DQRID : D960404.1
Start DateStart TimeEnd DateEnd Time Data Quality Metric
02/12/1994000004/04/19962359Suspect
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Subject:
SGP/MWR/B1/B4/B5 - Reprocess: Error in MWR calibration
DataStreams:sgpmwrlosB1.a1, sgpmwrlosB4.a1, sgpmwrlosB5.a1, sgpmwrlosB5.a0, sgpmwrlosB1.a0,
sgpmwrlosB4.a0
Description:
The effect of this error is small.  At most, it results in a bias of
about -0.015 cm in precipitable water vapor and -0.015 mm in liquid
water path during clear sky conditions.  The error is largest when the
brightness temperatures are small (i.e. clear skies and low PWV).

The error results from failing to correctly account for the effect of
the Teflon window covering the radiometer mirror.  Although the
contribution of the window is subtracted when the tip curve data are
reduced to determine the true zenith brightness temperature, it is not
added back in when the zenith brightness temperature is used to
calibrate the noise diode.  This would still not be a problem if the
contribution of the window where not subtracted (again) during zenith
line-of-sight (LOS) operations.  But it is and the net effect is to
subtract the contribution of the window twice.

The calibrations ('Noise Injection Temperatures') are off by a factor
of 1.00164 and 1.00217 for the 23.8 and 31.4 GHz frequencies,
respectively.

The magnitude of the error is equal to the emissivity of the window
multiplied by the difference between the brightness temperature and the
temperature of the window.  The latter is taken to be equal to the
temperature of the internal blackbody target (which is about 10 deg C
above ambient.)  The emissivity of the window is 0.00164 at 23.8 GHz
and 0.00217 at 31.4 GHz.  For a reference temperature of 292.6 K and
brightness temperatures of 32.3 and 20.8 K at 23.8 and 31.4 GHz
respectively, this amounts to errors of -0.43 and -0.59 K at the
respective frequencies.  The average PWV for this date (5 April 1995)
was 1.4 cm.

At higher levels of PWV and for cloudy conditions, the brightness
temperatures are higher and so the error is smaller.

I will adjust the calibrations of all SGP radiometers to account
for this problem by the end of tomorrow (4 April 1996).
Suggestions: 
Measurements:sgpmwrlosB4.a1:
  • liq
  • vap
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sgpmwrlosB1.a1:
  • liq
  • vap
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sgpmwrlosB1.a0:
  • liq
  • vap
more
sgpmwrlosB4.a0:
  • liq
  • vap
more
sgpmwrlosB5.a1:
  • liq
  • vap
more
sgpmwrlosB5.a0:
  • liq
  • vap
more

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