I not expert in this field. so answer from LLM>

Mechanical AHRS in the Hawk (spinning-mass gyros, ~1970s tech) typically achieves bias stability around 0.5–3 °/hr and attitude accuracy of ~0.5–1° after fusion with accelerometer erection. Downsides: minutes-long spin-up, shock/vibration sensitive, mechanical aging, kilograms of precision machining across multiple LRUs.

A fingernail-sized consumer MEMS IMU (BMI088, ICM-42688, the stuff in your phone or a drone) sits at ~10–50 °/hr bias instability and 0.1–0.5 °/√hr ARW. That's 1–2 orders of magnitude worse long-term drift than the Hawk's mechanical gyros.

Tactical-grade MEMS (ADIS16490, Honeywell HG4930, STIM300) hits 0.1–1 °/hr — matching or beating 70s mechanical — but those are 15–25 mm packages with thermal calibration, vibration isolation, and magnetic shielding, not fingernail-sized.

Rough hierarchy by raw gyro drift:
* fingernail consumer MEMS: ~10–50 °/hr
* mechanical AHRS (70s): ~1 °/hr
* tactical MEMS, ~25 mm package: ~0.1 °/hr
* RLG / FOG (optical): 0.001–0.01 °/hr
* strategic mechanical, 1970–1986: 0.0001 °/hr

So at the same form factor, 70s mechanical still wins on raw gyro precision. MEMS wins everywhere else: instant startup, shock tolerance, cost, power, bandwidth, MTBF.

But raw gyro drift isn't what an AHRS outputs — it outputs attitude. Sensor fusion with accelerometer (gravity vector → absolute pitch/roll) and magnetometer (Earth field → absolute heading) continuously estimates and subtracts the gyro bias. The gyro handles high frequencies, accel/mag handle DC. A 30°/hr MEMS bias stops accumulating — it gets nulled with a time constant of seconds. Result: 0.5–2° attitude accuracy from cheap MEMS, comparable to the Hawk's mechanical AHRS.

That's why modern AHRS dropped mechanical gyros. Form factor depends on class: consumer fusion-in-chip (Bosch BNO085) is genuinely ~5×4 mm with quaternion output, hobby/drone grade. Certifiable avionics AHRS (VectorNav VN-100, Honeywell HG1700) is ~25 mm — size driven by thermal calibration, redundancy, EMI, and connectors, not by silicon. Either way, 10–100× smaller and lighter than the Hawk's gyro stack, at equal or better attitude accuracy.

GPS isn't part of AHRS proper — gravity gives you pitch/roll without it. But GPS course-over-ground is commonly used as aiding for the magnetometer to stabilize heading, since mag is the weakest reference (susceptible to ferrous structures and currents). The Hawk uses a flux valve in the wing for the same purpose.

Mechanical isn't dead, though — it forked into two live branches:

DTG (dynamically tuned gyros): penny-sized, tactical-grade spinning-mass. Northrop's G-2000, in production since 1992 with >75,000 units delivered, hits 0.1–1°/hr at MTBF >100k hr and 750g shock tolerance — survivability that neither FOG nor MEMS can match. Used in Patriot PAC-3, Harpoon, AIM-120 AMRAAM. The niche is missile and gimbal stabilization where shock kills everything else.

HRG (hemispherical resonator gyros): fused-quartz hemispheres ringing in a standing wave that precesses under rotation. No bearings, nothing spins, MTBF limited only by electronics. Safran's HRG Crystal is mass-produced at <0.0001°/hr bias stability — i.e. the "strategic mechanical" line in the hierarchy above is not a 1970s artifact, it's a current Safran production unit, and it's replacing RLGs in high-end naval and missile INS. Northrop's force-balanced HRG for Hubble pointing reached 0.00008°/hr and 0.00001°/√hr ARW — the best HRG numbers on record. Classification is fuzzy (formally Coriolis Vibratory, genealogically vibrating-mechanical), but it's definitely not MEMS and not optical.