GNSS Reflectometry

The NASA Cyclone Global Navigation Satellite System (CYGNSS) is a confirmed spaceborne mission scheduled for launch in October 2016 that is specifically designed to study the surface wind structure in and near the inner core of tropical cyclones (TC). CYGNSS consists of a constellation of eight small observatories carried into orbit on a single launch vehicle. Each observatory carries a 4-channel bistatic radar receiver tuned to receive GPS navigation signals scattered from the ocean surface. The eight satellites are spaced approximately twelve minutes apart in the same circular, low inclination orbit plane in order to provide frequent temporal sampling in the tropics.  CYGNSS is expected to provide unprecedented temporal resolution and spatial coverage, under all precipitating conditions, and over the full dynamic range of wind speeds experienced in a TC. Mission simulations predict a median(mean) revisit time of 3(7) hours at all locations between 35^o N and 35^o S latitude..

The CYGNSS mission is currently in Phase D assembly, integration and test of the 8 Observatories and Deployment Module, followed by integration onto the launch vehicle. An update on the status of the mission will be presented, including the latest hardware developments, the science algorithms planned for ground processing, and simulations of the expected impact on tropical cyclone forecast skill due to the CYGNSS measurements.

In response to an European Space Agency (ESA) announcement of opportunity for climate change relevant science aboard the ISS, the GEROS-ISS (GEROS hereafter) proposal was submitted in 2011 and accepted by ESA to proceed to Phase A. GEROS-ISS is an innovative experiment primarily focused on exploiting reflected signals of opportunity from Global Navigation Satellite Systems (GNSS) at L-band to measure key parameters of ocean surfaces. GEROS will utilize the U.S. American GPS (Global Positioning System) and pioneer the exploitation of signals from Galileo and possibly other GNSS systems (GLONASS, QZSS, BeiDou), for reflectometry and occultation, thereby improving the accuracy as well as the spatio-temporal resolution of the derived geophysical properties.

The primary mission objectives of GEROS are: (1) to measure the altimetric sea surface height of the ocean using reflected GNSS signals to allow methodology demonstration, establishment of error budget and resolutions and comparison/synergy with results of satellite based nadir-pointing altimeters and (2) to retrieve scalar ocean surface mean square slope (MSS), which is related to sea roughness, wind speed and direction, with a GNSS spaceborne receiver to allow methodology testing, establishment of error budget and resolutions. Secondary objectives include the generation of the 2D MSS or directional MSS retrieval and the associated proof-of-concept scientific data product.

Secondary mission objectives, which increase the scientific value of the GEROS data, but are not driving the instrument developments, are: (1) to further explore the potential of GNSS radio occultation data (vertical profiles of atmospheric bending angle, refractivity, temperature, pressure, humidity and electron density), particularly in the Tropics, to detect changes in atmospheric temperature and climate relevant parameters (e.g., tropopause height) and to provide additional information for the analysis of the reflectometry data from GEROS and (2) to assess the potential of GNSS scatterometry for land applications and in particular to develop products such as soil moisture, vegetation biomass, and mid-latitudes snow/ice properties to better understand anthropogenic climate change.

A Science Advisory Group (SAG) was formed by ESA Mid 2013 and the initial definition of the GEROS mission and system requirements was completed end of 2013. Two industrial phase A studies were started end of 2014, complemented by the scientific study GARCA (GNSS-R – Assessment of Requirements and Consolidation of Retrieval Algorithms) to develop an End2End Simulator for the preparation of the GEROS-Mission and to perform Observing-System Simulation Experiments (OSSE) to assess the oceanographic significance of the expected measurements and to demonstrate the usefulness of the GEROS concept.

The presentation will give an overview on the current status of the GEROS experiment, review the science activities within the international GARCA study and related ESA-supported science activities.

Wetland dynamics is crucial to address changes in both atmospheric methane (CH₄) and terrestrial water storage. Among the multitude of CH₄ emission sources, wetlands constitute the largest contributor with the widest uncertainty range, since  atmospheric CH₄ variability is linked to climate-driven fluctuations of methane emissions from natural wetlands.  Directly relevant to terrestrial water storage and thus sea level change, the increase or decrease in wetland extent is closely dependent on the regional wetland hydrology that can be significantly impacted by changes in temperature and precipitation. Yet, both spatial distribution and temporal variability of wetlands remain highly unconstrained despite the existence of remote sensing products from past and present satellite sensors.

An innovative approach can be possible with the Global Navigation Satellite System Reflectometry (GNSS-R), which is a bistatic radar concept that takes advantage of the ever increasing number of GNSS transmitting satellites to yield many randomly distributed measurements with broad-area global coverage and rapid revisit time. The technique measures the forward reflection/scattering from an area on the Earth’s surface surrounding the specular reflection point, collected from a receiver on a fixed or moving platform. Preliminary aircraft and satellite experiments have shown that the GNSS-R signal from inundated wetlands is coherent and strong, thus allowing a clear distinction between wetlands and dry land. This evidently highlights a new potential method for wetland mapping and monitoring to be significantly aided by an adequate distribution of GNSS-R receivers, specifically designed for a frequent global coverage.  CYGNSS, a NASA Venture-Class mission consisting of eight observatories, is one such constellation. Although designed for hurricane intensification studies, reflections will be collected everywhere in a tropical latitudinal band which is suitable for wetlands studies. Additionally, ESA is currently studying the GEROS GNSS-R experiment on the ISS, the expected data from which is valuable to assess the new capability for wetland mapping and monitoring.

Hence, this communication presents the science motivation for mapping of wetlands and monitoring of their dynamics, and shows the relevance of the GNSS-R technique in this context, relative to and in synergy with other existing measurement systems. Additionally, the communication discusses results of our data analysis on wetlands in the Ebro delta of Spain, the Mississippi in the United States, and the Mekong delta of Vietnam, specifically from the initial analysis of satellite data acquired by the TechDemoSat-1 mission launched in 2014. Finally, recommendations are provided for the design of a GNSS-R mission specifically to address wetlands science issues.

TechDemoSat-1 (TDS-1) is a small UK satellite that was launched in July 2014 to demonstrate a variety of UK-developed technologies. The SGR-ReSI (Space GNSS Receiver- Remote Sensing Instrument) makes up part of Sea State Payload Suite that was one of the 8 payloads on the satellite.

The SGR-ReSI is a new GNSS receiver designed to collect and process GNSS reflections on-board, enabling the collection of large quantities of measurements. It comprises of a reprogrammable GNSS receiver, a data recorder and a coprocessor able to generate “Delay Doppler Maps” in real-time within the instrument. A nadir-pointing high gain antenna and calibrated low noise amplifier help with the collection of weak reflected signals. Partners in the National Oceanographic Centre have worked closely with SSTL to develop models and algorithms able to retrieve useful measurements relating to the sea surface.

After the launch of TDS-1, the instrument has been successfully commissioned, and demonstrated the collection of both processed and raw data over sea, land and ice. Preliminary validation has shown a definite correlation between the measurements over the ocean and wind speed, but more work on calibration in an on-going ESA supported campaign is expected to improve the match.

In addition to sensing sea state, the experiment has been collecting measurements over the land and ice, and showing some intriguing results. Many of the Level 1b and L2 datasets have been made available over the web for research.

This sensor technology holds a lot of promise for the future, as it is small, low power, effectively passive, but with many of the advantages of an active sensor, and can be accommodated on small satellites. As such it could be an enabling technology for a new global meteorological and sea state service.

GNSS-Reflectometry (GNSS-R) is a ground breaking ocean remote sensing technique that exploits reflected signals from Global Navigation Satellite Systems (GNSS) to retrieve geophysical information about the ocean surface such as near‑surface winds above the ocean. Adopting a bistatic radar configuration, signals emitted by GNSS satellites flying in Medium Earth Orbit (MEO) are received by a GNSS-R receiver on a Low Earth Orbit (LEO) observatory utilizing both a zenith antenna to receive the direct signal from the GNSS and a nadir antenna to acquire the earth-reflected signal. The reflected signal originated from a glistening zone on the ocean surface sited around the Specular Point (SP), the geometrical point on the Earth surface where GNSS signals are forward scattered in the specular direction. The two signals are correlated for different shifts in time (delay) and frequency (Doppler) relative to the specular point (SP) to produce a so-called Delay Doppler Map (DDM) of forward‑scattered electromagnetic power over the surface.

This paper gives an overview of recent results obtained for wind speed and ocean roughness retrieval with 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. We present examples of onboard-processed delay Doppler Maps, showing excellent DDM data quality for wind speed up to 27.9 m/s. The relationship between observed GNSS-R signals, wind speed, wind direction and ocean roughness is explored using global collocated matchup datasets with METOP ASCAT scatterometer winds, ECMFW numerical wind model output and WaveWatch3 numerical wave model output. A preliminary assesment of the effect of sea state over GNSS-R reflections is also presented. Several physically based and empirically tuned Geophysical Model Functions are proposed, that make it possible to retrieve wind speed without bias and with a precision of the order of 2 m/s even without calibration. This work demonstrates the capabilities of low-cost, low-mass, low-power GNSS-R receivers ahead of their launch on the NASA CYGNSS constellation in 2016.