BELA and Laser Altimetry
A fundamental task of exploratory space missions is to characterize and measure the figure, topography, and surface morphology of the target planet. A state of the art tool for this task is a laser altimeter because it can provide absolute topographic height and position with respect to a object-centred co-ordinate system. The technology of laser altimetry for interplanetary spaceflight is new in Europe. The BepiColombo Laser Altimeter (BELA) will be the first such instrument developed for a European space mission. It will fly on the European Space Agency's BepiColombo mission to Mercury and will measure the
• figure parameters of Mercury to establish accurate reference surfaces
• topographic variations relative to the reference figures and a geodetic network based on accurately measured positions of prominent topographic features
• tidal deformations of the surface
• surface roughness, local slopes and albedo variations, also in permanently shaded craters near the poles
BELA will form an integral part of a larger geodesy and geophysics package, incorporating radio science and stereo imaging. Although stand-alone instruments in their own right, the synergy between these instruments will dramatically enhance the scientific capability. The synergy will cover the problems of planetary figure and gravity field determination, interior structure exploration, surface morphology and geology, and extend into the measurements of tidal deformations.
The BepiColombo mission comprises two spacecraft, the Mercury Planetary Orbiter (MPO) to be built by ESA and the Mercury Magnetospheric Orbiter (MMO) to be built by JAXA. The two spacecraft will fly to Mercury together in a coupled system until reaching Mercury orbit. The MMO will then be released into a 400 km x 19200 km orbit to allow detailed study of the magnetospheric interaction between the planet and the solar wind. The MPO will then descend to a 400 km x 1500 km orbit which is optimum for remote-sensing of the planet's surface.
The BepiColombo Laser Altimeter is one of the main experiments onboard the MPO.
The BELA Instrument
Electronics Unit (ELU)
The ELU is an electronics box which houses the Power Converter Module (PCM), the Digital Electronics Module (DPM) and the Rangefinder Electronics Module (RFM). It is being integrated by DLR-PF in Berlin.
The PCM takes the spacecraft power supply (which is nominally 28 Volts) and converts the voltage to several other voltages that BELA needs to drive its electronics. The board is redundant so that a failure of one PCM will not lead to a complete failure of the experiment. The PCM is being developed by the Instituto de Astrofisica de Andalucia (IAA) in Granada in cooperation with Spanish industry (CRISA).
The DPM is the on-board computer which controls the behavior of the experiment. It tells BELA when to fire, it flags the time when the laser was commanded to fire, it adjusts operational parameters, controls temperatures, reports on BELA’s status (housekeeping), collects the data from the receiver, generates telemetry packets and send them to the spacecraft. The DEB uses a LEON-2 processor which is tolerant to radiation. A software has been established which allows us to use a command language similar to the C language. The DEB is being developed by DLR-Berlin.
The RFM is the board that evaluates the signals to determine a range to the target. The signals from the APD detector are digitized and then analysed to determine when the laser fired with respect to an onboard oscillator. The RFM knows roughly when to expect the return pulse to come back from the target. It opens a window (the range window) and collects data from the APD via the analogue electronics board (AEB). These data are digitized and analysed using a specific algorithm to determine the time delay between emission and reception with respect to the oscillator. The RFM then computes when to expect the next pulse and repeats the cycle.
The RFM has several modes depending upon how much data BELA can transmit to ground. In its simplest mode, the RFM simply returns the pulse delay plus some auxiliary data. It can however return the fully digitized range window and return this to the DPM.
The RFM needs software in a field programmable gate array (FPGA) which is used to identify the outgoing and the incoming pulses. The software also provides additional information (pulse width, pulse shape and pulse amplitude) to assist in the interpretation of the data.
The RFM has been developed by RUAG Space (Switzerland) under contract from the University of Bern.
Laser ElectronicsUnit (LEU)
The LEU controls the laser head box (LHB) and contains a large number of capacitors which pulse the laser diodes in the LHB. It takes 28 V from the spacecraft via the ELU to provide this pulse. The system can pump the system at 10 Hz.
The LEU is being developed by Cassidian Optronik under contract from the Max-Planck-Institut fuer Sonnensystemforschung in Göttingen.
Baseplate Unit (BPU)
The BPU comprises a baseplate (BP) upon which several elements are mounted. These are the laser head box (LHB), the receiver telescope (RTL), and the focal plane assembly (FPA).
The LHB is being developed by Cassidian Optronik under contract from the Max-Planck-Institut fuer Sonnensystemforschung in Göttingen.
Receiver Baffle Unit (RBU)
The BELA project is a joint project of the Physikalisches Institut of the University of Bern and the Deutsches Zentrum fuer Luft- und Raumfahrt - Institut fuer Planetenforschung. Other partners are the Max-Planck-Institut fuer Sonnensystemforschung, and the Instituto de Astrofisica de Andalucia.
Many persons have helped develop BELA. The core hardware team currently comprises
Nick Thomas (Project Lead - Hardware Phase)
Karsten Seiferlin (Overall Project Manager)
Daniele Piazza (Overall System Engineer)
Alain Peteut (Electrical Engineer)
Anthony Servonet (Test Engineer)
Tim Bandy (Integration and Test Support Engineer)
Lisa Gambicorti (Optical Engineer)
Julian Gouman (PhD student)
Kaustav Ghose (PA/QA)
Tilman Spohn (Project Lead - Operations Phase)
Harald Michaelis (Transmitter Manager)
Hauke Hussmann (Operations Supervisor)
Thomas Behnke (Electrical System Engineer)
Kay Lingenauber (Structural and Test Engineer)
Rolf Schröter (Software Development)
Kai Wickhusan (Software Development)
Uli Christensen (Project Lead - Laser)
Reinald Kallenbach (Laser Manager)
Henry Perplies (Laser PA/QA)
Luisa Lara (Project Lead - Power Converter)
Jose (Chema) Castro (Power Converter Manager)
Miguel Herranz (Electrical Engineer)
The industrial teams involved include
RUAG Space Switzerland
Cassidian Optronik, Germany
FISBA Optik, St. Gallen
Kallenbach, R., E. Murphy, B. Gramkow, M. Rech, K. Weidlich, T. Leikert, R. Henkelmann, B. Trefzger, B. Metz, H. Michaelis, K. Lingenauber, S. DelTogno, T. Behnke, N. Thomas, D. Piazza, and K. Seiferlin, (2013), Space-qualified laser system for the BepiColombo Laser Altimeter, Applied Optics, 52, 8732-8746 doi:10.1364/AO.52.008732.
Chakraborty, S., N. Thomas, M. Affolter, J. Neubert, S. Graf, D. Piazza, T. Beck, M. Gerber, G. Roethlisberger , K. Seiferlin and K. Gunderson, (2012), High accuracy alignment concept for receiver and transmitter in a laser altimeter system, Applied Optics, 51, 4907, doi: 10.1364/AO.51.004907.
Beck, T., A. Bieler, and N. Thomas, (2012), Numerical thermal mathematical model correlation to thermal balance test using adaptive particle swarm optimization (APSO), Applied Thermal Engineering, accepted, doi:10.1016/j.applthermaleng.2012.01.027.
Beck, T., B.S. Luethi, G. Messina, D. Piazza, K. Seiferlin, and N. Thomas, (2011), Thermal Analysis of a Reflective Baffle Designed for Space Applications, Acta Astronautica, 69, 323-334, doi:10.1016/j.actaastro.2011.03.014.
Thomas, N., T. Beck, S. Chakraborty, M. Gerber, S. Graf, D. Piazza, G. Roethlisberger, (2011), A wide‐beam solar simulator for simulating the solar flux at the orbit of Mercury, Measurement Science and Technology, Meas. Sci. Technol., 22, 065903, doi: 10.1088/0957-0233/22/6/065903.
Gunderson, K. and N.Thomas, (2010) BELA receiver performance modeling over the BepiColombo mission lifetime, Planetary and Space Science, 58, 309-318.
Gunderson, K.S., N.Thomas, T.Spohn, and K.Seiferlin, (2004),A tradeoff investigation for the BepiColombo Laser Altimeter design, Instruments, Science, and Methods for Geospace and Planetary Remote Sensing. Edited by Nardell, Carl A.; Lucey, Paul G.; Yee, Jeng-Hwa; Garvin, James B. Proceedings of the SPIE, 5660, 117-127.
Space Research & Planetary Sciences Division Web Site