[amsat-bb] Re: Asymmetric doppler curves?
g0mrf at aol.com
g0mrf at aol.com
Mon Jun 11 15:33:56 PDT 2012
Dear Domenico, Dave, Miguel Trevor and Ivo.
Thank you all for your thoughts and references. James Miller's Plan 13
etc is an excellent resource as, I discovered is Ivo's thesis.
I have reached the conclusion that extracting more information from
Doppler measurements in the limited time available may be considered a
little tenuous. A reasonable approximation of satellite velocity and
altitude should be straightforward. However I think trying to measure
the Earths rotational speed suffers from one major source of error and
that is the stability / calibration of the satellite transmitter and
the groundstation receiver. To accurately measure the difference or
asymmetry in Doppler between AOS and LOS (e.g. AOS may be fc +10,000Hz
while LOS could be fc - 10,200) depends entirely on knowing the exact
center frequency of transmission. Without that, the valid data becomes
lost within the errors. However, hopefully some of the students will
find it an inspiring addition to their study of gravitational fields.
Regards and thanks
From: i8cvs <domenico.i8cvs at tin.it>
To: Amsat - BBs <amsat-bb at amsat.org>; G0MRF <G0MRF at aol.com>
Sent: Sun, 10 Jun 2012 3:25
Subject: Re: [amsat-bb] Asymetric doppler curves?
Hi David, G0MRF
If you read the old issues of OSCAR-News from AMSAT-UK you
will realize that a serious traking program like PLAN-10 written
by James Miller G3RUH take into calculation all perturbations due
to inclination and velocity of the satellite due to Earths rotational
speed added / subtracted at either end of the pass and so doppler.
----- Original Message -----
From: <g0mrf at aol.com>
To: <amsat-bb at amsat.org>
Sent: Sunday, June 10, 2012 1:20 AM
Subject: [amsat-bb] Asymetric doppler curves?
> Hi all.
> I'm doing a small demonstration for a physics class using Doppler
> measurements on the HO-68 beacon.
> Hopefully we'll be able to calculate spacecraft velocity and from that
> result, go on to calculate orbit altitude. However, in thinking
> this I realised that there is a potential source of error. We are
> stationary !! - OK, it's obvious really, but I've never seen this
> mentioned in topics like Doppler correction programs and I've not seen
> it visually in displayed Doppler curves.
> The issue is that while a spacecraft with zero eccentricity will have
> constant velocity, the speed relative to an observer on Earth at AOS
> and LOS will be different for each half of the pass depending on the
> observers latitude and the inclination of the satellite.
> The worse case would be something travelling East to West or West to
> East as the velocity of the satellite would have the Earths rotational
> speed added / subtracted at either end of the pass.
> Has anyone seen this effect? Perhaps on the ISS? I think it could
> as much as 7% which may be measurable. - But not on HO-68 which is
> polar orbiting....
> Just want to make the most of my 45 minutes.
> David G0MRF
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