Pergamon

Acra Asironauirca Vol 39, No. 14. pp 303-313, 1996

Copynght $0 1997 Published by Elsewer Saence Ltd Prmted in Great Bntam All rights reserved

PII: SOO94-5765(96)00148-8 0094-5765196 515 00 + 0 00

THE LASER RANGEFINDER ON THE NEAR EARTH ASTEROID RENDEZVOUS SPACECRAFT

Timothy D. Cole, Andrew F. Cheng The Johns Hopkins University Applied Physics Laboratory

Laurel, Maryland 20723-6099

Maria Zuber MIT Department of Earth, Atmospheric and Planetary Science

Cambridge, Massachusetts 02139-4307

David Smith NASA Goddard Space Flight Center

Greenbelt, Maryland

ABSTRACT In 1999, the Near Earth Asteroid Rendezvous (NEAR) mission will place a spacecraft with five facility instruments, including the NEAR Laser Rangefinder (NLR), into a low-altitude (-50 km) orbit about the asteroid 433 Eros. The NLR is a high-fidelity altimeter capable of 3 1.5cm resolution and c2-m accuracy, and will be operate-d continuously while at Eros. The NLR design is quite robust and is the first spaceborne altimeter to have continuous inflight calibration capability. Slant range is measured using precise time-of-flight epochs originating with laser pulses from a solid-state 1.064 pm transmitter to detection of surface backscatter by a phomdiode-based receiver. Total mass of the NLR is 4.9 kg and average power consumption is s15.1 W. This paper describes the design of the NLR instrument and preliminary performance based on prelaunch calibrations and tests. All requirements imposed on the NLR by the NEAR altimetry mission were met. When combined with tracking da@ NLR data will permit detaikd geophysical modeling. The resulting global grid will achieve an absolute accuracy of -1Om to the center of mass and will reveal aspects associated with asteroid evolution and genesis.

Copyright 0 1997 Published by Elsevier Science Ltd

INTRODUCTION On 17 February 1996, the first NASA Discovery mission commenced with the launch of the NEAR spaccc& on a Delta 7925-8 vehicle. Discovery programs are small planetary missions that have very specific technical goals and programmatic guidelines that emphasize small, low-cost, quick-turnaround space missions that provide significant science returns. ‘Ibe objective for the NEAR mission is to answer questions relating to the nature and origin of

near-Earth asteroids (NEAs). These objects are of interest from two perspectives. First, undemtanding such “primitive” bodies provides information relating to the nature of early solar system processes and conditions. It is believed that near-Earth asteroids contain clues to the nature of the building blocks from which the inner planets, including the Earth, were formed. Secondly, NEAs are the primary source of large bodies that have repeatedly collided witi the Earth such as the asteroid that collided 65 million years ago with globally catastrophic results.

The asteroid to be visited is known as 433 Eros, the largest and most observed NEA since its discovery in 1898. Closest approach to Earth by Eros is 0.13 AU (20 million km). Radar and occultation studies estimate Eros as having ellipsoidal dimensions of 36 x 15 x 13 km and a rotational period of 5.27 hours. ‘.2 Doppler-tracking of the orbiting spacecraft will provide necessary information to de&mine the mass distribution (gravity field) of Eros.’ Tracking data, when combined with NLR altimetry data, will provide accurate topography and gravity damts. Assuming constant density, comparison of the NLR- derived datasets with the gravity field resulting from use of the asteroid’s shape model would indicate correlation between surface topography with the interior gravity field, and hence an indication of Eros’ homogeneity. To thoroughly analyze the structure, morphology, and evolution of Eros, precise mapping of altimetry data to the asteroid’s center of m&s is critical to permitting correlation of NLR data to other NEAR instruments. With the anticipated surface coverage of Eros and the expected performance of NLR, the altimetry data& will drastically revolutionize tbe determination of the asteroidal shape. 4

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NEAR LASER RANGEFINDER (NLR) The NLR design is simple yet versatile to support contingencies associated with prolonged operation in the deepspace environment.’ Weight growth, instrument costs, and schedule were continuously aked throughout the l&month instrument development period at APL to meet programmatic as well as technical specifications. Configuration control was maintained at the red-line level, and parts approval included components screened to MlL-STD- 883 (avionics level). Substantial testing at the subsystem and system levels was performed to eliminate failure modes. Where economical in design and cost, critical operations were made functionally redundant, especially those functions that are software implemented.

NLR Design Overview

A significant aspect of the NLR configuration is the fact that the receiver optics act as a direct-detection “photon bucket” -- this drastically simplified the design and development of the optical receiver. Since imaging is not required, the NLR optics could tolerate optical aberrations providing the freedom to select the receiver optical design best suited for low weight and manufacturing ease.

The rangefinder configuration is a bistatic design employing a gallium arsenide (&As) diode.-pumped chromium and neodymium-doped yttrium aluminum garnet (Cr.Nd:YAG) laser transmitter, built by McDonnell-Douglas Corporation6 (St. Louis, MO), and an enhanced-silicon avalanche-photodiode (APD) detector at the focal plane. The YAG laser operates at its primary emission of 1.064 pm (h,) and the receiver uses a narrowband (Ah = 7nm) spectral filter centered at h, to reduce background noise. Figure 1 presents a photo of the NLR flight instrument. The figure shows the various subsystems comprising the NLR: laser resonator (LRA), laser power supply (LPS), laser receiver (optical telescope&ties/door, calibration fiber-optic, analog electronics), medium- voltage power supply (MVPS), and digital processing unit (DPU). Note the separate TX and RX optics (bistatic configuration) which permitted parallel subsystem development. A door on the RX provides protection from contaminates in propellants during cruise phase and from particulates during prelaunch activities. The six windows are AR-coated fused silica and provide SO% of the collection area of the open aperture. In the unlikely event that the pyro release mechanism fails, the NLR will still be capable of performing at the required 50-km altitude.

NLR Design Approach

The approach to the NLR design emphasized receiver aperture area, Am, over transmitter power, Pt , bawd

on the range advantage attainable with apertum diameter according to the simplified range equation,

Rmax = I(& PB Arx)/(SNRP NEP)I l/2 (1)

Estimates of reflectivity, pR, at our wavelength range from 0.05 to 0.20 for Eros, an S-class asteroid. For reliable operation (probability of detection >0.95 with low false alarm, <10e6) a minimum signal-to- noise ratio, SNRp, of 18 dl3 is assumed.6 Using these values, estimates of Rmax are in the 100-150 km range. The lower estimate is a result of using Webb’s approximation to the current output of the APD device whereas the larger result came from radiometric analyses. ’

Figure 1. Photograph of NLR instrument showing major subsystems: laser transmitter (LRA, LPS). receiver (telescope, baffles, door/windows, analog section), digital processing unit (DPU), medium-voltage power supply (MVPS) and the calibration fiber-optic. Not shown is the fiber-optic spool that supports a 100-m length of fiber. Note the bistatic (separate TX and RX) design. The two power supplies were heavily filtered, shielded and located separate from the NLR on the spacecraft to minimize electrical interference to the NLR and other NEAR instruments.

Although the NLR design uses straightforward time- of-flight (TOF) range determination (i.e., direct- detection), existence of continuous calibration and digital processing provided the NLR with rather impressive and unique capabilities.’ Calibration is accomplished by optically sampling each transmitted pulse and injecting it into a 100-m fiber-optic spool. This sampled pulse is directed into the receiver optics to provide “true” end-to-end calibration. This feature proved to be a valuable debugging tool throughout integration and test. It permits detection of rangewalk due to threshold-level changes and oscillator drift throughout the mission.

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HU ADS_90 SPACECRAFT PWR (33.5 V$C’ I A

Thermal Mon.

& Cntrl Beam 9.3x Expander

FIBER-OPTIC DELAY ASSEMBLY, FODA

hybnd

Baffles I-

Htr I

305

Figure 2. Block diagram of the NLR instrument depicting the subsystems and functional signal interfaces. The transmitter (TX) and receiver (RX) run asynchronously. Laser pulses are produced at one of four prescribed repetition rates (l/8, 1,2, and 8 Hz) and the RX responds to each transmitted pulse via the fixed-delay signal (calibration signal) and backscatter from Eros’ surface.

The calibration capability also permits evaluation of NLR functionality throughout the cruise phase.

Digital processing permits selection of thresholds and variation in range-gate length to thwart noise floor increase and T-O noise while maintaining receiver sensitivity. “T-O” is the time the optical transmit pulse is generated by the laser. The attendant current surge(s) and/or the presence of a large optical signal in proximity of a sensitive detector, combine to produce a short-term increase in receiver noise. T-O gating eliminates this error source similar to how range-gating discriminates target backscatter from clutter. Figure 2 illustrates the block diagram of the NLR altimeter. The figure presents the subsystems and provides the functional signal interfaces. In the figure, Htr represents heater power and control to maintain survival and operable temperatures.

NLR operation can be traced through use of Figure 2. A FIRE command is issued by the DPU at a selected pulse repetition frequency, PRF (118, 1, 2, 8 Hz). The FIRE command enables the TX, ftres the diode-pump, and produces an optical pulse -192 ps subsequent to TX enable. As the optical pulse is emitted, a portion is simultaneously directed into the Fiber-Optic Delay Assembly (FODA). A START signal is generated by detecting an optical pulse within the TX that starts the TOF counters (range and calibration).r After a fixed delay, a pulse emerges and is directed through the receiver optics to the detector. This produces a STOP signal that halts the calibration counter. The emitted pulse from the TX illuminates the surface of Eros and is backscattered towards the spacecraft. The RX telescope intercepts a portion of this return signal and if sufficiently bright, triggers the comparator and produces a STOP signal which halts the range counter. The DPU then reads the

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content of the counters, formats the results, and resets the counters for the subsequent measurement.

Transmitter Subsystem (TX) 9 The solid-state l.Of%+tm transmitter consists of two subsystems: the laser resonator assembly (LRA) and the laser power supply (LPS). These two subsystems were designed simultaneously and derivatives have been successfully used in previous space programs, including BP, MOLA, Clementine, and STS-72 (SLA-1). Modifications were necessary for NEAR to reduce power consumption and to meet NLR specifications, but the underlying design (and many components) went unchanged.

1. Laser resonator assembly (LRA) The LRA is a proven polarization-coupled U-cavity design. Figure 3 is a photograph of the NEAR LRA. An internal resonator aperture reduces high-order modes and an external beam expander (9.3X Galilean telescope) reduces angular divergence to 235 prad (in vacuum). The resonator uses a 20-bar GaAs diode array to optically side-pump a Cr:Nd:YAG zigzag slab. Through this zigzag pumping, the optical energy is uniformly distributed throughout the YAG medium, thereby increasing absorption path length. The slab is located in a cross-porro cavity to provide boresight stability. Bach porro prism assembly includes a Risley wedge to allow for beam alignment of the standing-wave cavity during assembly.

POLARJZNGCUBE

-I uJKNk.K cult. _

QNd:YAG dab

PUtROPRISM

93XGALuEAN

IiNK QSWlTtIX

mRROPlusM

Figure 3. LRA optical component configuration.

The laser is operated in Q-switch mode using a lithium niobate (LiNbO3) Pockel cell. Polarization compensation is accomplished in the Q-switch leg by a half waveplate. At its output, the resonator produces 12-ns optical pulses (FWHM) at energies -15.3 mJ as exemplified in Figure 4. LiNb03 is pyroelectric and in space requires attention to regarding control of thermal gradients during operation.

Figure 4. Temporal response of LR4 optical pulse.

2. Laser power supply (LPS) The LPS provides the control logic and power necessary to enable and fire the LRA. A hybrid component is used to develop the +2800 V to operate the Q-switch. The LPS also provides the 190-ms pump current (-55 A) to the diode array. The LPS uses a signal, ENABLE, to start charging of the storage capacitors that produce the diode drive current. When a FIRE command is received from the DPU, the switching DC/DC converter within the LPS is disabled based on the maximum ranging duration to teduce noise coupling to the receiver during expected returns.

Receiver Subsystem (RX) 1. Optical assembly A lightweight Dall-Kirkham telescope is used with the receiver to focus laser backscatter onto an 80@ t.tm-dia. photodiode hybrid unit. Figure 5 is an illustration of the telescope, designed and fat&a&d by OCA Applied Optics Inc. (Garden Grove, CA) based upon APL specifications.” Predominant comatic behavior of a Dali-Kirkham configuration occurs with off-axis sources; however, the NLR will operate with paraxial return light, thus aberrations ate not an issue.”

Time-of-flight (TOF) measure.ments am made digitally with an APL-designed GaAs application- specific integrated circuit (ASIC)‘2. Two ripple counters are implemented in this TOF chip to provide measurement of the calibration return (via II-bit counter) and range to Eros (21-bit counter). Counter sizes were selected based on design and mission requirements. The II-bit counter, providing a

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maximum delay of 4257.8 ns, is ample for the actual NLR calibration delay (558 ns); 11 bits was selected as the original design used a fiber-optic delay of I km versus 100 m. The 21-bit counter (20 bits range with 1 bit overflow indication), was sized according to power requirements and dissipation of the ASIC device. Therefore, the NLR, as implemented supports one-way ranging to -327 km. (Of course, altering the clock frequency can improve resolution or increase range depending upon whether its frequency is increased, or decreased, respectively.)

-3.114”-

Figure 5. Dali-Kirkham design (scaled) for NLRP3.4 receiver telescope.

During ranging operation, the receiver sees two optical pulses per transmitted pulse. The first, arriving approximately 558 ns after laser firing, is the 7.2-pJ calibration pulse routed through the FODA. Detection of this pulse by the receiver halts the calibration counter. The next return is the backscatter from the asteroid, which halts the range counter. The threshold level used to eliminate the noise floor is set by command from the ground or through the auto- acquisition sequence. After both TOF counters are stopped, the DPU reads counter values and formats them into science data packets for transmission over the 1553 bus as requested by the spacecraft data collection process.

Final mass for the NLR telescope assembly was 167.4 g. Acceptance testing of this telescope indicates 98% of the focused energy is located within a lOO+m Airy disk. This IOO-pm spot size is easily accommodated on the 800 pm active diameter of the NLR detector. The 7-nm FWHM spectral filter is located in the convergence cone of the telescope to reduce weight and cost. Typically, spectral filter passbands strongly depend on temperature and angle of incidence. Thermal effects were minimized by selecting the deposition process by OCA” that reduces passband shift by a factor of 10. Sensitivity to angle of incidence was ti by selecting a wide enough passband to allow light from all acceptance angles of the telescope (4”-to-9”) to pass with minimal attenuation.‘*

2. Analog receiver section

The NLR analog section uses four fundamental stages: (1) APD detector, (2) Video amplifier, (3) Integrator stage (Bessel-type filter), and (4) Comparator. The APD used in the analog receiver is a hybrid part that combines an enhanced silicon APD with temperature compensation and transimpedance amplification. This unit is designated as a wide-FOV detector with a measured bandwidth response of 50 MHz and responsivity of 770 kV/W. This particular part provides ~5% gain variation over a -8 to +40 Y temperature range, however, in the NLR configuration, the analog receiver housing IS thermally controlled to +2MlO ‘C. Optical energy inputs (peak power) may range from a detectable 9 nW to a maximum (saturation) of 0.5 W based on IO-ns width at 1.064 pm.

The video amplifier stage consists of a wideband operational amplifier to provide a gain of +50 at 75- MHz bandwidth. The integrator stage uses a seven- pole Bessel filter developed by K&L Microwave. This lowpass filter (30 MHz) integrates return pulses that may be dilated in response to target-surface topology. The filter optimizes the probability of detection for anticipated surface slopes and limits high-frequency noise response of the analog receiver electronics. I4

The comparator stage determines whether an input signal has sufficient energy to generate a STOP signal. The comparator operates using any of eight preset threshold levels set by command and/or auto- thresholding. The output of this comparator is the STOP signal. In this design, 24dB dynamic range of signal input levels exists to adjust for noise sensitivity. This particular device exhibits very low propagation delay (c 2 ns) and very little overdrive dispersion, making it ideal for precise timing applications.

Digital Processing Unit (DPU) I5 1. Hardware

The digital processing unit (DPU) is implemented on a single board and uses a 480 MHz oscillator, the TOF ASIC, an Actel 1020A field-programmable gate array (FPGA), a RTX2010 microcontroller, an analog-to-digital converter (ADC), and a 1553 chip set. Memory units installed on this single digital board include: 2K x 16 PROM, 32K x 16 EEPROM, and 32K x 16 SRAM.

The APL-designed TOF chipI implements two ripple counters and attendant logic to support both ranging and calibration. Although successfully operated at 1.7 GHz, for the NLR, the TOF chip need only operate at

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480 MHz. The FPGA permitted several functions (bus arbitration, control logic, addressing, interrupt logic) to be implemented on a single chip. A 1553 chip-set and isolation transformers were required to implement the NLR as a remote terminal on the bus architecture.

Through use of MIL-STD 1553 bus architecture, illegal commands to the NLR are rejected (protocol error) and electrical errors (incorrect parity) ate detected and corrected. To interface the NLR to the spacecraft by 1553 bus required the inclusion of logic that represents 1553 remote terminals within the DPU electronics. The 1553 interface is redundant atrl consists of two channels, A and B. NLR data ate collected in either real time (virtual channel 3, or VC3), or stored for subsequent retrieval using the onboard solid-state recorder (VCO). Major frames are 1s in duration (there are eight minor frames per major frame), and the NLR is synchronized using the “Synchronize with data word mode code command,” which is sent at the start of each major frame (minor frame 0); discrete l-Hz signaling is not supplied to the NLR. Mission elapsed time is sent after the start of each major frame. It is this information that is used to time-tag the NLR data. In addition to collecting data, the capability to perform a subsystem-to-subsystem transfer is available for NLR range words to be directed to the navigation subsystem at 1 Hz.

2. Software

The NLR software was programmed in JH-FORTH for the embedded RTX-2010 microcontroller. On the basis of a minor frame interrupt (every 125 ms), this software determines when to fire the transmitter, initialize parameters (T-O mask, threshold, and range- gate), and read and format calibration and range. For every TX pulse, range data, spacecraft time, atxi instrument configuration parameters are formatted in a science packet and accumulated for 56 shots (112 for PRF = 2 Hz) for transmission as one packet via a downlink. Transmitter firing rates are demrmined through uplink commands, and timing is controlled by NLR software to insure transmitted pulses ate coincident with 1553 minor frames. An internal microcontroller timer and interrupt maintain minor frame counting with periodic updates from the 1553 “Sync-With-Data” pattern, which occurs at 1 PPS. The ~-HZ mode occurs for a 2-s burst followed by a 14-s quiescent period to reduce thermal stresses within the TX. Following the 14-s pause, the NLR is recon@ured to the nominal l-Hz rate. During TX firings, the analog section is momentarily disabled via a programmable range gate to prevent firing noise (T-O noise) from prematurely triggering the receiver and halting the TOF counters. This range gate is a IO-bit programmable timer implemented in the

FPGA with a timing resolution of 41.67 ns and a maximum delay time of 42.628 ps.

The microcontroller is programmed to support several modes. In the FAILSAFE mode, the TOF counter is started by a delayed FIRE command as opposed to an electronic trigger from the TX photodiode (START signal from Figure 2). The FIRE command is delayed because of a variable delay in the TX optical output, nominally 192 ps. This delay is determined by initiating a CALIBRATION mode, which varies the T-O mask through the lo-bit programmable timer until a reasonable calibration range is detected Since the counter has a 500-ns resolution, the maximum range error due to a failed START pulse would be 500 ns. The T-O mask, along with the range gate, is configurable using uplink commands.

The ONE-SHOT mode is invoked when a failure to the calibration signal path is detected. As implemented, the DPU always expects two returns (STOPS) per transmitted pulse, or one pulse and a range overflow. The first pulse, calibration, stops the first counter in the TOF chip and sets up the second counter for a stop from the backscattered return. If the calibration signal does not arrive (e.g., broken calibration fiber), the DPU will falsely associate the backscattered pulse with the calibration counter and the range counter will constantly overflow. ONE- SHOT permits the NLR to be operated with just the typical single return. In this way, range measurements are preserved although not calibrated.

An AUTO-ACQUISITION mode exists to allow autonomous noise-floor measurements for the analog section. This mode configures the NLR to step through each threshold level and collect 16 samples of range and calibration data at an ~-HZ rate. Collecting range data at 8 Hz minimizes the influence of relative movement between the NLR and asteroid terrain lending to high correlation between data points. This is important as range rate is calculated for each new sample at each threshold and is compared to a pmde.tennined difference (sent as a command argument) to determine the system noise floor, atrl hence, the operating threshold for reliable returns.

REQUIREMENTS AND SPECIFICATIONS

Specifications for the NLR arc presented in Table 1. Also presented in the table are measurements for the specified parameters. Mission specifications represent design criteria that must be met to provide adequate data accuracy and resolution within the operating restrictions for the spacecraft. Instrument specific- ations were derived from mission requirements atxl available technologies. Design constraints, associated with spacecraft resources, are defined in the

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specification’s column of Table 1. The NLR met all requirements and specifications.

PERFORMANCE TESTING OF THE NEAR LASER RADAR The NLR was subjected to numerous tests ” to satisfy both instrument and spacecraft requirements. At the instrument level, the NLR went through design verification testing. These tests ensured that the instrument components met interface and performance requirements. The NLR was environmentally tested to evaluate operation and performance at various operating environments.

NLR Functional Test

Figure 6 presents the data for a functional test performed on the NLR prior to environmental tests. The receiver was sequenced through the range of threshold levels, O-to-7, and range counts were averaged over multiple laser firings to produce this graph. As threshold is increased, correspondingly more optical energy is required to indicate the arrival of a return pulse. The calibration curve indicates a range-walk of -4 m from level 2 to 7. The corresponding walk seen for simulated range is - 2 m.

This difference is a result of calibration range being a one-way value whereas simulated range takes the time-of-flight and divides by two to obtain range-to- target. In this particular test, calibration data was unreliable for level 0 and was not plotted. For the particular values of simulated range, measurements overflowed our NLR counters for levels 0 and 1, thereby indicating insufficient power level (actually, signal-to-noise) for signal detection. Once again, with threshold level 2, reliable range measurements were obtained.

NLR Environmental Tests

Performance data were collected following each axis of vibration and instrument alignment was verified before and after vibration using two optical methods.

The first method measured the relative change between optical reference cubes mounted on the TX, RX, and instrument base. The alignment of these cubes was measured using a Theodolite having 2- arcsecond precision. TX-to-RX alignment was also determined by observing the TX beam in the RX far- field FOV. This was performed by mapping the TX central lobe to the electronically determined FOV of the RX.

Table 1 presents the final values determine from this effort.

175-

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0 I 2 3 4 5 6 7

THRESHOLD LEVEL

Figure 6. Functional test of NLR using simulated backscatter (level, delay, and dilation)

The operating temperature of the instrument deck is projected to be between -29’C and +WC and survival range is projected to be between -34°C and +6(X. However, the NLR components had to maintain different temperature limits based on cautions issued by the TX manufacturer, through blanketing arxI heater control. To test the adequacy of the NLR thermal design, the following thermal ranges were devised and tests conducted.

Operational 0°C to +20°C (TX) with blankets limits: and heaters

+lOY! to +30°C (RX) with blankets

and heaters

-29’C to +WC (DPU)

Survival -5°C to +25”C (TX) with blankets limits: and heaters

-34’C to +6O”C (RX and DPU)

To evaluate. the NLR over these ranges, the entire integrated instrument was placed into a thermal- vacuum (T-V) chamber and opemted over six thermal cycles which lasted five days (three days of actual T-V testing). Special T-V chamber fiber-optics were employed to route the TX signal out of the chamber to the ground-support equipment (GSE) and to route the resulting test signal back into the chamber to the RX. This permitted the continuous performance evaluation.

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max. range (altitude) range accuracy range resolution albedo (reflectivity) operational period, lifetime vibration levels (random, sinusoidal) deck ambient thermal range (operational; non-op) supply voltage

50 km

I 6m 16m 0.05 to 0.2 1 yr. (continuous), 4 yrs 13.6 grms (60 set), 15 g (thrust) 7.5 g (lateral) - 29oc to +55 oc; - 34 OC to +60 OC 22 to 34 VDC, 33.5 nominal

>lOOkmb <2ma 31.48 cma N/A N/A 13.6 grms (60 set), 15 g’s (thrust) 7.5 g’s (lateral) a - 29 oc to + 55 oc; - 34 OC to + 60 oCa 22 to 34 VDC, 33.5 nominala

E-M compatibility tests CE07A,07B; CSO1,02,06 CE07A,07B; CS01,02,06a

INSTRUMENT: pulse energy >5mJ@ 1.064pm 15.3 mJa energy jitter < 10% c 1 %a pulsewidth 10 ns c tpw < 20 ns 12 nsa pulse-width jitter < 2 ns I nsa wavelength spread f 3 nm f 1 nma PRF rates l/8, 1 (nominal), 2, 8 Hz l/8, 1, 2, 8 Hza R-gates: T-O mask T-O masks (there are two gates, 0.00 - 42.628 ps (41.67 ns); (resolution) having coarse and fine delay 0.00 - 511.5 pi (500 ns) a

steps) TEMOO mode > 90 % 91 %a (W Gaussian fit) divergence c 300 pad ( l/e2 -points)

235 prada

calibration power jitter f5% <54ba calibration timing jitter < lm < 31.48 cma thermal control <*2oc i20ca shots (lifetime) > 1.8B NIA effective aperture, U# N/A 7.62 cm, ~73.4~ spectral receiver bandwidth < 10nm 7 nma temporal receiver bandwidth s 100 MHz 30 h4Hza APD dark voltage 1195 ~Vrms (24 MHz) 150 ~Vrms (24 MHz) a APD responsivity 2770 kVlW 775 kVtWa optical receiver FOV >9OOwad 2.9 mradb threshold levels N/A 8(2” * 16mV)a datarates 51 bps, 6.4 bps variable, incl. 51 & 6.4 bpsa boresight, TX-to-RX < 50 ccrad, < 750 md < lOwada, <25mda

PHYSICAL: mass 2 5 kg 4.9 kga average power I 22 w 15.1 wa imush current 55A 4.1 Aa volume (1 x w x h) N/A 14.75” x 9” x 8.5” a

(note: N/A -- not applicable) Table 1. NLR instrument specifications and requirements.

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At each thermal plateau, an abbreviated performance (limited performance, LP) test was conducted. Figure 7 present calibration data obtained during the thermal- vacuum test for NLR. For this graph, we plotted counts rather than range to highlight any slight thermal dependencies. Although range would have been adequate, the use of counts avoided systematic biases associated with software updates that were occurring during T-V testing.

For Fig. 7, several hundred (> 200) points were collected at each threshold for each of the three plateau temperatures: - 29 “C (circle), + 25 “C (triangle), amI +55 “C (square). The deviation of hot (+55 “C) to cold (-29 “C) is seen to be relatively benign. Threshold-induced range walk is observed, but performance appears to be relatively insensitive to thermal effects as represented by this thermal-vacuum test scenario. Finally, we observed variance of these data to be quite small, approximately 1 to 2 counts (refer to error bars in graph).

273

269

r f 267

u” 265

263

261

259 f I 0 1 2 3 4 5 6 I

THRESHOLD LEVELS

Figure 7. Functional test of NLR using simulated backscatter (level, delay, and dilation).

NLR End-to-End Testing (Open-Air Testing) Following environmental qualification, an end-to-end system test was conducted to verify that the NLR operated as a ladar system. This test was conducted in a moderately controlled test area at APL. An extended test laboratory was reahzed by using a 213-m (700- foot) hallway. The test was conducted during 8-9 August 1995. Using sand-blasted aluminum (Lambertian scatterer) and a silicate rock as ranging targets. The test characterized the range walk of the system and provided data useful to estimate the maximum operational range (> 100 km).

Range simulation, based on return energy levels, was accomplished using only one of the six receiver door

uretary missions 311

windows inline with neutral density filters totaling an optical density (OD) of 6.1 at 1060 nm. With the additional 1OdR loss introduced by using only one window in the RX door to receive the backscatter versus the entire aperture, total attenuation for this test was 71 dR relative to having the full RX aperture (effectively 3” diameter). The NLR was configured to fire at 1 Hz, and data were collected at various ranges to the targets. These ranges varied from 600 to 690 ft. and actual ranges were determined using a NIST- traceable lOO-ft surveyor’s rule. A composite of the targets used during this test along with test results is presented in Figure 8.

The graphs have range data from the NLR with overflow and rejected returns edited. The NLR was operated at various threshold levels. The arrows on Figure 8 indicate which target was illuminated during the data collection. The targets are denoted as Al- target (sand-blasted aluminum plate) and Rock-target (uncharacterized rock used as an appropriate target due to its texture and its curvature). Data were collected at one range (Rl) for the aluminum target and two separate ranges (R2 and R3) for the rock target. Of note is the illuminated spot on the targets indicating where the YAG beam was impinging the target during data collection. (An &mm camcorder mconled these images using a typical intrinsic silicon CCD array which is sensitive to the near-infrared.)

Figure 8. Free-air test (700-ft hallway) within APL campus; The arrows indicate which target was illuminated during data collection.

Figure 9 provides details of the upper-right hand graph in previous figure. This graph is the calibration data for the collection period and correlates directly to the time axis of the range plot (upper-left hand graph).

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Range walk patterns (staircase) within the calibration data are clearly seen as expected with this system when changing the threshold level. The steps change by 1 count (31.48 cm) with each change in threshold change. The nominal threshold value, 4, is indicated in this figure as 268.5 counts (k 0.5 count).

The test started at level 4 and obtained data at threshold levels 5, 3, 2, and 1.

Lesser detail of the range walk is shown in the range plot (upper-left graph in Figure 8) because of the ordinate scaling, however, range walk can be seen in the range data at the tip of the arrow associated with the aluminum target.

270.0 -

269.5 -

269.0 - ‘;i 5 266.5’

H L

266.0 -

iti 267.5 - a ,g 267.0 - c f 266.5’

; 266.0 -

s 265.5 -

265.0 -

Range-walk

264.5 -

264.0- , , I I I I 200 250 300 350 400 450

CALIBRATION SAMPLE

Figure 9. Detail of calibration data collected during the “open-air” test. Jitter in calibration delay was fo.5 count. Range walk, expected of leading-edge detection schemes, is obvious in this data as the NLR threshold was varied from level 5 to 1.

This correlates in time with the prominent staircase exhibited in the calibration data (Figure 9) and is given in higher detail in Figure 10.

Figure 10 presents higher detail showing the range to each of the three targets and associated range jitter (* 1 count, or 31.48 cm). Range-walk in the range data is highlighted in the figure. Compare to the range- walk seen in the corresponding calibration data (Figure 9.)

700.0 -

690.0 -

Rock-target

660.0 -

670.0 -

660.0 -

650.0 -

640.0 -

630.0 -

620.0 -

610.0 -

\ Range-walk

600.0 - Rock-target (R3

590.0-’ , , , , , , , 200 250 300 350 400 450 500

Figure 10. Detail of actual range data that correlates in time (sample number) with the calibration data of Figure 9.

Results from the “open-air” test provided: (1) confirmation of the NLR operation as a ladar system, (2) indication of the range bias (-1 ft.), (3) determination of the range and calibration jitter to expect, and (4) because actual range data exist, overall electronic delay could be determined. This delay measured 27 ns.

SUMMARY

Throughout testing, NLR performance was quite successful. We observed extremely quiet operation with noise levels on the order of l-count bit. Altering threshold values produced the expected result of increasing range walk with larger threshold values. Several scenarios were simulated with successful results through the versatility of the test support equipment @SE). Radiometric analyses atnd requirements are summarized in Table 1. The NLR performance met or exceeded all specifications. The inclusion of an “open-air” lasing test provided significant data and confidence in the ability of the NLR to perform as required. Prior to this test, the NLR had been operated as a laser stimulating test equipment and a receiver being stimulated by test equipment. Issues relating to power levels, alignment, and other attributes associated with actual end-toend testing were not addressed until this all- inclusive test was performed.

Although the NLR was a relatively simple laser radar compared to designs commonly in use, the combined requirement of operating an instrument in space for a

Low-cost planetary mlsslons 313

prolonged period under strong design constraints (weight, cost, schedule) contributed significantly to the complexity of this instrument.

ACKNOWLEDGMENTS

We wish to acknowledge those who provided information regarding their inputs to the of the NLR design: M. Boies for the fiber-optic assemblies, A. Reiter regarding the analog receiver, A. El-Dinary for the functionality and programming of the RTX microcontroller and D. Rodriguez who designed and developed the DPU board. Finally, we wish to express appreciation for the efforts of F. Davidson for his review of our front-end receiver design. The NLR instrument was developed as part of NASA’s Near Earth Asteroid Rendezvous mission.

REFERENCES

1. R. Binzel, T. Gehrels, and M. Matthews eds., ASTEROIDS II, The University of Arizona Press, Tucson, AZ, USA, 1989.

2. A. Santo, System Overview section,- PROGRAM: Fault Protection Desien Review, 26 January 1995.

3. R. G. Williams, Navigation section, NEAR Laser Rangefinder (NLR) Critical Design Review, 29-30 November 1994.

4. M. T. Zuber, D. E. Smith, A. F. Cheng, and T. D. Cole, “The NEAR laser ranging experiment,” Joumul of Geophysical Research -- Planets, to be published 1996.

5. T. D. Cole et al., “Laser rangefmder for the Near- Earth Asteroid Rendezvous (NEAR) mission,” Lidar Techniques for Remote Sensing II, SPIE volume 2581. pp. 2-26, EUROPTO, Paris, France, 25 September 1996.

6. T. D. Cole, System Design section, NEAR Laser Rangefinder (NLR) Preliminary Design Review, 29 June 1994.

7. T. D. Cole and F. Davidson, “Perfotmance Evaluation of the NEAR Laser Rangefinder,” to be presented at SPIE Conference on Photonics for Space Environments IV, August 1996, Denver, CO.

8. R. C. Moore, NEAR Laser Rangefinder Transmitter Energy Monitor, S2F-94-0145, 25 April 1994.

9. C. Culpepper, M. Kushina, C. Wiswall and T. Cole, “Laser Transmitter for the Near Earth Asteroid Rendezvous Spacecraft,” IEEE/OSA CLEO/QELS ‘95, Baltimore, 23 May 1995.

10. T. D. Cole, Approval of Purchase Requisition 94/10145/S2A, Space-Qualified Telescopes Requisition, APL Memorandum S2A-94- 0104 Rev. 3, 12 July 1994.

11. H. N. Bums, NEAR Laser Radar (I%&) vAnalvsis, Report to APL, Bums Engineering, Inc, 21 November 1994.

12. J. Penn, GaAs Design for Laser Rangefinder Timer, S2R-94-050, 3 March 1994.

13. T. Swenson, Specification of Microplasma 1064 nm Narrow Band Filter, Optical Corporation of America, 29 August 1994.

14. A.S. El-Dinary, NEAR Laser Ranger Test Plan, APUJHU Report 7361-3007, version 1.0, 1 August 1995.

15. D. Rodriguez, NEAR Laser Rangefmder Digttal Processor -- Electrical Design Data Package, S2F-94-0315,6 October 1994.

16. R. C. Moore, NEAR NLR Flight Software Requirements Specification, APUJHU Report 7352-9069, October 1994.

17. R. A. Reiter, “A direct design approach to developing the analog receiver electronics for the NEAR laser rangefinder,” to be presented at SPIE Conference on Photonics, Space Environments IV, August 1996, Denver, CO.

18. M. T. Boies and T. D. Cole, “Design, development, and flight qualification of the optical receiver for the NEAR laser rangefinder,” to be presented at SPIE AEROSENSE ‘96, section on LASER RADAR APPLICATIONS, SPIE Proceedings volume 2748, April 1996.