Captjetblast From Argentina, joined Aug 2001, 277 posts, RR: 0 Posted (8 years 6 days 2 hours ago) and read 714 times:
I understand that the Innertial Navigator System (INS) flies the plane from one waypoint to another, selecting VOR's automatically and following flight plans as loaded in memory. I'm sure there are more lots of things it does I don't know.
But during transatlantic flights, over the ocean, no VOR's available, how does it manage to reach the waypoints? By detecting and correcting deviations based upon gyros, is that all?
The_Greq From Netherlands, joined Sep 2000, 179 posts, RR: 2 Reply 1, posted (8 years 6 days 2 hours ago) and read 692 times:
I am told that there are NO waypoints above the oceans.
Above the Atlantic, there are several flight routes. (I thought 8)
These flightroutes change by the day or even after 12 hours. But I am not sure.
The routes are between Gander (US/CANADA) and Shannon(IRELAND).
It depends on the weather conditions which routes are opened by the authorities. Between Gander and Shannon the pilots have to report their position, heading, speed, and so on, to one of the two stations.
I 747-300 KLM-pilot told me they really have an sound what goes off after 15 minutes. Such kind of thing you use when you boil an egg.
Tom775257 From United Kingdom, joined Dec 2000, 153 posts, RR: 0 Reply 2, posted (8 years 6 days 2 hours ago) and read 692 times:
Please forgive me as I have had far too much lager to make much sense, but will try...(and if you ask why mid afternoon, I'm a student)
I will first say the only aircraft I fly have crappy mechanical gyros that drift like a boat without anchor in a strong current, and aren't used as such for position finding (only atitude and direction)..however:
My learned knowledge regarding an INS (and please others correct me if wrong!!!)....an inertial nav system. These are mainly found in older aircraft e.g. 747 classics etc. that haven't had avionics updates. The INS is by definition linked to gyroscopes to provide acceleration information. From this it can work out through calculations (after alignment) where the aircraft is. You asked does it suffer drift? Yes, but not too much, I believe they use laser gyros, which I assume suffer less drift than standard gyros. Across oceans radio beacons are not followed as far as I know.
More modern aircraft e.g. 757/767 use a flight management computer (FMC) hooked up to an inertial reference system (IRS) to provide nav info and drive the autopilots (with input from the MCP) along with the normal nav radios. These too will suffer drift or map shift over time away from radio beacons. (The FMC can automatically update its position from radio beacons after getting within range)
Going more modern still, the 777 FMC I believe collects nav info from radio beacons, an IRS system (5 laser gyros I think) and a GPS unit...GPS obviously doesn't suffer drift!)
Hope this info is useful. I do not deal with these type sytems at all, so all my info regarding this is from kind people on this site, and on pprune. Others: please correct me if wrong.
Cheers, Tom.
747Teach From United States, joined Nov 2000, 176 posts, RR: 2 Reply 4, posted (8 years 5 days 22 hours ago) and read 663 times:
Captjetblast: In regards your question about INS. Inertial reference guidance systems are a navigation system that measures the earth's rotation and the vehicle's movement, and thereby determines the vehicle's position on the face of the earth. In the system I'm familiar with, each system has in its guidence unit 3 ring laser gyros, 1 each for each aircraft axis, and 3 accelerometers, 1 for each axis. These sensors send information to the navigation computer (housed inside the same box with the gyros and accelermometers), which performs the mathematics to calculate the aircraft's position. In addition, the computer memory also holds thousands of waypoint codes for most of the worlds airports and other navigational references. Up to 99 of these can be used at a time to set up a course. This memory is updated monthly. Each navigational system has a control display unit (CDU) so the crew can interface with the reference unit, and a Mode Select Unit (MSU) to turn the unit on and select the mode of operation. Most large transport aircraft operate with 3 complete INS systems.
The key to INS operation is, the crew must tell the INS system where it is (in latitude and longitude) when they initialize the system. Once the system knows where it is, it will keep track of itself, and the aircraft's movement, hence it's position. It is crucial that the aircraft not be moved during the "alignment" period, so the gyros can get a good reading on the earth's rotation. Once all three systems are aligned and placed in the navigate mode, they will constantly measure the aircraft's movement in all three axes.
Inertial reference systems are position "keeping" systems, rather than position "finding" systems like radio beacon or GPS. They need no outside reference to navigate. Typically, all three systems "talk" to each other (called "triple-mixing"), and can arrive at an "mean" position that should be extremely accurate. I believe there is one system on the market that advertises a drift of 1 nautical mile per hour in navigate mode. So a mean position after a 10 hour Atlantic crossing should have a circular error probability of no more than 10 nm. And that's without any updating or outside reference. If DME or other RNAV stations are available, the system can update from those points, and produce an even more accurate fix.
Hope this helps you out. The mathematics of how the computer figures out "local vertical," and does the trig and calculus to determine rotation rate gets pretty deep very quickly, so I didn't go into them here. Suffice to say the strapdown ring laser gyro systems are pretty accurate for commercial use, and are useable throughout the world without outside assistance. In conjunction with a properly operating autopilot system, they can fly the aircraft pretty much anywhere you want to go. The more advanced FMC's are performing much the same task, but with more automated functions and more integration of the various systems. Regards.
310_engineer From Belgium, joined Dec 2000, 165 posts, RR: 0 Reply 5, posted (8 years 5 days 22 hours ago) and read 662 times:
Captjetblast:
You are messing to things up. The INS and the FMS are two different things.
First the INS (inertial navigation system) consists two or three identical units (INU's), where each unit provides independantly and autonomicly following navigation information:
-references off attitude and direction : roll, pitch and platform heading (hereby the INS replaces the older vertical and directional Gyro's)
-navigational references:
* position (lattitude and longitude)
*Ground speed
*true heading
*Drift angle
*track angle error
*cross track distance
*distance to next waypoint
*and time to reach next waypoint
-if it receives TAS information from the ADC the INS can also provide windspeed and winddirection.
Basicly the INU is a platform that has been stabilized and kept perfectly horizontal by two Gyro's, and that carries accelerometers. On the axes there are synchros placed to measure the pitch, roll and yaw of the aircraft, this information goes to instruments and other systems who need it.
The INS does not need any info from ground stations(ie VOR's, NDB's,...)
More modern versions are IRS, and ADIRS (where the ADC and the IRU are combined in one computer)
The drift allowed between to INU's is 3+3T in Nm, where T is the flighttime. ie flight time is 5hrs, max drift is 3+3*5=18Nm.
Now the FMS (Flight management system) is also a navigation aid, but...:
The Fms uses information from the INS and works a little like a more precise system as it also takes info from VOR, ADF,etc... and can control the Auto pilot.
This can be done because a flightplan can be entered.
By dataloading almost anything can be loaded in this big mind such as routes, star's, Nav-frequencies etc...
When flying over big sufaces without any ground stations, the FMS will keep its position information from the INS and as soon as it receives a VOR or ADF info it will correct it's aircraft current position.
This was really basicly explained as the whole system is very complicated and can not be explained in 5 mins, but with this and together from what other techs or pilots will tell you, I think you could get an idea of it.
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