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NASA NTRS1977

NASA ground-based and space-based laser ranging systems

The development of NASA laser ranging systems is discussed with reference to applications such as orbit determination, gravity-field studies, and analyses of polar motion, earth tides, and tectonic plate motion. Several laser ranging designs are described noting the first satellite laser ranging experiment, the Moblas II and III systems, and the laser ranging satellites presently in orbit. Primary error sources are identified as atmospheric delay, optical signal-to-noise ratios, the electron multiplier, time-interval measurements, and target noise. Plans for Shuttle-based laser ranging systems are reviewed noting simulation studies of 12-day Shuttle missions.

Fitzmaurice, M. W.↗

NASA NTRS1979

Characterization of the Q-switched MOBLAS Laser Transmitter and Its Ranging Performance Relative to a PTM Q-switched System

A prototype Q-switched Nd:YAG laser transmitter intended for use in the NASA mobile laser ranging system was subjected to various tests of temporal pulse shape and stability, output energy and stability, beam divergence, and range bias errors. Peak to peak variations in the mean range were as large as 30 cm and drift rates of system bias with time as large as 6 mm per minute of operation were observed. The incorporation of a fast electro-optic cavity dump into the oscillator gave significantly improved results. Reevaluation of the ranging performance after modification showed a reduction in the peak to peak variation in the mean range to the 2 or 3 cm level and a drift rate of system time biases of less than 1 mm per minute of operation. A qualitative physical explanation for the superior performance of cavity dumped lasers is given.

Degnan, J. J., III↗

NASA NTRS1981

Effects of the gaseous and liquid water content of the atmosphere on range delay and Doppler frequency

When high precision is required for range measurement on Earth space paths, it is necessary to correct as accurately as possible for excess range delays due to the dry air, water vapor, and liquid water content of the atmosphere. Calculations based on representative values of atmospheric parameters are useful for illustrating the order of magnitude of the expected delays. Range delay, time delay, and phase delay are simply and directly related. Doppler frequency variations or noise are proportional to the time rate of change of excess range delay. Tropospheric effects were examined as part of an overall consideration of the capability of precision two way ranging and Doppler systems.

Flock, W. L.↗

NASA NTRS1983

Video Target Tracking and Ranging System

Proposed target tracking and ranging system uses two automatic video target trackers to keep two TV cameras trained on object being tracked. Microcomputer calculates range and range-rate information by triangulation. Input data for calculation are position coordinates of two cameras and pan and tilt aiming angles of two cameras. System is useful for target ranging at distances up to about 1,000 feet (300 m) in such applications as vehicle collision avoidance, traffic monitoring and surveillance. Also substitutes for short-range radar in situations where radar signal can not be tolerated.

Freedman, L. A.↗

NASA NTRS1983

A FMCW Radar Ranging Device for the Teleoperator Maneuvering System

A frequency-modulated continuous wave radar system is under development in the Communications Systems Branch of the Information and Electronic Systems Laboratory at Marshall Space Flight Center. The radar unit is being designed for use on the teleoperator maneuvering system. Its function is to provide millimeter-level accuracy in range and range rate measurements out to a range of thirty meters. This will facilitate soft docking with accuracy. This report is an updating of previous developments reported on this system. An innovation in the system is the utilization of a standard reference signal generated by shunting a portion of the radar energy into a shorted coaxial delay line. The regular radar target return signal is constantly compared with the reference signal to provide internal error compensation. Within a five meter range, a limit imposed by present laboratory dimensions, the radar system exhibits reliable accuracy with range error less than 0.2%.

Mcdonald, M. W.↗

NASA NTRS1986

Performance of NASA laser ranging systems during MERIT

The performances of the NASA satellite laser ranging (SLR) systems operating during the 1983-1984 MERIT campaign (SAO 1-2, Moblas 1-8, Hollas, MLRS, TLRS 1-2, and NLRS) are compared. Data for the single shot rms precisions and ranges per pass are given for these systems after most of them were upgraded with Quantel lasers. The Moblas 4-8 and the Hollas systems operated with high signal strength returns; single shot rms precisions of 3.5 to 4 cm and average ranges per pass of 2404 to 2606 points were achieved as compared with the values of 7.5 and 756, respectively, for the TLRS-2 system. In addition to the SLR systems, the NASA lunar laser ranging systems (McDonald 2.7M, MLRS, and NLRS) and their capabilities are discussed together with the history of the lunar laser ranging data quality.

Coates, Robert J.↗

NASA NTRS1989

Dynamic-Range Compression For Infrared Imagery

Photorefractive crystals covering detectors prevent saturation. To make full use of information in image, desirable to compress dynamic range of input intensity to within region of approximately linear response of detector. Dynamic-range compression exhibited by measurements of attenuation in photorefractive GaAs. Effective dynamic-range-compressor plate, film, or coating reduces apparent contrast of scene imaged on detector plane to within dynamic range of detectors; original image contrast or intensity data recovered subsequently in electronic image processing because range-compression function and inverse known.

Cheng, Li-Jen↗

NASA NTRS1987

Discrete filtering techniques applied to sequential GPS range measurements

The basic navigation solution is described for position and velocity based on range and delta range (Doppler) measurements from NAVSTAR Global Positioning System satellites. The application of discrete filtering techniques is examined to reduce the white noise distortions on the sequential range measurements. A second order (position and velocity states) Kalman filter is implemented to obtain smoothed estimates of range by filtering the dynamics of the signal from each satellite separately. Test results using a simulated GPS receiver show a steady-state noise reduction, the input noise variance divided by the output noise variance, of a factor of four. Recommendations for further noise reduction based on higher order Kalman filters or additional delta range measurements are included.

Vangraas, Frank↗

NASA NTRS1990

Kalman filter based range estimation for autonomous navigation using imaging sensors

Rotorcraft operating in high-threat environments fly close to the surface of the earth to utilize surrounding terrain, vegetation, or man-made objects to minimize the risk of being detected by the enemy. Two basic requirements for obstacle avoidance are detection and range estimation of the object from the current rotorcraft position. There are many approaches to the estimation of range using a sequence of images. The approach used in this analysis differes from previous methods in two significant ways: an attempt is not made to estimate the rotorcraft's motion from the images; and the interest lies in recursive algorithms. The rotorcraft parameters are assumed to be computed using an onboard inertial navigation system. Given a sequence of images, using image-object differential equations, a Kalman filter (Sridhar and Phatak, 1988) can be used to estimate both the relative coordinates and the earth coordinates of the objects on the ground. The Kalman filter can also be used in a predictive mode to track features in the images, leading to a significant reduction of search effort in the feature extraction step of the algorithm. The purpose is to summarize early results obtained in extending the Kalman filter for use with actual image sequences. The experience gained from the application of this algorithm to real images is very valuable and is a necessary step before proceeding to the estimation of range during low-altitude curvilinear flight. A simple recursive method is presented to estimate range to objects using a sequence of images. The method produces good range estimates using real images in a laboratory set up and needs to be evaluated further using several different image sequences to test its robustness. The feature generation part of the algorithm requires further refinement on the strategies to limit the number of features (Sridhar and Phatak, 1989). The extension of the work reported here to curvilinear flight may require the use of the extended Kalman filter.

Sridhar, Banavar↗

NASA NTRS1990

Flight data acquisition methodology for validation of passive ranging algorithms for obstacle avoidance

The automation of low-altitude rotorcraft flight depends on the ability to detect, locate, and navigate around obstacles lying in the rotorcraft's intended flightpath. Computer vision techniques provide a passive method of obstacle detection and range estimation, for obstacle avoidance. Several algorithms based on computer vision methods have been developed for this purpose using laboratory data; however, further development and validation of candidate algorithms require data collected from rotorcraft flight. A data base containing low-altitude imagery augmented with the rotorcraft and sensor parameters required for passive range estimation is not readily available. Here, the emphasis is on the methodology used to develop such a data base from flight-test data consisting of imagery, rotorcraft and sensor parameters, and ground-truth range measurements. As part of the data preparation, a technique for obtaining the sensor calibration parameters is described. The data base will enable the further development of algorithms for computer vision-based obstacle detection and passive range estimation, as well as provide a benchmark for verification of range estimates against ground-truth measurements.

Smith, Phillip N.↗

NASA NTRS1991

Tunable 2.1-micron Ho lidar for simultaneous range-resolved measurements of atmospheric water vapor and aerosol backscatter profiles

An eye-safe tunable differential-absorption lidar system has been developed for the range-resolved measurement of aerosol backscatter and water vapor in the atmosphere. The lidar uses a flash-lamp-pumped, Q-switched, 10-mJ solid-state Ho:YSGG laser that is continuously tunable over a 20/cm wavelength range near 2.084 microns. Both path-averaged and range-resolved measurements were performed with the Ho differential-absorption lidar system. Preliminary measurements have been made of the temporal variation of atmospheric aerosol backscatter and water-vapor profiles at ranges out to 1 km. These results indicate that the Ho lidar has the potential for the eye-safe remote sensing of atmospheric water vapor and backscatter profiles at longer ranges if suitably enhanced in laser power and laser linewidth.

Cha, Sungdo↗

NASA NTRS1992

Critical range evaluation using a diagonal flat plate

A novel technique is presented to image stray signals in RCS measurement ranges. In this technique, the scattered fields of a flat plate in the diagonal plane are measured in a range for various frequencies and look angles. The scattered field data is then processed to generate an inverse synthetic aperture radar (ISAR) image of the diagonal flat plate. It is shown that scattering mechanisms associated with various stray signals can be identified by using the ISAR image. This leads to better understanding of the range and possible chamber improvements. Scattering mechanisms as small as 90 dB below the plate broadside scattered field level have been observed in the image domain due to the high directivity and low sidelobe characteristics associated with the diagonal flat plate backscattered fields. The results obtained from evaluating the two compact range facilities at The Ohio State University ElectroScience Laboratory are presented to illustrate the virtues of this new range evaluation technique.

Lee, Teh-Hong↗

NASA NTRS1991

A fast algorithm for image-based ranging

Image-based ranging has emerged as a critical issue in the low altitude operation of flight vehicles such as rotorcraft and planetary landers. These flight regimes require ranging systems for recovering the geometry of the terrain and obstacles for use with guidance algorithms. The development of a ranging equation combining image irradiance together with various order spatial partial derivatives and the vehicle motion parameters is discussed. The ranging equation is in the form of a polynomial in scene depth. Two-dimensional linear filters are then used to compute the coefficients of this polynomial to result in a fast image-based ranging algorithm. Performance of the algorithm is demonstrated using laboratory images.

Menon, P. K. A.↗

NASA NTRS1952

Effect of fuel density and heating value on ram-jet airplane range

An analytical investigation of the effects of fuel density and heating value on the cruising range of a ram-jet airplane was made. Results indicate that with present-day knowledge of chemical fuels, neither very high nor very low fuel densities have any advantages for long-range flight. Of the fuels investigated, the borohydrides and metallic boron have the greatest range potential. Aluminum and aluminum hydrocarbon slurries were inferior to pure hydrocarbon fuel and boron-hydrocarbon slurries were superior on a range basis. It was concluded that the practical difficulties associated with the use of liquid hydrogen fuel cannot be justified on a range basis.

MISSILES↗

NASA NTRS1993

Sequential ranging: How it works

This publication is directed to the users of data from the Sequential Ranging Assembly (SRA), and to others who have a general interest in range measurements. It covers the hardware, the software, and the processes used in acquiring range data; it does not cover analytical aspects such as the theory of modulation, detection, noise spectral density, and other highly technical subjects. In other words, it covers how ranging is done, but not the details of why it works. The publication also includes an appendix that gives a brief discussion of PN ranging, a capability now under development.

Baugh, Harold W.↗

NASA NTRS1993

Eighth International Workshop on Laser Ranging Instrumentation

The Eighth International Workshop for Laser Ranging Instrumentation was held in Annapolis, Maryland in May 1992, and was sponsored by the NASA Goddard Space Flight Center in Greenbelt, Maryland. The workshop is held once every 2 to 3 years under differing institutional sponsorship and provides a forum for participants to exchange information on the latest developments in satellite and lunar laser ranging hardware, software, science applications, and data analysis techniques. The satellite laser ranging (SLR) technique provides sub-centimeter precision range measurements to artificial satellites and the Moon. The data has application to a wide range of Earth and lunar science issues including precise orbit determination, terrestrial reference frames, geodesy, geodynamics, oceanography, time transfer, lunar dynamics, gravity and relativity.

Degnan, John J.↗

NASA NTRS1993

Two color satellite laser ranging upgrades at Goddard's 1.2m telescope facility

The ranging laboratory at Goddard's 1.2 m telescope tracking facility has recently been upgraded to include a single photoelectron sensitive Hamamatsu streak camera-based range receiver which uses doubled and tripled Nd:YAG frequencies for satellite laser ranging. Other ranging system upgrades include a new continuum laser, which will deliver up to 30 millijoules (mJ) at both 532 and 355 nm at a pulsewidth of 30 picoseconds (FWHM), and replacement of both ranging and tracking computers with COMPAQ 386 based systems. Preliminary results using a photomultiplier-tube based receiver and waveform digitizer indicate agreement within the accuracy of the measurement with the theoretical Marini and Murray model for atmospheric refraction. Two color streak camera measurements are used to further analyze the accuracy of these and other atmospheric refraction models.

Zagwodzki, Thomas W.↗

NASA NTRS1993

Two wavelength satellite laser ranging using SPAD

When ranging to satellites with lasers, there are several principal contributions to the error budget: from the laser ranging system on the ground, from the satellite retroarray geometry, and from the atmosphere. Using a single wavelength, we have routinely achieved a ranging precision of 8 millimeters when ranging to the ERS-1 and Starlette satellites. The systematic error of the atmosphere, assuming the existing dispersion models, is expected to be of the order of 1 cm. Multiple wavelengths ranging might contribute to the refinement of the existing models. Taking into account the energy balance, the existing picosecond lasers and the existing receiver and detection technology, several pairs or multiple wavelengths may be considered. To be able to improve the atmospheric models to the subcentimeter accuracy level, the differential time interval (DTI) has to be determined within a few picoseconds depending on the selected wavelength pair. There exist several projects based on picosecond lasers as transmitters and on two types of detection techniques: one is based on photodetectors, like photomultipliers or photodiodes connected to the time interval meters. Another technique is based on the use of a streak camera as an echo signal detector, temporal analyzer, and time interval vernier. The temporal analysis at a single wavelength using the streak camera showed the complexity of the problem.

Prochazka, Ivan↗