Analytic Descriptions of the Ionospheric Impact on Space-Based Synthetic Aperture Radar

Lead Research Organisation: University of Birmingham
Department Name: Electronic, Electrical and Computer Eng

Abstract

Current space based observations of the Earth, whilst providing consistent and global coverage of land use, do not accurately measure forest biomass. This is because high frequency electromagnetic radiation (including X-band radar, optical and infra red sensors) measure scattering from surface features, such as leaves, and do not measure the biomass contained beneath. Although the situation is better with lower frequency C-band radar (such as Envisat) and even lower frequency L-band radar (such as PALSAR), the scattering that they measure still saturates at low levels of biomass. To overcome this limitation, longer wavelength (~1m, P-band) signals, which penetrate deeper into the forest, are needed. The backscatter from such radars saturate at higher levels of biomass, thus enabling accurate measurement. Low frequency SAR also has potential military applications, most notably as a counter to camouflage, concealment under foliage, and deception, and in planetary exploration missions.The overriding disadvantage of using long wavelengths, apart from antenna design issues due to their proportionally larger size, is the degrading impact of the ionosphere. The ionosphere is a highly variable and turbulent medium which at these frequencies primarily affects the phase of a radar signal with amplitude affects due to diffraction. The degrading effects are most prevalent at high and equatorial latitudes and in the evening sector. Judicious choice of the orbit may mitigate the ionospheric impact but this is not always possible for operational reasons, including the requirements of other payloads. PALSAR is an example of a satellite in an orbit which is affected by ionospheric turbulence as well as gradients. In this project, the generic problems identified above will be addressed by a three pronged attack. (a) The development of novel analytical expressions of the effect of the ionosphere on SAR imaging. (b) Comparison and verification of the analytic expressions through numerical simulation (facilitated through a full-diffraction parabolic-method). (c) Comparison and verification of the analytical expressions through comparison with experimental SAR images of known calibrated targets which have been imaged through the turbulent equatorial ionosphere.The analytic theory will be verified by a full simulation of the ionosphere that includes diffraction effects, ideally required for P-band frequencies and below. The experimental validation of our model will use L-band PALSAR imagery (made available by ESA), since there is no P-band SAR in orbit; for this, calibrated corner reflector targets will be used. To link analytic, numerical and experimental data we will need a measure of the TEC and ionospheric strength of turbulence. This will be provided through Global Positioning Satellite (GPS) measurements of signal phase. Further, we will utilize satellite beacon measurements of scintillation at 150 MHz and 400 MHz to link measurements at L-band (SAR and GPS) to frequencies most pertinent to a low frequency SAR. The measurements will be made in the equatorial region where the effects are largest.Once the analytic theory has been developed and verified, it will be applied to biomass measurement accuracy estimates and more generally to the design of SAR systems optimized to mitigate the ionosphere. Algorithmic developments and improvements will follow.

Planned Impact

Economy: Background: The UK space industry is an important national asset and benefits our economy in many ways. In this project we will be developing a number of new techniques which should provide wealth creation opportunities. Ensuring Impact: Our exploitation has a number of interwoven but independent threads. Through our project partner, QinetiQ, who are licensing a valuable software resource, and Astrium, who are supporting this proposal we hope to take the outputs of these studies forwards in both the civilian and military domains. We will brief and liaise with ESA (especially in the BIOMASS context) to enable pan-European benefits. ESA are tangibly supporting this proposal with data and have expressed a strong desire to be kept up-to-date with the project outcomes. Timescales: It is expected that this project will have impact within the course of the project with enduring long term impact on the design and operation of low frequency remote sensing satellites for civilian and military purposes. Society: Background: The impact of climate change, and Government's response to it, is probably one of the most significant challenges that can be addressed by science and engineering. This proposal will assist in the development of more effective policy making by increasing the accuracy of the quantification of the Earth's biomass and hence the knowledge of the speed and amount of global warming. An additional potential benefit is to UK MoD since low frequency SAR can also be used to penetrate foliage and detect concealed weapons, such as nuclear warheads, whose use would have untold economic and political impact. Ensuring Impact: Through the PI's links to central government departments we will seek to influence policy wherever possible. The PI advises MOD in a number of ways and has a number of excellent conduits to achieve this. The intended provision of a CASE studentship from dstl will enhance this link. Timescales: It is expected that this project will have an enduring long term impact on climate change research and related policy. Knowledge and Training: Background: We seek to develop new ionospheric mitigation techniques which may be the subject of patent applications. The research may also lead to new scientific advances. As already described, this proposal has triple academic relevancy. In the first it is relevant to the scientific community studying the climate, the carbon cycle etc. In the second it is relevant to the SAR radar community striving to improve the design of cutting edge low frequency radars. For both of these communities this research aims to develop new and novel techniques which will enhance the design and exploitation of the space radars. In the third, it is relevant to the community of ionospheric radio scientists. Ensuring Impact: Whilst not a research council requirement a final report will be written to facilitate knowledge transfer to Industry and ESA. Training: The project will enhance the researcher Co-I's ionospheric expertise. The identified studentship (to start in 2011) provides the first element of the training associated with this project. It is our intention to initiate a further studentship in 2012. These students are our successors and, in this inevitably growing field, the space radar experts of the future. Journal or conference papers will be a major output from each WP as means to transfer the knowledge to the academic community. Given the cost of collecting the data within this programme it will be made available to the wider UK academic community to further other studies and thereby gain maximum benefit. We hope that this interface project will act to glue together the communities identified above. Resources to Facilitate Impact: The cost in time and effort in promoting these new ideas has been well recognized in the time that the PI is giving to this project and by an appropriate T&S allowance.

Publications

10 25 50
 
Description Predictions of global climate change rely heavily on the global biomass, most notably forests. Space-based global measurement of forest biomass requires a foliage-penetrating, low-frequency synthetic aperture radar (SAR) which can suffer from ionospheric degradation. An important objective for this project was to analytically quantify the effect of the ionosphere on SAR imagery and hence biomass measurement. Theoretical analysis was supported by a significant experimental programme. The experiment involved imaging Ascension Island and corner reflectors placed on the Island using the PALSAR-2 satellite. Ascension Island was chosen because it is located in a region where ionospheric scintillation occurs. Simultaneously comparison data were collected on a number of GPS receivers. These data provide a unique resource that can be used for exploring the relationship between space-based SAR imaging and the effects of the ionosphere. It has been shown that the order parameter of K-distributed SAR clutter rises in direct proportion to the ionospheric strength of turbulence, CkL. By comparing two SAR images of the same area it is, therefore, possible to measure CkL using this new theory. Simulation and experimental analysis has shown that this technique can be used two orders of magnitude below the point where defocusing occurs. This technique may, therefore, provide a new method for mapping CkL over large areas. Given that the ionosphere can defocus a SAR image, it is important to establish how corrections for the defocussing can be made. An experiment was, therefore, conducted using two ground-based GPS receivers on a varying baseline to simulate both the defocusing and its correction. This novel technique showed that, as expected, the region over which an autofocus correction can be made is limited by the ionospheric decorrelation and provides a firm and rigorous foundation for the development of autofocus algorithms. Finally, a theoretical study, supported by data from the earlier PALSAR-1 satellite, showed that amplitude scintillation, induced by ionospheric irregularities, causes a systematic error as well as a random error, that results in average biomass figures that are too high. This backscatter enhancement (or RCS- enhancement) is common to all phase screens and was analytically parameterised for SAR imaging using the one-way S4 index. (The S4 index is a normalised estimate of the signal amplitude variability.) This radiometric calibration error was also demonstrated experimentally on PALSAR imagery. A method of measuring the amplitude scintillation index S4 from a SAR image was also developed, which might be used to correct for the systematic bias error. The change in image contrast for both monostatic and bistatic SAR was theoretically determined in terms of the S4 index. Experimental confirmation of the theoretical predictions is in its early stages.
Exploitation Route The primary wealth creation beneficiaries are companies and institutions designing, optimising and building space-based synthetic aperture radars (SAR) for biomass applications. The primary societal beneficiaries will be those that are affected by global warming, since this is strongly affected by the quantity of biomass and coordinated in the UK through DECC. The understanding and models developed here provide important inputs to the ESA BIOMASS mission and ESA will be progressively briefed. The secondary wealth creation beneficiaries will be the defence industry building space-based surveillance radars. The secondary societal beneficiaries will be MOD. DSTL have been briefed in some detail and this will continue through a close working relationship. The tertiary wealth creation beneficiaries will be industry involved in planetary exploration missions requiring surface and sub-surface radar measurements through an ionized atmosphere:- There has been no progress in this respect. The primary knowledge beneficiaries will be scientists and engineers interested in the accuracy with which biomass measurements can be made. The secondary knowledge beneficiaries will be the ionospheric community of scientists studying equatorial scintillation physics and morphology. The tertiary knowledge beneficiaries will be the community of planetary scientists. Three journal papers have been published, with two further drafted.. These papers have been/will be published in a variety of journal in order to attract the requisite audience and maximise impact.
Sectors Aerospace, Defence and Marine,Agriculture, Food and Drink,Environment,Security and Diplomacy

 
Description Informed development of MOD Space Radar Policy.
First Year Of Impact 2013
Sector Aerospace, Defence and Marine
Impact Types Policy & public services

 
Description Defence Scientific Advisory Committee (DSAC)
Geographic Reach National 
Policy Influence Type Membership of a guidance committee
 
Description Wideband Ionospheric Sounder Concept
Amount £95,836 (GBP)
Funding ID DSTLR-1000078884 
Organisation Defence Science & Technology Laboratory (DSTL) 
Sector Public
Country United Kingdom
Start  
End 09/2013
 
Description Wideband Ionospheric Sounder Concept
Amount £95,836 (GBP)
Funding ID DSTLR-1000078884 
Organisation Defence Science & Technology Laboratory (DSTL) 
Sector Public
Country United Kingdom
Start  
 
Description Collaboration - JAXA 
Organisation Japanese Aerospace Exploration Agency
Country Japan 
Sector Public 
PI Contribution Collaboration for the provision of data. Satellite data, after satellite is launched in 2013.
Start Year 2011
 
Description Collaboration with FOI, Sweden 
Organisation Fauna & Flora International
Country United Kingdom 
Sector Charity/Non Profit 
PI Contribution After a visit by Belcher and White to Sweden the use of two 5m corner reflectors was secured
Collaborator Contribution Provision of corner reflectors and the skills to install them
Impact One research paper.
Start Year 2011
 
Description PI with JAXA on the PALSAR2 mission 
Organisation Japanese Aerospace Exploration Agency
Country Japan 
Sector Public 
PI Contribution As a consequence of the failure of the ALOS1 satellite the project is awaiting the lauch of ALOS2 with the PALSAR2 radar onboard. Paul Cannon has recently been appointed as a PI for this new satellite whoich should be launched this summer.
Collaborator Contribution Provision of data and advice on the instrument.
Impact Two papers, two in review.
Start Year 2013