A high precision laser altimeter was developed under the Autonomous Landing and Hazard Avoidance (ALHAT) project at NASA Langley Research Center. The laser altimeter provides slant-path range measurements from operational ranges exceeding 30 km that will be used to support surface-relative state estimation and navigation during planetary descent and precision landing. The altimeter uses an advanced time-of-arrival receiver, which produces multiple signal-return range measurements from tens of kilometers with 5 cm precision. The transmitter is eye-safe, simplifying operations and testing on earth. The prototype is fully autonomous, and able to withstand the thermal and mechanical stresses experienced during test flights conducted aboard helicopters, fixed-wing aircraft, and Morpheus, a terrestrial rocket-powered vehicle developed by NASA Johnson Space Center. This paper provides an overview of the sensor and presents results obtained during recent field experiments including a helicopter flight test conducted in December 2012 and Morpheus flight tests conducted during March of 2014

Amzajerdian, Farzin

Barnes, Bruce W.

Pierrottet, Diego F.

NASA Technical Reports Server

A long distance Laser Altimeter for terrain relative navigation and 
spacecraft landing 
Diego F. Pierrottet 
Coherent Applications, Inc., 20 Research Drive, Suite 500, Hampton, Virginia, 23666, E-mail: 
d.f.pierrottet@cailabs.net 
Farzin Amzajerdian, Bruce Barnes 
NASA Langley Research Center, Hampton, Virginia 23682 
 
ABSTRACT 
A high precision laser altimeter was developed under the Autonomous Landing and Hazard Avoidance (ALHAT) 
project at NASA Langley Research Center.  The laser altimeter provides slant-path range measurements from 
operational ranges exceeding 30 km that will be used to support surface-relative state estimation and navigation 
during planetary descent and precision landing.  The altimeter uses an advanced time-of-arrival receiver, which 
produces multiple signal-return range measurements from tens of kilometers with 5 cm precision. The transmitter is 
eye-safe, simplifying operations and testing on earth.  The prototype is fully autonomous, and able to withstand the 
thermal and mechanical stresses experienced during test flights conducted aboard helicopters, fixed-wing aircraft, 
and Morpheus, a terrestrial rocket-powered vehicle developed by NASA Johnson Space Center.  This paper provides 
an overview of the sensor and presents results obtained during recent field experiments including a helicopter flight 
test conducted in December 2012 and Morpheus flight tests conducted during March of 2014.  
Key words: ALHAT, Lidar, range finder, altimeter 
INTRODUCTION 
As exploration on the surfaces of small planetary bodies continue, greater demands are placed on the technology 
driving the Guidance, Navigation, and Control (GN&C) of the landing vehicle.  Entry, descent and landing (EDL) 
challenges include the removal of the vehicles' kinetic energy in a thin atmosphere, coupled with requirements for 
precision delivery of payloads to potentially hazardous terrain.   
In support of future human or robotic exploration of Near Earth Objects (NEO), Lunar, and Mars missions, 
development of enhanced GN&C related technologies that enable the delivery of higher mass vehicles to surfaces at 
higher altitudes and with greater accuracy are neededi.  Accurate estimates of range and descent rate are needed for 
precision soft landing. NASA Langley Research Center, under the Autonomous Landing and Hazard Avoidance 
Technology (ALHAT) project is developing advanced, laser-based (lidar) sensors to meet NASA’s planetary exploration 
needs for precise, safe, and soft landing of payloads.  ALHAT is comprised of several systems fused for this purpose.  
These systems include; an Autonomous Flight Manager (AFM), GN&C algorithms, sensor algorithms for Terrain 
Relative Navigation (TRN), and commercial off the shelf (COTS) sensors such as IMUs, star trackers and altimeters 
representing the actual lander sensors that will provide vehicle state and attitude knowledge.  The primary function of the 
Laser Altimeter (LA) described in this paper is to provide information for the implementation of TRN.  The LA is a key 
ALHAT sensor that will measure range to the surface, and compare them to onboard reconnaissance data which will 
update the onboard navigation state with enough precision and accuracy to maintain minimal position error and dispersion 
along the descent trajectory.  Currently, the performance objectives for the altimeter are met using laser based technology. 
The receiver provides multiple range measurements from a single laser pulse, suitable for use in high altitude 
aircraft applications and test flights, where the laser beam may encounter several cloud layers or dust. This paper 
provides an overview of the sensor and presents results obtained during recent field experiments. 
https://ntrs.nasa.gov/search.jsp?R=20140006392 2019-08-29T14:03:39+00:00Z
SENSOR DESCRIPTION 
The LA was built in a compact and rugged housing in order to support ALHAT testing during hot weather, and aboard 
various high vibration test platforms.  Figure 1 shows the dimensions of the prototype sensor.  The housing was built 
around a Nd:YAG laser with an attached OPO to produce pulses at 1574 nm wavelength.  This wavelength is preferred 
because it is eye-safe at the operational pulse energies, which simplifies the safety logistics during field tests.  To 
accommodate the high ambient temperatures associated with tests sites such as summers in Death Valley, California, or 
the Nevada Test Site, an air cooled COTS laser was modified with active cooling and control system that maintains the 
laser temperature constant and thermally stable during ambient temperatures as high as 50-degrees C. The active cooling 
system requires convective heat removal using forced air, and thus approximately one third of the volume of the altimeter 
chassis is attributed to the eye-safe laser and the cooling requirements associated with field tests.  Future implementations 
of a space qualified system will not have these requirements, which will allow significant mass and volume reductions to 
the system.  The current system provides a proof of design for key sensor components. 
 
Figure 1.  Laser Altimeter chassis showing optical and electronic interfaces. 
 
As seen on Fig. 1, the chassis has several interface connectors and display on the side of the unit.  The connectors include 
output pulse monitor (OPM) for timing synchronization and diagnostics, signal monitor, GPS input (an option used during 
standalone testing), laser enable interconnect, Ethernet for input command and output telemetry, input power, and status 
display.  On the front of the unit, the receiver lens sits above the smaller transmitter port.  Immediately above the receiver 
lens is a USB bore-sight camera used for convenience in alignment and pointing.  The chassis can be mounted on a 
standard tripod for ground based testing.  To mitigate the high vibration environment of helicopters or rocket based 
Vertical Test Beds (VTB), six vibration isolation mounts are used to attach the sensor to the platform. 
The transmitter laser produces pulses of approximately 3 nsec, at pulse rate frequencies up to 50 Hz.  The pulse is spatially 
magnified to a collimated output diameter of approximately 30 mm, reducing its beam divergence. The optical receiver 
uses a 50 mm aperture, placed in a bistatic configuration, meaning the transmitter and the receiver field of view are 
mounted in parallel to each other.  The field of view of this receiver is slightly larger than the transmitter to accommodate 
spatial pointing jitter of the transmitter.  Figure 2 is a diagram describing the configuration of the primary system 
components. 
 
Figure 2. Laser altimeter system architecture block diagram. 
The detector is a thermally controlled InGaAs avalanche photo-diode, capable of very high sensitivity and low noise 
for optimized signal to noise ratio.  The detector with its pre-amplifier and signal conditioning circuit are all 
mounted in a monolithic assembly for electronic and mechanical robustness.  The detection circuitry is connected to 
the electronic receiver module where pulse detection is performed.  The receiver includes a constant false alarm rate 
(CFAR) circuit designed to maintain the false alarm rate to below 2%. 
As stated previously, the laser thermal stability is maintained by thermo-electric cooling system.  Power for the 
cooling system, laser, detector bias voltage, and system electronics are generated internally in the Power Module, 
from a common inline filter of the host vehicle power.   
The LA obtains slant range measurement from the ground by measuring the round trip time of flight of a laser pulse.  Each 
line of sight range measurement can directly feed the navigation system of a vehicle and significantly improve vehicle 
state calculations.  The LA is designed to provide range measurements from over 30 km with 2.5 cm digitizer 
precision.  An advanced time of flight (TOF) high speed counter was developed that provides a count resolution 
equivalent to 2.5 cm range.  The counter is initiated by high speed detector monitoring the outgoing pulse of the 
laser transmitter.  Once a signal is detected, the counter bits are latched and a second counter is initiated without 
losing a single clock cycle.  This second counter is identical to counter 1, and continues to count up until a second 
signal is detected. At this point, counter 2 bits are latched and counter 1 is again initiated.  This “Ping-Pong” 
architecture can continue to receive many signals from a single laser pulse.  Through software, the maximum 
number of detected signals was set to 3, where the last signal detected is used by the vehicle’s GN&C.  After each of 
the counter bits are latched, the system microprocessor collects the data and converts it into engineering units.  The 
data is stored internally, and is also placed in telemetry packets and sent to the host vehicle. All commands, data 
processing and external interfacing of the sensor are handled within the chassis by the Command and Data Handing 
(C&DH) unit.  During an operational flight, the LA will interface to and operate asynchronously with the navigation 
subsystem.  Commands will be received from the navigation subsystem, and output data products and system status 
are sent to the navigation subsystem.   The LA generates data products at 50 Hz and this data and status are 
packaged and returned to the navigation system at 5 Hz.  Data is time-stamped relative to a pulse-per-second (PPS) 
signal from the navigation system with one - microsecond precision.  All output data is recorded by the navigation 
subsystem and a subset of this information is transmitted via telemetry during flight for display and analysis to a 
ground station.   
FIELD TESTING 
In preparation of the various ALHAT field tests, the LA was characterized and tested at NASA LaRC, where 
measurements were taken from the top of the Gantry at a platform height of approximately 60 meters above the 
ground.  NASA LaRC sits on the Hampton Roads peninsula, surrounded by the Chesapeake Bay to the east and the 
James River to the west.  From the Gantry’s viewpoint, on a clear day, the Westin Hotel in Virginia Beach can be 
seen.  Figure 3 shows the line of sight distance measured from the Gantry to the Westin by the LA.  The 
measurements show a range measurement precision of 18 cm, and although a truth measurement is not available, the 
distance is confirmed from satellite images of the area.  The data also shows outliers with a 2 meter offset. This 
offset is due to an intermittent bit error in the counter, which was subsequently repaired. 
 
Figure 3. Line of sight range measurements obtained from the Gantry at NASA LaRC, to the Westin in Virginia Beach on 
a clear day in Hampton Roads. 
The laser altimeter has participated in several ALHAT field tests. The first field test was carried out in 2009, where 
the sensor was mounted on a gimbal inside a Beech B200 Super King airplane, and flown over the Nevada Test Site. 
The primary purpose of this test was to demonstrate the TRN algorithm developed for the ALHAT system.  Figure 4 
is a sample of the raw data collected by the altimeter.  The repetitive patterns of range correspond to several loops in 
a race track flight pattern over hills and craters over the desert. 
In 2010, ALHAT performed a field test campaign from NASA Dryden, in California.  The sensor was configured to 
collect data together with the 3-dimensional Flash Lidar, and the high precision Navigation Doppler Lidar, mounted 
in an instrumentation pod which in turn was carried by an Erickson Sky Crane helicopter.  This test provided further 
advances to the three instruments and their role within the ALHAT system. 
 Figure 4. Sample of flight test data from Beech B200 aircraft flown over Nevada Test Site in 2009. 
A similar test series was performed in 2012, this time at Kennedy Space Center, where a field was created, complete 
with rocks and craters, simulating potential hazards around a landing site.  For this test, the sensors could operate in 
an autonomous configuration, with real time telemetry outputs that would be provided to the navigation computer.  
Figure 5 shows the instruments mounting location underneath NASA’s Bell UH-1H Huey test helicopter. 
 
Figure 5  Laser altimeter mounted underneath the helicopter together with the Flash Lidar, and the Navigation Doppler 
Lidar at Kennedy Space Center. 
 Figure 6 The Morpheus/ALHAT test configuration. The laser altimeter is mounted on the lower support frame of the 
vehicle. 
Finally, the ALHAT sensors are mounted on the Morpheus Vertical Test Bed and operated in a total autonomous 
configuration, feeding sensor data to the ALHAT navigation computer.  The laser altimeter was mounted on the 
bottom support frame of the vehicle as shown in Fig. 6. It was wrapped with a protective thermal blanket to mitigate 
the rocket plume temperatures.  This field test helps to further advance the ALHAT system, raising the sensor’s 
technology readiness level.  At the time of writing this paper, three Morpheus free flights have been performed with 
the sensors on board in an open loop configuration to the navigation computer.  In the coming weeks, two closed 
loop flights are scheduled to complete the test series. 
 
Figure 7  Morpheus/ALHAT field testing at Kennedy Space Center. Altimeter data shown on right 
 
 
SUMMARY 
A prototype long distance Laser Altimeter instrument has been developed and tested as a navigation sensor for 
future space vehicles.  This Laser Altimeter is capable of range measurements from over 30 km with 2.5 cm 
resolution.  For convenient operation and testing from different platforms, the instrument is designed to be eye-safe 
and detect multiple returns with a single laser pulse so that the ground signal can be discriminated from signals from 
clouds or other clutter targets.  The rugged and compact design has allowed the laser altimeter to be tested on 
different platforms: fixed-wing aircraft, helicopter, and a rocket-propelled free-flyer vehicle. The latest tests onboard 
Morpheus free-flyer vehicle will demonstrate the Laser Altimeter as an integrated sensor of a planetary landing 
system capable of executing precision navigation to designated target location. Three open-loop flights have already 
been conducted in preparation for the closed-loop flights scheduled for May 2014.  The size, mass, and power of the 
current prototype system can be further reduced for an actual space flight unit by changing its operational 
wavelength from eye-safe 1.57 micron to 1.06 micron and simplifying the sensor’s thermal management design in 
accordance with the specific requirements of space environmental. 
Acknowledgments 
The authors are grateful to ALHAT project manager, Chirold Epp, NASA Johnson Space Center, for his guidance 
and support. We also would like to thank NASA's Advanced Exploration Systems (AES) program office for their 
continued support. The authors also acknowledge the ALHAT and Morpheus team members from NASA Johnson 
Space Center for facilitating the Morpheus flight tests at NASA Kennedy Space Center.  
.       
                                                          
iDennehy, C. J. (2010). Present Challenges, Critical Needs, and Future Technological Directions for NASA's GN&C Engineering 
Discipline. AIAA Guidance, Navitgation, and Control Conference. Toronto.  
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A Long Distance Laser Altimeter for Terrain Relative Navigation and Spacecraft Landing

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