Receiving APT signals (Automatic Picture Transmission) is not necessarily complicated. Still, some anterior knowledge comes in handy, particularly to avoid desillusions later. On the following pages, I'll try and describe some of the problems that appear in each part:
A key element is of course the receiver. Below, I'll discuss some of the options available:
With the advent of wide range VHF equipment (particularly of the 'handie'-type), many people entertain the illusion of receiving satellites. Many times, the desillusion afterwards is at least as big. Why? Because the signal transmitted by the weather satellites is not really compatible with the handies. The bandwidth of the birds is too wide for modern equipment. The resulting signal is quite noticeably distorted.
Even though the image may be acceptable (most of the information lost is in the clouded part), another part of the image is used to tell the decoding software how to process (synchronize) the image. It's just this part that is most distorted by the lack of bandwidth. The software just fails to find the image in the audio received (even though some of the software promises to compensate for the loss).
On the web, and in some publications, exist procedures on how to modify the equipment to get acceptable reception. If you decide to dedicate the receiver to satellite operation, this can be a valid solution, but if not, consider the following:
Ah well, if communications equipment is too narrow, surely broadcast FM receivers are wide enough! And surely, the discriminator is wide enough to receive the signal without linearity problems. Here another problem pops up: By using a receiver as wide as an FM receiver (150 kHz), you are inviting other signals to come in at the same time.
Even if there is no interference, you a getting much more noise into the receiver than in a special receiver, and this reduces the sensitivity of the receiver in a way you cannot receive the weak satellite signal in an acceptable way. Compare the power of the satellite with a flashlight at 800 km!
And, now there is actually a lot of interference! A constellation of some 30+ satellites is using nearly the same frequencies as the metsats (OrbComm satellites), and interfere very heavily!
Again, it is possible to modify the receiver, but keep in mind that those things are designed for low-cost commercial work: 20,000 watt transmitters at short range.
A very tempting solution is the large offer for 'scanners', that cover la huge range of frequencies - some up to a few gigahertz. This is a very valid option, and can offer you monitoring other frequencies that are of interest.
The only problem here is not buying the cheapest: Many of these scanners only offer wideband and narrowband filters. Only scanners that offer and intermediate filter (around 50 kHz) are of use and are in fact nearly ideal.
On the market exist a large variety of receivers specially designed for APT reception. These are not as cheap as the above options, but are generally well designed for the job. Mind: quite a few of them do not have adequate filters to eliminate the latest OrbComm interference! Look out...
The majority of these receivers have fixed channels, on the most popular frequencies: 137.3, 137.4, 137.5, 137.62 and 137.85 MHz. Some of them can be remotely controlled.
Even these receivers may have some problems - I had to modify the discriminator of one of the more known brands to eliminate a linearity problem and get a good reception during the complete pass. Still, these receivers are generally a safe bet.
Again, there are quite a few sources for kits. It's a very nice solution, as those kits contain all components necessary for a successfull construction. Some of those may be hard to obtain separately. The construction process is relatively foolproof, and problems are generally limited to after-construction calibration, which is a common problem to most homebrew work.
The last possibility, is to completely homemake the receiver. You can either get one of the several circuits available on the web, or even design your own. This has the advantage of the learning experience, and the satisfaction of doing it. Without some measuring equipment, and some experience, I would advise against it, though.
|(c) John Coppens ON6JC/LW3HAZ|