Inertial Reference System
by Ghost Aviator
Table of Contents
- Introduction — The Laser Gyro Revolution
- Inertial Reference Unit (IRU) — Outputs
- The Laser Gyro — Principle (Sagnac Effect)
- Construction and Operation
- Limitations — Drift, Lock-in, Dither
- Platform vs. Strap-Down (IRS)
- Alignment — Establishing the Trihedron
- Advantages of IRS over INS
- INS vs. IRS Comparison
- Practice Question & Detailed Answer
1. Introduction — The Laser Gyro Revolution
The laser gyro has caused a technological revolution in inertial reference and navigation systems. This solid-state, high-precision angular rate sensor is ideally suited for a highly reliable strap-down configuration — it eliminates gimbals, bearings, torque motors, and other moving parts.
Inertial Navigation means determination of a vehicle's location without external references. Strap-down inertial navigation goes further — navigation without a mechanically stabilised platform. The laser gyro/rate sensor and high-speed microprocessors allow a mathematical (rather than mechanical) stable platform.
2. Inertial Reference Unit (IRU) — Outputs
The IRU is the heart of the IRS. It provides all required inertial reference outputs for the aircraft's avionics. Primary sources of information: three laser gyros and three inertial accelerometers — plus initial position, barometric altitude, and TAS entered externally.
| Category | Output Parameters |
|---|---|
| Primary Attitude | Pitch and Roll |
| Heading | True and Magnetic |
| Accelerations | Lateral, Longitudinal, Normal |
| Angular Rates | Pitch, Roll, Yaw |
| Inertial Velocity | N/S, E/W, Ground Speed, True Airspeed, Vertical Rate |
| Position | Latitude, Longitude, Inertial Altitude |
| Wind Data | Wind Speed, Wind Angle, Drift Angle |
| Calculated Data | Flight path angle/acceleration, along/across track accel, inertial pitch/roll rate, vertical accel, potential vertical speed |
- Baro altitude — stabilises the vertical navigation channel; prevents vertical velocity and inertial altitude from drifting
- TAS — allows the IRU to calculate wind speed and direction (wind = inertial velocity − TAS vector)
- Initial position — present position is calculated from distance and direction travelled from start position; the IRU cannot determine position from scratch
IRS inertial information is used by: Flight Management Computer, Flight Control Computer, Thrust Management Computer, Stability Augmentation System, Weather Radar, Anti-skid / Auto-brake, ADI, HSI, VSI, RDMI, FDR.
3. The Laser Gyro — Principle (Sagnac Effect)
The laser gyro measures rotation by comparing two laser beams created and directed to travel in opposite directions within a very narrow triangular tunnel.
- When stationary: both beams travel equal path lengths → cancel each other out → no output
- When rotating: one beam takes longer (longer path) and the other takes shorter → the path lengths change → frequency changes
- The frequency difference between the two beams = measure of angular rate
- This frequency difference is easily and accurately measured along optical paths
"The change in frequency, caused by the change in path length due to rotation of the gyro, is known as the SAGNAC effect."
Lasing is achieved by running high voltage through helium–neon gas between anodes and cathode, transforming atoms into light in the pinkish-orange part of the visible spectrum.
4. Construction and Operation
Construction
- Triangular block of temperature-stable glass
- Small tunnels drilled parallel to the perimeter of the triangle
- Three mirrors in each corner to achieve rotational path for two counter-rotating beams
The Three Mirrors — Specific Roles
| Mirror | Function |
|---|---|
| Mirror 1 | Makes micro-adjustments to keep the physical light path accurately aligned |
| Mirror 2 | Partially transparent — allows laser light to be detected on photo-cell detectors. Includes a prism that redirects/flips the beam to cause interference with the direct beam. |
| Mirror 3 | Standard high-reflectivity mirror |
Detection — Fringe Pattern
- Mirror 2's prism redirects beam → both beams meet → interference (alternately cancel and reinforce)
- This creates a fringe pattern
- A photoelectric cell detects the direction and speed of fringe pattern movement
- Direction of pattern movement = direction of rotation
- Speed of pattern movement = rate of rotation → converted to angular rate signal
Path length and frequency: If path length decreases, light is compressed → frequency increases. If path length increases, frequency decreases.
5. Limitations — Drift, Accuracy, Lock-In, Dither
Drift
Principal error source, as with conventional gyro INS. In the laser gyro, noise is the cause — derived almost entirely from imperfections in the mirrors and coatings (not mechanical bearing imperfections as in a conventional gyro).
Accuracy
Accuracy is directly influenced by the length of the optical path. A small percentage increase in path length leads to a substantial increase in accuracy.
Lock-In (Laser Lock)
Dither Motor (Piezo-electric)
- A piezo-electric dither motor vibrates the laser ring about its input axis through the lock-in region
- This "breaks" the lock-in by ensuring the gyro never stays at zero rotation rate
- The dither motor motions are decoupled from the output of the ring laser gyro — so they do not contribute error to the navigation solution
- The photoelectric sensor can then detect smaller fringe pattern movements
6. Platform vs. Strap-Down (IRS)
graph TD A["INS — Platform System"] --> B["3 rate-integrating gyros
3 accelerometers
on mechanically stabilised gimbal platform"] C["IRS — Strap-Down"] --> D["3 laser gyro/rate sensors
3 accelerometers
bolted directly to aircraft chassis"] D --> E["High-speed microprocessor
maintains stable platform
MATHEMATICALLY"]
| Aspect | INS (Platform) | IRS (Strap-Down) |
|---|---|---|
| Gyro type | Rate-integrating (mechanical) | Ring laser gyro (optical) |
| Platform | Mechanical gimbals, motors, bearings | Mathematical (no moving parts) |
| Gravity compensation | Mechanical levelling of platform | Microprocessor subtracts local gravity from vertical acceleration |
| Earth rate compensation | 15.04°/hr torquing of gyros | Same — compensated at 15.04°/hr |
| Transport rate / Schuler tuning | V/R torquing; 84.4-min Schuler cycle | Same — Schuler tuning required for oscillation errors |
| Calibration | Manual | Automatic (computer-based) |
| Reliability | Lower (many moving parts) | Much higher (solid-state) |
7. Alignment — Establishing the Trihedron
- Finding True North: Aircraft stationary → the only rate of change is Earth rotation → system detects Earth rotation vectors → True North computed
- Latitude verification: Operator enters initial latitude. Computer assesses rotational vectors it is experiencing and compares with entered latitude. If there is a discrepancy, crew is alerted.
- Memory function: The IRS remembers its position at landing — on startup it will indicate any errors in initial position input (lat or long) to the crew
- Mathematical levelling: Computer completes full mathematical levelling process
- Process = "Establishing the Trihedron"
Unlike the INS, the IRS (with memory) can flag discrepancies between the entered position and its remembered landing position. This is an additional safeguard against gross position entry errors.
8. Advantages of IRS over INS
| Advantage | Detail |
|---|---|
| Activation time | Almost no spin-up time — ~1 second activation for the rate sensor |
| Manoeuvring | Insensitive to 'g', attitude, rolling and pitching manoeuvres |
| Construction | Mechanically simple, highly reliable (no gimbals, bearings, torque motors) |
| Dynamic range | Wide dynamic range |
| Drift | Very small drift rates — greatest errors induced by the operator (position entry) |
9. INS vs. IRS — Key Comparison Summary
- Double integration principle (acceleration → velocity → distance)
- Initial position must be entered manually
- Aircraft must be stationary during alignment
- Schuler tuning (84.4-min period)
- Earth rate compensation at 15.04°/hr
- Transport rate compensation (V/R)
- Coriolis and centrifugal compensation
- Accumulated position error with time
- No mechanical platform → more reliable, lighter, smaller
- Faster alignment (~10 min vs. INS ~15 min)
- Laser gyros have very small drift rates
- Memory function for position verification on restart
- Insensitive to vibration and manoeuvres
Practice Question & Detailed Answer
- A: Dither is specifically for breaking lock-in at low rates, not for enhancing accuracy at all rates.
- B: Maximum rotation rate sensing is a function of the optical path length and detector bandwidth — not dither.
- C: Laser frequency is stabilised by path length adjustment (mirror 1 micro-adjustment), not dither.
Reinforce Chapter 19: Inertial Reference System
Test your knowledge and practice actual exam questions for Navigation — Instrumentation.