  The velocity of light is invariant under transformations that alter
space-time metrics, while leaving Maxwell's equations invariant. A
one-parameter special conformal invariance group of the equations exposes an
ambiguity in current interpretations of the Doppler effect. Comparisons between
Doppler measurements and direct measurements of velocities and positions of
distant spacecraft could determine the value of the group parameter. The metric
is a conformal metric and not a Minkowski metric if the group parameter is
found to be non-zero. In this case, current understandings of the physics of EM
wave transmission, the Doppler effect, and Hubble's relations, must be
substantially revised.
