What is happening is that the satellite is receiving a transmission at the SGLS uplink frequency for the channel from a tracking station in the UK. The ground station's transmitter frequency is very precise and very stable. When the satellite acquires the signal, it locks onto the frequency and scales it up in the ratio 256/205 to produce the downlink frequency.

The satellite transmitter's frequency is in a continuous state of change, driven by the Doppler shifted frequency received from the ground station. The satellite sees the same sort of Doppler curve that a ground station sees from a satellite.

At the ground station itself, the received frequency at the point of Closest Approach (CA) will be the channel frequency - 2247.500 MHz or 2272.500 MHz in the case of the STSS Demonstrator satellites.

The image shows a plot of both SV2 (the two curves on the left) and SV1. The first trace is SV2 in frequency locked mode with a steep Doppler curve as a result. At 03:33:50, the transmitter unlocks and the uplink Doppler shift is removed. The trace becomes a normal Doppler curve as the transmitter returns to its free running mode at about 2272.518 MHz.

After the receiver frequency was changed, the third line is the start of the SV2 pass in free running mode with the transmitter at 2247.500 MHz. The curve shape is similar to the unlocked portion of the SV2 trace. The frequency scale on the right relates to the SV1 trace.

The ground station is about 250 kilometres from here. The satellite's speed along the ground track is a 5-6 kilometres per second so there can be up to forty seconds difference in time between CA at the ground station and CA here depending on the relative geometry of a pass. The satellite transmission frequency goes through the channel frequency a few seconds BEFORE closest approach here. By the time CA occurs at the Zarya location, the frequency is a few kiloHertz below the channel frequency (2272.500 MHz in the case of SV2). The situation is even more complicated for a receiver further away from the ground station.

The diagram below illustrates the difference between a tracked pass of STSS Demonstrators SV2 where the receiver is free running, and one where its transmitter is locked to the uplink frequency.

The free running transmission frequency tends to wander a little but is generally close to 2272.515 MHz. Once locked to the uplink, the transmitter frequency becomes very stable and, at closest approach to the ground station it is precisely 2272.500 MHz.

It is a few kHz different when it comes closest here. The main effect of being only about 250 km from to the source of the uplink is that Doppler curve makes the closes approach look as though it occurs when the satellite is roughly equidistant from the two sites but slightly biased away from the transmitter site due to the disparity between the uplink and downlink frequencies. In the example above, the satellite reached CA at the ground station before it reached here.