[patrickmckeen]

What this adds

A sensor's Bias had two things: a starting value and a random-walk rate, and the walk started from the starting value. That conflates two different errors, and it leaves out a third:

• the constant offset a device is built with (the turn-on bias, which calibration can remove), which should stay put;
• the random walk (rate random walk in gyro terms), which should start at zero and accumulate;
• the bias instability, the slow wander that an Allan-variance plot shows as its floor, which is neither constant nor a random walk: it stays within a band but is correlated over a characteristic time.

Bias now keeps the three parts separately, with bias as their sum so everything that reads or writes it keeps working:

• offset: the bias given at construction, constant.
• walk: starts at zero, steps with standard deviation std_bias · sqrt(Δt) as before.
• instability: a first-order Gauss-Markov process with standard deviation instability_std and correlation time instability_time (seconds), the standard stand-in for the flicker floor. It decays towards zero over the correlation time and is topped up each step so its standard deviation stays where configured, exactly for any step length.

A bias built the old way (value and rate only) produces the same sequence as before. On the estimator side, the random-walk rate the filter assumes for a bias state covers the instability with the usual 2·σ²/τ term, so a filter with a random-walk bias model still tracks a bias that wanders. The same applies to actuator biases, since they share the class.

This stacks on #234.

Tests

testing/test_sensors/test_bias_instability.py, 5 tests: the offset stays constant and the walk starts from it; without an instability the sequence equals the old random walk draw for draw; the instability is stationary with the configured standard deviation and lag-one correlation exp(−Δt/τ) over 40 000 steps; the estimator's rate includes the instability term; and the configuration is validated and survives sizing and copies. The full suite passes.

🤖 Generated with Claude Code

[github-actions]

Test Results

1 579 tests  +5   1 577 ✅ +5   3m 54s ⏱️ + 1m 28s
    1 suites ±0       2 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit a82a3e9. ± Comparison against base commit b4c6bc2.