First, accuracy and reliability — Page 262, Lesson 310

We’re now into the heart of the Inertial Reference System, and I want to walk you through the accuracy and reliability features, then the platform alignment, then the advantages. This is the part where the system proves itself. First, accuracy and reliability. The system uses several compensation techniques. Integration — the principles are used exactly as in the older INS, so the accelerations are integrated to get velocity, then integrated again to get position. Gravity — the microprocessor subtracts the effect of local gravity from any vertical acceleration, to compensate for local effects. So if the aircraft is sitting on the ground, the accelerometer feels the full pull of gravity; the computer removes that so it doesn’t interpret it as vertical motion. Earth rotation — the earth’s rotation rate is compensated for at 15.04 degrees per hour, just as with a gyro INS system. That’s the rate at which the earth turns, and the system must account for it or it would think it was rotating when it isn’t. Transport compensation — the transport rate. Schuler tuning is again required to compensate for oscillation errors as the system is transported over the earth. This relates to pendulum theory, which results in an 84.4 minute error cycle, as described in the older INS. So as you fly over the curved earth, the local vertical tilts, and Schuler tuning makes the system behave like a pendulum with that 84.4 minute period, so it doesn’t oscillate out of control. Calibration — automatic calibration, completed automatically by the computer to enhance the overall accuracy of the system. No pilot action needed. Now, platform alignment. The system, like the INS, requires finding true north to achieve alignment. This is achieved when the aircraft is stationary on the ground, and the only rate of change is that associated with the movement of the earth. True north is then found. So while parked, the only rotation the gyros sense is the earth’s rotation, and from that the system derives true north. Latitude — the initial latitude must be put into the system by the operator. The computer, after assessing the rotational vectors it is experiencing, compares the latitude it finds with that entered by the operator during initialization. However, note this: with this system, the inbuilt memory function remembers its position at landing, and will indicate to the crew any errors of initial position input — latitude or longitude — upon startup. So if you enter the wrong present position, the system will flag it. Alignment — the computer, after confirming the latitude, completes a full mathematical levelling process. Initial latitude and longitude must be entered manually as a present position to assist this alignment. And critically — the aircraft must NOT be moved during this process. This process is called Establishing the Trihedron. That’s the term you need: the trihedron is the set of three mutually perpendicular axes the system establishes as its reference frame. Now the advantages. Activation — almost no spin up time; one second activation for the rate sensor. Manoeuvring — insensitive to “g” attitude, rolling, and pitching manoeuvres. Construction — mechanically simple and highly reliable. Range — wide dynamic range. Drift — very small drift rates; the greatest errors are induced by the operator. And there’s a practice question at the end about dither in a laser gyro — we’ll take that one interactively.