Beginner question on doppler

Discussion in 'Satellite and Space Communications' started by WB4SPT, May 10, 2017.

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  1. WB4SPT

    WB4SPT Ham Member QRZ Page

    New to the satellite sport, be kind. :) I just got my first copies of AMSAT.

    On the LEO sats. Do folks account for uplink dopper shift by varying the Tx freq, or just let the other party deal with it on the Rx side? Let's say this is SSB mode.
     
  2. N8HM

    N8HM Premium Subscriber QRZ Page

    It depends:

    1. You can use computer control and control both uplink and downlink frequency.
    2. If you aren't computer controlled, you'll want to tune the higher of the two frequencies and keep the lower frequency constant while in QSO. For a Mode V/u (J) satellite with a 145 MHz uplink and 435 MHz downlink, you'll tune your receiver while keeping your transmit frequency constant. For a Mode U/v (B) satellite with a 435 MHz uplink and 145 MHz downlink, you'll tune your tranmsitter while keeping your receive frequency constant.
     
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  3. N8HM

    N8HM Premium Subscriber QRZ Page

    Dupe post
     
  4. WB4SPT

    WB4SPT Ham Member QRZ Page

    I suppose this gets more interesting with 3 parties in the QSO. Each with their own doppler shifts. Seems like computer control or not, each of the 3 will be on a different Rx freq, correct?
     
  5. W5PFG

    W5PFG Premium Subscriber QRZ Page

    It's really not so bad. Happens often.

    To answer your original question a little differently, if you are manually adjusting for Doppler shift, you always adjust the higher frequency:

    For FO-29 & SO-50, you'd adjust the downlink (70cm) not the 2m uplink

    For AO-85 & AO-7, you'd adjust the uplink (70cm) not the 2m downlink
     
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  6. K6LCS

    K6LCS Subscriber QRZ Page

    Here's a shot from my sat presentation, showing how much more pronounced the Doppler
    phenomenon is on 440 versus 2M ... Also posted is the actual equation for determining the
    numbers for yourself.
     

    Attached Files:

  7. WB4SPT

    WB4SPT Ham Member QRZ Page

    Thanks for that. I wonder if someone has the "long form" of that formula? Like, each QTH lat/long and the ephemeris of the sat? I'm sure my garmin has it buried in there deep.
     
  8. NJ4Y

    NJ4Y Premium Subscriber QRZ Page

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