View Full Version : How does an INS take account of wind drift?

17th Apr 2003, 23:47
I can understand how an INS can give you position info given its start position and all of the accelerations etc experienced during flight.

BUT how does it deal with wind drift assuming the INS has no link into any aids (like VORs) to verify its position.

Or does wind drift make so little difference it can be ignored.

Just curious.

18th Apr 2003, 00:26
The INS/IRS or whatever the aircraft has, incorporates 3 gyros, representing the 3 D axis, so it can sense yaw associated with wind drift. Think about how sensitiive these gyros are when they sense the earth's rotation when erecting on the ground. With windspeeds of 100+ kts in the jet stream, you better believe they take that into account, otherwise all you would need is a compass and a stopwatch to figure your position on the map.

18th Apr 2003, 00:38
Exactly so. Inertial Navigation relies on nothing other than the deflection of masses under a gravitational field; no VORs, no radio, no nothing.

For example, a/c takes off and crosswind hits from the right at 30kt. A/c accelerates left, to 30kt, and keeps going left. INS senses the acceleration. If it senses no other change in that direction, it will incorporate this drift of 30kt left into its calculated position.

18th Apr 2003, 14:21
Don't get confused between Inertial Navigation Systems and Inertial Reference Systems. An IRS only knows where it is, an INS not only knows where it is but also where it should be. Any difference between the two is an error -- drift. The INS is working the desired track between two waypoints from the flight plan. If wind blows the aircraft off course, the INS detects the drift and can compute windspeed and direction from this. Most Inertial Navigation Systems can be coupled to the Autopilot/Flight Dirtector and any tendency to drift is automatically corrected. Just add autotuning to an autocoupled INS and Bingo! - you get an FMS.

Through difficulties to the cinema

18th Apr 2003, 17:31
Thanks for the replies. I can sleep well now.

18th Apr 2003, 19:13
I've worked on and used INS's for some 28yrs now, in both military and civil jets, and I must admit, I understand them differently from the above posts. So, for what it's worth, here's my two pennorth:

The inertial platform senses accelerations from a given state or rest or movement. The associated computer integrates the accelerations and provides velocity and distance data. There is obviously a lot more to it, but basically, all the inertial system will give you is those values.

Using other aircraft inputs, and variations in inertial data. the cleverer systems can give you position on the earth's surface.

Now using the inertial position, inertial direction of travel and inertial distance covered and then comparing them with Air Data, e.g. true airspeed, compas direction, etc, the inertial system will provide drift, groundspeed and wind data.

More simply, the INS knows where you are going and how fast you are going. The ASI provides the TAS and the compas provides the direction you are pointing in. Put the lot together and you can work out wind speed and drift. On it's own, with pure inertial data, it is impossible to work out drift and wind speed.

Hope this helps,

21st Apr 2003, 14:17
In current airline systems TAS is an input, but not compass heading. In fact most modern jets don't have a magnetic compass system, only the standby compass is magnetic. Magnetic Heading is an output from the Inertial System based on True Heading corrected by magnetic variation derived internally from a database of the earth's magnetic field; the database being updated at regular intervals as the earth's magnetic field changes. True heading is the direction the aircraft is pointing, with the INS strapped down on the airframe. The accelerations along each of the axes derives the direction and velocity of motion. Any difference between the direction the aircraft is moving and the direction it is pointing gives the drift angle. The difference between track and heading provides the basis for deriving wind and drift. Although CAS, TAS and Altitude are measured by barometric means and computed for display on electrical ASI/Machmeters and Altimeters by the DADC, Vertical Speed is usually inertially derived and presented on an electrically driven IVSI.

Through difficulties to the cinema

22nd Apr 2003, 10:37
"Although CAS, TAS and Altitude are measured by barometric means and computed for display on electrical ASI/Machmeters and Altimeters by the DADC, Vertical Speed is usually inertially derived and presented on an electrically driven IVSI."

Veering off topic here, but....
Aircraft with IRS's usually need ADC info for the IVSI. In fact, the IVSI won't work without it on 767's, 747-400's, etc... (try pulling the relevant ADC breaker and see what happens to your IVSI). The IVS is a combination of both inertial and barometric info. The baro provides long term correction. In many cases, the air data is not fed directly to the indicator, but via the IRU (so it may not be immediately evident that the air data is being provided to this instrument).

Back on course...
IRS's compute position relative to the ground (if the aircraft drifts due to crosswind, the IRS's still know where the aircraft is relative to the ground). It's only when a display of windspeed/wind angle/etc is required in the cockpit that conventional air data is required.


23rd Apr 2003, 21:23
18greens you are right it doesn't and can't.
Airbedane has it correct, to give you drift, wind speed and direction (basically the same thing in effect) it needs imput from the "conventional" instruments.
All an IRS can tell you is the accelarations it experiences from the moment you intialise it, it can't tell the difference from an acceleration due to an aircraft engine or due to 100kt winds.

24th Apr 2003, 20:11
and as the IRU only knows the GS, where does it get the TAS? That's right, air data...


24th Apr 2003, 23:54
cc. Homed in on the "youwerewrong" part. Gotta make the coffe breaks longer around here! :D


27th Apr 2003, 02:59
As I said it can't work out wind spd etc without conventional instrument imput!

Reverend Doctor Doug
30th Apr 2003, 12:57
So what are you guys trying to say then?

You cant expect me to believe that the 3 little guys sitting in the autopilot control room, as seen in the CBT, making all the calculations and flying the airplane, don't really exist.

I'm shattered. How can i have faith in the system now!!!!

6th May 2003, 05:52
An INS alone gives two displays in a double window. One is Lat/Long, the other is Track and Groundspeed.

7th May 2003, 18:14
hopharrigan is right - the INS gives track/groundspeed.

If you remember the days of playing with a whizz wheel for your exams, if you know the track/groundspeed, and TAS/heading, you can calculate wind direction/speed.

Since a GPS knows track/groundspeed, it could also be used to calculate w/v.