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Inertial Navigation System: Difference between revisions
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Dead-reckoning is a navigation technique where you know where you started, what direction you flew, how fast you flew, and how long you flew. You can then, on a map, trace your position.[http://www.navfltsm.addr.com/basic-nav-general.htm] The inertial navigation system (abbreviated "INS") uses accelerometers to find your velocity and direction, and computers compile in the time and an entered start position to calculate your current position. | Dead-reckoning is a navigation technique where you know where you started, what direction you flew, how fast you flew, and how long you flew. You can then, on a map, trace your position.[http://www.navfltsm.addr.com/basic-nav-general.htm] The inertial navigation system (abbreviated "INS") uses accelerometers to find your velocity and direction, and computers compile in the time and an entered start position to calculate your current position. | ||
Inertial navigation systems are entirely self-contained in the aircraft and can be used anywhere -- regardless of the presence of radio navigation aids or GPS. However, it accumulates error over time (an accurate one is accurate to about 0.6 nm after one hour.) To combat that, most INS systems can update their position and velocity using radio NAVAIDS and/or GPS. In fact, one of the most powerful navigation systems is when GPS data is used to update the INS, called GPS/INS navigation. GPS doesn't drift, but it only updates once a second and isn't as consistend. However, the INS will "fill in the gaps" in the GPS signal, producing a reliable, accurate, and real time signal for the location, orientation, and velocity of the aircraft. Additionally, an INS can be used as part of CAT 3 ILS equipment. | |||
Inertial navigation systems usually have to be aligned on the ground, with the aircraft in a completely stationary position. The pilots (or GPS) gives them their initial system. Alignment for strapdown systems (see below for the types) usually takes 5-10 minutes, and is longer at higher latitudes -- sometimes systems won't align above certain latitudes, but you don't find international airports there. Gimballed systems align by powering up and leveling the platform (with motors.) Strapdown systems, however, sense gravity to get the attitude and sense the earth's rotation for true heading (they get magnetic heading by correcting with a database of magnetic variation) and to verify the entered latitude is approximately correct. However, there are some systems (like Honeywell's align-in-motion[http://www51.honeywell.com/aero/common/documents/White_Paper_Align_in_Motion.pdf]) that can align the system while in flight, often using GPS and compass readings. These aren't very common in current airliners. Additionally, many INS systems have an attitude-only mode where they can be aligned in flight to only give the plane's attitude, as only a full align on those systems can give navigational capability. Some can re-align at a ground stop quickly since most of the alignment has already been done -- however, they have to have not been turned off. | |||
==Two Types== | ==Two Types== | ||
There are two primary types of inertial navigation systems, gimballed and strapdown.[http://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-696.pdf] | There are two primary types of inertial navigation systems, gimballed and strapdown.[http://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-696.pdf] | ||
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<li>Fake alignment (using a timer) | <li>Fake alignment (using a timer) | ||
<li>Multiple unit mixing | <li>Multiple unit mixing | ||
<li>Fake attitude-only mode | |||
<li>Other small tweaks | <li>Other small tweaks | ||
<li>'''Ready to use in an aircraft''' | <li>'''Ready to use in an aircraft''' | ||
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<li>Fake a drift model (a better fake than the alignment... | <li>Fake a drift model (a better fake than the alignment... | ||
<li>Add a faked radio updating system | <li>Add a faked radio updating system | ||
<li>More realistic alignment simulation (gimballed/strapdown differences, etc...) | <li>More realistic alignment simulation (gimballed/strapdown differences, attitude-only mode (in air align) etc...) | ||
</ol> | </ol> | ||
==References== | ==References== | ||
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<ol> | <ol> | ||
<li>A good reference on aircraft navigation | <li>A good reference on aircraft navigation | ||
<li>Information on Honeywell's Align-in-motion | |||
<li>Detailed overview of INS systems | <li>Detailed overview of INS systems | ||
<li>Information on the IMU on Apollo 11 and gimbal lock | <li>Information on the IMU on Apollo 11 and gimbal lock | ||