Written by Håvard Grip, Ingenuity Mars Helicopter Chief Pilot at NASA’s Jet Propulsion Laboratory
On the 91st Martian day, or sol, of NASA’s Mars 2020 Perseverance rover mission, the Ingenuity Mars Helicopter carried out its sixth flight. The flight was designed to broaden the flight envelope and show aerial-imaging capabilities by taking stereo pictures of a area of curiosity to the west. Ingenuity was commanded to climb to an altitude of 33 toes (10 meters) earlier than translating 492 toes (150 meters) to the southwest at a floor velocity of 9 mph (four meters per second). At that time, it was to translate 49 toes (15 meters) to the south whereas taking pictures towards the west, then fly one other 164 toes (50 meters) northeast and land.
Telemetry from Flight Six reveals that the primary 150-meter leg of the flight went off with out a hitch. However towards the tip of that leg, one thing occurred: Ingenuity started adjusting its velocity and tilting forwards and backwards in an oscillating sample. This conduct endured all through the remainder of the flight. Previous to touchdown safely, onboard sensors indicated the rotorcraft encountered roll and pitch excursions of greater than 20 levels, massive management inputs, and spikes in energy consumption.
How Ingenuity estimates movement
Whereas airborne, Ingenuity retains monitor of its movement utilizing an onboard inertial measurement unit (IMU). The IMU measures Ingenuity’s accelerations and rotational charges. By integrating this data over time, it’s attainable to estimate the helicopter’s place, velocity, and perspective (the place it’s, how briskly it’s shifting, and the way it’s oriented in area). The onboard management system reacts to the estimated motions by adjusting management inputs quickly (at a fee of 500 instances per second).
If the navigation system relied on the IMU alone, it could not be very correct in the long term: Errors would shortly accumulate, and the helicopter would finally lose its manner. To take care of higher accuracy over time, the IMU-based estimates are nominally corrected regularly, and that is the place Ingenuity’s navigation digicam is available in. For almost all of time airborne, the downward-looking navcams takes 30 footage a second of the Martian floor and instantly feeds them into the helicopter’s navigation system. Every time a picture arrives, the navigation system’s algorithm performs a collection of actions: First, it examines the timestamp that it receives along with the picture with the intention to decide when the picture was taken. Then, the algorithm makes a prediction about what the digicam ought to have been seeing at that individual time limit, when it comes to floor options that it might acknowledge from earlier pictures taken moments earlier than (sometimes attributable to colour variations and protuberances like rocks and sand ripples). Lastly, the algorithm seems to be at the place these options truly seem within the picture. The navigation algorithm makes use of the distinction between the expected and precise areas of those options to right its estimates of place, velocity, and perspective.
Flight Six anomaly
Roughly 54 seconds into the flight, a glitch occurred within the pipeline of pictures being delivered by the navigation digicam. This glitch precipitated a single picture to be misplaced, however extra importantly, it resulted in all later navigation pictures being delivered with inaccurate timestamps. From this level on, every time the navigation algorithm carried out a correction primarily based on a navigation picture, it was working on the idea of incorrect details about when the picture was taken. The ensuing inconsistencies considerably degraded the knowledge used to fly the helicopter, resulting in estimates being consistently “corrected” to account for phantom errors. Giant oscillations ensued.
Surviving the anomaly
Regardless of encountering this anomaly, Ingenuity was capable of keep flight and land safely on the floor inside roughly 16 toes (5 meters) of the meant touchdown location. One purpose it was ready to take action is the appreciable effort that has gone into guaranteeing that the helicopter’s flight management system has ample “stability margin”: We designed Ingenuity to tolerate important errors with out changing into unstable, together with errors in timing. This built-in margin was not absolutely wanted in Ingenuity’s earlier flights, as a result of the automobile’s conduct was in-family with our expectations, however this margin got here to the rescue in Flight Six.
One other design determination additionally performed a task in serving to Ingenuity land safely. As I’ve written about earlier than, we cease utilizing navigation digicam pictures through the ultimate part of the descent to touchdown to make sure easy and steady estimates of the helicopter movement throughout this essential part. That design determination additionally paid off throughout Flight Six: Ingenuity ignored the digicam pictures within the ultimate moments of flight, stopped oscillating, leveled its perspective, and touched down on the velocity as designed.
Trying on the larger image, Flight Six ended with Ingenuity safely on the bottom as a result of various subsystems – the rotor system, the actuators, and the facility system – responded to elevated calls for to maintain the helicopter flying. In a really actual sense, Ingenuity muscled by way of the state of affairs, and whereas the flight uncovered a timing vulnerability that can now should be addressed, it additionally confirmed the robustness of the system in a number of methods.
Whereas we didn’t deliberately plan such a anxious flight, NASA now has flight knowledge probing the outer reaches of the helicopter’s efficiency envelope. That knowledge can be rigorously analyzed within the time forward, increasing our reservoir of information about flying helicopters on Mars.
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