Space weather environment and effects monitoring
Commercial-off-the shelf (COTS)-based microelectronics are playing an increasing role in spacecraft systems design. However, flight experience has shown that such devices are particularly susceptible to the deleterious effects of the severe ionising radiation environment encountered in Earth-orbit. The primary objective of this research programme is to investigate single-event-effect and total-ionising-dose phenomena as they occur in state-of-the-art COTS technologies flown on-board Surrey's spacecraft, and to evaluate mitigation and protection strategies. To aid this process, simultaneous measurements of the ionising radiation environment experienced by these satellites are made through various ionising radiation monitoring instruments developed by the group, e.g. CEDEX - the cosmic-ray energy deposition eXperiment.
Current research is directed towards minimising the size of these instruments down to "credit-card" size so as to provide a means for routinely monitoring the radiation environment inside operational spacecraft. In related projects, work is progressing in analysing radiation effects in the advanced micro-electronic data handling systems and in studying the anisotropy in the proton flux in the South Atlantic anomaly.
The Planetary Environments Group also supports SSTL with help, advice and practical ground-based testing of components in the context of high-energy particle interactions with spacecraft systems. Currently, this is being done in the context of missions planned for medium and high-Earth orbit.
Global total ozone mapping and UV imaging
The Planetary Environments Group, in conjunction with SSTL and the Chilean Air Force (FACH), designed and developed a miniaturised ozone layer monitoring experiment (OLME), which was flown on-board the 50 kg FASAT-Bravo, launched in July 1998 into an 820 km altitude Sun-synchronous orbit.
The purpose of the OLME is to monitor the distribution and concentration of ozone in the Earth's atmosphere at a small fraction of the cost of conventional instruments. The experiment comprises two instruments: the ozone ultraviolet backscatter imager (OUBI) - a dual dye-coated CCD-based system, capable of taking "snap-shots" of the UV backscattered from the atmosphere at a wavelength of 313 and 380 nm with a ground-resolution of ~3 km, and the ozone mapping detector (OMAD) - a four-channel UV-enhanced photodiode-based radiometer operating at 289, 313, 334 and 380 nm wavelengths with a ground resolution of ~150 km.
The group has been using the OMAD instrument to generate daily global "maps". These maps clearly show the formation and extent of the ozone "hole" which appeared in the Antarctic spring of 1998 and 1999. OMAD's "UV-albedo" maps show the global extent of cloud belts and ice sheets.
Research this year has focussed on correlating the OMAD data set with NASA's total ozone mapping spectrometer (TOMS) data and searching for other "signatures" in the data including looking for atmospheric pollution.
Thermal IR imaging
The Planetary Environments Group is now developing a small, un-cooled, solid-state thermal imaging system for future spacecraft. The instrument uses up to three thermal IR bands to provide meteorological data on land, sea and cloud-top temperatures, sea-surface temperatures and localised "hot-spots", such as forest fires and volcanic eruptions.
Earth observation image calibration and radiometry
The Planetary Environments Group provides research support for SSTL's Earth observation imaging missions. Currently, the group is aiding SSTL in the optical and radiometric calibration of the multi-spectral cameras, which are flown on the disaster monitoring constellation spacecraft. The first of these, ALSAT-1, was successfully launched in November 2002, and is producing stunningly detailed very wide swath-width images.
CMOS camera technology and machine vision
The Planetary Environments Group pioneered the use of miniature CMOS video cameras in space with the launch of such a camera on-board the 1998 Thai-Paht micro-satellite mission. This mission paved the way for the use of such cameras for remote-inspection nano-satellites - a concept which the group has been developing since 1995 - culminating in the launch of the first such mission, SNAP-1, on 28 June 2000.
One of the principal objectives of the SNAP-1 mission was to demonstrate the ability of nano-satellites to act as automatic "eyes-in-the-sky" to allow astronauts to examine the outside of their space vehicles for damage, etc. To test this concept, SNAP's four ultra-miniature CMOS cameras were used to provide "video telemetry" on its own deployment, as well as to image the deployment of the Tsinghua-1. The whole 60-second imaging sequence was controlled automatically by SNAP's Machine Vision System. The very first picture, taken just two seconds after deployment, shows the Russian Nadezhda satellite which had carried 6.5 kg SNAP-1 into orbit, and a few seconds later, the cameras showed the Tsinghua-1 deploying into space.
The group is currently working on the next generation of CMOS camera based systems for attitude sensing, visual inspection, optical navigation and multi-camera/multi-spectral Earth observation.
The first of these instruments is an eight-channel radiometric imaging sensor, which has been developed as part of the NigeriaSat programme. This is aimed at such applications as ocean colour sensing, land use monitoring and mineral prospecting. Work is also in progress on a single-camera passive optical navigation/rendezvous system designed for a remote inspector satellite.
Bi-static synthetic aperture radar (SAR) imaging
The SERS group have developed a novel method by which two s fly in a specific formation to accomplish a SAR imaging mission. Such missions normally require too much volume and power to be compatible with a platform. However, we have found a near-nadir viewing bi-static arrangement which cuts the power requirement down to ~100W and will fit into a standard SSTL enhanced bus.
The transmitting satellite will be the "master", with the receiver satellite "slaved" off it. The satellites, together, view a swath of 30 km, from 700 km altitude, with a ground sample resolution of 30-meters.
Work is in progress in designing the radar transmitter and receiver and the synchronization system needed to operate these over separated platforms.
After the success of the SNAP-1 nano-satellite launched in June 2000, the Planetary Environments Group is now developing a pico-satellite (i.e. a sub-1 kg satellite) called PALMSAT.
PALMSAT is an ultra low-cost satellite suitable for student-based research and project activities. The satellite will carry highly integrated "credit-card" sized systems which should enable it to carry out a remote inspection mission - similar to SNAP-1.
The group is developing an autonomous micro-air-vehicle: MASSIVA (Mars surface sample imaging VTOL aircraft) which is designed to explore the surface of Mars.
CEDEX radiation monitor
The GSTB-V2/A CEDEX payload
The CEDEX payload is a radiation environment monitor designed to provide an assessment of the single-event effect (SEE) hazard on a spacecraft due to protons and heavy ions.
CEDEX (cosmic-ray energy deposition experiment) monitors high-energy (>40 MeV) proton fluxes, and provides a detailed LET spectrum for the cosmic-ray ion environment through its 512‑channel pulse-height analyser. It does this by measuring the charge deposited in its prime PIN diode detector. The charge channel bin width is ~ 0.05 pC (equivalent to 1 MeV energy deposited). The LET range of the instrument is approximately 32 to 8400 MeV cm2 g-1.
CEDEX's high pulse-processing speed (8.7 microseconds per particle hit) means that it can cope with the very high particle fluxes encountered in the heart of the inner (proton) Van Allen belt, and during intense solar particle events. Its large area (3 cm x 3 cm) PIN‑diode detector enables good counting statistics to be acquired.
As well as recording all hits in its prime detector, the CEDEX monitor also uses a second detector to form a particle telescope, so that particle direction information can be obtained. The spacing of these detectors is set by the payload's mechanical housing so as to give approximately a 60o full angle of view. As well as giving directionality, the LET of particles travelling through both detectors can be more accurately determined as there is less ambiguity in the particle's path-length through the detectors.
In addition, CEDEX carries four experimental dose-rate sensors, situated behind domes of shielding thickness equivalent to approximately 2 mm, 4 mm, 6 mm and 13 mm of Aluminium (actually 2 mm Al, 4 mm Al, 2 mm Cu, and 4 mm Cu).
CEDEX's connections to the outside world are a dual-redundant CAN-bus for data transfer, and a nominal 28 V power bus. The unit power consumption will not exceed 3W, and the mass, including shielding, is 2 kg.
The GSTB-V2/A CEDEX is based on the TiungSAT-1 CEDEX payload, launched in 2000 and currently operational in low-Earth orbit. This payload in turn has flight heritage through Surrey's series of Cosmic-Ray Experiment (CRE) payloads flown on KITSAT-1 (1992), PoSAT-1 (1993), and AMSAT-OSCAR-40 (2000). These payloads follow on from the Defence Research Agency's (now known as QinetiQ) CREDO payload flown on UoSAT-3 (1990), and have provided continuous monitoring of the Earth's radiation environment since April 1990.
The CREDO, CRE and CEDEX payloads each detect SEE-inducing particles (protons and heavy‑ions) by means of PIN diode detectors. Particles passing through the PIN diode detector lose energy by creating electron-hole (e-h) pairs. The resulting charge is proportional to the total energy deposited by the particle, which is itself related to the particle's Linear Energy Transfer (LET). This charge is measured, and a running total is kept of the number of particle events in each charge or LET "bin".
The UoSAT-3 CREDO payload occupied one SSTL "micro-tray" 35 cm x 35 cm x 2.6 cm, and had an array of ten 1 cm2, 300 micron thick PIN diodes as its main detector, and could count up to 1000 particle hits per second. It had 9 logarithmically-spaced "bins" covering a normal-incidence LET range of 32.2 - 6430 MeV cm2 g-1 (0.1 pC - 20 pC).
In contrast, Surrey's CRE payloads occupy half a micro-tray and have a single 3 cm x 3 cm, 300 micron thick detector PIN diode, which can count up to 200,000 particle hits per second. It has a similar LET range of approximately 64 - 8360 MeV cm2 g-1 (0.2 pC - 26 pC), monitored through 512 virtually linearly spaced channels.
The TiungSAT-1 CEDEX payload is a compact version of the CRE (it fits in a box 13 cm x 9 cm x 6 cm and has a mass of 800 g), with new electronics and with the addition of a particle "telescope" to give improved LET and directional information.
The sensing elements comprise two 1 cm2 PIN diodes at 2 cm spacing, mounted co-axially with a main 3 cm x 3 cm PIN diode detector to give a 60o field-of-view particle telescope. A brass dome provides screening for electrons and low energy protons and heavy-ions.
The instrument provides 512 pulse-height "bins" covering a LET range of 32 to 9600 MeV cm2 g-1 (0.1 to 31 pC). Each bin may record up to ~16 million events. The event processing time is 5 microseconds, with a fully programmable integration time (in 100 ms steps).
Particle hits are automatically sorted into coincident and non-coincident events, and after 8 integration periods, the results are compressed and sent to the spacecraft computer via the built-in CAN interface.
The UoSAT-3 CREDO payload began operations in 800 km Sun-synchronous orbit (SSO) in April 1990 and produced virtually continuous measurement of the LEO space radiation environment until October 1996. The PoSAT-1 CRE, launched in 1993 into 790 km SSO, has maintained this programme of monitoring up to date (October 2004).
The KITSAT-1 CRE, whilst not in continuous use due to power-budget restrictions, provided a series of measurements at 1320 km altitude for almost a decade.
In December 2001, measurements were obtained from a CRE operated on-board the AMSAT-OSCAR-40 spacecraft launched in November 2000 into a highly elliptical Earth orbit (HEO). These gave a complete cross-section of the Van Allen belts from low-altitude out to ~10 Earth-radii. Unfortunately, problems with the spacecraft related to a kick-motor explosion have precluded other measurements being taken.
The TiungSAT-1 CEDEX payload began operations in September 2000 and is currently active.
The GSTB-V2/A CEDEX is essentially the same as the TiungSAT-1 CEDEX payload, with a few modifications to reflect the requirements of the new spacecraft bus:
The mechanical housing of the GSTB-V2/A CEDEX payload differs from that for TiungSAT-1 so that it can be accommodated in the high radiation environment on the exterior surface of the GSTB-V2/A spacecraft. The new housing is based on a standard SSTL nano-tray - but with extra radiation shielding. The particle telescope is housed in a copper cylinder to give the appropriate level of screening of electrons and low-energy particles.
Some obsolete parts (such as the Phillips 87C592 CAN micro-controller) have been replaced by currently available equivalents (e.g. the Infinion C515).
The power supply has been changed from accepting a regulated 5 V rail to accepting a nominal 26.5 to 38 V unregulated power rail to match the requirements of the GSTB-V2/A bus.
All telemetry and telecommands are now handled via the CAN interface to avoid the requirement for point-to-point links. The TiungSAT-1 CEDEX only had a single CAN link. The GSTB-V2/A CEDEX has a dual redundant CAN link to make full use of the platform's dual CAN bus.
Adjustments have been made to the analogue pulse processing chain to improve the low LET response of the system, so that a lower minimum LET threshold can be set. Thus, instead of using a "CR‑RC" pulse shaper with baseline-restoration, a CR-(RC)4 shaper is used to provide a smoother, pseudo‑gaussian pulse. This has necessitated the pulse-processing time to be extended from 5 to 8.7 microseconds.
The telescope now comprises two precision 3cm x 3cm PIN diodes rather than a single precision 3 cm x 3 cm PIN diode with two commercial-off-the-shelf 1 cm2 PIN diodes as on TiungSAT-1. This allows both detectors to be used in determining energy deposition as well as directionality on GSTB-V2/A. All electronic components have flight heritage on SSTL spacecraft.
Four experimental dose rate monitors have been added. These also comprise small PIN diodes configured to give a direct photocurrent output. The diodes are placed behind domes of aluminium (2 mm and 4 mm thick) and copper (2 mm and 4 mm thick), to give a representative series of measurements of the dose-rate inside the spacecraft at different shielding depths.
The CEDEX detector diodes are connected to a charge amplifier and a "CR-(RC)4" (constant peaking-time) pulse-shaping circuit, which in turn is connected to an event-driven, hardware-logic controlled multi-channel analyser (MCA), comprising a fast semi-flash 10-bit analogue-to-digital converter, which is used to address a self-incrementing memory. As only the nine most‑significant bits are used, the instrument allocates the event to one of 512 "channels", each of which can record in excess of 16 million particle-strike events.
The instrument has a resolution of ~0.05 pC of charge (~1.1 MeV energy) deposited per channel, giving a total range of ~0.1 pC to ~26 pC of charge deposited, equivalent to a normal incidence LET range of ~32 to ~8400 MeV cm2 g-1.
Events which trigger both detectors are counted as coincident events, and others as non-coincident. Both coincident and non-coincident events are recorded and marked appropriately.
Events are recorded internally in static-RAM in contiguous 150-second periods (selected via telecommand), and after four such periods, the results are compressed and sent to the spacecraft's OBC via the built-in CAN interface. The OBC handles the time-stamping of the data.
This process is repeated indefinitely, unless the payload is turned off. In normal operation it is powered on continuously.
CEDEX has a built-in DC-DC converter to generate its internal power rails. It connects to the spacecraft's unregulated 28V power bus. Thus, the spacecraft interfaces are 28 V, ground, and the dual-redundant CAN interface for data output, and telecommand input. CEDEX contains a temperature sensor and the output from this is monitored as part of CEDEX's data output.
Thermal IR camera
The traditional approach to Earth observing thermal IR (TIR) instrument design usually focuses on improved radiometric, spatial, and spectral resolution and hence, typically relies on expensive and bulky cryogenically-cooled detector technology. This approach yields instruments that are very precise but are cost- and/or size-limited to flight on single large-scale satellite platforms. Whilst these systems serve the bulk of the TIR user community, there are numerous user groups still ill-served due to the temporal resolution limits inherent in a single satellite platform.
Over the past year, the Surrey Space Centre has been striving to remedy this shortfall by developing a novel TIR imager, affordable and compact enough to produce for flight on multiple Surrey-class micro-satellite platforms, therefore providing high temporal and global TIR data.
Leveraging the latest low-cost miniaturization developments in the terrestrial un-cooled TIR detector industry has been the key to reducing instrument cost and size. To date, this research programme has resulted in a preliminary COTS-based instrument design.
The current micro-satellite compatible design would yield a 500-metre ground sample distance over a 150-kilometre swath width, from a 710 kilometre altitude. The noise equivalent noise temperature difference is expected to be less than 1 K for a 300 K ground-scene.
Laboratory evaluation of the imager components is currently underway.
Upon completion of this research programme, it is hoped that a novel, low-cost, and compact TIR imager prototype, capable of performing many high-temporal global thermal anomaly and change detection missions, will have been demonstrated.
This work has been sponsored in part by the USAF European Office of Aerospace Research and Development (EOARD) and is the PhD research of Brian Oelrich.
Microsatellite radar altimeter payload design for global sea state monitoring
Microwave radar altimeters have been proved an effective method for geographic, geodetic and oceanographic remote sensing during the last two decades. Compared with passive remote sensing methods, like CCD, altimeters have unique properties in geometric resolution independent of sensor altitude and all-weather imaging. From 1973, nine radar altimeters have been flown on USA, French and Japanese satellites, but never on a micro/minisatellite. However with the fast development of high performance solid state devices which decreases the on-board power consumptions dramatically and the maturing of small satellite platform & payload implementation technology, the design of light weight, low power, and highly accurate small satellite borne altimeters have now become very possible. This gives SSTL, who has pioneered the design of cost-effective micro/minisatellites, a unique opportunity to widen its research on remote sensing from passive to active techniques.
The main aim of this project is to provide near real time altimetry measurement results - significant wave height(SWH) and wind speed, to world wide users for shipping routes measurement. As the first phase of this project, the objective of this PhD study is therefore to finish the following aspects of work.
First is to finish a detail feasibility study for a microsatellite altimeter, analysis the performance and requirements of each sub-system respectively so that to identify the most significant problems in the whole platform and payload design. The feasibility study shows for a power limited microsatellite environment, the DC-RF efficiency of power amplifier which is one of the most power hungry sub-systems in the whole payload, is very critical. Along that, antenna off-pointing error compensation and correction is also very important in increasing received SNR.
Bear in mind that the project application is for near real time world wide altimetry broadcasting, a 12 satellite constellation is proposed and simulated. This constellation is based on the consideration of altimetry measurement grid and minima waiting time for any possible user. The simulation shows the longest waiting time occurs for the users in equator, for that case, it is less than one satellite period which is around 100 minutes depends on the altitude.
Highly efficient power amplifier design has been demonstrated as the key part of the whole payload design, especially from hardware point of view. Shown in the previous feasibility analysis, its DC-RF efficiency directly decides the capability of the radar altimeter transmitter, received SNR, and consequently the satellite size, payload and platform cost. However, it is also well known the higher the efficiency, the higher the phase distortion we may expected. It is quite common this error may larger than 40 degree. Whist a detail understanding of the relationship between altimetry measurement especially SWH measurement, and the phase distortion is still unclear. Therefore in my work, first I try to outline this relationship by running a simulation with a model that consider the errors from both the signal generator and the power amplifier. The simulation results show the power amplifier influence is more significant than that of signal generator in SWH estimation, and phase errors influence is worse for lower SWH condition. It is recommended from the simulation that the phase error in the power amplifier should be controlled under 0.2 rad. In this payload design, class-F is chosen as the amplifier operation mode due to its high efficiency and less harmonic frequencies elements. A large signal model is set up in the Comms simulation package, followed by a S band amplifier design simulation and implementation with 75 per cent and 45 per cent efficiency respectively. A new design principle is proposed during the class-F simulation, which has never been mentioned in the previous literature. Based on this, an adaptive feedback group delay equalizer is proposed as a solution for the phase error compensation within the whole chirp signal band-width. A very simple but effective phase error detection and calculation circuit is designed, simulated and built up. The test branch results are very satisfying, its small size and lower power consumption makes it very suitable for the a compact microsatellite environment.
In summary, the possibility of a medium resolution microsatellite borne radar altimeter for optimising shipping routes is investigated in this study. A 12 satellites constellation is proposed for achieving near real time altimetry broadcasting. The key payload design problems are identified in a thorough feasibility study, the restriction corresponding to these main problems is quantified via the SWH estimation simulation. A feedback linearization method is proposed as a promising solution for the compact microsatellite design with high power efficiency requirements, demonstrated by both simulation and hardware implementation results.
Onboard signal analysis
The RF spectrum is a limited resource which cannot easily be globally monitored. RF emissions are regulated by diverse bodies in different regions of the world. Regulations are not globally homogeneous and in some cases not strictly observed.
Satellite operations using the crowded VHF and UHF bands at Surrey Space Centre (SSC) have highlighted the usefulness of LEO RF signal analysis. SSC satellites have been used to make global measurements of RF signal levels in VHF uplink channels. This can provide information on specific occurrences of RFI and general channel usage statistics. This sort of information can be useful for regulation against and avoidance of sources of RFI. It can also be used by regulatory bodies when considering frequency allocations both for spacecraft operations and other applications.
It would be desirable to have a payload in LEO capable of making similar measurements to those already taken over a wide frequency range. A LEO system which can perform this task is not currently available in the civilian arena. This research investigates the techniques that might be employed by such a payload on board a small satellite given the inherent limitations to hardware resources.
The system must perform rapid spectral analysis over a wide frequency range. The range of frequencies in which we are interested are in the VHF/UHF band. This suggests that we can only detect signals from sources where line of sight exists to the spacecraft or from sources which are just over the horizon.
A satellite in LEO travels at great speed which means that signals from ground based sources may only be detectable for a few minutes at most. In addition, we are assuming that the majority of signals we are likely to detect will be either continuous or 'bursty' in nature. Therefore, we must be able to carry out a spectral analysis over a wide frequency range in a few seconds to capture the majority of signals. This requirement is beyond the capability of conventional spectrum analysers.
The chirp Fourier transform is a variation of the general Fourier transform. Analogue implementations perform chirp spectral analysis at very high frequency coverage rates. Compressive receivers employ such techniques for military applications where a certain frequency range must be covered with 100 per cent probability of intercept (POI) of signals of interest. These are generally pulsed signals of very short duration, possibly a fraction of a millisecond. It is assumed, for the purposes of this research, that the majority of signals we are dealing with have much longer durations and that we do not require such high frequency coverage rates. It is proposed that the system use a novel implementation of chirp spectral analysis employing digital techniques to perform the required spectral analysis. This would not be as fast as an analogue implementation but would still offer advantages over traditional methods. Using modern digital devices which can be reprogrammed would improve the flexibility of the system.
This research will demonstrate the following:
- The operation of a small satellite payload with unique capabilities
- A unique implementation of chirp spectral analysis employing a digital filter using the overlap-add method
GPS multipath mitigation
Multipath mitigation by harmonic analysis
GPS receivers capable of supporting multiple antennas can be used to determine a vehicle's attitude in addition to its position. One problem with using this technique onboard a satellite is that the abundance of equipment, typically located on the space-facing facet of the spacecraft, creates a poor multipath environment for the GPS antennas. In addition, an uncontrolled "line bias" can exist in the measurements; for example, when the GPS receiver is reset. This research focuses on the novel mathematical algorithm, developed to map static multipath. This new technique is unique in that it operates well even when the data does not evenly cover the hemisphere, while simultaneously overcoming the problem of the line bias. The basic principal of this new methodology is based on classical weighted least squares estimation, spherical harmonic reduction and analysis of variance.
GPS carrier phase difference measurement
- A baseline consists of 2 GPS antennas
- Measure the carrier phase difference transmitted by the visible GPS satellites at a instant between these 2 GPS antennas
Space flight experiment setup
- Experiments carried out by using UoSat 12 satellite
- Launched on April 21, 1999 by a Russian launcher from the Baikonur Cosmodrome
- UoSat 12 has demonstrated Surrey's new mini satellite bus with GPS orbit and attitude determination
- The outcome of this research is to generate a correction map to mitigate multipath
- The accuracy of this correction map will be investigated statistically
The multipath mitigation algorithm has solved 4 key problems in GPS attitude determination, namely:
- Signal Confusion arising from multipath
- Resolve errors arising from line bias
- Allows for an arbitrary distribution of data points
- Copes with noise
There are two limitations to the approach we have adopted:
- We are unable to identify the DC component of the multipath map
- The system is unable to model abrupt changes in the multipath map
For quantitative remote sensing, a fundamental aspect prior to using the data is to calibrate the data, either using on-board standards or vicarious calibration targets in various locations on the Earth's surface.
The remote sensing group is already involved in the development of methods for the vicarious calibration of the DMC satellite constellation and is also in the process of examining the use of lightweight, low cost solutions for on-board calibration of the current range of micro and mini satellites.
We are currently carrying out two specific calibration programmes. A relative calibration to equalise the detector response (20,000 detectors) over the wide swath of this pushbroom sensor. Also we have an absolute calibration based on fieldwork with the University of Arizona over the Railroad Valley (RRV) site in Nevada. This was completed in July 2004.
The image above is from the calibration software showing the variation in response across a flat field Antarctic image for the port imager. The variation in shape is due to the drop off in radiation following the COS squared rule.
Additional images have been taken of deep space, the Pacific Ocean at night and Greenland, to derive specific parameters regarding the sensor performance.
Comparison of two Antarctica images separated by one day in time and with different exposure settings, show that the difference in coefficients (to remove this cross-track variation) vary by less than 1 per cent which is of the order of the noise standard deviation.
GPS reflectometry uses a downward looking GPS antenna on a spaceborne, airborne or ground based experiment platform to collect the GPS signals reflected from the Earth's surface.
The GPS signals are generated by the various GPS constellations developed by the United States and Russia, which provide a wealth of signals which drives the growing market in satellite navigation systems.
In the case of reflectometry we are not interested in the navigation solution. We only wish to determine the effect that the Ocean surface has on the returned signal and how we can derive physically meaning parameters about the Ocean surface or near surface environment by analysis of these reflected signals (including wind speed, wind direction, mean square slope and significant wave height).
The image above shows the configuration of the UK-DMC GPS Reflectometry experiment. The block diagram illustrates the antenna and electronics layout. This instrument was launched in September 2003 and has been producing regular reflectometry results over the Pacific Ocean.
The image above shows the footprint of the GPS signal on the ocean surface. We can measure both the delay of the reflected signal from the ocean surface and the doppler characteristics and generate delay doppler maps (such as the one shown) of the returned signal over the zone showing the returned power distribution. This is actual data derived from the UK-DMC, the specular point is the peak of the power distribution.
Data request and delivery
An important element of the current research is to develop novel, automated ways of interacting with large constellations of satellites, using distributed networks of ground stations for a heterogeneous mixture of data end-users. This encompasses all aspects of the ground segment, including data request mechanisms; optimisation of data requests; automatic handling of data processing and data quality assessment. As well as the interaction with autonomous client agents that replace the need to constantly monitor the request status for the data end-users. The interactions with the Mission planning System are also being explored, including low bandwidth data request and data delivery using SMS/MMS messaging to mobile devices.
Optimisation of data requests to a multi-satellite, multi-ground station constellation
The aim of the research is to develop a methodology to produce an optimised schedule for a constellation of satellites based on a set of prior constraints. In this case the constraints of our two types.
Constraints of the satellites which include power, memory, download opportunities, current memory allocation or any operational scenario (orbit maneouvres) that would affect the operation
Constraints of the ground station in terms of capacity, operational problems and failures. Different optimisation strategies and algorithms are being explored to determine a suitable automated optimisation scheme. The diagram below illustrates a high level structural model of the operation.
Data delivery and request management architecture
Another active area of research is how to request and manage the data obtained from the satellite systems and deliver it in a timely manner.
As part of this research we are investigating the more established technologies such as web services and peer to peer networks to determine which software elements we can adapt to a transparent image gathering and data delivery network.
Some of the factors driving the development include:
- The system needs to be extremely flexible and provide a simple (minimal) interface to allow access to a multiplicity of data providers
- Each connection to a data provider must provide a sufficient level of security and access determined by the status of the user
- The system should allow for simple methods to add, modify or remove data sets without changes to the system architecture or operation
- The system should be capable of handling both location and non-location based information
- The user should be capable of updating items of a distant data source if that capability is required
Low bandwidth communication using SMS/MMS messaging
We are currently exploring the use of low bandwidth mobile devices (mobile phones and pDA's) for requesting data from the disaster monitoring constellation (DMC) and providing both status and small sub-sectioned / sub-sampled images to the user in a field environment. The aim is to provide a rapid turnaround in data delivery for specific targets which require timely information. In this case for disasters or short duration events where the client requesting the data is in the field environment without access to computer facilities.
This requires new interfaces to the mission planning system (currently based on a web service interface) and automatic processing algorithms to produce the required product and sub-section or sub-sample the data as required. Additionally, once the data is avaiable, the interface on the mobile system should enable the user to request a view of another sub-sectioned element of the data and to "move" around the image. All data entry is carried out using a dedicated client on the mobile device that communicates using either short formatted SMS messages and retrieves data using an MMS message format.
Autonomous client agents for request control
The overall aim of all the ground segment elements is to make the system as automatic and as efficient as possible. The objectives are to increase throughput and satisfy the client requirements for data delivery. In a situation where there are numerous clients, some requesting a single image with high priority, others requesting multiple images over multiple targets with a certain regularity, a manual control over the data request system to a constellation of satellites and ground stations can soon become unmanageable.
The aim of this research project is to develop a methodology to allow an external user to contract for a certain data delivery (for example multiple images over the UK for one year). Once agreed a process is set up to carry out the data acquisition that has limited autonomy based on the Service Level Agreement between the client and the data provider (in terms of priority and guarenteed service levels). Once a client process is kicked off, it will automatically request data at a specified priority, this will be handled by the optimised shceduling algorithm of mission planning and sent to one of the satellites of the constellation.
Once the data is collected it is returned to the ground station and pre-processed by another process which checks data quality. It does this by communicating with the client process to determine the data quality checks it should perform. Once the checks are completed a breakdown of the data quality is provided to the client process. If parts of the image are suitable (for example parts are not cloud covered) then the client process can change the area to acquire to try and capture the part of the image that was cloud covered. If time is running short the client process can also increase its priority if it is competing with other autonomous clients also requesting data.
Only if it seems that a request may not be fulfilled will the owner of the client process be informed and any manual action taken to try and correct the situation.
Super-resolution is a technique to allow the construction of a higher resolution image of an area, based on the merging of information from two lower resolution images. The technique works on the basis that two lower resolution images taken over the same site, will not be prefectly aligned. This slight mis-registration of two images contains important information on boundaries and the presence of ground elements that have the same dimensions (or smaller) as the data pixels themselves.
The super-resolution process involves several steps, the two most important being the registration of the two images (to estimate the whole and fractional pixel offsets in X and Y) and then the interlacing of the two low resolution images to create the higher resolution grid product.
There are several methods of calculating the registration of the two images. Many are based on interpolation techniques. An alternative that has been implemented is based on phase correlation.
Using the phase correlation method we can determine the offsets in X and Y required to align our two images, which is a necessary first step prior to the interlacing of the images to produce the higher resolution output. In the image below we can clearly see the correlation peak, compared to a low (noise) background for a real image pair.
Careful analysis of the phase peak shows that the peak correlation is not exactly at a boundary of an integer number of pixels (this would be extremely unlikely) but has a fractional component. By careful analysis of this component we can determine the fractional sub-pixel offset within the image in both the x and y directions and use this in our interlacing algorithm to construct our high resolution image.
The second stage of the process is the construction of the higher resolution image from the multiple low resolution images. Both simulated and real data have been used. The image on the right compares simple interlacing methods with super-resolution methods based on Walsh functions to re-create a higher resolution image. Note the poorly defined edges using a standard interpolation method, compared to the Walsh function.
The image below shows one of the satellites in the disaster monitoring constellation (DMC). This has two banks of cameras which produce a substantial overlap at Nadir.
Volcanic sulphur dioxide monitoring
The project concentrates on the problem of sulphur dioxide emissions from volcanoes and addresses the issues of the required sensitivity of the instrumentation required to detect large, medium and even small eruptive events.
The aim is to build a suitable instrument to allow us to detect and quantify such events and the impact they have on both the local environment and in the case of large eruptions the global climate.
The image above shows the eruption cloud of sulphur dioxide emitted from a volcano in Mexico. The instrument will be sensitive to wavelengths in the ultra-violet, which is very demanding due to the low signal levels and high levels of atmospheric scattering. Work is in progress to fully define the instrumental parameters.