An IMU measures motion. An INS computes where you are from those measurements. The right choice depends on one question: do you already have a navigation stack, or do you need position out of the box?
Photonic vs FOG vs MEMS gyroscopes
Three technologies dominate rotation sensing: vibrating-silicon MEMS, fiber-optic gyros, and the newer chip-based photonic gyros. OSCP builds and sells both MEMS and photonic IMUs, so this is the honest version of the comparison, including where each one loses.
New to inertial sensors? Start with [What is an IMU?]
How each one measures rotation
MEMS. A microscopic silicon structure vibrates. Rotation couples some of that motion into a second axis, the Coriolis effect, and the coupling is read out electrically. Made by the million on wafers.
FOG. Light travels both ways around hundreds of metres of coiled optical fiber, and rotation shifts the two beams relative to one another, the Sagnac effect. No moving parts at all, and the benchmark for stable, quiet rotation sensing.
Photonic. The same Sagnac physics, with the optical system built on a photonic integrated circuit. Optical measurement with semiconductor-style manufacturing.
The comparison that matters
| Property | MEMS | FOG | Photonic |
|---|---|---|---|
| Physics | Vibrating mass, Coriolis | Sagnac, fiber coil | Sagnac, photonic circuit |
| Typical gyro grade | Consumer to tactical, about 0.5 °/hr at best | Tactical through strategic, 0.5 down to below 0.001 °/hr | Tactical shipping at 0.5 °/hr, with a navigation-grade prototype |
| Moving parts in sensing | Yes, a vibrating structure | None | None |
| Vibration and shock | Sensitive, with errors under g-loading | Highly resistant | Highly resistant |
| EMI | Susceptible, electrical readout | Highly resistant, optical path | Highly resistant, optical path |
| Size and power | Smallest, milliwatts to about 1 W | Large, often tens of watts | Compact, single-digit watts |
| How it is made | Wafer-scale, scales with volume | Precision coil winding, labour-intensive | Semiconductor processes, follows a chip cost curve |
| Best fit | Volume platforms, short denial windows, well-aided systems | Long-endurance navigation where size and power allow | Long GNSS denial on SWaP-limited platforms, and FOG replacement |
For the grade names used here, see the grade ladder in [What is an IMU?]
Where each honestly wins
Choose MEMS when size dominates, GNSS outages are short, or rich aiding such as odometry or vision is always available. A good tactical MEMS IMU, and our MK2M2 is one, is the right answer for a lot of robotics, construction and volume UAV work. Its limits appear in long denial windows and high-vibration environments.
Choose FOG when you need proven strategic-grade performance now and the platform can carry the size and power. Ships, submarines and high-end aircraft have run FOGs for decades for good reason. The pain is bulk and a labour-intensive supply base that does not scale with demand.
Choose photonic when you need optical-class stability but the platform cannot host a FOG, such as drones, small subsea craft and ground vehicles, or you are designing for volume and cannot build a programme on labour-intensive coil winding. That middle ground is exactly why the category exists.
The trap to avoid: comparing only the headline number
Two IMUs can quote the same degrees per hour and behave completely differently on a vibrating platform. Bias stability is measured on a bench, and your vehicle is not a bench.
Ask how the gyro behaves under vibration and g-loading, what the angular random walk is, and how bias repeats over temperature. This is where the no-moving-parts technologies quietly earn their money, and why our published road test, a 21 minute drive with GNSS positioning denied and a 1 Hz speed input, showed a 3.6x position-error gap between photonic and MEMS units whose specs look close on paper.
Frequently asked questions
Is a photonic gyro just a small FOG?
Same physics, different construction. A FOG measures the Sagnac effect through a precision-wound fiber coil. A photonic gyro builds the optical system on a chip, which changes the size, the power and the manufacturing economics.
Will photonic gyros replace MEMS?
Not at the low end. MEMS wins on size and cost for consumer and short-outage uses. Photonic competes upward, into territory where the alternative was a large FOG.
Which does OSCP recommend?
We sell both, so the recommendation follows the mission. MK2M2, the MEMS unit, for well-aided platforms where size and weight lead. MK2E2, the photonic unit, when the denial window is long or the environment is harsh. For navigation-grade drift budgets, ask about the MK2Z, our early-stage photonic prototype.
Keep reading
Related: What is an IMU?
An IMU measures how a vehicle is moving: its rotation and acceleration, using nothing outside the vehicle. What is inside one, what 6, 9 and 11-DoF mean, and what actually separates a good IMU from a poor one.
Previous: How to read an IMU datasheet
IMU datasheets bury the story in a dozen numbers measured under conditions your vehicle will never see. Here is what each spec really tells you, which ones drive navigation performance, and the traps that catch even experienced buyers.