ASTRA 1F TLE Problem
Bruno Tilgner (Bruno_Tilgner@compuserve.com)
Sat, 15 Nov 1997 07:19:44 -0500
Hello everybody,
The following 6 satellites make up the cluster of the ASTRA direct TV
broadcast satellites:
Name COSPAR CATALOG
---------------------------
ASTRA 1A 88109B 19688
ASTRA 1B 91015A 21139
ASTRA 1C 93031A 22653
ASTRA 1D 94070A 23331
ASTRA 1E 95055A 23686
ASTRA 1F 96021A 23842
These satellites are co-located at a nominal longitude of 19.2
degrees east, so they can all be captured with a single dish.
During an ongoing photographic project of recording the short-term
movements of these satellites and their light curves, persistent
problems were encountered to determine the position of ASTRA 1F.
In fact, recent orbital elements show it several degrees further
east so that it would be impossible to appear on the same image
with the other 5 satellites. During the last few weeks a number
of photos were taken on film and with CCD cameras by three different
observers on different dates and at different locations.
All pictures (several hundred) show 6 objects, whereas the TLEs used
would allow only 5, given the field of view of about 0.5 degree
by 0.5 degree.
I have checked older TLE sets and found the following evolution
of the position of ASTRA 1F:
Date Longitude
--------------------
17 Dec 96 19.24 E
28 May 97 19.18 E
07 Jun 97 19.29 E
03 Jul 97 21.19 E
10 Jul 97 21.81 E
18 Jul 97 22.61 E
02 Oct 97 21.78 E
16 Oct 97 23.14 E
30 Oct 97 24.95 E
10 Nov 97 26.58 E
--------------------
Whilst all other members of the ASTRA family have stayed near their
nominal position at 19.2 E, ASTRA 1F seems to have drifted by more
than 7 degrees during the summer, starting some time in the second
half of June.
This is a totally unrealistic result which can only be attributed
to faulty TLEs. Can anybody offer an explanation why the TLEs for
just one of these 6 satellites should get out of hand by so much ?
On a related subject: The SDP4 code in two FORTRAN and one PASCAL
implementations give the position of geosynchronous satellites
with enough precision so they can be found in a telescope. However,
the code is not capable of correctly computing short-term movements,
in the order of one to several hours. To this end, an accuracy of
about 1 arcminute would be necessary. The ASTRA satellites offer
a unique opportunity (at least from the eastern hemisphere) to
observe these movements in real-time and to compare them with
computed positions. The SDP4 model tends to result, at least for
these satellites, in a larger eastward drift in longitude than is
actually observed.
Bruno Tilgner
Saint-Cloud, France
Bruno_Tilgner@compuserve.com
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