Download Chirped amplitude modulation ladar for range and

Chirped amplitude modulation ladar for range and Doppler
measurements and 3-D imaging
Barry Stann*, Brian C. Redman, William Lawler, Mark Giza, John Dammann
Army Research Laboratory (ARL), 2800 Powder Mill Road, Adelphi, MD 20783
Keith Krapels
Office of Naval Research, 875 N. Randolph St., Ste. 1425, Arlington, VA 22203-1995
ABSTRACT
Shipboard infrared search and track (IRST) systems can detect sea-skimming anti-ship missiles at long ranges, but
cannot distinguish missiles from slowly moving false targets and clutter. In a joint Army-Navy program, the Army
Research Laboratory (ARL) is developing a ladar to provide unambiguous range and velocity measurements of targets
detected by the distributed aperture system (DAS) IRST system being developed by the Naval Research Laboratory
(NRL) sponsored by the Office of Naval Research (ONR). By using the ladar's range and velocity data, false alarms and
clutter objects will be distinguished from incoming missiles. Because the ladar uses an array receiver, it can also
provide three-dimensional (3-D) imagery of potential threats at closer ranges in support of the force
protection/situational awareness mission. The ladar development is being accomplished in two phases. In Phase I, ARL
designed, built, and reported on an initial breadboard ladar for proof-of-principle static platform field tests. In Phase II,
ARL was tasked to design, and test an advanced breadboard ladar that corrected various shortcomings in the transmitter
optics and receiver electronics and improved the signal processing and display code. The advanced breadboard will
include a high power laser source utilizing a long pulse erbium amplifier built under contract. Because award of the
contract for the erbium amplifier was delayed, final assembly of the advanced ladar is delayed. In the course of this
year’s work we built a “research receiver” to facilitate design revisions, and when combined with a low-power laser,
enabled us to demonstrate the viability of the components and subsystems comprising the advanced ladar.
Key Words: Ladar, Lidar, fm-cw, chirped AM, range-Doppler, tracking, 3D imaging, cruise missile, force protection,
IRST
1.0 INTRODUCTION
Shipboard infrared search and track (IRST) systems are capable of detecting sea-skimming anti-ship missiles at long
ranges. Since IRST systems cannot measure range and line-of-sight (LOS) velocity, they have difficulty distinguishing
missiles from slowly moving false targets and clutter. Radars can provide range and velocity measurements to
distinguish high-speed targets from slowly moving false targets and clutter. Radars, however, have difficulty operating
in the marine boundary layer against near surface targets due to scattering from the sea surface and ducting in the
marine boundary layer. In addition, ships desire to minimize the use of their radars to reduce their chances of being
detected by enemy combatants. The Office of Naval Research (ONR) has requested that the Army Research Laboratory
(ARL) develop a ladar to augment the performance of the distributed aperture system infrared search and track (DASIRST) system under development at the Naval Research Laboratory (NRL). Unlike radar, the small beamwidth of a
ladar enables it to operate against near surface (3 m - 10 m above the sea surface) targets such as sea-skimming cruise
missiles. The small beamwidth also makes the ladar less detectable by enemy combatants than radar. The ladar will
measure the target's three-dimensional (3-D) position and velocity. The measured velocity of the target will be used to
classify the target as a valid missile target or a false target. In addition to the cruise missile tracking mission, ONR has
requested that the ladar be able to provide high resolution 3-D images of closer, extended targets for force
protection/situational awareness. By using a receiver based on Intevac Inc.'s Short Wave InfraRed (SWIR) band
Electron Bombarded Active Pixel Sensor (EBAPS), ARL's ladar can perform both missions.
*
[email protected]; phone 1 301 394-3141; fax 1 301 394-5270; arl.army.mil
The principles of operation for the chirped AM ladar for range and velocity measurements, the mission requirements,
the ladar performance model, the performance predictions for the ladar breadboard, and the results of the field tests at
ARL and at the Navy's Chesapeake Bay Detachment (CBD) facility with the initial chirped AM ladar breadboard were
presented at this meeting last year.1 Since that time, ARL was tasked to design, and test an advanced breadboard ladar
that corrected various shortcomings in the transmitter optics and receiver electronics and improved the signal processing
and display code. To reach the required ranges for the target track and force protection, the advanced breadboard will
include a high power laser source utilizing a long pulse erbium amplifier built under contract. Because award of the
contract for the erbium amplifier was delayed, final assembly of the advanced ladar is delayed. Our plan for improving
the receiver electronics included building a “research receiver” in a larger enclosure to facilitate experimentation. Work
with this “research receiver” allowed us to incorporate and test the following improvements: an order-of-magnitude
increase in photocathode quantum efficiency through the purchase of full-spec image tubes, more uniform and greater
modulation depth across the chirp bandwidth, a more compact chirp generator, improved thermal management, quasireal-time data processing and display, and quasi-real-time image motion compensation. We also developed a 1 W
illumination source for the “research receiver” based on a laser diode built by Quintessence Photonics Corporation that
features separate oscillator and amplifier sections. This laser is efficiently modulated with a low power microwave
amplifier and if a planned fiber coupled version is developed it will serve as a cost effective exciter for the high power
erbium amplifier and may simplify the amplifier design by eliminating some preamplifier stages. This paper reviews the
theory, planned design, and upgrades of the various subsystems and components for the advanced Army-Navy ladar and
discusses testing of a short-range ladar built around the “research receiver” and the illumination source.
2.0 ARCHITECTURE AND THEORY OF THE ARMY-NAVY LADAR
A block diagram of ARL’s chirped AM ladar architecture using an image tube as a receiver is shown in Figure 1. The
principle of operation is briefly summarized here but is presented in detail elsewhere. 2 The first stage of the transmitter
is a chirp generator that produces a microwave signal whose frequency increases linearly with time over some period
which is the frame time of the ladar. The start frequency for the chirp generator is typically in the low 10s of MHz and
the stop frequency is in the 100s of MHz; the difference in the start and stop frequency ΔF sets the range resolution ΔR
equal to c/ ΔF where c is the velocity of light. The chirp generator output modulates the current driving a low-power
diode laser that then yields an amplitude modulated light signal. This light is fed into an erbium fiber amplifier and the
output is fed into an optical system which directs the amplified light onto the target scene.
Light reflected from the target is imaged on to the cathode of an image tube (EBAPS, electron bombarded active pixel
sensor). Photoelectrons emitted from the cathode into the tube vacuum are accelerated toward the detector array of the
active pixel sensor. Upon striking the detector additional hole-electron pairs are created in the detector effectively
causing current gain. A delayed version of the chirp waveform (local oscillator (LO) voltage) is applied to the cathodeanode voltage of the EBAPS tube to modulate the tube's gain. Since the output current of an EBAPS pixel is linearly
proportional to the tube's gain and to the received laser power, the output current of an EBAPS pixel will be
proportional to the product of the LO voltage and the received laser power in the pixel. This arrangement, therefore,
provides mixing of the received laser's intensity modulation with the LO voltage. Since the received laser signal is
delayed with respect to the LO voltage due to the round-trip time to the target, the received laser signal will be at a
slightly lower frequency than that of the LO. Because the integration time of the detector in the EBAPS provides low
pass filtering, the sum frequency of mixing is lost and the difference frequency or intermediate frequency (IF) is
retained.
For the linear chirp waveform, the IF is linearly proportional to the round-trip delay time to the target, and therefore,
also to the target range. By taking the magnitude of the temporal Fast Fourier Transform (FFT) (i.e., the magnitude
spectrum) of the detector's output over a chirp period, the range and signal amplitude for the target(s) in that detector's
instantaneous field-of-view (IFOV) can be measured. By using separate chirp generators on the transmitter and receiver
with a variable delay between them to generate the transmitter and LO chirps, the IF can be kept within the limited
bandwidth of the detector and readout circuitry of the EBAPS for targets at long ranges. The magnitude spectrum is
computed for every pixel to form a 3-D intensity image of the target.
In summary, ARL's chirped AM ladar uses optical direct detection with coherent mixing in the radio-frequency (rf)
domain. The intensity of the transmitted laser beam and the gain of the receiver are modulated with the chirp rf
waveform over a wide bandwidth. Mixing at the receiver recovers an IF signal which contains all of the amplitude,
range, and phase information for the scatterers in a pixel’s IFOV. The Fourier transform over all pixels maps the data
space into a 3-D ladar image. In contrast to optically coherent FM/CW ladars for which the carrier is the laser light
wave and the frequency of the laser light wave is shifted linearly with time, for chirped AM ladars, the frequency of the
laser's light wave is not changed, only the frequency of the laser's intensity modulation is changed. Also, in contrast to
optically coherent FM/CW ladars for which the LO is a laser light wave, for chirped AM ladars, the LO is an RF
voltage waveform that modulates the receiver gain.
Fig. 1. Block diagram of the chirped AM ladar architecture with an image tube receiver.
As explained in detail elsewhere, to recover Doppler for cruise missile tracking, samples from a contiguous series of
chirps arranged in two-dimensional (2-D) data space are collected.3 Time samples within a chirp sample the IF signal,
and each chirp samples the change in start phase of the IF signal from chirp-to-chirp. The time rate-of-change of the IF
signal's start phase corresponds to the Doppler frequency shift due to the target's line-of-sight (LOS) velocity. The chirp
bandwidth and chirp rate are designed such that the target moves less than one range resolution cell during the entire
measurement time over the number of chirps comprising the range-Doppler data space. The chirp rate and chirp center
frequency are also designed such that the highest Doppler frequency shift, corresponding to the maximum target
velocity, is sufficiently sampled (Nyquist Theorem) by the chirp rate. The temporal FFT is performed on each
individual chirp, and the results for each sequential chirp are arranged as sequential rows in a 2-D array. An FFT is then
performed along each column of this array. The magnitude of each element of the resulting complex 2-D array is then
taken. The result is a 2-D array for which the columns represent IF and therefore, range cells and the rows represent
Doppler frequency shift and therefore, velocity cells. For a target moving at a constant velocity along the LOS, there
will be a sharp peak in amplitude in this array at the range-Doppler resolution cell corresponding to the target's range
and LOS velocity. The peak amplitude is proportional to the signal strength.
3. GENERAL
As mentioned in the introduction, we built a “research receiver” to facilitate redesign and testing of the receiver and
other components and software for the advanced Army-Navy ladar breadboard. We also built a 1 W illuminator
designed around a commercial diode laser featuring separate oscillator and amplifier sections. During the fiscal year we
were invited to participate in an exercise to collect foliage penetration (FOPEN) data which required a ladar with a
shorter maximum range (100 m), higher range resolution (.5 m), and a larger field of view (4-6 degrees). As a result, the
“research receiver” and illuminator work transitioned into an effort to build a full-up ladar for FOPEN data collection
(FOPEN ladar). In the remainder of the paper we structure the text to discuss work to build the advanced Army-Navy
ladar and the FOPEN ladar in two separate sections. The section covering the advanced breadboard will include a
discussion of the components only unique to that breadboard while the section on the FOPEN ladar will cover the
balance of the components.
4. ADVANCED ARMY-NAVY LADAR
4.1 Planned transceiver layout
Figure 2 shows the planned layout of the advanced breadboard ladar transceiver. The chirp generator, DFB diode laser,
Mach-Zehnder electro-optic modulator (EOM), located on an "arm" projecting from the side of the receiver telescope
supply a low power “seed” light signal to the high power erbium-doped fiber amplifier (EDFA). Light output from the
EDFA is fed into beam expander comprised of a small concave lens and the transmitter telescope. An enclosure
mounted on the transmitter telescope contains pointing control hardware for insuring that the output beam is
appropriately routed through the telescope. The EDFA is located off of the transceiver breadboard to decrease its weight
for mounting on a tripod or gimbal. The receiver electronics is mounted on an “arm” at the output of the receive
telescope. The operator station control computer/electronics and the data capture, processing, storage, and display
computer/electronics are located off of the transceiver breadboard. At this time the chirp generator, DFB diode
laser/Mach-Zehnder modulator, and receiver modules are built and tested. We have tested most of the
computer/electronics functions during work with the “research receiver”. The pointing and focus control assembly is
also completed and tested. Some improvements to the mechanical structures that support the telescopes are necessary
and new microwave cables need to be built.
High Power Long Pulse EDFA
Fig. 2. Layout of the Army-Navy advanced breadboard ladar transceiver.
4.2. Transmitter optics
As shown in figure 2, the fiber amplifier's output is routed via a fiber pigtail from the amplifier to a transmitter box
attached to the top Meade Schmidt-Newtonian telescope. The transmitter box contains a picomotor equipped New
Focus tip-tilt mount on a Thorlabs miniature x-y translation stage for providing alignment control of the fiber output
coupler to the telescope. The tip-tilt and translation stages are mounted on a miniature rail at an angle to the optical axis
to implement the off-axis beam expander configuration described below. The fiber output coupler is moved along the
rail to provide focusing/defocusing of the transmitted beam output by the telescope from a diffraction-limited,
collimated beam divergence of 20.7 rad to one matching the full-frame receiver's maximum FOV of 7.56 mrad. When
collimated, the beam expansion ratio is set by the ratio of the telescopes focal length (1016 mm) to the transmitter
focusing lens focal length (16 mm) and is 63.5 X. Thus, the output beam diameter at the telescope aperture is 95.25 mm
for the input beam diameter of 1.5 mm.
In order to eliminate the diffraction ring pattern caused by the spherical aberrations due to the large diameter spherical
lens in the previously used Galilean beam expander, we investigated several designs for the transmitter beam expander
that included aspheric lenses and/or mirrors. The use of aspheric optical surfaces, however, is generally expensive. We
found a design by Jeffrey Oseas which uses a combination of refractive and reflective optics, all with spherical surfaces,
but with excellent correction for spherical aberrations. 4 In Oseas' design shown in Figure 3, a lens with spherical
surfaces at the input to the beam expander is tilted to provide compensation for the spherical aberration of the off-axis
spherical secondary and primary mirrors. The "right" choice of curvatures and tilts for the spherical surfaces results in
excellent correction of spherical aberrations.
In a variation of Oseas’ design, we decided to use a spherical lens (or combination of spherical lenses) to focus the input
laser beam off-axis but in the image plane of a Schmidt-Newtonian telescope. As shown in Figure 3, from focus, the
expanding laser beam is directed off-axis to the spherical primary mirror by the planar secondary mirror. The primary
mirror then collimates the expanded laser beam and directs it to one side of the secondary through the Schmidt corrector
plate at the output of the beam expander telescope. In the Meade LXD55 SN10 telescope we are using, the Schmidt
corrector plate corrects for the spherical aberration over a large area of the focal plane, and does a good job of
eliminating the diffraction rings due to spherical aberration even with the laser beam directed off-axis. In this case, the
Schmidt corrector plate is an aspheric refractive optic at the output of the beam expander, rather than a spherical
refractive optic at the input to the beam expander as in Oseas' design. Despite the aspheric corrector plate, large aperture
Schmidt-Newtonian telescopes are produced inexpensively (< $1.4 K for the Meade telescope we are using) for the
amateur astronomy market.
Fig. 3. Transmitter beam expander design to eliminate spherical aberration induced diffraction rings.
4.3 Receive optics
The receiver optical system consists of a Meade LXD75 SN-10 Schmidt-Newtonian telescope with an input aperture of
25.4 cm (10") diameter with an 8 cm diameter central obstruction and a 1.016 m focal length (F/4). For short range
operation, the Schmidt-Newtonian telescope can be replaced with any small C-mount camera/video lens to obtain
various aperture diameters and focal lengths. A 10 nm full width at half-maximum (FWHM) narrowband optical filter
centered at 1550 nm is mounted in the optical path before the receiver sensor. The transmission of the telescope at 1550
nm is only about 50% since the anti-reflection coatings on the Schmidt corrector plate were designed for visible light.
The narrowband optical filter's peak transmission is about 60%, thus, the overall receiver transmission is only about
30%.
4.4 Seed laser
The seed laser module for the EDFA is a distributed feedback (DFB) diode laser fiber coupled to a waveguide MachZehnder electro-optical modulator, the output of which is fiber coupled to the EDFA. The DFB laser provides 20 mW of
light output at 1548 nm which is attenuated to about 4mW at the output of the modulator when modulated. An rf source
providing 40 mW of power attains a modulation depth of about 95% over the operating frequency range of 25 to 400
MHz; far greater modulation bandwidths are achievable because the electro-optical modulator bandwidth is 10 GHz.
The seed laser components are built into a 4” X 6” box along with a thermo-electric cooler and laser driver using buck
converter technology which yields high prime power efficiency and low heat build-up.
4.5 High power Erbium amplifier
A contractor is presently working to build a breadboard of a high power “long pulse” erbium amplifier for use in the
advanced ladar. The desired specifications for the erbium amplifier and the rationale for them are presented in detail in
an earlier reference3. For the reader’s convenience a summary of the specifications is presented here.




Operate at “L” band (1590 nm).
Achieve 10 mJ pulses at 3 kHz repetition frequency with continuous pumping.
Produce 10 mJ distortion-free 200 μs pulses at 1 kHz repetition frequency with pulse pumping at rated pump diode
powers.
Produce 10 mJ distortion-free 67 μs pulses with three times overdrive of the pump diodes.
At this writing the contractor demonstrated operation of a two stage preamplifier that yields approximately rectangular 1
mJ pulses with continuous pumping and a triangular seed pulse.
5. FOPEN LADAR
5.1 FOPEN ladar transceiver
Figure 4a is a photo of the FOPEN ladar transceiver which is built on a 1’ X 2’ optics board. The microwave chirp
generator is mounted on the right-hand side of the board, the receiver is located in the center, and the laser illuminator is
located on the left. Figure 4b shows the rear of the transceiver and reveals the microwave cabling connecting the
synthesizer to the receiver and illuminator that handle the laser modulation, receiver LO signal, high voltage gating
signal, CMOS sensor clock and vertical synch signals. The transceiver runs on 120 VAC; the AC to DC power supplies
are all mounted on the bottom of the optics board. Not shown is the PC that performs all data acquisition, signal
processing, and data display. In the following text we describe the design and performance of the major transceiver
components in detail.
Fig. 4. a. Transceiver front view.
b. Transceiver rear view.
5.2 Chirp generator
The chirp generator consists of two synchronized chirp generators-one for the laser illuminator and one to provide the
receiver LO. The start of the LO chirp signal can be delayed by some pre-set time after the start of the laser chirp signal
resulting in a shorter apparent round-trip delay and, thus, a lower IF. This moves the start of the range swath of the
ladar’s three-dimensional image to a desired offset range in both the imaging and tracking modes of operation. For the
imaging mode ranges in particular, the IF will normally exceed the sampling rate (frame rate) of the EBAPS thus
offsetting the range swath is often required. Each chirp generator uses an Analog Devices AD9858 Direct Digital
Synthesizer (DDS) chip. Synchronization between the two is implemented using an FPGA that controls the frequency
update signals to the DDSs. Various programmable registers and counters in the FPGA control the start frequency,
frequency step size, dwell time, and delay attributes of the chirp waveforms generated by the DDSs. Each DDS is
capable of 400 MHz bandwidth and feeds a low power microwave amplifier (100 mW rf output) for driving the
respective illuminator or receiver. This year we revised the chirp generator design to improve performance and reduce
size. By using rf surface mount components, the unit’s size decreased from a 19” rack mount box to the present
standalone box of approximately 5”x7”. Other improvements include moving the 1 GHz master oscillator, which
provides the clock for the DDS’s and, ultimately, the full system, to a location inside the synthesizer enclosure to reduce
cabling and the overall footprint.
We are using a personal computer (PC) to setup and control the chirp generator, high voltage supply, and the EBAPS
through a communication channel now based on the Total Phase AardvarkTM SPI/USB interface host adapter board that
is built into the chirp generator. Setup data entered on the PC are transferred sequentially to unique registers in all
devices. The FPGA handles the critical timing signals, such as triggering the rf chirp generation and the gate for the
high-voltage and the rf modulation. The FPGA also monitors the video vertical synchronization (v-sync) signal from the
EBAPS camera and the digital frame grabber card status during data collection. Registers in the receiver printed circuit
board and the EBAPS store the voltage settings for the high voltage source and the read-out parameters, respectively.
5.3 Laser Diode Illuminator
Figure 5 is a close-up of the laser diode illuminator used for the FOPEN ladar. The heart of the illuminator is a diode
laser built by Quintessence Photonics Corporation that has separate oscillator and amplifier sections. This design
delivers 1 W at 1550 nm at a single frequency and with a single mode. Supply currents for the oscillator and amplifier
are nominally .7 A and 4 A, respectively. A laser driver built by Analog Technologies supplies the oscillator current and
a laser driver by Wavelength Electronics, Incorporated (outside of the laser enclosure) supplies the amplifier section.
The laser is mounted on an aluminum block that is thermo-electrically (TE) cooled to a few degrees below ambient; the
TE cooler is driven by an Analog Technologies product. Output from the laser is collected with a Melles Griot
collimator mounted in an X-Y-Z stage and directed toward the target scene. By defocusing the collimator, the laser will
flood a roughly rectangular area that approximately matches the receiver FOV for a 128X128 image when the focal
length for the receiving lens is 50 to 75 mm. Because of the laser’s single mode operation, the illumination field is
considerably much more uniform than for high power broad area lasers used in a similar fashion and could probably be
tailored to fit the receiver FOV more exactly with a custom lens design. An important benefit of the separate
oscillator/amplifier design is the ability to amplitude modulate the laser output over a broad bandwidth with a low
power microwave amplifier. In our design we use a low-priced 1 W amplifier built by Mini-Circuits that has been
removed from its original housing and mounted in the laser enclosure. We removed the transformer from the
amplifier’s output circuit and installed our own based on a classical transmission line design with a 4:1 transformation
ratio. With this circuit we obtain a very flat modulation index out to about 440 MHz which is adequate because the
bandwidth of the modulation circuit for the receiver is limited to about 330 MHz. This was our first attempt to modulate
the Quintessence laser and we performed nearly no experimentation to extend the bandwidth therefore it is likely that
more bandwidth and performance can be achieved.
RF amplifier
Laser
TE cooler controller
XYZ stage
Transmission line
Xformer
Oscillator controller
Amplifier controller
Fig. 5. Diode laser illuminator.
5.4 Receiver
The receiver is built around the EBAPS image tube that is manufactured by the Intevac Corporation. It has an InGaAs
photocathode with good quantum efficiency (15% to 38% depending on the particular tube and Schottky barrier bias
voltage) in the SWIR band from 950 nm to 1650 nm at variable gains of up to 300. However, for the initial breadboard
testing reported last year, we were able to use only an EBAPS tube that was rejected from Intevac's production line,
provided to us free of charge under our Cooperative Research and Development Agreement (CRDA) with Intevac. This
tube had a quantum efficiency of only about 2%, and had several blemishes, but was sufficient for proof-of-principle
testing. This year we purchased three new tubes- two full spec units and one subspec unit characterized by minor
blemishes. The tube anode (EBAPS) is a Kodak KAC-9638 silicon CMOS FPA that is thinned and electron bombarded
on the backside to provide nearly 100% fill factor. The KAC-9638 can be set to read out only over a specified region of
interest in the focal plane (windowing) at vastly increased frame rates. Adjacent pixels in the window can also be
averaged together (binned) to further increase the frame rate. The photocathode also has a Schottky barrier deposited on
the vacuum side of the photocathode. The photocathode quantum efficiency depends on the Schottky barrier bias
voltage, which is nominally biased at a voltage from about +2 V to +6 V depending on the particular tube.
As discussed in prior literature, the challenge to using the EBAPS as a receiver required adding the LO voltage to the
usual gated high voltage across the cathode to anode without disrupting operation of the EBAPS. To achieve this we
designed a new printed circuit board (header board) that supplied power, clock and control signals to the EBAPS and
fed out the digital image data. We took particular care to insure that all leads to ground from the EBAPS were very
short and direct and used a socket for the EBAPS with as short as possible paths from the EBAPS package to the circuit
board pads. Additionally, we built a circuit board that added the LO voltage to the high voltage dc supply. This board
contains circuitry to isolate the output of the preceding microwave amplifier from the high voltage and the transients
induced by the gating of the high voltage supply.
This year we revised the printed circuit layout of the header board to incorporate changes that further improved
performance and provided some simplification. We retained successful design features of last year’s board including
internal ground and power planes, microwave and low frequency circuitry positioned on opposite sides of the board, and
short path connection to ground of any grounded leads on the tube. We also revised the header board to accept a new
socket for the tube that is more mechanically robust than last year’s socket which tended to wear-out through multiple
insertions and removals of the tube causing intermittent connections. We found that inductive beads were still necessary
on all supply and control lines, but not on the outgoing data lines thereby providing some simplification. We changed
the PC layout to accept SMA connectors to replace the solid copper coax that brought in the CMOS sensor clock signal
and picked up the sensor’s vertical sync signal. We also added a wideband rf transformer at the clock input to the tube
to effectively isolate the tube ground from the clock driver ground which is positioned in the receiver box. This step
also reduced instances of the EBAPS stopping.
A close-up of the top view of the receiver module is shown in Figure 6. Here the microwave board is soldered to the
back of the header board. Spring-loaded pins on the microwave board protrude through the header board to contact pads
on the rear of the tube that are connected to the cathode. The microwave amplifier in the photo generates a large signal
LO voltage applied to the tube. It is positioned close to the tube to eliminate large variations in signal amplitude with
frequency that occurs when driving a large impedance mismatch such as the tube anode-to- cathode impedance through
long coaxial cables. The amplifier is capable of providing 50 W of power into a 50 ohm load over a band from 20 MHz
to 500 MHz. With the tube load impedance, the amplifier provides up to about 250 Vp-p over a band from 10 MHz to
330 MHz and is capable of operating stably with this load. The high voltage supply is mounted on the rear wall of the
receiver housing. Ancillary circuitry consisting of line drivers for the clock and Vsynch signals and regulators for the
EBAPS and the high voltage supply are in the gold colored box mounted on the side of the receiver. The fan which was
added this year prevents heat build-up from the microwave amplifier. We have conducted tests showing that the EBAPS
operates very reliably over the chirp bandwidth; the mean output of the tube sensor over a chirp is very constant thereby
further reducing a “self clutter signal” over last year’s design; and the modulation index is improved because more rf
voltage can be applied across the tube without causing interference or stopping the EBAPS.
Fan
Microwave board
Microwave amplifier
High voltage supply
Line drivers
Fig. 6. Close-up photo of receiver.
5.5 Operator station control and display software
We set the operating parameters for the ladar high voltage supply, EBAPS, and synthesizer using the “Master
Controller” code which is written in Labview. For the high voltage supply, control inputs are available for setting the
cathode-to-anode voltage and the Schottky barrier bias and reading feedback from the supply on the state of these
settings. The EBAPS parameters are also set from an array of control inputs, the most important being the amplifier gain
and offset, frame rate, window size and position, and pixel binning. Control inputs for the chirp generator include the
number of frequency steps in the chirp, the chirp start and stop frequencies, the gate width, and drive levels for the
illuminator and LO. Separate clickable buttons are provided to load the high voltage, EBAPS, and synthesizer settings.
Data processing and display is performed in the Open Inventor TM programming environment. Starting this code bringsup five windows, the three most important include-one showing processes that the CPU is currently working, a twodimensional display showing EBAPS frame data in real-time, and a three-dimensional display of the ladar data. The
two-dimensional display allows the system user to operate the system passively or actively i.e. with the laser on) but
with no LO applied to the image tube. The user can then manually set the light level and receive lens focus without
concern for possible damage to the tube that may occur with the LO applied continuously. With the LO on, the user
simply clicks a button on the side of the two-dimensional window to collect successive EBAPS frames comprising a
raw ladar data set. The data is immediately processed into a ladar image and displayed in the three-dimensional window
where a user can zoom and arbitrarily rotate the data. Buttons to the side of the three-dimensional window allow the
user to shift the threshold and apply a median filter which further improves the estimate of pixel range and therefore the
image appearance. Once the three-dimensional image is formed, the user can return to the two-dimensional image move
a pointer to a pixel in the image click and display graphs of the raw IF data and range-gated data (Fourier transformed
data) for that pixel. By moving the pointer into the range-gated plot the user can set the range swath shown in the threedimensional image by clicking on the desired ranges.
In the prior year’s field tests we tried to image a boat on the Chesapeake. Despite the calm conditions there was some
motion of the boat and ladar transceiver jitter that smeared the ladar imagery. To mitigate this problem, we implemented
the data acquisition, frame-to-frame registration, and 2-D and 3-D ladar imagery display in a quasi-real-time manner in
the Open InventorTM programming environment. Previously the XCAP TM software supplied with the Epix frame
grabber had been used to control the frame data capture. Starting a data acquisition had required switching back and
forth between the Master Controller window and the XCAPTM window to set up and initiate data acquisition manually.
With a moving target, the delays induced by this switching back and forth between windows often resulted in the target
moving out of the image frame before data capture started. In the new data acquisition, control, and display architecture,
after setting up the chirp parameters and sending them to the DDS in the Master Controller, the operator switches to and
remains in the Open Inventor window to select the image capture area, and to start image capture and display. The start
of image capture now occurs with a mouse click that indicates the position of the center of a user determined correlation
region window, which is described below. This mouse click then automatically sends a start data acquisition instruction
to the Master Controller via a file transfer.
The data frames are acquired sequentially in real-time to a memory buffer. Once the user defined set of memory buffers
is filled, the software immediately begins the frame-to-frame re-registration process. The user selects the size and
position within the image frame for a correlation window, as mentioned above, placing it around a high contrast area of
the target for which the image is to be stabilized against target and ladar platform motions. The user may also turn on 2D spatial median filtering to be applied to each captured image frame to improve the image signal-to-noise ratio (SNR)
prior to re-registration. This filtering was implemented to eliminate residual image jitter after re-registration of +/- 1
pixel due to noise found in most test imagery. A 2-D spatial FFT of the portion of the image in the correlation window
is performed for each of the first and second frames. Multiplying these two FFTs and performing the inverse FFT
produces the 2-D correlation between the two frames within the correlation window. The location of the correlation
peak indicates the number of pixels and the direction that the second frame must be shifted in order to align it with the
first. This shifting is handled in the software by shifting the position of pointers to the image data array rather than
shifting the data in the array in order to minimize the processing time. Subsequently, the correlation window's center is
shifted by the correlation offset to "track" the correlation region's motion, and the correlation and shifting process is
repeated for all subsequent frames in the buffer. Thus, all frames are re-aligned with the first frame such that the
correlation regions for all frames overlap.
In laboratory tests collecting 128 data frames of 640 x 512 pixels each with a 64 x 64 pixel correlation region, the total
time of processing from the end of data collection to completion of 3-D image processing including registration was
about 0.3 seconds. Since the data acquisition time for 128 frames at 50 frames per second (fps) is about 2.5 seconds, the
additional 0.3 seconds of processing time is only about 12% of the data acquisition time. Having a latency time between
end of data acquisition and completion of image processing for display that is a fraction of the data acquisition time is
what we mean by quasi-real-time operation.
5.6 FOPEN breadboard ladar test results
The performance goal for our FOPEN ladar is to form 128 X 128 images to 100 m over a 64 m range swath with .5 m
range resolution at a 1 Hz frame rate. Modelling showed that this was possible with 1 W transmitter power and a 5 cm
diameter receiver aperture although only in conditions where the solar background is about one-tenth of the peak value
(assumes a 10 nm solar filter). Here we show the results of operating the ladar on ARL’s outdoor rooftop range in figure
7. The image is of the top of a wooden step ladder positioned 25 m downrange. Using the diagnostic features of the
display code, we observed that the signal-to-noise (S/N) on the ladder supports is about 20 and the range accuracy is
well under 1 inch as evident in the side view of the image. The dark regions at the very top of the ladder are from a
metal bracket which had strong specular returns that saturated the receiver and suppressed the range information. This
image was collected during a clear Fall day around 1600; operating at noon generally saturated the receiver.
Front
Side
Fig. 7. Ladar image of top of a wooden step ladder.
In this first outdoor test the ladar had a number of shortcomings that reduced its performance below the theoretical.
After “zooming-in” on the image, we noted that the angular resolution was poor. This is attributable to the visible
camera lens used in the receiver which focuses to about a 40 μm spot at a wavelength of 1.55 μm; this spot is
substantially larger than the 12 μm pixel size of the EBAPS. To attain high range accuracy at 100 m as achieved in the
image at 25 m, we require that the S/N improve 16 fold. This increase should be obtainable if the following
shortcomings in the breadboard are corrected. At the time of this test, the laser amplifier was temporarily operated with
a laser driver producing only 2 A as opposed to the 4 A specified in the data sheet. As a result, the laser generated
approximately one-third of its rated output. We also could not reduce the size of the illumination field of the laser to
match the FOV of the receiver; this caused approximately a two-fold loss in laser power illuminating each pixel. As
another problem, we measured the losses in the receiver lens to be about 50 percent. Upon carefully observing the twodimensional image data from the tube in the absence of any light, we noticed that the first 10 pixels in a row were less
noisy relative to the remainder of the pixels in the row. By using the feature in the display code to plot the IF waveform
and Fourier transform of the IF waveform, we determined that the S/N in the received image was about three times
higher in the first 10 pixels in a row. We believe that by using a new image tube this loss in S/N can be recovered.
If all these losses are recovered, we should gain a factor of 36 (3 X 2 X 2 X 3) in S/N at 100 m which should yield a
quality image. As discussed in section 5.3, the laser illuminator is now built with a 4 A driver for the amplifier and now
yields it’s full output power thus the ladar will gain the threefold S/N over the original test. The illuminator’s bandwidth
is also improved over the initial design and as a result the IF waveform does not slowly drop in amplitude over the last
half of the modulation band. This will increase the signal-to-noise by some small factor. We have ordered a new lens
designed for operation at 1.55 μm which has a 50 mm focal length. With this focal length the receiver FOV will cover
the illumination field of the laser and gain the two fold loss in S/N. The new lens will be better than 85 percent efficient
thus nearly correcting for the losses in the old lens; unfortunately the aperture diameter is only 35 mm which will collect
only 50 percent of the light relative to the original 50 mm diameter lens so no S/N gain is expected through the lens
change. However, we hope to obtain improved angular resolution. We have also purchased a new image tube that
through better promised manufacturing techniques and screenings will yield low noise performance over the entire array
and thus improve the S/N threefold.
6. CONCLUSIONS
ARL is still working to build an improved EBAPS-based advanced breadboard ladar for cruise missile tracking and
force protection. This advanced breadboard incorporates the following improvements in the transmitter: a high power
long pulse erbium amplifier to acquire targets at the required ranges (10 km cruise missile tracking, 2-3 km force
protection) and a transmitter beam expander with a new design that eliminates spherical aberration induced diffraction
rings to produce a smoother illumination field. The following improvements will be implemented in the receiver: a new
image tube with high quantum efficiency and improved image quality, more uniform and greater modulation depth
across the chirp bandwidth, a more compact chirp generator, and improved thermal management. Software
improvements for signal processing and display include quasi-real-time data processing and display and quasi-real-time
image motion compensation. Initial laboratory testing of the advanced breadboard ladar demonstrated the capability of
the system to acquire, process, and display 3D imagery in near real-time, including near real-time frame-to-frame reregistration to compensate for platform and target motion. This capability will be particularly useful for long range field
tests with targets on the water at NRL’s CBD facility since these targets will be in constant motion due to waves and
currents. When the erbium amplifier is delivered from the contractor, ARL will perform final assembly of the ladar and
conduct a field test.
As a spin-off effort, ARL built a short-range breadboard ladar for FOPEN data collection that incorporates the same
improvements in the receiver and software as the advanced Army-Navy ladar. Tests of the ladar validated changes to
the receiver and other components and software also used in the advanced breadboard ladar. Imaging tests were
promising despite shortcomings in the laser illuminator power, receive optics focusing ability and transmission losses,
and degraded quality of the image tube. The laser illuminator now operates at full power and a new receive lens and
image tube have been purchased. Once these purchased items are installed and tested, we will conduct a FOPEN data
collection at Fort A.P. Hill.
ACKNOWLEDGMENTS
The ARL authors thank CDR Keith Krapels, Ph.D. of ONR for his support in terms of funding, field test support, and
technical review for this joint Army-Navy program.
REFERENCES
1.
2.
3.
4.
1
Brian C. Redman, Barry Stann, William Lawler, Mark Giza, John Dammann, William Ruff, William Potter,
Ronald G. Driggers, Jose Garcia, John Wilson, Keith, "Chirped AM ladar for anti-ship missile tracking and
force protection 3D imaging : update", Proceedings of SPIE Volume: 6214, Laser Radar Technology and
Applications XI, 2005.
2
Stann, B.L., W.C. Ruff, Z.G. Sztankay, “Intensity-modulated diode laser radar using frequency modulation/
continuous wave ranging techniques,” Optical Engineering, vol. 35, no. 11, pp 3270-3278, 1996.
3
Ibid.
4 Oseas, Jeffrey M., "RELATIVELY INEXPENSIVE WIDE−APERTURE LASER−BEAM EXPANDER,"
NASA Tech Brief, Vol. 26, No. 10, October 2002, from JPL NEW TECHNOLOGY REPORT NPO− 30432.