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R: A doppler 2.4GHz question



----- Original Message -----
From: Graham Shirville <g.shirville@btinternet.com>
To: <amsat-bb@amsat.org>
Sent: Thursday, December 16, 2004 9:28 PM
Subject: [amsat-bb] A doppler 2.4GHz question

> Hi All,
>
> We are still working on the possibility for a digital ATV installation for
> the ISS and this would probably use the DVB-S system and have a downlink
in
> the 2400-2450MHz band.
>
> One of the questions being asked is about how robust the ground segment RX
> system would be in the presence of the doppler shifts that occur at that
> frequency.
>
> The next question that then comes is "what is the worse case - i.e. the
> maximum rate of change" for the received frequency?
>
> Now I know this will be when the ISS is approaching TCA on an overhead
pass
> but I have no idea how to calculate the actual rate of change.
>
> Any help would be much appreciated. We know that Formula 1 racing cars
> coming to sudden stops is not a problem for their headcams and other
digital
> cameras but this around a 700Hz per sec change and I suspect that we may
be
> an order of magnitude more than that!
>
> cheers and happy holidays!
>
> 73
> Graham G3VZV
> ----

Hi Graham, G3VZV

I selected the ISS  Rev. numbar 34711 of today 12/17/2004 wich pass has a
max elevation of 49.955 over Naples and using InstantTrak I have manually
computed (T key) the range and elevation very close to my TCA at the
following times with intervals of 1 second in between to calculate the speed
of approach expressed in km/sec

UTC =10:07:31        Elevation= 49.955       Range 1 = 462.156 km
UTC =10:07:32        Elevation= 49.589       Range 2 = 464.427 km
UTC =10:07:33        Elevation= 49.194       Range 3 = 466.486 km

As you can see the ISS just started to go away from the point of closest
approach TCA of my QTH going to LOS at the following speeds:

Range 2-Range 1 =  2.271 km/sec
Range 3-Range 2 =  2.059 km/sec

Considering a frequency of 2400 MHz and the speed of light 299792 km/sec
the doppler shift at 10:07:32 can be calculated as follows expressed in Hz

                 6
2400 x 10   x ( 2.271 / 299792 ) = 18180.60 Hz

And after one second at 10:07:33 the doppler shift is:

                  6
2400 x 10   x (2.059 / 299792 )=  16483.42 Hz

The rate of change of doppler shift at 2400 MHz expressed in Hz/sec for the
above pass at TCA between  10:07:32 UTC and 10:07:33 UTC  for the ISS is

18180.60 - 16483.42 = 1697.18 Hz/sec

As you suspected the order of magnitude is more than 700 Hz/sec and for
latitudes at wich a pass is more overhead the rate of change of doppler
shift is even greater than 1697.18 Hz/sec

Best 73" de

i8CVS Domenico

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