1. Assessing passband errors
in MW Sounding Data
Roger Saunders2
Bill Bell2, Qifeng Lu1, Carole Peubey3, Niels Bormann3, Steve English3,
Alan Geer3, Enza Di Tomaso3 and Richard Renshaw2
National Satellite Meteorological Center, CMA, Beijing
Met Office
ECMWF
Thanks to: Michail Tretyakov

2. FY-3A MWTS & AMSU channels
FY-3 & AMSU-A are
heterodyne radiometers
FY-3: 4 channels. free running LOs
AMSU-A:14 channels.
channels 9  phase locked
channels 1-8 free running
MWTS Channels
Channels < 57 GHz . BW ~ 300 MHz
Required frequency stability :
~ 5MHz for channels < 57 GHz *
* Peubey et al, ECMWF Tech memo 643
(based on simulations of forecast degradations
resulting from frequency drift)

3. MWS Biases - CASE 1 FY-3A MWTS
Brightness temperature biases (state dependent)
found to be due to pass band uncertainties
(30-55Hz errors in pre-launch measurements)
Due to n change in LO micro-cavity
[Lu et al, JTECH, 2011]
Mechanism: vertical shift in weighting functions
results in a state dependent bias.

4. Optimisation of pass band centre frequency estimates
Pass band centres:
design spec.
measured
optimised
Shifts exist relative to
pre-launch
measurements
Residual biases for
ch 3 and 4
© Crown copyright Met Office

5. MWTS Radiometer Non-linearity
In general the response of a MW
radiometer will be slightly
non-linear wrt the
measured scene temperature.
If perfect linearity is assumed between the
2 calibration points (cold space and a warm target)
then an error (bias) results

6. MWTS Radiometer Non-linearity
Adjust passband:
variance reduced
Corrected for
non-linearity:
leaves locally
unbiased data

7. Improved FY-3A MWTS Data Quality
First Guess Departures / K

8. Improved AMSU-A Data Quality
First Guess Departures / K
(obs – model equivalents,
based on T+9 hour forecasts)