Tomographic Scanning Imaging Seeker

The tomographic scanning (TOSCA) imaging seeker concept is presented. The system is based on conical scan reticle seeker optics modified with an eccentrically rotating circular aperture. After signal preprocessing, an image is extracted using tomography reconstruction techniques. Simulation results are provided to show the reconstruction quality. The concept, using a single pixel and a simple rotating axis scan mechanism, allows for a simple, low-cost, software-driven imaging sensor. Imaging capabilities against air targets are discussed. 1.0 INTRODUCTION 1.1 Historical Development The discovery of infrared (IR) radiation was done more than two centuries ago by Herschel [1], and the use of this radiation in military applications was proposed almost a century ago by Lindemann [2]. Technical difficulties, notably in material sciences, prevented its use during World War II despite both Allied and German efforts [2,3], and it was not until 1953 that the first successful test of an IR seeker was made with the AIM-9 “Sidewinder” missile [4]. Simplicity was a success factor in missile design for several decades, but more complex and hostile scenarios as well as efficiency requirements have pushed the requirements for seeker performance to a point where imaging of the scene has been found advantageous, and several missile systems are now being fielded with imaging sensors, notably the new AIM-9X Sidewinder, IRIS-T, ASRAAM and Python-5. The highly sophisticated seekers in these missiles contribute to a significant portion of the total system cost, and the TOSCA concept is looked upon as a low-cost alternative to traditional imaging sensors. Although the TOSCA principle is an imaging concept based on a (non-imaging) seeker design, it is not limited to missile seeker applications. Detector Fixed reticle Tilted, rotating secondary mirror Incoming light Figure 1: Principle of the conical scan reticle FM seeker. Hovland, H. (2005) Tomographic Scanning Imaging Seeker. In Emerging EO Phenomenology (pp. 2-1 – 2-8). Meeting Proceedings RTO-MP-SET-094, Paper 2. Neuilly-sur-Seine, France: RTO. Available from: http://www.rto.nato.int/abstracts.asp. Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 01 OCT 2005 2. REPORT TYPE N/A 3. DATES COVERED 4. TITLE AND SUBTITLE Tomographic Scanning Imaging Seeker 5a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) Norwegian Defence Research Establishment Postboks 25, NO-2027 Kjeller NORWAY 8. PERFORMING ORGANIZATION REPORT NUMBER 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S) 11. SPONSOR/MONITOR’S REPORT NUMBER(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release, distribution unlimited 13. SUPPLEMENTARY NOTES See also ADM202114, RTO-MP-SET-094. Emerging EO Phenomenology (Naissance de la phenomenologie et de la technologie electro-optique)., The original document contains color images. 14. ABSTRACT 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT UU 18. NUMBER OF PAGES 8 19a. NAME OF RESPONSIBLE PERSON a. REPORT unclassified b. ABSTRACT unclassified c. THIS PAGE unclassified Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18 Tomographic Scanning Imaging Seeker 2 2 RTO-MP-SET-094 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED 1.2 The Con-scan Reticle Seeker The TOSCA seeker concept is based on the con scan reticle seeker. A sketch of the basic conical scan FM seeker optics is shown in figure 1. Focussing optics projects a target image onto a fixed reticle consisting of transparent and non-transparent sectors. The optics have an off-axis tilt and are rotating, scanning the image in a circular movement onto the reticle. A detector is placed behind the reticle, and detects light passing through the transparent fields of the reticle. The transparent fields of the reticle are traditionally distributed evenly around a central point, as seen in figure 2. Figure 2: Typical conical scan FM reticle and two signals produced by it. The nutating optics scans the target image in a circular fashion in such a way that a centred target image follows a circular trajectory around this central point. Assuming the target image is a slightly blurred spot, a centred target will produce a sinusoidal signal with constant carrier frequency. If a target is slightly offaxis, the circular image scan trajectory will be moved sideways. The width of the fields crossed by this circle closer to the reticle centre will be smaller, leading to a higher signal frequency in that region than at the opposite side, where the distance to the reticle centre is bigger, and hence the fields are wider. This leads to a frequency modulation of the carrier signal. The phase and amplitude of the frequency modulation can be extracted to determine the hot spot position. The concept of hot spot tracking was found suitable in air target scenarios, as the background in shortand mid-wave IR is relatively homogeneous in comparison with the typically high contrast aircraft targets. In more complex environments, such as cluttered background and/or when countermeasures are present, simple hot spot tracking may not give a satisfactory performance. Several proposals have been made to counter these issues. In addition to pure counter-countermeasures techniques, proposals have been made for multi-pixel reticle sensors [5,6], and a combination of a two-colour reticle seeker using sophisticated statistical methods known as independent component analysis [7,8]. The first proposal involves a fairly complex setup, and the second method is only capable of resolving a very limited number of point sources. 2.0 THE TOSCA CONCEPT One fundamental property of the con-scan system is that the image projected onto the reticle in a con-scan system maintains its orientation at all times. The target image movement is thus translational. The reticle, with its transparent and non-transparent fields can be considered as a superposition of several knife-edges across which the target image is scanned, as shown in figure 3. We will now examine what happens when the target is scanned across such a knife-edge. Assuming the target signature changes are negligible during a scan, the time derivative of the signal is only due to the amount of the target image that crosses the knife-edge per unit time. This amounts to a line integral along the knife-edge at any given time. This means that the time derivative of the signal is proportional to a line-scan of the target. Using the so-called Fourier slice theorem [9], the Fourier transform of the derivative signal are found to lie on a line in the Fourier space of the image plane. The orientation of these lines is identical to the knife edge normal. Tomographic Scanning Imaging Seeker RTO-MP-SET-094 2 3 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED This was the first key point in the development of the TOSCA concept. The second key point was the fact that the target orientation remains constant, whereas the knife-edges appear at regularly distributed angular orientations. It is thus possible to fill the Fourier plane with points lying on lines around the origin with evenly distributed orientations. The discovery of these elements lead to the development of the TOSCA principle in 2004 [10]. Time t = t0 Time t = t0+∆t Figure 3: Details in the conical scan process: The image retains its orientation at all times, whereas the knife-edges scan the target at well defined, evenly distributed angles. 2.1 The Specialized TOSCA Seeker One problem with the basic system was that the image was the superposition of several knife-edges, all seeing the scene and giving contributions at the same time. This produces aliasing artefacts. Another problem is the limited extent of the knife-edges, which produces effects that are difficult to handle. The reason is that a bright source may exist in the outskirt of the scene, and produce significant line artefacts that are difficult to predict. The solution to these two issues lay the foundation of the specialized TOSCA [11]. The hardware in the specialized TOSCA differs from the general one in that a circular aperture, rotating together with the focussing optics, the centre of the circle being located at the focal point of a centred incoming beam. Ideally, this will reduce the effective field of view in such a way that it stays time invariant (assuming the sensor orientation is fixed). The aperture diameter should be set such that only one knife-edge is overlapping the aperture at any time. The resulting configuration is shown in figure 4. Fixed reticle