That sound recording made from the Mars Express tracking data is nifty, but I don't understand the way that the signal's frequency is shifting. The caption to the signal animation reads
The lander can be seen in the animation starting from about 342 s after the start time and disappears at about 1085 s. This shows Mars Express picking up on the Phoenix signal and tracking it while closing in on the lander; the closest Mars Express got to Phoenix was 1550 km.
As Mars Express flew away, the lander deployed its parachute, separated from it and landed, the signal from the lander was cut off.
What I see in the animation is:
- at about 342 sec a spike appears at between 7 and 8 KHz.
- the spike moves to lower frequencies over time.
- shortly before 700 sec it reaches its lowest frequency, less than 1 KHz.
- then the spike moves to the right.
- The spike disappears at 1085 seconds, I guess with landing.
I don't understand why the frequency goes down, then up again. I would have assumed that the frequency would be highest at the beginning, with Mars Express approaching Phoenix, then when it passes its closest approach the frequency would have suddenly shifted down, as Mars Express draws away. Can someone explain to me why the frequency changes as it does?