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Stopping the periodic shift
As the systemic velocity of the source in the observatory frame varies with
time in a predictable way, the radio-observatories tune the local
oscillator frequency in order to stop the shifting of the observed
frequency scale. For instance, the relation between the observatory
frequency and the LSR (Local Standard of Rest) frequency is
 |
(29) |
because
 |
(30) |
Thus adding
to the local oscillator frequency makes
the signal frequency appear as if it was measured in the LSR frame. All the
velocities are then expressed in the LSR frame instead of the observatory
frame.
However, there is a single local oscillator frequency. Let's assume that
the correction is done at the tuned signal frequency, i.e.,
 |
(31) |
The correction which is applied to another frequency is then
 |
(32) |
while the correction that should have been applied to another frequency is
 |
(33) |
The difference (or error) is
 |
(34) |
The correction is thus exact only in
, i.e., for the tuned
signal frequency1. All the other signal and image
frequencies oscillates with time with a frequency amplitude which linearly
increases with the distance to the reference channel. In other words, this
corrects only the global frequency shift, not the dilatation around the
reference channel.
Next: Stopping the periodic dilatation
Up: Earth movements
Previous: Periodic shift and dilatation
Contents
Gildas manager
2023-06-01