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Re: AO40Rcv / Instatrack - InstaTune question



At 02:56 AM 6/10/2001 +0100, Gunther Meisse wrote:
>I am running AO40Rcv to demod the telm from AO-40 & it works like a dream. I
>am tracking with InstaTrack / InstaTune. I have entered the beacon freq in
>the IT AO-40 database. I get what I think is a strange Doppler Shift number.
>At ph 4 I had a negative number of about -30000 increasing, now at 20.8 I am
>increasing from -26431 toward 0. The direction is correct but the value
>strikes me as very high, especially since it did not travel through 0.

Those are plausible numbers. Remember, Doppler shift is proportional to frequency. 2401 MHz is more than 16 times as high as 146 MHz. A familiar LEO Doppler shift of 3 kHz at 2 meters would be nearly 50 kHz at S band.

Doppler passes through 0 when the range (distance from you to the satellite) is steady for an instant, because the satellite's velocity is perpendicular to the line from you to the satellite. If you look at the orbit diagram picture from InstantTrack, you can visualize when that would happen.

If you use InstantTrack's "Satellite Position Table (Ephemeris)" feature, you can see how the Doppler shift changes during a pass.

>Should I have the 145 Mhz IF in the database in place of the actual 2Ghz
>value?

Definitely not. If you did that, InstantTrack would underestimate the Doppler shift by a factor of more than 16.

73  -Paul
kb5mu@amsat.org

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