Single photon sensitive 3D imaging lidars have multiple advantages relative to conventional multiphoton lidars. They are
the most efficient 3D imagers possible since each range measurement requires only one detected photon as opposed to
hundreds or thousands in conventional laser pulse time of flight (TOF) or waveform altimeters. Their high efficiency
enables orders of magnitude more imaging capability (e.g. higher spatial resolution, larger swaths and greater areal
coverage). In our Single Photon Lidars (SPLs), single photon sensitivity is combined with nanosecond recovery times
and a multistop timing capability, This enables our lidars to penetrate semiporous obscurations such as vegetation,
ground fog, thin clouds, etc. Furthermore, the 532 nm operating wavelength is highly transmissive in water, thereby
permitting shallow water bathymetry and 3D underwater imaging.
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The first space application of photon-counting lidars occurred shortly after the landing of Apollo 11 on the
Moon. Various scientific groups used lasers to range to retroreflector panels left on the lunar surface by the
astronauts. Because of the great distances involved (384,000 km one-way) and practical limitations on laser
energy and telescope aperture, the detected signals were necessarily at the single photon level. The Lunar
Laser Ranging (LLR) effort has continued uninterrupted over the past four decades and has allowed
scientists to study solar system dynamics, Earth-lunar interactions, and lunar properties. It has also served
as a testbed for relativistic theories. Since the mid-1990's, the author has applied the photon-counting
technique to a number of new space applications. These include: (1) an eyesafe satellite laser ranging
system which presently tracks high altitude (6000 km) satellites with sub-cm precision at kHz rates with
only 60 microjoules of transmitted energy; (2) an airborne, high resolution 3D imaging lidar which operates
day or night and can be scaled to globally and contiguously map extraterrestrial moons from 100 km orbits;
(3) an upcoming NASA mission for mapping the Earth's surface in 3D from a 600 km orbit; and (4)
interplanetary ranging and time transfer via two-way laser transponders. The present paper provides an
overview of these efforts.
Sigma Space Corporation has recently developed a compact 3D imaging and polarimetric lidar suitable
for use in a small aircraft or mini-UAV. A frequency-doubled Nd:YAG microchip laser generates 6
microjoule, subnanosecond pulses at fire rates up to 22 kHz. A Diffractive Optical Element (DOE)
breaks the 532 nm beam into a 10x10 array of Gaussian beamlets, each containing about 1 mW of laser
power (50 nJ @ 20 kHz). The reflected radiation in each beamlet is imaged by the receive optics onto
individual pixels of a high efficiency, 10x10 pixel, multistop detector. Each pixel is then input to one
channel of a 100 channel, multistop timer demonstrated to have a 93 picosecond timing (1.4 cm range)
resolution and an event recovery time of only 1.6 nsec. Thus, each green laser pulse produces a 100
pixel volumetric 3D image. The residual infrared energy at 1064 nm is used for polarimetry. The scan
pattern and frequency of a dual wedge optical scanner, synchronized to the laser fire rate, are tailored
to provide contiguous coverage of a ground scene in a single overflight. In both rooftop and
preliminary flight tests, the lidar has produced high spatial resolution 3D images of terrain, buildings,
tree structures, power lines, and bridges with a data acquisition rate up to 2.2 million multistop 3D
pixels per second. Current tests are aimed at defining the lidar's ability to image through water
columns and tree canopies.
The first successful photon-counting airborne laser altimeter was demonstrated in 2001 under NASA's
Instrument Incubator Program (IIP). This "micro-altimeter" flew at altitudes up to 22,000 ft (6.7 km)
and, using single photon returns in daylight, successfully recorded high resolution images of the
underlying topography including soil, low-lying vegetation, tree canopies, water surfaces, man-made
structures, ocean waves, and moving vehicles. The lidar, which operated at a wavelength of 532 nm
near the peak of the solar irradiance curve, was also able to see the underlying terrain through trees
and thick atmospheric haze and performed shallow water bathymetry to depths of a few meters over
the Atlantic Ocean and Assawoman Bay off the Virginia coast.
Sigma Space Corporation has recently developed second generation systems suitable for use in a small
aircraft or mini UAV. A frequency-doubled Nd:YAG microchip laser generates few microjoule,
subnanosecond pulses at fire rates up to 22 kHz. A Diffractive Optical Element (DOE) breaks the
transmit beam into a 10x10 array of quasi-uniform spots which are imaged by the receive optics onto
individual anodes of a high efficiency 10x10 GaAsP segmented anode microchannel plate
photomultiplier. Each anode is input to one channel of a 100 channel, multistop timer demonstrated to
have a 100 picosecond timing (1.5 cm range) resolution and an event recovery time less than 2 nsec.
The pattern and frequency of a dual wedge optical scanner, synchronized to the laser fire rate, are
tailored to provide contiguous coverage of a ground scene in a single overflight.
The present paper reports on the design and performance of a scanning, photon-counting laser altimeter, capable of daylight operations from aircraft cruise altitudes. In test flights, the system has successfully recorded high repetition rate returns from clouds, soils, man-made objects, vegetation, and water surfaces under full solar illumination. Following the flights, the signal was reliably extracted from the solar noise background using a Post- Detection Poisson Filtering technique. The passively Q-switched microchip Nd:YAG laser 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 produces several microjoules of green energy in a subnanosecond pulse at rates approaching 10 kHz. The illuminated ground area is imaged by a 14-cm diameter, diffraction-limited, off-axis telescope onto a segmented anode photomultiplier. Each anode segment is input to one channel of fine range receiver (5-cm resolution), which records the times-of-flight of individual photons. A parallel coarse receiver provides a lower resolution (greater than 75 cm) histogram of all scatterers between the aircraft and ground and centers the fine receiver gate on the last set of returns.
The lunar laser ranging (LLR) has been dependent on the retro-reflectors placed on the Moon since the past three decades before. In spite of the technical improvement, regular observations are performed by a limited number of stations due to the weak echo. To allow more opportunities for observations, it is most effective to place modern device on the Moon. Since a Japanese second lunar probe is planned to be launched in 2006, the ideas are collected for scientific purposes. It is described here that an optical transponder on the Moon would enhance the possibility of LLR observations to a large degree. It is also proposed to have three radio sources on the Moon to determine the angular component perpendicular to the Earth-Moon vector and librations. One of the most important scientific targets for the proposed mission is a relativistic experiment which was not attained by a conventional LLR.
SLR2000 is an autonomous and eyesafe single photon-counting satellite laser ranging station with an expected single shot range precision of about one centimeter and a normal point precision better than 3 mm. The system will provide continuous 24 hour tracking coverage. Replication costs are expected to be roughly an order of magnitude less than that of current manned systems, and the system will be about 75% less expensive to operate and maintain relative to the current manned systems. Computer simulations have predicted a daylight tracking capability to GPS and lower satellites. Computer and hardware simulations have demonstrated the ability of our current correlation range receiver and autotracking algorithms to extract mean signal strengths as small as 0.0001 photoelectrons per pulse from solar background noise during daylight tracking. The initial SLR2000 system concept was developed in 1994 , and the technical approach was refined in later years . However, significant funding for the project was not provided by NASA until August 1997. During the first year of funding, prototypes of several "enabling' components, without which the system is not feasible, were successfully developed. These include: (1) a sensitive, high speed, quadrant microchannel plate photomultiplier; (2) a moderate power microlaser transmitter; (3) a "smart" meteorological station; (4) a high speed range gate generator; and (5) a high speed, high resolution event timer. Once the key specifications on these advanced components were largely met and system feasibility had been established, attention then turned to the detailed engineering design and procurement of more conventional elements of the system such as the shelter and protective dome, arcsecond precision tracking mount, telescope, and optical transceiver. The principal challenge during this second phase was to keep prototype fabrication and replication costs as low as possible to meet our cost goals. Prototypes of the various SLR2000 components and subsystems have either been developed or are well into the detailed design! build phase. The system is scheduled to conduct field tests in the 2000-2001 time frame. The primary driver for schedule is a fixed level of funding available each year to support SLR2000 development. A fairly detailed engineering overview of the SLR2000 system was presented approximately one year ago at the 1 1th International Workshop on Laser Ranging in Deggendorf, Germany, and has recently been published in the Workshop Proceedings [31. In addition, the SLR2000 project maintains a web site at the following URL address: http://cddisa.gsfc.nasa.gov/920_3/slr2000/slr2000.html Thus, only a brief overview of engineering status and a summary of recent developments (i.e. within the past year) on the various subsystems will be given here. The reader is referred to earlier publications [1-3] for more detail on the overall system.
Satellite range measurements which include differential time of flight measurement between simultaneous doubled and tripled Nd:YAG laser pulses are being made at the NASA/Goddard Space Flight Center's 1.2m telescope tracking facility. A description of the streak camera-based range receiver is given along with the differential time of flight measurement. Typical streak camera return waveforms are displayed from satellite tracks which include: ADEOS/RIS, AJISAI, GFZ-1, STELLA, and TOPEX.
SLR2000 is an autonomous and eyesafesatellite laser ranging station with an expected single shot range precision of about one centimeter and a normal point precision better than 3 mm. The system will provide continuous 24 hour tracking coverage. Replication costs are expected to be roughly an order of magnitude less than current operational systems, and the system will be about 75% less expensive to operate and maintain relative to the manned systems. Computer simulations have predicted a daylight tracking capability to GPS and lower satellites with telescope apertures of 40 cm and have demonstrated the ability of our current autotracking algorithm to extract mean signal strengths as small as 0.000 I photoelectrons per pulse from background noise. The dominant cost driver in present SLR systems is the onsite and central infrastructure manpower required to operate the system, to service and maintain the complex subsystems (most notably the laser and high precision timing electronics), and to ensure that the transmitted laser beam is not a hazard to onsite personnel or to overflying aircraft. In designing the SLR2000 system, preference was given to simple hardware over complex, to commercially available hardware over custom, and to passive techniques over active resulting in the prototype design described here. This general approach should allow long intervals between maintenance visits and the "outsourcing" of key central engineering functions on an "as needed" basis. As a result, many of the signal extraction techniques and engineering designs employed here may have application in remotely operated or even spacebome lidar applications. SLR2000 consists of seven major subsystems: (1) Time and Frequency Reference Unit; (2) Optical Subsystem; (3) Tracking Mount; (4) Correlation Range Receiver; (5) Meteorological Station; (6) Environmental Shelter with Azimuth Tracking Dome; and (7) System Controller. The Optical Subsystem in tum consists of a 40 cm aperture telescope and associated transmit/receive optics, a passively Q-switched microlaser operating at 2 KHz with a transmitted single pulse energy of 135 μJ, a start detector, a quadrant stop detector for simultaneous ranging and subarcsecond angle tracking, a CCD camera for automated star calibrations, and spectral and spatial filters to reduce the daylight background noise. The meteorological station includes sensors for surface pressure, temperature, relative humidity, wind speed and direction, precipitation type and accumulation, visibility, and cloud cover. The system operator is replaced by a software package called the "pseudooperator" which, using a variety of sensor inputs, makes all of the critical operational decisions formerly made by onsite personnel. Keywords: laser ranging, microlasers, laser altimetry, lidar, single photon detection, meteorological instrumentation, satellites, autonomous instruments
Since its first successful demonstration in 1964, the precision of Satellite Laser Ranging (SLR) has improved by three orders of magnitude, i.e., from a few meters to a few mm. Each technological improvement has been rapidly followed by a new scientific capability. To date, centimeter accuracy SLR measurements to the passive LAGEOS satellites by a ground-based network of approximately 40 stations have; (1) helped to define a Terrestrial Reference Frame accurate to a centimeter globally; (2) measured the motions of tectonic plates and detected regional crustal deformations near the plate boundaries; (3) helped define the terrestrial gravity field; and (4) monitored variations in the Earth's gravity field, the orientation of the Earth's spin axis, and its rate of rotation and related them to angular momentum exchanges and/or large mass redistribution within the land, ocean, atmosphere system. By providing few centimeter precision orbits measurement to altimetric satellites such as ERS-1 and 2 and TOPEX/POSEIDON, SLR has enabled precise measurements of global ocean, circulation, wave heights, ice topography, and even mean sea level rises on the order to two mm/yr. In addition to providing useful test of general relativity, centimeter accuracy SLR measurements to five retroreflector packages placed on the lunar surface by US and Soviet landers have helped to define the planetary reference frame, provided ultraprecise lunar ephemerides, defined the lunar librations, and constrained models of the Moon's internal structure.
NASA's Satellite Laser Ranging Network was originally developed during the 1970's to track satellites carrying corner cube reflectors. Today eight NASA systems, achieving millimeter ranging precision, are part of a global network of more than 40 stations that track 17 international satellites. To meet the tracking demands of a steadily growing satellite constellation within existing resources, NASA is embarking on a major automation program. While manpower on the current systems will be reduced to a single operator, the fully automated SLR2000 system is being designed to operate for months without human intervention. Because SLR2000 must be eyesafe and operate in daylight, tracking is often performed in a low probability of detection and high noise environment. The goal is to automatically select the satellite, setup the tracking and ranging hardware, verify acquisition, and close the tracking loop to optimize data yield. TO accomplish the autotracking tasks, we are investigating (1) improved satellite force models, (2) more frequent updates of orbital ephemerides, (3) lunar laser ranging data processing techniques to distinguish satellite returns from noise, and (4) angular detection and search techniques to acquire the satellite. A Monte Carlo simulator has been developed to allow optimization of the autotracking algorithms by modeling the relevant system errors and then checking performance against system truth. A combination of simulator and preliminary field results will be presented.
The study reviews the research and development of a prototype laser used to study one possible method of short-pulse production and amplification, in particular, a pulsed Nd:YAG ring laser pumped by laser diode arrays and injected seeded by a 100-ps source. The diode array pumped, regenerative amplifier consists of only five optical elements, two mirrors, one thin film polarizer, one Nd:YAG crystal, and one pockels cell. The pockels cell performed both as a Q-switch and a cavity dumper for amplified pulse ejection through the thin film polarizer. The total optical efficiency was low principally due to the low gain provided by the 2-bar pumped laser head. After comparison with a computer model, a real seed threshold of about 10 exp -15 J was achieved because only about 0.1 percent of the injected energy mode-matched with the ring.
The design, implementation, and evaluation of a high-resolution vidicon-based reconfigurable imaging system for integration into a photon-counting streak camera that can be readily coupled to a standard interface and computer have been achieved. Experimental results are reported which demonstrate that the design goals are met, providing the capability to measure differential time to better than 3 picosecond accuracy. Augmented by real-time calibration, the accuracy, linearity, noise levels, and stability of the system are adequate to support dual wavelength laser ranging.
Satellite laser ranging (SLR) has been used for over two decades in the study of a variety of geophysical phenomena, including global tectonic plate motion, regional crustal deformation near plate boundaries, Earth''s gravity field, the orientation of its polar axis and rate of spin, lunar dynamics and general relativistic studies. The subcentimeter precision of the technique is now attracting the attention of a new community of scientists, notably those interested in high- resolution ocean, ice, and land topography. Over the next several years, the international SLR network will provide an essential link between the geocentric terrestrial reference frame (as presently defined by the international VLBI and SLR networks) and two new oceanographic satellites, ERS-1 and TOPEX-Poseidon, which will range to sea and ice surfaces using microwave altimeters. The combined SLR/altimetry data set will provide precise orbits, improved gravity models, and estimates of the marine geoid. The latter are necessary to infer the dynamic sea surface topography and will enable measurements of parameters important to an understanding of global change, such as mean sea level and ice sheet thickness. Laser tracking of oceanographic satellites from multiple sites as they overfly special calibration towers equipped with tide gauges will also provide periodic estimates of microwave altimeter bias. The few-centimeter precision orbits determined by the SLR network will be used as ''ground truth'' data in the intercomparison and performance evaluation of developmental space radio-navigation systems such as GPS (TOPEX/Poseidon) and PRARE (ERS-1). Future spaceborne two-color SLR instruments, such as NASA''s geoscience laser ranging system (GLRS), can monitor the tectonically-induced motions of tide gauges by bouncing laser pulses off of collocated retroreflectors. Similar systems can measure the barometric loading over the open ocean. When used as transmitters in spaceborne or airborne altimeters, the narrow beamwidths and short pulsewidths available from lasers can provide high spatial resolution (both horizontal and vertical) topographic data over land and ice in support of a diverse set of science applications.
A demonstration-prototype CO2-laser heterodyne spectrometer operating at 9-12 microns and suitable for long-term space missions is described and illustrated with extensive diagrams, drawings, photographs, and graphs of test performance data. The spectrometer has total volume 0.63 cu m, mass 30 kg, and power requirement 60-70 W, compatible with miniature-class Space Shuttle experiment payload specifications. It comprises three modules: (1) an optical front end with reflecting optics, a 2-GHz BW HgCdTe photomixer, and a 0-2-GHz 40-dB RF preamplifier; (2) a local oscillator with an RF-excited waveguide CO2 laser, a 75-percent-efficiency RF amplifier, a stepper-driven grating mode selector, and an etalon stabilized for over 30,000 h of use; and (3) an RF-filter-bank spectral-line receiver with a 25-MHz RF channel, 1.6-GHz IF spectral coverage, onboard instrument control, a serial link to the host computer, and highly integrated design.
A billion-shot flashlamp developed under a NASA contract for spaceborne laser missions is presented. Lifetime-limiting mechanisms are identified and addressed. Two energy loadings of 15 and 44 Joules were selected for the initial accelerated life testing. A fluorescence-efficiency test station was used for measuring the useful-light output degradation of the lamps. The design characteristics meeting NASA specifications are outlined. Attention is focused on the physical properties of tungsten-matrix cathodes, the chemistry of dispenser cathodes, and anode degradation. It is reported that out of the total 83 lamps tested in the program, 4 lamps reached a billion shots and one lamp is beyond 1.7 billion shots, while at 44 Joules, 4 lamps went beyond 100 million shots and one lamp reached 500 million shots.