You're a hundred yards into a cave and you lost GPS signal a while back. Now what?

That's the problem I'm solving with ATLAS, a handheld environmental tricorder I've been developing that packs nine sensors, dual RP2350 processors, and now a breadcrumb navigation system that can guide you home even when satellites can't see you.

The Compass Was Just the Start

ATLAS already had a working compass thanks to a BNO085 IMU. I actually tried rolling my own sensor fusion first with a raw gyro and accelerometer. It worked, but not well enough to trust in the field. The BNO085 runs Hillcrest's sensor fusion on-chip with gyro bias correction and magnetic calibration. It basically doesn't care about orientation. Tilt it, flip it, shove it in your pocket. The heading stays solid. That gave me a reliable compass, but a compass alone doesn't get you un-lost.

Breadcrumb Navigation

The concept is simple. When you activate trail recording, ATLAS starts dropping GPS waypoints at adaptive intervals, faster when you're driving, slower when you're standing still. The trail lives on the SD card so it survives a power cycle. When you want to get back, you have two options:

Backtrack. Follow your breadcrumbs in reverse, waypoint by waypoint. The nav screen shows a compass ring with a direction arrow pointing to the next crumb. Walk that direction, arrive within 5 meters, and it advances to the next one. You retrace your exact path.

Direct. Ignore the trail entirely. The arrow points straight back to where you started. As the crow flies. Useful when you know the terrain between you and your starting point is passable and you just need a heading.

Dead Reckoning When GPS Drops

Here's where it gets interesting. GPS is great until it isn't. Dense tree canopy, canyon walls, caves, urban canyons. When ATLAS loses GPS fix for more than 8 seconds, the system engages dead reckoning using the BNO085 heading and last known speed to project your position forward.

But I didn't want some arbitrary confidence timer that just counts down and hopes for the best. I built an error-budget model based on the actual sensor characteristics:

• BNO085 heading uncertainty: ~5° typical
• Speed estimation error: ~±25% (using last GPS speed, no wheel encoders here)
• Lateral drift rate: speed × sin(5°) per second
• Along-track drift rate: speed × 0.25 per second

At walking speed (5 km/h), that works out to about 0.37 meters of position error per second. So after 5 minutes of dead reckoning you've accumulated roughly 110 meters of uncertainty and the system reports CONF:93>. After 15 minutes, you're at about 65%. After 30 minutes, around 30%.

At that point, yeah, you're a bit lost until GPS comes back online. But the arrow is still pointing in broadly the right direction, which is a lot better than nothing when you're trying to find the mouth of a cave.

The moment a GPS fix returns, error resets to zero and confidence snaps back to 100%. No accumulated drift baggage.

What's On Screen

The nav display shows a compass ring with N/E/S/W cardinal markers rotated by your actual magnetic heading, so north always points north regardless of which way you're facing. Inside the ring, a direction arrow points toward your target. Below that: distance to target, bearing, waypoint progress (for backtrack mode), and the confidence readout.

CONF:GPS 8s means you have a real fix with 8 satellites. CONF:87% E:35m means you're on dead reckoning with an estimated 35 meters of position error. You always know exactly how much to trust the arrow.

Why Not Just Use Your Phone?

Fair question. Your phone has better GPS, more processing power, and Google Maps. But your phone also has a glass screen, a battery that dies in the cold, needs cell service for most of its tricks, and isn't something you want to pull out in a downpour or drop on a rock face. ATLAS is a ruggedized tool that boots in seconds, runs all week on a single charge, and gives you environmental data your phone can't: CO2, radiation, barometric pressure, UV index, alongside navigation. Different tool for a different job.

Why This Feature Matters

I think this is one of the most important features on the entire device. Nine environmental sensors are great for data collection, but a nav system that guides you home when you're disoriented is the kind of thing that can genuinely save a life. People get lost in the woods every year, and for some reason they almost always make it home. Almost always. I'd like to improve those odds.