Estel Cardellach

Altimetric Analysis of the Sea-Surface GPS-Reflected Signals

We describe the development and implementation of a method for extracting altimetric information using the Passive Reflectometry and Interferometry System (PARIS), i.e., from GPS signals after their reflection off the sea surface. We have formalized one idea laid out in the description of a bistatic system for ocean altimetry using the GPS signal, by Hajj and Zuffada (Jet Propulsion Laboratory), and have extended it to real situations encountered in PARIS aircraft experiments. Second, we have developed the corresponding algorithms to produce real-time altimetric observables to be used in dedicated digital signal processors. Finally, we have applied this method to estimate sea-surface height from one flight experiment in the North Sea off the coast of Norway.

Tutorial on Remote Sensing Using GNSS Bistatic Radar of Opportunity

In traditional GNSS applications, signals arriving at a receivers antenna from nearby reflecting surfaces (multipath) interfere with the signals received directly from the satellites which can often result in a reduction of positioning accuracy. About two decades ago researchers produced an idea to use reflected GNSS signals for remote-sensing applications. In this new concept a GNSS transmitter together with a receiver capable of processing GNSS scattered signals of opportunity becomes bistatic radar. By properly processing the scattered signal, this system can be configured either as an altimeter, or a scatterometer allowing us to estimate such characteristics of land or ocean surface as height, roughness, or dielectric properties of the underlying media. From there, using various methods the geophysical parameters can be estimated such as mesoscale ocean topography, ocean surface winds, soil moisture, vegetation, snowpack, and sea ice. Depending on the platform of the GNSS receiver (stationary, airborne, or spaceborne), the capabilities of this technique and specific methods for processing of the reflected signals may vary. In this tutorial, we describe this new remotesensing technique, discuss some of the interesting results that have been already obtained, and give an overview of current and planned spacecraft missions.

Sea surface state measured using GPS reflected signals

We discuss an airborne experiment aimed to establish the potential of the PARIS concept (PAssive Reflectometry and Interferometry System) to retrieve small features in the sea surface topography. The date and location were chosen to coincide with a TOPEX/POSEIDON (T/P) overflight. The signals of the Global Positioning System (GPS) reflected off the sea surface are tracked and compared to the directly received ones, to compute the relative delays. The features detected in the peak tracking are likely caused by topographic and sea roughness variations. While very promising, these results open the challenge to use additional information to appropriately separate both contributions.

Mediterranean Balloon Experiment: ocean wind speed sensing from the stratosphere, using GPS reflections

Abstract The MEditerranean Balloon EXperiment (MEBEX), conducted in August 99 from the middle–up stratosphere, was designed to assess the wind retrieval sensitivity of Global Navigation Satellite Systems Reflections (GNSSR) technology from high altitudes. Global Positioning System reflected signals (GPSR) collected at altitudes around 37 km with a dedicated receiver have been inverted to mean square slopes (MSS) of the sea surface and wind speeds. The theoretical tool to interpret the geophysical parameters was a bistatic model, which also depends on geometrical parameters. The results have been analyzed in terms of internal consistency, repeatability and geometry-dependent performance. In addition, wind velocities have been compared to independent measurements by QuikSCAT, TOPEX, ERS/RA and a Radio Sonde, with an agreement better than 2 m/s. A Numerical Weather Prediction Model (NWPM, the MM5 mesoscale forecast model) has also been used for comparison with varying results during the experiment. The conclusion of this study confirms the capability of high altitude GPSR/Delay-map receivers with low gain antennas to infer surface winds.

Consolidating the Precision of Interferometric GNSS-R Ocean Altimetry Using Airborne Experimental Data

This paper revises the precision of altimetric measurements made with signals of the Global Navigation Satellite Systems (GNSS) reflected (GNSS-R) off the sea surface. In particular, we investigate the performance of two different GNSS-R techniques, referred to here as the clean-replica and interferometric approaches. The former has been used in GNSS-R campaigns since the late 1990s, while the latter has only been tested once, in 2010, from an 18-m-high bridge in static conditions and estuary waters. In 2011, we conducted an airborne experiment over the Baltic Sea at 3-km altitude to test the interferometric concept in dynamic and rougher conditions. The campaign also flew a clean-replica GNSS-R instrument with the purpose of comparing both approaches. We have analyzed with detail the data sets to extract and validate models of the noise present in both techniques. After predicting the noise models and verifying these with aircraft data, we used them to obtain the precision of altimetric measurements and to extrapolate the performance analysis to spaceborne scenarios. The main conclusions are that the suggested noise model agrees with measured data and that the GNSS-R interferometric technique is at least two times better in precision than a technique based on using a clean replica of the publicly available GPS code. This represents a factor of at least four times finer along-track resolution. A precision of 22 cm in 65-km along-track averaging should be achievable using near-nadir interferometric GNSS-R observations from a low earth orbiter.

Altimetry with GNSS-R interferometry: first proof of concept experiment

The Global Navigation Satellite System Reflectometry (GNSS-R) concept was conceived as a means to densify radar altimeter measurements of the sea surface. Until now, the GNSS-R concept relied on open access to GNSS transmitted codes. Recently, it has been proposed that the ranging capability of the technique for ocean altimetric applications can be improved by using all the signals transmitted in the bandwidth allocated to GNSS, which includes open access as well as encrypted signals. The main objective of this study is to provide experimental proof of this enhancement through a 2-day experiment on the Zeeland Bridge (The Netherlands). In the experiment, we used a custom built GNSS-R system, composed of high gain GPS antennas, calibration subsystem, and an FPGA-based signal processor which implemented the new concepts, an X-band radar altimeter and a local geodetic network. The results obtained indicate that the new approach produces a significant improvement in GNSS-R altimetric performance.

GEROS-ISS: GNSS REflectometry, Radio Occultation, and Scatterometry Onboard the International Space Station

GEROS-ISS stands for GNSS REflectometry, radio occultation, and scatterometry onboard the International Space Station (ISS). It is a scientific experiment, successfully proposed to the European Space Agency in 2011. The experiment as the name indicates will be conducted on the ISS. The main focus of GEROS-ISS is the dedicated use of signals from the currently available Global Navigation Satellite Systems (GNSS) in L-band for remote sensing of the Earth with a focus to study climate change. Prime mission objectives are the determination of the altimetric sea surface height of the oceans and of the ocean surface mean square slope, which is related to sea roughness and wind speed. These geophysical parameters are derived using reflected GNSS signals (GNSS reflectometry, GNSS-R). Secondary mission goals include atmosphere/ionosphere sounding using refracted GNSS signals (radio occultation, GNSS-RO) and remote sensing of land surfaces using GNSS-R. The GEROS-ISS mission objectives and its design, the current status, and ongoing activities are reviewed and selected scientific and technical results of the GEROS-ISS preparation phase are described.

Phase Altimetry With Dual Polarization GNSS-R Over Sea Ice

This paper evaluates the potential use of reflected signals from Global Navigation Satellite Systems as a source of opportunity for the retrieval of absolute ellipsoidal heights over sea ice. Accurate estimation of the surface level would be helpful for the determination of the ice thickness, a key parameter for classification and characterization of sea ice masses. Our analysis is based on altimetric estimations from the coherent differential phase between direct and both cross- and co-polar reflected signals. For this purpose, GPS waveforms have been collected from a fixed platform in Greenland, monitoring the complete process of sea ice formation and melting during a 7-month period. The variability of coherent phase samples and polarimetric measurements are compared with in situ observations to make a realistic rough characterization of the ice cover. The retrieved sea ice surface height estimates are then evaluated against an Arctic tide model, ice surface temperature from moderate-resolution imaging spectroradiometer, and the laser altimetry product from ICESat.

PARIS Interferometric Technique proof of concept: Sea surface altimetry measurements

We report preliminary results of an aircraft experiment aimed to proof the PARIS Interferometric Technique. The experiment was performed in the Gulf of Finland during a two hours flight. We installed a PARIS Interferometric Receiver together with a GOLD-RTR instrument to collect reflected C/A, P(Y) and M-code GPS signals. The collected data has been analyzed to produce altimetric observables with both techniques.

Ionospheric calibration of radar altimeters using GPS tomography

We compare TEC measurements from the NASA Radar Altimeter and DORIS instrument on board TOPEX/POSEIDON with GPS TEC estimates, and evaluate different GPS data analysis strategies. We verify that global tomographic GPS analysis using a voxel grid is well suited for ionospheric calibration of altimeters. We show that a 1-day fit of 20-second-averaged NRA ionospheric correction data versus GPS tomographic TEC data has a bias of 3.4 TECU and a root mean square deviation of 3.2 TECU. Tomographic inversion using simulated data from the Parametrized Ionospheric Model highlights the strong correlation between GPS bias constants, electronic densities at the highest layer, and unmodeled protonospheric TEC. This suggests that GPS TEC estimates at the TOPEX/POSEIDON altitude are more accurate if the bias constants are estimated and if a layer above TOPEX/POSEIDON is added to the grid.