Martin Unwin

Detection and Processing of bistatically reflected GPS signals from low Earth orbit for the purpose of ocean remote sensing

We will show that ocean-reflected signals from the global positioning system (GPS) navigation satellite constellation can be detected from a low-earth orbiting satellite and that these signals show rough correlation with independent measurements of the sea winds. We will present waveforms of ocean-reflected GPS signals that have been detected using the experiment onboard the United Kingdoms Disaster Monitoring Constellation satellite and describe the processing methods used to obtain their delay and Doppler power distributions. The GPS bistatic radar experiment has made several raw data collections, and reflected GPS signals have been found on all attempts. The down linked data from an experiment has undergone extensive processing, and ocean-scattered signals have been mapped across a wide range of delay and Doppler space revealing characteristics which are known to be related to geophysical parameters such as surface roughness and wind speed. Here we will discuss the effects of integration time, reflection incidence angle and examine several delay-Doppler signal maps. The signals detected have been found to be in general agreement with an existing model (based on geometric optics) and with limited independent measurements of sea winds; a brief comparison is presented here. These results demonstrate that the concept of using bistatically reflected global navigation satellite systems signals from low earth orbit is a viable means of ocean remote sensing.

Antennas for Modern Small Satellites

Modern small satellites (MSS) are revolutionizing the space industry. They can drastically reduce the mission cost, and can make access to space more affordable. The relationship between a modern small satellite and a ldquoconventionalrdquo large satellite is similar to that between a modern compact laptop and a ldquoconventionalrdquo work-station computer. This paper gives an overview of antenna technologies for applications in modern small satellites. First, an introduction to modern small satellites and their structures is presented. This is followed by a description of technical challenges in the antenna designs for modern small satellites, and the interactions between the antenna and modern small satellites. Specific antennas developed for modern small-satellite applications are then explained and discussed. The future development and a conclusion are presented.

Spaceborne GNSS reflectometry for ocean winds: First results from the UK TechDemoSat-1 mission

First results are presented for ocean surface wind speed retrieval from reflected GPS signals measured by 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. Examples of onboard-processed delay Doppler Maps reveal excellent data quality for winds up to 27.9 m/s. Collocated ASCAT scatterometer winds are used to develop and evaluate a wind speed algorithm based on Signal-to-Noise ratio (SNR) and the Bistatic Radar Equation. For SNR greater than 3 dB, wind speed is retrieved without bias and a precision around 2.2 m/s between 3–18 m/s even withoutcalibration. Exploiting lower SNR signals however requires good knowledge of the antenna beam, platform attitude and instrument gain setting. This study demonstrates the capabilities of low-cost, low-mass, low-power GNSS-R receivers ahead of their launch on the NASA CYGNSS constellation in 2016.

Spaceborne GNSS-Reflectometry on TechDemoSat-1: Early Mission Operations and Exploitation

GNSS-Reflectometry is a new technique that shows promise for many earth observation applications including remote sensing of oceans, land, and ice. A payload has been developed that is low size and power, and suitable for use on small satellites. The first flight of the SGR-ReSI GNSS Reflectometry Instrument is on the TechDemoSat-1 mission, launched in July 2014. The instrument has been operational since its commissioning in September 2014, and has been collecting delay Doppler maps routinely over many different surfaces. Preliminary work has been undertaken to develop and validate wind speed inversion algorithms against ASCAT measurements with promising results. Measurements over land and sea ice are also showing interesting geophysical characteristics This paper describes the instrument, early operations, data dissemination through the Measurement of Earth Reflected Radio-navigation Signals By Satellite (MERRByS) website and preliminary data assessments in preparation for further data exploitation.

A Compact Multipath Mitigating Ground Plane for Multiband GNSS Antennas

This paper presents the design of a novel multipath mitigating ground plane for global navigation satellite system (GNSS) antennas. First, the concept of a compact low multipath cross-plate reflector ground plane (CPRGP) is presented. In comparison with the choke ring and electromagnetic band gap (EBG) ground planes, the proposed CPRGP has compact size, low mass, wide operational bandwidth, and simple configuration. The proposed CPRGP is then integrated with a circularly polarized dual-band GNSS antenna in order to assess the multipath mitigating performance over two frequency bands. Measurement results of the proposed CPRGP with GNSS antenna achieves a front-to-back ratio (FBR) over 25 dB at L1 (1.575 GHz) and L2 (1.227 GHz) bands and maximum backward cross-polarization levels below -23 dB at both bands. Antenna phase center variation remains less than 2 mm across both L1 and L2 bands. Furthermore, the performance comparison of the proposed CPRGP with the commercially available pinwheel antenna and the shallow corrugated ground plane is presented, showing the advantages of CPRGP for high precision GNSS applications.

Antennas for small satellites

Small low-cost satellites, pioneered at Surrey, are revolutionizing space. This paper gives an overview of antenna technologies for applications in small satellites. First, an introduction to small satellites and their structure is presented. This is followed by a description of the technical challenges of antenna design for small satellites. Various antennas for small satellite applications are illustrated. A conclusion and future work at Surrey Space Centre (SSC) and Surrey Satellite Technology (SSTL) is presented in the end.

Stand-alone spacecraft attitude determination using real flight GPS data from UOSAT-12

Abstract This paper presents the results of an experimental attitude determination system employing GPS (Global Positioning System), using phase difference data logged in orbit on the UoSAT-12 minisatellite, which was launched in 1999. The basics of GPS attitude determination are described, and two attitude determination algorithms are described and compared: one based on least squares estimation, and the other based on Kalman Filtering. The algorithm developed for resolving integer ambiguities in the carrier phase differences is tested to achieve instantaneous attitude from a set of only four measurements collected from two antenna baselines. A quaternion estimator based on a standard extended Kalman filter (qEKF) uses all GPS measurements to enable continued attitude determination taking into account the spacecraft dynamics. The attitude information derived from on-board magnetometers and a horizon sensor is used to evaluate the results of GPS attitude determination. Most recently, the LSQ attitude algorithms have been demonstrated in orbit and compared with the qEKF method.

Low-Cost Dual-Band Circularly Polarized Switched-Beam Array for Global Navigation Satellite System

This paper presents the design and development of a dual-band switched-beam microstrip array for global navigation satellite system (GNSS) applications such as ocean reflectometry and remote sensing. In contrast to the traditional Butler matrix, a simple, low cost, broadband and low insertion loss beam switching feed network is proposed, designed and integrated with a dual band antenna array to achieve continuous beam coverage of ±25° around the boresight at the L1 (1.575 GHz) and L2 (1.227 GHz) bands. To reduce the cost, microstrip lines and PIN diode based switches are employed. The proposed switched-beam network is then integrated with dual-band step-shorted annular ring (S-SAR) antenna elements in order to produce a fully integrated compact-sized switched-beam array. Antenna simulation results show that the switched-beam array achieves a maximum gain of 12 dBic at the L1 band and 10 dBic at the L2 band. In order to validate the concept, a scaled down prototype of the simulated design is fabricated and measured. The prototype operates at twice of the original design frequency, i.e., 3.15 GHz and 2.454 GHz and the measured results confirm that the integrated array achieves beam switching and good performance at both bands.

GPS Receiver Operations on the Disaster Monitoring Constellation Satellites

The Disaster Monitoring Constellation (DMC) is an international Earth observation programme to provide a rapid global remote sensing service for the monitoring and mitigation of natural and man-made disasters. Although the Global Positioning System (GPS) was originally designed for terrestrial and air applications, satellite operations have benefited greatly from the use of on-board GPS receivers. This paper describes the GPS receiver operations on the DMC satellites, performance analysis, lessons learned, and upgrades planned for the future.

Effects of ground plane on the performance of multipath mitigating antennas for GNSS

Effects of different types of ground planes on the multipath mitigation and cross polarisation performance of a planar GNSS (L1 band) antenna have been investigated and presented. Simulations are done for circular patch antenna mounted on different types of ground planes including a finite metal disc ground plane, metallic cup and choke ring. Co-polar and cross polar radiation patterns are plotted for performance comparison. In addition to this, a novel technique is suggested to reduce the height of a choke ring antenna.