Spaceborne Systems Research Articles

Investigating landslides with space-borne Synthetic Aperture Radar (SAR) interferometry

This paper is addressed to readers without advanced knowledge of remote sensing. It illustrates some current and potential uses of satellite Synthetic Aperture Radar interferometry (InSAR) for landslide assessment. Data acquired by SAR systems can provide 3D terrain models and be used to assist in regional scale investigations, e.g. aimed at evaluation of susceptibility of slopes to failure. Under favourable environmental conditions, the innovative Permanent Scatterers (PS) technique, which overcomes several limitations of conventional SAR differential interferometry (DInSAR) applications in landslide studies, is suitable for monitoring slope deformations with millimetric precision. The PS technique combines the wide-area coverage typical of satellite imagery with the capability of providing displacement data relative to individual image pixels. With the currently available radar satellites, however, only very slow ground surface displacements can be reliably detected and measured. The presented case study of a landslide from the Liechtenstein Alps indicates that the most attractive and reliable contribution provided by this remote sensing technique lies in the possibility of (i.) wide-area qualitative distinction between stable and unstable areas and (ii.) qualitative (relative) hazard zonation of large, slow landslides based on the identification of segments characterised by different movement rates. Since only the radar line of sight projection of the displacements can be detected, a quantitative exploitation of the PS data is possible only where sufficient ground truth is available. In site specific or single landslide investigations the PS data can represent a very useful complementary data source with respect to the information acquired through ground based observations and in situ surveying. However, the difficulties associated with the feasibility assessments of the applicability of SAR data to local scale problems, as well as with the interpretation of PS results, require a close collaboration between landslide experts and specialists in advanced processing of radar satellite data. The interpretation of the exact geotechnical significance of small, radar sensed ground surface deformations is challenging, especially where ground truth is lacking. Although any ground deformation is potentially of interest to an engineering geologist, detection of movements in both vertical and horizontal directions is needed in the case of landslides to evaluate slope failure mechanisms. With their high radar viewing angles, however, the current space-borne systems can detect only a fraction of the horizontal component of movement. It is expected that the upcoming SAR dedicated missions with new sensors and different acquisition geometries, combined with the rapid developments in the field of advanced radar data processing, will allow a full 3D reconstruction of deformation data and help to further reduce the current limitations of the PS and similar DInSAR approaches.

Mapping recharge from space: roadmap to meeting the grand challenge

Fields of diffuse recharge flux play pivotal roles in: (1) linking surface and subsurface hydrologic systems, (2) controlling the biogeochemistry of terrestrial systems, and (3) determining the sustainability of well withdrawals from aquifers. This hydrologic flux is intimately related to the distribution and functioning of vegetation cover and type. In turn, it also plays a significant role in the distribution of vegetation. Until now, recharge has been mostly estimated as residual of either surface or subsurface water balance. In situ instruments for measurement of this quantity are difficult to implement and maintain. Long-term and spatially explicit (mapped) monitoring of recharge flux have been elusive goals. A review is presented of the possible spaceborne and airborne remote sensing and data interpretation techniques that may address the grand challenge of recharge mapping on large scales. The approaches rely on microwave remote sensing in order to measure land surface states in the presence of atmosphere and vegetation cover that attenuate and disturb the signal as it travels from the surface to the sensor. The emphasis is on radar systems that also have beneficial ground spatial resolution characteristics. Finally, examples of two candidate space-borne systems are presented as feasible approaches to the required measurements.

Glaciers and Ice Sheets Mapping Orbiter concept

We describe a concept for a spaceborne radar system designed to measure the surface and basal topography of terrestrial ice sheets and to determine the physical properties of the glacier bed. Our primary objective is to develop this new technology for obtaining spaceborne estimates of the thickness of the polar ice sheets with an ultimate goal of providing essential information to modelers estimating the mass balance of the polar ice sheets and estimating the response of ice sheets to changing climate. Our new technology concept employs VHF and P‐band interferometric radars using a novel clutter rejection technique for measuring surface and bottom topographies of polar ice sheets from aircraft and spacecraft. Our approach will enable us to reduce signal contamination from surface clutter, measure the topography of the glacier bed at better than 1 km intervals with an accuracy of 20 m, and paint a picture of variations in bed characteristics. The technology will also have applications for planetary exploration including studies of the Martian ice caps and the icy moons of the outer solar system. Through the concept developed here we believe that we can image the base and map the three‐dimensional basal topography beneath an ice sheet at up to 5 km depth.

Performance improvement for constellation SAR using signal processing techniques

A new concept of spaceborne synthetic aperture radar (SAR) implementation has recently been proposed - the constellation of small spaceborne SAR systems. In this implementation, several formation-flying small satellites cooperate to perform multiple space missions. We investigate the possibility to produce high-resolution wide-area SAR images and fine ground moving-target indicator (GMTI) performance with constellation of small spaceborne SAR systems. In particular, we focus on the problems introduced by this particular SAR system, such as Doppler ambiguities, high sparseness of the satellite array, and array element errors. A space-time adaptive processing (STAP) approach combined with conventional SAR imaging algorithms is proposed which can solve these problems to some extent. The main idea of the approach is to use a STAP-based method to properly overcome the aliasing effect caused by the lower pulse-repetition frequency (PRF) and thereby retrieve the unambiguous azimuth wide (full) spectrum signals from the received echoes. Following this operation, conventional SAR data processing tools can be applied to focus the SAR images fully. The proposed approach can simultaneously achieve both high-resolution SAR mapping of wide ground scenes and GMTI with high efficiency. To obtain array element errors, an array auto-calibration technique is proposed to estimate them based on the angular and Doppler ambiguity analysis of the clutter echo. The optimizing of satellite formations is also analyzed, and a platform velocity/PRF criterion for array configurations is presented. An approach is given to make it possible that almost any given sparse array configuration can satisfy the criterion by slightly adjusting the PRF. Simulated results are presented to verify the effectiveness of the proposed approaches.

Spectral Subtraction: A New Approach to Remove Low‐ and High‐Order Speckle Noise

We present a novel subtraction algorithm (SSA) technique to remove speckle noise. It consists of a low-order and a high-order SSA and is based on a three-dimensional image spectroscopy in which the three-dimensional data cube is available and thus the speckle noise introduced by the wave-front error can be efficiently subtracted. For the low-order SSA, speckles up to the second or third order can be totally subtracted, leaving the residual speckles dominated only by the third or fourth order, respectively, and imaging contrast is increased consequently; for the high-order SSA, speckles up to the fourth or fifth order can be subtracted, leaving the residual speckles dominated only by the fifth or sixth order, respectively, and the performance is further improved. This is the first demonstration that such high-order speckles could be subtracted. Since the SSAs are conducted over a wide spectral band, a white-light image can be re-assembled from the three-dimensional data cube. The white-light image would increase the single-to-noise ratio and reduce the exposure time, which are crucial for the search of faint companion objects. Combined with a coronagraph, the SSA can provide an extra contrast gain for the coronagraph imaging, relax the requirement for the wave-front quality (no adaptive optics correction is required for a space-borne imaging system), and significantly increase the performance of exoplanet imaging and biomarker spectroscopy.

A novel approach for wide‐swath and high‐resolution SAR image generation from distributed small spaceborne SAR systems

In this paper we investigate the possibility to produce high‐resolution synthetic aperture radar (SAR) images of wide areas from the data collected by the distributed small spaceborne SAR systems. In particular, we focus on the data processing techniques for overcoming the Doppler ambiguity effects characterizing ground echoes and achieving high‐resolution SAR images of wide areas. A novel approach is proposed, the key idea of which is to retrieve the unambiguous azimuth wide (full) spectrum signal from the received ground echoes by using a space–time adaptive processing (STAP) algorithm to overcome the Doppler aliasing effect induced by the lower pulse repetition frequency (PRF); following this operation, conventional SAR data processing tools can be applied to fully focus the SAR images. The STAP‐based approach has the advantages of simplicity, robustness and high efficiency. We also discuss the array configuration requirements, and give a platform velocity/PRF criterion for array configurations.

Towards Formulation of a Space-borne System for Early Warning of Floods: Can Cost-Effectiveness Outweigh Prediction Uncertainty?

The three most important components necessary for functioning of an operational flood warning system are: (1) a rainfall measuring system; (2) a soil moisture updating system; and, (3) a surface discharge measuring system. Although surface based networks for these systems can be largely inadequate in many parts of the world, this inadequacy particularly affects the tropics, which are most vulnerable to flooding hazards. Furthermore, the tropical regions comprise developing countries lacking the financial resources for such surface-based monitoring. The heritage of research conducted on evaluating the potential for measuring discharge from space has now morphed into an agenda for a mission dedicated to space-based surface discharge measurements. This mission juxtaposed with two other upcoming space-based missions: (1) for rainfall measurement (Global Precipitation Measurement, GPM), and (2) soil moisture measurement (Hydrosphere State, HYDROS), bears promise for designing a fully space-borne system for early warning of floods. Such a system, if operational, stands to offer tremendous socio-economic benefit to many flood-prone developing nations of the tropical world. However, there are two competing aspects that need careful assessment to justify the viability of such a system: (1) cost-effectiveness due to surface data scarcity; and (2) flood prediction uncertainty due to uncertainty in the remote sensing measurements. This paper presents the flood hazard mitigation opportunities offered by the assimilation of the three proposed space missions within the context of these two competing aspects. The discussion is cast from the perspective of current understanding of the prediction uncertainties associated with space-based flood prediction. A conceptual framework for a fully space-borne system for early-warning of floods is proposed. The need for retrospective validation of such a system on historical data comprising floods and its associated socio-economic impact is stressed. This proposal for a fully space-borne system, if pursued through wide interdisciplinary effort as recommended herein, promises to enhance the utility of the three space missions more than what their individual agenda can be expected to offer.

Spaceborne imaging radar-C/X-band synthetic aperture radar (SIR-C/X-SAR): a look back on the tenth anniversary

The spaceborne imaging radar-C, X-band synthetic aperture radar (SIR-C/X-SAR) missions have resulted in important scientific discoveries and provided new insights into Earth system processes. Analyses of SIR-C/X-SAR engineering-mode data have also led to new measurement and mission concepts. The multifrequency, multipolarisation capability provided by SIR-C/X-SAR is unsurpassed from a spaceborne system, making the data set valuable for algorithm development and assessment of optimal imaging parameters more than a decade after the missions were flown.

Space Weather: The Solar Perspective

The term space weather refers to conditions on the Sun and in the solar wind, magnetosphere, ionosphere, and thermosphere that can influence the performance and reliability of space-borne and ground-based technological systems and that can affect human life and health. Our modern hi-tech society has become increasingly vulnerable to disturbances from outside the Earth system, in particular to those initiated by explosive events on the Sun: Flares release flashes of radiation that can heat up the terrestrial atmosphere such that satellites are slowed down and drop into lower orbits, solar energetic particles accelerated to near-relativistic energies may endanger astronauts traveling through interplanetary space, and coronal mass ejections are gigantic clouds of ionized gas ejected into interplanetary space that after a few hours or days may hit the Earth and cause geomagnetic storms. In this review, I describe the several chains of actions originating in our parent star, the Sun, that affect Earth, with particular attention to the solar phenomena and the subsequent effects in interplanetary space.

Reliability Measurement of Mass Storage System for Onboard Instrumentation

Advances in spaceborne vehicular technology have made possible the long-life duration of the mission in harsh cosmic environments. Reliability and data integrity are the commonly emphasized requirements of spaceborne solid-state mass storage systems, because faults due to the harsh cosmic environments, such as extreme radiation, can be experienced throughout the mission. Acceptable dependability for these instruments has been achieved by using redundancy and repair. Reconfiguration (repair) of memory arrays using spare memory lines is the most common technique for reliability enhancement of memories with faults. Faulty cells in memory arrays are known to show spatial locality. This physical phenomenon is referred to as fault clustering . This paper initially investigates a quadrat-based fault model for memory arrays under clustered faults to establish a reliable foundation of measurement. Then, lifelong dependability of a fault-tolerant spaceborne memory system with hierarchical active redundancy, which consists of spare columns in each memory module and redundant memory modules, is measured in terms of the reliability (i.e., the conditional probability that the system performs correctly throughout the mission) and mean-time-to-failure (i.e., the expected time that a system will operate before it fails). Finally, minimal column redundancy search technique for the fault-tolerant memory system is proposed and verified through a series of parametric simulations. Thereby, design and fabrication of cost-effective and highly reliable fault-tolerant onboard mass storage system can be realized for dependable instrumentation.

Spaceborne Polarimetric SAR Interferometry: Performance Analysis and Mission Concepts

We investigate multichannel imaging radar systems employing coherent combinations of polarimetry and interferometry (Pol-InSAR). Such systems are well suited for the extraction of bio- and geophysical parameters by evaluating the combined scattering from surfaces and volumes. This combination leads to several important differences between the design of Pol-InSAR sensors and conventional single polarisation SAR interferometers. We first highlight these differences and then investigate the Pol-InSAR performance of two proposed spaceborne SAR systems (ALOS/PalSAR and TerraSAR-L) operating in repeat-pass mode. For this, we introduce the novel concept of a phase tube which enables (1) a quantitative assessment of the Pol-InSAR performance, (2) a comparison between different sensor configurations, and (3) an optimization of the instrument settings for different Pol-InSAR applications. The phase tube may hence serve as an interface between system engineers and application-oriented scientists. The performance analysis reveals major limitations for even moderate levels of temporal decorrelation. Such deteriorations may be avoided in single-pass sensor configurations and we demonstrate the potential benefits from the use of future bi- and multistatic SAR interferometers.

Space technologies for short-term earthquake warning

Recent theoretical and experimental studies explicitly demonstrated the ability of space technologies to identify and monitor the specific variations at near-earth space plasma, atmosphere and ground surface associated with approaching severe earthquakes (named as earthquake precursors) which appear several days (from 1 to 5) before the seismic shock over the seismically active areas. Several countries and private companies are in the stage of preparation (or already launched) the dedicated spacecrafts for monitoring of the earthquake precursors from space and for short-term earthquake prediction. The present paper intends to outline the optimal algorithm for creation of the space-borne system for the earthquake precursors monitoring and for short-term earthquake prediction. It takes into account the following: 1. Selection of the precursors in the terms of priority, considering their statistical and physical parameters. 2. Configuration of the spacecraft payload. 3. Configuration of the satellite constellation (orbit selection, satellite distribution, operation schedule). 4. Different options of the satellite systems (cheap microsatellite or comprehensive multisatellite constellation). Taking into account that the most promising are the ionospheric precursors of earthquakes, the special attention is devoted to the radiophysical techniques of the ionosphere monitoring. The advantages and disadvantages of such technologies as vertical sounding, in-situ probes, ionosphere tomography, GPS TEC and GPS MET technologies are considered.

Nanocomposite and nanostructured tribological materials for space applications

Satellites and space-born systems have a number of moveable mechanical parts, whose reliability is severely limited by degradation of lubricants and excessive wear. Many systems may remain in space for 10–30 years and, therefore, exposed to atomic oxygen, solar radiation, energetic particles, and temperature cycling from cryogenic to 400 °C. Furthermore, these systems are frequently tested on the ground and stored for many years under controlled environments before launching. Also, reusable launch vehicles are so planned that they will operate with space–terrestrial cycling and with temperature spikes in excess of 800 °C during re-entry. A “chameleon” tribological coating concept was developed to address this challenge. This approach relies on the coating to change its surface (both chemistry and structure) to self-adjust to the environment and thus achieve long durability. The first “chameleon” coatings were made of WC, WS 2, and diamond-like carbon (DLC); they provided superior mechanical toughness and performance in space/terrestrial environmental cycling. In order to address the temperature variation, the second generation of “chameleon” coatings were made of yttria stabilized zirconia (YSZ) in a gold matrix with encapsulated nanosized reservoirs of MoS 2 and DLC. Encapsulation of MoS 2 nanoparticles into Al 2O 3 matrix and high temperature lubrication with low melting point glassy ceramic phases were also explored. “Chameleon” coatings with various chemistries are discussed along with their characterization by various analytical, mechanical, and tribological methods. Coating toughness was remarkably enhanced by a grain boundary sliding mechanism. Unique friction and wear performance was demonstrated by testing in controlled humidity air, dry nitrogen, vacuum, 500–600 °C air, and in environmental cycling.

Space weather effects of October?November 2003

The period between late October and early November of 2003 included a series of large solar events that resulted in some of the most extreme space weather of the current solar cycle. Space weather has been defined as the conditions on the sun and in the solar wind, magnetosphere, ionosphere, and thermosphere that can influence the performance and reliability of space-borne and ground-based technological systems. Adverse conditions in the space environment can cause disruption of satellite operations, communications, navigation, and electric power distribution grids, leading to a variety of socioeconomic losses (National Science Foundation 2000). The space weather events of October–November 2003 were initiated by three very large sunspot clusters that became visible on the Sun over a period of approximately 7 days beginning on 18 October, 2003. One of these clusters, region 486, became the largest sunspot group of the current solar cycle and the largest seen since 1991. This sudden increase in sunspots induced a series of solar flares and coronal mass ejections (CMEs), (Department of Commerce 2004). Solar flares are explosions on the surface of the Sun that produce a quick directed high-speed blast of charged particles and electromagnetic energy. One of the largest solar flares ever seen on the Sun occurred on 28 October. It was the third largest detected since regular monitoring began approximately 30 years ago and the largest since April 2001 (US Department of Commerce 2004). This activity was accompanied by a CME. CME’s are huge bubbles of plasma ejected from the Sun that speed into space. They disrupt the flow of the solar wind and produce disturbances that can strike the Earth with sometimes-catastrophic results (http://science.nasa.gov/ssl/pad/solar/ cmes.htm). This particular CME was one of the fastest on record in that it left the sun with a speed of 2,500 km/s, five times the average speed (Murtagh 2004). Figure 1 illustrates the observed active regions on 27 October together with the images of the solar flare and CME recorded on 28 October as observed by the SOHO spacecraft (http://sohowww.nascom.nasa.gov/). These events resulted in a severe geomagnetic storm in the Earth’s environment at 06:00 UT on 29 October. The disturbance levels on the Sun remained high for several days with a second powerful CME occurring at approximately 21:00 UT on 29 October. Figure 2 illustrates the planetary 3-h Kp indices for the period of 29 October 2003 to 1 November 2003. The Kp index is derived from 13 global magnetic observatories. It is a measure of the disturbance level of the geomagnetic field by auroral ionospheric currents. Kp values greater than 4 represent disturbed conditions. As the Kp values increase, the severity of the storm increases. Kp values of 9 represent the most extreme disturbed conditions and occur only a few times during the course of each 11-year solar cycle (http://www.sec.noaa.gov/NOAAscales). Note that Kp values of 9 were reached multiple times during 29–30 October in response to the CME activity.

Integrated adjustment of CHAMP, GRACE, and GPS data

Various types of observations, such as space-borne Global positioning system (GPS) code and phase data, accelerometer data, K-band range and range-rate data, and ground-based satellite laser ranging data of the CHAllenging Minisatellite Payload (CHAMP) and GRAvity Climate Experiment (GRACE) satellite missions, are used together with ground-based GPS code and phase data in a rigorous adjustment to eventually solve for the ephemerides of the CHAMP, GRACE, and GPS satellites, geocenter variations, and low-degree gravity field parameters. It turns out that this ‘integrated’ adjustment considerably improves the accuracy of the ephemerides for the high and low satellites, geocenter variations, and gravity field parameters, compared to the case when the adjustment is carried out stepwise or in individual satellite solutions.

TOTAL-DOSE AND SINGLE-EVENT EFFECTS IN SILICON-GERMANIUM HETEROJUNCTION BIPOLAR TRANSISTORS

We present an overview of radiation effects in silicon-germanium heterojunction bipolar transistors ( SiGe HBT). We begin by reviewing SiGe HBTs, and then examine the impact of ionizing radiation on both the dc and ac performance of SiGe HBTs, the circuit-level impact of radiation-induced changes in the transistors, followed by single-event phenomena in SiGe HBT circuits. While ionizing radiation degrades both the dc and ac properties of SiGe HBTs, this degradation is remarkably minor, and is far better than that observed in even radiation-hardened conventional Si BJT technologies. This fact is particularly significant given that no intentional radiation hardening is needed to ensure this level of both device-level and circuit-level tolerance (typically multi-Mrad TID). SEU effects are pronounced in SiGe HBT circuits, as expected, but circuit-level mitigation schemes will likely be suitable to ensure adequate tolerance for many orbital missions. SiGe HBT technology thus offers many interesting possibilities for space-borne electronic systems.

THE EFFECTS OF SPACE RADIATION EXPOSURE ON POWER MOSFETS: A REVIEW

Power MOSFETs are a commonly used device for many switching and power control applications. Their upper frequency limit spans a fairly broad range, from 1 MHz to 10 MHz. These devices are frequently used in spaceborne electronic systems where they encounter radiation exposure during operation. This paper reviews the current technology, its high frequency capability, the future trends for power MOSFET technology, and the degradation that the power VDMOS technology experiences in the space radiation environment.

The СПРУТ-VI Instrument Complex for the Mir Orbital Station

The basic elements of the СПРУТ instrument package program for orbital stations are presented. The characteristics of the СПРУТ-VI equipment developed for the Mir orbital station within the framework of this program are described. This equipment allows the simultaneous recording of the characteristics of near-earth space (electron, proton, and nucleus fluxes, magnetic fields, and low-frequency electromagnetic waves) and their effect on the elements of spaceborne equipment and systems. The mass of the equipment is 16 kg; the power consumption is no more than 20 W.

Radiation characteristics of a high-emissivity cylindrical–spherical cavity with obscuration

We have calculated, to first order, the apparent emissivity of the bounding diffuse surfaces of a high-emissivity cylindrical-spherical cavity enclosure. Our calculations indicate that to achieve emissivities close to a perfectly absorbing blackbody cavity along the bounding surfaces of the spherical enclosure, the radius of the sphere must be equal to or greater than a factor of 4 times the cylinder radius R(S) > or = 4R(C). Furthermore, to achieve emissivities approaching a blackbody cavity along the lower bounding surfaces of the cylindrical enclosure, the length of the cylinder must be a factor of 4 times greater than the radius of the cylinder L > or = 4R(C). In addition, we present the mathematical framework necessary to calculate radiant transfer within a cavity enclosure that contains obscuration. These results can be applied to the design of high-emissivity blackbody calibration cavities and to the reduction of stray light in terrestrial and spaceborne optical systems.

Global mapping of attenuation at ku- and ka-band

The propagation of radio waves for Earth-space slant path at C-band and higher frequencies is dominated by precipitation in the atmosphere. At a given frequency, attenuation depends on the length of the radio path, the size distribution, and the phase state of the hydrometeor profile. Using the observations from the Tropical Rainfall Measuring Mission (TRMM) spaceborne Ku-band (13.8 GHz) radar at low Earth orbit of 350 km above Earth, global attenuation maps are produced at the Ku-band frequency. A simple microphysical model for precipitation developed using hydrometeor size distributions and thermodynamic phase state is used to estimate attenuation and reflectivity observations at Ka-band (35 GHz) where numerous high-bandwidth satellite applications are being planned including the next-generation space-based radar for the Global Precipitation Mission (GPM). Differences in the microphysical structure in convective and stratiform precipitation are also incorporated in the model. The results show substantial attenuation variation in a 12-month period at both Ku- and Ka-bands over the various regions of the globe, including the contrast between land and ocean. The estimates of attenuation made at Ku- and Ka-band will be useful in the design and development of spaceborne systems.