Re: planscan: Computing optimal directions for performing a plane scan

From: Greg Roberts via Seesat-l <>
Date: Sun, 14 Feb 2016 22:44:06 +0200
Evening all

Many thanks Cees for the software you wrote for doing a plane scan.
It looks very interesting and detailed and I will have a better look at
it tomorrow but several points come to mind which I think I need
some clarification. I plan to try it after I do a little shuffling of your
data to fit the formatting used by my system but that is a trivial
matter and I can use about 80% of your output ( although I do
not use RA/Dec for the scan - but its ideal for when I need to measure
an image where I need the RA/Dec of approximate centre of field
of view - I currently have to write this down whenever a satellite
is seen as the CoSatrack program displays RA/Dec/azimuth and

The program scan appears to start at a time specified by the observer
and one needs to specify the satellite altitude and the program then
predicts for maximum brightness. Now when I do plane scans for
the KH-11 satellites I often only have a partial pass due to shadow
entry - how is this avoided in your program ? i.e there is no sense
in doing a plane scan on a point inside the earths shadow. The second
point is the altitude is required. Now if I do a plane scan on a Molniya
type orbit I have an altitude range of something like (say) 800 km to
12000 km - how is this problem overcome and what altitude should
one use ?

You are correct in that my CoSatrak system tracks using an ascii listing
of time, azimuth and elevation at a determined interval-usually 30
seconds. The software driving the system then does interpolation between
these points to more or less give a smooth track instead of stepping
every defined time. When near culmination on a reasonably high
elevation pass the azimuth can change quite rapidly so stepping might
cause ones field centre to "be off".

The program - actually two - that I use were written for me about 15
years ago by Thierry Marais and run in DOS - my CoSaTrak system runs
under Dos because one cannot have the interrupts etc  caused by
Windows to the operation of the computer which could cause the
program to loose counts when moving from one position to another.

Thierry is still around - I last had contact with him about two-three
years ago but from what I recall he had lost the code he wrote :-((

Anyway the software first produces a table of date, time, satellite
x,y,z coordinates, followed by azimuth, elevation and height above
the earths shadow and can cover two-three days. I select a time and
position where the satellite is at a reasonable azimuth and elevation
and clear of the earths shadow. I do not go too close to the shadow
entry point in case the satellite has done a change and perhaps
dropped its altitude above my location. I also try and avoid a position
that is too high - say above 60 degrees elevation, as this can cause
problems with the movement of the scope. This position chosen
agrees with the predicted details for the element set used, even if
the satellite is no longer at that position in the orbit plane. The second
part of the program then takes this position as the "pivot" point and
I start the computation of the plane scan say an hour earlier for a
total plane scan of say three hours. Obviously on a low altitude
satellite one will not have a plane scan lasting three hours with the
satellite above the horizon for the satellites I plane scan. I simply
delete the entries that are below say 5 degrees elevation, so I scan
on a specific point that I know should be reasonably clear of the
earths shadow and this works very well and if the satellite is bright
enough has never failed to work. The software also has provision for
defining the field of view and reading an element database and listing
all satellites and their track angle as they cross the field of view for
the position currently being looked at. I do not use this as there is a
limit on how many satellites it will show , so unless I remove all the
junk and only leave the brighter satellites I will soon run out of the
satellites that can be shown - Ive forgotten how many it is - besides
I am too busy watching the images taken and noting which frames
show satellites to make a note of which satellite it is.

So that is the story. When I had an SLT mount a few years ago I was
experimenting with the freeware program RTGUI by Robert Sheaffer
available at If my failing memory is still okay it can
control most GOTO telescopes using an ascii table of positions of
such objects as comets,asteroids and neo for example and I think
it could do satellites also. This was a few years ago and the program
has been further improved so who knows what its capabilities are
now with regard to satellite tracking, so this is something worth looking

Hope this helps

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Received on Sun Feb 14 2016 - 14:45:06 UTC

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