Inertial Navigation Systems — Page 244, Lesson 284

Let's pick this up with the alignment mode, because that's where the excerpt starts. When the INS is in alignment mode, the platform is being levelled and aligned — and that alignment process is called gyro compassing. When both of those processes are complete, you'll see the READY NAV light illuminated. That's your cue that the equipment can now be switched into the Nav mode, and the aircraft is free to taxi without degrading the accuracy of the INS. So the key point there: you don't lose accuracy by taxiing once READY NAV is lit. Now, there may be occasions when full navigation computing is not available, but the gyros are still serviceable. This could happen in the event of a computing malfunction — so that earth rate and transport wander corrections cannot be calculated — or it could happen after an alignment failure in flight. Let me unpack those two terms. Earth rate is the rotation of the Earth itself, which the platform must compensate for. Transport wander is the apparent drift of the gyros caused by the aircraft moving over the curved surface of the Earth. Both corrections are computed by the navigation computer, and if that computer fails, those corrections can't be made. However, on many aircraft the gyros are used as primary attitude information as well as for inertial navigation, and it may be possible to retain that gyro information. This is done by selecting ATT REF on the MSU — that's the Mode Selector Unit. So ATT REF is the attitude reference mode. Now here's what selecting ATT REF actually does. It disconnects computing and loses alignment, if that hasn't already happened anyway. The accelerometers now act as gravity switches, just as they do during the levelling phase of alignment. And the gyros become gravity-tied in the long term — they become what we call earth gyros. The system now gives you attitude information and a limited form of heading. The gyros are normally very accurate, but there is no correction for earth rate and transport wander, so the heading needs to be reset periodically to an independent source — usually magnetic. In effect, the gyros are acting as a super-accurate form of DGI — that's a Directional Gyro Indicator — and as an attitude indicator. Now let's talk about the electrical supply. Should the aircraft electrical supply to the INS cease for any reason, the INS will automatically switch to its own battery pack. For as long as a satisfactory level of power is being supplied by the internal battery, the INS BATT light will be illuminated on the Control and Display Unit — the CDU. As the power from the battery starts to fail, the BATT warning light on the Mode Selector Unit will illuminate, indicating that the INS is about to fail. And here's the critical operational point: if you're halfway across the Pacific Ocean at this time, this could spoil your whole day, because the INS cannot be re-levelled and/or re-aligned in flight. For that, the aircraft must be stationary, and the exact position must be known. Finally, the Control and Display Unit — the CDU — is shown at Figure 18.17. If you've studied the radio syllabus, you'll notice the similarity between this CDU and the control/display panel of a similar vintage VLF/Omega receiver. Although the inputs for the two equipments are vastly different, the presentation of navigational information to the pilot is more or less identical in both cases. So to tie it all together: you have the alignment mode with READY NAV, the ATT REF fallback mode that turns the gyros into a super-accurate DGI, and the battery backup with its two warning lights — INS BATT on the CDU and BATT on the MSU. Each of those is a distinct indication telling you a different stage of what's happening with the system.