Scene setting for the ESA hydroGNSS GNSS-Reflectometry scout mission

HydroGNSS is a mission concept selected by ESA as a Scout candidate, and consists of a 40 kg satellite that addresses land hydrological parameters using the technique of GNSS Reflectometry, a form of bistatic L-Band radar using satnav signals as the radar source. The four targeted essential climate variables (ECVs) are of established importance to our understanding of the climate evolution and human interaction, and comprise of soil moisture, inundation / wetlands, freeze /thaw (notably over permafrost) and above ground biomass. The technique of GNSS Reflectometry shows potential over all geophysical surfaces for low cost measurement of ocean winds, ocean roughness, soil moisture, flood & ice mapping, and other climate and operational parameters. SSTL developed and flew the SGR-ReSI GNSS remote sensing instrument on the 160 kg UK TechDemoSat-1 (TDS-1) in July 2014 and, with sponsorship from ESA, collected data until TDS-1’s drag-sail was deployed in May 2019. TDS-1 was a precursor for NASA’s CYGNSS mission which uses the SGR-ReSI on its 8-microsatellite constellation for sensing hurricanes. The datasets from TDS-1 have been released via the MERRByS website, and include ocean wind speed measurements and ice extent maps from National Oceanography Centre’s C-BRE inversion. At the same time, researchers recognised the benefits of GNSS reflectometry over land, including the unique capability to sense rivers under forest canopies to a high resolution. HydroGNSS has been proposed for the ESA Scout mission opportunity by a SSTL and a team of partners with a broad range of experience in GNSS technology, GNSS-Reflectometry modelling and applications, and Earth Observation from GNSS-R measurements. The instrument takes significant steps forward from previous GNSS-R experiments by including capability in dual polarisation, dual frequency and coherent reflected signal reception, that are expected to help separate out ECVs and improve measurement resolution. The satellite platform is the 40 kg SSTL-Micro, which has improved attitude determination and a high data link to support the collection of copious quantities scientific data with a short time delay. HydroGNSS builds upon the growing GNSS-R knowledge gained from UK-DMC, TDS-1, and ORORO / DoT-1, and is anticipated to generate a new research data set in GNSS Earth Observation, specifically targeting land and hydrological applications. State of the art satellites that target soil moisture such as ESA SMOS and NASA SMAP are highly valued by scientists and operational weather forecasters, but will be expensive to replace. As evidenced by TDS-1 and CYGNSS, HydroGNSS will be able to take GNSS-R measurements using GNSS signals as a radar source, reducing the size of the satellite platform required. The forward scatter L-band nature of the measurement means that they are complementary to other techniques, and HydroGNSS brings further new measurement types compared to TDS-1 and CYGNSS. The small size and low recurring cost of the HydroGNSS satellite design opens the door to a larger constellation that can further improve spatial and temporal global hydrological measurements to an unprecedented resolution, invaluable to the better understanding of our climate

Overview of ESA’s Earth Observation upcoming small satellites missions

The “New Space” paradigm, has enabled the creation of many new opportunities in the space sector like the development of a large number of missions based on small and nano-satellites. The European Space Agency (ESA) is supporting these new development approaches and technology advancements, including use of Commercial-Off-The-Shelf (COTS) components to enable missions based on small and nano satellites. ESA’s Earth Observation Programmes Directorate (ESA-EOP) is already involved not only in the implementation of technologies exploiting the capabilities offered by small and nano-satellites as a complement to the EOP scientific and application-driven flagship satellites, but also in the quick validation of new approaches like A.I, super resolution or more in general in orbit data processing. ESA-EOP developments in the area of small and nano satellites are spread in three different programmatic lines, each with its own objectives: Scout and F-sat Missions and the InCubed Programme. This paper presents the overall ESA-EOP small missions strategy providing a brief insight on the genesis of each programmatic line and their selection processes including an update of the status of the first initiatives and missions under development or study

Fast Precise Point Positioning based on real-time ionospheric modelling

Summary of main results of new technique Fast Precise Point Positioning developed in the framework of the “Enhanced Precise Point Positioning (EPPP) GNSS multifrequency user algorithm” ESA funded project. -The precise ionospheric corrections facilitate the resolution of undifferenced carrier phase ambiguities, ambiguity validation and integrity monitoring. -The FPPP performance is shown in terms of accuracy, convergence time and integrity, with actual GPS and simulated Galileo data. -Very limited bandwidth requirements for future EPPP users (less than 300 bps for dual-frequency GPS data).Postprint (published version

An Introduction to the HydroGNSS GNSS Reflectometry Remote Sensing Mission

HydroGNSS (Hydrology using Global Navigation Satellite System reflections) has been selected as the second European Space Agency (ESA) Scout earth observation mission to demonstrate the capability of small satellites to deliver science. This article summarizes the case for HydroGNSS as developed during its system consolidation study. HydroGNSS is a high-value dual small satellite mission, which will prove new concepts and offer timely climate observations that supplement and complement the existing observations and are high in ESAs earth observation scientific priorities. The mission delivers the observations of four hydrological essential climate variables as defined by the global climate observing system using the new technique of GNSS reflectometry. These will cover the world's land mass to 25 km resolution, with a 15-day revisit. The variables are soil moisture, inundation or wetlands, freeze/thaw state, and above-ground biomass

GNSS Interference in L-Band SAR Missions - Assessment and Mitigation

Synthetic Aperture Radar (SAR) satellites commonly make use of onboard Global Positioning System (GPS) receivers for precise orbit and baseline determination. In view of the extreme SAR transmit power levels, interference from SAR signals may inhibit proper GPS tracking and poses a particular challenge to space missions using L-band SAR signals with frequencies adjacent to or even overlapping the GPS frequency bands. Within this study, the impact of simulated SAR signals on direct and semi-codeless GPS signal tracking is assessed in a signal simulator test bed using two commercial-off-the-shelf geodetic-grade receivers. A high robustness of GPS tracking to both adjacent-band and in-band SAR interference is obtained within the tests using representative chirp signals. For SAR signals next to or overlapping the GPS L2 band, proper tracking of the GPS L1 C/A code, GPS2 L2C, and semi-codeless L1/L2 P(Y)-code tracking is retained for interference powers up to \SI{90}{db} above the natural GPS signal power. Apparently, a high level of immunity to high-power pulsed signals with repeat periods in the (sub-)ms regime is already provided by the automatic gain control of the receivers and/or a saturation of the analog-to-digital converters in the frontend that mimic an explicit pulse blanking. On the other hand, the addition of an external pulse blanking synchronized with the chirp pulses was found to be of marginal value. This unexpected result can presumably be understood by low power ``noise'' in the synthetic SAR signals that adds an additional signal outside the spectral and temporal limitations of the actual chirp signal and dominates the overall interference when simulating very high chirp signal powers

Relative positioning of formation-flying spacecraft using single-receiver GPS carrier phase ambiguity fixing

In recent years, differential carrier phase-based relative positioning (or “baseline determination”) with precision at the millimeter and submillimeter levels has been demonstrated for the GRACE, TanDEM-X and Swarm missions in offline processing. Specific features of such missions have included the use of spacecraft of similar shapes placed in almost identical orbits as well as the use of consistent geodetic-class GPS receivers. These elements have proven to be advantageous for the computation of baseline solutions with such precision levels. Particularly, they have allowed to fully leverage the use of differential GPS techniques, including the estimation and use of carrier phase integer ambiguities. Similarly, the aforementioned spacecraft and orbit characteristics have made it possible to tightly constrain the relative dynamics of formations in the generation of reduced-dynamic solutions. Other than the former examples, prospective formation-flying mission proposals, such as SAOCOM-CS and PICOSAR, may comprise spacecraft with very different characteristics, including dissimilar GPS/ GNSS receivers. Such cases may no longer provide favorable conditions for relative orbit determination strategies. As an alternative, absolute orbit solutions may be computed individually for each spacecraft and used for the generation of precise baseline products. This study aims at the assessment of the potential of single-receiver ambiguity fixing for the generation of precise baseline solutions. Results using flight data from the GRACE, TanDEM-X and Swarm missions exhibit baseline accuracy better than 5 mm (3D RMS) for a one-month test period in June 2016. As such, the presented solutions may be considered for prospective formation-flying remote sensing missions with baseline precision requirements at the subcentimeter level. Likewise, the method is considered of particular interest for future multi-spacecraft formations and swarms that require efficient determination of a large number of individual baselines

Precise Orbit and Baseline Determination for the SAOCOM-CS Bistatic Radar Mission

This paper presents an overview of the SAOCOM-CS synthetic aperture radar (SAR) interferometry mission and the use of GPS/GNSS for precise orbit and baseline determination. Compared to X-band SAR interferometry missions, the use L-band SAR in SAOCOM-CS poses less stringent (sub-centimeter rather than mm-level) requirements on the baseline accuracy. At the same time, however, the use of dissimilar spacecraft and GNSS receivers implies a less-than optimal precondition for carrier phase differential GPS (CDGPS) navigation and is likely to degrade the baseline determination accuracy compared to ideal conditions. Based on GPS data collected in previous formation flying missions, the achievable baseline performance in SA-OCOM-CS is assessed and recommendations for the most suitable processing scheme are given

Fast Precise Point Positioning based on real-time ionospheric modelling

Summary of main results of new technique Fast Precise Point Positioning developed in the framework of the “Enhanced Precise Point Positioning (EPPP) GNSS multifrequency user algorithm” ESA funded project. -The precise ionospheric corrections facilitate the resolution of undifferenced carrier phase ambiguities, ambiguity validation and integrity monitoring. -The FPPP performance is shown in terms of accuracy, convergence time and integrity, with actual GPS and simulated Galileo data. -Very limited bandwidth requirements for future EPPP users (less than 300 bps for dual-frequency GPS data)

Fast Precise Point Positioning based on real-time ionospheric modelling

Summary of main results of new technique Fast Precise Point Positioning developed in the framework of the “Enhanced Precise Point Positioning (EPPP) GNSS multifrequency user algorithm” ESA funded project. -The precise ionospheric corrections facilitate the resolution of undifferenced carrier phase ambiguities, ambiguity validation and integrity monitoring. -The FPPP performance is shown in terms of accuracy, convergence time and integrity, with actual GPS and simulated Galileo data. -Very limited bandwidth requirements for future EPPP users (less than 300 bps for dual-frequency GPS data)