One of the critical aspects of Bird Tracking is handling of Doppler shift during the pass of satellite in your area.
1. What is Doppler shift/Doppler Effect?
The Doppler effect (or Doppler shift), named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer. The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession.
2. Why does it happen?
The relative increase in frequency can be explained as follows. When the source of the waves is moving toward the observer, each successive wave crest is emitted from a position closer to the observer than the previous wave. Therefore each wave takes slightly less time to reach the observer than the previous wave. Therefore the time between the arrival of successive wave crests at the observer is reduced, causing an increase in the frequency. While they are traveling, the distance between successive wavefronts is reduced; so the waves "bunch together". Conversely, if the source of waves is moving away from the observer, each wave is emitted from a position farther from the observer than the previous wave, so the arrival time between successive waves is increased, reducing the frequency. The distance between successive wavefronts is increased, so the waves "spread out".
3. How to effectively handle the doppler shift for bird VO-52 (HAMSAT)?
The success of bird tacking depends on how effectively you tackle the doppler shift while satellite is approaching you and while it is moving away from you.
Below given is the table which explains you can tackle the doppler shift for VO-52 (HAMSAT) while up-linking (transmitting) on UHF 435.250 Mhz, Keep the downlink fixed at 145.900 Mhz.
When Sat is approaching you----- 435.245
When Sat is nearer to you -------- 435.250 145.900
When Sat is goes away from you- 435.255
The other way could be to keep the Uplinking frequency fixed at 435.250 and shift the frequencies while as satellite approaches you as described below:-
145.905----When Sat is approaching you
435.250 145.900----When Sat is nearer to you
145.895 --- When Sat is goes away from you
As you get used to these shifting during the advanced stages you can shift both uplink and downlink simultaneously.
Happy Bird Tracking
Next post i shall give the frequency shifts for AO-51