Using APRS in Weather and SKYWARN Applications

OVERVIEW: APRS is an ideal tool for reporting weather conditions via packet. It is compatible with both human entry as well as automatic weather station entry of current conditions. With the optional $14 WX registration, APRS will accept serial data from the PEET Bros ULTIMETER or DAVIS home weather stations and automatically insert your wind, temperature and optionally rain, humidity and barometric pressure into your position/weather packet. You can even mount an ULTIMETER remotely with only a TNC and Radio, and have it periodically report the remote weather conditions. Use the MAPS-OVERLAY-OTHER command to overlay the WX-APRS.POS file on the map to see all known APRS WX stations..

APRS is idealy suited for SKYWARN with the following features:

In addition to weather data included in position reports, the station STATUS packet is also available for broadcasting additional info. These STATUS packets can be reviewed on the LATEST display. The APRS BULLETIN feature is very powerful as an alerting mechanism to all stations on the net. Even non-packet voice stations making a weather report can be placed on the map (like an object) by another APRS packet operator. His station will appear similar to any other APRS weather station, except that his report will include a marker indicating that he was manually placed on the map by another operator.


Based on work by KD4UYR in Clearwater FL, the MAPS-OVERLAY-NWS command searches the NWSposns.POS file and temporarily displays all NWS stations within the boundaries of the current APRS map display (up to 250 miles). You are given the option of a one-time map overlay or to add the sites to your P-LIST.

The WX-GET-METAR command is used for loading a file of NWS METAR reports. This file can be obtained from most of the commercial data servers or some BBS's, or on the internet at APRS will scan the NWS METAR file looking for each station that you have on your APRS P-LIST. For each match, it will parse the Winds, temperature, dew point and barometric pressure onto the P-list. It takes the remainder of the report and places it on the L-list. The time of the entries in the P and L-list will be the time of the NWS report (in local time). The date is assumed to be the current date.

Once the data is loaded, you can uplink selected stations to the APRS net as desired. Be considerate, though, and not overload the channel. Once everyone on the net sees the reports, the uplinking station should consider Quitting the uplink in order to reduce QRM.

DEMONSTRATION: To see how APRS works in a SKYWARN or other reporting net, load the backup file SKYWARN.BK using the FILES-LOAD command. To demonstrate the NWS capability, center your map on Illinois at the 128 mile range, enter the MAP-OVERLAY-NWS command and select the Add option to add them to your P-List. Next, enter the WX-GETmetar command to load the sample METAR data file. Look at the P and L-lists to see what is going on...

METRIC MEASUREMENTS: You may select wind speed in KPH and temperature in Celsius by toggling the WX-METRIC command. This will force the WX display at the top of the screen to display metric and will cause the manual WX entry values to be accepted in Metric. You can make this permanent, by saving a CONFIG file. The on-air APRS WX protocols, however, still will be in MPH and F. Also, the ALARMS are compared to MPH and F values.

WX ALARMS: If a WX report comes in that exceeds your alarm values, the station is plotted in RED and an ALARM sounds. The alarms are WIND, TEMP, and RAIN. These settings can be used to warn of the passing of a warm or a cold front. Use APRS and these alarms to make an almost un-attended SKYWARN network! Install an APRS computer at the NWS site, and let the NWS operators set the alarm levels. Then without transmitting a single packet (there is rarely a licensed HAM on the premises) the APRS screen will show SKYWARN data and where the alarm conditions are!

CLEARING ALARMS: When an alarm occurs, the station is drawn in RED and the map is re-drawn to center on that station. Also, the station is marked on the P-list with the ALARM marker. TO clear the alarm, simply hook the station on the map, and then UNhook it (hit ENTER twice), or use the WX-ALARMS-CLEAR command. To clear the A on the P-list, hook the station and hit the A key. Normally, the ALARM on the P-list will only sound if that station MOVES. Since a WX station should not move, there is really no problem in leaving the A on the Plist until convenient to remove it.

WEATHER WARNINGS AND WATCH BOXES: You can use the OBJ-AREA-LINE command to draw a box plus-or-minus X miles from a line between points A and B. The line and width are perfect for WATCH and WARNING boxes. If baseline lengths longer than 400 miles are needed you must combine two or more boxes. Use the normal OBJ-AREA-LINE command and indicate the +/- offset of the sidelines in a 3 digit number of Miles enclosed in braces 75 anywhere in the COMMENTS field.

HOME WEATHER STATION INTERFACE: With the optional $14 WX registration, APRS inserts the ULTIMETER or DAVIS conditions in your position report for unattended weather reporting. ULTIMETERS that output data with a * output wind in MPH and those with a # in KPH. APRS detects the difference and does the conversion. Also APRS averages the wind value over the last 4 samples. Thats why the display UPDATE period is set to 15 seconds, so that the resulting wind speed transmitted represents a 1 minute average.

RAIN VALUES: Each WX instrument has one or more rain counters which are incremented every 0.1 or 0.01 inch. APRS asks you which gage you have and converts both to 0.01 inches on the air. Since rain counts have no meaning without knowing when they were last reset, APRS only compares the difference in rain for the last 60 minutes (rXXX) and the last 24 hours (pXXX) (P for Precipitation). A reading of r123 means 1.23 inches fell in the last hour. After 60 minutes, this value will return to zero. A reading of p164 means 1.64 inches have fallen since the same hour yesterday. For REMOTE ULTIMETERS where there is only a TNC and no PC, the P value means rain since MIDNIGHT and an uppercase Pxxx is used. Also since there is no RAIN per hour value, that field is replaced with the raw rain counter #xxxx.

FORMATS: The following formats are the way that APRS re-formats the data from the attached WX unit and transmits the data on the air. For remote Ultimters, where there is no PC, and the TNC is just transmitting the raw Ultimeter format, then these formats represent the way that APRS parses and displays the received data on the P-LIST.

   _CSE/SPDtXXXrXXXpXXXgXXXdU-II        Original U-II auto
   _CSE/SPDtXXX#XXXU2r                  Original U-II remote
   _CSE/SPDtXXXrXXXpXXXbXXXXhXXgXXXdDvs  Davis WX station
   _CSE/SPDtXXXrXXXpXXXbXXXXhXXgXXXdU2k  U-500/2000 with APRS
   _CSE/SPDtXXXPXXXbXXXXhXXgXXXU2kr      Remote Ultimeter Logger
   _CSE/SPDtXXX#XXXbXXXXhXXgXXXUpkm      Remote Ultimeter Pkt Mode


Where: CSE/SPD is wind direction and sustained 1 minute speed
t is in degrees F
r is Rain per last 60 minutes
p is precipitation per last 24 hours (sliding 24 hour window)
P is precip per last 24 hours since midnight
b is Baro in tenths of a mb
h is humidity in percent. 00=100
g is Gust (peak winds in last 5 minutes)
# is the raw rain counter for remote WX stations
See notes on remotes below
% shows software type d=Dos, m=Mac, w=Win, etc
type shows type of WX instrument

With the WX interface enabled, the normal APRS decaying of position/wx reports still holds as long as nothing changes (except the wind). This way, redundant WX reports do not cloud the channel. If any value, Position, temperature, rain, barometric pressure, or comments change, then the periodicity is reset to the value set in the alt-SETUP-POSrate command. Since the wind is the most changing parameter, the WX-WIND command can be used to set a Wind threshold. Whenever the wind is above that threshold, the reporting period is also reset to the POSrate value. Nominally, you might want to set the POSrate value to about 4 minutes or so. This way, during changing WX conditions, or wind above your threshold, the WX is reported every 4 minutes. Otherwise, the period rapidly decays back to the maximum APRS delay period (default is 20 minutes). For a weather station, you might want to change this MaxPeriod to maybe 10 minutes or so. Use a text editor to change this value in your CFIGxxx.APR file. There it is expressed in seconds. Of course, a WX/position report will be transmitted by the APRS station at anytime, in response to an APRS or WX query.

ULTIMETER INTERFACE CABLE: Peet Bros sells an interface cable with a DB-25 connector to RJ-11. (Note, this is a DB-25, not a DB-9). Since the U-II has no negative supply, a 5 K pulldown resistor is added to the DB-25 connector between Pins 2 and 3. This uses the -5 or -12 volts on your TXD line to bias the RXD line. When I plugged in a standard phone line connector to the U-II serial data output (unmarked connector on their junction box, or side of the Display unit), I found the data output on the green while using the black wire as ground. If your connector was crimped on backwards, red is data and yellow is ground. The Peet Bros cable includes transient protection and RF filtering to protect your computer as well as providing the bias resistor. Remember, the annemometer is like an antenna, high in the air. It can radiate RFI and take lightening charges... The U-2000 is directly RS-232 and needs no pull down resistor. Be sure the U-500/2000 is in DATA-LOGGER mode.

DAVIS INTERFACE: You must have the DAVIS Weather Link serial interface option before your Davis station will output serial data. Simply connect their connector to your PC. You MUST use the APRS alt-SETUP-OTHER-AGL command to set your average ground level. This converts your baramoter readings to their equivalent value at sea level... Your objective is to find an exact AGL that will result in your BARO readings matching the readings on your instrument or local NWS. THis AGL will probably NOT be your true AGL.

SAVE A COMM PORT! You can use a simple 89 cent switch to allow other devices to share a single COMM port with your WX station. Since the WX stations only use ONE pin and GROUND, simply add them to your MODEM cable with a simple SPDT switch as follows:

       ULTIMETER-II   >---------*--------*  sw1
                                |        <----*--------------> RXD
       MODEM   }      >------------------*
         or    }                |
       other   }                *-----//\//\-------*
       serial  }                        5.6k         |
       device  }      <------------------------------*-------< TXD
               }      * * * -----------------------------* * * other pins
               }      *--------------------------------------* GND

Actually, you can wire this to any serial device that you do NOT use while running APRS. I chose my MODEM that is on COM2. Note the series resistor as mentioned in the text to provide the -V pull-down for the U-II output.

REMOTE ULTIMETER OPERATION: Only the Ultimeters from Peet Bros can be used as remote WX stations (using only the WX station, a TNC and a radio). This is because they output continuous data withing needing a PC to interrogate it periodicaly. There are two ways to time the transmission of this data for the packet channel depeneding on what Ultimeter you are using. We call these PACKET mode or DATA LOGGER mode:


U-II - These units output 14 bytes continuously beginnning with * or #
U-2000 (old) - These units output 44,48,or 52 bytes beginning with !!

For the DATA LOGGER mode, the TNC must do the scheduling of packets just like the TNC does for continuous GPS data. That is why this mode is often called the GPS mode even though GPS has nothing to do withe Weather. You must program your TNC to think that the WX data is GPS data. Simply program the stand-alone-TNC to recognize the #, * or !! instead of the usual $GPRMC data typically used for GPS. Then set the timing for each transmission to be about once every 5 minutes. Be sure to SET ECHO off if your ULTIMETER-II uses a resistor on the TXD line to bias the RXD line! (Otherwise you will echo all heard packets back onto the channel! KD4GOE figured that one out!).


U-500 - Outputs 44,48,or 52 bytes beginning with $ULTW

This mode was designed for APRS packet since the WX unit only outputs a single data line every 5 minutes. Since it is already timed to once every 5 minutes, all the TNC has to do is be in UI CONVERSE mode and it will send the packet. Be sure to use a TNC that can be configured to power-up in UI-CONVERSE. Try UIMODE or PMode-CONV depending on your TNC. ALso you must set LFIgnore to ON so that the LF after the CR is not transmitted.. Since the Kantronics TNC's do not have LFIGNORE, you must set SENDPAC to $0A (linefeed) and set CR off! If there are dozens of REMOTE U-500's, hopefully their clocks are staggered enough that they wont all transmit at EXACTLY the same time.

CAUTION: Only U-500's and only U-2000's manufactured after April 97 should be used in packet mode. U-2000's prior to that date will not show wind in the packet mode.

WX POSITION: In both cases, you must put the unit's POSIT in the BText using the "_" WX symbol. This way the TNC transmits two packets, one is a BText WX posit with no WX data, and the other is raw WX data with no posit! APRS cannot display the WX on the map until it has received a WX posit beacon. So, set the beacon to go about once every 20 minutes. Once the posit is received it will be displayed as a Blue dot unless the site is also acting as a WIDE-RELAY digipeater, and it will show GREEN if the delimiter between the LAT/LONG is set to a backslash instead of the usual forward slash.

WX LOGGING: Use the WX-LOG command to turn WX-LOGging on and off. It will show on the control panel when on. If ON, then on-air reports are saved when received every 10 minutes or so, and your own report will be saved at the rate set by the Set-Pos-Rate command.

AUTOWX.EXE: This is a program written by Les, N5KOA to also interface the Ultimeter to a TNC with a PC and send APRS data. I have not used nor tried this program. Contact Les Blalock, N5KOA, 1711 Pagewood, Odessa, TX 79761. (915) 332-7112 day, 332-1942 fax, 367-0408 eve or on the Internet

DIRECTION FINDING: Since your station can only send one position report, If you use the BEAMHEADING command to enter a beam heading on a Fox or Jammer, then your WX station is disabled. To restore your WX station, restart the program.

1995 prices:

The ULTIMETER-II is available from PEET Bros 1-800-USA-PEET (872-7338) for $199 plus $20 for their Serial interface cable and $68 for the optional rain guage (self emptying, 0.1 inches) plus shipping of $8.75. (For APRS, their $69 software is not required but it is a good package for doing the usual home WX logging and analysis) See ads in QST, CQ, Popular Mechanics, etc.

The ULTIMETER-2000 is also available for $379 and includes Barometric pressure. Outside Humidity/Temp sensor is $110.

The higher sensitivity .01 rain gauge is $90. And a wireless remote rain gauge is $139. afs

The ULTIMETER-500 $239. is the same as the U-2000 but without the barometer and humidity.

The DAVIS Weather Monitor II is available for about $395. Rain gage is $75, and outdoor Humidity/Temp sensor is $125. For APRS you will also need their WeatherLink for $165.

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