Keeping Our Sense of Direction: Dealing with a Dead Sensor

As the season turned into winter at Jezero Crater, conditions became increasingly challenging for Ingenuity, which was designed for a short flight test campaign during the much warmer Martian spring. Increasing amounts of dust in the atmosphere, combined with cooler daytime temperatures and shorter days, affected Ingenuity’s power budget to the point where it was unable to keep warm during the Martian nights. In its new winter operations paradigm, the Ingenuity is effectively shutting down overnight, letting its internal temperature drop to around minus 112 degrees Fahrenheit (minus 80 degrees Celsius) and letting the onboard electronics reset. This new way of operating poses risks to Ingenuity’s electronics, many of which are not designed to survive the temperatures they are being exposed to at night. In addition, extreme temperature cycles between day and night tend to cause stresses that can result in component failure.

Over the last few suns on Mars, the Ingenuity team has been busy recommissioning the helicopter for flight, going through a series of activities that include pre-flight verification of sensors and actuators and a high-speed rotation of the rotor. These activities revealed that one of the helicopter’s navigation sensors, called an inclinometer, had stopped working. A navigation sensor that doesn’t work sounds like a big deal — and it is — but it’s not necessarily the end of our Mars flight.

navigation sensors

When Ingenuity is flying, the onboard flight control system closely tracks the helicopter’s current position, speed and orientation. It does this with the help of a sensor suite consisting of:

  • an inertial measurement unit (IMU), which measures accelerations and angular rates in three directions
  • a laser rangefinder, which measures the distance to the ground
  • a navigation camera, which takes pictures of the ground below

Data from these sensors is processed by a set of algorithms implemented in Ingenuity’s navigation computer. For the algorithms to work correctly, they must be initialized before takeoff with an estimate of Ingenuity’s roll attitude and pitch. That’s where the inclinometer comes in.

The inclinometer is composed of two accelerometers, whose sole purpose is to measure gravity before turning and taking off; the sensed gravity direction is used to determine how Ingenuity is oriented relative to the downward direction. The inclinometer is not used during the flight itself, but without it we are forced to find a new way to initialize the navigation algorithms before takeoff.

Representing the inclinometer

The Ingenuity sensor suite offers some redundancy when it comes to detecting attitude on the ground. The IMU contains accelerometers, which – like the accelerometers inside the inclinometer – can be used to estimate the initial attitude. Unlike the inclinometer, the IMU is not built specifically to detect static orientation, so your initial attitude estimates will generally be slightly less accurate. However, we believe that an IMU-based initial attitude estimate will allow us to take off safely and therefore provide an acceptable return that will allow Ingenuity to resume flight.

Taking advantage of this redundancy requires a patch to the Ingenuity flight software. The patch inserts a small snippet of code into software running on Ingenuity’s flight computer, intercepting garbage packets received from the inclinometer and injecting replacement packets constructed from IMU data. For the navigation algorithms, everything will be as before, the only difference is that the received inclinometer packets do not originate from the inclinometer.

Anticipating that this situation could arise, we prepared the necessary software patch ahead of last year’s arrival on Mars and kept it on the shelf for that eventuality. So we can move quickly with the update, and the uplink process for Ingenuity is already underway.

returning to service

If all goes well, over the next few pays, the team expects to finish uplinking and apply the software patch, which will be followed by commissioning activities to ensure the new software is operating as planned. Barring additional surprises, we anticipate that Ingenuity will take to the skies for Flight 29 – a southwesterly repositioning move designed to keep us within Perseverance’s communication range – in the near future.

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