[amsat-bb] Making the linear satellites more accessible

[Mac A. Cody]

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