First, Integration — Page 262, Lesson 310

We're now into the heart of the Inertial Reference System, and I want to walk you through the refinements that give this system its "proved accuracy and reliability." These are the error-compensation techniques the computer performs, and then we'll move into how the system aligns itself on the ground. Let's start with the compensations. First, Integration. The IRS uses integration principles exactly as the older INS system did. That's the mathematical process of summing accelerations over time to get velocity, and velocity over time to get position. Next, Gravity. The microprocessor subtracts the effect of local gravity from any vertical acceleration. Why? Because if the aircraft is sitting still on the ground, the accelerometers feel the full pull of gravity. If we didn't remove that, the system would think we're accelerating upward at 1g. So the computer removes the local gravity value to compensate for local effects. Third, Earth rotation. The system compensates for the earth's rotation rate at 15.04 degrees per hour — the same value used in a gyro-based INS. The earth is spinning beneath the aircraft, and the system must account for that apparent motion. Fourth, Transport compensation. This is the transport rate — the rate at which the aircraft moves over the earth's surface. Here, Schuler tuning is again required to compensate for oscillation errors as the system is transported over the earth. This ties back to pendulum theory, which produces an 84.4 minute error cycle, exactly as described in the older INS. That's the natural period of a pendulum whose length equals the earth's radius — and the system is tuned so its errors oscillate over that period rather than growing unbounded. Finally, Calibration. This is automatic calibration — completed automatically by the computer to enhance the overall accuracy of the system. No pilot action needed; the computer does it itself. Now, Platform Alignment. The system, like the INS, must find true north to achieve alignment. This is done when the aircraft is stationary on the ground, because then the only rate of change the system senses is that associated with the movement of the earth. From that, true north is found. Latitude must be entered by the operator. The computer then assesses the rotational vectors it's experiencing — the earth's rotation components — and compares the latitude it finds with the one you entered during initialization. Here's a nice feature: the system has an inbuilt memory function that remembers its position at landing. On startup, it will indicate to the crew any errors in the initial position input — whether latitude or longitude — that you've entered. So it cross-checks you. Then comes Alignment proper. After confirming the latitude, the computer completes a full mathematical levelling process. Initial latitude and longitude must be entered manually as a present position to assist this alignment. And here's the critical warning: THE AIRCRAFT MUST NOT BE MOVED DURING THIS PROCESS. This process is called Establishing the Trihedron — that's the three mutually perpendicular axes of the reference frame being set up mathematically. Now the Advantages of this system. Activation: almost no spin-up time — just one second activation for the rate sensor. Compare that to a spinning gyro that needs time to come up to speed. Manoeuvring: it's insensitive to "g" attitude, and to rolling and pitching manoeuvres. Construction: mechanically simple and highly reliable — no spinning mass to wear out. Range: it has a wide dynamic range. And Drift: very small drift rates — in fact, the greatest errors are induced by the operator, not by the hardware. That figure shows the triangular path of the device — the laser gyro's ring — which does not rotate, but the two beams of light are caused to travel different path lengths, which is how it senses rotation. Now, the book gives us a practice question here about dither in a laser gyro. Let's think about what dither does. The options are: enhance accuracy at all rotational rates; increase the maximum rotational rate; stabilize laser frequencies at peak power output; or break the frequency lock which would prevent small rotational rates from being sensed. The correct answer is d — dither breaks the frequency lock that would prevent small rotational rates from being sensed. That's the classic problem: at very low rotation rates, the two counter-propagating beams lock together in frequency and the gyro goes blind to small rotations. Dither — a mechanical oscillation — breaks that lock. These are the book's practice questions — let's try them one at a time.