Posts: 2685
Joined: Sun Jul 16, 2000 3:44 am


Thu Nov 02, 2000 3:00 pm

A United 777 captain explained to me how their primary navigation tool is GPS. However, this system is backed-up with an IRS. Could someone please explain how these systems work together? How does the IRS differ from the INS? Any information regarding these systems is appreciated. Thanks.
Posts: 209
Joined: Wed Oct 25, 2000 12:28 am


Thu Nov 02, 2000 8:07 pm

The basic difference between INS and GPS is how they obtain the planes cuurent position.
INS (Inertial Navigation System) is a independent on-board navigation system. It does not need any outside inputs or signals for calculating the present position. Before moving the airplane it has to be aligned to the aiplanes parking position. (Thats why you might have noticed the gate's coordinates displayed near the parking position).
Describing the way INS works probably would be to scientific. Basically it measures accelerations and rates and then computes its current position based on these signals. This system also supplies data to the aircraft's attitude indication systems. The drawback of INS is that over a longer period of operation, it gets inaccurate. As a rule of thumb, the error will be about 2 nautical miles per hour of operation.
The GPS (Global Positioning System) needs signals from outside to calculate the present position. These signals are provided by satellites. This system's position is within a tolerance of about 100-something feet. GPSs' disatvantage is that it still is a military system, without a 100% guarantee of availability. For that reason it is not approved for sole means of navigation. It can and is used as a supplementary system on modern aircraft.
For example, the INSs' long term deviation mentioned above is corrected with GPS signals.
Posts: 540
Joined: Sat Oct 02, 1999 12:10 am

GPS Precision Add-on

Fri Nov 03, 2000 2:29 am

Dear A320FO, just to add that the GPS system works perfectly with a precision of 3 feet, but only the army can work it out until this precision. When the system was put in the civil property like all kind of navigation and positioning helpers, different leisure and activities, etc... its precision standard was set to 300ft approximatively and this for "military" safety reasons.


Fri Nov 03, 2000 9:01 am

I'd like to add a little bit to the excellent posts herein. Modern airliners' such as the 777 integrates a large number of navigational instruments to obtain its positional awareness.

The FMS integrates various navigation sources in the following order (lower order is used only if the better source fails or pilot deselect):

1. Localizer Update (during an ILS approach, LOC update has highest priority) mixed with GPS/IRS data
2. GPS data filtered by IRS data
3. DME/DME update mixed with IRS data
4. VOR/DME update mixed with IRS data
5. IRS data alone

There are a few more combinations but I think you get the idea.

IRS data is mixed with everything is because IRS data is very stable with fast update rate. GPS solution, on the other hand is updated only every 1 second or so.

As far as GPS accuracy is concerned, the US military has dropped the intentional dithering (degraded C-code vs military P-code) so you should get very accurate GPS data all the time now.

Best Regards,
Posts: 136
Joined: Thu Nov 02, 2000 9:31 am

IRS INS - The Difference

Fri Nov 03, 2000 12:07 pm

IRS - Inertial Reference System
INS Inertial Navigation System

INS is the older type system
IRS is the newer system

Both pieces of kit do the same thing, by sensing the movement of the aircraft, and continuously calculating the aircraft's position, speed etc.

The systems sense movement according to the following:

INS uses mechanical gyros, which move physically within the system itself, and with the use of accelerometers. This suffers cumulative errors as detailed in previous messages.

IRS uses laser ring gyros, that have no moving parts, and sense the difference in the frequency of the laser beam reflected in the system, which in turn allows the computers to calculate, position, movement, speed etc. THE IRS system suffers LESS cumulative errors than INS - hence the difference in terminology - Hope this expains the pioint well!

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