Geostationary satellites
Greg Roberts (grr@da.saao.ac.za)
Thu, 7 Mar 1996 10:37:40 +0200
I find the mail referring to geostationary satellite visual observations
of great interest. For the past 20 years I have been observing these
satellites using either a 30 inch or 20 inch reflector at our Sutherland
observing outstation of the South African Astronomical Observatory. This
is not official work and I only do it on a casual basis when observing
conditions are too poor for my normal astronomical observing. I have kept
a list of satellites observable from my location but this has not been
updated for several years now so is probably not of much use and I will
try and update it as soon as possible.
Unfortunately the telescope used has a very narrow field of view-about 10
minutes of arc so unless the satellite is close to the equatorial plane
I stand a good chance of missing it,especially if the inclination is more
than a degree or so. With an image intensifier I can get down to about
magnitude 15.5-16 on a good night. For those interested the technique is
to park the telescope at the declination corresponding to the equatorial
belt from my location - about 5.3 degrees North Dec, then use the telescope
slow motion in hour angle to scan in Right Ascension. Once I locate a
moving object I switch the telescope drive off then watch the stars drift
through the field with the satellite stationary. By reading the telescope
hour angle and declination circles I am able to determine the satellites
position and hence sub-satellite point if needed.
Although I havent done much geostationary satellite scanning in recent years-
a result of getting old making me lazier ! - I will check out the published
positions of military satellites etc and see what is there. With luck the
satellite might be at a node and hence in my field of view, otherwise I will
have to scan in declination as well as hour angle.
One interesting grouping I found recently was at the location for the ASTRA
satellites. In my 10 minute arc field of view I can see 5 satellites
simultaneously,two being brighter than the rest and one being somewhat fainter
than the other four. What is even more interesting is that they drift
relative to one another and I have actually seen one satellite merge with
another. Looking at the orbital parameters there is only a spread of about
6 kilometres in altitude so I would think these satellites come very close
to one another at times! Now as to brightness - when I first pick them up
at the start of the night they are rather faint - perhaps magnitude 10 to 14
but at local midnight, when the phase angle at the satellites is about
maximum I would guess they are about 2 magnitudes brighter.As dawn approaches
they get fainter again so for those with small telescopes try and observe
them at maximum phase angle. For me the satellites are almost on my meridian
with an hour angle of 7.2 minutes west, so maximum phase occurs around local
midnight .
"Technical" details are :
Norad catalog numbers 19688,21139,22653,23331 and 23686 corresponding to
ASTRA 1A,1B,1C,1D and 1E. Objects 19688 and 23331 appear to be the
brightest and are suspected of being the two that merge every night. I
suspect the faintest satellite is #22653. However I would imagine these
satellites are all of similar if not identical size so the variations in
brightness amongst them are due to reasons other than physical size.
Occassionally I have used the 20 inch reflector for viewing medium altitude
satellites such as the GLONASS and NAVSTAR satellites.,but even then the
satellites still rapidly whizz across the field of view . If Im quick enough,
and press the slow motion controls in the correct sense etc I can actually
follow these satellites . Using this same telescope I was able to track
Apollo 11 and its assorted debri out to about 200 000 miles - the objects
were easy and it was only the brightness of the moon that prevented me from
tracking them further out, so that is my present "distance" record. (in those
days the telescope was not fitted with an image intensifier). With todays
large amateur telescopes and CCD cameras it should be possible to exceed
this kind of distance with great ease.
My next two observing runs at Sutherland are the first week in April and
the last in May so Ill have a go at the "classified" locations and will keep
readers informed of the result.
Good viewing
Greg