# Re: FT-847 AO-40 and Doppler Uplink Correction?

• Subject: Re: [amsat-bb] FT-847 AO-40 and Doppler Uplink Correction?
• From: Chris Hill <Chris.Hill@xxxxxxxxxxxxxxxx>
• Date: Mon, 23 Jul 2001 08:33:45 +0800

```Hi Jens,

No, simpler than that.  On the uplink, we want to place a signal into the
satellite at a certain frequency, in order to have the transponder generate
a downlink where we want it.

If the satellite is moving towards us, then our signal will arrive higher
in frequency than we are transmitting, so we compensate by deliberately
transmitting lower in frequency:

So if we wanted to put a signal into the satellite at 435.573 MHz, and
Doppler shift at UHF was +3 kHz, we'd transmit at 435.570 MHz.

Exactly the same rule applies, even if the satellite is now moving away
from us, except that now the Doppler shift is a negative number.  If we
still wanted to put a signal into the satellite at 435.573 MHz, but Doppler
shift at UHF was now -3 kHz, we'd transmit at 435.576 MHz.

What I was trying to emphasise before, is that the same rule applies, just
keep the sign of the Doppler shift (positive or negative) in mind, not just
the magnitude of the Doppler.

I hope this helps a bit.

73 Chris vk6kch

At 22:19 22/07/2001 +0200, you wrote:
>At 06:15 22-07-2001, you wrote:
>
>>[On the uplink, you want to compensate so that your signal arrives at the
>>satellite on the correct frequency.  Therefore, you need to subtract the
>>Doppler shift (at UHF) from your TX freq.  Subtracting a negative number is
>>the same as adding...  confused yet :-).  ]
>
>Ehhhh.... let me see..
>
>The doppler on the up- and downlink SUBTRACTS because the transponder is
>inverting..
>Right, or.. ?
>
>Cheers, Jens
>
>73 de OZ1LRG
>
>

+--------------------------------------------------------------+
|  Chris Hill               mailto:chris.hill@crhtelnet.com.au |
|  CRH Telnet Pty Ltd                       A.C.N. 008 922 530 |
|  Phone  : +61 8 9310 3066              Fax : +61 8 9310 1117 |
|  Postal : PO Box 557 Willetton WA 6155 Australia             |
+--------------------------------------------------------------+

----
Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

```

AMSAT Home