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SATPC32 - Doppler Correction off on ISS

Discussion in 'Satellite and Space Communications' started by N7AY, Nov 21, 2021.

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  1. N7AY

    N7AY Premium Subscriber QRZ Page

    I have my PC clock accurate and doppler corrections sufficiently frequent but for some reason my doppler correction gets off on some birds (ISS is the worst) then the pass is close to directly overhead. It gets so far off it breaks the link. Is that normal? How to correct this?
  2. N7AY

    N7AY Premium Subscriber QRZ Page

    .... and my keps are up to date.
  3. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    In my experience, yes. I haven't found any tracking program to be bullet proof. Doppler is changing rapidly at zenith.
  4. K6CLS

    K6CLS Ham Member QRZ Page

    I agree, predicted Doppler is just guidance; your radio may be off frequency, the satellite radio may be off, who knows. That's why I have one hand on the VFO.
  5. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    In my mind the solution is the next level of technology; Having a bandpass spectrum waterfall. Either buy an IC-9700 or add an external SDR with monitor that samples your radio IF.
  6. KB1PVH

    KB1PVH XML Subscriber QRZ Page

    Open the CAT tab and select 10 instead of the default 1 in the Speed section. I've also changed the SSB/CW interval setting from the default 70 I think it is to 20. Give those settings a try and see what happens.
  7. N7AY

    N7AY Premium Subscriber QRZ Page

    My understanding is limited to intuition mostly but it would seem to me that if SATPC32 knows where I am (I told it), where the satellite is and how fast it's traveling that calculating a sufficiently accurate doppler adjustment wouldn't be difficult. My rotor/controller can move my yagis fast enough to track the bird and I believe SATPC32 can frequency update my ic9700 fast enough to track it calculated doppler shift. This shouldn't be difficult I wouldn't think. I had my SSB/CW Interval set to 50. I'll change it to 20 and give that a try. My Speed was at the default "1", I'll give "10" a try.

    Part of me is thinking that SATPC32 might not be tracking the ISS correctly. I think this because it commonly (but not always) reports differing MaxE elevations for the ISS. The one reported in the main window and one in the CountDown window often don't agree with each other. Sometimes they differ by a lot - as much as 20-30 degrees. Makes me wonder if SATPC32 is using an incorrect elevation and thus calculates incorrect doppler corrections. I have see this problem mildly on other FM birds but it by far worse on the ISS.

    Another clue is that other satellites don't have this problem. Especially of interest are the linears when they pass close to the zenith i don't see this problem at all. SSB is a lot less tolerant of frequency shift but I don't see this problem with 3-4 different linear birds.
  8. KB1PVH

    KB1PVH XML Subscriber QRZ Page

    The ISS does boost maneuvers occasionally so I just got in the habit of updating my keps everytime I fire Satpc32 up using the nasabare file from AMSAT.
  9. KS1G

    KS1G Ham Member QRZ Page

    Behavior is "normal". SatPC32 (and every other tracker with which I have any familiarity) are open loop; they rely on the accuracy of the keplerian elements, algorithm that propagates the orbit forward in time, time, operating frequencies of the spacecraft, and operating frequencies of the ground station. (Our antennas for LEO have a wide enough beam I'm not too worried about antenna pointing). At TCA, the rate of doppler shift is maximum, so any errors are magnified. Good suggestions made up thread to use most current available data (esp for ISS) and update frequency adjustments at shorter intervals.

    FM sats can give us closed-loop Doppler correction for the downlink by tracking the FM center frequency. I think (have not used) some SDR packages have an AFC capability and I recall some folks here suggesting in IC-9700 discussion threads activating AFC on FM downlinks. The greater doppler for the ISS FM repeater on the downlink, so if your setup allows it, enabling AFC could have some real benefit or at leats be an interesting comparison vs. PC-based tracking.

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