Uncertainty of Radiometer Calibration Loads and Its Impact on Radiometric Measurements

Abstract

We present an uncertainty analysis of radiometer calibration. The procedure can be used to determine the uncertainty in the nominal brightness temperature of the unknown scene. A total power radiometer requires frequent calibration with known reference loads that are connected to the radiometer. Our analysis includes uncertainties from the radiometer calibration loads and from the connecting network (CN) that is required to multiplex calibration loads and scene to the radiometer input. We show the design and analysis of three calibration loads and how their uncertainties propagate from load terminals to the radiometer calibration plane and to the scene. All three loads, including a cryogenic load, are simple, inexpensive, and show great stability and accuracy. We give an uncertainty calculation example for our three calibration loads and for the CN. We validate our model and the long-term stability of the loads through measurements. The analysis is done at 52 GHz, but the analysis and the construction of the loads are generic and easily scalable to other frequencies.

Calibrating ground‐based microwave radiometers: Uncertainty and drifts

“…LN2cals of the HATPRO‐G4 are mainly affected by three error sources: (i) the uncertainty of the refractive index of liquid nitrogen; (ii) nonzero reflectance of the radiometer's horn antenna; and (iii) the entrainment of oxygen into the bath [ Maschwitz et al , ; Pospichal et al , ; Paine et al , ]. Kaisti et al [] showed that the increase of the LN2 boiling point due to hydrostatic pressure is smaller than 0.05 K for filling heights of the cryogenic load of a few centimeters (maximum 7 cm above the target) as used in our measurement setup.…”

“…In addition to the resonant effect, Paine et al [] found that LN2cals are affected by the entrainment of oxygen into the liquid nitrogen bath inducing an increase of the cold reference's temperature up to 0.4 K during a LN2cal, depending on wind conditions and the length of the calibration activity. Kaisti et al [], who combined models and observation, estimated the accuracy of the cryogenic target to be ±1.25 K.…”

Radiometric Resolution Analysis and a Simulation Model

“…These loads are connected to the radiometer through a lossy connecting network. We describe the uncertainty and stability of these loads and the effect of the connecting network in [6]. In the measurements and in the example simulation presented in this paper the scene is a matched load (ML) inside a temperature stabilized radiometer box with a set brightness temperature of T ML = 300.15 K.…”

“…The solved scene is then also given at the calibration plane from where it needs to be converted to the scene terminals. We describe these conversions in [6] and here we only consider the analysis at the calibration plane.…”

“…All measured brightness temperatures were converted to the radiometer input i.e., to its calibration plane [6]. We make the comparison between the simulation and the measurement at this plane.…”