On Mon, Sep 28, 2009 at 7:04 AM, William Leijenaar [email protected] wrote:

Hello AMSATs,

I agree that APRS can have a higher power transmittter, because of its small amount of time to get the data broadcasted. However the workability with the HT and whipe antenna is only an advantage for the downlink. For the uplink there will be no advantage.

Personally I believe that making a satellite easier accessable will also decrease its functional efficiency. This is especially the case for satellites with limited user access, like single channel FM satellites. This also includes APRS. The same issue is valide for a geostationary satellite, with the addition that a geostationary satellite would be overloaded in short time by more and more stronger stations as fixed antennas can be used. Only the uplink power level would be the parameter of "competition". Resulting in a privat chat satellite for only few (high power) users.

Satellites for emergency communication sounds very interresting, and personally I believe it can give a great advantage when doing it well. The question is only what will work well in an emergency situation. It will depend on the needs and the availability of equipment in the effected area. I believe that digital communication will be of limited use as you need also a computer, modem, keyboard, screen, software etc besides your radio. Only one thing missing and you will not be "heared". I don't say it is impossible, but to make the system work the ground stations need to be made more easier somehow. Maybe HT APRS in combination with voice to text conversion (and opposite) would be an idea ?

In some cases, the groundstation situation might not be as complicated as you describe. The popular TH-D7A Kenwood HT, though no longer manufactured, has integrated APRS messaging with a TNC on board. So, too, does Yaesu's new VX-8R. And there are mobile 2m radios with similar capabilities. Moreover, if there were a wide satellite network of APRS birds, we could hope that even more new HTs would include this feature, since the cost of implementing a modem is quite low these days.

Nevertheless, there is a real trade-off here between the power that digital downlinks provide (and therefore the simplicity of the receiving system) and the demands that they would make in accommodating the digital mode of the signals. Another point in their favour might be that unattended operation is more practical. An APRS message packet can be set up to be sent every 2 minutes, leaving the operator free to do other things. Similarly, the radio (or computer) stores incoming messages.

Besides the technical difficulty there is also the (human) organising factor. I heared some ideas about an easy to access geostationary satellite with high power downlink. That would be great, but without any communication control it would be like the FM LEOs where everyone talks at the same time and nobody is able to get there message through.

The same might well be true with a 1200 bps digital voice channel along the lines of what we've discussed above. But I think the APRS traffic, consisting in very brief bursts of data, would be easier to deal with.

It might be solved with a central control centre on a safe place (maybe by the Amateur Radio Emergency Service ?) that has control over the satellite radio and uses it as a remote radio "ear" in space. In this way it is also possible to use one frequency simplex system.

Just some ideas,

Me too.

73, Bruce VE9QRP