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Amsat-UK's Oscar News, 1989 Jun, No. 77 p.16-17



James Miller G3RUH

 Oscar-13 switches regularly between the mode B, L, J and S transponders,
and OFF.  This article describes how the schedule is determined.

 The principal constraint is set by the requirement to maintain a positive
power budget to the battery system.  AO-13 is ultimately solar powered, so
an ATTITUDE SCHEDULE has first to be determined to keep solar illumination
around 90% or better.  This corresponds to Sun angles (the angle of the Sun
above or below the spacecraft's equator) better than +/- 26 degrees, and
certainly no worse than +/- 30 degrees.

 This attitude schedule has a regular 6 month cycle; 3 months at 180/0, 3
months at 210/0, in sympathy with the Sun's annual movement.  The
nomenclature 180/0 means attitude longitude 180, latitude 0,  refered to
the orbit plane.  180 longitude means the spacecraft antennas are Earth
pointing at apogee, i.e. at MA = 180 degrees.

 Next ANTENNA SQUINT is examined.  Squint is the angle between the
spacecraft's antenna boresight, and a user.  There's an infinity of users
of course, so representative geographical latitudes 45N and 35S are chosen,
and plots are made of squint vs MA for these two latitudes for every
visible orbit over an 11 day period.  See facing pages.

 Now a MODE L PERIOD can be chosen.  The spacecraft's 23cm RX helix's 3 db
beamwidth is +/- 19 degrees, so having an operating period where the squint
exceeds this makes mode L operation unnecesarily laboured. Looking at the
plots for an ATTITUDE 180/0, you can see that between MA 100 and 150,
Northern and Southern hemisphere users will have squint less than 19
degrees, whilst from 110 and 145, an ON period of 94 minutes, the value is
better than 15 degrees (only 1.8 db down on the antenna plot).

 The remaining time can be allocated to MODE B.  However, twice a year AO-
13 goes into ECLIPSE, so the transponders must be shut off before and after
to minimise battery drain during eclispe, and allow battery charge
recovery.  Short eclipses (< 28 minutes) happen around perigee, hence there
is an OFF period from MA 240 - MA 3.   The other period can be as long 90
minutes typically in the MA 30-70 region.  These are so long that
significant thermal changes take place, and while the effects of this are
being evaluated, prudence dictates a substantial OFF period for the 3 week
"long" eclipse season.  (In 1989 April it was MA 0 - 100)

 Finally, MODE J can operate along with mode L, but MODE S must be on with
mode B.  Mode S needs low values of squint too, so it is natural to have it
ON after mode L.  However mode L loads AO-13 with a negative power budget
(witness battery recharge when L goes OFF), so a recovery period is needed
before Mode S is activated.  S too is a heavy current user, and can only be
operated concurrently with mode B for about 30 minutes.   Mode B has a net
positive budget at good Sun angles.

 So, that's how a SCHEDULE is established.  The plan is made a month or two
in advance, to allow time announcements to spread, and to help user's to
plan ahead.

 The AO-13 command team do not work in a vacuum, and SUGGESTIONS are always
welcome.  Over the last year we've had several sensible suggestions from
regular users about mode L, B and S scheduling; all have been implemented.

 Fig 01 

 Fig 03 The top pair of plots show antenna squint as a function of MA, AO-13's position around its orbit, assuming the spacecraft's attitude is 180/0 (Earth pointing at apogee) for typical Northern and Southern hemisphere latitudes.  Fig 02
 Fig 04 Similarly, the lower plots are for an attitude of 210/0. For details refer to adjacent article. Plots generated by adding about 20 lines of code to program PLAN13. Footnote 1994 ------------- In 1993 the AO-13 70cm transmitter failed, so mode-L is no longer in use. Instead, equivalent time is given to the S-band transponder, which has similar low squint constraints. Typically the S-band beacon is on for 2 MAs, then the S transponder for 30 MAs, then 5 MAs of beacon again. Either side of this 37 MA sequence, the mode S transponder is on simultaneously with mode B.

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Created: 1994 Nov 14 -- Last modified: 2005 Oct 31