The imaging performance of the SRC on Mars Express

Transcription

1 Planetary and Space Science 56 (2008) The imaging performance of the SRC on Mars Express J. Oberst a,, G. Schwarz b, T. Behnke a, H. Hoffmann a, K.-D. Matz a, J. Flohrer a, H. Hirsch a, T. Roatsch a, F. Scholten a, E. Hauber a, B. Brinkmann a, R. Jaumann a, D. Williams c, R. Kirk d, T. Duxbury e, C. Leu f, G. Neukum f a German Aerospace Center, Institute of Planetary Research, Rutherfordstr. 2, D Berlin, Germany b German Aerospace Center, Remote Sensing Technology Institute, Oberpfaffenhofen, Germany c School of Earth and Space Exploration, Arizona State University, Tempe, USA d United States Geological Survey, Flagstaff, USA e Jet Propulsion Laboratory, Pasadena, USA f Institute of Geosciences, Freie Universität, Berlin, Germany Received 3 November 2006; received in revised form 21 September 2007; accepted 27 September 2007 Available online 17 October 2007 Abstract The Mars Express spacecraft carries the pushbroom scanner high-resolution stereo camera (HRSC) and its added imaging subsystem super resolution channel (SRC). The SRC is equipped with its own optical system and a framing sensor. SRC produces snapshots with 2.3 m ground pixel size from the nominal spacecraft pericenter height of 250 km, which are typically embedded in the central part of the large HRSC scenes. The salient features of the SRC are its light-weight optics, a reliable CCD detector, and high-speed read-out electronics. The quality and effective visibility of details in the SRC images unfortunately falls short of what has been expected. In cases where thermal balance cannot be reached, artifacts, such as blurring and ghost features are observed in the images. In addition, images show large numbers of blemish pixels and are plagued by electronic noise. As a consequence, we have developed various image improving algorithms, which are discussed in this paper. While results are encouraging, further studies of image restoration by dedicated processing appear worthwhile. The SRC has obtained more than 6940 images at the time of writing (1 September 2007), which often show fascinating details in surface morphology. SRC images are highly useful for a variety of applications in planetary geology, for studies of the Mars atmosphere, and for astrometric observations of the Martian satellites. This paper will give a full account of the design philosophy, technical concept, calibration, operation, integration with HRSC, and performance, as well as science accomplishments of the SRC. r 2007 Elsevier Ltd. All rights reserved. Keywords: Mars; Mars Express; Instruments; Cameras 1. Introduction The European Space Agency (ESA) Mars Express (MEX) was successfully launched on 2 June and entered Mars orbit on 25 December The spacecraft has a payload of seven instruments, including the high-resolution stereo camera (HRSC) and its super resolution channel (SRC). The goal of the HRSC, a nine-sensor pushbroom imager, is to obtain color stereo images covering large areas, whereas the SRC is designed to show high-resolution Corresponding author. Tel.: ; fax: address: Juergen.Oberst@dlr.de (J. Oberst). detail within the HRSC scenes, like a magnifying lens. With SRC being part of the HRSC/SRC experiment, it makes full use of the digital HRSC circuitry as well as of the HRSC ground operation and data system. The HRSC was conceived in the late eighties, and was first launched towards Mars on board the ill-fated Russian Mars 96 spacecraft that failed to reach its Mars transfer trajectory (Neukum et al., 1996; Jaumann et al., 2007). Later, when the opportunity opened for a reflight of HRSC on ESA s MEX mission, the SRC was added (Neukum et al., 2004a). At the time of writing (September 2007), the HRSC and SRC have jointly operated successfully through the nominal MEX mission and the beginning of /$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi: /j.pss

2 474 J. Oberst et al. / Planetary and Space Science 56 (2008) the extended mission, with the SRC having acquired more than 6940 image frames, comprising Mars surface views, limb profiles of the Martian atmosphere, images of the moons Phobos and Deimos, as well as a number of calibration images. While a companion paper discusses the properties and imaging data from the HRSC (Jaumann et al., 2007), this paper provides a technical description of the SRC, its calibration, operations, and performance. For practical purposes, we will refer to the HRSC only if necessary for the understanding of SRC performance in the following. Otherwise, we shall discuss the SRC as a separate camera unit. The paper concludes with a discussion of the prospects for science, while showing examples of recently obtained SRC data. 2. Scientific motivation and technical constraints HRSC with its high-resolution (10 m) color stereo imaging capabilities remained scientifically attractive for the reflight on MEX in However, the payload selection decisions for new missions to Mars such as Mars Global Surveyor and Mars Reconnaissance Orbiter (Malin, 2006) demonstrated the increasing interest of the scientific community in images with very high resolution. Discussions within the HRSC science team led to the idea of adding an extra imaging channel with a nominal pixel resolution of about 2 m to the existing HRSC design aimed at photo-geological studies of small-scale morphologic features. These high-performance requirements of this add-on to HRSC had to be accommodated within the given constraints of only 2 kg of mass and a power consumption of 5 W. As a consequence, SRC was conceived as a lightweight frame imaging device bore-sighted with the nadir direction of the HRSC instrument and attached to its commanding and data handling system. SRC would practically act as a tenth sensor channel of HRSC and not require additional digital electronics. Moreover, the well-calibrated HRSC data could be used to provide context and reference information for the SRC images, which therefore would not require excessive geometric accuracy, radiometric resolution, and contrast. This approach made it possible to develop SRC rapidly within stringent constraints of size, mass, time and costs. Though thorough calibration activities or small design modifications may have been desirable, these were not possible within the tight development schedule. Contracts with industry led to the assembly of a flight model and a flight spare model (Fig. 1, see also Jaumann et al., 2007, Fig. 1). 3. Camera description Based on these basic constraints on mass, size, and power, the SRC optics, its CCD detector, and its electronics were selected. No further sub-systems had to Fig. 1. HRSC (upper optics) and SRC (lower optics) flight models. be developed as the SRC is controlled by the HRSC digital unit with its command interpreter and telemetry handling. Here, the SRC benefits from the HRSC digital signal chain processing concept, including the capability for real-time image compression on the fly, which, however, has been rarely used during the mission. Both the SRC and HRSC deliver their data to a mass memory provided by the spacecraft. Thermal control for both imagers is provided by the HRSC. See Table 1 for SRC instrument parameters Optics To accommodate the high-resolution optical system with its required large focal length, the SRC optical system was based on a compact and low mass Maksutov Cassegrain design (which uses a primary mirror and a meniscusshaped reflector plate) with an optics diameter of 9.1 cm, a focal length of 980 mm, and an f number of 11 (Fig. 2). However, while the large focal length is realized within a comparably short optical tube, the required secondary mirror located in the center of the tube reduces the effective aperture of the system by 10 20%. The optics was designed under the explicit assumption of thermal equilibrium, with the implicit possibility that the presence of temperature gradients might lead to astigmatic deformation of the primary mirror and degradation of images. Tests were carried out in a thermal vacuum chamber, in which thermal gradients associated with an optics exposure to open space were simulated. Indeed, a deformation of the primary mirror and a defocusing of the

3 J. Oberst et al. / Planetary and Space Science 56 (2008) Table 1 SRC parameters Mass 2163 g Volume 2200 cm 3 Power consumption Standby Imaging Power supply Launch loads Shock and vibration levels Expected lifetime Cruise and in-orbit 1.8 W 5.3 W 75% efficiency 25 g quasistatic load More than 4 years Thermal environment Storage 0 to +50 1C (T min, T max ) Cruise 20 to +60 1C (T min, T max ) In-orbit 15 to +15 1C (T min, T max ) Thermal control Heaters Temperature sensors 118MF2000A Table 1 (continued ) Non-linearity o1% Electronics Technology Chip on board Exposure 0.5 ms y, max: 8.33 s Detector read-out rate 2.1 MHz Data rate 2.1 M samples/s (at 8 bit resolution) Data buffer 8 Mbytes Sampling technique Correlated double sampling Gain settings Fixed gain A/D conversion 14 bits Radiometric resolution 8 or 14 bits System gain 5.3 e /DN Electronic noise 5.3 e (without CCD) Dynamic range 1:560 (55 db) Redundancy concept Electronics Electrical interfaces Power converter General performance goals FOV per pixel Total FOV Resolution on ground Expected coverage Motion compensation Baffling Imaging strategies Dynamic range Spectral range None Nominal/redundant command interface RS232 Nominal/redundant power control I/F 2 arcsec (9 mrad) 0.75 arc (13 mrad) 2.3 m per pixel from 250 km 1% of the Martian surface None None Single and repetitive images 1:560 (55 db) nm Optics Type Maksutov Cassegrain mirror optics with dioptric telelens Focal length Nominal: 975 mm; in-flight: mm Central obscuration 10 20% f number 11 Entrance pupil diameter 91 mm Net transmission of all 0.5 components MTF 0.25 at 55.5 lp/mm Stray light/baffling No baffle Focus adjustment Under vacuum CCD detector Detector type Kodak KAI 1001 Number of pixels 1024 lines 1024 columns Number of active pixels 1008 lines 1018 columns Pixel size 9 mm 9 mm Pixel fill factor 55% Spectral range nm Quantum efficiency 0.40 (peak at 700 nm) Charge transfer efficiency Blooming Anti-blooming protection Full well capacity 30,000 e Dark current 0.5 na (140 ms, 40 1C) Total rms noise 52.2 e at 300 K and 0.5 ms exposure time Zero level/bias 3000 DN Responsivity 11.5 mv/e Fig. 2. SRC optical system. Ray tracing diagram (top) and 3-D view (bottom). images were demonstrated. Consequently, small heaters were added to the camera optics, and it was confidently assumed that the appropriate and uniform operating temperature range could be achieved in orbit about Mars. (This, unfortunately, has turned out not to be the case.) 3.2. CCD detector The detector is a high-resolution interline Kodak CCD sensor featuring high reliability and performance, as well as small size and low cost. The average quantum efficiency is approximately 0.35, with peaks of 0.4 near 700 nm (Fig. 3). From the pericenter height of 250 km, the detector pixel size of 9 mm 9 mm combined with the telescope focal length of 980 mm yields a ground footprint of about 2.3 m 2.3 m. The interline architecture of the CCD and the fast pixel read-out time on the order of a few nanoseconds obviates the need for a mechanical

4 476 J. Oberst et al. / Planetary and Space Science 56 (2008) Detector quantom efficiency, % Wavelength, nm Fig. 3. SRC CCD quantum efficiency, taken from CCD data sheet. shutter. As the buried channels between the active pixel lines comprise portions of the CCD, the effective light collection area is restored by microlenses in front of the CCD pixels. We estimate that a pixel fill factor better than 80% is achieved. Some edge pixels of the sensor area are masked for measuring the dark current during image taking, leaving 1008 lines and 1018 columns collecting image information. Note that in spite of the fast pixel readout time, the recovery of the full image takes approximately 0.5 s. The CCD response (Fig. 4) to increasing exposure time was studied under room temperature. Measurements (which were corrected for dark current) were made under constant illumination condition by a flat-field LED (630 nm). To first order, the response is linear to 1% within most of the dynamic range (above 10% and below 90% of the total range) (Fig. 4 bottom). Estimates of noise floor and the a priori unknown effective system gain were made using photon transfer measurements. Standard deviations of the signal s S (DN) were estimated from statistics of small groups of pixels on the CCD array and presented in a log log plot as a function of average signal S(DN) (Fig. 5). With increasing light levels, the total RMS noise increases linearly in the log log plot with a slope of 1/2, as the uncertainty in collected charges per pixel is equal to the square root of the number of incident photons, according to Poisson s statistics, N e ¼ O(S e ). With the unknown gain factor G (in dimensions of e/dn) we have N DN G ¼ O(S DN G). Forming the logarithm, we find log ðn DN GÞ¼log ð p ðs DN GÞÞ (1) and log ðn DN Þ¼1=2 log ðs DN Þ 1=2 log ðgþ. (2) By carrying out a linear fit, we obtain the coefficient for effective system gain of 5.3 e/dn from the slope intercept Fig. 4. Signal response to increasing exposure time (top) and linearity error model (residuals after linear fit, bottom). Measurements were made under room temperature and illumination by an LED. (e.g., Stark et al., 1992). For near-zero signal levels ( darkness ), the total RMS noise (e.g., shot noise generated by dark current and additional noise sources of the signal chain) is approximately constant at approx. 9.8 DN, or 52 electrons, in agreement with the specification from KODAK. Figs. 4 and 5 combine the results of one measurement sequence Electronics To maintain a compact camera design, the detector electronics are located close to the CCD detector and attached to the electronics boards and the interface

5 J. Oberst et al. / Planetary and Space Science 56 (2008) controller by a rigid-flex interconnection (Fig. 6), a design that has previously been used for the ROLIS camera on the Rosetta lander (Michaelis et al., 2000). The analog-todigital converter (ADC) can carry out digitization at low noise level and high speed, with two full SRC images per second. Considering complete full well capacity of the CCD of 30,000 electrons and using the system gain of approximately 5 e/dn, as was determined from Fig. 5, this translates to a maximum number of 6000 DN and a dynamic range of 14 bits which fit comfortably into the ADC. Unfortunately, the rather large noise floor of the Fig. 5. Photon transfer measurements, i.e., signal standard deviation as a function of average signal. The left part of the plot (at low signal levels) reveals the constant noise floor or total RMS noise at zero signal level, to be approx. 9.8 DN, or 52 electrons. The right part of the plot is dominated by shot noise which increases linearly in this log log plot with a characteristic slope of 1/2. The system gain can be determined from the intercept with the y-axis (see text for details). sensor (which is optimized for high-speed video applications) limits the effective dynamic range to 8 bits or 55 db. Hence, while full resolution 14-bit images can be generated for detailed camera performance testing, for routine imaging only the upper 8 bits are returned. This effectively reduces data rate, but also the dynamic range of the image Signal-to-noise considerations Due to the given size and mass constraints which limited the admissible optics diameter, it was clear from the beginning that the noise budget would be the limiting factor of the SRC performance. To make a rough calculation of the flux of solar photons reflected from the surface and resulting image signal-to-noise levels to be expected near Mars, we use F ¼ LAO, (3) with F, L, A, and O being the surface-reflected solar flux per pixel, the radiance of the surface, the ground pixel size, and the solid angle subtended by the telescope. For the SRC, the ground pixel area is A ¼ðpr=f Þ 2, (4) where p is the pixel size on the sensor, r the distance to the surface, and f the focal length of the receiver telescope. The solid angle subtended by the SRC optical receiver, as seen from the source is O ¼ðd=rÞ 2 ðp=4þ, (5) where d is the diameter of the optics and r is the distance from the planetary surface. The radiance of the source is computed assuming Lambertian scattering: L ¼ ae cosðiþ=p, (6) where E stands for the solar irradiance onto the surface, a is the albedo given normal illumination, and i the off-zenith angle of the Sun. Near Mars and at visible wavelengths, E Fig. 6. SRC readout electronics (left) and the complete instrument after integration (right). The length of the main optical tube is 190 mm, the diameter is 118 mm.

6 478 J. Oberst et al. / Planetary and Space Science 56 (2008) Table 2 SRC typical signal levels Albedo s/c height (km) s/c velocity (km/s) Footprint (m/pix) Exp. time (ms) Signal level (DN) S/N Pericenter case Bright areas Dark areas Off-pericenter case Bright areas Dark areas Typical operation case (accepting motion smear) Bright areas Dark areas Computed using optics aperture: 91 mm; focal length: 980 mm; pixel size: 9 mm; quantum efficiency: 0.4; total system transmission (to include optics central obscuration and pixel fill factor): 0.25; gain factor: 5.3 e/dn; read-out noise: 52.2 e ; solar incidence angle: 451; center wavelength: 550 nm; wavelength window: 500 nm. is approximately W/cm 2 /mm or if the solar spectrum is integrated over all wavelength, E ¼ 0.59 kw/m 2.Aswe wish to make only a rough estimate, we use the solar irradiance for a center wavelength of 550 nm and assume a constant radiance over the optics and CCD sensitivity bandwidth of 7250 nm, i.e., E ¼ W/cm 2. As the MEX ground track velocity near pericenter is approx. 3.5 km/s, the detector exposure times must be limited to 0.6 ms if motion smear is to be avoided completely, considering the small SRC ground pixel size. During operation, however, it has turned out to be more practical to use longer exposures, such as 5 ms, and to accept motion smear in order to achieve more acceptable SNR, as is demonstrated here and in Table 2. For bright areas with a typical surface albedo of 0.25, using the SRC parameters for telescope and detector (see brief summary of parameters in Table 2), we obtain a signal of roughly S ¼ 1000 electrons. With a gain of 5.3 e/dn, this translates to approximately 190 DN. Using qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi SNR ¼ S= ðs þ R 2 Þ, (7) (where R is the readout noise of 52.2 electrons), we obtain a signal-to-noise ratio SNR ¼ 16. The SNR improves (SNR ¼ 35) when we sacrifice spatial resolution and take data off-pericenter, because the ground track velocity becomes smaller and longer exposure times can be used. For an exposure of 5 ms over bright areas, SNR becomes approx. 80. Typical cases are shown in Table Ground tests and calibration measurements As the SRC had to be developed within a short time frame and within stringent budget constraints, only a limited number of tests could be carried out prior to launch. In addition to functional tests, dark current and flat-field measurements were carried out both for the flight and the flight spare model. Flat-fields were obtained by imaging of an integrating sphere, which provided the required diffuse uniform illumination. In addition, a number of dark exposure images were selected to generate a matrix for the correction of the dark signal nonuniformity (DSNU). The dark-corrected flat field images were used to produce a flat field correction matrix of pixel response non-uniformity (PRNU). The ground tests also revealed that the CCD had a number of blemish pixels which did not respond to incoming light. These are normally marked in the calibrated images, and removed in the subsequent processing by median filtering. As the SRC was not intended for radiometric or photogrammetric precision measurements, no further calibration measurements were carried out. Outdoor end-to-end tests with the SRC flight model were carried out through 2002, in which several ground-based images of the Moon and Mars were captured. The flight spare model was tested in May 2003 by imaging various horizon features in the vicinity of the DLR facility, Berlin (Fig. 7). However, these images were obtained under haze and turbulent air conditions, and little quantitative information on camera performance could be obtained. 5. In-flight tests Following launch, test images were obtained of Earth, Moon, and star fields for performance tests and calibrations. Star observations were typically executed in longexposure point-and-stare mode, performed jointly with other MEX instruments. After arrival in Mars orbit, several more star calibration sequences were obtained HRSC/SRC alignment The star observations acquired during cruise in July 2003, were used for alignment calibrations. Twenty-four observations of Delta Scorpii were recorded with fixed spacecraft pointing. Nine more stars were observed when the SRC and HRSC were slewed through the constellation Scorpius. The SRC alignment angles relative

7 J. Oberst et al. / Planetary and Space Science 56 (2008) Fig. 7. Images obtained by the SRC during outdoor tests showing the airport tower of Berlin Schönefeld, from a distance of approx. 8 km (left). Lines across the SRC image are electric wires. The color image (which was obtained by a commercial digital camera) is given for reference, airport tower indicated by an arrow. to the spacecraft axes were determined to be , , 90.01, for x, y, and z, respectively. The HRSC nadir channel was found to be aligned with SRC column An SRC image extends from HRSC nadir sensor pixel positions (Fig. 8) Focal length Using cruise images of a small star field containing seven stars, the focal length of the camera was re-estimated as f ¼ mm, with measurement residuals of up to 2 pixels. This value is significantly higher than the value measured during vacuum pre-flight tests (975.0 mm), probably owing to gravity relaxation. Note that an error of 10 mm in focal length translates to a displacement of approximately 7.5 SRC pixels in every image corner. Later in Mars orbit, the magnification factor of the SRC with respect to the HRSC was estimated from position measurements of surface features using a digital image matcher. We estimate this magnification factor to be Assuming that the nominal focal length of the HRSC is correct, this corresponds to an SRC effective focal length of mm. This value though not in perfect agreement with the directly determined SRC focal length confirms that the SRC focal length increased after launch Point spread function (PSF) From the first in-flight tests, it became apparent that the SRC images had noticeable degradations, suggesting that the SRC optics suffered from astigmatism. While some symmetric point spreading is to be expected in every optical system, strong blurring coupled with ghost effects are observed. The calibration images of stars are particularly Fig. 8. SRC CCD and spacecraft coordinate systems and alignment between SRC and HRSC. suited to the study of the different observed patterns of the PSF. On some occasions, stars show a distinct splitting. Other star images show triple point sources, with all three peaks having comparable signal levels (Fig. 9). On the other hand, from studies of images containing several stars in different parts of the image it appears that at a given time the multi-peak PSF is spatially uniform throughout the field of view. From measurements of image coherence in various image samples, we estimate that the effective resolution of images is reduced over nominal by a factor of approximately 2 3. Unfortunately, only little data are available for systematic studies of the underlying physical cause of the varying character of the images. For example, it would have been desirable to have thermal sensors on the camera optics to study temperature changes during imaging. However, due to constraints in costs and schedule

8 480 J. Oberst et al. / Planetary and Space Science 56 (2008) Fig. 9. Typical SRC star images, showing the variability of point spread effects. no such sensors could be realized on the spacecraft, and therefore, a characterization of the SRC s thermal environment during operation is not possible Blemish pixels During cruise, the MEX spacecraft was exposed to particle bombardment originating from the most powerful solar flares recorded in history (e.g. gov/apod/ap html). The SRC CCD sensor was severely affected, as is manifested by an increased number of warm, hot, or dead pixels that show irregular or even no response to incoming light. Dark signal uniformity (DSU) images taken in Mars orbit show that more than 1000 pixels (0.1% of the total CCD array) differ by more than 5 DN from the mean DN, whereas a few days after launch the number of such blemish pixels was only 79 (Fig. 10). Consequently, the DSNU matrix previously obtained on the ground had to be updated for use in the flight data calibrations Stray light Unlike the HRSC, the SRC does not have a baffle, due to size and mass constraints. As a consequence, some stray light is present in the SRC images. Quantitative estimates on the spatial extent and levels of stray light can be obtained from images, in which Phobos is seen in front of dark sky (Fig. 11). As the effect is minor (less than 1% of the maximum signal) and visible only in images of extreme contrast, no dedicated corrections are performed. Fig. 10. Statistics of blemish pixels right after launch (solid line) and after particle bombardment due to solar flare activity in October 2003 (dashed line). The plot gives on the y-axis the number of pixels which vary from the mean by more than xdn Absolute calibration Observations of stars with known magnitudes were used for an absolute radiometric calibration of the camera. Specifically, the measurements of star brightness were used to obtain an improved estimate of the optical system transmission, for which only rough estimates had been available before launch. Nine selected stars with known magnitudes ranging from 1.1 to 6.4 were imaged with identical exposure times of 400 ms. A plot of star magnitudes and the logarithm of measured DN values

9 J. Oberst et al. / Planetary and Space Science 56 (2008) current pixels taken from the covered areas on the CCD are averaged to obtain the DSU level and multiplied with the DSNU. This product is subtracted from the raw data. Finally, the image data are divided by the PRNU (i.e., the flat field). For selected scenes of interest where extra effort appears worthwhile, additional correction steps are applied Coherent noise Fig. 11. SRC stray light map obtained from observations during the Phobos flyby in orbit The size of Phobos is 510 pixels. Contour lines represent constant image brightness in DN per ms exposure. Frequency domain analyses revealed that the SRC images are affected by additive fabric patterns, which vary from image to image both with respect to frequency and intensity. Hence, images have to be analyzed individually to determine the sinusoidal characteristics of the interference, before this interference can be subtracted from the image. This is done within the Fourier domain, where single peaks within the mean signal level correspond to interference frequencies (Gonzalez and Woods, 2002; see Fig. 13 for a typical example). As a rule, the coherent noise effects remain limited to small amplitudes. Nevertheless, it is obvious that the coherent patterns have to be removed prior to PSF correction (see below). Otherwise, these patterns would be boosted in intensity during subsequent image restoration Correction of PSF effects Fig. 12. Measured DN values of stars plotted versus star magnitudes (symbols). The measured data agree with predictions from our SRC camera model (line). show a linear trend with slope and magnitude in agreement with the predictions (Fig. 12). 6. Image calibrations and corrections 6.1. Standard radiometric calibration The radiometric calibration is carried out as part of the overall systematic processing of the SRC images. The dark PSF effects are most obvious in Phobos images, which show the satellite s blurred limb and small ghost craters on its surface. Efforts have been made to obtain a full understanding of the image distortions. The SRC instrument design relies on thermally balanced conditions, and ground tests had predicted a considerable thermal impact on the SRC imaging performance. Numerical modeling of thermal mirror deformations and ray tracing confirms that a deformed mirror can cause a bifurcation of point sources in SRC images. However, owing to the complexity of this multi-parameter problem of astigmatism, the model images cannot be brought into satisfactory quantitative agreement with the actual image characteristics. Laboratory studies of the flight spare (which features a slightly modified optical system) also remained inconclusive. As a consequence, the images must be corrected by a phenomenological approach. We concentrate on a flexible and robust restoration scheme with the goal of resolving morphological image features rather than producing radiometrically accurate image pixels. For this purpose, selected sequences of star images each acquired in a single orbit are averaged to improve their signal-to-noise levels. These stacked PSF data are then used in a conventional transform-domain image restoration approach (Gonzalez and Woods, 2002). Restoration tests have been carried out using synthetic images with various levels of contrast and details. The synthetic images are degraded with the obtained PSFs, restored, and compared with their original counterparts

10 482 J. Oberst et al. / Planetary and Space Science 56 (2008) Fig. 13. SRC images (left: during a local dust storm, right: Phobos, captured in orbit 413) affected by coherent electronic noise. The corresponding Fourier spectra of the images, in blue, show pronounced spectral peaks. (Fig. 14). For software verification, traceable PSFs (point duplication or uniform image smear) are used. Though test results with these simple PSFs are satisfactory, tests with the realistic PSFs of the SRC show that the restoration gain is very limited in these cases. In particular, fine-grain information cannot be retrieved satisfactorily. Our best results can be obtained where small and isolated ghost craters are removed from a bleak background, as can be seen in some of the Phobos images (Fig. 15). More work is required to improve our PSF estimates and to establish advanced restoration schemes, e.g. by an adaptive Richardson Lucy-type algorithm (Molina et al., 2001). Several members within the HRSC/SRC team are currently involved in further SRC image restoration efforts (Duxbury, 2006) Motion smear Owing to the limited light-gathering power of the SRC optical system, choices must be made between short image exposures with low signal levels or long exposures, which will result in motion smear. Motion smear can be modeled by an adapted point spread function which can be derived by stretching of the static PSF (see section above) by an amount known from the spacecraft motion vector and the image exposure time. Hence, the motion smear removal becomes part of the overall PSF correction. This adapted PSF is derived by repetitive shifting and stacking of the static PSFs. However, due to the large distortion already introduced by the static PSF, the relatively small motion smear has a minor impact on the visual appearance of the images Operations The SRC is usually operated during an HRSC imaging sequence. The coaligned cameras are mostly nadir-pointed during pericenter passes, where they reach their maximum possible resolution. Off-nadir pointing towards specific targets (which require a re-orientation of the spacecraft) is possible, but large nadir offsets (4101) are rarely used. SRC images can be taken in any of three modes (Fig. 16): spot imaging mode, where individual images are acquired at pre-selected locations along track, or raster imaging mode where sequences of images are taken at constant time steps, or contiguous mode, where SRC images are obtained in rapid succession with approximately 5% overlap. Note that it takes approximately 0.5 s to read out one SRC frame. Typically, seven images are obtained, as the internal SRC data buffer is full after 3.5 s. Specific operations modes for HRSC are available which make the acquisition of larger numbers of SRC images possible, but at reduced imaging capabilities of the HRSC (see image examples further below). These modes are used rarely. The spot imaging mode requires the precise timing and commanding of each individual SRC image, otherwise, anticipated targets will be missed. Thus, the usefulness of this mode hinges on the accuracy of the predicted orbit and target parameters. In raster or continuous mode imaging, timing requirements are less severe. To reduce the complexity of imaging planning, the SRC exposure times are typically fixed to 5 ms for Mars observations. The long exposures are used to obtain acceptable signal, while we accept significant motion smear (compare to signal-to-noise considerations earlier in the text and Table 2). No attempt is made to adjust exposure in response to variations in actual ground track velocity or anticipated albedo of the target area to be imaged. Like the HRSC, the SRC requires advance planning and uploading the commanding sequences during visibility of MEX from Earth. The commands include one or more

11 J. Oberst et al. / Planetary and Space Science 56 (2008) Fig. 14. Synthetic SRC image (upper left), generated from a HRSC scene (obtained during orbit 756), convolved with the typical SRC point spread function (upper right; point spread function shown enlarged within the small embedded image). The image after image processing intended to remove the point spread effect (lower left). The difference image (lower right) between original (upper left) and resulting image (lower left), representing what was removed in processing. time-driven imaging sequences, as well as instructions for intermediate data storage within the spacecraft mass memory and data downlink. The advance planning must include the management of resources, such as power budget, mass memory utilization, and downlink rates (cf. Hauber et al., 2002; Jaumann et al., 2007) Limb sounding Limb images are obtained by inertially fixed pointing of the spacecraft and its body-mounted cameras towards the Mars horizon. The HRSC and SRC typically perform two limb soundings during each month, or more, when the orbit pericenter is in darkness and regular nadir imaging not possible Flyby imaging Owing to the unique elliptical orbit of MEX, reaching out beyond the orbit of the Martian satellite Phobos (mean orbit radius approx km), close encounters with this Martian satellite are possible with small adjustments in the MEX orbit. During a flyby of Phobos, the camera is pointed at an inertially fixed point in the sky, and images

12 484 J. Oberst et al. / Planetary and Space Science 56 (2008) Fig. 15. An attempt is made to correct this SRC image of Phobos for point spread effects: before correction (left), after correction (center), the difference image (right). Note that double craters visible in the original are effectively removed. slewed across the ecliptic plane of the two Martian satellites to search for scattered light from possible trail particles. Images were taken at high solar phase angles, approximately 301 off from the Sun. However, owing to the difficulties of accounting for internal camera stray light, analyses are still ongoing. 7. Image products 7.1. Product levels Fig. 16. SRC operating modes: spot imaging mode (top), raster imaging mode (center), and contiguous mode (bottom). are taken when the moon is predicted to enter the camera field-of-view. As the Phobos surface is darker than that of Mars, longer image exposure times of 20 ms are typically selected. The image planning software (which was updated in the summer of 2005 for more flexibility in the planning of the flybys) allows us to check and adjust the inertial pointing vector for visibility of background stars, which are essential for astrometric measurements. The inertial pointing vector can also be set for coverage of selected areas on Phobos. Unfortunately, only 1 3 faint (+6omo+9) stars typically fall within the narrow field of view of the SRC, which are typically not visible using the exposure times used for imaging Phobos. Therefore, to make these faint stars visible, the first and last image of a flyby sequence (usually before and after Phobos enters and leaves the field of view) are taken at longer exposure times (approx. 500 ms) Search for dust trails Recently, the SRC, together with HRSC, engaged in a search for Phobos and Deimos dust trails. Cameras were The routine processing of individual SRC images follows closely the processing sequence of HRSC images (Jaumann et al., 2007). Following the HRSC naming scheme, the processing begins with raw telemetry data, followed by level 1 (unpacked and decompressed data), level 2 (radiometrically corrected data), and level 3 (geometrically corrected data). The SRC images are reprojected to a reference surface (MOLA terrain models, Smith et al., 2001) using nominal time tag, orbit, and pointing data. After a proprietary phase of 6 months, HRSC and SRC images are made available to the general science community. At the time of writing, all image data from the nominal mission have been released. Specifically, for SRC, the level 2 and 3 images, including calibration information and navigation data, are submitted to PDS and ESA SRC/HRSC mosaics Selected radiometrically and geometrically corrected level 3 SRC images are finally merged into mosaics. The images are brightness-stretched using histogram adjustment techniques to obtain a single-byte representation and to avoid brightness discontinuities at the image margins. They are inserted into a north-south trending superresolution strip centered within the HRSC image (Fig. 17) to show additional surface details. Alternative products are side-by-side comparisons of SRC and HRSC products

13 (Kirk et al., 2003, Figs. 18 and 19, see also Jaumann et al., 2007, Fig. 11). Mosaics are shared among the experiment science team but are not part of the regular data delivery to PDS or to ESA SRC/MOC stereo Overlapping SRC and Mars Orbiter Camera-Narrow Angle (MOC-NA) images (Malin et al., 1992 for technical details) have the potential to make good stereo pairs. Although the effective resolution of the SRC images is limited compared to what MOC can accomplish, one can construct impressive red-blue stereo anaglyphs with SRC and MOC-NA images. A digital terrain model was constructed for images obtained in the Olympus Mons caldera (Kirk et al., 2003, Figs. 19 and 20) and allows us to study the morphology of fracture systems within the area. Unfortunately, since both MOC-NA and SRC images cover very small areas and since both cameras are only occasionally pointed off-nadir as needed for stereo, SRC/MOC-NA overlaps are very rare. Nevertheless, the ability to combine the two types of images provides very unique topographic coverage of Mars at effective spatial resolutions of a few meters. 8. Scientific return Through the course of the MEX mission, the SRC has captured fascinating details of surface morphology, including crater rims, hill slopes, tectonic fractures, fluvial channels, glacial moraines, wind streaks, dune fields, and dust devil tracks. The SRC has also proven to be useful for statistics of small craters and for observations of Phobos. Here, we highlight useful studies that have been made using SRC images Active geology dust devils J. Oberst et al. / Planetary and Space Science 56 (2008) An active dust devil was imaged by the HRSC on the southern highlands Peneus Patera volcano on September 11, 2005, and by chance it was also captured in a 23-frame continuous-mode SRC mosaic that transects the Peneus Patera caldera (Fig. 21). The SRC mosaic reveals an abundance of dark dust devil tracks, indicating that this caldera is filled with dust and that volcanism likely ceased in the caldera long ago. The dark nature of the tracks suggests that the surface underlying the brighter dust may be basalts emplaced in the caldera from an ancient effusive episode. This dark material in the central upper section of the Peneus caldera floor is also revealed in the corresponding HRSC color image as a blue violet unit. While the dust devil is barely visible in the HRSC image, from the SRC images, it is possible to make an estimate of the dust devil diameter (42 m) and height (1360 m) (Stanzel et al., 2006). Both bright and dark dust devil tracks have been imaged by the SRC, not only in the plains across Mars, but also on Fig. 17. SRC mosaic of a dune field in the Nili Patera area of Syrtis Major (orbit 1098). The SRC ground pixel size is 2.5 m, and the mosaic is 2.7 km wide.

14 486 J. Oberst et al. / Planetary and Space Science 56 (2008) Fig. 18. HRSC image (left) with SRC footprints and the corresponding SRC image (right), taken in Mars Express orbit The HRSC scene is approx. 5.1 km wide. The images show erosion and layering near a table mountain in Simud Vallis. Fig. 19. Comparison of HRSC (left), SRC (center) and MOC (right) images within the Caldera of Olympus Mons (Mars Express Orbit 37). The resolution of the HRSC image is 19 m/pix, SRC: 4.7 m/pix, MOC: 6.25 m/pix. The total width of the SRC frame is 4.8 km, and the width of the fractures ranges from about 220 to 600 m. The SRC image and the MOC image make a stereo pair. elevated spots such as the Tharsis volcanoes. These mosaics will be useful for studies of the global distribution of dust devil activities on Mars Topographic information from shadow measurements One advantage of the SRC over both MOC and HRSC is the ability to obtain topographic information (in the form of scarp heights) using shadow measurements. This is due to the high solar incidence angles afforded by some MEX orbits, while illumination conditions for MOC-NA images, taken from the sun-synchronous orbit of the Mars Global Surveyor, are usually non-ideal for shadow measurements. We use two SRC mosaics covering parts of Hadriaca Patera and make shadow measurements on scarps to assess the thickness of some of the eroded flank layers. Our results (Fig. 22) show that the scarps of eroded layers on Hadriaca Patera s flanks range from 40 to 70 m height, with slightly higher scarps on the southern flank. These results are consistent with scarp heights on eroded layers measured from MOLA tracks (NASA, 2003) correlated with MOC-NA images M (60 m) and

15 J. Oberst et al. / Planetary and Space Science 56 (2008) E (60 90 m). The new SRC imagery supports the earlier interpretation of these layers as eroded pyroclastics rather than lava flows (e.g., Greeley and Spudis, 1981; Crown and Greeley, 1993) Small craters SRC observations (Fig. 19) are used for determining the age of one of the Olympus Mons calderas by means of crater frequency statistics (Hartmann and Neukum, 2001; Neukum et al., 2004b; Werner et al., 2005, 2006; Werner, 2006). Crater counts were performed on both SRC and HRSC imagery, and reveal a cumulative crater frequency of N(Dp1 km) ¼ 6.49e-5 and a corresponding surface age of 113 million years (Fig. 23). In spite of the described reduced effective resolution of SRC images by a factor of 2 3, the range of useful crater diameters extends down from HRSC resolution of 10 m/pixel by a factor of two. Smaller craters not only improve the overall statistics but become especially important on younger surfaces and on units of smaller area extent where the number of large-sized craters is not sufficient for age determinations. Identification of smaller craters may help in the currently ongoing discussions about the relevance of secondaries in the record of craters o500 m (Hartmann, 2005; McEwen et al., 2005; Werner et al., 2006) Atmospheric profiles Fig. 20. Topographic model (oblique view over shaded relief) showing fractures within the Olympus Mons Caldera obtained from the SRC/ MOC stereo images in Fig. 19. The model has a grid spacing of 15 m. Heights in the perspective view are exaggerated by a factor of 4. Studies of the Martian atmosphere by limb monitoring are prominent science goals of the HRSC/SRC imaging experiment. Owing to spacecraft motion in combination with inertially fixed pointing of the suite of MEX instruments, the limb images obtained by the HRSC scanner are severely distorted, and their interpretation is not immediately obvious. In contrast, the array images from the SRC do not require geometric corrections and can Fig. 21. Contiguous SRC mosaic within a HRSC false color image (left) and magnified portions (right) capturing an active dust devil in Peneus Patera (images from orbit 2133).

16 488 J. Oberst et al. / Planetary and Space Science 56 (2008) Fig. 22. Orbit 528 HRSC view (50 m/pix) of Hadriaca Patera (center) and SRC mosaics of flank regions near the caldera (Orbit 550 SRC at left, orbit 528 SRC at right). The SRC images are used to estimate scarp heights from shadow measurements in these low-sun images. The solar elevation angles for the orbit 550 and orbit 528 SRC mosaics are 161 and 181, respectively. Scarps of eroded layers were found to be between 40 and 90 m in height, consistent with MOLA measurements in the area. be readily analyzed. The high spatial resolution of the SRC reveals the highly stratified vertical structure of the limb as it depends on location, time of day and season (Fig. 24), features not observed in such detail previously (e.g., Jaquin et al., 1986; Chassefiere et al., 1992; Titov et al., 1997; Pearl et al., 2001) Phobos/Deimos observation results As of September 2007, a total of 63 Phobos flyby maneuvers have been executed, from which useful SRC images were returned. The images obtained during the flybys (Fig. 25) were used to determine the astrometric

17 J. Oberst et al. / Planetary and Space Science 56 (2008) positions of Phobos with accuracies of km (Oberst et al., 2006). Also, images of Deimos, the second Martian satellite, were obtained from far range. The hitherto available Phobos and Deimos orbit predictions differ substantially among each other and also do not agree with the actually observed positions of the satellites. Hence, the astrometric data obtained by the SRC have already initiated new efforts for Phobos and Deimos orbit modeling (Lainey et al., 2005). 9. Conclusions SRC images show that the intended high resolving power of the camera has not been reached to the expected extent. With images being plagued by astigmatism in combination with electronic noise and low signal levels, we have been able to improve their quality by applying restoration algorithms in post-processing efforts. The results are encouraging, and it appears worthwhile to continue the efforts to improve the effective resolution of the SRC images to an extent approaching the camera s original design goal. As of 1 September 2007, the SRC has obtained more than 6940 images, including Mars surface views, atmospheric limb views, and Phobos/Deimos images. There has been much science return from special targetof-opportunity observations which have not been anticipated, for example, the abundance of Phobos images and the incidental capture of dust devils. In addition, the SRC has demonstrated the great value of high-resolution surface views within some well-defined context frames in order to further our understanding of the geomorphology and evolution of Mars. Acknowledgements Fig. 23. Crater size frequency distribution within the Olympus Mons Caldera. Typically, sizes for craters as small as 4 pixels can be measured and included in the statistics. In this case, using SRC (with the effective image resolution reduced over nominal by a factor of 2 3), the crater distribution can be extended to small crater sizes of approximately 20 m. Data were obtained from images shown in Fig. 19. We wish to congratulate the European Space Agency and her industry partners to the successful launch and continuing operation of the Mars Express mission. The prime contractors for the HRSC/SRC experiment were EADS Astrium Friedrichshafen and DJO Jena- Optronic GmbH. We very much appreciate the thoughtful Fig. 24. SRC view from the dark hemisphere towards the bright limb (left), range: 1260 km, 12 m/pix taken in orbit 602. SRC view from above the fully illuminated hemisphere towards the terminator and limb (right), range: 6325 km, 60 m/pix, orbit 724.

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