APRS & Metro Skywarn:WX-net Possibilities
by Donn Baker, WA2VOI donn.baker@unisys.com
Summary
Simply put, there is NEVER enough weather (WX) data. There are always too few WX stations with many miles between them.
This is a problem when storms can be as small as a few 100 yards wide. The Skywarn program helps make up for the lack
of WX stations by using spotters; BUT spotters are not always available; they may not be where the storms are; and
aren't generally available at night. In fact, in the Metro Skywarn training classes, we discourage leaving home to
spot at night for obvious safety reasons.
Some of these problems could be solved with additional remote, automatic WX stations throughout east-central Minnesota
and west-central Wisconsin. This network would "fill-in the gaps" between WX stations, and supplement Skywarn spotters;
it can't replace a core of well-trained Skywarn spotters.
WX station network
The WX-net would be implemented using Automatic Position Reporting System (APRS) packet radio and commercially available
"home" weather stations. While these inexpensive automatic WX stations don't provide the same quality of data that a
professional station can, they do provide wind speed and direction, air temperature, and possibly relative humidity
and rain fall.
Once available, the data needs to be reported to the Weather Service office and throughout the entire metro area.
Setting up a network from scratch to support to do this is a daunting expensive task. Fortunately, amateur radio
already provides the basic pieces necessary for such a network with APRS or Automatic Position Reporting System.
APRS
The basis for APRS is geography; or location and position. Although originally designed to report vehicle positions,
we want to use APRS for automatic WX reports and possibly storm tracking. The system is real-time, and automatic,
which makes it ideal for WX reports.
While APRS is similar to packet, there are some important differences between it and general packet operation. First,
APRS has special protocols within AX-25. It uses Unnumbered Information (UI) frames which broadcast information
rather than establishing a connection as does packet. This means that there are no ACKs (acknowledgments) when
information is received. Assurance of delivery is by repetition of the data, either by rebroadcast or by hearing
the data from several sources in a network. This makes network reliability very important.
Second, APRS is NOT intended for conversations... the information content is the position of an object, or, in
our case, WX report data, not "talk." There is a provision for passing bulletins to stations; or talking to one
or more stations, but its really not set up to do that. Use regular packet for conversations.
Which brings up the third major difference between APRS and regular packet: APRS does NOT coexist with normal
packet conversations on the same frequencies... it simply can't run on the same frequency with connected protocol
packet services. Mutual interference merely disrupts standard packet, but destroys the data flow in APRS. An APRS
network requires its own frequency.
Twin Cities APRS network
Currently, there is a small Twin Cities APRS network consisting of maybe a dozen stations on a more or less regular
basis. Only two of these currently are WX stations: WA2VOI in St. Anthony and WBØNKX in Faribault. These two
stations operate 7 days by 24 hours (7 x 24). At least two other WX stations are, or have been in the network
from time to time, but there is no regularity.
There are several mobile stations that are very regular; KFØZH-9 between Shakopee and Owatanna: WRØP-9 and
N5NTE-9 in the Cambridge area: and KBØXR in the Plymouth area. An unknown number of stations monitor the APRS network.
WX Servers
While APRS is the basis for the infrastructure of a WX net, certain pieces are missing, which I'll call WX servers.
These specialized WX Servers provide wide-area digipeaters so that data collected in one place can be seen throughout
the area covered be the network.
The best way to picture these WX Servers is to think of handi-talkies and repeaters. Handi-talkies are great; everyone
has one. But they have limited range. To become truly useful, FM repeaters are necessary. The WX Servers are the
"repeaters" for the APRS WX-net! The network needs to be reliable, and that means 7 x 24 operation. The best way
to provide the needed reliability is to sponsor and place a few specialized nodes, which then form the backbone of
the WX net. Each WX Server also supports an automatic WX station. The network thus has a minimum number of WX
stations... which need to be supplemented with additional WX stations.
WX Servers allow participation by all Amateurs &the community. Once a WX Server is located in an area, any amateur
can run APRS and put up their own WX station with a minimum of equipment. It doesn't have to operate 7 X 24, nor does
it have to be a big, expensive radio since it only has to transmit to and receive from the nearest WX Server.
Individual hams can participate in the WX net whether or not they are in Skywarn. APRS stations can provide services to
the WX-net without a having a WX station... if they operate on a full time basis (they leave the radio and TNC on,
whether or not they are actually "using" APRS software) they fill-in the network. This provides additional paths for
the data to pass through, helping to ensure that all stations in the network see the WX data.
We will need to get stations in those places where we don't have spotters and along the "front lines" where the storms
approach. Unfortunately, many of these places are far from the established APRS network. We'll have to build up the
WX-net as we go. This isn't as bad as it sounds as it can as simple as a properly placed WX Server, or a small beam
antenna pointed in the right direction at an existing WX Server.
APRS WX Stations
Several companies make reasonable priced WX stations for the "home" market. APRS supports those made by Peet Brothers
and Davis Instruments. Peet Brothers has two models: the Ultimeter 500, priced about $250, and the Ultimeter 2000,
priced about ~ $380. The optional relative humidity sensor is another ~ $120, and the optional Rain gauge is ~ $100.
Davis Instruments' Weather Monitor II is ~ $395, with relative humidity sensor an option at ~ $125, and a rain gauge
for ~$75. To work with APRS, the Weather Monitor II requires Davis' WeatherLink software and cable, costing $165.
OK, Let's Do It !
The Metro Skywarn Board of Directors decided at the December 1997 Board meeting to support the WX-network in the Twin
Cities. A "Steering Committee" was established by the Board to oversee the planning and implementation of the WX-net.
The current plan calls for determining just where WX Servers need to be by March 1998. At the same time, we're trying
to locate trustees, such as ham clubs or individuals, to license stations and sponsors willing to donate money or
equipment for building and installing the WX Servers.
To guide the overall network development, a set of Technical Standards will be published by April. These standards
will cover TNC setup, the use of aliases, UNPROTO-path set up, etc. to provide the correct flow of WX data through
the network. A major need is to locate sites and obtain usage agreements for the WX Servers. We'd like to do this
for at least two or three critical need WX Servers so they can be operational by May. We have set a goal of expansion
of WX Servers and WX Stations outside of the Metro area by Spring 2000.
APRS Information
Bob Bruninga, WB4APR, is the original author of APRS. He maintains complete info for the DOS-versions at
http://web.usna.navy.mil/~bruninga/aprs.html
TAPR (Tucson Amateur Packet Radio) organization provides a central location for almost anything packet related,
including APRS.
Their general homepage is http://www.tapr.org/
APRS software download is http://www.tapr.org/tapr.html/sigf.html
Getting On Track with APRS," by Stan Horzepa, WA1LOU who conducts the packet column in QST. The book is available
from the ARRLhttp://www.arrl.org/)or from Radio City here in town.
APRS Frequencies
Most APRS activity is on VHF (i.e., 2m). 145.79 MHz, 1200 baud is used in much of US; with 144.39 MHz, 1200 baud
being used in Canada. There is a move afoot to move the US operation to 144.39 also. There are currently no
standard frequencies established for 223 MHz or 440 MHz. The current Twin Cities APRS network runs on 145.79 MHz, 1200 baud.
Radios for APRS
Almost any FM radio will work for APRS, and usually no modifications are necessary. Single channel crystal-controlled
radios work just fine, as APRS is single frequency operation. Power levels for APRS radios are surprisingly low...
10 watts seems like a good choice, especially in a well planned network. Network coverage has to be improved by
adding stations and digipeaters, NOT by increasing the power output, or using high-gain antennas. (Think of the
FM voice repeater network. Handi-talkies don't increase power or add big antennas... they talk to repeaters.
Improving an APRS network is easier than improving FM voice: only one frequency, and added stations increase
both reliability and expand coverage.)
TNCs
For home stations, almost any TNC will work for APRS, but the use of "APRS" TNCs is encouraged. "APRS" TNCs include
firmware that directly supports APRS with several special commands, unique to UI (Unnumbered Information) frames.
There are several on the market, and most manufacturers also have available firmware upgrades for older TNCs to
provide the APRS command support. These TNCs are no more expensive than "regular" TNCs (~ $150).
Computers & software for APRS
A computer is needed to display data from the APRS network. Generally, an IBM-type system, with a 386 processor is considered the minimum , but a 286 or even 8088-based system will "work." The APRS software is available in DOS and Windows-based versions. There is a separate Macintosh version. APRS software is "freeware," but registration has advantages, mostly in convenience of operation. The cost is minimal, and can be reduced further if your club registers several copies at a time.
