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R: FT-847 AO-40 and Doppler Uplink Correction?




----- Original Message -----
From: Jens H. Jensen <topcat@mail1.stofanet.dk>
To: <amsat-bb@AMSAT.Org>
Cc: <amsat-bb@AMSAT.Org>
Sent: Monday, July 23, 2001 4:37 AM
Subject: Re: [amsat-bb] FT-847 AO-40 and Doppler Uplink Correction?


> At 22:42 22-07-2001, you wrote:
>
> >I hope this helps a bit.
>
> Ok, Chris, but aren't you simplifying it a bit ?
>
> What if we take the uplink as a reference:
>
> Uplink frq = X (23cm)
> Downlink frq = 2X (13cm)
> Approximate frq's, without doppler, just for example!
>
> Uplink at frequency X, doppler on uplink -10Khz (example!)
> Uplink frq that the satellite sees : X - 10Khz Doppler
>
> Transponder Inverting : receives 10Khz too low, becomes 10Khz too high on
> downlink after inversion (at the sat, before doppler)
>
> Downlink frq about 2X (example) + 10 Khz - 20Khz doppler (because downlink
> frq twice as high)
> Downlink frq that the uplinking/downlinking station sees/hears : 2X -
10Khz.
>
> This will look (subjectively) as half the doppler, compared to a one-way
> downlink (beacon).
>
> Doppler subtracts here (subjectively, seen from the rx/tx station), as
> opposed to an non-inverting transponder, where it would ADD. ??
> I reckon this is the reason for the invention of the inverting
transponder,
> or are there other reasons?
>
> Anyone more experienced have a comment?
>
> 73 de OZ1LRG
>

Hello  Jens,

You are right

Suppose to have a satellite wich receive our uplink from 435,400 to 435,500
MHz using an onboard transponder having its downconverter with a local
oscillator of 446,100 MHz .

During conversion our LSB  435,400 signal became 446,100-435,400=10,7 MHz
and our 435,500 MHz signal became 446,100-435,500=10,6 MHz

In other words,using a local oscillator higher than the IF value we inverte
the band  because the lower uplink frequency 435,400 MHz is converted up in
to the higher IF frequency of 10,7 MHz and the higher uplink frequency
435,500 is converted back in to the lower IF frequency of 10,6 MHz

In addition,using an inverting conversion,our LSB uplink frequency band
435,400 to 435,500 is  inverted as well  in to a USB  frequency band  from
10,7 to 10,6 MHz respectively in to the IF passband.

More important for us,using an inverting convertion our uplink signals are
converted in to the satellite transponder with frequency variations that are
opposite to that the satellite see by Doppler during  his approach or  going
away from our location.

If the satellite is approaching  he see our frequency higher than the
frequency we really transmit on the earth but  it converts this frequency
variation  with opposite sign subtracting the Doppler shift   from the
frequency to be downlinked back to the earth.

As an example if the satellite approach to us,our frequency is seen higher
(+) by the satellite but,before to be downlinked, it is converted lower  in
frequency in to the inverting satellite transponder and exactly by the same
amount of  KHz  (-)

On the other side,since the satellite is approaching to us we on the earth
also see his downlinked frequency higher in frequency respect that really is
trasmitted by the satellite (+) but using an inverting transponder our
uplink  Doppler  shift  as seen by the satellite is effectively subtracted
(-) in to its inverting transponder  by the positive (+) Doppler variation
that we see on the earth during the satellite approach.

The net effect of  an inverting transponder is to compensate for Doppler and
this compensation is greater as closer  are in value both the uplink and
downlink  frequency bands.

In other words,receiving AO40 in 2401 MHz  the Doppler compensation is
greater uplinking  in 23 cm  than uplinking in 70 cm.

The reverse would happen using a non inverting transponders because the
uplink Doppler shift as seen by the satellite add directly to that received
by us in downlink and this is the reason that non inverting transponders are
not suitable for our satellite communications.

The beacon frequency is a special case because its signal is generated by
a  satellite oscillator and not by a convertion process in to a transponder.

For this reason,the Doppler shift,as observed in equal time intervals,is
greater for the beacon frequency than for  a translated signal and this
because the beacon frequency cannot be compensated for Doppler.

As an example the Doppler shift for the FO-20 beacon is approximately 20 KHz
in a pass and is 15 KHz in a QSO via its inverting transponder.

73 de i8CVS Domenico


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