Stephan Esterhuizen

Spaceborne GNSS-R from the SMAP Mission: First Assessment of Polarimetric Scatterometry over Land and Cryosphere

This work describes the first global scale assessment of a Global Navigation Satellite Systems Reflectometry (GNSS-R) experiment performed on-board the Soil Moisture Active Passive (SMAP) mission for soil moisture and biomass determination. Scattered GPS L2 signals (1227.6 MHz) were collected by the SMAP’s dual-polarization (Horizontal H and Vertical V) radar receiver and then processed on-ground using a known replica of the GPS L2C code. The scattering properties over land are evaluated using the Signal-to-Noise Ratio (SNR), the Polarimetric Ratio (PR), and the width of the waveforms’ trailing and leading edges. These parameters show sensitivity to the effects of the Earth’s topography and Above Ground Biomass (ABG) even over Amazonian and Boreal forests. These effects are shown to be an important factor in precise soil moisture and biomass determination. Additionally, it is found that PR shows sensitivity to soil moisture content over different land cover types. In particular, the following values of the PR are found over: (a) tropical forests ~−1.2 dB; (b) boreal forests ~0.8 dB; (c) Greenland ~2.8 dB; and (d) the Sahara Desert ~3.2 dB.

TOGA, a prototype for an optimal orbiting GNSS-R instrument

Remotely sensing the Earths surface using GNSS (Global Navigation Satellite System) signals as bi-static radar sources is one of the most challenging applications for radiometric instrument design. As part of NASAs Instrument Incubator Program, our group at JPL is building a prototype instrument, TOGA (Time-shifted, Orthometric, GNSS Array), to address a variety of GNSS science needs. Observing GNSS reflections is major focus of the design/development effort. The TOGA design features an electronically steered antenna (ESA) array which forms simultaneous high-gain beams in multiple directions. Multiple FPGAs provide flexible digital signal processing logic to process both GPS and Galileo reflections. A Linux operating system based science processor serves as experiment scheduler and data post-processor. This paper outlines the TOGA design approach as it applies specifically to observing science quality GNSS-R signals from low Earth orbit.

Spaceborne GNSS-R from the SMAP mission: First assessment of polarimetric scatterometry

NASAs Soil Moisture Active Passive (SMAP) mission has been tuned to perform a Global Navigation Satellite Systems Reflectometry (GNSS-R) experiment. The motivation of this study is to assess the capabilities of GNSS-R for soil moisture determination, biomass monitoring and cryosphere studies. The use for first time of the Polarimetric Ratio (PR) from a spaceborne platform shows significant sensitivity to soil moisture. Additionally, the investigation for first time of the leading and trailing edges width sensitivity to Above Ground Biomass (AGB) and rough topography shows promising results. Better understanding of these effects in the reflected waveforms will improve the development of retrieval algorithms.

GNSS-R from the SMAP and CyGNSS missions: Application to polarimetric scatterometry and ocean altimetry

Global Navigation Satellite Systems Reflectometry (GNSS-R) ocean applications includes scatterometry and altimetry. In this work, an investigation is performed on polarimetric scatterometry over ocean surface using data from a GNSS-R experiment on-board the Soil Moisture Active Passive (SMAP) mission, and on ocean surface topography from the Cyclone Global Navigation Satellite System (CyGNSS) mission using new retrieval algorithms. The former one provides global coverage because of the Sun Synchronous Orbit (SSO), while the latter one focuses on tropical latitudes providing a spatial sampling of 32 swaths. First results from SMAP over the Artic Sea show clearly sea ice effects on the reflected waveforms.

Relay communications support to the ExoMars Schiaparelli lander

The European Space Agencys ExoMars Trace Gas Orbiter (TGO) arrived at Mars on October 19, 2016, three days after releasing the Schiaparelli Lander on a ballistic trajectory to Meridiani Planum. During the separation event, and subsequently during Schiaparellis Entry, Descent, and Landing (EDL), the NASA-provided Electra Ultra-High Frequency (UHF) payload onboard TGO was used to record signals from the Schiaparelli Lander for post-processing on the ground to recover both tracking of the landers carrier signal and reconstruction of the landers 8 kb/s telemetry. In addition, ESAs Mars Express orbiter recorded the Schiaparelli signal, with ground post-processing providing independent tracking of the lander carrier signal, and the Giant Metrewave Radio Telescope near Pune, India was configured to provide real-time detection of the lander carrier signal. While an anomaly in the latter stages of EDL led to loss of the lander, these critical event data sets, and in particular the telemetry reconstruction enabled by the TGO Electra recording, proved essential in enabling detailed diagnosis of the anomaly. While the loss of the lander during EDL precluded the planned surface relay operations, the preparations for that activity provide important lessons learned for future Mars relay support scenarios.

An innovative direct measurement of the GRAIL absolute timing of Science Data

The Gravity Recovery and Interior Laboratory (GRAIL), a NASA Discovery mission, twin spacecraft were launched on 10 September 2012 and were inserted into lunar orbit on 31 December 2011 and 01 January 2012. The objective of the mission was to measure a high-resolution lunar gravity field using inter-spacecraft range measurements in order to investigate the interior structure of the Moon from crust to core. The first step in the lunar gravity field determination process involved correcting for general relativity, measurement noise, biases and relative & absolute timing. Three independent clocks participated in the process and needed to be correlated after the fact. Measuring the absolute time tags for the GRAIL mission data turned out to be a challenging task primarily because of limited periods when such measurements could be conducted. Unlike the Gravity Recovery and Climate Experiment (GRACE), where absolute timing measurements are available using the GPS system, no absolute timing measurements were available on the far side of the Moon or when there were no DSN coverage periods. During the early cruise phase, it was determined that a direct absolute timing measurement of each spacecraft Lunar Gravity Ranging System (LGRS) clock could be directly observed by using a DSN station to eavesdrop on the Time Transfer System (TTS) S-band inter-satellite ranging signal. By detecting the TTS system directly on earth, the LGRS clock can be correlated directly to Universal Time Coordinated (UTC) because the TTS and LGRS use the same clock to time-tag their measurements. This paper describes the end-to-end preparation process by building and installing a dedicated hardware at Goldstone station DSS-24, selecting favorable lunar orbit geometries, real time signal detection and post processing, and finally how the absolute timing is used in the overall construction of lunar gravity fields.