Navigation

[Vicar]

How do military planes navigation systems differs from those of civial aircraft - other than the different types of 'radar' they used relative to the GPS ?

Compasses they used, do they follow the point to the True North or the (North)where magnetic fields are the most concentrated??

Is there a formula to convert Mag. Flds North to True North? For civil/transportation aircraft(uses Mag Flds North), i heard they just need to press a button to convert it to True North.

If there's is some problems with the panel on the aircraft such that it does not help you 'navigate', how would you navigate then? -celestrial naviagation-? Pilot's i've come across and spoke to doesn't seems clear of celestrial navigation or it's just my luck.

Thanks

Vicar

 

[Toro]

I've never flown civilian aircraft, but I'll take a shot at what I can specific to the Strike Eagle...

How do navigation systems differs - other than the different types of 'radar' they used relative to the GPS ?

The radar we use has nothing to do with navigation, it's for finding other aircraft or ground targets. We have the basic TACAN, DME, ILS equipment (no VOR) for approaches. We also have what's called an ALG mode (Autonomous Landing Guidance) where we can punch in a designated ground coordinate and the computer will set us up an an ILS to that point -- I've never even heard of this being used, though. We have an INS, but more of a backup if our GPS fails. We also have a tactical situation display (TSD) - a moving map basically. Different sized maps, from low fly charts to airway charts are loaded up and the aircraft and flight route are superimposed to give you a God's eye view of where you are and where you're going. There's also all kinds of cool gizmos for maintaing precise timing to targets, but I won't get into it.

Compasses...do they follow the point to the True North or the (North)where magnetic fields are the most concentrated??

Magnetic north

Is there a formula to convert Mag. Flds North to True North?

I don't know of any button that does it for you, but the conversion is easy. Just about everywhere has a magnetic variation that can be found by looking on most navigation charts. You'll see, for example, a "3E" or "5W" which would mean 3 degrees East variation or 5 degrees West variation. If the variation is east, you subtract it from the true mag heading on the map - add West variations. So, if you determine a heading on a map to be 180 and the magnetic variation in that location is 4E, your magnetic heading would be 176.

If there's is some problems...that it does not help you 'navigate', how would you navigate then? -celestrial naviagation-?

Flying by the stars? The possibility of a Strike Eagle losing all avionics would be unlikely, but if it did happen we (like every other aircraft) have the old backup whiskey compass. It bounces all around and it's inaccurate in turns, but it's there nonetheless. Realize also that we almost never fly single ship, so if our avionics $hit out on us, we just have our flight lead/wingman lead us home.

 

[Vicar]

Thanks, that was an excellent reply Toro.

The possibility of a Strike Eagle losing all avionics would be unlikely, but if it did happen we (like every other aircraft) have the old backup whiskey compass

Old backup whisky compass? like those you could get in retailers? those roational kind? Where are these compasses located? Nv seen them b4 on a Strike Eagle(simulaton)...

Since planes are using the Mag North..:
If you enter the center of a magnetic field, an ordinary compass - those you can get in small stores outlets - they tend to spined non-stop like the compass had gone crazy. Does it affect the compass on the plane too? There are claims from pilots who flew thru a burmuda triangle, that the numbers on their avionics keeps changing at a very fast pace... so does their digital compasses... could the burmuda trigangle be a result of a mag. field say maybe clash, lost or something....


a Million Thanks...

arigato~

Vicar

 

[Toro]

Old backup whisky compass? like those you could get in retailers? those roational kind? Where are these compasses located?

In the F-15E and T-38 (and I would guess most fighters), they are located at the top portion of the canopy bow in front of the pilot. They call them whiskey compasses because I guess at one point they were actually filled with whiskey (not sure the liquid that's in them now). They are rotational, but the 'face' is actually the side of the compass so you look at it straight on as opposed to your standard hiking compass that you would look down at.

As far as a magnetic field affecting the compass on an aircraft, I don't know for sure, but I would imagine that any type of magnetic disturbance would affect it just like a normal compass. For a definite answer you're going to have to ask someone a lot smarter than me.

 

[Sonic Cruzer]

Why yes, I AM a Rocket Scientist!

Interesting questions.

As far as Headings and Courses between True north and Magnetic North, Toro's got you covered. It's a simple matter of adding or subtracting a specified Magnetic Variation (AKA Declination.)

The algebra for calculating Declination isn't too bad, but the formula usually contains constants that are extracted from mathematical models of the earth's magnetic field. This type of equation is known as a LeGendre Polynomial. At worst, the math is 3rd semester college Calculus (vector alegbra & series). I recall doing some of these calculations in Jr-year college Geophysics. For the most part, we were encouraged to use the wide variety of models and simulations available online.

Here's a website that will allow you to calculate the Magnetic Declination for any Lat/Lon/Altitude. It also gives you some other interesting Magnetic Field Parameters. The explanation that comes with the results page is High School/undergrad level.
It's a neat site. If you don't know your own Lat/Long you can input your ZIP code.

Compute Values of Earth's Magnetic Field (Version 4.0)

In terms of the Earth's Magnetic Poles, there is a Magnetic Lat/Long coordinate system. This is used only by the scientific community and shouldn't be confused with anything a pilot would navigate by. For shi+s and grins you may still want to try a model that converts regular Lat/Long (based upon true north) to MLat/MLong.


Geomagnetic Coordinate Transformation


You also asked about magnetic field disturbances and compasses. Again Toro is correct. Your compass is at the mercy of any signifigant magnetic field disturbance, be it a local disturbance in the Earth's Magnetic Field or Electromagnetic Interference (EMI) caused by electronic devices in the cockpit.

In most aircraft the magnetic compass is purposely located away from the radios and other electronic equipment in the cockpit. In fact, jets often have magnetic sensors for their compasses mounted in the outboard wing sections for this same purpose. (i.e. T-37 Flux Valve)

Most magnetic compass installations in aircraft have a card beneath them called the Compass Correction Card. Once a compass is installed in an aircraft (and periodicallly after that) a test known as a compass swing is performed. You turn on all the aircraft systems (avionics, engine in particular) and point the airplane at the known Magnetic North.

[Test is usally performed on a compass rose painted on the ground or with aid of a surveyor standing outside and away from the a/c. At Boeing, we have these large turntable fixtures reminiscent of the old train depot turntables. The jet is towed on to the turntable, which is then rotated for the compass swing.]

You compare what the compass actually says and jot that down on the compass correction card. For example if pointed at 360 deg Magnetic the wet compass says 354 deg, the pilot will know that with all the sources of airplane generated EMI, he/she must actually fly 354 to head Magnetic north (360).