First, thanks to those that replied to my question about the level of activity on the linear transponder satellites. What was saidwaspretty much what I suspected. I was just looking for some verification from others before expressing some thoughts about making the linear satellites more accessible to amateur radio operators. I apologize up front for this lengthy posting. Please accept it in the spirit of experimentation, reducing the level of congestion on a few satellites, and possibly increasing the ranks of the amateur satellite community. I'm sure that there will be those ready to step forward and correct me on any of the assumptions or statements that I make. Please be understanding.
There are currently two FM repeater satellites (three, counting the sporadicly active LilacSat2). SO-50 and AO-85 are heavily used, with frequent congestion. Meanwhile, there are a dozen linear transponder satellites that are relatively lightly used with bandwidth for multiple simultaneous channels. I understand that it makes sense to launch linear transponders, rather than FM repeaters, due to the better utilization of resources.
In my estimation, the relative lack of utilization of the linear birds, compared to the FM birds, is the cost barrier for the radio equipment required to be able to work them. The added challenges of finding other stations in the band and stricter Doppler accommodation also exist, but can be mastered with practice. Allow me, though, to confine my comparison to voice-mode communication only, for reasons I'll explain shortly.
A pair of inexpensive hand-held FM transceivers and a pair of Yagi antennas (or an Arrow or Elk antenna) are all that is needed to successfully work the FM birds. In fact, that is what I'm using right now. An expenditure of about $200 is needed, possibly less. On the other hand, to properly work the linear birds using voice requires an SSB transceiver (if capable of full duplex operation), or that and another SSB receiver, otherwise. The attendant expense appears to approach $1000, possibly more.
One suggestion is to work the linear satellites using CW mode. While this could definitely be less expensive, it exchanges one barrier for two others. The first is the need to know Morse code. The second is the apparent lack of regard that many in the satellite operator community appear to exhibit for CW operations on the satellites. The cause appears to be a lack of discipline by some CW operators in both power management (signals hogging the transponder power or 'flipping the bird') and signal frequency management (failing to properly compensate for Doppler shift, thereby encroaching on other QSOs). Both of these discipline problems occur at the expense of the SSB operators resulting a lot of public ire towards CW ops - Ironically, I also hear that such discipline problems also occur with some SSB ops, too. Consequently, I don't see CW ops as a particularly viable solution to lowering the barrier to using the linear birds.
I also don't see allowing other digital modes on the linear birds to be a viable solution either. Most digital modes are propagated via SSB transceivers. This raises the cost bar once again, which doesn't solve the problem. There are a few digital modes, such as Hellschreiber, that don't ride on SSB, but they are not popular and would exclude QSOs with voice-mode operators. The closest may be the successful use of OpenDV and an appropriate digital modulation scheme. That is all still very experimental.
What I propose, then, is the use of well-disciplined DSB (double sideband) modulation as a means of lowering the cost bar while providing interoperability with SSB voice stations that are working the satellites. What I mean by 'well-disciplined' DSB is as follows: 1) Baseband audio bandwidth sharply limited to 2.5 KHz, resulting in RF bandwidth or 5 KHz. This would realistically enable three to fifteen simultaneous 'channels' depending upon the linear bird being used. 2) Power output would be limited to just a couple of Watts. This is all that is really necessary to access the linear birds in most situations. It would reduce the possibility of signal interference. The cost of the PA would be relatively low. 3) The RF oscillator would be accurate and computer controlled (e.g. Silicon Labs Si570), which would enable active Doppler compensation via software with CAT control (e.g. gpredict).
Current SDR technologies already enable inexpensive, multi-band receiver solutions. The desired sensitivity and selectivity of the SDR receiver ultimately drives the receiver cost. The performance of the receiver, then depends upon the budget of the station builder. Solutions for every budget are now available, ranging from dirt cheap hardware derived from RTL-SDR dongles to more capable devices (e.g. SDRplay, AirSpy, FunCube Dongle).
At the sacrifice of some bandwidth, the following advantages can be had with using DSB modulation: 1) DSB modulation is relatively easy to achieve. The band-limited audio is fed into a double-balanced mixer, bandpass filtered for harmonics, and fed into a PA for transmission. The resulting hardware is comparatively inexpensive to construct and to tune. 2) Sideband inversion caused by some linear birds is no longer an issue. Having both sidebands present in DSB modulation means that the correct sideband will always be available for reception. 3) Interoperability with SSB stations would be maintained, as DSB modulation is a superset of SSB modulation.
Thanks again, for reading this long posting. I'm sure that there will be those that will consider my suggestions to ill-advised, or even outright heresy!
73,
Mac Cody / AE5PH