Perseverance on Mars

This activity allows students to develop strategies for maintaining a straight trajectory without external cues such as GPS or the magnetic field. It simulates the autonomous navigation challenges encountered by robots on Mars.

On the surface of Mars, the Perseverance rover faces a navigation challenge that would stump most humans: how do you drive in a straight line when there is no magnetic field for a compass, no GPS satellites in orbit, and no roads or landmarks? Mars lost its global magnetic field about 4 billion years ago, so magnetic navigation is impossible. Instead, Perseverance relies on an inertial measurement unit (IMU) containing accelerometers and gyroscopes, combined with star trackers and visual odometry from its cameras. But inertial sensors suffer from drift: small errors accumulate over time, causing the estimated heading to gradually deviate from the true heading. This experiment simulates the Martian navigation challenge by asking students to walk 150 steps in as straight a line as possible using only their smartphone sensors, without looking at the ground or using a compass. By testing different sensor strategies, students discover firsthand why autonomous navigation is one of the hardest problems in robotics.

Learning objectives:

The student explores different methods for moving in a straight line using only the sensors available in a smartphone. By successively testing different FizziQ instruments (gyroscope accelerometer) while moving over 150 steps, the student evaluates the effectiveness of each method and thinks about emergency solutions that can be used by a robot like Perseverance on Mars.

Level:

Middle school

FizziQ

Author:

Duration (minutes) :

40

What students will do :

- Attempt to maintain a straight-line trajectory using only smartphone sensor feedback
- Compare the effectiveness of different sensors (accelerometer, gyroscope, light meter) for heading maintenance
- Understand the concept of sensor drift and its consequences for navigation
- Connect the experiment to the real navigation challenges of Mars rovers
- Evaluate the strengths and limitations of inertial navigation systems

Scientific concepts:

- Autonomous navigation
- Inertial systems
- Sensor drift
- Space robotics
- Data integration

Sensors:

- Accelerometer (lateral acceleration detection)
- Gyroscope (angular velocity / heading change)
- Light sensor (optional, for sun-based orientation)

What is required:

- Smartphone with the FizziQ application
- A clear outdoor space for walking in a straight line
- FizziQ experience notebook

Experimental procedure:

Find a large, open outdoor space (at least 100 meters long) where you can walk freely in any direction.
Mark a starting line and a target direction (e.g., toward a distant landmark). Place markers every 30 meters along the straight path for reference.
Trial 1 (No sensor): Close your eyes (or wear a blindfold with a partner guiding for safety) and try to walk 150 steps in a straight line. Mark your ending position.
Trial 2 (Accelerometer): Open FizziQ and select the lateral (Y-axis) accelerometer. Try to maintain zero lateral acceleration as you walk 150 steps with eyes closed. Mark your ending position.
Trial 3 (Gyroscope): Use the FizziQ gyroscope (yaw/heading component). Try to maintain zero angular velocity (no turning) as you walk 150 steps with eyes closed. Mark your ending position.
Trial 4 (Light sensor): If sunny, use the FizziQ light sensor to maintain a constant light reading (keeping the sun at a fixed angle). Walk 150 steps. Mark your ending position.
For each trial, measure the lateral deviation (perpendicular distance from the intended straight line) at the endpoint.
Also measure the total distance walked along the intended direction to check for distance accuracy.
Create a comparison table: method, lateral deviation, final heading error, ease of use.
Rank the methods from most to least effective for maintaining a straight course.
Discuss why even the best sensor method shows significant drift after 150 steps.
Research how Perseverance actually navigates on Mars and compare its methods with your experiment.

Expected results:

Without any sensor aid, most people will deviate 10-30 meters laterally over 150 steps due to unequal leg lengths and unconscious turning tendencies. With the accelerometer, the deviation may improve to 5-15 meters, but lateral acceleration is difficult to interpret in real time. The gyroscope typically gives the best results (3-10 meters deviation) because heading changes are directly measured, but gyroscope drift accumulates steadily. The light sensor works well on sunny days if the sun's position provides a clear directional reference, but cloud cover renders it useless. The key takeaway is that all inertial methods suffer from drift, explaining why real rovers must combine multiple sensors and periodically recalibrate using external references.

Scientific questions:

- Why do humans tend to walk in circles when blindfolded?
- What is sensor drift, and why does it accumulate over time?
- How does Perseverance navigate on Mars without GPS or a magnetic compass?
- What is visual odometry, and how does it complement inertial navigation?
- Why is the gyroscope generally more effective than the accelerometer for maintaining heading?
- Could you use the stars for navigation on Mars, as sailors did on Earth?

Scientific explanations:

Autonomous navigation without external cues is one of the major challenges of space robotics. On Mars, robots like Perseverance cannot rely on a global magnetic field (Mars does not have one) nor on a GPS system.

This experiment simulates these constraints by exploring the sensors that can be used to maintain a straight heading. Three main approaches can be tested with FizziQ: 1) The accelerometer: in theory, maintaining zero acceleration perpendicular to the direction of travel should guarantee a straight line.

In practice, the double integration required to go from acceleration to position amplifies errors, causing significant drift. 2) The gyroscope: by measuring the rotation around the vertical axis, we can detect any deviation from the straight line.

More accurate than the accelerometer for short movements, it also suffers from long-term drift. 3) The light meter or camera: in the absence of a magnetic field, Mars rovers often use the position of the Sun or the stars as a directional reference, supplemented by visual cues.

This “celestial” navigation is particularly reliable but depends on lighting conditions. These methods are complementary and generally combined in Martian navigation systems.

Perseverance uses advanced visual navigation called “Visual Odometry” which compares successive images to determine its movement, supplemented by an inertial unit (accelerometer and gyroscope). The main difficulty remains cumulative drift: even a minimal error of 1° can cause a deviation of 2.6 meters after 150 steps (approximately 100 meters).

This experiment illustrates why Martian missions progress relatively slowly: Perseverance travels only 100-200 meters per Martian day to maintain its navigational precision.

Extension activities:

Frequently asked questions:

Q: Is it safe to walk 150 steps with eyes closed?
R: Always have a partner walk alongside you for safety, watching for obstacles. Choose a flat, open area free of hazards. The partner should not give directional guidance but should prevent collisions or falls.

Q: The gyroscope readings drift even when I hold the phone still. Is this normal?
R: Yes, gyroscope drift is a fundamental limitation of MEMS sensors. Typical smartphone gyroscopes drift at 1-5 degrees per minute, which accumulates significantly over a 2-3 minute walk.

Q: The accelerometer shows large spikes that make it hard to maintain zero lateral acceleration. Why?
R: Each step produces acceleration spikes in all directions. The lateral acceleration from individual steps is much larger than the small systematic drift you are trying to detect. Low-pass filtering or averaging would help but is difficult in real time.

Q: How accurate is the Perseverance rover's navigation?
R: Perseverance uses a combination of IMU, visual odometry (tracking features in camera images), and periodic position fixes from orbital imagery. It can typically maintain its position estimate to within a few meters over drives of 100+ meters.