Laser altimetry provides a high-resolution, high-accuracy method for measurement of the elevation and horizontal variability of Earth-surface topography. The basis of the measurement is the timing of the round-trip propagation of short-duration pulses of laser radiation between a spacecraft and the Earth's surface. Vertical resolution of the altimetry measurement is determined primarily by laser pulsewidth, surface-induced spreading in time of the reflected pulse, and the timing precision of the altimeter electronics. With conventional gain-switched pulses from solid-state lasers and sub-nsec resolution electronics, sub-meter vertical range resolution is possible from orbital attitudes of several hundred kilometers. Horizontal resolution is a function of laser beam footprint size at the surface and the spacing between successive laser pulses. Laser divergence angle and altimeter platform height above the surface determine the laser footprint size at the surface, while laser pulse repetition-rate, laser transmitter beam configuration, and altimeter platform velocity determine the space between successive laser pulses. Multiple laser transitters in a singlaltimeter instrument provide across-track and along-track coverage that can be used to construct a range image of the Earth's surface. Other aspects of the multi-beam laser altimeter are discussed

Bufton, Jack L.

Harding, David J.

Ramos-Izquierdo, Luis

English

NASA Technical Reports Server

N94-15620
MULTI-BEAM LASER ALTIMETER
Jack L. Bufton
David J. Harding
Luis Ramos-lzquierdo
Laboratory for Terrestrial Physics
Goddard Space Flight Center
Greenbelt, MD
Laser altimetry provides a high-resolution, high-accuracy method for
measurement of the elevation and horizontal variability of Earth-surface
topography. The basis of the measurement is the timing of the round-trip
propagation of short-duration pulses of laser radiation between a spacecraft and
the Earth's surface. Vertical (i.e. surface elevation) resolution of the altimetry
measurement is determined primarily by laser pulsewidth, surface-induced
spreading in time of the reflected pulse, and the timing precision of the altimeter
electronics. With conventional gain-switched pulses from solid-state lasers and
sub-nsec resolution electronics, sub-meter vertical range resolution is possible
from orbital altitudes of several hundred kilometers. Horizontal resolution is a
function of laser beam footprint size at the surface and the spacing between
successive laser pulses. Laser divergence angle and altimeter platform height
above the surface determine the laser footprint size at the surface; while laser
pulse repetition-rate, laser transmitter beam configuration, and altimeter platform
velocity determine the spacing between successive laser pulses.
Multiple laser transmitters in a single altimeter instrument provide across-
track as well as along-track coverage that can be used to construct a range image
(i.e. topographic map) of the Earth's surface. Figure, 1 is an illustration of the
pushbroom laser altimeter instrument measurement concept that utilizes multiple
laser beams. This multi-beam laser altimeter (MBLA) contains modular laser
sources arranged in a linear, across-track array. Simultaneous or near
simultaneous measurements of range to the surface are possible by independent
triggering of the multiple laser pulse transmitters and reception by a single
telescope that is staring at nadir and is equipped with a multi-element linear
detector array in its focal plane. This arrangement permits alignment of each
transmitter output into a separate, dedicated receiver channel. The illustrated
configuration is a linear, contiguous across-track array of 30 beams which
produces a strip-image range map of the Earth's surface. This MBLA configuration
is one possible arrangement to accomplish high accuracy terrestrial topographic
mapping near the nadir track in a NASA Earth Probe mission devoted to global
topographic measurements. The configuration can be changed in accord with
required science products and available resources. Possible modifications in the
design include variation of footprint size and/or footprint spacing both along-track
and cross-track to produce the desired coverage or sampling density within the
sensor swath width.
The illumination pattern incident on the Earth's surface from any one
transmitter element is a two-dimensional circular pattern of laser irradiance, with a
Gaussian spatial distribution of illumination intensity, that is produced by a single
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transverse (spatial) mode of the laser cavity. The transmitted laser pulsewidth is
short (i.e. ~ 5 nsec full-width-at-half-maximum). The temporal distribution of laser
irradiance in the pulse is approximately Gaussian and is the result of multiple
longitudinal laser cavity modes produced by the gain-switched (Q-switched) laser
cavity. In an ideal altimeter application (e.g. measurement of a smooth water
surface) the backscattered laser pulse retains the shape of the incident pulse.
However in the general case, the height distribution (i.e. roughness) and slope of
the surface within the laser footprint produce spreading in time of the laser pulse
reflected to the receiver. After interaction of the laser footprint with a rough or
sloping surface, the backscattered pulsewidth may be expanded to several tens to
several hundreds of nsec. The spread pulse degrades range measurement
accuracy but analysis of the received pulse shape provides additional information
on surface structure.
Figure 2 illustrates the pulse spreading effect and portrays the measurement
approach in laser altimetry by providing the time varying amplitude of an altimeter
detector that observes both the transmitted and backscattered laser pulse. Pulse
spreading, by re-distributing the available pulse energy into a larger time interval,
acts to reduce the peak-power signal-to-noise-ratio, thus increasing the probability
of error for the range measurement. Pulse spreading also adds timing uncertainty
by slowing the rise time of the return signal. Variability in pulse rise time in turn
produces a time-walk effect when conventional threshold-crossing time-interval-
unit devices are used for the range measurement. The application of GHz-
bandwidth digitization or multi-stop time-interval measurement to the receiver pulse
waveform provides pulse shape data. Digitization is indicated by the horizontal
axis tick marks in Figure 2. The centroid Ts of the pulse shape data is used to
make a timing correction that provides the measure of range-to-surface. The
centroid is in effect the mean round-trip time-of-flight range to surface features
within the laser footprint, weighted by: (1) the input two-dimensional Gaussian
illumination pattern; (2) the reflectivity and areal extent of the surface features; and
(3) the laser altimeter receiver transfer function.
The pulsewidth (or rms pulse spreading) that is derived from digitizer or
multi-stop timing data is used to assess the magnitude of surface slope and/or
surface structure within the footprint. Pulse spreading data taken together with
along-track and across-track slope information provided by adjacent range pixels,
enables calculation of the sub-pixel (footprint) slope or roughness. An analytical
expression has been developed (Gardner, 1991)to express pulse spreading
(mean square pulse width) in terms of the laser altimeter system parameters, beam
curvature, nadir angle of observation, surface slope, surface roughness, and laser
receiver operating signal-to-noise ratio.
The total area under the received pulse is proportional to pulse energy and
is a measure of surface reflectance at the monochromatic 1 I_m laser wavelength.
Effective use of this reflectance data requires normalization by laser transmitter
energy and consideration for atmospheric transmission. Reflectance data acquired
with the pushbroom scan pattern of the laser altimeter provide an imaging
capability that supplements the ranging functions. Since this image is acquired
with an active sensor that transmits and receives only near nadir (180 ° phase
function, i.e. backscatter mode), the surface illumination angle is fixed within 1° of
zenith and the resultar_t image is free of bidirectional reflectance effects that exist in
passive images with variable solar illumination geornetries. Surface slope effects
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on this reflectance image can also be directly characterized from the associated
laser altimeter along-track and across-track range measurement record.
The functional block diagram of the multi-beam laser altimeter instrument
appears in Figure 3. The laser transmitter module, receiver telescope, detector
package, ranging and waveform electronics, GPS receiver, and pointing attitude
measurement components form the major instrument subsystems. These
subsystems are packaged into a common structure that provides a rigid platform for
the laser transmitter, receiver optical components, and dual star cameras. The size
of this structure is primarily dependent on telescope aperture (~ 0.9 m). The key
component of the laser altimeter instrument structure is a lightweight, rigid optical
bench illustrated in the perspective view of the MBLA Instrument in Figure 4 and
the cross-sectional view of Figure 5. The altimeter telescope primary mirror is
attached to the nadir-viewing side of the optical bench and the laser transmitter
modules, detector package, and dual star cameras are attached to the opposite
side. This construction ties all the transmitter and receiver optics together for
maintenance of arc sec alignment. Beryllium is the material of choice for
fabrication of the telescope optics and structure, optical bench, laser module cases,
and the star camera mounting brackets. Beryllium provides a rigid optical platform,
superior thermal diffusivity for removal of waste heat, an athermal optical train, and
a minimum total mass. With these beryllium components, the design illustrated in
Figure 5 has a mass of ~ 60 kg. Mass of the altimetry electronics, instrument
computer, thermal, power, and GPS receiver subsystems bring the instrument total
to ~ 100 kg.
The pulsed transmitter is based on high-power neodymium (Nd)-doped
solid-state laser crystals and employs the Q-switching technique to concentrate
laser energy in a short pulse. Each of the 30 laser transmitter modules illustrated
in Figure 5 is optically-pumped by separate AIGaAs laser diode arrays that are
coupled into the Nd laser crystal by fiber-optic cables. This results in an all-optical
laser module that is separated from the electronic and power supply components of
the laser subsystem. The majority of thermal dissipation for the laser modules can
thus be grouped together and placed at a remote radiator location in the spacecraft
instrument. The laser module design is a scaled version of present-day
commercial diode-pumped Nd laser technology that is in use in NASA airborne
laser altimeter systems. The illustrated array of laser transmitter modules is
capable of producing ~ 7000 pulses-per-sec and requires average input electrical
power of 1 kW when operational.
Optical backscatter from the Earth's surface is collected by the MBLA
telescope that is fixed in orientation at the nadir track of the spacecraft. A series of
two optical lenses and optical bandpass filters are used to collimate, filter, and then
focus the backscattered radiation on the detector plane. Each laser transmitter
element is angle-mapped into a silicon avalanche photodiode detector array
element for a continuous two-dimensional range and reflectivity image of the
surface. Energy measurements are made for the transmitted and received laser
pulses and are affected by the refiectivity and transmission of the various optical
surfaces in the instrument as well as detector sensitivity. Optical contamination and
degradation for long term exposure to the space environment are potential
problems. An on-board laser diode emitter can be utilized as a calibration source
and coupled with fiber optics into the optical detectors. This method will maintain
calibration for the energy measurement for surface reflectance studies.
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Pointing attitude knowledge is generated by dual star cameras. Each camera
is a second-generation star tracker that employs a 2-dimensional CCD array that is
capable of simultaneous tracking of as many as five stars. On-board Kalmann-
filtering is utilized to compare stellar angular position data with a star catalog and
provide an output pointing attitude estimate; in principle eliminating the need for an
inertial reference unit. Both star cameras are capable of 1 arc sec (total angle)
pointing knowledge limited by the quality of the star catalog. The dual star camera
system can provide the 2 arc sec total angle (1-sigma) knowledge required for sub-
meter accuracy altimetry. Pointing angle data are continuously generated with-
respect-to the stellar inertial reference frame. Laser pointing angles are tied to the
star camera data through beam angle sensors which sample a portion of one or
more output laser pulses and measure the angle (at the arc-sec level) of reflection
from reference mirrors on the star cameras. Output laser pulse samples are also
coupled with retroreflectors directly into the receiver telescope in order to assess
alignment shifts between telescope and lasers.
The pulse timing data, waveform digitizer, and pulse energy data form the
basic laser altimeter dataset for each laser pulse. These data points accumulate in
a buffer in digital form, are formatted into data blocks or files, and then enter the
altimeter platform data stream for recording or telemetry. The expected data rate of
the laser altimeter resulting from 1000 - 7000 laser pulse measurements a
secondis estimated to range from 10 - 50 kbps. On-board processing is planned
for the ranging centroid correction and waveform shape data products. Data
telemetry involves range-to-the-surface, waveform shape products, sensor
housekeeping, pointing attitude, and GPS position data. Post processing on the
ground is used to correct the altimetry data for the precision orbit and spacecraft
pointing attitude. The basic data products are a gridded topographic map
containing >1010 surface elevation measurements with the selected horizontal
resolution (grid size) and an image of Earth's surface reflectance at 1 _m
wavelength with a similar level of detail. These products will be developed with
minimal ground processing, stored on compact optical disks, and made available
to the scientific community in a timely fashion. Analysis and use of the topographic
data will be done by the end user in the scientific community.
REFERENCE:
Gardner, C.S., "Generalized Target Signature Analysis for Laser Altimeters",
Applied Optics, 1992.
ACKNOWLEDGMENTS:
The definition of the MBLA instrument was made possible through the efforts of a
number of individuals in the Laboratory for Terrestrial Physics (LTP) at the Goddard
Space Flight Center and' in the aerospace industry. Notable among these efforts
were those of David Rabine in the University Research Foundation of the Maryland
Advanced Development Laboratory, Nita Walsh of Science Systems and
Applications, Inc., Ying Hsu of the Optical Corporation of America, David
Thompson of Spectrum Astro, Inc., and Tom Baer, Spectra Physics, Inc.
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MULTI-BEAM LASER ALTIMETER MISSION CONCEPT
EARTH PROBE
SPACECRAFT
400 km
SUN-SYNCHRONOUS
6am/6pm
POLAR ORBIT
SUNLIGHT
LASER PULSES (30)
1 p.m
WAVELENGTH
NADIR
TRACK
SENSOR
FOOTPRINT
30 m diam.
9OO m
SWATH
WIDTH
ACROSS
TRACK
DIRECTION
Fig. 1
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