Satgen63  Satellite Uplink and Downlink Problems     12 June 90 by GM4IHJ

The only packet satellite presently operational , is JAS1-B. It also
operates in Analog Voice/SSB mode, and it is being heavily utilized.
In consequence , many of the users are encountering the problems of mode J
2m uplink and 70 cms downlink, for the first time. The following review
may therefore be appropriate, and some of it is likely to apply to all the
new mode J Pacsats and Uosats.
MODE J PROPAGATION , QRM and SATELLITE PROBLEMS   Operating mode J has
never been easy, and the main problems are of three types :-
Propagation. We are now encountering the peak of the northern hemisphere
Sporadic E season. During daylight trans European terrestrial contacts
will be possible on many days . The effect is not precisely predictable,
but from the satellite users point of view, it will be apparent that when
the ionosphere refracts and scatters terrestrial signals back to earth, it
also blocks the path to space for other 2m signals. This effect is usually
noticed as a slight drop in the satellites 70cm downlink, accompanied by
total failure to get a 2m signal up to access the satellite. Quite often
the path is blocked only for one half of the orbit and not the other half.
This effect was first written up in ORBIT magazine for Nov/Dec 80. Aurora
and Field Aligned Irregularities in the Ionosphere, can also produce
similar effects. The 2m signal going up always suffers most.

QRM   A brief tune around any JAS1-B orbit over Europe produces several
regular sources of interference. On June 11th at I noted French FM Voice
Amateur Radio activity on 435.796 and on 435.837. It was very strong,
blocking large portions of the passband above the beacon, and presumably
uplinking from the satellite section of the 2m band. I also noted computer
type qrm in two places in the passband , which did not go off , when I
switched everything off in the house except my receiver. Clearly external
computer qrm , can be a problem.

SATELLITE PROBLEMS   Unfortunately it must also be recorded that JAS1-B
makes a lot of its own QRM. When modes JA and JD are on simultaneously I
can hear 4 copies of the JD signal in the JA passband , and in addition
there are also 4 spaced copies of the JA beacon in the passband, whether
mode JD is on or not. These spurious signals are the result of non
linearity in the JAS1-B transponder and they appear to be permanent (
whether JAS1-B is heavily loaded or not ).

So please be aware that the path up to the packet satellites is not an
easy one. Former Oscar 8J operators had to learn all about it in the 70s.
Modern operators will find the subject well reviewed in old Amsat NA
publications. Meanwhile the other pacsats have been producing a few
surprises. Lusat has been using 435.125 MHz BPSK down on some orbits. This
is supposed to be a Raised Cosine channel, but unlike the Webersat raised
cosine it demodulates easily. Even more confusing, it does not sound like
raised cosine, whereas the Lusat 435.150 downlink has recently sounded
like raised cosine, and has been almost impossible to print. I am
presently hurrying to build the modem to allow me to use G4BMK's Spectrum
analysis program on one of my PCs, to get a clearer idea of what is going
on. Dove is still unmodulated carrier on S Band .73 de GM4IHJ @ GB7MAC