Philip Jales

Spaceborne GNSS reflectometry for ocean winds: First results from the UK TechDemoSat-1 mission

First results are presented for ocean surface wind speed retrieval from reflected GPS signals measured by the Low-Earth-Orbiting UK TechDemoSat-1 satellite (TDS-1). Launched in July 2014, TDS-1 provides the first new spaceborne Global Navigation Satellite System-Reflectometry (GNSS-R) data since the pioneering UK-Disaster Monitoring Mission experiment in 2003. Examples of onboard-processed delay Doppler Maps reveal excellent data quality for winds up to 27.9 m/s. Collocated ASCAT scatterometer winds are used to develop and evaluate a wind speed algorithm based on Signal-to-Noise ratio (SNR) and the Bistatic Radar Equation. For SNR greater than 3 dB, wind speed is retrieved without bias and a precision around 2.2 m/s between 3–18 m/s even withoutcalibration. Exploiting lower SNR signals however requires good knowledge of the antenna beam, platform attitude and instrument gain setting. This study demonstrates the capabilities of low-cost, low-mass, low-power GNSS-R receivers ahead of their launch on the NASA CYGNSS constellation in 2016.

Spaceborne GNSS-R Minimum Variance Wind Speed Estimator

A Minimum Variance (MV) wind speed estimator for Global Navigation Satellite System-Reflectometry (GNSS-R) is presented. The MV estimator is a composite of wind estimates obtained from five different observables derived from GNSS-R Delay-Doppler Maps (DDMs). Regression-based wind retrievals are developed for each individual observable using empirical geophysical model functions that are derived from NDBC buoy wind matchups with collocated overpass measurements made by the GNSS-R sensor on the United Kingdom-Disaster Monitoring Constellation (UK-DMC) satellite. The MV estimator exploits the partial decorrelation that is present between residual errors in the five individual wind retrievals. In particular, the RMS error in the MV estimator, at 1.65 m/s, is lower than that of each of the individual retrievals. Although they are derived from the same DDM, the partial decorrelation between their retrieval errors demonstrates that there is some unique information contained in them. The MV estimator is applied here to UK-DMC data, but it can be easily adapted to retrieve wind speed for forthcoming GNSS-R missions, including the UKs TechDemoSat-1 (TDS-1) and NASAs Cyclone Global Navigation Satellite System (CYGNSS).

Spaceborne GNSS-Reflectometry on TechDemoSat-1: Early Mission Operations and Exploitation

GNSS-Reflectometry is a new technique that shows promise for many earth observation applications including remote sensing of oceans, land, and ice. A payload has been developed that is low size and power, and suitable for use on small satellites. The first flight of the SGR-ReSI GNSS Reflectometry Instrument is on the TechDemoSat-1 mission, launched in July 2014. The instrument has been operational since its commissioning in September 2014, and has been collecting delay Doppler maps routinely over many different surfaces. Preliminary work has been undertaken to develop and validate wind speed inversion algorithms against ASCAT measurements with promising results. Measurements over land and sea ice are also showing interesting geophysical characteristics This paper describes the instrument, early operations, data dissemination through the Measurement of Earth Reflected Radio-navigation Signals By Satellite (MERRByS) website and preliminary data assessments in preparation for further data exploitation.

The First Application of Stare Processing to Retrieve Mean Square Slope Using the SGR-ReSI GNSS-R Experiment on TDS-1

The bistatic radar technique of Global Navigation Satellite System-Reflectometry (GNSS-R) is capable of measuring wind and wave parameters using a passive instrument on-board a small satellite platform. In this paper, data from the Space GNSS Receiver-Remote Sensing Instrument (SGR-ReSI) experiment on-board TechDemoSat (TDS-1) are analyzed to perform geophysical parameter retrievals. Stare processing utilizes the high-spatial overlap between successive delay-Doppler maps (DDMs) and the typical Level 1B TDS-1 data product, to achieve multiple looks at the same surface point. The Stare processing approach is detailed as a method to recover the mean square slope (mss) of the scattering surface. This is achieved by fitting a slope probability density function (pdf) to measurements of a surface point over a time series of DDMs. The results of colocations with the global WaveWatch3 (WW3) model are shown. Results show Pearson correlation coefficients of 0.684 between TDS-1 mss and WW3 mss values and 0.742 when compared in dB units. The latter result indicates better correlation for low values of mss with a tail-off in sensitivity for rougher seas. Further work and improvements to the implementation of Stare processing are discussed.

The SGR-ReSI and its application for GNSS reflectometry on the NASA EV-2 CYGNSS mission

As part of the EV-2 Cyclone Global Navigation Satellite System (CYGNSS) mission team, Surrey will be providing the Delay Doppler Mapping Instrument (DDMI) for eight Observatories designed and built by the University of Michigan and Southwest Research Institute (SwRI). Following the success of the GPS Reflectometry Experiment on the UK-DMC 1 satellite launched in 2003, Surrey has developed the SGR-ReSI as a move towards operational reflectometry and other applications. The Space GPS Receiver Remote Sensing Instrument (SGR-ReSI) is a COTS-electronics based GNSS receiver which can support up to eight programmable front-ends. It allows collection of raw sampled data but also is capable of processing the reflections into Delay Doppler Maps in real time. The first flight of the SGR-ReSI will be on the UK TechDemoSat-1 to prove the instrument and its various applications. The SGR-ReSI on CYGNSS has a different configuration to that on TechDemoSat-1 which is needed to focus on the requirements for operational cyclone sensing.

Development of low-cost spaceborne multi-frequency GNSS receiver for navigation and GNSS remote sensing

This paper describes the development of a new generation of low-cost spaceborne multi-frequency global navigation satellite systems (GNSS) receivers for navigation and GNSS remote sensing applications. The spaceborne GNSS receiver-remote sensing instrument (SGR-ReSI) uses reflectometry to gather data, which may be used to derive information about the Earth: ocean, atmosphere, land, snow and ice. First, a review of the GNSS remote sensing including GNSS-reflectometry and radio occultation is presented. Then, the science and operational needs of GNSS receivers for the remote sensing of ocean, atmosphere, land, snow and ice are discussed. The design and development of a new generation of low-cost spaceborne multi-frequency GNSS receivers for navigation and GNSS remote sensing are described. Detailed results and designs of dual-band antennas and arrays for navigation, radio occultation and GNSS-reflectometry are presented. GNSS receiver designs, including both the software and hardware, are also explained. The processing algorithms and modelling techniques are described. Some initial results are also illustrated. This paper ends with discussions of future flight opportunities and a conclusion.

An Assessment of Non-geophysical Effects in Spaceborne GNSS Reflectometry Data From the UK TechDemoSat-1 Mission

An assessment of non-geophysical effects in spaceborne global navigation satellite system reflectometry (GNSS-R) data from the UK TechDemoSat-1 (TDS-1) mission is presented. TDS-1 was launched in July 2014 and provides the first new spaceborne GNSS-R data since the pioneering UK-disaster monitoring constellation experiment in 2003. Non-geophysical factors evaluated include ambient L-band noise, instrument operating mode, and platform-related parameters. The findings are particularly relevant to users of uncalibrated GNSS-R signals for the retrieval of geophysical properties of the Earth surface. Substantial attitude adjustments of the TDS-1 platform are occasionally found to occur that introduce large uncertainties in parts of the TDS-1 GNSS-R dataset, particularly for specular points located outside the main beam of the nadir antenna where even small attitude errors can lead to large inaccuracies in the geophysical inversion. Out of eclipse however, attitude adjustments typically remain smaller than 1.5°, with larger deviations of up to 10° affecting less than 5% of the overall sun-lit data. Global maps of L1 ambient noise are presented for both automatic and programmed gain modes of the receiver, revealing persistent L-band noise hotspots along the Equator that can reach up to 2.5 dB, most likely associated with surface reflection of signals from other GNSS transmitters and constellations. Sporadic high-power noise events observed in certain regions point to sources of human origin. Relevant conclusions of this study are that platform attitude knowledge is essential and that radiometric calibration of GNSS-R signals should be used whenever possible. Care should be taken when considering using noise measurements over the equatorial oceans for calibration purposes, as ambient noise and correlated noise in delay–Doppler maps both show more variation than might be expected over these regions.

Preparation for the first flight of SSTL's next generation space GNSS receivers

This paper discusses the new GNSS receiver architecture being designed by SSTL, and its preparation for use in space. The first application is the SGR-ReSI experiment that will be flying on the TechDemoSat-1 mission in 2013. This will allow the collection of GPS Reflected signals for validating ocean models, and also an early prototype service to notify users about wind and waves. The SGR-Axio is an advanced GNSS receiver based upon the SGR-ReSI and intended to replace the SGR-10 on low Earth orbit satellites. It has many advanced capabilities such as multi-antenna, dual frequencies and multisignals. The SGR-Axio has been designed to accommodate the Chip Scale Atomic Clock to provide an improved timing service. The paper also discusses CYGNSS, a new NASA satellite project that deploys the SGR-ReSI onto 8 small satellites for sensing hurricanes.

Satellite-borne remote sensing: Using Space GPS/GNSS receiver for reflectometry

This paper is a review on Surrey Satellite Technology Ltds developments in Space GNSS receivers. Hardware implementations of the UK-DMC as well as the imminent flying SGR-ReSI are briefly summarised. Past sets of data obtained by the disaster monitoring satellite, UK-DMC, are re-processed in order to acquire for Satellite Based Augmentation System (SBAS) signals present at the time data was taken. A coverage analysis is also implemented using Satellite Tool Kit (STK), indicating the importance of tracking SBAS signals as an addition to the classic GPS signals. Some of the results of the SBAS acquisitions are presented in this paper.