Marek Peca

Photon counting altimeter and lidar for air and spaceborne applications

We are presenting the concept and preliminary design of modular multipurpose device for space segment: single photon counting laser altimeter, atmospheric lidar, laser transponder and one way laser ranging receiver. For all the mentioned purposes, the same compact configuration of the device is appropriate. Overall estimated device weight should not exceed 5 kg with the power consumption below 10 W. The device will consists of three main parts, namely, receiver, transmitter and control and processing unit. As a transmitter a commercial solid state laser at 532 nm wavelength with 10 mW power will be used. The transmitter optics will have a diameter at most of 50 mm. The laser pulse width will be of hundreds of picoseconds order. For the laser altimeter and atmospheric lidar application, the repetition rate of 10 kHz is planned in order to obtain sufficient number of data for a distance value computing. The receiver device will be composed of active quenched Single Photon Avalanche Diode module, tiny optics, and narrow-band optical filter. The core part of the control and processing unit including high precision timing unit is implemented using single FPGA chip. The preliminary device concept includes considerations on energy balance, and statistical algorithms to meet all the mentioned purposes. Recently, the bread board version of the device is under construction in our labs. The concept, construction, and timing results will be presented.

Time-to-Digit Converter Based on radiation-tolerant FPGA

Architecture of a time-to-digit converter (TDC) is presented. TDC is an electronic device which measures time of arrival of discrete electronic pulses, with respect to reference time base. Our work on TDC is motivated by its applications in field of long-range laser distance measurement and time synchronization. Unlike earlier time interpolation methods, we have chosen all-digital approach based on pulse propagation through tapped delay line. We do not expect it could outperform recent invention of time interpolation using narrow-band filter excitation [1], [2]. However, our approach relies on a standard digital circuitry only. With space applications in mind, we are implementing the TDC into a space qualified, radiation-tolerant field-programmable gate array (FPGA). On top of related works [4] and [5] on all-digital TDCs, delay line, we try to gather more complete information about the sampled pulse. It is done by sampling of whole bit vector, corresponding to all of the delay line taps. A calibration method based on random pulse source is discussed, including preliminary results. Impact of physical FPGA cell placement on resulting time measurement granularity is observed. Actually measured jitter distribution is compared to normal distribution function, giving an insight of absolute accuracy limit of our approach within the given FPGA platform.

Ultrasonic Localization of Mobile Robot Using Active Beacons and Code Correlation

Ultrasonic localization system for planar mobile robot inside of a restricted field is presented. System is based on stationary active beacons and measurement of distances between beacons and robot, using cross-correlation of pseudorandom binary sequences (PRBSes). Due to high demand of dynamic reserve imposed by range ratio in our specific task, both code- as well as frequency-divided media access has been utilized. For the same reason, 1-bit signal quantization has been abandoned in favor of higher resolution in receiver analog-to-digital conversion. Finally, dynamic estimation of the position is recommended over analytic calculation. The final solution uses the extended Kalman filter (EKF), equipped with erroneous measurement detection, initial state computation, and recovery from being lost. EKF also performs data-fusion with odometry measurement. Unlike the approach in majority of works on mobile robot localization, a model, actuated solely by additive process noise, is presented for the data-fusion. It offers estimation of heading angle, and remains locally observable. Simplistic double integrator model of motion dynamics is described, and the importance of clock dynamics is emphasized.

SPEJBL—The biped walking robot

Abstract Construction of a biped walking robot, its hardware, basic software and control design is presented. Primary goal achieved is a static walking with non-instantaneous double support phase and fixed trajectory in joint coordinates. The robot with two legs and no upper body is capable to walk with fixed, manually created, static trajectory using simple SISO proportional controller, yet it is extendable to use MIMO controllers, flexible trajectory, and dynamic gait. Distributed servo motor control over a CAN fieldbus is used. Important points in construction and kinematics, motor current cascaded control and fieldbus timing are emphasized. The project is open and full documentation is available.

Single photon lidar demonstrator for asteroid rendezvous missions

We present compact single photon lidar demonstrator dedicated for asteroid rendezvous missions. The instrument provides crucial data on altitude and terrain profile for altitudes exceeding 5km with a precision of less than 10 cm fulfilling the Rayleigh criterion. Transmitter and receiver optics designs are discussed, control and processing electronics based on a single rad-hard compatible FPGA (Field Programmable Gate Array) is analyzed. The FPGA electronics subsystems are TDC (Time-to-Digit Converter), laser trigger pulse generator and gate generator. Indoor calibration procedures of the whole demonstrator chain are proposed and evaluated. The calibration covers positioning of receiver and transmitter optics related to detector and laser, aligning of transmitter and receiver optical common paths. The retrieving strategy of terrain elevation profile is proposed and via indoor tests validated. Theory for surface slope and scanning is established, simulation and measurement results are compared and discussed.

Clock composition by wiener filtering illustrated on two atomic clocks

Estimation instead of feedback loops is recommended to obtain a composite clock. Wiener filtering approach to clock ensembling is introduced and demonstrated on the simplest case of two clocks. Design procedure dealing with clock system non-stationarity, non-observability and numerical issues, is given. Impact of causality to unexpected performance degradation is discussed.

Programmable delay controller allowing frequency synthesis and arbitrary binary waveform generation

Design of a programmable delay controller (PDC) within an aerospace-compatible field-programmable gate array (FPGA) fabric is presented. Although PDC is a common digital block nowadays, the possibility to use it for low-jitter arbitrary frequency generation constrained only by minimum edge-to-edge time still seems to be uncovered. The novel idea of the developed PDC is seamless line delay switching at sampling frequencies corresponding to the generated output frequency, unleashing a possibility of arbitrary binary waveform (frequency) generation. The maximum frequency is constrained only by the FPGA fabric performance, and by idle delay of available multiplexers. Glitch-free operation with no unintentional edges is employed for proper PDC control signal switching. The overall output signal jitter is composed solely of the jitter of input signal and propagation jitter of the delay elements (σmax = 4.1 ps RMS). Measured resolution of the PDC is ±Δτmax/2 = ±7.5 ps. Measured temperature drift of the PDC is ~ 30 ps K_1. An ability of PDC to generate fractional frequency from the input has been demonstrated on a simplified, low-resolution variant, delivering 33.3MHz out of 50 MHz input.

Photon counting Lidar for deep space applications: concept and simulator

The paper presents the concept and preliminary design of a single photon counting laser altimeter (lidar) dedicated for deep space rendezvous missions towards an asteroid. Overall estimated device weight and volume should not exceed 2 kg and 2 dm2 with the power consumption below 20 W. The altimeter range evaluation precision should be on the order of 5 cm. A complex software simulator of the entire laser ranging and photon counting link has been developed and verified. The simulator framework incorporates both the three-dimensional space-craft flight mechanics and topographical maps of the surface. Altimeter performance metrics are proposed and evaluated considering the topographical surface slope, environmental conditions, footprint coverage, gate range settings, and spacecraft-to-asteroid range and velocity. The asteroid topographical mapping and landing scenarios are discussed; specific spacecraft-to-asteroid approach phases are simulated and range reading maximum repetition rate is determined for each operation phase. Photon counting approach enables additional functions to laser altimetry, namely the one way laser ranging and laser time transfers over interplanetary distances and absolute radiometry at the laser wavelength.

Photon counting Lidar for deep space applications: demonstrator design

The paper presents design of a single photon lidar device suitable for space-borne applications. The device is composed of radiation tolerant components or components that have a radiation tolerant equivalent. The design is modular so that the same core can be utilized for several photon counting applications including laser altimeter, atmospheric lidar, laser transponder or one-way laser ranging receiver. The transmitter consists of a pulsed solid state diode pumped microchip Nd:YAG laser with second harmonic generator operating at 532 nm. Several laser versions may be employed { an externally triggered laser with repetition rate of up to 4 kHz or a free-running laser at 10 kHz. The receiver is equipped with a 1nm bandwidth optical band-pass interference filter and an active quenched SPAD detection module. A radiation tolerant K14 SPAD detector package developed at Czech Technical University with resolution of 20 ps rms is foreseen for space applications. Alternatively, commercial SPAD module lacking the radiation tolerance with higher detection efficiency can be used in air-borne or ground applications. The core module based on a radiation tolerant FPGA is reaching a timing precision of 30 ps. The overall ranging performance of the device is as good as 5 cm resolution for ranges of several kilometers. Besides the design and construction of the device, some performance test results will be presented.

Single photon time transfer link model for GNSS satellites

The importance of optical time transfer serving as a complement to traditional microwave links, has been attested for GNSSes and for scientific missions. Single photon time transfer (SPTT) is a process, allowing to compare (subtract) time readings of two distant clocks. Such a comparison may be then used to synchronize less accurate clock to a better reference, to perform clock characterization and calibration, to calculate mean time out of ensemble of several clocks, displaced in space. The single-photon time transfer is well established in field of space geodesy, being supported by passive retro-reflectors within space segment of five known GNSSes. A truly two-way, active terminals work aboard of Jason-2 (T2L2) - multiphoton operation, GNSS Beidou (Compass) - SPTT, and are going to be launched within recent ACES project (ELT) - SPTT, and GNSS GLONASS – multiphoton operation. However, there is still missing comprehensive theoretical model of two-way (using satellite receiver and retroreflector) SPTT link incorporating all crucial parameters of receiver (both ground and space segment receivers), transmitter, atmosphere effects on uplink and downlink path, influence of retroreflector. The input to calculation of SPTT link performance will be among others: link budget (distance, power, apertures, beam divergence, attenuation, scattering), propagating medium (atmosphere scintillation, beam wander, etc.), mutual Tx/Rx velocity, wavelength. The SPTT model will be evaluated without the properties of real components. These will be added in the further development. The ground-to-space SPTT link performance of typical scenarios are modeled. This work is a part of the ESA study “Comparison of optical time-transfer links.”