Multi-wavelength PPG
Green, red and infrared LEDs read blood flow through the wrist, not just one. The extra wavelengths beyond a standard green-only sensor are what make SpO2 possible and keep heart rate and HRV accurate during movement, not just at rest.
Single-lead ECG
Captures the heart's actual electrical signal rather than inferring it optically. Used to flag rhythm irregularities and to continuously calibrate the PPG sensor against a ground-truth reading.
SpO2 (blood oxygen)
Calculated from the ratio of red-to-infrared light absorption in the blood. A drop here is one of the earliest signals of respiratory strain, poor sleep quality, or altitude effects, long before it's felt.
Continuous skin temperature
archie builds a personal baseline over weeks, so a shift of even a few tenths of a degree is detectable: a signal for oncoming illness, recovery status, or cycle-related change, read against your own normal, not a population average.
Bioimpedance (BIA)
A barely perceptible electrical current is passed through tissue and its resistance measured, estimating body fat, muscle mass and hydration, the kind of reading that normally needs a separate scale.
6-axis IMU
Accelerometer and gyroscope combined. Classifies movement type, counts reps with real form data rather than a generic estimate, and infers sleep stage from micro-movement patterns through the night.
Barometric altimeter
Tracks fine changes in elevation: floors climbed, hill gradient, stair effort, adding real context to intensity that step count alone can't capture.
GPS / GLONASS
Dual-constellation positioning via HERE Maps for accurate route, pace and distance outdoors, with stronger signal reliability in dense urban streets and under tree cover than single-constellation GPS.
No single sensor here is unique. What's defensible is combining all eight with archie's memory of you specifically, so a temperature shift, a dip in HRV, and a change in how you're talking are read together, not in isolation.
