In-line Water Vapor Radiometer for Precision Deep Space Doppler Tracking

Precision Doppler tracking supports many investigations, especially determinations of gravity fields and interior structures of solar system bodies. Analysis of past measurements, acquired on a 34m diameter antenna and calibrated with an external, co-pointing, water vapor radiometer, reveals that errors are dominated by water vapor fluctuations over the short term (30-100 sec) and by antenna mechanical noise over the long term (1000-10000 sec). A new effort is underway to develop a water vapor radiometer that is integrated with the downlink antenna's electronics, allowing the radiometer to receive through the same antenna beam thus providing better line-of-sight calibration.

The design uses a broad band septum polarizer (22 - 32GHz) and a pseudo-correlation radiometer topology to allow reception of both spacecraft and radiometer signals. Rather than a traditional Dicke switch radiometer approach that interrupts received signals, the design uses a magic-T hybrid to split the receive signal and inject a reference. The split signals are run through two low-noise amplifiers and then split again to feed additional magic-T hybrids. The split signals are summed with the reference signal in-opposite-phase to recreate the downlink signal and with the reference signal in-phase to provide the radiometer signal. Backend processing will extract the spacecraft signal as well as the radiometer brightness temperatures at three frequencies: 22.2, 23.8, and 31.4 GHz.

Initially, an existing feed package on a 34m antenna will be modified to add this functionality. System performance tests, looking for improved Doppler at short time scales, will be conducted. Ultimately, a smaller, stiffer, antenna is needed to also improve Doppler at long time scales by reducing antenna mechanical noise. Consideration is being given to designing an integrated Doppler/radiometer feed for use on a 12m-class antenna.