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Re: Misnomers - Please Don't Use
I'm climbing back up on my soapbox here. This is going to be a long one, so if you want to cut to the chase, you can skip to the last paragraph.
The FAA definition of dead reckoning (as taken from the Gleim Commercial Pilot manual):
"Dead reckoning is the navigation of your airplane solely by means of (human) computations based on true airspeed, course, wind direction and speed, groundspeed, and elapsed time. Simply, dead reckoning is a system of determining where the airplane should be on the basis of where it has been." Notice that "should" is underlined - it doesn't say determining where the airplane is, but where is should be.
This definition pretty much agrees on the two that you presented - i.e. navigating without direct external reference. However, what is not provided in the definitions is the fact that you have no way to truly measure groundspeed, wind speed and course - you can only estimate. The FAA also defines that any use of electronic navigation (i.e. any positioning system) to gain the above information is NOT dead reckoning - it is electronic navigation. If you try to use any electronic navigation aids during the dead reckoning portion of your pilot test, you will fail, as the FAA deems this as NOT dead reckoning.
Dead reckoning is ILLEGAL to use as a primary means of navigation under intrument flight rules. Some inertial navigation systems are 100% certified to use under IFR as a primary means of navigation.
Therefore... according to the FAA's definition (which is the one I subscribe to, being a commercially certified pilot and all), inertial navigation (and I would argue direct measurement of the ground using a wheel) is NOT dead reckoning.
Getting back to the IFR thing... IFR navigation is very much like our robot's autonomous mode. You know where you are when you enter the clouds, but while you're in the clouds, you have NO reference to the outside world (via your eyes) so you can't 100% truly verify your position over the ground. (for the robot, you know where you start, but without sensors, you can't truly know where you end up). This is where the difference between dead reckoning and inertial navigation comes in. Let's say you're in solid IMC (instrument meteorilogical conditions). You're calculating your position via dead reckoning and then the wind shifts. You would have NO IDEA that you're being blown off course into the mountain that's to the right of your route. (For the robot, you think you're going along in a straight line, when in actuality, you're in a shoving match with another robot.) However, with an inertial navigation system, the system would detect the subtle lateral acceleration and keep you on course. (For the robot, the navigation system would detect the lack of motion and could do something about it.)
In other words, the genrally accepted definition (amongst pilots, anyway) is that TRUE dead reckoning is BLIND between reference points. i.e. you only know where you start, where you THINK you're going, and how fast you THINK you're going. Then you calculate where you SHOULD be. Then, once you're not blind again (i.e. once you decide to reference the outside world), you can see how good you did, and then dead reckon to the next point. Even though you correct yourself at a few discrete points, you are blind between reference points (this is the definition of dead reckoning). If you have direct measurements of your course at any arbitrary instant in time, this is NOT defined as dead reckoning.
So... If you are measuring position constantly (by any reliable means of measurement), that is NOT dead reckoning. If you ever turn on motors to move and stop measuring, that IS dead reckoning. I guess it depends on how you defing "reliable means of measurement". I would agree that after a long period of time, and cheap INS would amount to dead reckoning. However, you mentioned that it was accurate to within an inch over the course of the autonomous period. I would say that you have a reliable means of measurement, and therefore are not dead reckoning.
(I'm climbing back down again, for the time being).
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An ounce of perception is worth a pound of obscure.
Last edited by Chris Hibner : 13-01-2004 at 18:15.
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