Iridium satellite status situation

MALEY, PAUL D. (paul.d.maley1@jsc.nasa.gov)
Tue, 5 May 1998 12:43:39 -0500

Hello everybody:

There have been a number of messages about Iridiums recently. Let me see if
I can provide an even more current update.

The four failed IR's are still: 11,20,21,27.

However, if a satellite (other than the 4 above) does not flare or flare at
the correct magnitude (and is not observed to be tumbling), it more than
likely is still a working spacecraft, regardless of the orbit it is
currently in.  Once an IR is observed to be flashing, it is likely that the
thruster control has failed, and the satellite is likely to cease
functionality. At certain times, a satellite will either go into or be
intentionally placed in 'thruster control mode'. As of today, IR 5 and IR 48
are in this mode. It is not always possible to tell by looking at the
elements that this has occurred, even though changes in the drag term may be
a good signal.  When an IR goes into the thruster control mode, the
deadbands increase to 3.5 to 4.0 degrees which will cause the mirror/flare
angles to be way off target.  The deadbands will need to be tightend up in
order to have a fully functional satellite system made up of spacecraft with
similar deadbands.

Reports of a satellite going 'off line' that appear on SEESAT should be
factored into those programs such as GSOC where there is a lot of public
access. However, confirmation (as has already occurred with IR 5) is
important before deleting a satellite. The satellite should be deleted until
the signal is given that it is back in working order (if that occurs).

Two months ago I sent emails to several observers at high latitudes to
report observed vs predicted flare magnitudes so that I can keep track of
these variations. From the variety of reports received on the net, it is
probably useful to ask that any observer who is capable of making ACCURATE
observed maximum flare magnitudes to report them as convenient. You can do
so directly by using my email address below. In order for the observations
to have maximum impact, the elsets used should not be more than a few days
old.  

Requested info includes:

1.Date 
2.Time (UT)
3.IR number/USSPacecom or Int'l ID
4.predicted maximum magnitude (compared to star/planet whose brightness is
known)
5.observed maximum magnitude  (   "   )
6.age of element set used (if known), or epoch

For ease of reporting, this can be done in a summary after a few weeks time
rather than reports on individual events.  Because there are so few bright
calibration sources in the sky, I have recommended looking at flares that
are -1 or fainter. This opens up the possibility of observing more flares,
though they are not spectacular. I would also appreciate reports of double
flaring which I and others have seen from some Iridiums. The double flaring
should be very specific regarding the moment of the first flare and the
number of seconds between the max of that flare and the max of the second.
In some cases the first flare occurs on the predicted position and the
second one thereafter. The process has also been observed to occur in
reverse order. 

In summary, the Iridium constellation offers an opportunity for SEESAT
observers to provide interesting information on the status of its individual
satellites.

Paul

Paul D. Maley
United Space Alliance
DO5/Cargo Operations
NASA Johnson Space Center
Houston TX 77058 USA

tel. 281-244-0208
email: paul.d.maley1@jsc.nasa.gov
latitude 29.6049 north, longitude 95.1086 west