Physics – Optics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.p42a0557s&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #P42A-0557
Physics
Optics
7594 Instruments And Techniques
Scientific paper
Measurements by the Mars Orbiter Laser Altimeter (MOLA) on board the Mars Global Surveyor (MGS) may be used to provides a radiometric measurement of Mars in addition to the topographic measurement. We will describe the principle of operation, a mathematical model, and the receiver calibration in this presentation. MOLA was designed primarily to measure Mars topography, surface roughness, and the bidirectional reflectance to the laser beam. To achieve the highest sensitivity, the receiver detection threshold is dynamically adjusted to be as low as possible while keeping a predetermined false alarm rate. The average false alarm rate is monitored in real time on board MOLA via a noise counter, whose output is fed to the threshold control loop. The false alarm rate at a given threshold is a function of the detector output noise, which is the sum of the photodetector shot noise due to the background light seen by the detector and the dark noise. A mathematical model has been developed that can be used to numerically solve for the optical background power given the MOLA threshold setting and the average noise count. The radiance of Mars can then be determined by dividing the optical power by the solid angle subtended by the MOLA receiver, the receiver optical bandwidth, and the Mars surface area within the receiver field of view. The phase angle which is the sun-Mars-MOLA angle is available from the MGS database. MOLA also measures simultaneously the bidirectional reflectance of Mars via its 1064nm laser beam at nadir with nearly zero phase angle. The optical bandwidth of the MOLA receiver is 2nm full width at half maximum (FWHM) and centered at 1064nm. The receiver field of view is 0.85mrad FWHM. The nominal spacecraft altitude is 400km and the ground track speed is about 3km/s. Under normal operation, the noise counters are read and the threshold levels are updated at 1Hz. The receiver sensitivity is limited by the detector dark noise to about 0.1nW, which corresponds to <2% the maximum radiance during daytime from the brightest area on Mars. The results from the mathematical model agree well with the pre-launch measurements at several calibrated optical power levels. The radiance of sunlit Mars estimated with this technique correlates well with the measurement from the MGS Thermal Emission Spectrometer (TES) and the Hubble Space Telescope at similar wavelength. Reference [1] J. B. Abshire, X. Sun, and R. S. Afzal, 'Mars Orbital Laser Altimeter: receiver model and performance analysis,' Applied Optics, Vol. 39, No. 15, pp. 2449-2460, May 2000.
Abshire James B.
Neumann Gregory A.
Sun Xiaosong
Zuber Maria T.
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