Raul Onrubia

Precision Bounds in GNSS-R Ocean Altimetry

Global navigation satellite systems (GNSS) signals offer a promising opportunity to perform mesoscale altimetry with worldwide coverage using small satellites. However, unlike the signals used in classical altimeters, the GNSS signals have smaller bandwidth and lower transmitted power. This paper analyzes the achievable height precision of the Global Navigation Satellite System Reflectometry (GNSS-R) altimeters based on the Cramér-Rao bound for time-delay estimation. Precision bounds for the composite signals GPS L1 and L5 and Galileo E1, E5, and E6 in a spaceborne mission scenario are assessed, and the optimum receiver bandwidths which minimize them are obtained.

Dual-Polarization GNSS-R Interference Pattern Technique for Soil Moisture Mapping

The interference pattern technique (IPT) consists of the coherent addition of the direct and reflected global navigation satellite systems (GNSS) signals in the receiving antenna. The detected power oscillates (fading), and the amplitude of these oscillations is very sensitive to the soil reflection coefficient at the specular reflection point. Therefore, variations of the reflection coefficient can be mapped, and thus dielectric constant variations, from which soil moisture can be retrieved. This work extends the use of the IPT technique from vertical polarization (V-Pol) to horizontal polarization (H-Pol). Moreover, the IPT equations are reformulated to facilitate the combination of dual-polarization retrievals. Simulations of the interference patterns at V- and H-Pol are presented for different soil moisture conditions. An upgrade of the SMIGOL GNSS-R instrument for dual-polarization observations is presented. This instrument was deployed in a flat, dry grassland in Yanco, Australia, in order to validate the proposed concepts. Finally, a comparison between the data retrieved from the SMIGOL instrument and the ground-truth soil moisture data is presented showing a good agreement between them and rainfall information.

Retrieval of Significant Wave Height and Mean Sea Surface Level Using the GNSS-R Interference Pattern Technique: Results From a Three-Month Field Campaign

Since 1993, when the European Space Agency (ESA) proposed the use of Global Navigation Satellite Systems reflected signals for sea mesoscale altimetry, a wide range of applications have appeared. This paper focuses on the retrieval of significant wave height (SWH) and the mean sea surface level (MSSL) from a ground-based experiment using the interference pattern technique (IPT). Two different observables on the IPT are analyzed: the oscillation frequency and the angle where coherency is lost. The point where coherency in the reflection process is lost can be related to the Rayleigh criterion for smooth surfaces and helps to determine the SWH. Spectral analysis on the interference pattern helps to determine the MSSL. A three-month field campaign was performed on the “Pont del Petroli” pier, Badalona, Spain, to see how the reflected GNSS signals were affected by coastal sea state and check previous assumptions. Results from this field experiment are shown, confirming that the SWH can be retrieved with accuracy of 6 cm and the MSSL with 4 cm. Estimations of both parameters are obtained every 30 min approximately.

Improving the Accuracy of Soil Moisture Retrievals Using the Phase Difference of the Dual-Polarization GNSS-R Interference Patterns

Soil moisture (SM) is a key parameter in the climate studies at a global scale and a very important parameter in applications such as precision agriculture at a local scale. The Global Navigation Satellite Systems Interference Pattern Technique (IPT) has proven to be a useful technique for the determination of SM, based on observations at vertical polarization (V-Pol) due to the Brewster angle. The IPT can be applied at both V-Pol and horizontal polarization (H-Pol) at the same time, observing the Brewster angle only at V-Pol. This letter presents a measurement technique based on tracking the phase difference between V-Pol and H-Pol interference patterns to improve the accuracy of the Brewster angle determination and, consequently, that of the SM retrievals. This technique benefits from the different phase behavior of the reflection coefficients between H-Pol and V-Pol in the angular observation range. To be sensitive to the phase difference, the Rayleigh criterion for smooth surfaces must be accomplished. This technique is not sensitive to topography as it is intrinsically corrected. Experimental results are presented to validate the proposed algorithm.

3 Cat-2: A P(Y) and C/A GNSS-R experimental nano-satellite mission

3Cat-2 is a 6 U CubeSat mission designed to perform ocean altimetry by means of Global Navigation Satellite Systems Reflectometry (GNSS-R). In this paper 3Cat-2 mission is presented. The main payload is the novel dual-band altimeter P(Y) & C/A ReflectOmeter (PYCARO) which has already been tested from a pier and from an aircraft over the Mediterranean Sea. In addition, an experiment from the DLR/SNSB Balloon-borne EXperiments for University Students (BEXUS) stratospheric balloon in North Sweden will be performed on October 2013 for the ultimate optimization of the payload parameters.

Simulation and Analysis of GNSS-R Composite Waveforms Using GPS and Galileo Signals

Nowadays, several global navigation satellite system (GNSS) services coexist at L-band and some more will be available in the near future. These new signals were originally conceived to enhance the location accuracy, but they also offer a promising opportunity for improved GNSS-reflectometry (GNSS-R) retrievals. This work gives closed form expressions for GNSS-R waveform simulation using the composite GPS L1 and L5 and Galileo E1, E5, and E6 signals as function of the receiver bandwidth. The impact of the observation geometry and sea state on the waveform shape is studied in airborne and spaceborne conditions. Finally, the altimetric height bias using the derivative method is analyzed as function of the receiver bandwidth.

SNR Degradation in GNSS-R Measurements Under the Effects of Radio-Frequency Interference

Radio-frequency interference (RFI) is a serious threat for systems working with low power signals such as those coming from the global navigation satellite systems (GNSS). The spectral separation coefficient (SSC) is the standard figure of merit to evaluate the signal-to-noise ratio (SNR) degradation due to the RFI. However, an in-depth assessment in the field of GNSS-Reflectometry (GNSS-R) has not been performed yet, and particularly, about which is the influence of the RFI on the so-called delay-Doppler map (DDM). This paper develops a model that evaluates the contribution of intra-/inter-GNSS and external RFI effects to the degradation of the SNR in the DDM for both conventional and interferometric GNSS-R techniques. Moreover, a generalized SSC is defined to account for the effects of nonstationary RFI signals. The results show that highly directive antennas are necessary to avoid interference from other GNSS satellites, whereas mitigation techniques are essential to keep GNSS-R instruments working under external RFI degradation.

DME/TACAN Impact Analysis on GNSS Reflectometry

Global navigation satellite system reflectometry (GNSS-R) is becoming a widely accepted technique for remote sensing. The interferometric technique (iGNSS-R) correlates the direct signal received from a satellite and the same signal reflected on the Earths surface, whereas the conventional technique (cGNSS-R) correlates the reflected signal with a locally generated replica of the transmitted code. As GNSS signals are received below the noise level, this technique is extremely sensitive to radio frequency interference. The distance measurement equipment, and the TACtical air navigation systems are two radio navigation systems that transmit in the GPS L5, and Galileo E5 bands with powers up to 3.5 kW. This work studies in depth the impact of these systems on iGNSS-R, and cGNSS-R instruments. This study is then applied to a hypothetical reflectometer that will be placed in the international space station: The GEROS experiment. It is shown that the received power in space will be strong enough to degrade the systems performance by increasing the noise floor, but the sea altimetry precision will still be accurate enough for scientific studies.

Assessment of back-end RFI mitigation techniques in passive remote sensing

Radio-Frequency Interference (RFI) is a growing problem specially for those systems that work with low power signals such as passive remote sensing instruments. Consequently, RFI mitigation techniques are currently under development. This works aims at evaluating back-end mitigation algorithms in terms of their probability of detection and mitigation performance. Results show that Wavelet Denoising (WD), and Multiresolution Fourier Transform (MFT) are the best techniques in most scenarios, specially for GNSS-based instruments.

Soil Moisture mapping using forward scattered GPS L1 signals

This work presents a novel technique for the determination of soil moisture obtaining 2-D Soil Moisture (SM) information with a single instrument. Both the instrument and the retrieval algorithm used, which is based on inferring the reflection coefficient of the terrain by direct and forward scattering polarimetric measurements of Global Navigation Satellite Systems (GNSS) Signals, are briefly described. Some preliminary results of a field campaign performed on La Pobla de Mafumet (Tarragona, Spain) are presented. This instrument and retrieval algorithm can be used for different applications, such as, an input for the irrigation algorithm (Smart-Irrigation) or forest fire prevention among others.