Development of a Laser Range Finder for the Antarctic Plateau

The development of sensing subsystems is crucial to the operation of highly autonomous robots in harsh environments. This paper will describe the methodologies used and the preliminary measurement campaigns, with the relevant results obtained, which were carried out for the realisation of a high performance laser range finder. The sensor will be installed on an autonomous rover (RAS), lodged on a specially conditioned housing in front of the vehicle allowing for the greatest visibility and optimum protection. The RAS will be used for scientific campaigns and logistic support operations specifically in Antarctica and possibly in other snowy environments. Preliminary tests have been already carried out in Glaciers and Snow fields campaigns. A problem is the very poor S/N ratio in sunlight conditions, caused by the total backscattering of clean snow surface. Additional difficulties come from the behaviour of icy surfaces and from the poor transparency of the atmosphere caused by wind-driven ice microcrystals. The technological solutions chosen to overcome these aspects will be analysed. INTRODUCTION The exploration of harsh environments, especially for wide area operations and measurement campaigns, is a reference field for highly autonomous robotics. The requirements of these applications lead to the development of a wide spectrum of technologies with special emphasis on the sensing subsystems, crucial point for a reliable world representation and thus for correct operation planning by the robot intelligence subsystem. In harsh environments the availability of a sensor (intended as the total time of reliable operation) can be low and then the best solution is to achieve a good redundancy for each needed sense. Range finding is one of the crucial points for the robot operation, giving the distance and other information between the mobile platform and the other objects of the scenario and has been achieved in the RAS (Robot for Antarctica Surface) project with three separate systems: stereo vision, millimetric wave radar, laser radar. The research project, funded by the Italian Ministry of Research in the frame of the PNRA (National Program for Researches in Antarctica) is well positioned from the point of view of national and international co-operation. The activity is conducted in conjunction with CNES (Centre National d'Etudes Spatiales) and LAAS (Laboratory for Analysis and Architecture of Systems) in the frame of the CONCORDIA initiative, the Italian and French Joint venture for the realisation of an international scientific base for the development of research activities in the interior of the Antarctica continent (in the site named DOME C). In particular CNES is interested in the development of autonomous or semiautonomous vehicles for planetary exploration, similar to the recent experience of JPL (Jet Propulsion Laboratory, Pasadena) with the Rover Sojourner on Mars, and in this respect the Antarctica site offers a number of significant conditions for testing the most critical technologies. Again for space applications, ENEA (Italian National Committee for New Technology, Energy and Environment) is also exploring the possibility to coordinate a common experiment with the Italian Space Agency (ASI) and other high technology operators like Tecnomare to test on the RAS remote control techniques when long and very long communication delays occur. Proceedings of EARSeL-SIG-Workshop LIDAR, Dresden/FRG, June 16 – 17, 2000 EARSeL eProceedings No. 1 149 RAS is a large robotics platform, built on a snowcat (Kässbohrer PB 260A, shown in picture 1) which is intended to support human operations in the Antarctic environment, ranging from the automation of measurement campaigns to carrying out potentially dangerous tasks in logistics operations, for instance reconnaissance during traverses with man-driven vehicles. An autonomous vehicle has been recognised as a great safety improvement when travelling over crevassed surfaces in order to identify the best path for other vehicles. The project is therefore aimed at the study and development of the autonomy aspects and at the capability to make decisions relevant to the specific tasks assigned to the robot (typically navigation). To achieve these aims the robot needs to operate on a robust world representation based on complete metrology information, reliable sensing of its own dynamic and kinematic data, semantic interpretation of the detected objects surrounding the vehicle, knowledge of meteorological conditions (the latter allowing to adequately weight the reliability of data coming from the different sensors), and to have an easy and stable communications subsystem. Starting from this information, a high level, rule based control system can be realised. Picture 1: Antarctica traverse operation of Kässbohrer RAS motion platform This paper aims to present the problems encountered in icy environments with a laser sensor. ENEA worked for several years on the various techniques centred on the use of laser for range measuring. The most important problem to be faced within this experience is to cope with the extreme light intensity of the Antarctic environment, partially, but not to the same extent, met also in glaciers at higher latitudes. This requirement, coupled with the need to ensure eye-safe operation, since these machines are also to operate in the presence of men, and a long-range operation (some tens of metres) represents a significant challenge for the realisation of the sensor itself. The final testing of the fully functional device is planned for the 2000-2001 winter campaign in the Arctic circle. The campaign in the final target environment, the Antarctic plateau, will be conducted one year later. METHODS The topological radar upon which the range finder is based is described briefly in this section together with the relevant basic theory and the environmental characteristics peculiar to its use. Topological radar: description and theory Here a brief description of the demonstration unit, designed and developed in our laboratory, is given. The instrument consists of an incoherent diode laser sensor, suitable to be equipped with a mechanical scanning camera. A modulated beam sounding technique is used for applications at Proceedings of EARSeL-SIG-Workshop LIDAR, Dresden/FRG, June 16 – 17, 2000 EARSeL eProceedings No. 1 150 medium or low target distances, since the heterodyne technique employing a frequency downconversion allows an indirect measurement of the round trip time delay of the sounding beam through a measurement of the phase delay of the signal photocurrent. The transceiving component of the unit is shown in Figure 1. The telescope and the laser semiconductor are used in a monostatic configuration. The absolute range measurement is performed by using a beam modulation technique carried out at 10 MHz frequency, so that unambiguous measurements can be obtained up to 15 m. Since the sensor should be able to operate in a wide range of environmental conditions, a minimisation of the intense background light, expected during daylight operation, is required on the receiving optics before reaching the detector. This has been obtained by introducing a very narrow bandwidth interference filter (10 Å), combined with an iris which restricts the instantaneous field of view approximately to the dimensions of the beam spot on the remote target. A portion of the modulated laser beam is reflected by a beamsplitter and two prisms on a photodiode generating a sinusoidal signal with constant amplitude and phase. This acts as a reference for the instantaneous phase measurement of the current signal coming from the detector. The sinusoidal signal generated by the beating of the reference and the signal beam, which is detected by the avalanche photodiode, has an amplitude depending on the reflectivity of the scene and a phase depending on the distance of the target (1). The sensor can be operated in combination with a mechanical scanning camera, which sounds the scene with a raster of collimated and focused modulated beam synchronised with an analogue to digital converter and acquisition system. Figure 1: The laser range finder layout An evaluation of the optical level available on the avalanche photodiode of the described transceiver can be carried out as follows: let PL be the power of the transmitted laser beam, R the distance of the target, ρ the overall target reflectance. Neglecting the power reflected in the mirror-like mode, we obtain (refer to Table 1 below for a full parameters definition) for the optical power on the detector: Ps = PL ρ ε T D F(θ) e / 8R (1) where F( θ ) = γ cos θ + φ S ( θ ) (2) is the angular distribution of the backscattered signal, which adds to the cosine Lambertian distribution a retroreflective component φS(θ) weakly dependent on θ when φ is near to the unity. Typical practical values valid for a large variety of target surfaces are listed in Table 1. Table 1: Typical parameters for operation of a laser range finder Optical wavelength λ = 670 nm Proceedings of EARSeL-SIG-Workshop LIDAR, Dresden/FRG, June 16 – 17, 2000 EARSeL eProceedings No. 1 151 Transmitted laser power PL = 2 mW Carrier modulation depth M = 0.8 Beam modulation frequency (Hz) f = ω/2π = 10 Distance from the target R = 1-15 m Atmospheric absorption coefficient α = 0.89 / km Incidence angle of the sounding beam θ = 0° Target average reflectivity ρ = 35 % Lambertian fractional power γ = 85 % Retroreflective fractional power S (θ) = 15 % Aperture of the receiving optics D = 0.05 m Collection efficiency of the receiver ε = 50 % Transmission of the interference filter T = 60 % Quantum efficiency of the detector η = 80 % Detector noise factor Γ = 1.5 Pixel sampling time τ =10 ms In order to determine the spatial resolution of a modulated laser sensor, the "phase noise" of the measurement process must be accounted for. This consists of the uncertainty in the phase that originates from a temporal jitter of the signal and can be evaluated as the inverse of the product of the modulation frequency, depth of modulation m and the current signal-to-noise ratio SNRi of the generated current. The phase noise ∆R is then represented by the formula ∆R = c/(2mωSNRi) (3) Assuming the arrival of the photons on the photocathode as a Poisson random process, and taking into account only the shot noise of the signal, the current signal-to-noise ratio has the following expression: SNRi = (Psη τ /hν Γ ) (4) where h is Planck’s constant and ν is the optical frequency, τ = 1/2B and B is the bandwidth of the detecting electronics; η represents the quantum efficiency and Γ is the noise factor associated with the avalanche amplification process in the detector. The scene background Daylight operation of the designed scanning transceiver requires the use of several optical elements to reduce as much as possible the level of stray light reaching the detector from the scene background through the receiving optics. This is due to the scattered sunlight in the field of view (FOV) of the transceiver. The amount of spurious light can be estimated by evaluating the ratio of the sunlight transmitted by an interference filter with a wavelength bandwidth ∆λ and the total sunlight irradiance. Approximating the sun with a 6000°K blackbody radiator, this ratio is