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Qualitative impact of the MRTCAL frequency dependent
calibration
Differences between MIRA- and MRTCAL-calibrated spectra are expected
as a consequence of the different calibration bandwidths. One value of the
calibration parameters are derived and applied per 1.35GHz natural
hardware unit when MIRA calibrates FTS200 spectra. In contrast, MRTCAL
derives and applies the calibration in steps of 20MHz (in the current
default settings for the automatic online data processing; this value can
be customized by the user).
The MRTCAL default is intended to improve the quality of the baseline,
as can be seen on Fig. 1
to 4. The average spectra computed from the
same 15 minutes On-The-fly scan is displayed in each of the four
figures. The only difference between the four spectra are the way
MRTCAL calibrates the raw data.
- In Fig. 1, the calibration parameters
are derived and applied per natural hardware unit (i.e., every 1.35GHz
as defined by the FTS units). This gives a staircase look to the spectra.
- In Fig. 2, the calibration parameters
are derived every 1.35GHz. But they are linearly interpolated before
being applied. The staircase look is greatly decreased.
- In Fig. 3, the calibration parameters
are derived and applied every 20MHz. No linear interpolation is done
before application. The staircase look and the baseline oscillations
disappeared. The atmospheric line around 110.8GHz now appears in
absorption as it should because the reference position was localized
about 1 degree away from the OTF observations.
- Finally, in Fig. 4, the calibration
parameters are derived every 20MHz and linearly interpolated before
being applied. The improvement with respect to the previous solution
exists even though it is not obvious on this plot.
MRTCAL proposes by default the fourth solution. It thus delivers better
behaved baselines. The MRTCAL default is also intended to improve the
line calibration accuracy in regions where calibration parameters vary
quickly as a function of frequency, as shown in
Sect. 5.
Figure 1:
Spectra averaged over a 15 minute On-The-Fly scan observed
with a combination of E090 and FTS200 under average summer weather
(
mm of precipitable water vapor). The simultaneously
observed LSB and USB spectra are shown on the bottom panel and a
5GHz window near the 3mm band edge is zoomed on the top
panel. The calibration parameters are here derived and applied by
MRTCAL per natural hardware unit (1.35GHz bandwidth for each FTS
unit, i.e., same as MIRA calibration scheme).
|
Figure 2:
Same as Fig. 1 except that the
calibration parameters are now linearly interpolated by being
applied.
|
Figure 3:
Spectra averaged over a 15 minute On-The-Fly scan observed
with a combination of E090 and FTS200 under average summer weather
(
mm of precipitable water vapor). The simultaneously
observed LSB and USB spectra are shown on the bottom panel and a
5GHz window near the 3mm band edge is zoomed on the top
panel. The calibration parameters are here derived and applied by
MRTCAL every 20MHz.
|
Figure 4:
Same as Fig. 3 except that the
calibration parameters are now linearly interpolated by being
applied. This spectra is the default delivered by MRTCAL.
|
Next: Observations dedicated to the
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Gildas manager
2023-06-01