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Layout, Design and Integration of a Measurement Setup in the Context of the AIV of the BELA Transmitter Laser
Abstract
The space probe BepiColombo will carry a laser altimeter for measuring the surface elevations of Mercury. This laser altimeter, called BELA (BepiColombo Laser Altimeter), is the first suchlike instrument developed, integrated and qualified in Europe. The instrument must fulfil high requirements. The specifications related to those requirements must be tested and verified in a special measurement setup that has been designed in scope of this work. The laser parameters to be verified are pulse energy, temporal pulse shape, wavelength, beam quality and divergence, beam profile and especially pointing stability. Pointing stability is of dominant importance as fluctuations or drifts of the beam axis can falsify the range measurements. The measurement setup for pointing stability is based on a concept published by laser altimeter verification teams from NASA.
The designed concept uses a 4000 mm off-axis parabolic mirror as focusing element and a stable reference laser providing a reference signal. This reference laser uses the same optical elements as the transmitter laser and thus compensates for the disturbances introduced by them. As detector a CCD camera is used which allows determination of both spot centroids and so the relative movement of the beams to each other. To adjust the beam diameters in the focal plane a 20 x beam expander was selected for the reference laser. The balancing of the beam intensities on the CCD is realized with a wavelength dependent filter unit. Bandpass filters and baffling assure a minimal influence of background illumination effects. Stiff optomechanical elements and shrouds minimize the influence of vibration and temperature fluctuations.
Considered as critical was the long focal length of the focusing mirror. Vibrations of the optical elements and the entire setup might dominate the expected small pointing stability value of the transmitter laser of 50 µrad. The mechanical stability of the setup was verified with a substitute laser and proved as feasible. Furthermore beam sizes in the focal plane were calculated and tested to be small enough for fitting into the CCD.
The final tests with the transmitter laser will be performed in a thermal vacuum chamber and in a cleanroom environment. A thermal shroud for the transmitter laser is necessary therefore.
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The Mercury Laser Altimeter (MLA) is one of the payload science instruments on the MErcury Surface, Space ENvironment, GEochemistry,
and Ranging (MESSENGER) mission, which launched on August3, 2004. The altimeter will measure the round-trip time of flight
of transmitted laser pulses reflected from the surface of the planet that, in combination with the spacecraft orbit position
and pointing data, gives a high-precision measurement of surface topography referenced to Mercury’s center of mass. MLA will
sample the planet’s surface to within a1-m range error when the line-of-sight range to Mercury is less than 1,200km under
spacecraft nadir pointing or the slant range is less than 800km. The altimeter measurements will be used to determine the
planet’s forced physical librations by tracking the motion of large-scale topographic features as a function of time. MLA’s
laser pulse energy monitor and the echo pulse energy estimate will provide an active measurement of the surface reflectivity
at 1,064nm. This paper describes the instrument design, prelaunch testing, calibration, and results of postlaunch testing.
The Ice, Cloud and Land Elevation Satellite (ICESat) mission will measure changes in elevation of the Greenland and Antarctic ice sheets as part of NASA's Earth Observing System (EOS) of satellites. Time-series of elevation changes will enable determination of the present-day mass balance of the ice sheets, study of associations between observed ice changes and polar climate, and estimation of the present and future contributions of the ice sheets to global sea level rise. Other scientific objectives of ICESat include: global measurements of cloud heights and the vertical structure of clouds and aerosols; precise measurements of land topography and vegetation canopy heights; and measurements of sea ice roughness, sea ice thickness, ocean surface elevations, and surface reflectivity. The Geoscience Laser Altimeter System (GLAS) on ICESat has a 1064 nm laser channel for surface altimetry and dense cloud heights and a 532 nm lidar channel for the vertical distribution of clouds and aerosols. The predicted accuracy for the surface-elevation measurements is 15 cm, averaged over 60 m diameter laser footprints spaced at 172 m along-track. The orbital altitude will be around 600 km at an inclination of 94° with a 183-day repeat pattern. The on-board GPS receiver will enable radial orbit determinations to better than 5 cm, and star-trackers will enable footprints to be located to 6 m horizontally. The spacecraft attitude will be controlled to point the laser beam to within±35 m of reference surface tracks at high latitudes. ICESat is designed to operate for 3–5 years and should be followed by successive missions to measure ice changes for at least 15 years.
The Mercury Laser Altimeter (MLA), developed for the 2004 MESSENGER mission to Mercury, is designed to measure the planet’s topography by laser ranging. A description of the MLA optical system and its measured optical performance during instrument-level and spacecraft-level integration and testing are presented.
Simple procedures and formulas for tracing the characteristics of a spherical Gaussian beam through a train of lenses or mirrors are described which are analogous to those used in geometrical optics to trace repeated images through an optical train.
LLNL plans to specify optical components for the National Ignition Facility according to ISO 10110, the new international standard for preparation of optics drawings. The standards have been approved by the international optics community and represent a fairly comprehensive language for describing optical components. We will describe our plan for implementation and experience to date in doing so.
The BepiColombo Laser Altimeter (BELA) has been selected for flight on board the European Space Agency's BepiColombo Mercury Planetary Orbiter (MPO). The experiment is intended to be Europe's first planetary laser altimeter system. Although the proposed system has similarities to the Mercury Laser Altimeter (MLA) currently flying on board NASA's MESSENGER mission to Mercury, the specific orbit and construction of the MPO force the use of novel concepts for BELA. Furthermore, the base-lined range-finding approach is novel. In this paper, we describe the BELA system and show preliminary results from some prototype testing.
As the inner end-member of the planetary system, Mercury plays an important role in constraining and testing dynamical and compositional theories of planetary formation. With its companions Venus, Earth and Mars, it forms the family of terrestrial planets, a category of celestial objects in which each member holds information essential for retracing the history of the whole group. For example, knowledge about the origin and evolution of these planets is one of the keys to understanding how conditions to support life have been met in the Solar System and, possibly, elsewhere. This quest is all the more important as terrestrial-like objects orbiting other stars are not yet accessible; our own solar system remains the only laboratory where we can test models that are also applicable to other planetary systems. The exploration of Mercury is therefore of fundamental importance for answering questions of astrophysical and philosophical significance, such as: ‘Are terrestrial bodies a common feature of most planetary systems in the Galaxy?’