View Full Version : IRS update in flight.

21st May 2002, 17:06
I understand that the IRS/INS cannot be updated in flight- and can only be aligned on the ground when the aircraft is stationary.

My question is how then does the IRS/INS keep the radial error down- if it cant update its position in flight?? I know that the FMC provides cross cuts and the most reliable source of updating comes from DME-DME and then DME-VOR. What happens to this information? Does it by-pass the IRS completely, and is only mixed in the FMC along with GPS information etc?



Alex Whittingham
21st May 2002, 17:57
My understanding is that the FMS takes account of the IRS position and the possible circle of uncertainty caused by the system drift. It compares this to DME/DME crosscuts with their much smaller potential error and computes a likely position between the two but heavily biased to the DME/DME position, ideally within all the circles and ranges of uncertainty. If the double DME crosscut is lost it assumes the position is the IRS position with the last known bias applied to it. This means, in short, that the INS/IRS is just allowed to drift and the FMS calculates its position on the basis of all available information.

Which IRS/INS position is taken depends on type. The L1011 had three INS and the FMS 'triple mixed' the positions of the three systems to arrive at an averaged INS position. I understand the B737 uses only the left hand IRS as an FMS position input under normal circumstances.

The old INS could be position updated in flight, I don't know about IRS. You used to select POS and HOLD coming up to a beacon overhead, insert the lat and long of the beacon, and release the HOLD button when overhead.

Finally, the radio updates into the FMS vary. The B737 only uses DME/DME, some systems will also use DME/VOR and a few very old systems will also use VOR/VOR.

oxford blue
21st May 2002, 21:28
Because INS/IRS/FMS is still a fairly recent discipline, there is no totally consistent answer that holds good for everyone. Each company develops it's own solution, and as with VCRs (remember the competing claims of Betamax, Phillips, and VHS?) eventually one solution will dominate the market (like Microsoft, for instance), and everyone will do it that way.

At present, there are only 2 serious contenders - Honeywell, who build the systems for the Boeing, and the Airbus consortium. Whilst I think that Airbus produce excellent airframes and engines, I think that the Honeywell approach will eventually become the industry norm (at least, for a while - until it all changes).

With the Boeing 737-400 system (which is the one that the JAA use for examination), you have a left IRS and a right IRS, both independently fed into a Kalman filter. The IRS can only have a present position fed into it during initialisation, ie, on start-up. From that moment on, all of the IRS input is treated as sacrosanct within the IRS itself. If it's going to get mangled, that happens downstream, within the FMS's Kalman filter.

As you advance the TO/GA levers on the take-off roll, the FMS is automatically updated with the data-base present position of the piano keys of the take-off runway. The raw IRS positions are unchanged.

Once airborne, DME/DME positions are computed. The FMS does NOT automatically move the IRS positions to the DME/DME. What is does is, over a period of time take out a small amount of the apparent fix error on each fix. In broad terms, it finds out the history of the actual DME fixes compared with each separate IRS. The computer knows that each IRS will generate a combination of bounded and unbounded errors. The bounded errors will be a sinusoidal Schuler with a period of 84.4 minutes and the unbounded errors will be a combination of a ramp and a parabola.

Knowing all this, if the Kalman filter gets enough fixes (which it will over a reasonable period of time) it can then do a curve-fitting exercise during which it can build up the history of the fixes compared with the raw IRS positions. It knows the format of the differential equations linking the differences - one will be sinusoidal and the other will be a ramp - all it has to do is solve the coefficients in the formulae - it's a form of regression analysis.

Using this, it can build a mathematical model of the error growth rate in both the left and the right IRS, to predict where the next DME/DME fix will come. The more good DME/DME fixes it gets over time, the more refined this model becomes.

If, at any stage, it loses DME/DME, it uses the last model it had of the error growth rate to predict the probable error of the raw left and right IRS positions. If there is no other information to suggest which is the more reliable one (which there usually isn't, once DME has been lost), then it takes the left one.

However, at the end of the day, it is only the FMS position which gets changed. The raw left and the raw right IRS positions continue to be computed until you switch the thing off.

Simple, really!!!

21st May 2002, 22:17
Oxford Blue... I just love people like you who can even make people like me understand such complex theory!

Cornish Jack
23rd May 2002, 11:09
Essentially the IRS is carrying two positions - one which is the product of it's computation from the accelerometers (based on its initialisation position) and one derived from the updates received from wherever (FMS etc.). These two positions are inevitably going to be different over time due to IRS drift and the amount of that drift can be resolved with the aircraft parked at an accurately known ground position at the end of flight. The acceptable amount of drift differs between installations but, as an example, the L1011 computation was based on a maximum acceptable error of 3 + 3T in nautical miles, where T was hours from initialisation.

oxford blue
23rd May 2002, 11:21
Sorry to sound pedantic, Cornish Jack, but I don't think that it's the IRS which holds both the raw and the refined positions. That's what used to happen on the older system of INS, where you actually updated the INS present position.

With the Honeywell Boeing system, it is the FMC which holds the refined position. All the IRS holds is the raw position. Thus each IRS has its own raw position, but the FMS holds 4 positions at the same time, left IRS, right IRS, DME/DME, and FMC position (the final computation used). If you select the "Present Position" page on the CDU you can actually see and compare all 4 positions.

It may be all different on the L1011, of course.

24th May 2002, 07:40
It is hard to generalise when there are so many different systems in use. Just to try and keep things simple but clear - In any Inertial Navigation System the inertial navigation is done by an INU or an IRU that is fed with present position on start up. Initially this present position and the position displayed on the control panel are the same, but as the flight proceeds the displayed position is the inertial position [I]with updates added to remove errors introduced by platform drift. The inertial position remains in the computer as the basis for all calculations. Between the platform and the display a navigation computer continually adjusts the displayed position to include position updates entered from sources external to the platform. The external source may be a pilot input as described by Alex Whittingham or automatic updates within the navigation system using DME/DME cross checks as explained by oxford blue, but in either case the inertial position remains in the computer until the INS is finally shut down at the conclusion of the flight.

Manual in-flight update by overflying beacons cannot be accurately judged due to the width of the 'cone of confusion' at altitude. I think back to our 707's using, say Shannon VOR, to update a Carousel INS after crossing the Atlantic, when they would be up at 30,000 feet where the "cone" over the beacon may be up to ten nautical miles in radius - which was of course, the most commonly reported error distance reported by the crew! At the end of the flight the difference was written up as a defect and, with the 'Carousel' system, the first step is "Update Flushing" to remove any in-flight updates from the system so that the actual Inertial Position appears in the display. This is usually found to be accurate and the defect lies in an inaccurate update.

To return to lonerider's question, in the case of Honeywell systems at least, all navigation calculations are performed in the Inertial Reference System's own computer. The IRS calculates where the aircraft actually is from the inertial position as modified by fixes, the FMS calculates where the flight plan requires the aircraft to be and, if there is a difference, the FMS produces outputs to the AFCS or Flight Director to enable the aircraft to acquire and maintain the desired flight path.

This arrangement combines the benefit of the short term accuracy of the inertial sensors with the long term accuracy of multiple position fixes from ground based navigation aids or from GPS.

Through difficulties to the cinema

24th May 2002, 10:40
Any boffins out there who understand the workings of the French 'Litton' IRS system as fitted to some Airbus.
Any info' appreciated.

24th May 2002, 12:18
For once, the airbus system seems to operate fairly similarly, at least in terms of IRS.

Each of the two Flight Managemant and Guidence Computers, which are normally synchronised, computes its own position from a mean weighted average of the three IRS positions, called Mix IRS and a computed radio or GPS position. If one IRS drifts abnormally, an algorithm that decreases the influence of the drifting IRS within the Mix position is used.

The overall system, FMGS (which looks after the FMGCs) selects the most accurate of these, taking into account the estimated accuracy and integrity of each positioning equipment.

Each IRS also computes an hybrid position that is a mix IRS/GPS position called GPIRS. Each FMGC receives these three hybrid positions and one is selected according to figure of merit and a priority system.

If the GPIRS data does not comply with an integrity criteria, the GPS mode is rejected and radio position updating is used, as described in earlier posts.

Each FMGC computes a vector from its Mix IRS position to the radio or GPIRS position called a bias. Each FMGC continuously updates its bias, if a radio or GPIRS postion is available. If an FMGC loses its radio/GPIRS position, it memorises the bias and uses it to compute an FM position, which equals the Mix IRS position plus the bias. The FM position is also updated on departure to the runway threshold position. The priority for updating FM position in flight is given to GPIRS, then to IRS-DME/DME, then IRS-VOR/DME, then IRS only.

For simplicity, the crew's primary check on Navigation is an accuracy level display on one of the screens, either HIGH or LOW, depending on whether the FMGS calculated Estimated Position Error is less than or greater than the Required Navigation Performance for the flight phase. Most of the time, GPS PRIMARY is displayed as the main Nav mode, with NAV ACCURACY HIGH.

IRS drift rates on my machine (A330) tend to be around 0.2 nm/h over an 8 hour flight.

Hope this helps answer the question!