T. D. COLE
142 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998)
I
NEAR Laser Rangefinder: A Tool for the Mapping and
Topologic Study of Asteroid 433 Eros
Timothy D. Cole
n 1999, after a 3-year transit through space, the Near Earth Asteroid Rendezvous
(NEAR) spacecraft will place a scientific payload consisting of five instruments into
a low-altitude orbit (»35 km) about the asteroid 433 Eros for 1 year. One instrument,
the NEAR Laser Rangefinder (NLR), will use infrared laser pulses to provide
astrophysicists with precision altimetry data, measurements that were previously
unavailable from asteroid observations. These data will accurately map Eros’s topology,
identify and characterize small-scale surface features, and precisely determine overall
volume and mass once they are combined with navigation data. Objectives associated
with the NLR science mission are presented along with performance specifications and
instrument design details. The method by which NLR performance was analyzed is
described, as are tests used to verify its performance and operability. During
performance testing, an “end-to-end” test was conducted, where the integrated NLR
instrument was operated in free space using a 216.4-m hallway. Test results fully
verified all instrument interfaces and indicated that NLR performance parameters were
well within all specifications. In addition, range noise and biases were repeatable to 1
count, which is the minimum level possible for a direct-detection rangefinder.
(Keywords: Acceptance testing, APD detector, Laser altimeter, Laser Rangefinder.)
INTRODUCTION
On 17 February 1996, NASA’s first Discovery
mission began as the Delta II rocket rose from complex
17B carrying the Near Earth Asteroid Rendezvous
(NEAR) spacecraft. The overall NEAR mission objective
is to provide information about the origin and
nature of near-Earth asteroids, whose characteristics
are suspected to provide clues about the formation of
the inner planets, including the Earth, and whose
composition is reflective of material as it existed soon
after the “big bang.” Interest in asteroids has been
further heightened by the realization of their potential
for terrestrial impacts; the Earth’s geologic record
contains evidence of many such events, including the
cataclysmic collision that occurred some 65 million
years ago.
Astrophysicists have derived their knowledge of
asteroids from Earth-based observations, distant spacecraft
flybys, and analyses of meteorites. From these
data, a framework was developed to theoretically
model asteroidal dynamics, structure, and thermal
evolution. Indeed, sizes and shapes of asteroids contain
important clues about their thermal, collisional, and
JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998) 143
NEAR LASER RANGEFINDER
dynamic histories and their internal
structures. The NEAR mission
offers an opportunity to dramatically
improve our understanding of
asteroids by gathering data in close
proximity to one. In addition to
gathering data typical of previous
observations, the NEAR mission
will obtain asteroid data never
possible before, data whose gathering
requires close-range operation
such as high-precision altimetry.
High-precision altimetry data
will contribute significant insight
to asteroid evolution by describing
surface characteristics and global
parameters such as volume and
mass. The altimeter instrument,
the NEAR Laser Rangefinder
(NLR), is designed to operate continuously
throughout the 1-year
orbit and to produce precision altimetry
data over altitudes <327
km from Eros’s surface. This laser
altimeter detects round-trip time,
or time of flight (TOF), using a
high-power pulsed laser with precise (2.08-ns resolution)
timing measurements, which provides range resolution
of »32 cm. With onboard calibration capability
and from instrument-level tests prior to launch, the
NLR range accuracy is also »32 cm.
The target asteroid is 433 Eros, one of the largest
and most intensively studied near-Earth asteroids.
Astrophysicists have estimated Eros to be 10–20 km in
size, with a rotational period of roughly 5.27 h and an
albedo of 0.15. Available Earth-based observations1
indicate that Eros is a highly elliptical body measuring
36 ´ 15 ´ 13 km. These data, however, do not rule out
the possibility that Eros may actually be a “rubble pile”
consisting of two or more small, gravitationally bound
bodies. Figure 1 illustrates the geometry involved for
NEAR. The instruments continually observe Eros by
attitude rotation. The rotation plane is constrained by
the requirements to maintain solar arrays, which are
mechanically fixed, facing toward the Sun and to keep
the high-gain antenna within an acceptable angle of
Earth’s position.
The NLR design approach is described in this article.
The approach used to analyze the operating performance
of the altimeter is presented, and results from
this analysis are summarized. To validate the NLR
design and to ensure its successful operation subsequent
to launch and exposure to deep space over a total
period of 4 years, several instrument-level and integration
tests were devised and performed. These included
an operational test of the NLR to verify correct system
operation “end-to-end” and to characterize instrument
bias and performance. End-to-end testing also provided
information associated with operational peculiarities
of the NLR and was vital to the check-out of the
instrument’s interface to the spacecraft.
MISSION SPECIFICATIONS
The objective of the NEAR laser ranging investigation
is to obtain accurate, high-resolution altimetry
measurements that can be correlated with navigation
and gravity data to provide quantitative insight into
the internal structure, rotational dynamics, and evolution
of Eros. According to Dr. Maria Zuber, the NLR
science team leader, NEAR altimetry data and orbital
tracking data will allow the volume and mass of Eros
to be estimated to a precision of 0.01% and 0.0001%,
respectively. Comparison between the NLR-derived
data set and the predetermined gravity field (estimated
from Eros’s shape and spacecraft orbit perturbations)
will permit correlation of surface topography to the
local gravity field. Although the mean density measurement
will be limited by the accuracy of our topographic
field, resultant volume and mass values can be
estimated with an accuracy significantly improved
over that for any other asteroid previously observed.
Specifications for the NLR were derived from NLR
mission requirements and spacecraft constraints. To
provide the desired volume and mass measurement
precision, single-shot altimetry resolution and accuracy
relative to the asteroid center of mass must not exceed
6 m. For small-scale topology, altimetry sample density
Figure 1. NEAR will place a spacecraft into orbit about the elliptical asteroid 433 Eros.
The asteroid’s dimensions have been estimated at 36 ´ 15 ´ 13 km. NEAR’s optical
instruments, including the laser radar, are continuously pointed toward the asteroid’s
surface. Critical angles must be preserved as the spacecraft orbits Eros and rotates to
have the instruments face the asteroid’s surface. These angles are governed by the
requirement to maintain solar illumination of the power arrays and to remain connected
with the Earth’s communications network, the Deep Space Network.
Earth
Sun
12 m
Q
T. D. COLE
144 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998)
(spacing between altimetry measurements on Eros’s
surface) must be almost contiguous along the direction
of the subsatellite track on the asteroid. Over the range
of spacecraft altitudes expected with a spacecraft velocity
of 5 m/s relative to Eros, the sampling density will
be more than adequate given the altimeter firing rate
of 1 Hz and a laser transmitter divergence of 235 mrad.
Figure 2 illustrates the extreme expected orbit geometries
and resulting sampling densities. In addition to
the sampling density along the track, global parameter
estimation requires that the cross-track resolution, that
is, the spatial resolution perpendicular to the subsatellite
tracks, must be »500 m. Cross-track resolution is
governed by orbit-to-orbit separation and orbit (NLR)
mission duration; therefore, it was not considered a
direct requirement for the NLR instrument.
All NLR measurements will be registered in an
absolute center-of-mass reference frame, allowing precise
registration with data from other NEAR sensors.
Any offset between center of mass and center of figure
for the asteroid will be determined by correlating gravity
with topography data. The offset will reveal whether
internal density differences are uniformly distributed
and will help verify if Eros consists of two or more
gravitationally bound bodies. Table 1 summarizes the
requirements for the NLR based on NLR science
objectives and on estimated characteristics associated
with Eros.
NLR SYSTEM DESIGN: A MODULAR
APPROACH
To meet the accelerated schedule for the NEAR
program, the NLR was developed as a modular system
(Fig. 3). We selected a bistatic configuration, which
permitted parallel development of the transmitter and
receiver. The NLR instrument operates by transmitting
laser pulses to the asteroid surface and measuring
the TOF between outgoing light energy and optical
energy backscatter from the asteroid surface toward the
receiver. This direct-detection approach uses leadingedge
detection based on Neyman–Pearson thresholding,
thereby greatly simplifying wave form processing
requirements. Not only are leading-edge altimeters
relatively simple to implement, but the approach reduces
range scintillation generated when the extent of
the target range exceeds the transmitted pulse width,
a condition that could be encountered when mapping
irregular surfaces of an asteroid.
Basic Subsystems and Operation
The NLR required 19 months to develop, from start
of design through completion of flight qualification.
The transmitter and receiver units were developed and
tested separately; subsystems were tested prior to integration
using National Institute of Standards and
Technology (NIST)-traceable equipment. Although
schedule constraints precluded fabrication of brassboard
models, breadboard and engineering models
were produced and used extensively to debug design
and packaging issues.
A block diagram of the NLR instrument (Fig. 4)
shows its five subsystems: the laser transmitter subassembly
with a fiber-optic delay assembly (FODA) and
laser power supply (LPS); the optical receiver; the
analog electronics with a detector, processor boards,
and a medium-voltage power supply (MVPS); the digital
processing unit (DPU); and the low-voltage power
supply (LVPS). Red lines in Fig. 4 indicate the optical
paths for laser signals; red words indicate the primary
signals for the electronics. The optical receiver, analog
electronics, LVPS, MVPS, and DPU are collectively
Figure 2. Mission geometry of the NEAR spacecraft during its
encounter with the near-Earth asteroid 433 Eros. Two extreme
orbital configurations are shown to demonstrate different perspectives.
(a) The two extremes for the NEAR orbit geometry for
the NLR. (b) The instrument’s footprint and spacing dimensions.
Case 1 shows the spacecraft orbit position relative to Eros’s major
axis, with significantly overlapped sampling. Case 2 shows ground
track sampling, which is nearly contiguous.
5.88 m
5 m spacing
Case 1
Track
Laser spot size
2 m
Case 2
(b)
Center of mass
50 km
Case 1
Case 2 Eros
6.5 km
18 km
(a)
Track
JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998) 145
NEAR LASER RANGEFINDER
Douglas Aerospace Corp. The
transmitter, once completed and
qualified by McDonnell Douglas
Aerospace Corp., was delivered to
APL for final integration and acceptance
testing at the instrument
level. The integrated NLR instrument
was subsequently tested and
subsequently flight qualified at
both instrument and spacecraft
levels.
NLR operation is described by
Fig. 4. The measurement sequence
begins with a “Fire” command issued
by the DPU at a selected
pulse repetition frequency. This
command enables the transmitter,
which fires a 15-ns optical pulse
toward the asteroid and simultaneously
directs a portion of this
pulse into the FODA for calibration
TOF purposes. A photodiode at the transmitter
output detects the laser pulse and sends an electronic
START signal command to both of the receiver
Table 1. Specifications for the NLR instrument, determined through direct
measurement or inferred from related measurements.
Parameter Specification Measurementa/ Estimateb
Max. range (altitude) 50 km >100 kmb
Albedo (reflectivity) 0.10–0.22 0.15b
Range accuracy #6m <6 mb
Range resolution #6m 31.22 cma
Instrument weight (max.) 5 kg 4.9 kga
Instrument power (avg.) <22 W 15.1 Wa
Sample (grid) spacing Contiguous Contiguous (overlapped)a
Data rates <10 to <56 bps Variable; 6.4–51 bpsa
Operational period 1 year (continuous) N/Ac
Lifetime 4 years N/Ac
aActual measurements; bestimates using indirect measures; cnot applicable.
(a) Detector assembly
(b) Laser transmitter
(c) Mirror assembly
(d) Digital/LVPS
Figure 3. The integrated NLR instrument showing key subsystems. (a) The detector assembly uses an enhanced avalanche photodiode
hybrid mounted at the receiver focal plane (the detector is on the opposite side as shown). (b) The laser resonator and associated laser
power supply are based on previous designs. (c) Receiver optics (mirror assembly) consist of an all-reflective, lightweight Dall–Kirkham
telescope made of aluminum (athermal design). (d) Receiver electronics are shown mounted in the NLR chassis; both the digital
processing unit and the low-voltage power supply (LVPS) are shown. The calibration unit, a 109.5-m length of spooled optical fiber, is
shown on top of the transmitter in the center photograph (black cylinder). Not shown is the avalanche photodiode power supply (+550
V DC) and the laser power supply; both were remotely located to minimize interference with the sensitive receiver electronics.
referred to as the NLR receiver. The receiver was
designed, fabricated, and tested at APL. The transmitter
was obtained through subcontract from McDonnell
T. D. COLE
146 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998)
gallium arsenide (GaAs) TOF counters (range and
calibration).
As a result, during operation, the NLR receiver
should see two optical pulses per transmitted pulse.
The first, arriving 558 ns after the laser fires, is a
calibration pulse routed through the fixed delay provided
by the FODA. Detection of this pulse by the
receiver halts the calibration TOF counter. The second
return is the optical backscatter from the asteroid,
which halts the range counter. To minimize noise,
received optical signals are compared with a threshold
level set either by ground command or through an
auto-acquisition (calibration) sequence. After both
TOF counters are stopped, the DPU reads the counter
values and formats their contents (and other data) into
NLR science data packets for transmission over the
1553 bus, as requested by the spacecraft data collection
process. Consequently, what is measured is elapsed
time between the START indication from the laser
and the arrival of STOP signals as produced within the
receiver. Terms used in Fig. 4 are defined and described
in subsequent sections. The figure also illustrates the
electrical interface between the spacecraft and the
instrument. (Heater control and receiver door release
circuits are not shown.)
NLR Transmitter
The laser resonator assembly (LRA) is a solid-state
laser based on a proven polarization-coupled U-cavity
design2 (Fig. 5). The gain medium is a Cr:Nd:YAG
zigzag slab, side-pumped at 809 nm using a 20-element
gallium arsenide (GaAs) diode array (having thermal
sensitivity of Dl/DT = 1 nm/4°C) with a peak power
of 620 W in a pulse width of 200 ms. The opposite side
of the slab is coupled to a heat sink for thermal control.
An antireflection coating applied to the long dimension
of the slab improves pump-power coupling efficiency,
and a high reflectance coating on the heat-sink
side reflects pump energy back into the slab. This
zigzag pumping yields a uniform distribution of optical
energy throughout the gain medium, increasing absorption
path length and conversion efficiency.
An antireflection coating at both slab end faces
reduces optical losses for rotated polarization states. A
quarter-wave plate placed after the slab end face
maintains polarization. The gain medium is located in
a cross-porro cavity to provide boresight stability; a
Risley wedge in each porro prism assembly permits
cavity alignment during assembly. An internal aperture
reduces higher-order modes, and a 9.3´ Galilean
telescope acts as an external beam expander to reduce
Figure 4. The five subsystems of the NLR instrument consist of the transmitter subsystem, the low-voltage power supply (LVPS), the
optical receiver, the analog electronics, and the digital processing unit. Gray areas indicate these subsystems. Red lines indicate optical
signals (laser light). Thresholding is used both to start the time-of-flight (TOF) counters (START) and to stop the same counters (STOP).
APD = avalanche photodiode; mC = microcontroller; FPGA = field-programmable gate array; MVPS = medium-voltage power supply;
TLM = telemetry data.
Nd:YAG (diode-pumped)
Q-switched laser
Laser power
supply
Spacecraft
Amp
Analog
electronics
+
Digital processing
unit
“Fire”
± 5 V ±15 V
+ 15 V
Data/control
Spectral
filter
Backscattered
pulse
9.33 Galilean optic
Transmitter subsystem
Q-switch
Transmit
pulse
STOP
Fiber-optic delay assembly (FODA)
Detector
+Vref
Detected laser
pulse
Diode drive
Data
“Ready”
START
Spacecraft power (33.5 V DC)
LVPS subsystem
+
•
•
TLM data
•
DC/DC converters
GaAs
TOF
480-MHz
oscillator
FPGA
Data
acquisition
RTX 2010
C
1553
interface
MVPS
Threshold
Thermal
control
APD
Amplification
and filtering
Optical
receiver
–
–
Data/control
m
JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998) 147
NEAR LASER RANGEFINDER
angular divergence. (Output beam divergence was
specified as £300 mrad; our final divergence, in vacuum,
is 235 mrad.)
The laser operates at 1.064 mm and is Q-switched
for pulsed-mode operation using a lithium niobate
(LiNbO3) Pockels cell. A half-wave plate provides
polarization compensation. Placement of the Q-switch
in the output segment of the beam path minimizes the
potential for optical damage to the crystal by locating
it within the low circulating power portion of the
resonator. Because LiNbO3 is pyroelectric,
thermal gradients must be
controlled during operation in vacuum
environments. For this reason,
the NLR has redundant operational
and survival heater circuits to
reduce thermal variations to 22 to
14°C of the thermal set point.
To provide thermal isolation,
the LRA is mounted to the NLR
housing using Vespel shoulder
washers. Figure 6 is a photograph of
the actual LRA assembly depicting
the laser and associated resonator
optics. Note the lightweight 9.3´
output optic. Mechanical interface
for the LRA uses a three-point kinematic
mounting configuration.
Table 2 presents specifications
for the NLR. Also presented are
measurements (and estimates) for
each of these values. The laser
transmitter was extensively characterized
as part of the NLR instrument
test sequence. The system
reliably produced 15-ns (60.82
ns) optical pulses each having 15.3
mJ (62 mJ) energy. This optical
pulse was coupled to the FODA
and was measured at the LRA
output coupler as 7.2 pJ. (All
power measurements used NISTtraceable
power meters accurate
to 65%). Prelaunch NLR testing
demonstrated that the NLR
passed all specifications.
The output beam was characterized
using a fast (picosecond
response) detector to detail the
longitudinal mode structure (Fig.
7). Characterization of the near
field included measurement of
beam diameter (defined where
amplitude decreases to 1/e2 of
maximum), modal structure, and
energy distribution. Far-field testing
consisted of beam divergence,
jitter, and wander. Divergence significantly affects the
science data through surface sampling size. This parameter
was measured in both air (ambient, at 135
mrad) and vacuum (235 mrad) (Fig. 8). Gaussian
correlation coefficients along both the x and y axes
were 0.91; therefore, performance was better than the
specified 0.90. The difference in divergence is related
to output telescope alignment with and without atmosphere.
Although the index of refraction changes by
only 3 parts in 10,000,3 it was enough in a high-power
Turning
prism
Polarizer
cube
LiNbO3
Q-switch Risley
Wavelength
plate
Porro
prism
Porro
prism
Risley
Aperture
Wavelength
plate
Wavelength
plate
Corner
cube
Cr:Nd:YAG slab
(lasing medium)
Output
beam
Galilean telescope
Figure 5. NLR laser resonator cavity configuration is a polarization-coupled U-shaped
cavity design.2 The Cr:Nd:YAG zigzag slab is side-pumped by a diode laser array to
optimize conversion efficiency.
Polarizing cube
Cr:Nd:YAG slab
Porro prism
Corner cube
9.33 Galilean
telescope
Porro prism
LiNbO3 Q-switch
Figure 6. The LRA optical component configuration. Note the lightweight structure, made
of aluminum, used to mount the elements for the Galilean beam expander. One side uses
metallic Mylar for electromagnetic compatibility/electromagnetic interference purposes
(bottom of unit). Mechanical mounting is 3-point kinematic.
T. D. COLE
148 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998)
optic using air-gap spacing to make a measurable difference
in the wavefront shaping.
The LPS provides the control logic and power
necessary to enable and fire the LRA. The LPS also
provides the 190-ms pump current (45 A) to the diode
array. A hybrid component develops the 12800 V
necessary to operate the Q-switch. Upon receiving a
“Fire” command from the DPU, the LPS uses a signal
from the NLR logic within the DPU to begin charging
the storage capacitors that produce the diode drive
current. To reduce noise coupling to the receiver
during expected arrival of laser backscatter, the switching
DC/DC converter within the LPS is disabled for
the maximum ranging distance (327 km). Subsequent
to maximum TOF, the LPS is enabled to prepare for
the next pulse.
NLR Receiver
The NLR receiver design uses a lightweight Cassegrain
telescope, a spectral filter (centered at the laser
wavelength), a single-element avalanche photodiode
(APD) hybrid detector, amplification filtering, voltage
biasing and thresholding circuitry, and appropriate
power supplies (1550 V DC for the APD from the
MVPS, and 615 V DC and 65 V DC from the LVPS).
The receiver design was based on previous APL
APD-based detection circuit designs and on APL developments
in TOF systems using high-speed (2 GHz)
GaAs applications-specific integrated circuitry (ASIC).
A significant aspect of our configuration is that the
NLR receiver optics act as a direct-detection “photon
bucket.” This approach drastically simplified the design
and development of the optical receiver because
Table 2. NLR design parameters.
Parameter Specification Measurementa/ Estimateb
Transmitter pulse energy >5 mJ @ 1.064 mm 15.6 mJa
Transmitter energy jitter <10% <1%a
Transmitted pulsewidth, tpw 10 ns < tpw < 20 ns 15 nsa
Transmitted pulsewidth jitter <2 ns 0.82 nsa
Transmitter wavelength broadening ±3 nm ±1 nma
PRF rates 1/8, 1 (nominal), 2, 8 Hz 1/8, 1, 2, 8 Hza
Range gate (resolution) N/Ac 0.00–42.63 ms (41.67 ns)a
T-0 event mask (resolution) N/Ac 0.00–511.50 ms (500 ns)a
TEM00 mode (% Gaussian) >90% 91%a
Divergence (1/e2 ) <300 mrad 235 mrada
Beam waist (near-field) N/Ac 22.93 6 0.12 mma
Beam centroid jitter (shot-to-shot) <50 mrad 16.31 6 24.39 mrad a
Beam centroid wander <300 mrad 4.81 ± 31.25 mrad a
Calibration power jitter 65% <5%a
Calibration timing jitter <1 m <31.22 cma
Thermal control 62oC 62°Ca
Shots (lifetime) >31.5 ´ 106 >1 ´ 109 (TBD)
Effective aperture, f/# N/Ac 7.62 cm, f/3.4a
Spectral receiver bandwidth <10 nm 7 nma
Temporal receiver bandwidth #100 MHz 30 MHza
APD dark voltage #195 mVrms (24 MHz bandwith) 150 mVrms (24 MHz bandwidth) a
APD responsivity ³$770 kV/W 775 kV/Wa
Optical receiver FOV >900 mrad 2.9 mradb
Threshold levels N/Ac 8 (2n ´ 16 mV, n = 0 to 7)a
Data rates 51 bps, 6.4 bps Variable, including 51 and 6.4 bpsa
TX-to-RX alignment shift <1100 mrad (700 mm effective diameter 345.0 mrada (pre- to post-values
APD) vibration test)
Note: APD = avalanche photodiode; FOV = field of view; PRF = pulse repetition frequency; TBD = to be determined; T-0 = laser firing
time; TEM00 = transverse electromagnetic wave fundamental mode.
aActual measurements; bestimates using indirect measures; cnot applicable.
JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998) 149
NEAR LASER RANGEFINDER
the system is not required to produce an image. Consequently,
the optics could tolerate high levels of
aberrations as long as the resultant spot size remained
within the physical and alignment bounds of the APD
detector. This gave us the freedom to select the receiver
optical design best suited for low weight and manufacturing
ease, specifically, an f/3.4 Dall–Kirkham
design (Fig. 9).
Our telescope is a two-mirror aluminum Dall–
Kirkham arrangement using an athermal design. The
primary mirror is 3.5 in. (8.89 cm) (see Fig. 9) with
an overall field of view of 3 mrad. The Dall–Kirkham
layout is simple to manufacture but suffers primarily
from coma arising from off-axis light. However, since
the NLR operates with paraxial light, comatic aberration
is minimal and does not influence the performance
of the receiver.
Since there are no bright sources near the NLR line
of sight when viewing Eros, specifications of surface
roughness for the aluminum mirrors were relaxed; we
used diamond-turned surfaces, which are relatively
inexpensive for the mechanical and optical quality
provided. No superpolishing was required; only an
antioxidant layer was deposited onto the surfaces to
maintain reflectivity. Stray light contamination was
controlled by a system of baffles in the receiver support
structure to prevent off-axis light from reaching the
primary mirror in less than two reflections. The tworeflection
tolerance reduced the number of baffles, and
hence, the weight of the receiver housing. Baffles,
made of lightweight magnesium, were also knifeedged
to minimize direct reflections into the receiver
telescope.
The final mass of the telescope assembly (Fig. 9) was
167.4 g. Acceptance testing indicated that 98% of
the focused energy was located within a 100- mm
central (Airy) disk, easily accommodated by the 700-
mm active diameter of the APD detector.
To further reduce optical background noise, we
placed a narrowband (7 nm) spectral filter in the
convergence cone of the telescope, a location selected
to reduce weight and cost. Spectral filter passbands are
strongly dependent on temperature and angle of incidence.
To minimize thermally induced performance
variation, the filter was fabricated using a proprietary
deposition process4 that reduces passband shift by a
factor of 10. We reduced angular sensitivity by selecting
a wide enough passband to allow light from all
angles of the telescope, where the converging cone of
light at the detector extends from 4 to 9°, to pass with
minimal attenuation. The relatively low orbital velocity
of the spacecraft (5 m/s) imposed negligible Doppler
requirement on filter bandpass performance.
A windowed door over the entrance aperture of the
NLR receiver protects the optics from contamination.
Our primary concern was contamination from byproducts
arising from propellant burns during the transit
Figure 7. Temporal response of an LRA optical pulse. Each
temporal unit represents 10 ns; full-width half maximum measures
15 6 2 ns. Longitudinal modes are present because the
cavity is single mode only through aperturing.
Figure 8. Ambient far-field pattern of a typical NLR pulse (135 6
2 mrad at 1/ e2 points). The beam pattern, which approaches a
Gaussian intensity distribution, was acquired during ambient
testing at the nominal pulse repetition frequency (1 Hz) and
spacecraft voltage (33.5 V). Vacuum divergence measured 235 6
2 mrad.
Figure 9. Dall–Kirkham, athermal (aluminum) design for the NLR
f/3.4 receiver telescope. All optical surfaces were diamondturned
for NLR use. Overall weight of this telescope was 167.4 g.
100 mV
10 ns
Spectra
filter
8.89 cm
7.91 cm
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150 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998)
phase, but the door also provided mechanical
and contamination protection
during prelaunch activities. The door is
a deployable element meant to be
opened once, which was successfully accomplished
on September 24, 1997.
Both transmitter and receiver optical
sections were kept under a positive purge
using research-grade nitrogen prior to
launch with a purge valve in the receiver
door for pressure relief. (The transmitter
did not require a door; however, the
transmitter was nevertheless kept under
purge until final pressure equalization
was reached subsequent to launch.) The
door-release mechanism uses redundant
pyrotechnic wire cutters with a tempered
beryllium oxide wire. Six silica-coated windows
are located on the receiver door to provide approximately
50% of the total collecting area in the event
of a door-release failure at Eros. This 50% collection
area will permit the NLR to operate at the planned
orbital altitude (50 km).
Analog Electronics
The purpose of the analog electronics is to convert
backscattered optical energy from the asteroid surface
into a digital STOP signal, permitting round-trip TOF
measurements to be computed. Four fundamental stages
compose the analog electronics (Fig. 10): the APD
hybrid detector, a video amplifier, an integrator stage
(Bessel-type lowpass filter), and a programmable comparator.
The detector and amplifier are installed on a rigidflex
detector board where the detector electronics use
50-V matched lines; the integrator and comparator are
installed on a separate, analog signal processing board.
The APD is a hybrid device combining an enhanced
silicon APD with temperature compensation and transimpedance
amplification. The enhancement process
pushes maximum response of the APD detector slightly
beyond 1 mm. The particular APD used for the NLR
is a wide field-of-view (FOV) detector with a measured
bandwidth response of 37 MHz and responsivity of 770
kV/W. (For coherent laser radars, electronic bandwidth
of the baseband signal relates to the FOV used in
observing the returned signal.) Assuming 15-ns pulses
at 1.064 mm, our detector circuit operates with a
minimum detectable power of 9 nW and maximum
input power of 0.5 W. Gain variation is less than 5%
over the temperature range from 28 to 140°C due to
compensation; however, our analog electronics housing
is thermally controlled to 20 6 10°C.
The video amplifier stage consists of a wideband
amplifier used to provide a gain of 150 over a 75-MHz
bandwidth. The filtering, or integrator, stage is a
seven-pole lowpass Bessel filter with 3-dB cutoff at
30 MHz. The lowpass filter is used to integrate return
pulses that may be spatially dilated by interaction of
the transmitted laser pulse with the asteroid’s topology.
The use of this filter optimizes the probability
of detection of anticipated surface slopes while it
limits high-frequency noise response of the analog
electronics.5
A comparator stage determines whether an input
signal has sufficient energy to generate a STOP signal.
The comparator operates using one of eight preset
threshold levels, which are set by ground command or
auto-thresholding. These threshold levels vary from 16
to 2048 mV; the lowest threshold is below the receiver
noise floor, and the highest setting is just above the
signal strength associated with the calibration input.
Therefore, threshold levels increase as 2n Vth , where
n = 0, . . . ,7 and Vth is the threshold voltage (16 mV).
Thresholding permits an input dynamic range of 24 dB
to compensate for instrument noise sensitivity and
variations of return signal strength. The comparator
exhibits very low propagation delay (<2 ns) and very
little overdrive dispersion, making it ideal for precise
timing applications such as leading-edge detection.6
Digital Processing Unit
To save weight and to minimize high-speed lines to
the TOF chip, the DPU uses a single rigid-flex multilayered
board (see Fig. 11). The board contains
a GaAs TOF chip, a radiation-hardened RTX-2010
FORTH-language microcontroller, a dual-frequency
stable oscillator, a field-programmable gate array
(FPGA) chip, a redundant-channel 1553 chip set and
attendant transformers, digital memory, an analog-todigital
(A/D) converter and sampler, and control logic.
7 The dual-frequency oscillator produces two output
frequencies: 480 MHz 60.01% and 48 MHz 60.01%,
with short-term (100 ms) frequency stability of 1:108,
but only the 480-MHz frequency is used. The required
48-MHz clock is generated using the 480-MHz
Figure 10. Analog processing block diagram. The analog signal was generated
by a hybrid avalanche photodiode detector (APD) that included a transimpedance
amplifier and thermal compensation. Filtering (integrator) was used to counteract
pulse dilation effects. The filter used was a 7-pole low-pass Bessel filter (integrator).
The threshold circuit (comparator) operated at one of eight levels, which are
binary multiples of 16 mV.
Integrator
Threshold
Receive
optics
APD
hybrid
Video
amplifier
51V
+
+
–
–
Comparator
STOP
+V
Reference
Reference
select
0.1 F m
JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998) 151
NEAR LASER RANGEFINDER
frequency within the TOF ASIC. The FPGA uses this
48-MHz clock and produces 24-MHz, 2-MHz, and
500-kHz output clocks for the 1553-bus controller,
RTX microcontroller, and FPGA internal range-gating
counters, respectively.
The GaAs TOF chip is an APL-designed ASIC
with one 11-bit counter (used for calibration) and one
21-bit counter (used for range measurement with 1-bit
overflow). Both counters operate at 480 MHz. The
TOF counters are enabled with a “start” pulse from the
transmitter (START) and are stopped when a detected
return pulse arrives from the receiver comparator
(STOP). The RTX-2010 is a parallel 16-bit microcontroller
that operates at 2 MHz to conserve power. The
microprocessor chip contains three on-chip timers, a
dual-stack architecture, and an interrupt controller
with the ability to handle five external interrupts.
The 1553-bus interface to the DPU consists of two
1553 bus transformers (channels A and B), a 1553
dual bus transceiver, and a bus protocol controller.
The controller responds to bus commands sent by the
Figure 11. Digital processing unit (DPU) single-board implementation contains a GaAs time-of-flight chip, a radiation-hardened FORTHlanguage
microcontroller, memory, an analog-to-digital (A/D) converter and sampler, redundant-channel 1553 chip set and attendant
transformers, a dual-frequency stable oscillator, a field-programmable gate array (FPGA) chip, and control logic. The photograph shows
DPU and low-voltage power supply boards as they appeared in the NLR chassis. An optical bench was mounted to this chassis, and
the receiver and laser transmitter were aligned and mounted to this bench. Various in/out (I/O) ports are identified.
spacecraft command telemetry processor (CTP). The
bus controller handles data transfers, commands, and
telemetry that transfer between the NLR and the
CTP. Data bus arbitration is handled by our FPGA,
which simplified digital hardware design and fabrication
by incorporating several functions such as address
decoding, memory arbitration, clock generation, receiver
range gating, receiver enable (denoted T-0
masking), and transmitter and receiver configuration
control.
T-0 masking is necessary to eliminate spurious noise
that is produced with the formation of the laser optical
pulse. T-0 masking is a disabling of the range counters
(both calibration and return signal counters) for a few
cycles of the natural response of the receiver circuit at
the time the optical pulse is formed within the laser
transmitter. Without such masking, opportunities for
false STOP signals are possible, and actual range
measurements would not be possible. A second rangegating
function is implemented to gate the return signal
to improve the sensitivity of the NLR receiver.
Receiver
GaAs
time-of flight
chip
1553
transformer
1553
transformer
STOP
Control
480-MHz
oscillator
Laser
transmitter
RTX 2010
microcontroller
Actel A1280 FPGA
(8000 gates, 140 I/O pins)
Control
Data
48 MHz
Ack
Ack
Control
Control
Data
2 MHz
8-bit
A/D converter
Memory
2K 3 16 PROM
32K 3 16 EEPROM
32K 3 SRAM
Multiplexer
Control
Control
Data
Control
Data
Data
1553 bus
protocol
controller
1553
dual bus
transceiver
24 MHz
Data
Control
Control
Data
Redundant spacecraft
1553 bus interface
START
Off-the-shelf devices
Application-specific
integrated circuit
APL-programmed device
Specially designed optics
1553 interface
with spacecraft
Interface to
transmitter
and receiver
DPU board
Unregulated bus voltage
(from spacecraft)
Low-voltage
power supply
T. D. COLE
152 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998)
The sought-after signal is the STOP indication,
which is used to indicate a detected optical pulse above
the set threshold. STOP terminates either the calibration
or range TOF counter, depending on system configuration,
and the corresponding range is read out.
Software
The NLR software was programmed using the
FORTH programming language for the RTX-2010
microcontroller. The software interfaces with the
spacecraft CTP via the 1553 bus, executes commands
from the ground control, formats science telemetry and
instrument housekeeping commands, and controls
instrument operation. The software was implemented
in a multitasking environment running four different
NLR programs8: (1) “NLR_PROCESS” handles
transmitter/receiver control and data formatting;
(2) “TELEMETRY_PROCESS” implements necessary
1553 protocol for data transfer; (3) “COMMAND_
PROCESS” handles incoming instrument commands;
and (4) “DUMP_PROCESS” transfers blocks of NLR
memory to the telemetry stream for transmission to the
ground.
“NLR_PROCESS” is the main controlling program
for the instrument. This task has the highest priority
and requires 5 ms to execute. On the basis of a minor
frame interrupt (every 125 ms) arriving by the 1553
bus, this program determines when to fire the transmitter,
initialize receiver parameters (i.e., T-0 mask,
threshold level, and receiver range-gate), and read and
format calibration and range counter values (i.e., the
altimetry measurements). Measurement data, spacecraft
time, and NLR configuration parameters are
formatted into a science packet for each transmitter
shot, with 56 shots (112 for a pulse rate of 2 Hz)
accumulated into each of these packets for transmission
to ground via the downlink.
“NLR_PROCESS” is programmed with several
calibration algorithms. Operating during a contingency,
or “fail-safe,” mode, the TOF counter is started by
a delayed transmitter fire command as opposed to an
electronic trigger from the transmitter photodiode.
This fire command is purposely delayed due to an
inherently variable delay in the transmitter optical
output, nominally 192 ms. We can determine this delay
by initiating a calibration mode that varies the T-0
mask through a 10-bit programmable counter until a
reasonable calibration range is detected. Since the
counter has a 500-ns resolution, the maximum range
error due to a failed START would be 74.9 m.
The calibration algorithm configures the NLR to
step through each threshold level and collect 16 samples
of range and calibration data at a rate of 8 Hz.
Collecting range data at 8 Hz minimizes the influence
of relative movement between the NLR and asteroid
terrain, permitting 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 predetermined difference (sent as a command argument)
to determine the system noise floor and, hence,
the operating threshold for reliable range returns. The
8-Hz mode occurs for a 2-s burst followed by a 14-s
quiescent period to reduce thermal stresses within the
transmitter. Following the 14-s pause, the NLR automatically
reconfigures to the nominal 1-Hz rate.
In-line Optical Calibration
The NLR is the first laser altimeter to feature inline
calibration, which was implemented using a
109.5-m single-mode fused silica optical fiber. This
fiber optic delay assembly (FODA) is optically connected
between the transmitter and receiver using
small turning mirrors and graded-index lens assemblies.
A minute portion of each transmitted pulse is
injected (using internal transmitter scattering at the
corner cube, see Fig. 5) into the FODA, producing a
constant optical delay of 529.20 ns between transmitted
pulse and reception of that pulse by the receiver
optics; the measured delay after integration into the
system was 558.33 ns, indicating an additional system
bias of 29.13 ns. By directing each transmitted optical
pulse directly to the receiver optics, “true” end-to-end
calibration is possible, which allows detection of
range-walk, an error caused by threshold-level changes
or oscillator drift. This self-calibration feature, therefore,
ensures high quality of the NLR data and permits
evaluation of the NLR instrument functionality
throughout the mission. With the existence of the
FODA, we essentially provide the NLR with a virtual
fly-along laser radar target. This allows us to operate
the NLR and obtain calibration measurements based
on actual TOF readings using laser output power until
our arrival at Eros.
In addition to instrument operation, the FODA
proved invaluable for performance evaluation and
debugging during prelaunch integration and test
phases. Using the calibration range counter during
testing allowed us to determine the quality of the instrument
measurements based on threshold level selected,
power levels used, operational mode selected,
and various environmental conditions.
NLR PERFORMANCE ANALYSIS
Performance was estimated using several approaches,
including the basic radiometric modeling (Neyman
–Pearson detection) and Webb’s9 photoelectron
output approximation. Determination of the adequacy
of the NLR design requires evaluation of performance
given parameter values describing Eros, NLR mission
parameters (e.g., orbit altitude), and the NLR design
implementation. (Table 1 summarizes these parameters
JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998) 153
NEAR LASER RANGEFINDER
describing the NLR design.) Analytical evaluations
used these values to examine expected NLR performance
through link margin calculations.
Radiometric performance of the APD and associated
analog electronics was particularly important. Using
Neyman–Pearson detection statistics, given the
required detection probability (Pd = 0.95) and false
alarms (<10–2), our performance model indicates ranging
operation well beyond the required 50 km (Fig.
12). At 232 km, our analysis indicates »135 equivalent
signal photons (Ns) arriving at the NLR detector
(APD) surface, providing a detection probability Pd of
0.1 with false alarms of less than 0.01. Our detection
scheme uses single-shot statistics; no multi-shot averaging
is performed. At this range, we set the threshold
level at 7s0 (s0 is the receiver noise floor power level),
providing a false alarm count of 6 ´ 1024. Figure 12
illustrates detection probability and the number of
source photons received at the detector as a function
of range to the asteroid.
Because the NLR uses leading-edge detection, pulse
dilation due to interaction with the surface will introduce
range error and can lead to performance loss as
a result of receiver filter mismatch. This is especially
true when we observe Eros at an angle, us, from nadir
or when we range to a sloped surface ( up, from the local
horizon), or when significant surface roughness (Zrms)
exists. Pulse dilation is defined as the temporal error
in return pulse detection caused by elapsed time between
the arrival of the initial and final backscattered
photons for a given transmitter pulse, leading to error
in measured TOF. Figure 13 illustrates these concepts
for a divergence of ud, nadir altitude h0, mean slope of
Figure 12. Plot of NLR performance based on parameter values describing Eros, mission
geometry, and the NLR design. Avalanche photodiode output current was based on
Webb’s approximation. Probability of detection Pd indicates adequate operation beyond
50 km. At 223 km, analysis indicates »135 equivalent source photons ( Ns) arriving at the
detector surface ( Pd = 0.1). Statistics assumed single-shot operation. False alarm was
less than 6 ´ 1024 using a threshold set at 7´ receiver noise floor (s0).
us, and an off-nadir look angle of up. An analysis10 was
performed that estimates pulse dilation as a range error,
Z( us, up), within a footprint. Figure 14 is a plot of
Z( us, up) using values of pointing angles and a range
of slope values presumed for the NLR mission. The loci
where we match the science requirement of 6-m error
are shown in Fig. 14.
Pulse dilation can also occur as the result of beam
wave-front curvature or variations in localized surface
albedo. Additional range errors can arise from electronic
delay, timing errors, and incorrect correlation of
attitude with range sample. Accounting for the various
sources of range error with root-sum-square error analysis,
we determined that the NLR instrument will
generate a topographic field that is accurate to »10 m
with respect to Eros’s center of mass. (This is an estimate
of the accuracy using the entire NLR data set
within the ground-based data center. The NLR measures
range to the asteroid’s surface, not to its center
of mass, which must be estimated through numerous
measurements made while NEAR orbits Eros.)
NLR TESTING
The NLR was tested at the instrument and spacecraft
levels to ensure that it satisfied or exceeded all
instrument and spacecraft requirements and that no
adverse conditions existed that would jeopardize or
corrupt spacecraft or other instrument operations.
Instrument testing included functional, environmental,
and end-to-end testing.
Functional testing used ground support equipment
(GSE) to simulate return signals for the NLR to operate
with over simulated pulsewidths
(dilation), return levels
(simulating changes in
range or albedo), and operational
modes. Fiber-optic
links allowed us to test
throughout integration although
the transmitter was a
Class IV laser, which would
have precluded open-air testing
in unprotected areas.
Environmental testing
was performed to evaluate
the NLR operation and performance
under various operating
conditions (variable
spacecraft bus voltages, environmental
temperatures and
gradients, and modes of operation).
Because the NLR
produces high currents within
its power supply to the
transmitter (»45 A), the influence
on other spacecraft
135 received photons
@ 223 km
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300
0
2000
4000
6000
8000
10,000
12,000
14,000
Pd
Mission requirement
Ns
NLR capability
Slant range (km)
Probability of detection (single shot)
Signal photons per pulse at detector
T. D. COLE
154 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998)
laser firings. This was verified during a post-launch
interference test that was conducted in May 1996.
Finally, a free-space laser radar test was conducted
to ensure end-to-end operation and to provide significant
data and confidence in the ability of the NLR
to perform as required. Prior to this test, the NLR had
been operated using GSE; the receiver was provided
with a “simulated” return signal, and the transmitter
was used only to provide timing for the return signal.
Issues such as power level sensitivity, optical alignment,
and attributes associated with overall system
implementation were not quantifiable until this endto-
end test was performed.
Functional Testing
A graded-index lens was attached to one of the
windows in the aperture door to provide access to the
receiver prior to launch. A cap at the output aperture
of the NLR transmitter was installed that had a fiberoptic
connector to allow access to the laser. Fiberoptic
links allowed direct access to the transmitter and
receiver to permit simulation of effects during the
environmental and functional tests without having to
resort to open-air testing. This simulation allowed
significant testing without affecting people around the
laser, especially during spacecraft integration activities.
Fiber optics routed the transmitted pulse from the
laser to the GSE, where a photodetector converted
the optical signal into an electrical one for simulation
of delay, signal level changes, and pulse stretching.
This modified signal was then sent to a GSE laser
source, transforming the electrical signal back into an
optical one. This modified optical signal could then
be directed to the receiver, and NLR performance
could be analyzed over a multitude of return signal
conditions.
Figure 15 presents data for a functional test performed
on the NLR. The receiver was sequenced
through a range of threshold levels, and range counters
were averaged over multiple laser firings at each of the
threshold levels. The data plots in Fig. 15 illustrate an
expected error caused by the use of leading-edge detection,
a condition denoted as range-walk. As the threshold
levels are increased, increasingly more energy is
required by the return pulse to signal its arrival (which
produces the STOP signal). Increased energy occurs
later within the return pulse; thus the range indicated
by the range counters increases with increased threshold
levels, introducing a bias, or range-walk. The calibration
curve shown in Fig. 15 illustrates a range-walk
of 4 m over operational thresholds. The corresponding
range-walk for range measurements is 2 m; the difference
between the counters is that calibration measurements
are one-way TOFs, whereas range is computed
by dividing TOF by 2. In the particular case shown,
calibration data at the lowest threshold were unreliable
Figure 13. Diagram explaining pulse dilation occurring as the
pulse interacts with the asteroid’s surface. Various parameters
describing the spacecraft’s look angle are presented in (a), which
give rise to pulse dilated returns, defined by (b). As the pulse is
transmitted from the NLR, the pulse duration is relatively sharp
and small, »15 ns. As the pulse incurs extended aspects of the
surface, significant backscatter dilation starts to occur. This “distributed”
return manifests itself by stretching the pulse over the
finite extent of the range gate used. (c) The details associated with
rough surface pulse stretching are shown. R= measured range;
zrms = surface roughness.
instruments (especially the magnetometer) was evaluated.
No interference was observed from the NLR
using data from all NEAR instruments during repeated
Spacecraft
r1
r0
x
h0
us
ud
up
Eros’s surface
(a)
Transmitted
pulse
Threshold level
Received
pulse
Time
R Dilation
(b)
z rms
Eros’s surface
NLR line of sight
Mean slope
(c)
Mean slope
z ( s, p) u u
z ( s, p) u u
JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998) 155
NEAR LASER RANGEFINDER
35
30
0
5
10
15
20
25
20
0
5
10
15
0
10
20
30
40
Pointing angle, p (deg)
u
u
Mean slope angle, s (deg)
u
u
Dilation, z( s, p) (m)
z( s, p) = 6 m u u
Figure 14. Surface plot of dilation z( us, up) with locus of points describing z = 6 m
shown to indicate maximum permissible z (which becomes range error) for the NLR
mission.
175
173
170
171
172
174
169
168
166
167
165
Calibration (m)
Simulated range (m)
0 1 2 3 4 5 6 7
Threshold level
Calibration
Range
1589
1585
1586
1587
1588
Figure 15. Data for a functional test performed on the NLR. The
receiver was sequenced through a range of threshold levels, and
range counters were averaged over multiple laser firings at each
of the threshold levels. The plots illustrate an expected error
caused by the use of leading-edge detection, a condition denoted
as range-walk. Threshold levels are described in the text and
represent the binary exponent multiple of the base threshold
voltage, 16 mV.
and therefore not plotted. For range, the measurements
were excessively noisy at the lowest two levels, indicating
insufficient energy in the return pulse for reliable
detection. However, once we used threshold levels set
for operation (levels from n = 2 to n = 6 in the threshold
equation given earlier, 2n Vth), reliable calibration and
range values were provided by the tests.
Environmental Testing
To ensure that the instrument
would survive launch stresses and the
NEAR space environment, each NLR
component was subjected to rigorous
environmental testing. Such testing
involved frequency-swept vibration
levels, mechanically induced from 10
to 100 Hz at sweep rates of 4 octaves/
min along the thrust-axis optical boresight.
(Maximum amplitude of 15 g occurred
from 30 to 40 Hz.) Along the
lateral axes, frequency range and rate
were identical with maximum amplitudes
occurring between 15 and 25 Hz
of 7.5 g. The NLR instrument was also
subjected to random vibration over
frequencies of 20–2000 Hz for 60 s
along each of the axes, with an overall
root-mean-square amplitude of 13.6 g.
Performance data were collected
following each axis of vibration, and
instrument alignment was verified before
and after each test using two optical methods. The
first method measured the relative change between
optical reference cubes mounted on the transmitter,
receiver, and instrument base. The alignment of these
reference cubes was measured with a theodolite having
2-arcsec precision. Transmitter-to-receiver alignment
for our bistatic configuration was also determined by
observing the transmitter beam in the receiver far-field
FOV. This measurement was made by mapping the
transmitter central lobe to the electronically determined
FOV of the receiver. Measurements indicated
that the shifts induced by the mechanical vibration
tests (345 mrad) were well within the acceptable range
(<1100 mrad), permitting reliable transmitter-toreceiver
alignment.
The operating temperature of the spacecraft is projected
to be between 229 and 155°C, and the survival
range is projected to be between 234 and
160°C. However, manufacturers’ specifications for
some of the NLR elements required that the components
be maintained at a different temperature range.
This was accomplished through blanketing and heater
control. We tested the adequacy of the NLR thermal
design by testing the transmitter (laser) from 0 to
120°C, the receiver optics from 110 to 130°C, and
the NLR electronics from 229 to 155°C. Survival
tests, during which the components are unpowered,
increased the thermal range of operation testing by
5°C in each direction (e.g., transmitter survival testing
range was increased to be from 25 to 125°C).
To evaluate the NLR over these ranges, we placed
the entire integrated instrument (including all power
supplies and flight cabling) in a thermal-vacuum
T. D. COLE
156 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998)
(T-V) chamber and operated it over several thermal
cycles. The T-V test lasted for 5 days: 3 days involved
actual testing and 2 days were required to attain stable
thermal and vacuum conditions for operation. To characterize
the NLR while it was in the chamber, the
previously described fiber-optic links were used to
route the transmitter signal out of the chamber to the
GSE and to return the modified optical signal back
into the chamber to the receiver, permitting continuous
performance evaluation. To prevent corona and
to ensure reliable operation of the NLR, the instrument
was soaked in vacuum (<1025 torr) for the initial
36 h before it was powered.
We evaluated calibration data during the T-V testing,
collecting over 200 range measurements at various
threshold levels for each of the three plateau temperatures
investigated, 229, 125, and 155°C. When
operation at 155°C was compared with operation at
229°C, a very slight thermal influence on NLR performance
was found; the range shifted by less than
0.5 ns (»15 cm).
End-to-End System Test
An end-to-end system test, conducted
under moderately controlled
conditions in a 216.4-mlong
hallway at APL, verified that
the NLR operated as a laser altimeter
system. The NLR was configured
to fire at 1 Hz at two distinct
targets, one at a time. One target,
a sand-blasted aluminum sheet,
could be characterized as a Lambertian
scatterer. The second target, a
silicate-rich rock, was use to represent
more of what we expect of
Eros’s surface. Figure 16 shows these
targets; the white circle on the targets
in Fig. 16a is the actual
Nd:YAG infrared beam; the camera
used to photograph is a charge coupled
device camcorder and is sensitive
to the near infrared. Data were
collected at one range for the aluminum
target and at two separate
ranges for the rock target. Target
ranges varied between 182 and 211
ft, with actual values determined
using a NIST-traceable 91.44-m
surveyor’s rule. The data, as represented
in Fig. 16a, illustrate the insignificant
noise and error levels associated
with NLR operation.
To simulate actual operating distances
(nominally 50 km) for the
NLR, we used neutral density filters
270
NLR calibration range (counts)
700
NLR range (counts)
680
600
620
640
660
200 250 300 350 400 450 500
Range sample
269
264
265
266
267
268
Range-walk
Range-walk
Range-walk
Al target
Rock target
Rock target
(a) (b)
200 250 300 350 400 450
Calibration sample
Figure 16. A free-air test (in a 216.4-m hallway) within the APL campus. (a) Evaluation
and (b) calibration of NLR. Photographs of the targets used (sand-blasted aluminum and
silicate rock) are also shown. The arrows indicate which target was illuminated during data
collection. (a) Note the presence of the infrared laser beam on the targets as a result of the
infrared sensitivity of the “photographs,” which were derived from frames of a commercial
charge coupled device camcorder. (b) An expanded version of the threshold changes
while the aluminum target was viewed (the plots in a and b are time correlated). Counts
are converted to range by using the speed of light, c. (Calibration requires division by an
index of refraction of the fiber optic, »1.5; range uses c, but must be divided by 2 to account
for round-trip elapsed time.)
and an aperture stop for a total attenuation of 71 dB.
The graphs in Fig. 16 show range data from the NLR
for operational threshold levels. As expected, at the
lowest threshold levels noise dominated, and at the
highest threshold levels no returns were detected.
Given the binary-increasing (octave) scaling used
to create the threshold levels (recall that comparator
voltage levels vary as 2n Vth), the operational threshold
at n = 5 is »9 dB above the receiver noise floor. With
the 71-dB attenuation, estimated link margin for the
NLR operating at 50 km to Eros is estimated to be 9–
12 dB. The 3-dB granularity cannot be improved upon
using these test data because the data were taken using
the eight-valued preset threshold scheme.
SUMMARY
The successful launch of the NEAR spacecraft from
Cape Canaveral on 17 February 1996 at 3:43 EST was
only the beginning. Three tests performed subsequent
JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998) 157
NEAR LASER RANGEFINDER
to launch indicated satisfactory operation of the NLR.
The actual Eros rendezvous is scheduled for February
1999. From the substantial surface coverage of Eros by
the NLR and the altimeter’s expected resolution and
accuracy performance, the resulting measurements
should significantly enhance our understanding of
asteroid structure. In addition to instrument-level testing,
the NLR underwent spacecraft qualification testing
as an integrated instrument. Figure 17 depicts the
NLR located on the NLR instrument deck (note the
nonflight, red caps that were used to protect the optics
prior to launch).
Although the NLR was a relatively simple laser
radar compared with designs commonly in use by terrestrial
laser rangefinders, the combined requirement
of operating an instrument in deep space for a prolonged
period under strong design constraints (weight,
cost, and schedule) contributed significantly to the
complexity of this instrument. Fortunately, radiation
effects for the NEAR mission are slight (10-krad total
dose); nevertheless, space exposure influenced our
selection and design approach for the electronics and
the optical components.
REFERENCES
1Binzel, R., Gehrels, T., and Matthews, M. (eds.), Asteroids II, The
University of Arizona Press, Tucson (1989).
2Near Earth Asteroid Rendezvous Laser Transmitter, Vol. 1: Technical and
Management, Proposal to RFP No. 389, McDonnell Douglas Aerospace—
West (13 Dec 1993).
3Bass, M. (ed.), Handbook of Optics, Vol. I: Fundamentals, Techniques, and
Design, McGraw-Hill, New York (1995).
4Swenson, T., Specification of Microplasma 1064-nm Narrow Band Filter,
Technical Application Note, Optical Corp. of America (29 Aug 1994).
5Davidson, F. M., and Sun, X., Reduced Electrical Bandwidth Receivers for
Direct Detection 4-ary PPM Optical Communication Intersatellite Links, NASA
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6Reiter, R. A., Timing Precision of the NEAR Navigation Laser Rangefinder
(NLR) Analog Electronics, APL Technical Memorandum S2A-93-0201,
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7Rodriguez, D., NEAR Laser Rangefinder Digital Processor—Electrical Design
Data Package, APL Technical Memorandum S2F-94-0315, JHU/APL,
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9Webb, P. P., “Properties of Avalanche Photodiodes,” RCA Rev. 234–278
(Jun 1974).
Figure 17. The NLR instrument (circled) as integrated into the
NEAR spacecraft. Note the red-tag covers used to protect optics
prior to launch. The total spacecraft mass was 816.5 kg (1800
pounds) (with propellant); the NLR weighed slightly less than 5 kg.
10Cole, T. D., and Davidson, F. M., “Performance Evaluation of the Near-
Earth Asteroid Rendezvous (NEAR) Laser Rangefinder,” Proc. SPIE,
Photonics for Space Environments IV 2811, 156–168 (1996).
ACKNOWLEDGMENT: This work was supported under contract N00039-95-
C-002 with the U.S. Navy. Thomas B. Coughlin is the program manager for the
NEAR program.
THE AUTHOR
TIMOTHY D. COLE is the section supervisor of the Electro-Optical Instrumentation
Section within APL’s Space Department. He holds academic degrees in
electrical engineering (B.S.E., M.S.E.E.) and technical management (M.S.).
Mr. Cole has been involved in laser radar technologies for both exoatmospheric
and terrestrial applications. He designed and led the engineering effort
that developed the NEAR laser altimeter. He has also been a lead engineer
and project manager on several space programs and led the development
of an ophthalmologic instrument in collaboration with the Hopkins Wilmer
Eye Institute. Before Mr. Cole came to APL, he performed analyses of and
research into long-wave infrared sensors and detector technologies. Currently,
he is a member of the NLR science team and holds a chair position at the
Center for Non-destructive Evaluation (CNDE). His e-mail address is
timothy.cole@jhuapl.edu.