Not everyone starting out with APRS wants to dive fully in, adding GPS receivers or weather stations or direction-finding equipment. This month we'll talk about using the various APRS functions which use manual data input.
Last time I mentioned automatic plotting of weather data from a home weather station. Let's simplify this a bit and say you own a regular thermometer mounted outside a window and a no-frills rain gauge. A storm has just blown through and you'd like to report your rain amount to the APRS station at the National Weather Service.
With APRS running, press W (Weather), E (Enter data). Notice these are single keystrokes. You don't have to press the <ENTER> key after each. At the bottom of the screen you will be prompted to enter a wind direction. If you know what it is, or can estimate it, enter the direction in degrees (for example, wind from the west would be 270). You can leave any entry blank if you don't have the data available. Next is a prompt for wind speed and another for peak wind gust. The next entries are for temperature and dew point (again, if you know them). Then you'll see a prompt for rain amount in hundredths of an inch. If you had an inch and a half of rain, you would enter 150. You could also enter a snowfall amount at this prompt by adding "S" after the number. You'll be asked for a pressure reading in tenths of millibars (some barometers have a millibar scale). For example, 29.90" converts to 1012 mb, so enter 10120. The last entry is for comments, where you can enter a short line of text such as "Rainfall between 7:30 and 7:55 PM."
When you have finished entering your weather data, APRS will format a packet for transmission. It will transmit it immediately, then begin lengthening the interval between subsequent transmissions. One of the fundamental principles of APRS is that new information is more valuable than old information. Eventually your weather data will become "old" and you will want to either update it or manually delete the data by entering a different kind of packet, which I'll discuss later.
Your weather packet is now being broadcast to your local APRS network, and you are also receiving weather packets from other weather stations, both manual and automatic. You have several options for displaying weather stations, discussed two months ago. To summarize briefly, press J (Just) W (Weather) to see just weather stations. Then press W (Weather) C (Callsigns) to see a list of options.
APRS can sound/display an alarm if limits of weather data are exceeded from any station. Press W (Weather) A (Alarms) to see a list of limits. You can choose to set an alarm condition if a temperature is exceeded, either high or low, if a wind speed is exceeded, or if a rain amount is exceeded.
Another APRS weather function is to display a wind streamline. Press W (Weather) D (Display) W (Wind Flow), and APRS will interpolate a 21 x 13 grid of wind directions and speeds based on observations received. This function isn't too practical unless you had a significant number of stations reporting to make interpolation more accurate. With a sizable reporting network, you could use this function to see the wind shifts associated with the passing of a front.
Two other weather functions, beyond the scope of this article, include the capability of plotting a tornado/severe thunderstorm watch box and the ability to fill a county with color to indicate a weather warning. For more information on these, consult the F1-F help files under WX.TXT.
Now, let's move beyond weather. There are several other data parameters you can transmit, including the location of objects and other information about your station. The routines for data entry are similar, and they all begin with the keystroke I (Input). In issue #2, we talked about inputting your latitude and longitude so that your station would plot on a map. This is done by pressing I (Input) M (My Data) P (Position). Generic station information is entered through I, M, S (Status). An estimate of your station's simplex coverage can be made by pressing I (Input) P (Power-Height-Gain). Follow the prompts, being careful to estimate your height above average terrain properly, and APRS will draw a circle around your station plot and also make that information available to other stations monitoring.
You can plot objects on your map and they will display on other stations'
ARPS maps as well. Say you wanted to make others aware of the location
of the annual Oshkosh Fly-In. With the map displayed, zoom in on the Oshkosh
area and place the cursor over the desired site. Then press I (Input)
(Add Object). You'll be prompted for latitude, but there will be a default
entry for your cursor location. Press <ENTER> to accept the default.
Do the same with longitude. Now you're asked for an object name or callsign.
You could enter "FLY-IN" and press <ENTER>. Next is a
list of available symbols to plot. Press the first letter of the symbol.
In this case, enter "A" and a new list of icons beginning with "A"
appears. Press "2" for "Airplane." The next prompt is for course
and speed. Enter numbers here for moving objects or zero for stationary
objects. You can then enter comments text if desired. Finally, you'll need
to enter a date and time stamp. This information is needed if the object
is moving and other stations wish to "dead-reckon" its future position
based on course and speed. Fortunately, a default entry is presented which
is current time. Press <ENTER> to accept the time. Then you have
a prompt asking if all your information is accurate. Press "Y" to
accept it, or "N" to start over if you made a mistake. APRS will
now plot the symbol on your map and transmit a packet so it will plot on
other users' maps.
You can experiment with the "dead-reckoning" function to estimate future positions by pressing C (Controls) D (Dead-Reckoning). To see if you have it enabled, press the <TAB> key. A small status window will display at the bottom of the screen. On the bottom line you'll see either "DR" which means it is enabled or "dr" which means disabled. If you add a moving object to the screen, APRS will assume that its course and speed are constant, and when you press the space bar to redraw the map, APRS will update its estimate of the object's current position. I've used this in SKYWARN applications, plotting information from NOAA weather radio, such as "At 2:55, National Weather Service doppler radar indicated a severe thunderstorm over Baraboo, moving northeast at 35 miles per hour." Try it.
A side note: remember, you don't need to have a packet station on the air to experiment with the APRS software. In fact, while you're learing the various functions, being off the air helps to keep the frequency clear of your "experiments!" Once at an Emergency Government demonstration, I showed someone how I could plot an ambulance travelling westbound on I-94 toward Madison at 75 mph. I forgot that I actually sent this position packet over the air. When I got home later that day and looked at my home display, the ambulance icon was over Nebraska, still heading west!
Next month we'll discuss using APRS for direction-finding, using the data entry techniques learned this time. There are three different methods APRS uses for DF-ing, and two of them can use data from stations using non-directional antennas! How can that be? Stay tuned.
A summary of APRS links to download software, in case you missed them earlier:
Latest APRS DOS version - ftp://ftp.tapr.org/aprssig/dosstuff/APRSdos/aprs8xx.zip
Extra maps for Wisconsin - ftp://ftp.tapr.org/aprssig/maps/PCMaps/WIMAPS03.ZIP
(Use PKUNZIP.EXE -d to unzip both files.)
APRS for Windows - ftp://msproul.rutgers.edu:/pub/hamradio/APRS/WinAPRS
MacAPRS - ftp://msproul.rutgers.edu:/pub/hamradio/APRS/MacAPRS
(Remember that this APRS series highlights the DOS version. Windows/Mac versions have different features!)
See you on 144.39!
This page originally appeared in August 1999 Badger
State Smoke Signals but was updated 2000 June 14.
Copyright 1999, 2000, Thomas C. Weeden, WJ9H