Mars Helicopter Ingenuity keeps offering engineering lessons on ways to deal with unexpected glitches in new technology.
On Thursday, NASA JPL Chief Pilot Havard Grip offered an instructive status update of the tenuous final moments of rotorcraft’s sixth flight conducted May 22 on the Red Planet.
At about 54 seconds into the flight, the craft began oscillating up to 20 degrees before landing safely due to a “glitch in the pipeline of images being delivered by the navigation camera,” he said.
“This glitch caused a single image to be lost, but more importantly, it resulted in all later navigation images being delivered with inaccurate timestamps. From this point on, each time the navigation algorithm performed a correction based on a navigation image, it was operating on the basis of incorrect information about when the image was taken.
“The resulting inconsistencies significantly degraded the information used to fly the helicopter, leading to estimates being constantly ‘corrected’ to account for phantom errors. Large oscillations ensued.”
Grip also offered an insight for engineers into how the rotorcraft navigates but also how Ingenuity was able to survive the oscillations and land about 16 feet from its intended landing spot.
Ingenuity navigates in part by tracking motion with an onboard inertial measurement unit (IMU) that measures accelerations and rotational rates. That way, it estimates its position, velocity and attitude. The IMU reacts and adjusts control inputs at a rate of 500 times a second.
But the IMU is not enough. The IMU estimates are nominally corrected by the navigation camera images. Downward looking navcams take 30 pictures a second of the Martian surface and each time an image arrives, the navigation system algorithm examines the timestamp of the image, then makes a prediction about what the camera should have seen at that moment in terms of surface features recognized in previous images taken moments before.
The algorithm then looks at where those features actually appear in the image. The difference between the predicted and actual locations of those features is used to correct the craft’s estimates of position, velocity and attitude. It was the glitch in the pipeline of images that threw off Ingenuity and caused it to start oscillating.
But NASA engineers put considerable advance effort into providing a stability margin for the copter’s flight control system to help it survive the oscillations and landing. “We designed Ingenuity to tolerate significant errors without becoming unstable, including errors in timing,” Grip said. “This margin came to the rescue in Flight Six.”
In addition, engineers designed Ingenuity to stop using navigation camera images during the final phase of descent to ensure smooth and continuous estimates of the copter’s motion. “That design decision also paid off: Ingenuity ignored the camera images in the final moments of flight, stopped oscillating, leveled its attitude and touched down at the speed as designed,” Grip said.
“Flight Six ended with Ingenuity safely on the ground because a number of subsystems—the rotor system, the actuators and the power system – responded to increased demand to keep the helicopter flying,” Grip said. “In a very real sense, Ingenuity muscled through the situation…”
NASA provided a short video of the last 29 seconds of Ingenuity’s sixth flight.
In a classic understatement, Grip noted that NASA “did not intentionally plan such a stressful flight.” Flight data will be used to expand insights about flying helicopters on Mars.
And NASA now has to address the timing vulnerability discovered on the flight.