can then replace the cover on the TNC. To program the TNC you’ll need some type of communication program, I used the one supplied with the Windows 95 and 98 (Hyper Terminal). Set the program for 2400 baud, 8 bits, One Stop Bit, and Parity None (2400,8,N,1). Connect your TNC to the comm port you are using on the computer, start your communications program, then turn on your TNC. If you press Shift *, the TNC will go into auto baud rate. You should see the TNC go into the command mode.
cmd:> intface terminal
cmd:> abaud 2400
cmd:> GPSHEAD 1 $ULTW (This is what you use if you have a Peet Brothers system, the $ULTW is the first part of the data stream sent by the weather equipment)
cmd:> BLT 1 EVERY 00:05:00 (This sets the time period for TNC to transmit the weather data. Peet Brothers equipment sends the data every five minutes in the packet mode).
cmd:> LTP 1 ARPS VIA WIDE, WIDE (this is the path for the data to take...hopping from station to station).
cmd:> MON OFF
cmd:> CD SOFTWARE (changes the carrier detect to software controlled)
cmd:> BTEXT ! xxxx.xxxxN/xxxx.xxxxW_Remote WX (replace the xxxx.xxxx with your lat/lon then add the _Remote WX then the APRS system will decode your weather data and place it on the WINAPRS maps).
cmd:>CTEXT STAND-ALONE WEATHER STATION. NO OPERATOR, DISCONNECTING
cmd:> B E 00:20:00 (This sends out a beacon every 20 minutes)
cmd:> INT GPS (This turns on the GPS mode)
cmd:> DAYTIME YYMMDDHHMM[SS] (to set the real-time clock)
Power off the TNC and then power it back on, this places the TNC in GPS-Beacon mode. You should be up and running.
This configuration works with my KPC 3 Plus TNC. But it should give you a good idea on how yours might be configured. You’ll see in the BTEXT you’ll need to enter your latitude and longitude for your site. (If you don’t have this, give us a call and we’ll bring our GPS out to get this information for you.)
TNC to WEATHER EQUIPMENT
The Davis Instrument and the Peet Brothers weather equipment has output that can be connected to your TNC. I choose the Peet Brothers equipment only because it allows you to go right from its equipment to the TNC without having a computer and software to interface the two together. The only problem I had when setting up the interface between the TNC and the weather equipment was I didn’t realize that there was a difference between a computer RS232 and a TNC RS232. What I had done was connected the output of the Peet Brothers Equipment to a computer and started my terminal program to watch the data flow. Thinking that the connection was the same, I connected it the output to the TNC but I never got an output through the TNC. Upon further investigation I found that the receive and transmit pins on the TNC were different than the computer. Once I made the change, the weather data started flowing.
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