Design and performance of a high-stability water vapor radiometer

The design of two new high‐stability microwave water vapor radiometers is presented along with a performance evaluation. The radiometers operate next to a spacecraft tracking station at NASA's Goldstone facility in California, where they will be used to calibrate tropospheric path delay fluctuations during an upcoming gravity‐wave search experiment (GWE) involving the Cassini spacecraft. Observing frequencies of the radiometers are 22.2, 23.8, and 31.4 GHz, and the antenna beam width is 1°. The instruments are room temperature Dicke radiometers with additive noise injection for gain calibration. Design highlights include: (1) a practical temperature control system capable of stabilizing the entire receiver to a few millikelvin from day to night; (2) redundant noise diode injection circuits with 30 ppm RF power stability; and (3) a voice coil actuated waveguide vane attenuator which is used as a high‐performance Dicke switch. Performance of the radiometers is evaluated from intercomparisons of the two radiometers and from continuous tip curve calibrations spanning nearly 1 year. Structure function analysis of the intercomparison data indicates that the brightness temperature stability of these radiometers is better than 0.01 K on 1000–10,000 s timescales. Analysis of tip curve calibrations indicates RMS errors of ∼0.05 K on 30‐day timescales and 0.15 K on 1‐year timescales.