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13th International Laser Ranging
Workshop
"Toward Millimeter Accuracy"
Submitted
Abstracts
Advanced Systems and Techniques (B. Greene and T. Murphy)
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| SLR2000: Progress and Future Applications | John Degnan |
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NASA Goddard Space Flight Center NASA's new SLR2000 system is an unmanned photon-counting satellite laser ranging (SLR) station. It is designed to autonomously track the full constellation of over twenty SLR satellites, which range in altitude from about 300 km to 20,000 km. Autonomous operation and a common engineering configuration are expected to greatly reduce station operations costs relative to the current manned systems. The system has also been designed with a goal of significantly lowering replication costs. All of the prototype components and subsystems have been completed and tested and have substantially met the original specifications. The prototype system is presently undergoing final assembly and integration in a dedicated shelter with an azimuth tracking dome synchronized to the optical tracking mount. The facility also features a number of security features such as security cameras and sensors designed to detect power or thermal control problems or entry by unauthorized personnel. Field tests are scheduled to begin later this year. The present paper provides an overview of the various subsystems and test results to date. The meteorological subsystem has operated successfully in the field for over two years and consists of several sensors which measure: (1) pressure, temperature, and relative humidity; (2) wind speed and direction; (3) ground visibility and precipitation; and (4) local cloud cover as a function of station azimuth and elevation (day and night). A "pseudo-operator" software program interprets the sensor readings and makes decisions related to system health and safety and modifies satellite tracking priorities based on local meteorological conditions. The prototype laser transmitter consists of a passively Q-switched microchip Nd:YAG laser followed by a passive multipass amplifier and nonlinear frequency doubling crystal. It produces about 200 microjoules of energy at 532 nm in a few hundred picosecond pulse at a laser fire rate of 2 kHz. The low energy laser beam is expanded to fill the entire 40 cm aperture of the off-axis telescope to produce an eyesafe energy fluence at the exit aperture. The quadrant microchannel plate photomultiplier detects incoming photons reflected from the satellite with a quantum efficiency of 13% and produces a precise "stop" pulse for the event timer, which has a measured timing resolution of 5 psec (corresponding to a range resolution of slightly less than one millimeter). The quadrant detector also generates an angular pointing error for driving the tracking mount servos, so that the central peak of the transmitter beam falls onto the satellite. During acceptance testing, the mount demonstrated a one sigma RMS tracking precision of one arcsecond or better over a wide range of satellites and orbits. The system receives updated tracking priorities and orbital predictions from a central processor at the Goddard Space Flight Center via the Internet or a backup modem and transmits satellite range data back to the processor over the same links. The system also transmits data related to the health and safety of the instrument, and, if necessary, various system diagnostics can also be run remotely by maintenance engineers. Recently, there has been great interest in adapting SLR2000 to serve as the ground terminal for a high data rate, space-to-ground laser communications link, perhaps in parallel with its satellite tracking function. The motivation for, and potential advantages of, such a dual mode system are discussed briefly. Oral paper; received August 15, 2002 |
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| Optimization of the Correlation Range Receiver Parameters in SLR2000 | John Degnan |
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NASA Goddard Space Flight Center The extraction of single photon satellite returns from the solar background during daylight tracking relies on the "temporal coherence" of the signal returns and is accomplished in SLR2000 by a "Correlation Range Receiver". In such a receiver, the range gate is divided into a number of equally sized "range bins", and the photon counts in each bin are summed over a sampling period called the "frame interval". The counts in each bin, resulting from many laser fires within the frame interval, are then compared to a "frame threshold". If the count exceeds the "frame threshold", the bin is judged to contain signal whereas, if the count falls below the threshold, that particular bin is deemed to contain only noise. The optimum choice of range bin size, frame interval, and threshold will vary from satellite to satellite and depend also on the current performance of the laser and detector as well as local meteorological and solar noise conditions. Thus, the correlation receiver design must be flexible enough to adapt to changing operating conditions and targets. It must also be able to deal with occasional data dropouts as might be caused by intervening clouds or telescope pointing errors. In order to optimize the parameters of such a receiver over a wide dynamic range of both signal and noise, the system must be controlled in real time by software that can combine a priori information on the satellite link with real time sensor data. Default values for bin size, frame interval and threshold are computed and tabulated in the software based on a nominal ranging link and solar noise model and the maximum expected range and time biases in the orbit prediction for a given satellite. The measured count rates are then compared to the nominal values and the receiver parameters adjusted via derived algorithms accordingly to achieve a high probability of signal detection combined with excellent noise rejection. These algorithms are based on the maximization of the "differential cell count", which is the difference between the number of cells per frame correctly identified as signal (maximum of one) minus the number of false alarm cells. The present paper reviews the methodology by which the default receiver settings are optimized and the manner in which they can be updated rapidly in the presence of sensor readings that deviate from their expected values. Numerical results are presented for several key satellites under a variety of atmospheric conditions. Oral paper; received August 19, 2002 |
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| Photon-Counting Airborne Microlaser Altimeter | John Degnan |
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NASA Goddard Space Flight Center Jan McGarry, Thomas Zagwodzki, Philip Dabney Under NASA's Instrument Incubator Program, we have recently demonstrated a scanning, photon-counting, laser altimeter, which is capable of daylight operations from aircraft cruise altitudes. The instrument measures the times-of-flight of individual photons to deduce the distances between the instrument reference and points on the underlying terrain from which the arriving photons were reflected. By imaging the terrain onto a highly pixellated detector followed by a multi-channel timing receiver, one can make multiple spatially-resolved measurements to the surface within a single laser pulse. The horizontal spatial resolution is limited by the optical projection of a single pixel onto the surface. In short, a 3D image of the terrain within the laser ground spot is obtained on each laser fire, assuming at least one signal photon is recorded by each pixel/timing channel. The passively Q-switched microchip Nd:YAG laser transmitter measures only 2.25 mm in length and is pumped by a single 1.2 Watt GaAs laser diode. The output is frequency-doubled to take advantage of higher detector counting efficiencies and narrower spectral filters available at 532 nm. The transmitter typically produces a few microjoules of green energy in a subnanosecond pulse at several kilohertz rates. The illuminated ground area is imaged by a diffraction-limited, off-axis telescope onto an ungated segmented anode photomultiplier with up to 16 pixels (4x4). The effective receive aperture is about 13 cm. Each anode segment is input to one channel of a "fine" range receiver (5 cm detector-limited resolution), which records the times-of-flight of the individual photons. A parallel "coarse" receiver provides a lower resolution (>75 cm) histogram of atmospheric scatterers and centers the "fine" receiver gate on the last set of returns, permitting the fine receiver to lock onto ground features with no a priori range knowledge. In test flights, the prototype system has operated successfully at mid-day at aircraft altitudes up to 6.7 km (22,000 ft), with single pulse laser output energies of only a few microjoules, and has recorded kHz single photon returns from clouds, soils, man-made objects, vegetation, and water surfaces. The system has also demonstrated a capability to resolve volumetrically distributed targets, such as tree canopies and underlying terrain, and has performed wave height measurements and shallow water bathymetry over the Chesapeake Bay and Atlantic Ocean. The signal photons are reliably extracted from the solar noise background using an optimized Post-Detection Poisson Filter. Poster paper; received August 20, 2002 |
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| Time Transfer by Laser Link | Etienne Samain |
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OCA The T2L2 experiment allows the synchronization of remote clocks on Earth and the monitoring of a satellite clock with a time stability of the order of 1 ps over 1000 s and a time accuracy better than 100 ps. The principle is based on the propagation of light pulses between the clocks to synchronize. T2L2 will be then able to measure the performances of ground clocks having a stability in the range of 3 10-15 over the visibility period of a satellite at 700 km. One can also accumulate the data of several successive passages of the satellite to reach a stability in the 3 10-17 range over ten days. T2L2 has been proposed for the Myriade micro-satellite and for the prototype of the Galileo project. Poster paper; received August 22, 2002 |
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| Millimeter Ranging Accuracy - the Bottleneck | Ivan Prochazka |
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Czech Technical University Karel Hamal The satellite laser ranging random and systematic ranging error budgets have been analyzed with the ultimate goal of the millimeter precision and accuracy. Several new contributors have been identified and investigated. Oral paper; received August 22, 2002 |
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| Installing TIGO in Concepcion | Stefan Riepl |
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Bundesamt fuer Kartographie und Geodaesie Hayo Hase, Armin Boer, Wolfgang Schlueter Eduardo Carvacho, Rodrigo Reeves, David Ramirez, Cesar Guaitiao Emma Chavez, Raul Escobar Carlos Bustamante, Roberto Aedo, Marco Avendano, Gonzalo Remedi During the last year the Transportable Integrated Geodetic Observatory (TIGO), and amongst it the SLR module, was in stand by mode for the shipment to Concepcion, Chile. The negotiations of the previuos years led finally to a diplomatic note exchange aiming at the joint operation of TIGO in Concepcion. Apart from the BKG the chilean side formed a consortium in order to share funding for infrastructure and man power, giving means to host TIGO for at least three years. The Consortium consists of the following institutions:
The present paper describes the final updates in the SLR module, as well as the shipment and setup procedure including first results and local survey data providing a connection between the various employed geodetic space techniques. Oral paper; received August 23, 2002 |
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| Overview of Data for the SLR2000 Tracking Mount Performance Testing | Donald Patterson |
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HTSI Jan McGarry This paper addresses performance specifications provided to the mount manufacturer and final test data sets obtained before acceptance of the mount. Outlined as well are the major dynamic tracking errors identified during the development and testing of the mount and the steps taken by the manufacturer and the customer to correct these problems in order to meet stringent pointing and tracking specifications. The paper concludes with a comparison of test data sets taken at the factory and similar test sets taken after the field installation of the mount. Oral paper; received August 23, 2002 |
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| Laser Tracking of Space Debris | Ben Greene |
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EOS Yuo Gao, Chris Moore, Y. Wang, A. Boiko, Ian Ritchie, J. Sang,
J. Cotter Mount Stromlo laser ranging site has tracked non-cooperative objects in space for several years, and has recently extended the technology to the laser tracking of small space debris objects. The technique has strong advantages in the determination of precise orbits for small and unstable objects. The application of laser ranging techniques to typical targets without reflectors, such as space debris, will be addressed. Oral paper; received September 12, 2002 |
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| The MLRO Project: a Status Report | Giuseppe Bianco |
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Agenzia Spaziale Italiana T. Oldham The Matera Laser Ranging Observatory (MLRO) has been developed by Honeywell Technology Solutions, Inc., for the Italian Space Agency, and installed at the Center for Space Geodesy "G. Colombo" near Matera, Italy. This paper gives a description of the project, outlining the major observational results achieved so far, and discusses possible improvements and future plans. Oral paper; received September 17, 2002 |
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| A satellite laser ranging system based on a micro-chip laser | J. Amagai |
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Communications Research Laboratory Hiroo Kunimori, Hitoshi KiuchiCommunications Research Laboratory Takunori Taira Presented by: Hiroo Kunimori We have developed a micro-chip laser (MCL) that consists of a passive Q-switched Nd: YAG laser pumped by a fiber-coupled laser diode which can be used in the first generation of a compact satellite laser ranging (SLR) system. We focus on the pulse-timing jitter of the MCL. We report on the characteristics of the MCL and its integration with a current SLR system and also provide some preliminary results of experiments with our revised model. We are now developing a dynamic tracking system with a function of reducing the effect of timing jitter in the signal from a Q-sw laser to less than 1 us. Poster paper; received October 04, 2002 |
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Last Updated:
October 4, 2002
