Calibration of a 35 GHz airborne cloud radar: lessons learned and intercomparisons with 94 GHz cloud radars
<p>This study gives a summary of lessons learned during the absolute calibration of the airborne, high-power Ka-band cloud radar HAMP MIRA on board the German research aircraft <i>HALO</i>. The first part covers the internal calibration of the instrument where individual instrument...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-03-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | https://www.atmos-meas-tech.net/12/1815/2019/amt-12-1815-2019.pdf |
| Summary: | <p>This study gives a summary of lessons learned during the absolute calibration
of the airborne, high-power Ka-band cloud radar HAMP MIRA on board
the German research aircraft <i>HALO</i>. The first part covers the internal
calibration of the instrument where individual instrument components are
characterized in the laboratory. In the second part, the internal calibration
is validated with external reference sources like the ocean surface
backscatter and different air- and spaceborne cloud radar instruments.</p>
<p>A key component of this work was the characterization of the spectral
response and the transfer function of the receiver. In a wide dynamic range
of 70 <span class="inline-formula">dB</span>, the receiver response turned out to be very linear
(residual 0.05 <span class="inline-formula">dB</span>). Using different attenuator settings, it covers
a wide input range from <span class="inline-formula">−105</span> to <span class="inline-formula">−5</span> <span class="inline-formula">dBm</span>. This
characterization gave valuable new insights into the receiver sensitivity
and additional attenuations which led to a major improvement of the absolute
calibration. The comparison of the measured and the previously estimated
total receiver noise power (<span class="inline-formula">−95.3</span> vs. <span class="inline-formula">−98.2</span> <span class="inline-formula">dBm</span>)
revealed an underestimation of 2.9 <span class="inline-formula">dB</span>. This underestimation could
be traced back to a larger receiver noise bandwidth of <span class="inline-formula">7.5</span> <span class="inline-formula">MHz</span>
(instead of 5 <span class="inline-formula">MHz</span>) and a slightly higher noise figure
(1.1 <span class="inline-formula">dB</span>). Measurements confirmed the previously assumed antenna
gain (50.0 <span class="inline-formula">dBi</span>) with no obvious asymmetries or increased side lobes.
The calibration used for previous campaigns, however, did not account for a
1.5 <span class="inline-formula">dB</span> two-way attenuation by additional waveguides in the airplane
installation. Laboratory measurements also revealed a 2 <span class="inline-formula">dB</span> higher
two-way attenuation by the belly pod caused by small deviations during
manufacturing. In total, effective reflectivities measured during previous
campaigns had to be corrected by <span class="inline-formula">+7.6</span> <span class="inline-formula">dB</span>.</p>
<p>To validate this internal calibration, the well-defined ocean surface
backscatter was used as a calibration reference. With the new absolute
calibration, the ocean surface backscatter measured by HAMP MIRA agrees very
well (<span class="inline-formula"><1</span> <span class="inline-formula">dB</span>) with modeled values and values measured by the GPM
satellite. As a further cross-check, flight experiments over Europe and the
tropical North Atlantic were conducted. To that end, a joint flight of
<i>HALO</i> and the French Falcon 20 aircraft, which was equipped with the RASTA cloud
radar at 94 <span class="inline-formula">GHz</span> and an underflight of the spaceborne CloudSat at
94 <span class="inline-formula">GHz</span> were performed. The intercomparison revealed lower
reflectivities (<span class="inline-formula">−1.4</span> <span class="inline-formula">dB</span>) for RASTA but slightly higher
reflectivities (<span class="inline-formula">+1.0</span> <span class="inline-formula">dB</span>) for CloudSat. With effective
reflectivities between RASTA and CloudSat and the good agreement with GPM,
the accuracy of the absolute calibration is estimated to be around
1 <span class="inline-formula">dB</span>.</p> |
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| ISSN: | 1867-1381 1867-8548 |