Planetary & Cometary Exploration Cameras on Orbiters and Landers

Planetary & Cometary Exploration

• 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

• Many moving parts
• System is really heavy  ~65 kg for some simple black and white pictures

• 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

• Advantage: extremely accurate intensity measurements

• 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

• Measured intensities are not too accurate

• 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

• Useful for separating atmosphere from surface

• 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