Does The Pioneer Anomalous Acceleration Really Exist?
  The analysis of the Pioneer 10 and 11 data demonstrated the presence of an anomalous Doppler frequency blue-shift drift which is interpreted as an anomalous acceleration. The Doppler frequency dirft follows by considering the motions of the Pioneers in the universe, i.e. it is of cosmological origin. There is no anomalous acceleration. 
Walter Petry