NASA Ingenuity encountered an “in-flight anomaly” on its sixth mission to Mars which caused it to fly erratically. Although the helicopter landed safely, the incident provided an unanticipated stress test for the system.
On May 22, 2021, the sixth test of Ingenuity was supposed to push the helicopter to its limits, but mission planners got a lot more than they anticipated.
Ingenuity was supposed to climb 33 feet (10 meters) and then fly for 492 feet (150 meters) in a southwesterly direction, followed by two additional trips of approximately 50 feet (15 meters) and 164 feet (50 meters) before landing at a specific spot.
The test flight started out smoothly, but at the end of Ingenuity’s first stretch, things started to get a little wacky. As Håvard Grip, the chief pilot of the Ingenuity Mars Helicopter, said in a paper produced for NASA, the helicopter suddenly seemed uncertain, adjusting its velocity and “tilting back and forth in an oscillating pattern,”
Throughout the flight, this strange behavior persisted. “Onboard sensors indicated the rotorcraft endured roll and pitch excursions of more than 20 degrees, large control inputs, and spikes in power consumption” before landing, Grip added.
To fathom what went wrong during this flight, we must evaluate how this self-driving car manages to maintain things in order. During flight, Ingenuity uses an onboard inertial measuring unit (IMU) to keep track of its location, speed, and orientation.
An onboard control system, in turn, reacts to these measurements, which are merely approximations. Indeed, the IMU would be insufficient for the task on its own, as errors would accumulate over time. To support this system, the helicopter employs its onboard navigation camera, which takes 30 photos of the Martian surface while in flight and sends them to the navigation system right away.
However, as Grip notes, this two-pronged technique is reliant on exact timestamps being provided to an algorithm:
Each time an image arrives, the navigation system’s algorithm performs a series of actions: First, it examines the timestamp that it receives together with the image in order to determine when the image was taken. Then, the algorithm makes a prediction about what the camera should have been seeing at that particular point in time, in terms of surface features that it can recognize from previous images taken moments before (typically due to color variations and protuberances like rocks and sand ripples). Finally, the algorithm looks at where those features actually appear in the image. The navigation algorithm uses the difference between the predicted and actual locations of these features to correct its estimates of position, velocity, and attitude.
An in-flight anomaly threw this delicate balance off at the 54-second mark of Ingenuity’s sixth trip. A glitch—a single lost frame—disrupted the constant flow of camera data, causing all subsequent navigation images to be transmitted with an incorrect timestamp.
As a result, the navigation algorithm was working with erroneous data about when the photos were collected. Working with corrupted data forced Ingenuity into a state where it had to make constant adjustments, resulting in the back-and-forth motions. Despite this, Ingenuity was able to continue its flight and arrive within a few feet of its intended landing spot.
NASA designed Ingenuity to tolerate significant errors without becoming unstable, including errors in timing,” Grip wrote. This “built-in margin was not fully needed in Ingenuity’s previous flights because the vehicle’s behavior was in-family with our expectations, but this margin came to the rescue” during the sixth flight. In addition, Grip said the helicopter “muscled through the situation” using its rotor system, actuators, and power system, which all responded to the increased demands of the wild flight.
The bizarre flight was not planned, but by pushing Ingenuity to its limits, the flaw will almost certainly produce some useful data. According to Grip, this “data will be carefully analyzed in the time ahead, expanding our reservoir of knowledge about flying helicopters on Mars.”