Hugo Carreno-Luengo

GNSS-R Derived Centimetric Sea Topography: An Airborne Experiment Demonstration

The results of two airborne experiments performed to test the precision and the relative accuracy of the conventional Global Navigation Satellite Systems Reflectometry (GNSS-R) technique employing only the C/A code are presented. The first and the second experiments demonstrate, respectively, a 17 cm precision for a 500 m flight altitude with a 8 km along-track spatial resolution, and a 6 cm precision for a 3000 m flight altitude with a 6.6 km along-track spatial resolution. In both, the Relative Mean Dynamic Topography (RMDT) is compared with results derived from traditional radar altimetry provided by Jason-2. The Root Mean Square (RMS) of the RMDT difference between both measurement systems is 48 cm for the first flight, and 198 cm for the second flight. During the second flight, the feasibility of the proposed technique to measure the sea slopes is demonstrated by superposing over the aircraft ground track the measured sea surface height with the geoid undulations, which are about 1 meter.

Experimental Evaluation of GNSS-Reflectometry Altimetric Precision Using the P(Y) and C/A Signals

This work describes a novel dual-band Global Navigation Satellite Systems Reflectometer (GNSS-R) that uses the P(Y) and C/A signals scattered over the sea surface to perform highly precise altimetric measurements. The results derived from two different ground-based field experiments over a dam and over the sea under different surfaces roughness conditions are presented. The analysis of the altimetric performance shows that the results obtained using the P(Y) code improve by a factor between 1.4 and 2.4 as compared to the results obtained using the C/A code, respectively, for high and mid-low satellites elevation angles.

First polarimetric GNSS-R measurements from a stratospheric flight over boreal forests

The first-ever dual-frequency multi-constellation Global Navigation Satellite Systems Reflectometry (GNSS-R) polarimetric measurements over boreal forests and lakes from the stratosphere are presented. Data were collected during the Swedish National Space Board (SNSB)/European Space Agency (ESA) sponsored Balloon Experiments for University Students (BEXUS) 19 stratospheric balloon experiment using the P(Y) and C/A ReflectOmeter (PYCARO) instrument operated in closed-loop mode. Maps of the polarimetric ratio for L1 and L2 Global Positioning System (GPS) and GLObal Navigation Satellite System (GLONASS), and for E1 Galileo signals are derived from the float phase at 27,000 m height, and the specular points are geolocalized on the Earths surface. Polarimetric ratio (Grl/Grr) maps over boreal forests are shown to be in the range 2-16 dB for the different GNSS codes. This result suggests that the scattering is taking place not only over the soil, but over the different forests elements as well. Additionally to the interpretation of the experimental results a theoretical investigation of the different contributions to the total reflectivity over boreal forests is performed using a bistatic scattering model. The simulated cross- (reflected Left Hand Circular Polarization LHCP) and co-polar (reflected Right Hand Circular Polarization RHCP) reflectivities are evaluated for the soil, the canopy, and the canopy-soil interactions for three different biomass densities: 725 trees/ha, 150 trees/ha and 72 trees/ha. For elevation angles larger than the Brewster angle, it is found that the cross-polar signal is dominant when just single reflections over the forests are evaluated, while in the case of multiple reflections the co-polar signal becomes the largest one.

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.

3Cat-2—An Experimental Nanosatellite for GNSS-R Earth Observation: Mission Concept and Analysis

Global navigation satellite system reflectometry is a multistatic radar using navigation signals as signals of opportunity. It provides wide-swath and improved spatiotemporal sampling over current space-borne missions. The lack of experimental datasets from space covering signals from multiple constellations (GPS, GLONASS, Galileo, and Beidou) at dual-band (L1 and L2) and dual-polarization (right- and left-hand circular polarization), over the ocean, land, and cryosphere remains a bottleneck to further develop these techniques. 3Cat-2 is a 6-unit (3 × 2 elementary blocks of 10 × 10 × 10 cm3) CubeSat mission designed and implemented at the Universitat Politècnica de Catalunya-BarcelonaTech to explore fundamental issues toward an improvement in the understanding of the bistatic scattering properties of different targets. Since geolocalization of the specific reflection points is determined by the geometry only, a moderate pointing accuracy is only required to correct the antenna pattern in scatterometry measurements. This paper describes the mission analysis and the current status of the assembly, integration, and verification activities of both the engineering model and the flight model performed at Universitat Politècnica de Catalunya NanoSatLab premises. 3Cat-2 launch is foreseen for the second quarter of 2016 into a Sun-Synchronous orbit of 510-km height.

Retracking considerations in spaceborne GNSS-R altimetry

The European Space Agency Passive Reflectometry and Interferometry System In-orbit Demonstrator (IoD) aims to perform mesoscale altimetric observations by measuring the Global Navigation Satellite System (GNSS) opportunity signals reflected over the sea surface. Altimetry based on GNSS reflectometry (GNSS-R) is significantly affected by satellite motion, since it requires relatively long integration times to reduce noise. We present the impact of the satellite motion on the GNSS-R observables and the need to retrack the waveforms. By using a detailed GNSS-R space mission simulator, the change of delay difference between the direct and the reflected signals during the incoherent averaging of the waveform has been investigated. Their effects on the waveform shape and the altimetric performance are presented comparing the aligned and non-aligned waveforms. Results show that the performance of spaceborne GNSS-R altimeter is seriously degraded without a proper alignment of the waveform samples.

Analysis of Spaceborne GNSS-R Delay-Doppler Tracking

For spaceborne Global Navigation Satellite System-Reflectometry (GNSS-R), the delay and Doppler frequency conditions dynamically change, so the compensation of the delay and Doppler errors using tracking is important to the altimetric and scatterometric performances. This work presents the characteristics of the delay and the Doppler frequency in spaceborne GNSS-R, such as the range of them, the Doppler spreading width, and the change rate. They are important considerations for design of the delay and Doppler tracking strategy. The characteristics of delay-Doppler conditions of spaceborne GNSS-R are presented with additional considerations for tracking design. The tracking methods are discussed, and the error impacts are visually demonstrated by using the spaceborne measured data from United Kingdoms Disaster Monitoring Constellation (UK-DMC) satellite.

Improvement of the PAU/PARIS End-to-end Performance Simulator (P 2 EPS) in preparation for upcoming GNSS-R missions

PAU/PARIS End-to-end Performance Simulator (P2EPS) has been developed for studying and design tools for GNSS-R space mission. For the purpose of accuracy and various scenario, P2EPS has recently improved in many features. Especially, the wide range of the simulation inputs enables to simulate the upcomming GNSS-R space missions. This paper presents the recent improvement of the P2EPS.

First Results of a GNSS-R Experiment From a Stratospheric Balloon Over Boreal Forests

The empirical results of a global navigation satellite systems reflectometry (GNSS-R) experiment onboard the Balloon EXperiments for University Students (BEXUS) 17 stratospheric balloon performed north of Sweden over boreal forests show that the power of the reflected signals is nearly independent of the platform height for a high coherent integration time Tc = 20 ms. This experimental evidence shows a strong coherent component in the forward scattered signal, as compared with the incoherent component, that allows to be tracked. The bistatic coherent reflectivity is also evaluated as a function of the elevation angle, showing a decrease of ~6 dB when the elevation angle increases from 35° to 70°. The received power presents a clearly multimodal behavior, which also suggests that the coherent scattering component may be taking place in different forest elements, i.e., soil, canopy, and through multiple reflections canopy-soil and soil-trunk. This experiment has provided the first GNSS-R data set over boreal forests. The evaluation of these results can be useful for the feasibility study of this technique to perform biomass monitoring that is a key factor to analyze the carbon cycle.

Spaceborne GNSS-R from the SMAP Mission: First Assessment of Polarimetric Scatterometry over Land and Cryosphere

This work describes the first global scale assessment of a Global Navigation Satellite Systems Reflectometry (GNSS-R) experiment performed on-board the Soil Moisture Active Passive (SMAP) mission for soil moisture and biomass determination. Scattered GPS L2 signals (1227.6 MHz) were collected by the SMAP’s dual-polarization (Horizontal H and Vertical V) radar receiver and then processed on-ground using a known replica of the GPS L2C code. The scattering properties over land are evaluated using the Signal-to-Noise Ratio (SNR), the Polarimetric Ratio (PR), and the width of the waveforms’ trailing and leading edges. These parameters show sensitivity to the effects of the Earth’s topography and Above Ground Biomass (ABG) even over Amazonian and Boreal forests. These effects are shown to be an important factor in precise soil moisture and biomass determination. Additionally, it is found that PR shows sensitivity to soil moisture content over different land cover types. In particular, the following values of the PR are found over: (a) tropical forests ~−1.2 dB; (b) boreal forests ~0.8 dB; (c) Greenland ~2.8 dB; and (d) the Sahara Desert ~3.2 dB.