Timothy D. Cole

Laser radar instrument for the Near-Earth Asteroid Rendezvous (NEAR) mission

In 1999 after a 3-year transit, the Near-Earth Asteroid Rendezvous (NEAR) spacecraft will enter a low-altitude (approximately 50 km) orbit about the asteroid, 433 Eros. Five instruments, including a laser radar, will operate continuously during the one-year orbit at Eros. The NEAR laser rangefinder (NLR), developed at the Applied Physics Laboratory (APL), is a robust rangefinder and the first spaceborne altimeter to have continuous inflight calibration capability. A bistatic configuration, the NLR uses a diode- pumped Cr:Nd:YAG transmitter and a leading-edge receiver with a 3.5-inch aperture Dall-Kirkham telescope. Detection is accomplished using an enhanced-silicon avalanche photodiode. From system tests, the NLR is capable of ranging in excess of 100 km to the asteroids surface. Measurements of the time-of-flight between laser pulse firings and detection of surface backscatter are made using an APL- developed receiver having range resolution of 31.48 cm and accuracy of 2 m. Total mass of the NLR is 4.9 kg and its average power consumption is <EQ 15.1 W. This paper reviews specifications for the NLR instrument, provides overall design details, and presents system performance using prelaunch test results.

Laser rangefinder for the near-earth asteroid rendezvous (NEAR) mission

The near-earth asteroid rendezvous (NEAR) mission is the first of the NASA discovery programs. Discovery-class programs emphasize small, low-cost, quick turnaround space missions that provide significant science returns. The NEAR spacecraft and ground control system are currently being developed and tested at the Applied Physics Laboratory (APL). The NEAR spacecraft will orbit, 433 Eros, possibly the most studied of the near-Earth asteroids. Subsequent to a 3-year cruise, the NEAR spacecraft is inserted into a 50-km-altitude orbit about Eros for 1 year to permit data collection in the infrared, visible, x-ray and gamma-ray regions. One instrument, the NEAR laser rangefinder (NLR), will provide altimetry data useful in characterizing the geophysical nature of Eros. In addition, ranging data from the NLR will support navigation functions associated with spacecraft station-keeping and orbit maintenance. The NLR instrument uniquely applies several technologies for use in space. Our configuration uses a direct-detection, bistatic design employing a gallium arsenide (GaAs) diode-pumped Cr:Nd:YAG laser for the 1.064-micrometer transmitter and an enhanced-silicon avalanche-photodiode (APD) detector for the receiver. Transmitter pulse energy provides the required signal-to-noise power ratio, SNRp, for reliable operation at 50 km. The selected APD exhibited low noise, setting the level achievable for noise equivalent power, NEP, by the receiver. The lithium-niobate (LiNbO3) Q-switched transmitter emits 12-ns pulses at 15.3 mJ/pulse, permitting reliable NLR operation beyond the required 50-km altitude. Cavity aperturing and a 9.3X Galilean telescope reduce beam divergence for high spatial sampling of Eross surface. Our receiver design is an f/3.4 Dall-Kirkham Cassegrain with a 7.62-cm clear aperture -- we emphasized receiver aperture area, Arx, over transmitter power, Pt, in our design based on the range advantage attainable according to the simplified range equation, Rmax equals [(Pt(rho) BArx)/(SNRp NEP)]1/2. Asteroid reflectivity, (rho) B, is estimated to be 0.05 at our wavelength. A reasonable power signal- to-noise ratio for reliable operation, SNRp, was assumed. To minimize our noise equivalent power, NEP, we carefully designed and selected the receiver components. The receiver circuit uses leading-edge detection of the laser backscatter. Our detector circuit is an enhanced-silicon APD hybrid using a video amplifier, an integrating Bessel filter, and a high- speed programmable threshold comparator. We accomplish time-of-flight (TOF) measurements digitally with an APL-designed GaAs application-specific integrated circuit. A radiation-hardened FORTH microprocessor controls range gating, data collection and formatting, and operational modes. Implementation of control and data communications between the spacecraft and rangefinder uses the MIL-STD 1553-bus architecture. Functional testing and calibration indicate exceptional performance; return power levels were reliably detected over several thresholds with 71-dB attenuation, while observed range jitter was equivalent to the resolution determined by the TOF GaAs chip (31.5 cm). This paper discusses NLR performance requirements, design implementation, and qualification testing. It also provides preliminary results from calibration and performance testing.

NEAR laser rangefinder light-weight packaging design

The NEAR laser range finder (NLR) design is a compact, light weight design with a high power laser transmitter and a high performance mirror receiver system. One of the main objectives of the NLR is to provide the in-situ distance measurement from the spacecraft to a near earth asteroid. An on board computer will compile this information to provide necessary navigation requirements for the NEAR satellite. Due to the weight budget constraint, the maximum weight limitation of the NLR has been a critical issue from the beginning of the program. To achieve this goal and meet the system design objectives, innovative designs have been implemented in the development of light weight optical, mechanism, and electronic packaging hardware. This paper provides details of the NLR electronic packaging design, thermal and structural designs.

Analysis of laser radar measurements of the asteroid 433 Eros

After a 5-year mission, a 4-year transit followed by a one-year mission orbiting the asteroid 433 Eros, the Near-Earth Asteroid Rendezvous-Shoemaker (NEAR) spacecraft made a controlled landing onto the asteroids surface on 12 February 2001. Onboard the spacecraft, the NEAR Laser Rangefinder (NLR) facility instrument had gathered over 11 million measurements, providing a spatially dense, high-resolution, topographical map of Eros. This topographic data, combined with Doppler tracking data for the spacecraft, enabled the determination of the asteroids shape, mass, and density thereby contributing to understanding the internal structure and collisional evolution of Eros. NLR data indicate that Eros is a consolidated body with a complex shape dominated by collisions. The offset between the asteroids center of mass and center of figure indicates a small deviation from a homogeneous internal structure that is most simply explained by variations in mechanical structure. Regional-scale relief and slope distributions show evidence for control of some topography by a competent substrate. It was found that pulse dilation was the major source of uncertainty in single-shot range measurements from the NLR, and that this uncertainty remains consistent with the overall 6-m range measurement system accuracy for NEAR. Analysis of NLR data fully quantified the geodynamic nature of this planetesimal, ergo, illustrating the utility of laser altimetry for remote sensing.

Spaceborne laser altimetry

Space-based laser altimeters are effective in providing topographic measurements critically important to the understanding of the formation and early evolution of planetary bodies. Using laser altimetry data, topographic grids can be produced that provide significant insight into the shape, internal structure and evolution of the subject body. Prime examples of space-based altimetry efforts are the Clementine and the Near-Earth Asteroid Rendezvous (NEAR) missions. Clementine spent two months sampling the Moon, and through its altimetry data, provided a glimpse of the lunar surface previously unseen. NEAR will place a laser altimeter (NLR) in orbit at the near-Earth asteroid 433 Eros for a one year observation period. Specifications for such altimeters are driven by mission requirements and host spacecraft constraints. Mission requirements usually prioritize observation objectives associated with other payload instruments, therefore, altimeter design must readily accommodate other payload instruments. Constraints placed on altimeters include mass, power, and volume; also for deep-space missions, data rates are limited and become an issue especially when imaging instruments are part of the mission. Altimeter performance specification and modeling to meet these requirements are described and approaches to verify instrument performance during pre-launch testing are provided. Lessons provided from laser altimetry missions indicate the technological progression to the next-generation laser altimeters.

Qualification of the NEAR laser transmitter

The qualification of the NEAR laser transmitter is discussed with emphasis placed on the three major problem areas encountered: (1) use and derating of discrete power supply components; (2) application of a non-hermetic, high voltage hybrid to the space environment; and (3) vibration testing of the laser optics train. Summary comments are made with respect to the predictability of these quality/reliability problems.

Control software for the Near Earth Asteroid Rendezvous laser rangefinder

The near earth asteroid rendezvous (NEAR) laser rangefinder (NLR), an instrument on the NEAR spacecraft, was designed to measure range from the NEAR spacecraft to the surface of the asteroid 433 EROS. The instrument consists of a laser transmitter, a calibration fiber, an optical receiver, analog electronics, power converting and conditioning electronics, and a digital processing unit. The digital processing unit controls configuration and operation of the transmitter and analog electronics. Software running in the processor handles communication between the spacecraft data bus and the NLR. The software includes functions for command handling, telemetry data formatting and data transfer to the command and data handling computer, transmitter control, measurement of the receiver noise floor, and correction of some timing delays. A brief overview of the software is given along with descriptions of auto-calibration sequences and test results.

Optical system development and performance testing of the NEAR laser rangefinder

The near earth asteroid rendezvous (NEAR) laser rangefinder (NLR) is a bistatic system using a diode-pumped Nd:YAG laser and a Dall-Kirkhamm telescope for a receiver. The NLR is one of a suite of five scientific data gathering instruments on the NEAR spacecraft. The NEAR mission is the first of NASAs Discovery Series of spacecraft. The NLR transmitter emits a 15.6 mJ, 15 ns pulse at 1064 nm. The receiver is capable of reliably detecting return signals from the asteroid as low as 1 fJ per pulse, which corresponds to an average power of 50 nW (20 ns pulse). The development and alignment approach of the bistatic system are discussed. The performance test results of the receiver, transmitter, and integrated rangefinder system are presented. Particular attention is given to the system alignment tests and an open air range verification test.

Pre-launch and post-launch testing of the Near Earth Asteroid Rendezvous (NEAR) laser rangefinder

The NEAR spacecraft was launched on February 17, 1996. Qualification tests conducted on the NEAR laser rangefinder allowed evaluation of the instruments performance and provided calibration data prior to launch. From these data, we were able to determine the system electronic delays, the receiver rangewalk, and the receiver noise floor. The first operational test occurred on April 25, 1996. This post- launch test of the rangefinder verified survival of the instrument and provided data on the calibration parameters listed above. This paper describes these parameters and their significance to rangefinder operations. An interference test was conducted on May 22, 1996. This test allowed engineers to evaluate the effect of laser operations on data from other instruments. The post-launch test and interference are described and the results from these test are presented.

Testing and space qualification of the NEAR laser range finder

The Near Earth Asteroid Rendezvous (NEAR) mission is the first mission of the NASA Discovery Program. The NEAR spacecraft, developed and tested by the Johns Hopkins University Applied Physics Laboratory (JHU/APL), embarked on a four year mission on February 17, 1996. During the three- year cruise phase, the satellite will fly near the asteroid Mathilde and will receive an energy boost during an Earth swing-by in 1998. In 1999 NEAR will begin its year long orbit around the asteroid 433 Eros to collect scientific data using several instruments including an imager, a magnetometer, an X-ray/Gamma-ray detector, and a laser altimeter. The NEAR Laser Rangefinder (NLR) will provide altimetry data for characterizing the topography of Eros from a distance of 42 km. The instrument was designed and tested to meet the requirements of the NEAR space environment. In this paper we review the NLR design, present the test philosophy, highlight the tests, and present test results.