Citation
Abstract
Brightness temperature measurements at 22.2 GHz, 23.8 GHz, and 31.4 GHz collected by a water vapor radiometer (WVR) embedded within the DSS-13 beamwaveguide (BWG) optics are presented and compared with simultaneous measurements from an identical reference radiometer located outside of the beam waveguide. The objective of these tests is to assess the feasibility of measuring tropospheric water vapor with an embedded microwave radiometer. Such a system potentially could reduce path-delay estimation errors associated with the antenna ofiset and beam mismatch of the current stand-alone WVR. The data show that the excess noise temperature of the BWG antenna is considerable|on the order of 6 to 12 K|and that the noise temperature varies greatly depending on azimuth and elevation positions, feedhorn gain and alignment, and ambient temperature. The data also show that the excess noise of the BWG is a very smooth function of these variables. For time scales of up to 300 seconds and spatial scales up to about 30 degrees, the excess noise during individual azimuth scans of the antenna while at a 20 degree elevation angle can be fit to within 0.03 K with a radiative transfer model that involves a three-harmonic Fourier expansion versus azimuth. Above 45 degrees of elevation angle, the excess noise can be fit to about 0.01 K for the same spatial and temporal scales. Side-lobe contamination is believed to determine these error levels. At longer time scales (3 days at a fixed zenith position), fits versus ambient temperature and surface humidity show residual errors on the order of 0.1 K. At short time scales of less than »300 seconds, the difierence between the embedded and the external measurements are shown to be dominated by the atmospheric variability and sampling difierences associated with antenna pattern mismatch and the spatial ofiset between the two systems. These results indicate that an embedded radiometer operating in conjunction with an external radiometer would ofier significant advantages over the stand-alone external radiometer on short time scales. Such measurements would fill a gap in present tropospheric path-delay calibration capabilities for time scales of less than 300 seconds.
Keywords
Details
- Volume
- 42-143
- Published
- November 15, 2000
- Pages
- 1–23
- File Size
- 696.7 KB