Cameras on Orbiters and Landers Nick Hoekzema
Purpose of the camera (I) Navigation, orientation - Solar sensors orientation with ~ 0.5º accuracy
- Star trackers to recognize constellations accuracy up to the `` range
- Feedback between cameras and gyros/rockets
Atmospheric research - Usually large FOV and high S/N is more important than high spatial resolution. Usually a few km/pixel is quite sufficient WAC cameras
- E.g., weather, cloud, and aerosol studies
Purpose of the camera (II) Surface geology - Usually high spatial resolution is more important than high S/N or large FOV NAC cameras
- Try to embed NAC image in WAC context image
Geochemistry spectral imaging Surface topography stereo cameras, laser altimeters Mapping needs: - very accurate positional measurements
- very accurate description of the body
- Image deformations by optics must be well known
Border conditions (I) Data rate - Earth remote sensing: many Gbytes/day if needed
- Deep space: be happy with a Gbyte/day
Weight, how much payload does a camera take? - Simple WAC & navigational cameras nowadays: few kg or even few hundred grams
- Some Earth observers have cameras of hundreds of kg
- Old fashioned pre CCD era cameras: tens of kg
- Omega (spectral imager) ~20—25 kg
Temperature environment - Dark current and response to incident light change with temperature unstable temperature environment ruins calibration
- Spectral imagers, IR cameras often need cooling
Border conditions (II) Power consumption - Small WAC cameras: few W or even less
- Old fashioned pre CCD era camera: tens of W
- Active system like MOLA laser altimeter: tens of W
- Huge, cooled, IR telescopes: hundreds of W
Weathering: CCDs don’t like cosmic rays, fast solar wind protons, etc Dimensions - some positional cameras and WACs fit into a matchbox
- High resolution cameras need a telescope much larger e.g., MOC ~ 0.5 X 0.5 X 0.9 m
- Some spy satellites had telescopes of several meters
Lander Stereo Cameras Most landers have stereo cameras Stereo information is needed to manouvre vehicle or manipulators
Past sixties & seventies Extremely high resolution space images from spy satellites. E.g., US Samos - Use television system for targeting
- Register high resolution images on film
- Drop film in capsule to Earth surface
- (Panic when capsule lands on wrong spot)
Obvious problems when you want to have pictures from planets other than Earth, then use television quality not so good
Lunar Orbiter 1966-1967 Great images (but the reproduction shown here is less than optimal) Although the optics were not impressive, objects of only a few meters are visible… and intensities are extremely well calibrated …because the S/C could be put into low lunar orbit… …and, most of all, because it exposed onto a 70 mm film!
Lunar orbiter (II) Essentially used a normal photo-camera The spacecraft developed and digitized its own films onboard Drawbacks Not used for interplanetary missions such as Viking or Voyager
Vidicon Telescope focuses images on a Vidicon Image is an imprint of variable electrostatic charge on the faceplate of the Vidicon Faceplate is then scanned and neutralized with an electron beam and variations in charge are read in parallel into a seven-track tape recorder They flew on numerous missions (Mariners, Voyagers, etc) They were heavy (Voyager camera system ~40 kg)
VIS: Viking orbiter vidicon cameras as an example But for many over/under exposed pixels, intensities are ~1% reliable Bit slow (i.e., the readout and digitization) Moving parts (shutter, filter wheel) Consume upto 35 watts
Facsimile Viking lander cameras Very different from vidicon principle Intensities from a small solid angle are measured by one or more photodiodes (viking facsimiles had 12) A nodding mirror is used to build an image pixel by pixel Drawback: slow, very slow, and contains moving parts
Present And then miniaturization gave birth to the CCD Each pixel stores a charge that is determined by the illumination incident on it End of exposure - charge is transferred to a storage register
- the CCD is freed up for the next exposure
First interplanetary use: 1986 Halley flyby of - Giotto CCD from MPAE!
- Vega
CCD: good and bad Few or no moving parts • Extremely lightweight Fast • Reliable • Small power consumption Can handle large contrasts - Originally ~5%
- Nowadays ~0.5% or better
Sensitive to damage from e.g., cosmic rays In short: If time, weight, maintenance, data transfer rates, and transport were no problem then old fashioned facsimile and film cameras would often still be the better choice
Framing cameras Use rectangular CCD to take pictures No Viking-like problems with fast phenomena like these dust-devils Since nowadays CCD may easily have several million pixels… …observing at high spatial resolution usually is less of a problem than… …sending images of a few Mbyte each in a reasonable amount of time Therefore add pixels prior to transmitting the image - Higher S/N ratios
- Lower spatial resolution
This procedure is called macro pixeling
Push broom scanners Examples: MOC (on Mars Global Surveyor) MISR (on Terra) HRSC (on Mars Express) ((Omega (on Mars Express))) compare Sumer (on SOHO) Push broom cameras scan the surface with line CCDs Images are built line after line as the spacecraft moves along its orbit Line CCDs may have many thousands of pixels Biggest problems usually: - Data rate
- Need for accurate correction for S/C movements and vibrations
High resolution MOC image MOC couldn’t quite resolve Pathfinder But the 2005 orbiter camera probably will S/N ratio is pretty awful: ~20--30
Multiple line push broom scanners Examples, MISR and HRSC Several line CCDs are mounted in parallel Each observe in different colors and/or angles stereo view in color Note the difference in optical depth between 0º and 60º
Stereo Remote Sensing Gives DEMs Very useful for aerosol and other atmospheric studies Some stereo cameras fly onboard airplanes are Air Misr and HRSCa Stereo remote sensing of Earth: - ATSR-2 onboard ERS
- POLDER onboard ADEOS
- MISR onboard TERRA
Stereo remote sensing of Mars from 2004 with HRSC on Mars Express
So what about the future? Scanning with a rectangular CCD? In fact a 1000 X 1000 pixel CCD is a set of 1000 line CCDs in parallel You might put a grating in front of it so that a spectrum is projected on the CCD Scan the surface with each of these ‘line CCDs’ This is a form of ‘spectral imaging’ Largest drawback: the data rate is enormous if done at high resolution Mars Climate observer was to use a simple, low data rate version of this principle (pity it was lost)
Laser altimeters MOLA (Mars Orbiting Laser Altimeter) gave a superb topographic map of Mars However, it also: - Probes the atmosphere
- Measures surface albedos
- Measures surface roughness
- Can look at dark surfaces
Will be a valuable tool on missions to Mercury, asteroids, Jovian moons, etc
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