Craig Underwood

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.

Attitude control for small satellites using control moment gyros

Abstract In this paper a new Attitude Control System is proposed, based on Control Moment Gyroscopes (CMG). These actuators can provide unique torque, angular momentum and slew rate capabilities to small satellites without any increase in power, mass or volume. This will help small satellites become more agile. Agility considerably increases the operational envelope and efficiency of spacecraft and substantially increases the return of earth and science mission data. In the proceeding sections the fundamental features of CMGs are presented. A low cost, miniature SGCMG designed for an enhanced microsatellite is analysed. Sizing of a proposed SGCMG indicates the advantages of using CMGs. The SGCMG is able to produce a torque of 9.82 mNm and this is confirmed through experiments performed on an air-bearing table. A comparison between the SGCMG and a RW demonstrates the mass and power savings that can be gained by using CMGs.

Design and testing of a control moment gyroscope cluster for small satellites

Experimental results on the performance of a control moment gyroscope cluster are presented. The goal is to design and evaluate a control moment gyroscope cluster for three-axis control for agile small satellites. The experimental data are compared with simulation (theoretical) results and both are used to verify the principles, advantages, and performance specifications of a control moment gyroscope cluster for a small satellite, in a practical way. Control moment gyroscope systems are considered in the literature to be more efficient devices, from an electrical power point of view, than current actuators such as reaction/momentum wheels. Experimental measurements are presented and then compared to two reaction wheels of different size. Control moment gyroscopes are shown to have a potential performance advantage over reaction/momentum wheels for spacecraft with agile requirements.

Observation and prediction of SEU in Hitachi SRAMs in low altitude polar orbits

In-orbit SEU (single event upset) data from three microsatellites are separated into galactic cosmic ray (GCR), South Atlantic anomaly (SAA) and solar flare upsets. Heavy ion and proton testing of the same devices is reported, and predictions using LET (linear energy transfer)-dependent ion cross sections and a two-parameter fit to proton cross section data are compared with in-flight data. SEU trends in memory devices from a single manufacturer, from 16 kb to 4 Mb, are identified. The increasing density of the memories is found to be accompanied by an apparent decreasing sensitivity to the heavy ion environment in polar orbit. Although the LET threshold decreases, the decrease in asymptotic cross section has a greater effect. However, the sensitivity to proton-induced upsets is nearly similar in all memories, so the proton effects become dominant by an order of magnitude. This has important implications for future spacecraft with large onboard memories and high performance processors. >

Microdosimetry code simulation of charge-deposition spectra, single-event upsets and multiple-bit upsets

An ion microdosimetry extension to the Monte Carlo High Energy Transport Code (HETC) has been developed to allow tracking of all the reaction products and has been applied to model charge-deposition spectra in pin diodes caused by atmospheric neutron spectra, as well as upsets in DRAMs from ground and space irradiation by protons. These cases cover sensitive zone sizes ranging from hundreds of microns to sub-micron. Angular distributions of both incident particles and reaction products are found to be important, particularly for the prediction of multiple-bit upsets in devices of small feature size.

Observed radiation-induced degradation of commercial-off-the-shelf (COTS) devices operating in low-Earth orbit

Observations of single-event effects (SEEs) and total-dose degradation are presented for the data-handling system of the S80/T and KITSAT-1 micro-satellites, which have been operating for six years in a 1,320 km altitude, 66/spl deg/ inclination orbit, and which are comprised almost entirely of commercial-off-the-shelf (COTS) components. The radiation environment inside both spacecraft is inferred from measurements made by the radiation monitoring payload carried on-board KITSAT-1. Ground-based radiation testing of samples of the COTS devices (including flight spares) using proton and heavy-ion beams, has allowed comparisons to be made between the observed performance of the devices under this radiation environment, and predictions made using standard models. The conclusion is that, whilst the accumulated radiation damage is likely to cause the spacecraft to fail in the near future, the use of COTS devices coupled with a not-too-conservative design, has enabled the satellite to carry out its intended mission rapidly and at very low cost.

Improving JPEG performance in conjunction with cloud editing for remote sensing applications

The authors propose an improved version of JPEG coding for compressing remote sensing images obtained by optical sensors onboard microsatellites. The approach involves expanding cloud features to include their cloud-land transitions, thereby simplifying their coding and subsequent compression. The system is fully automatic and appropriate for onboard implementation. Its improvement in coding stems from the realization that a large number of bits are used for coding the blocks that contain the transition regions between bright clouds, if present in the image, and the dark background. A fully automatic cloud-segmentation algorithm is therefore used to identify the external boundaries of the clouds, then smooth the corresponding blocks prior to coding. Further gains are also achieved by modifying the quantization table used for coding the coefficients of the discrete cosine transform. Compared to standard JPEG, at the same level of reconstruction quality, the new method can achieve compression ratio improvement by 13-161%, depending upon the context and the amount of cloud present in the specific image. The results are demonstrated with the help of several real images obtained by the University of Surrey, U.K., satellites.

The single-event-effect behaviour of commercial-off-the-shelf memory devices-A decade in low-Earth orbit

This paper presents the results of a 10-year study on radiation effects in commercial-off-the-shelf (COTS) memory devices operating within the on-board data handling systems of five low-Earth orbiting micro-satellites, designed and built at the University of Surrey (UoS). The ionising particle environment inside three of these spacecraft has been investigated concurrently using radiation monitoring payloads developed by UoS and the Defence Evaluation and Research Agency (DERA). Through the use of these monitoring instruments, and an allied programme of ground-based testing of the memory devices, the industry-standard computer models of the radiation environment have been verified, and the memory device behaviour has characterised with respect to single-event (SEEs) due to galactic cosmic-rays, geomagnetically trapped particles, and solar particles.

Observations of Internal Charging Currents in Medium Earth Orbit

The Galileo global navigation system will employ an array of satellites in medium Earth orbit (MEO). Internal charging is one of the primary hazards for any spacecraft in MEO, and accordingly, the Galileo test satellite, Giove-A, carries a detector, SURF, to undertake the measurement of internal charging currents at three different shielding depths. The currents are due to electrons stopped in three aluminum plates: the first plate is 0.5 mm thick and located under 0.5-mm Al-equivalent shielding, the second is 0.5 mm thick and located underneath the first plate, and the third is 1.0 mm thick and located underneath the second plate. Giove-A was successfully launched on December 28, 2005 into a 23 300-km circular 56deg inclination orbit. In this paper, we provide data on the internal charging currents observed in 2006, with particular emphasis on two large charging events, one in mid-April and one in mid-December. Comparisons are made to predictions using the DICTAT internal charging tool and the FLUMIC ldquoworst caserdquo trapped electron belt model. In general, the charging currents observed are safely within the standard DICTAT 3.5/FLUMIC 3.0 predictions but are exceeded in the most shielded plate on five calendar days during December 2006. The December event was unique in 2006 in that it was triggered by the arrival of a coronal mass ejection (CME) rather than being due to the effects of a fast solar wind stream flowing from a coronal hole. The currents measured in the December event were, however, an order of magnitude lower than those predicted using the ldquoanomalously large eventrdquo supplement to FLUMIC which models the most extreme electron enhancements associated with CMEs.

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.