SSTV & Spotters - Mississippi and Alabama
In North Mississippi and Alabama we have been experimenting with the transmission of Doppler radar images. The purpose
is to give the storm spotters as much information about the area in which they are located as possible. However, due
to political and other problems, we are not using this method on a full time basis. We are mixing the Doppler images
along with the RCM pictures, depending on the availability of the Doppler drop site.
The hardware setup is fairly simple and low cost. The Doppler drop site is located in the Emergency Management building
which is not open full-time. The Doppler images which are on the PC show no ID or copyright images(ALRAD) which would
prevent their use via ham radio. The various radar images are copied to disk using a screen capture program (Screen
Thief) to convert the image to a compatible format. The average file size runs about 110K bytes for TIFF format.
I use TIFF because that is the native form used by the transmission program (JVFAX). Since none of the programs used
are Windows compatible, a second PC is used for the transmission. The TIFF file goes directly to a floppy disk, which
is carried to the second PC. The transmission is in a usable audio format which is compatible with any form of ham
radio. We use any available ham radio which uses the same frequency that the spotters use. This is mainly two meters,
but 70cm or 80 meters can be used also. This makes the radar images readily available for either the net control
operator during quiet periods, or a separate repeater can be specified, as needed. Normally a second operator is needed
to run the transmissions, but the net control can do it also. The schedule for radar images, if used on the net
frequency, depends on the current need by the spotters. During maximum storm activity, events may happen too quickly to
schedule a transmission. For this reason I am looking for an automated transmission system, preferably using macros
under Windows 95. The end product should be on a dedicated repeater, specified as needed by the spotter distance.
For the spotters in the field, a portable lap-top computer is preferred due to elbow room in the vehicle, and VGA
display. The decoder can be any SSTV compatible system such as the MFJ-1278, but these types are too bulky for field
use. The JVFAX module is my preference, since the serial interface actually fits inside a RS-232 head shell. The radio
audio is fed from the external speaker jack to the interface. Images take two minutes to transmit, well under the
three minute repeater time-out. An audio split is easily made using a speaker mike. This way, the radio is immediately
available prior to, and after the SSTV transmission. If the hardware is available, transmission back to the net control
can be made. For this, a camera and digitizer are needed for the image capture to the computer. A Polaroid and a hand
scanner also makes a usable system. This way, any storm damage or other images can be available almost immediately to
the net control, or even news, fire, police and other agencies with a scanner.
The reason this system came about was a common problem with the wx net control. Generally, three or more teams are sent
out by the local SKYWARN group. The net control has a tendency to concentrate his information to only one of the
groups, and the other teams are practically forgotten. The other teams have to almost beg to get any information about
where they need to be positioned. Also, the information available to any of the teams depends entirely on the net
control's ability to read a radar image, or relay, and to convey it to the spotters. By using the radar image
transmission, all teams get the same information, and can make their own decisions about where they should be. Less
time is required by the control operator to provide go here/go there type information. This time can be used for
exchanging other types of information, such as damage reports, and FEMA traffic. The spotters only need to check in with
reports, instead of waiting in line to get a positioning schedule. For a future upgrade, I would like to see the
spotters using GPS locations for the net control to keep up with where they are.
The Doppler transmission system is in its infancy, and with further compression techniques, the time to transmit
should be less and less. With DSP capabilities, the transmission of packet should be compatible along with voice
transmissions, much like the telephone variety. For instance, using two subcarriers of 100 and 200 cycles would
provide the mark and space needed for digital transmission, without interference to the voice channel. Using a very
short packet length would just about prevent any extended key up time to the voice channel, and no apparent change in
voice operations would be noticed. I am just a minor user of the system, so I know there could be other, better, ways
of conducting a weather net. Many systems will evolve over the next few years, so stay tuned!
Tom, AA5MT@gate.kc5aug.ampr.org or aa5mt@kc5aug.ms.usa.noam
