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Meteorological satellites - Receiving

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:

The receiver

A key element is of course the receiver. Below, I'll discuss some of the options available:

Modification of commercial communication equipment
Modification of wideband FM receivers
'Scanners'
Decicated equipment
Homemade equipment

Modification of commercial communication equipment

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:

Normal use of the receiver
Modifying the receiver filter - some articles advocate removing and replacing it with a capacitor, the satellite signal will pass much better, but for normal terrestrial communications will loose selectivity and you will not be able to discriminate between close spaced local signals.
Discriminator
The satellite signal, after passing through the filter, arrives at the part of the circuit where it will be converted to audio signals. This discriminator is designed to handle comm signals - narrowband. No garantee can be made on the linearity needed to get a good grayscale image
The modification itself
Don't be fooled: modern equipment is very difficult to work on. The miniaturization makes it difficult to remove components without damaging the circuit, without the proper equipment to do so.

Modification of commercial FM broadcast receivers

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.

'Scanner' receivers

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.

Dedicated receivers

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.

Homebrew equipment and kits

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.


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