What is Doppler?
Doppler is the change of frequency that occurs when an object (such as a spacecraft) has motion relative to you. It manifests itself as the frequency of a received signal being HIGHER as the spacecraft approaches you, dropping and going LOWER than the nominal frequency as it recedes from you. The amount of shift increases with both the velocity and the frequency of the signal in question. On AO-40 Doppler can be as high as 30 KHz on S band. Note: the spacecraft receiver experiences Doppler on your signal too. However, as your uplink is lower in frequency than the downlink, this can often be ignored. Also, other stations far from you experience different amounts of Doppler, so you can’t just subtract Doppler and make a sked for that frequency!
