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

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

IEEE 4003-2021

27 páginasThe scope of this effort is to develop a standard for data and metadata content arising from spaceborne global navigation satellite system-reflectometry (GNSS-R) missions, which uses GNSS signals as signals of opportunity, as described in “The IEEE SA Working Group on Spaceborne GNSS-R: Scene Study.” In particular, this standard would provide a means for describing: a) The terminology assigned to GNSS-R data and products, such as the product levels. b) The structure and content of the data. This includes, but is not limited to, units of measure, data organization, data description, data encoding, and data storage format. c) The metadata. This includes and is not limited to metadata, methods and algorithms applied to the data, parameters related to the algorithms, citation information, instrument calibration and characterization, and description of the input signals. The purpose of this standard is to provide a set of specifications and recommended practices that can be used to describe any known and future spaceborne GNSS-R data set, allowing users to work with different GNSS-R data sets at the same time. The definition of such standard would also allow any software that uses these data to fully operate and ingest any spaceborne GNSS-R input data as they will conform to the same standard

Spaceborne receiver design for scatterometric GNSS reflectometry

Global Navigation Satellite System-Reflectometry (GNSS-R) is an innovative technique for remote sensing. It uses reflected signals from the navigation constellations to determine properties of the Earth’s surface. The primary focus of this work is the remote sensing of the ocean by measurement of surface roughness. The most significant unresolved challenge in spaceborne GNSS-R is to verify the accuracy of surface roughness measurements. Existing remote sensing techniques have typically relied on extensive data-sets to validate satellite measurements with the ground truth. This thesis provides a receiver design for collection of the required validation data-sets which can then form part of an operational system for surface roughness measurement. New receiver approaches were investigated through the design of a software receiver to postprocess existing data from the GNSS-R experiment on the UK-DMC satellite. This forms the reflections into Delay-Doppler Maps (DDMs) from which the surface roughness can be determined. The software receiver improves on existing implementations by targeting all available specular reflections using open-loop tracking. A new approach called Stare processing is analysed, which controls the receiver to remain focused at a fixed point on the Earth’s surface as the satellites move. This improves the surface resolution over using the full DDM. Additionally it is shown to be a viable approach for surface roughness measurement through a scattering model and the first demonstration on data collected from space. GNSS-R research has primarily focused on the established GPS navigation system. This research extends the measurement concept to the new Galileo GNSS. A receiver that can target multiple GNSS constellations will allow greater remote sensing coverage. The primary differences between Galileo and GPS are analysed and an approach is developed leading to the first spaceborne demonstration of Galileo-like signals for remote sensing. The system design for the GNSS-R receiver presented in this thesis was carried out in the context of Surrey Satellite Technology Ltd developing a GNSS navigation receiver called the SGR-ReSI, to be launched on the UK Technology Demonstrations Satellite TDS-1. The critical areas identified in the GNSS-R system design were implemented and tested on this receiver. The design overcomes the challenging constraints of GNSS-R in a small satellite platform: principally the mass, power and data downlink capacity. To achieve these, on-board data compression was developed through real-time DDM processing and reflection tracking. An algorithm for real-time DDM processing within the mass and power constraints was designed and demonstrated within the receiver and combined with open-loop reflection tracking. A ground-based test set-up was developed to test the design on existing spaceborne data, from the UK-DMC experiment, before the TDS-1 satellite launch

Spaceborne receiver design for scatterometric GNSS reflectometry

Global Navigation Satellite System-Reflectometry (GNSS-R) is an innovative technique for remote sensing. It uses reflected signals from the navigation constellations to determine properties of the Earth’s surface. The primary focus of this work is the remote sensing of the ocean by measurement of surface roughness. The most significant unresolved challenge in spaceborne GNSS-R is to verify the accuracy of surface roughness measurements. Existing remote sensing techniques have typically relied on extensive data-sets to validate satellite measurements with the ground truth. This thesis provides a receiver design for collection of the required validation data-sets which can then form part of an operational system for surface roughness measurement. New receiver approaches were investigated through the design of a software receiver to postprocess existing data from the GNSS-R experiment on the UK-DMC satellite. This forms the reflections into Delay-Doppler Maps (DDMs) from which the surface roughness can be determined. The software receiver improves on existing implementations by targeting all available specular reflections using open-loop tracking. A new approach called Stare processing is analysed, which controls the receiver to remain focused at a fixed point on the Earth’s surface as the satellites move. This improves the surface resolution over using the full DDM. Additionally it is shown to be a viable approach for surface roughness measurement through a scattering model and the first demonstration on data collected from space. GNSS-R research has primarily focused on the established GPS navigation system. This research extends the measurement concept to the new Galileo GNSS. A receiver that can target multiple GNSS constellations will allow greater remote sensing coverage. The primary differences between Galileo and GPS are analysed and an approach is developed leading to the first spaceborne demonstration of Galileo-like signals for remote sensing. The system design for the GNSS-R receiver presented in this thesis was carried out in the context of Surrey Satellite Technology Ltd developing a GNSS navigation receiver called the SGR-ReSI, to be launched on the UK Technology Demonstrations Satellite TDS-1. The critical areas identified in the GNSS-R system design were implemented and tested on this receiver. The design overcomes the challenging constraints of GNSS-R in a small satellite platform: principally the mass, power and data downlink capacity. To achieve these, on-board data compression was developed through real-time DDM processing and reflection tracking. An algorithm for real-time DDM processing within the mass and power constraints was designed and demonstrated within the receiver and combined with open-loop reflection tracking. A ground-based test set-up was developed to test the design on existing spaceborne data, from the UK-DMC experiment, before the TDS-1 satellite launch

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 UK's TechDemoSat-1 (TDS-1) and NASA's Cyclone Global Navigation Satellite System (CYGNSS).<br/

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

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