# 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: Sun, 22 Jul 2001 12:15:40 +0800

```Hi Hasan,

Using "435.630 up corresponds to 2401.360 down....without doppler" as our
example:

RX = 2401.360 MHz + Doppler_at_2.4GHz
= 2401.360 MHz + (-0.02687 MHz)
= 2401.33313 MHz

[Note that for the receive calculation, you need to _add_ the Doppler, not
subtract it.  When the satellite is moving away from you, the Doppler has a
negative sign, so adding a negative number is effectively subtracting the
magnitude of the Doppler shift.]

TX = 435.630 MHz - (Doppler_at_2.4GHz, corrected for UHF)
= 435.630 MHz - (-0.02687 MHz * 435.630 MHz / 2401.360 MHz)
= 435.630 MHz - (-0.02687 MHz * 0.1814097011693)
= 435.630 MHz - (-0.00487447867042 MHz)
= 435.63487 MHz

[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 :-).  ]

There's a good article related to this topic, by Paul kb5mu, dating back to
1994.  See:
http://www.amsat.org/amsat/features/one_true_rule.html

You've probably worked this out by now anyway...  did you have a successful
QSO?

73 Chris vk6kch

>I'm showing a doppler of about -26.87 Khz at 0300 utc tonight (predicted).
>I'm trying to pick a freq pair to put into the 847 that will help me find
>myself.
>
>435.630 up corresponds to 2401.360 down....without doppler.
>
>With -27 khz of doppler at 0300, what would be the predicted frequency I
>would find myself on actually?
>
>How much of the 27 khz of doppler do I need to make up for on the UPLINK of
>435.630, in order for me to actually show up on 2401.360?
>
>(I know this is a dumb question, but bear with me)
>
>Listening for the beacon is easy..just subtract doppler from 2401.332 and
>bingo...there it is. My question seems a bit more complicated....what
>adjustment do I apply from a non-doppler corrected uplink freq so that I
>come out on the "predicted" 2401 downlink freq?
>
>The SUB tune function of the 847 is made for VERY slow freq adjustments...I
>don't want to have to crank the knob for five minutes trying to get close. I
>want to use the direct keypad entry to get really close, and then use the
>SUB tuning for the minor corrections required to carry on a qso.
>
>Tnx any and all. 73
>
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