Satgen279 Satellite Profile No 8A Oscar 13 (AO13)   by GM4IHJ  30 July 94
BID of this msg is SGEN279  Please use this BID if you retransmit this msg

The launch of Phase 3C Oscar 13 on 15th June 1988 was a nail biting event.
Phase 3A had plunge into the Atlantic when its launch rocket failed, and
Phase 3B Oscar 10 had failed to achieve its planned high inclination
orbit. Fortunately , all went well with Oscar 13 and it attained a 57
degree inclination orbit. In that orbit it sees the UK ( and nearly half
the rest of the world at the same time ), for about 8 hours, then a few
hours later it sees UK briefly for one or two hours from the other side of
the world.
Unfortunately Oscar 13s, mode L and J links have failed. So the satellite
alternates between mode B( 435 up and 145 down), and, mode S ( 435 up and
2400 ) down. Actual mode switching times vary, but presently mode B is
available from mean anomaly 150 through 256/0 to mean anomaly 120 ( ie
missing only the period around the apogee high point). Mode S is available
by itself mean anomaly 122 to 145, and concurrent with mode B mean anomaly
90 to 120 and, 150 to 180.
The mode B transponder is switched to omni directional antennas from mean
anomaly 230 through 256/0 to 30 (ie around the perigee low point of the
orbit ) .At other times directional antennas are connected with actual
pointing data being given as Bahn Longitude and Latitude , where Blon/Blat
180/0 indicates antennas pointing at earth centre and in the orbit plane
when the satellite is at apogee high point , or, Blon/Blat 230/0 ( which
starts in September 94) where the antennas point 50 degrees clockwise of
earth centre , in the orbit plane as seen from above the north pole of the
earth at apogee. Most Oscar 13 software accepts inputs of Blon/Blat and
works out for you the squint angle between the line of sight satellite to
your station and, the line along which the antennas are pointing.
Mode B uplink band is 435.575 to 435.425 MHz which downlinks approximately
at 145.823 to 145.973 + or - doppler shift. Please note the frequency
inversion . Highest up = Lowest down, so you send LSB up to get USB down.
Suitable antennas are a minimum of a 12 element 70cm yagi uplink and, a 5
element yagi plus a pre amplifier downlink.
On mode S 435.64 to 435.61 up, gets you down on 2400.725 to 2400.695 down.
There is an S band beacon on 2400.64 MHz and the mode B beacon is on
145.8125 MHz. A 12 element yagi is suitable from mode S up, and a 20 turn
helix is a good antenna for mode S down via a converter 2400/144 Mz.
Oscar 10 and 13 operators have evolved several methods for combatting
changing squint angle and polarisation on mode B. Some stations use yagis
with switchable polarisation ( Horizontal or Vertical ). Some add
facilities for right hand and left hand circular polarisation switching.
While some of the most successful stations use separate right and left
hand circularly polarised antennas, switching from one to the other in a
pattern learnt by experience of operating at various squint angles. This
sort of thing is necessary because even at Blon/Blat 180/0 , the sats
antennas do not necessarily point at you when the sat is away from apogee,
and the further the sat gets from apogee the more serious, the problems of
squint angle and changing polarisation become.
There are several other "special" considerations which have to be observed
when using Oscars 10 and 13. These will be discussed in Satgen280 next
week.
ERROR CORRECTION. Sgen 275 third line of text. Uplink should be 435.016
for Oscar 21. Tnx to Ron 4X1MK for spotting this. 73 de GM4IHJ @ GB7SAN