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 Quote by Zman Am I missing something?
You are referring to Wikipedia! I think you are correct.

Welcome to these Forums Zman, you will find we treat Wikipedia articles with caution around here.

Of course, the PA is not measuring an acceleration nor distance from the Sun, I will copy from a link I included erroneously above, but because the edit facility timed out I was unable to repair it!

1. The PA is measured as a residual blue Doppler shift on signals returned back to Earth. The value of the frequency change or time acceleration is equal to:

ad = (2.92 ± 0.44) × 10−18 s-1.

2. This can be interpreted as an acceleration (either towards the Sun or the Earth) equal to
aP = (8.74 ± 1.33) × 10−10 m/s2.

3. The effect has been constant and equal for both spacecraft from 10AU - 90AU. (Pioneer 10 - Feb 2003) (Other effects swamped it when they were closer than than 10AU from the Sun.)

4. It does not show up in the orbital dynamics of the outer planets. This alone indicates to me that it cannot be modelled by modification in the gravitational field of the Sun. See Iorio's eprint Can the Pioneer anomaly be of gravitational origin? - answer: negative.

(However, as a caveat, remember the residuals in Uranus’ orbit that led to the discovery of Neptune? Once Neptune was discovered there was still a residual that led to a search for Planet X. Pluto was found and the search discontinued. However Pluto was not Planet X, it is 2 OOM too small, so a residual in Uranus’ orbital elements still appears to exist!)

5. That 'normal physics' from On-Board Systematics, (source The Study of the Pioneer Anomaly: New Data and Objectives for New Investigation Turyshev et al.), can so far explain a maximum of:

i Radio Beam Reaction Forc arb = (1.10 ± 0.10) × 10−10 m/s2. .
ii Anisotropic Heat Reflection aah = (−0.55 ± 0.55) × 10−10 m/s2. .
iii Differential Change of the RTG’s Radiant Emissivity are = 0.85 × 10−10 m/s2. .
iv Constant Electrical Heat Radiation as the Source: not viable.
v Helium Expulsion from the RTGs ahe = (0.15 ± 0.16) × 10−10 m/s2. .
vi Propulsive Mass Expulsion apme = ±0.56 × 10−10 m/s2.

This makes a maximum total of an = (2.1 ± 0.8) × 10−10 m/s2 that can be caused by normal physics leaving at least a minimum anomalous acceleration of ax = (6.6 ± 2.1) × 10−10 m/s2 to be explained.

This may be expressed as a minimum Doppler shift or clock drift of

ad residual = (2.20 ± 0.70) × 10−18 s-1.

6. Furthermore note that Hubble's constant in similar units (1/(Hubble Time) expressed in seconds) is equal to:

H = (2.4 ± 0.2) × 10−18 s-1 (with h=0.73) and where I have given H ±10% error bars, which is consistent with that unexplained residual ad residual in the PA.

The PA may therefore be cosmological and not local in nature and viable gravitational theories that predict such a clock drift should be given due consideration.

Garth