ALHAT – Getting There Safe, Even In The Dark


Being able to navigate in the darkest of night, whether while driving a car, flying an airplane or helicopter, even landing a probe on a distant, never visited solar body is something that we cannot do safely today. But, imagine how much better our ability to do such things would be if drivers, rescue pilots, or astronauts did not need to see their surroundings to be able to safely navigate or land?

For all of our vaulted technology, here on Earth today, none of that is fully possibility.

Now, imagine how much more difficult the challenge is of landing on the Moon, 238,855 miles away, a scientific probe – perhaps one investigating the existence of Lunar ice – on the Moon’s north or south pole, close to a deep crater within which the Sun has not, for billions of years, shone even once. Or, even more challenging, safely landing a crew of astronauts in this dark and shadowy place, in a challenging follow-on to the Apollo program of 40 years ago.

Now we are talking science fiction.

Except, we aren’t because NASA is solving this problem and, as only NASA can, in an ingenious way.

NASA may not have a mission to someplace at the moment, but that does not mean its engineers and scientists are sitting still. Like a horse pulling at the bit to go, NASA and its people are eagerly pushing the extremes of the envelope. Within the next few years, NASA’s engineers, technicians, and programmers will make commonplace the ability to see in any lighting condition, even in the dark, by creating accurate 3D maps of the terrain, and do it on the fly. The NASA project that is undertaking this effort is called ALHAT, which stands for Autonomous Landing Hazard Avoidance Technology, but to this person who has just seen it at work, it’s almost magic. Just as it should be with things that NASA does.

ALHAT is funded through two NASA offices. The first is NASA’s Advanced Exploration Systems program, or AES, which is operated under the NASA Headquarters Human Exploration and Operations Mission Directorate, or HEOMD, and is led by Jason Crusan at NASA headquaters. The second is NASA’s Technology Demonstration Missions program, or TDM, which operated under the NASA Headquarters Office of the Chief Technologist, or OCT, and is led by John McDougal at Marshall Space Flight Center. The Advanced Exploration Systems and Technology Demonstration Missions offices are tasked with anticipating and developing the next generation of technologies that space explorers (robotic and crewed) will need in the years to come. To that end, these offices have supported the ALHAT project and organized it to take advantage of the large pool of talent available throughout the NASA community of NASA centers and contractors, such as Johnson Space Center, Jet Propulsion Laboratory, Langley Research Center, Draper Laboratory, and Johns Hopkins University’s Applied Physics Laboratory.

The ALHAT project comprises several forword-thinking technology development initiatives that will enable the next bold step in future robotic and crewed NASA, as well as commercial, lander missions back to the Moon, onward to Mars, other planets, and asteroids. The ALHAT technology components together enable safe landing for future missions where NASA never could have gone during Apollo, such as deep craters at the Moon’s north and south poles where the sun hasn’t shined for millions or billions of years.

To fully demonstrate and space qualify ALHAT, the Advanced Exploration Systems program, which is also funding the development of the Morpheus lander at Johnson Space Center, would like to launch a Morpheus lander with the ALHAT system to the Moon within 5 years on the 2017 launch of the Space Launch System, or SLS, and then land Morpheus safely on one of the Moon’s poles, thanks to ALHAT. This would enable the first SLS launch to deliver real science beyond that of verifying that the “Big Rocket”, as some call it, is ready to extend our human space exploration efforts beyond low Earth orbit, where we have been confined to since the last Apollo mission, Apollo 17, returned to Earth on December 19th, 1972.

ALHAT can bring the Morpheus lander to a safe landing in the darkest of places by using a set of technologies, each of which is in its own right pushing the envelope. The ALHAT collage of technologies includes the Hazard Detection System, a Doppler lidar velocimeter, a laser altimiter, and a Library of Congress-sized volume of advanced algorithms, software, and path-to-space computer processors to crunch the data, all of which integrate with a lander’s onboard navigation instrumentation.

Using the Morpheus mission tentatively scheduled for the 2017 SLS launch as an example of how ALHAT works, it all begins with the Terrain Relative Navigation system, or TRN, when the Morpheus lander is still hundreds of kilometers away from the landing area. TRN consists of 3 different sensors; a passive optical camera; a flash lidar using that uses special optics to allow it to focus only on a small area of a few pixels; a laser altimeter. The Terrain Relative Navigation sensor suite takes measurements and compares its results to information stored onboard the lander. As the Morpheus lander gets closer to the Moon’s surface, ALHAT’s Doppler lidar and laser altimeter scan the lander’s flight path to determine the lander’s position and velocity. The goal is to improve the lander’s relative position and velocity, or what engineers call the navigation state. At about 1 km, or 2/3 of a mile, above the landing site, ALHAT’s Hazard Detection System, or HDS, which uses a flash lidar sensor, begins scanning a large region, roughly 100 by 100 meters or about 4 football fields, of the local terrain for a safe landing area, taking dozens of 3D images. The Hazard Detection System then stitches those images together into a mosaic and generate a digital elevation map, or DEM, with a 10-cm resolution of the landing area. The digital elevation map is used to determine possible safe landing locations for the Morpheus vehicle. The digital elevation map is also used as a reference by the Hazard Relative Navigation system, whose job it is to update the lander’s position and velocity relative to the chosen landing site. The software and computing architecture needed to capture and combine the dozens of mosaic images and crunch them to produce the digital elevation map, and then to identify a safe landing site are just a few of the cutting-edge technologies that makes the ALHAT system so advanced.

During all six of the Apollo landing missions, each of the astronaut crews experienced difficulty seeing the Moon’s surface because of the dust kicked-up by the Lunar Module’s engine. To avoid this problem, ALHAT uses inertial navigation beginning at about 100 meters above the lunar surface. ALHAT is able to do this because of the highly accurate position and velocity information provided earlier by its lidar sensors. This accurate information, along with the short time it takes for the lander to descend the final 100 meters to a safe landing, means that ALHAT will place a lander within a meter or two of the targeted landing location.

Picking a safe landing spot for a robotic or, more critically, a human-crewed lander is obviously important, but the process is not so simple. The onboard algorithms must consider the orientations and angles of the lander footpads, the onboard navigation knowledge, the sensor noise, and the clearances of the lander components. For instance, Apollo 15 landed just on the edge of a small crater – one that might have been deep enough for the lunar module to fall over upon landing if the Navigation knowledge had been in error by another meter. The Morpheus lander has a rocket engine nozzle that allows for only a 20 cm clearance above the surface. Were the nozzle to land first upon a rock, the whole lander could be lost.

For this mission, the end result is a safe landing for Morpheus, new exploration and science discoveries at a region of the Moon never before seen, and a successful delivery of a scientific payload for the first launch of the Space Launch System. NASA’s investment in the ALHAT technology is a key enabler of this mission.

Once again, NASA is dreaming big and the benefits to us in space and at home could be equally large. But then, NASA has been doing this for America since its birth.


  1. Jim Hillhouse, you have my most sincere appreciation for the gift of such an informative, educational, interesting, and enjoyable article. You have brought to light factors that I had not previously considered. I am particularly very pleased with your pro-NASA statements and outlook. In this NASA-bashing, “privatize NASA” era, your intelligent, malice-free statements have earned my most sincere gratitude. I look forward to the pleasure of reading your next article which I am sure will be equally well written.

    • Well said! It’s a real shame that the general public doesn’t get informed on the marvellous job NASA is doing.

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