Satgen111  Orbit in range Window timings     by GM4IHJ      11th May 91

Recent questions on packet have asked " Where is MIR. I cannot hear it".
The questioner is doing all the right things and listening on Mir's
143.625 and 145.55 FM downlinks, but there are two reasons he is hearing
nothing. Firstly only a window of 6 of Mir's 15 plus orbits per day , come
in range of UK, and secondly the Mir cosmonauts keep Moscow time and
usually (but not always), sleep between 2000 ut and 0500 ut.

Last week you only heard Mir if the cosmonauts stayed up late, because
their window of in range UK orbits occured between approx 2000 and 0400
ut.

This week however the window drifts about 168 minutes earlier ( steps
24 minutes approx each day). So already one or two evening passes around
1900 have been audible and by the end of the week passes around 1700 ut
will be audible.

To visualise what is happening, please remember that the orbit circle
tries to stay fixed in space almost but not quite in step with the stars.
But the Earth is bigger around the Equator than it is around its Poles. So
every orbit pass over the Equator gets a tug, of in Mir's case a little
over a third of a degree each orbit. In a day this pulls the orbit about 6
degrees . So Mir's orbit window appears to shift 6 degrees or so, = 24
minutes earlier in time , each day.

At 6 degrees per day perturbation, the Mir orbit goes through a complete
circle in about 60 days. So every 60 days for about 10 days, all Mir's
orbits in range UK occur during the cosmonauts normal sleep time.

Fortunately for us , the start of the Project Juno visit to Mir on 20th
May should coincide with a window between 1430 and 2230 ut approx, and end
, on 26th of May with a window between 1226 and 2026 ut 

PLEASE NOTE THAT ORBITS CAN OCCUR AT ANY 93 MINUTE SEQUENCE IN THE WINDOW.
A WINDOW OPEN AT 1226 DOES NOT MEAN FIRST ORBIT AT 1226. IT MEANS FIRST
ORBIT SOMETIME IN THE 93 MINUTE PERIOD AFTER 1226. THEN EACH SUCCESSIVE
ORBIT IN ROUGHLY 93 MINUTE STEPS THEREAFTER UNTIL THE WINDOW CLOSES. 

The perturbation of the orbit plane due to the Earth's bulge is far less
serious for satellites which cross the Equator at a steep angle ( sats
with a high angle of orbit inclination ). So the Radio sport satellites
take many months to be perturbed through a full circle , and hence orbit
windows occuring over night are much less frequent than they are for Mir.
BUT when they do occur the pattern of night time orbits extends over
weeks, not days. 73 de John GM4IHJ @ GB7SAN