[FrankM]

There is a new effort to identify what many call the anomalous deceleration of the Pioneer 10 & 11 spacecraft. Very few reports point out actually how they determined the deceleration was detected.

In brief, the Pioneer Anomaly is an unexplained deceleration of the spacecraft Pioneer 10 and Pioneer 11 as they exit the solar system. Technically speaking, it is a blue shifted Doppler frequency of 6x10^-9 Hz, which can be interpreted as a deceleration of 8.74x10-10 m/s^2 of the spacecraft away from the sun.

2005 Pioneer Anomaly Conference - What We Do | The Planetary Society

Previous NASA reports suggest that permittivity changes as a function of the distance from the Sun. Telemetry signals were "red shifted" close to the Sun and they are "blue shifted" away from Sun.

The last window for receiving the Pioneer 10 signal has passed and it was not detected. They aimed their radio telescopes where they thought the spacecraft should be based upon the consideration it was "decelerating", rather than looking where it would be if it had not.

March 2006: Update from the Pioneer Anomaly Team - What We Do | The Planetary Society

Basic physics teaches us that the speed of light varies as a function of the permittivity of the medium in which it is permitted to propagate.

Do you know why they interpret the "blue shift" as a deceleration rather than a change in the permittivity?