Amateur Radio Packet Systems and Skywarn
Carbondale, IL
Packet radio has some advantages and disadvantages. In my case, I found doing a central RTTY broadcast from a
high-power station would better serve my users. I wrote a program that captures and disseminates selected NWS
products based on product type and county codes. The system transmits the products on 146.565 MHz at 300 baud
using ASCII characters and standard Bell 103 tones (the kind a slow phone modem uses).
A standard PC modem (like a Hayes modem) will work. Actually, the old dumb 300 baud modems work the best. When you use
Hayes-compatible modems you need to send a little initialization string that tells the modem not to freak out when the
carrier is lost (i.e.. the end of the transmission). To receive the data you need a receiver (an old scanner or single
channel radio), a cable from the earphone jack of the radio connected to the red and green leads of the phone cable
that plugs into the modem, and a PC. The PC can be running the program I created (the same one that is used to capture
and transmit the data) or you can just run a dumb terminal program if you just want to watch the data scroll by).
Now, if I were building up this system from scratch, and say in a large metro area, I might consider packet or at
least a faster ASCII mode such as Bell 202 (1200 bps). The speed (300, 1200, 9600) really depends on the volume of
traffic you will be sending and on the length of the products. For instance, if it takes 3 minutes for a warning to
be transmitted (like at 60 speed baudot RTTY code) the warning is being "artificially" delayed. However, if it takes
12 seconds to transmit it at 1200 baud vs. 30 seconds at 300 baud the increased speed (1200) might not be worth the
expense of people having to purchase specialized equipment to receive the signal. However, if you plan to do a grand
system, it might be worthwhile purchasing specialized hardware like the BabyPac 1200 baud Bell 202 modem. The BabyPac
costs something like $35 and is a little Bell 202 modem built into a DB-25 connector housing. The BabyPac is a simple,
cost-effective way of getting 1200 baud data over the air and it's not totally proprietary since many TNCs will allow
you to receive straight ASCII (as opposed to AX25) data.
If enough people are interested, see if you can pool your resources/money and set up your own Contel NWWS satellite
receiver. The cost is probably prohibitive for a single person (unless you make a lot of money and don't have kids)
but shared between people (like a SKYWARN club) the fee would be reasonable. You than have the entire public products
feed at your disposal and can tell your software to automatically transmit whatever you want. You can even add a
dial-up modem to the system and let users call up the capture PC and download products that are not routinely
transmitted over the air (like agricultural bulletins for example).
For now, you might take advantage of the WSO and their AFOS system. In St. Louis, the closest city with civilization
to us, the hams are using software developed by a ham at the NWS in Indianapolis that takes information sent to the
AFOS local printer and saves it on a PC --for dissemination via packet. I think in the St. Louis situation, and don't
quote me on this because it's been a while since I've been up there, people have to request the products --unlike my
system that transmits them automatically.
The advantage of this system over packet is that just about everyone can receive it (without having to purchase a TNC)
and the data automatically comes to the user and can trigger alarms on the user's PC --as opposed to the user having to
manually go after the data. I invested about $800 in a transmitter, power supply, and repeater-grade 120W amplifier.
There is no sophisticated network to manage --just one transmitter. There are probably some equally good reasons for
people to use packet for such an application but in *my* case a direct broadcast system made the most sense.
Chris Hayes Novy - WA9V Internet: Chris@SIU.Edu Southern Illinois Univ.
BITNET: Chris@siucvmb Morris Library Phone: (618) 453-1683 (work)
Systems Administration (618) 457-6149 (home) FAX: (618) 453-3440
Carbondale, IL 62901-6632
