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The telescope main mission is the oil spill detection, mainly in ocean environment. Beyond the obvious functionality of this main mission, the oil spill detection can be very useful to detect crashed aircrafts in the ocean. As secondary missions, this telescope can be used to map deforestation, finding lost ships in the ocean and help the drug smuggling by finding camouflaged man-made objects. The Project current stage consists in preliminaries optical and mechanical projects in which the mirrors follow ITAR regulation. The telescope will operate in LWIR frequencies and will extract polarization information from the target. The preliminary optical project indicates that the telescope optical performance has Modulation Transfer Function greater than 45% at Nyquist frequency, native GSD of 25 meters and that the telescope is diffraction limited (wavefront error of 0.012 wavelengths and Strehl ratio of 99.38%). The preliminary mechanical project indicates that the maximum deflection caused by gravity induces to a despicable error (smaller than one over one hundred and forty) when compared to the mean wavelength of operation. The preliminary radiometric analyses indicates that the telescope has a NETD of 0.90 K. |
1.INTRODUCTIONThe purpose of this work is to develop a low-cost space telescope for Earth remote sensing that could be housed in a 12U CubeSat with a GSD of 25 meters at LWIR. It will be used the Three Mirror Anastigmatic concept and Commercial Off-The-Shelf components whenever it is possible. The telescope main mission is the oil spill detection, mainly in ocean environment. Beyond the obvious functionality of this main mission, the oil spill detection can be very useful to detect crashed aircrafts in the ocean (as the accident of the Air France 447, flying between Rio de Janeiro, Brazil, and Paris, France, which crashed at May 31, 2009). As secondary missions, this telescope can be used to map deforestation, finding lost ships in the ocean and help the drug smuggling by finding camouflaged man-made objects. The telescope will operate in LWIR frequencies and will extract polarization information from the target. The motivation for the initial choices we made for the project variables are the following:
The remaining of this paper is organized as follows. Section II describes the telescope project and its current stage. The next steps to finish the project will be listed in Section III. Finally, we conclude in Section IV. 2.THE PROJECTThe current stage of this work consists in preliminaries optical and mechanical projects in which the mirrors follow ITAR regulation. The ITAR regulation constrains the maximum aperture stop of the optics allowable to be exported to be smaller than 50 cm5. 2.1Preliminary Optical ProjectThe preliminary optical project took as hypotheses a focal plane array (FPA) with pixel pitch p of 17 microns, a wavelength λ ranging from 7.5 to 13.5 microns, a flight altitude H of 450 km, an aperture stop D of 187.5 mm and a focal length f of 306 mm. These data lead to the following values of GSD (ground sampling distance) and GRD (ground resolution distance): The GSD equals the FPA pixel projection on the ground. Fig. 4 illustrates this concept, where IGFOV represents the GSD. The GRD corresponds to the Rayleigh resolution criterion6. Rayleigh proposed that two objects are resolved when the maximum of the diffraction pattern of the first corresponds to the first minimum of the second, as illustrated by Fig. 5. In the preliminary optical project, the rate GRD/GSD = 1.23. Table 1 lists data from several operational satellites so the rate GRD/GSD can be analyzed. In this table, the columns named GSD and GRD were calculated using equations (1) and (2), and the column nominal GSD indicates the advertised GSD by the satellite operator. The satellites in in the hatched area of Table 1 are the ones in which the rate GRD/GSD > 1 and their average value is 1.23. The rate GRD/GSD of our project being the average value of several operational satellites shows that the nominal performance of the preliminary optical project is completely feasible to be achieved. Fig. 5 shows the preliminary optical project. The preliminary optical project indicates that the telescope optical performance has Modulation Transfer Function7 greater than 45% at Nyquist frequency, as shown in Fig. 7, native GSD of 25 meters and that the telescope is diffraction limited (wavefront error of 0.012 wavelengths and Strehl ratio of 99.38%). In order to achieve this performance, we designed the mirrors to be free-form ones, described by Zernike polinomials8 of order up to 36th. 2.2Radiometric ConsiderationsRef. 9 defines the noise equivalent temperature difference (ΝΕΔΤ or NETD) as the temperature difference between a target and the background that produces a signal equivalent to the noise. They state NETD is directly proportional to the square of the F-number (f/#) and is inversely proportional to the incident radiation. The thermal response time τ, also known as time constant, “expresses the physical time a bolometer needs to heat up and give an electrical output that equals or represents the input” 10. Ref. 11 states that there is a NETD/τ proportionality relationship and that it is accurate since the NETD is limited by 1/f noise. After 3 τ, which is the usual time the FPA manufactures use as integration time, the output signal will represent 95% of the input signal10. A Time Delay Integration (TDI) is an artifact that improves the SNR by a factor of For a telescope flying at 450 km of altitude, the dwell time of a ground resolution cell of 25 m is 3.5 ms. Setting the integration time to this value will produce an image with minimum motion blur. We assume a commercial microbolometer FPA12 (thermal response time of 14 ms and NETD of 40 mK at f/# =1.0) and the optics described at 2.1 with f/# = f/D = 1.632. Given the NETD/τ proportionality relationship11, it is possible to achieve, with the same technology of this commercial microbolometer, a FPA with thermal response time of 1.17 ms and NETD of 480.0 mK (14.0 x 40.0 = 1.17 x 480.0). The optics make the NETD to be 1278.44 mK. Assuming the polarization filter will divide the signal into 4 equal parts, then NETD = 5113.77 mK. Finally, with a TDI of 32 stages, we will have a NETD of 0.90 K. In this fashion, we propose a FPA with fast thermal response time (3 x 1.17 ms = 3.5 ms = 25 m GSD) and 32 stages of TDI, as illustrated in Fig. 8, where each block of same polarization has 640 columns by 8 lines. 2.3Preliminary Mechanical ProjectWe opted to use aerospace qualified aluminum (Al6061-T6) as subtract to both mirrors and structure. The reasons that drove this decision were the same ones listed by Ref 13: “A16061-T6, which is diamond turnable but also offers distinguished optical quality for IR-optics, especially if fine grain sized aluminum alloy like RSA-6061 is used”. The preliminary mechanical project, added by the light rays, is shown in Fig. 9, where the mirrors are painted in dark gray, the light rays in blue and the structure in light gray. Fig. 10 shows three perspective visions from the mechanical project. It was done a study on the mechanical project through finite element method8. The result of this study is shown in Fig. 11 and Fig. 12, where the maximum deflection δ caused by gravity is smaller than 74 nanometers. This deflection induces to a despicable error (smaller than one over one hundred and forty) when compared to the mean wavelength of operation (λ = 10.5 microns). 3.NEXT STEPSIt must be frozen the optical and mechanical projects. In order to do that, it is necessary to perform the following critical actions, among others:
After these steps, we foresee the following phases to conclude the project: 4.CONCLUSIONThe purpose of this work is to develop a low-cost space telescope for Earth remote sensing that could be housed in a 12U CubeSat with a GSD of 25 meters at LWIR. It was used the Three Mirror Anastigmatic concept and Commercial Off-The-Shelf components whenever it is possible. Section II described the telescope project and its current stage and the next steps to finish the project were listed in Section III. The telescope main mission is the oil spill detection, mainly in ocean environment. Beyond the obvious functionality of this main mission, the oil spill detection can be very useful to detect crashed aircrafts in the ocean. As secondary missions, this telescope can be used to map deforestation, finding lost ships in the ocean and help the drug smuggling by finding camouflaged man-made objects. The Project current stage consists in preliminaries optical and mechanical projects in which the mirrors follow ITAR regulation. The telescope will operate in LWIR frequencies and will extract polarization information from the target. It could be seen that the preliminary optical project indicated that the telescope optical performance has Modulation Transfer Function greater than 45% at Nyquist frequency, native GSD of 25 meters and that the telescope is diffraction limited (wavefront error of 0.012 wavelengths and Strehl ratio of 99.38%). The preliminary mechanical project indicated that the maximum deflection caused by gravity induces to a despicable error (smaller than one over one hundred and forty) when compared to the mean wavelength of operation. The preliminary radiometric analyses indicated that the telescope has a NETD of 0.90 K. We are still in the very beginning of this project, but the results we have so far strongly support our believe that the fulfillment of our goals is feasible. 5.ACKNOWLEDGMENTSWe would like to thank the Fundacao de Apoio a Pesquisa do Distrito Federal (FAP-DF) for its financial support. 6.6.REFERENCESChenault, D. B., Vaden, J., Mitchell, D., and Demicco, E.,
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