10 years to the first camera for shooting Mars in high resolution

Most likely, this event went almost unnoticed. Is that professionals and especially interested amateurs recalled that on June 2, 2003, the spacecraft of the European space agency Mars Express (Mars Express) went to the Red Planet, carrying for the first time a high-resolution stereoscopic camera on its board, HRSC). In general, of course, it is surprising that the Europeans were the first to do this: out of 20 successful Mars exploration missions, 13 were organized by the USA, 7 by the USSR (the last, Phobos-2, ended in 1988) and only one ( Mars Express) - by the European Space Agency.

The fact that the camera was originally developed for the Russian mission Mars-96 adds to the plot's sharpness: two full-fledged modules were made and calibrated. But after the Russian AMC, due to the failure of the accelerating unit, did not enter the departure trajectory, and collapsed upon entering the atmosphere of the Earth, the Institute of Space Sensor and Planetary Exploration modified the duplicate module in order to use it in the planned European mission. It is a pity, of course, that the laurels of the participants of dispelling the "Martian Sphinx" and drawing up stunning 3D maps of Mars did not go to the Russian apparatus, but the stereoscopic camera itself, thanks to which all this became possible, deserves a separate story.


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HRSC is a nine-channel CCD-based camera that allows you to take pictures with details up to 2 meters, as well as build digital elevation models. The main parts of the camera are: a digital unit (ensuring the supply of electricity to digital elements of the camera and sensors, a control processor and a data compression unit) and the actual elements of the camera (the frame on which the optics and sensors are attached, also serves as a radiator; the camera head, including lens lenses and optical bench; super-resolution channel mounted inside the frame).




Optics HRSC, located in the camera head, is an Apo-Tessar lens with a focal length of 175 mm (f / 5.6) mounted on a titanium base. The transparency coefficient varies from 0.37 for the blue channel (440 nm) to 0.68 in the panchromatic range. The frequency-contrast characteristic (MTF) of the optics for the central pixel (nadir) is 0.4 at 50 lp / mm, with a deviation of 20 ° from the nadir, the MTF is 0.33.

The CCD array consists of 9 Thomson THX 7808B elements arranged in parallel to work in the push-broom mode of shooting (about the principle of operation of which I described in detail in the article about HiRISE ). The electronics in the camera head consists of three modules located in the focal plane. Each module contains three CCDs and a preamplifier. Each of the 9 CCDs contains 5184 pixels with a physical size of 7 μm (for comparison: HiRISE has a pixel of 12 μm), which provides a surface resolution of 10 m per pixel with a flight height of 250 km.

HRSC allows you to take stereo images with participation from 3 to 5 sensor lines, including the front - in the nadir - behind (± 18.9 °), plus two internal rows of sensors (± 12.8 °):


Sensor capture angles: ND - nadir; S1, S2 - stereo 1 and stereo 2 (± 18.9 °); P1 and P2 - photometry 1 and photometry 2 (± 12.8 °); IR - near infrared channel (+ 15.9 °); GR - green channel (+ 3.3 °); BL - blue channel (+ 3.3 °); RE - red channel (-15.9 °). The angle of capture across the flight line for all 9 sensor lines is ± 6 °. SRC - Super Resolution Camera Coverage.

Stereo channels, photometry and nadir work in the spectral range of 675 ± 90 nm, blue - 440 ± 45 nm, green - 530 ± 45 nm, red - 750 ± 20 nm, near infrared - 970 ± 45 nm, SRC operates in the panchromatic range . Graph of spectral sensitivity for all channels:



Stereo shooting along the direction of motion of the spacecraft allows you to completely avoid the influence of atmospheric and light factors, and participation in the formation of the final image of at least 3 sensor lines gives a clear reconstruction to create a digital elevation model with an accuracy of 1 pixel. When using 5 panchromatic images, the final frame also contains the photometric characteristics of the relief.

A super-resolution channel, the Super Resolution Channel (SRC) is a separate optics and CCD array with the appropriate electronics. The SRC optical system is a telescope of the Maksutov-Cassegrain system with a focal length of 972 mm (f / 11), the axes of which are located parallel to the optical axis of the HRSC. The optical scheme used combines the Maksutov telescope with a teleoptic lens, which reduces the overall length of the structure and corrects chromatic aberrations in the Maksutov system. Image quality is at the level of the diffraction limit with a contrast of about 25% at a Nyquist of 55.5 lp / mm.

SRC sensors - a Kodak KAI 1001 CCD with a continuous scan of 1024 x 1032 px and a pixel size of 9 microns, which gives 2.3 m of surface per pixel at an altitude of 250 km. SRC is mechanically and electronically connected to HRSC and transmits data to Mars Express not directly, but by a separate (tenth) channel via HRSC interfaces. SRC has a special anti-blooming protection and electronic exposure control, allowing to reduce image blurring. Super resolution camera operation provides three modes: point, raster and continuous. In the point mode, the images are not made sequentially, in the raster mode - in a certain order, and in continuous mode the tape of images is formed.

Of course, with HiRISE with a spatial resolution of 0.3 m, the main focus of the scientific value of HRSC is on building digital terrain models (DTM). The source data for building DTMs are based on terrain-related images from 5 panchromatic channels: nadir, stereo, and photometric (front and back nadir). Comparing the intersection points of the profiles and the internal geometry of the camera, as well as the parameters of the Mars Express orbit, we get a grid of the coordinates of three-dimensional objects, which is further interpolated into a raster DTM. Example: Hebe Canyon in false colors (ESA / DLR image):



Anaglyph images are also easily extracted from data obtained by HRSC sensors. If to obtain a terrain model it is necessary to compare several levels of data, especially to eliminate the influence of topography, then anaglyph is simply a correction of the source data on a flat surface (Mars ellipsoid). If stereo data is taken from the same orbit, such an adjustment reduces the lateral parallax and gives an excellent stereo pair of epipolar images consisting of the red and green ranges of the RGB model. An example is Hebe Canyon (ESA / DLR image):



Summing up a brief summary of this story, I will note a few not quite obvious things. It would seem, why photograph Mars with a resolution of 10 m / pixel, if there are cameras capable of shooting with a resolution of up to 0.3 m / pixel? Despite a completely unimaginable resolution, HiRISE managed to remove only 1% of the surface. While the HRSC has almost completed the creation of a full three-dimensional map of Mars (90% for February 2013). The second feature - the ability to stereo in real time. This is the only camera that takes pictures of Mars not by shooting from different points of the orbit (when the influence of atmospheric, light and other factors is unavoidable), but by simultaneous scanning with different lines of sensors.

And finally, I’ll leave two images of the “Martian Sphinx” made by HRSC, so that the followers of the cult will finally be convinced that #European hides ;-)