Friday, August 17, 2012

A Compressive Sensing Capability for CheMin aboard Curiosity ?

While reading through the list of Instruments currently on Curiosity, I came across this one:

PI: David Blake, NASA Ames Research Center
Contributions of CheMin to MSL Science Objectives
An important science goal of the MSL mission is to identify and characterize past or present habitable environments as recorded in sediments and rocks. CheMin is a definitive mineralogy instrument that will identify and quantify the minerals present in rocks and soil delivered to it by the Sample Acquisition, Sample Processing and Handling (SA/SPaH) system. By determining the mineralogy of rocks and soils, CheMin will assess the involvement of water in their formation, deposition, or alteration. In addition, CheMin data will be useful in the search for potential mineral biosignatures, energy sources for life or indicators of past habitable environments. CheMin can unequivocally identify and quantify minerals above its detection limits in complex natural samples such as basalts, multicomponent evaporite systems, and soils.

....Energy­-Dispersive Mode: Focus on Characteristic Cobalt Radiation
The CCD-224 directly detects individual X-ray photons that are absorbed by the active silicon, producing a number of electron hole pairs equal to the energy of the X-ray in electron volts, divided by 3.65 (the energy of an electron-hole pair in the silicon lattice). For example, a cobalt K-alpha X-ray with an energy of 6.93 keV will produce 1,899 electron hole pairs. Single pixel events (those representing the absorption of a single photon into a single pixel of the CCD) are summed into a histogram of energy vs. number of counts. This histogram constitutes the energy-dispersive fluorescence spectrum of the sample. The table shows the energy range and resolution....

Modes of X­-ray Photon Detection for X­-ray Diffraction
A special case of X-ray detection by the CCD is the detection of Co K-alpha or Co K-beta characteristic photons from the primary source. When Co K-alpha (or Co-beta) photons are detected, the X,Y pixel location in the CCD is identified and the corresponding X,Y location in a 600x582 counting number array is incremented by one. This process results in a diffraction image. Various strategies are used in on-board data processing to optimize the quality or quantity of diffraction data returned (e.g., "single pixel" detection, and "split pixel" detection).
An additional 600x582 array stores an image of all of the photons detected by the CCD regardless of energy. This array acts very much like a piece of photographic film, recording the XRD pattern as well as background, energy-dispersive data, and Bremsstrahlung.

[Emphasis mine]

I really don't know what the various strategies are and what is possible on-board, i.e if there is the possibility of performing something in between "single pixel" and "split pixel" detection but it sure looks like the potential to use compressive sensing. 

The "single pixel" capability is equivalent to the sum of all the smaller pixels with the same weights while the "split pixel" could correspond to the sum of only certain pixels with different or equal weights. If the "split pixel" capability were to allow for setting individual pixels with weight to 0/1 or some real numbers before  the summing then compressive sensing ought to be looked into. 

In effect, the mode of operation that could be proposed would be close to that of the single pixel camera (How does the Rice one pixel camera work ? ). Here though, the "single pixel" is really one configuration (all mirrors are in) whereas the "split pixel would allow other configuration as shown here.

Image Credit: NASA/JPL-Caltech/Malin Space Science Systems 
This image was taken by Mastcam: Left (MAST_LEFT) onboard NASA's Mars rover Curiosity on Sol 3 (2012-08-09 05:09:32 UTC) . 
Full Resolution

No comments: