Inertial Navigation

Aircraft were originally navigated by the pilot looking outside, recognising a feature on the ground, and proceeding from there. This was known as Eyeball Mark 1, and is still In use today. Later, on, as it became necessary to knew where you where when you could not see any ground detail, radio direction finding and astronavigation were used. These modes of navigation had several disadvantages: ADE (Automatic Direction Finding) was inaccurate and useless in storms (the needles would point you directly at the storm centre), Loran suffered from dawn and dusk errors and both modes required a separate operator.

VHF aids such as VOR/DME were more accurate but had several disadvantages: (a) the aircraft had to fly to and over a facility which concentrated the traffic wonderfully, and (b) was of course useless on over water flights. Another disadvantage of astro and Loran were that they told you where you had been at the time the fix was plotted – in the case of astro this could be up to twenty minutes later, which nowadays is getting on for two hundred miles, and that’s not in Concorde. Also, astro was not much help in daytime, under cloud cover or in severe turbulence.

The advent of inertial navigation (INS) like most bright boffin’s ideas, was somewhat distrusted at first by the normally doubting aircrews: we had provision for Loran on our first 747s, but rapidly realised it wasn’t needed. I do not propose describing the technicalities of INS (that’s right, I don’t understand it) but hope to give a brief idea of INS application. The following refers to most 747s and similar commercial aircraft.

There are two or three totally independent INS sets which the crew load with the geographical co-ordinates (lat. and long.) at the airport of the moment with an accuracy which allows for the actual part of the airfield on which the aircraft is standing. These details are tabulated in airfield data on the aircraft. This is the only function that has to be carried out individually on each set; when loaded, the units compare their data and show an error light if one is incorrect and also if the loaded ‘present position’ differs from the position in the units memory from its arrival on the previous flight. If this: is satisfactory the three units proceed with their internal checks, refining their accuracy until ready for the next step. This is the    en. route positions (called way points) over which the aircraft will fly. These are geographic co-ordinates, up-to nine of which maybe loaded at one time.

These can be loaded in any one machine and inserted into all three simultaneously. The crew members check their own machine’s loading independently. Depending on the length of flight, nine may not be needed of course, also on many flights thirty or forty points may be needed to be loaded this is done when time allows as way points are passed over. The great attractions of the INS to the crews are: (a) instantaneous readout of present position, (b) time to next way point at present ground speed, (c) independence from external information, (d) ability to fly many different tracks, i.e. not concentrated in one narrow, high density channel of traffic, and (e) continuous presentation of outside wind speed and direction. This last data is most useful in conjunction with the weather charts in anticipating weather changes and turbulence. The INS units are couple d to individual autopilots any one of which can be in command .of the aircraft and will take the aircraft along the desired track, and then near the next way point will alert the crew and then turn the aircraft on to its next heading. What do we do? Watch it, do crosswords, and feel grateful. (Or so we are told).

Errors on Atlantic crossings are often less than ten miles, frequently three or so. At the destination the position: that each .INS indicates is noted and if the geographic co-ordinates of the .destination are loaded, as they will be, the direction und magnitude of any errors is read off and logged. The INS will store this error in each memory and correct for it on the next flight.

Malfunctions are recorded in the memory and can be read off in a list of 64 codes at any time. And finally, the gyros are of a much higher accuracy than any previously met and provide attitude aid direction references for flight instruments as well as autopilots.

This is a brief (but long enough) article to show that these machines with an average total failure rate of around one a year ( and there are still two more in each aircraft ) are vastly better that sliced bread.

Doug Mepham – G4ERA.

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Month on the Air - G3MGQ's popular monthly DX contest/expedition list.
Wilf Gaye Memorial Cup - The clubs annual operating event in the memory of Wilf Gaye M0GYE.
St. Richard's College Buildathon/STEM/ARISS - HERC attends St. Richard's Catholic College for their various events surrounding the Tim Peake ARISS contact.
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Sussex Electronics Radio Fair - SERF Sussex Electronics Radio Fair 2016.
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Experimenters Corner - A selection of Proteus projects by Bob Gornal (G7DME)
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Radio Rallies 2016 - An up to date list of radio rallies scheduled for 2016.
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