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**Session 117 - Instrumentation and Techniques.**

*Oral session, Thursday, January 18*

*Salon del Rey North, Hilton*

## [117.06] A Method of the Precision Doppler Measurements of Transverse Velocities

*L. M. Ozernoy (CSI/GMU and GSFC/NASA)*
Recent progress in techniques of measurements makes possible to implement
the relativisic Doppler effect
to measuring transverse velocities. The proposed method that makes it feasible
employs two key elements: (i) use of a stabilized gas absorption cell
placed before the spectrograph, which serves as a velocity metric and
(ii) elimination of a much larger longitudinal (radial) Doppler effect
and extraction of the transverse velocity component
via modulation the latter by the orbital motion of the Earth.
If \Delta\nu=\nu_max-\nu_min is the amplitude of the
frequency shift of some line (whose proper frequency is \nu_0)
during one revolution of Earth around the Sun,
the inferred tangential velocity of the source is given by
V_t= c^2øver 2v\sin b\Delta\nuøver \nu_l ,
where c is the speed of light, v is the Earth mean velocity, b is the
source's ecliptic latitude, \nu=\nu_l\left(1-V_t^2/c^2\right)^1/2
is the measured frequency that
includes both the longitudinal and transverse Doppler effects,
and \nu_l=\nu_0\left(1-V_l^2/c^2\right)^1/2\big /\left(1+V_l/c\right)
[1+(v/c)\cos (\widehatv, V_l)]
absorbs the entire longitudinal (radial) Doppler
effect; \nu_l is assumed to kept constant on the time scale of at least one
year.

The routine precision in an individual measuring velocity shifts is currently
7 m/s (iodine absorption cell), which enables one to determine transverse
velocities with uncertainty \sigma_V_t\sim35 km/s. Fourier analysis would
permit to extract the embedded periodicities with amplitudes of \sim1 m/s,
which translates into \sigma_V_t\sim5 km/s.

A telescope placed on a satellite
orbiting around the Earth would make possible a much quicker accumulation
of the periodic changes in the precision Doppler velocity measurements,
although at the expense of a smaller velocity v that modulates the data.
At radio wavelengths, the use of a cosmic megamaser source
within the object whose transverse velocity is to be measured
would also result in a larger uncertainty in V_t.
\vspace3mm

**Program
listing for Thursday**