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AO-27 Mode String



G'day,

A few weeks ago, someone asked what the mode string was for AO-27, so that
he could enter that into a satellite tracking program and therefore predict
at what time the satellite was actually operational.  A reply stated that
the satellite did not conform to a schedule based on the satellite's phase,
and a good, detailed explanation followed on what the TEPR states of the
satellite actually meant.

This prompted me to take a look at AO-27's orbital characteristics, to
determine whether it could actually be used here in Australia.  The short
answer is no (unless the TEPR programming is changed to support more
southerly latitudes...  we need about another 17 minutes of transponder
operation).

However, I believe that it _is_ possible to generate a mode schedule for
AO-27, based on the satellite's phase, which will be accurate for the
instant it is created, but subject to drift at about 1 minute per day...
this schedule can then be used in most popular satellite tracking programs,
just like we were used to with AO-13 etc.

The two main causes of drift that I have considered are due to the change
of angle of the earth's axis, causing TEPR state 4 to occur later each day,
and the argument of perigee changing (apsidal rotation), thus causing the
relationship between the satellite's orbital phase and end of eclipse to
drift.

Ideally, a small program is required which will allow end users to do their
own TEPR-to-schedule translation, however I thought I would publish a
"beta" set of figures for users in the northern hemisphere to make use of,
to see if the information is of any interest in the first place!

So here is is:

Beta Mode Schedule For AO-27 
============================

(calculated for 16:00 UTC, 10th July 1999, TEPR 4 = 42, TEPR 5 = 78):

000o196J242o

Where '0' is zero, 'o' is lower case 'oh' for "transponder off", 'J' is
when the Mode J transponder is ON.


I look forward to any feedback (positive or otherwise).


Good luck with it this weekend,


Chris vk6kch



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