Abstract: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 Earth's surface. Polarimetric ratio (Γ /Γ ) 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 OPEN ACCESSRemote Sens. 2015, 7 13121 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.

The 3 CAT-2 nanosatellite aims at demonstrating global navigation satellite system reflectometry (GNSS-R) techniques for spaceborne applications in the small form of a six-unit CubeSat. There are many challenges involved from a size, processing, and power perspectives. The proposed solution for the payload uses a software-defined radio (SDR) connected to a nadir looking array of dual-band and dual-frequency and dual-polarization antennas to capture the reflected GNSS signals and to a zenith looking patch antenna to capture the direct ones. The SDR is controlled by the payload computer, which retrieves the binary samples and processes the raw data to obtain delay-doppler maps (DDMs) via various techniques. DDMs are then compressed using the fully adaptive prediction error coder algorithm, producing an output more suitable for the limited downlink capabilities of these small platforms.

This article introduces 3 Cat-1, the first project of the Technical University of Catalonia to build and launch a nano-satellite. Its main scope is to develop, construct, assemble, test and launch into a low Earth orbit a CubeSat with seven different payloads (mono-atomic oxygen detector, graphene field-effect transistor, self-powered beacon, Geiger radiation counter, wireless power transfer (WPT), new topology solar cells and WPT experiment), all fitted in a single-unit CubeSat. On one hand, this is mainly an educational project in which the development of some of the subsystems is carried out by undergraduate and postgraduate students. The satellite demonstrates its capabilities as a cost-effective platform to perform small scientific experiments and to demonstrate some of the new technologies that it incorporates.

This work describes the attitude determination and control system (ADCS) of ³CAT-2, a six-unit CubeSat scheduled for launch this 2016. The ADCS of 3 CAT-2 aims at controlling the satellite in orbit and fulfilling the pointing requirements imposed by the mission. The attitude control system implemented in 3 CAT-2 will be used to point the antennas towards the Earth to perform altimetry tests, orient the solar panels towards the Sun to maximize power input when battery levels are critical, and reduce the tumbling motion of the satellite after the deployment phase. In order to guarantee pointing requirements for remote sensing purposes, an active three-axis attitude determination and control system is considered.

This paper provides a high level overview of a multi-frequency Ground-Based Synthetic Aperture Radar operating at X-, C-, L-and P-bands. The system core is implemented using a flexible high performance Software Defined Radio, aided by a custom radio-frequency front-end. The capabilities of the system are demonstrated by measuring dense time-series of a vegetated area. The benefits of lower frequencies appear as a significant increase in phase stability and coherences at P-and L-bands, which is explained by the increased vegetation penetration depth of these bands compared of C-and X-bands. It is concluded that multi-frequency measurements closely spaced in time are valuable and suggest new applications in vegetated areas.