EMBEDDED PSEUDO-CORRELATION WATER VAPOR RADIOMETER IN NASA’S DEEP SPACE NETWORK FOR ATMOSPHERIC DELAY CORRECTION

M. Salim¹, A. Tanner¹, A. Jongeling¹, S. Brown¹, L. Locke¹, J. Lazio¹

¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

Water vapor is the most variable of the major atmospheric constituents and a dominant factor in radio propagation delay [1]. Fluctuations in tropospheric water vapor directly degrade the precision of Doppler tracking and ranging of spacecraft signals, which in turn degrade Radio Science experiments and spacecraft navigation. The current approach to measuring water vapor fluctuations involves the use of an antenna that observes a slightly different line of sight than that to the spacecraft, leading to residual phase errors in the measured quantities. To mitigate this, we have developed the Embedded Water Vapor Radiometer (EWVR) and tested it on a Deep Space Network (DSN) antenna. This integration enables simultaneous reception of spacecraft telemetry and measurements of atmospheric brightness temperatures near the 22.2 GHz water vapor absorption line, ensuring corrections to propagation delays without disrupting communication.

The key innovation lies in the pseudo-correlation radiometer architecture [2,3], which provides simultaneous amplification of reference and antenna signals. The signal from the antenna is fed into the summing arm of a magic-T while a reference signal (matched load) is fed into the delta-arm. The two outputs of the magic-T then feed two low-noise amplifier chains, which are then recombined by a second magic-T to reconstruct antenna and reference outputs. The spacecraft downlink signal is tapped after the second hybrid antenna signal and passed to the communication receiver with unchanged performance. This leaves both antenna and reference signals available for the radiometer—which compares reference and antenna using a switch—essentially functioning as a Dicke switched radiometer. In this way, calibration occurs transparently, with no interference to the telemetry link.

Embedding the EWVR within the DSN antenna represents an important advance for both atmospheric science and mission operations. it provides continuous line-of-sight measurements of water vapor, a key element in the Earth’s radiative budget [1]. For Radio Science (or Gravity Science) experiments, the EWVR reduces the residual phase errors enabling higher precision determinations of planetary interior structures or other propagation effects within the Solar System. For mission operations, it enables improved spacecraft navigation accuracy by reducing unmodeled tropospheric errors. The pseudo-correlation design further demonstrates a modern radiometer architecture that achieves calibration stability while remaining fully compatible with mission-critical communication systems [2,3].

In summary, the EWVR leverages pseudo-correlation calibration to operate seamlessly within the DSN downlink chain. Notably, calibration occurs without degrading the DSN link, a critical feature for embedding radiometry into operational systems. In this presentation, we will describe the EWVR architecture, its integration into the DSN antenna, and the results of its performance evaluation.

References

1. M. Bevis, “GPS Meteorology: Remote sensing of atmospheric water vapor using the global positioning system,” J. Geophys. Res., vol. 97, Oct. 1992.

2. A. Tanner, J. Bosch-Lluis, and P. Kangaslahti, “Applications of the Pseudo-Correlation Microwave Radiometer,” Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA.

3. K. Cortés, R. Reeves, M. Figueroa, P. Kangaslahti, W. Ramírez, L. Mora, P. Cartes, D. Arroyo, G. Burgos, and B. Molina, “A Pseudo-Correlation MMIC-based 183 GHz Water Vapor Radiometer,” Univ. de Concepción, Chile; Jet Propulsion Laboratory, Pasadena, CA, USA.