Understanding Doppler Shift
Why Correct on the Uplink?
The standard convention in amateur satellite work is to correct for Doppler on your transmit (uplink) frequency while leaving the downlink constant. This means every operator monitoring the satellite's downlink hears your signal at a stable frequency, even though the satellite is moving. If everyone corrected their own downlink instead, each operator would hear others drifting — creating chaos on a shared transponder.
The S-Curve
Doppler shift follows a characteristic S-curve over a pass. The rate of change is highest at the point of closest approach (highest elevation), where the satellite's radial velocity changes most rapidly. This is why SSB/CW operators must tune more aggressively during high-elevation passes. Orbital Radar's Doppler gauge computes the corrected frequency in real time using range-rate data from SGP4 propagation.
Doppler Shift by Frequency
| Band | Centre Frequency | Max Shift | FM Impact |
|---|---|---|---|
| 2m VHF | 145.900 MHz | ±3.5 kHz | Absorbed by receiver BW |
| 70cm UHF | 435.000 MHz | ±10 kHz | May need manual tuning |
| 23cm SHF | 1296.000 MHz | ±30 kHz | Active correction required |
| 13cm SHF | 2400.000 MHz | ±56 kHz | Active correction required |