Satgen 382   Oscar 13 Shadow Dancer    by GM4IHJ         20th July 96

The peculiar orbit of Oscar 13 now takes it from a high point of 38534
kms , down to a low point of only about 220 kms. This produces some
odd effects ,not encountered before in a high altitude eliptical orbit
Molniya amsat. One of these effects occurs in respect of eclipse in
earth shadow.

The basic mathematics of whether the satellite is sunlit or in shadow
, depends on two features. First of these is the sum angle arrived at
by adding the radius of the solar footprint at the earth ( a near
constant of just under 90 degrees ), to the radius of the satellites
footprint ( a highly variable quantity of roughly 81 degrees at
apogee, down to about 16 degrees at perigee ). Which gives us a sum
total varying between approximately 171 and 106 degrees.

The second feature important to this problem , is the size of the
angle subtended at the earth centre between the solar sub point on the
earths surface and the satellites sub point. If this second angle is
greater than 171 degrees then the satellite cannot possibly see the
sun and is therefore in shadow. Which means that at satellite apogee
there is only a narrow 9 + 9 =18 degree band of over which the
satellite at apogee can get into earth shadow.

By contrast however , the satellite at perigee has a 74 + 74 =148
degree wide band into which it can get into earth eclipse shadow, out
of sunlight. You can be forgiven for thinking that this means that the
satellite goes into shadow more often and for longer periods around
perigee. But this ignores another feature of the eliptical orbit
whereby the satellite at perigee is moving very fast around the earth
, whereas at apogee it hardly changes position at all.

The resolution of this curious dance is that eclipses taking place
this July , occuring far from perigee, can last for hours. Whereas the
eclipses starting towards the end of October this year, which will
occur when the satellite is at or very near perigee, do not last
longer than 20 minutes or so. 

At present Oscar 13s perigees are northern hemisphere events, and this
may continue until December, unless the orbit parameters change
drastically. So northern hemisphere stations have a chance to study
Ao13s telemetry beacon, and look for tell tale signs of temperature
rise , and changes in satellite stability. Doppler rates on the
145.985 MHz beacon are quite dramatic at perigee as the satellite
flashes just above the earth, crossing continents in minutes, but
those used to tracking 70cm pacsats should have no problem following
Ao13. 

Will Ao13 be visible as it goes through perigee ? Yes it should be
provided it is sunlit over a darkened earth ( but please note the
above caution about perigee eclipses due October to December
inclusive). Satgen 383 next week will discuss the curious " turn back
on itself, track across your sky" which Ao13 can follow during these
visual opportunities.