Frequency and velocity scales
NOEMA operates with Doppler tracking. When the user provides a setup,
with a given reference rest frequency,
, and a source
velocity,
, within some referential (usually LSR referential) on
one side and an IF frequency,
, at which the rest
frequency should appear on the other side. The Doppler tracking
consists in modifying the LO1 frequency so that the two frequencies
stay aligned to within one tenth of the smallest channel spacing of
the spectral setup being observed.
The rest,
, and topocentric frequencies,
, are
related through:
 |
(39) |
where
is the Doppler factor between the chosen referential
and the topocentric referential, and
is the source systemic
velocity within this referential.
Since
(+ for an USB conversion, - for
a LSB conversion), we have:
 |
(40) |
so by setting
 |
(41) |
we ensure the Doppler tracking as defined above.
Table 4:
Parameters for receiver tuning. For the source referential TYPE_VEL, 1 means LSR, 2 heliocentric, 3 geocentric, 4 no doppler tracking, 5 unknown.
Parameter |
Freq |
Frame |
Notation |
SIC variable |
Reference rest frequency |
RF |
Rest |
 |
FREQUENCY |
Sideband |
|
|
|
ISB |
Reference IF frequency |
IF |
Topo |
 |
FIF1 |
LO1 frequency |
|
Topo |
 |
FLO1 |
Source referential |
|
|
|
TYPE_VEL |
Source velocity |
|
Ref. |
 |
VELOCITY |
Doppler factor |
|
|
 |
DOPPLER |
|
The frequency value of channel
from spectral window
are
defined as:
where
is the channel spacing of the backend in the
topocentric frame and
Table 5:
Parameters for spectral windows and corresponding SIC variables
Parameter |
Freq |
Frame |
Notation |
SIC variable |
Number of channels |
|
|
 |
N_CHANNELS |
Channel number of (2) |
|
|
 |
REF_NUM |
Center frequency |
IF |
Topo |
 |
LFCEN |
Reference channel |
|
|
 |
REF_CHANNEL |
Rest frequency |
RF |
Rest |
 |
F_OFFSET |
Frequency spacing |
|
Topo |
 |
LFRES |
Frequency spacing |
|
Rest |
 |
F_RESOLUTION |
|