MarkRight to the point there:>assuming our solar system, our galaxy and our universe are not >movingI guess, and this is only a guess ;-) the movement of the sun around the center of the galaxy would add such an infinitesimal value so it is regarded as zero. :-) Or it is used as a base for the IRS calculations. But back to your questions. I'll have to look in the book for a moment................ OKFor alignment both accelerometers and gyros are used. The Inertial Reference (IR. A part of the IRS) has to establish a level plane to true north as well as a heading to the same. This plane coordinates with a N/E plane. This is done by three inputs. Your entered position, the gravitation and the earth rotation. IR assumes the only acceleration is the gravity and the only movement is the earth movement. For positioning the IR uses an initial vertical for it's computations. A latitude is established based on this vertical to the earths surface, and the heading and direction to true north. I have to admit that the only thing the Guide says about the lomngitude is that it compares the last stored longitude with the one you entered. And that the earth rate sensing can't determine the longitude... Maybe someone else can fill in here...And to the drift...One reason for drift would be any slight error in entered latitude, resulting in an erroneous calculated earth rate. Discrepansies in the equipment is another reason for drifting.... but I'm just drifting away hereThe main calculation of the IRS is acceleration. The gyros are there to determine the relationship of the acceleration to the earth's surface. Given this, one could assume that even ground speed woould drift if position drifts because from acceleration the IRS integrates a speed and from that a position...God, I'm babbeling away :DHope it makes sense

Although IRSs are very accurate, after entering NAV mode, errors start accumulating. The errors result in 3 slightly differn't calculated present positions.Best Wishes,Randy J. [email protected]" A little learning is a dangerous thing"AMD XP 2100 |MUNCHKIN 512 DDR RAM |ECS[/b ] K7S5A MB |[b]GF3 64 MEG |WIN XP PRO |MITSUBISHI DIAMOND PLUS 91 19"

The IRS contains accelerometers and laser gyros that measure airplane movement in all three axis. These components replace conventional vertical gyros, directional gyros and magnetic compass systems. Signals from the acceleromometers and gyros are sent to the IRS computer. the computer users this info to calulate the airplanes attitude, headng, ground track ,ground speed and lat/long.Before the computer can calculate this info it must accomplish a preflight alignment. Alignment refers to the process used to determine the airplanes relationship to true north. The airplane must remain stationery during alignment. During alignment the sensed gravity force is used to calculate the airplanes attitude and the Earth's rotation force is used to calculate heading. Stored in the IRS is the magectic variation for all locations between North 73 degress, and south 60 degress. The magnetic variation is used to convert true heading to magnectic heading. Outside this latitude range only true heading is calculated. Before the IRSs can calculate navigation information, you must either use the CDU or IRS panel to enter your present position. After alignment is complete, the signals from the accelerometers and gyros are used to calculate present position. The IRS then sends the calculated position to the FMC.Best Wishes,Randy J. [email protected]" A little learning is a dangerous thing"AMD XP 2100 |MUNCHKIN 512 DDR RAM |ECS[/b ] K7S5A MB |[b]GF3 64 MEG |WIN XP PRO |MITSUBISHI DIAMOND PLUS 91 19"

Hi MarkFrom what I understand, acceleration in each axis and the laser ring gyros will drift as the factors of earth rotation and equiptment errors come into effect. Sinsoidal Schuler bounded errors will result for about 81- 85 minutes I think, don't quote me here. The other factor is ramp error over time. It is a math error and is solved through a coefficiential formula.I am not big on physicis but as for your question. Yes ground speed drifts as well. The amount is about 0.3 miles for 7-8 hours from the first initialisation. Not a big deal.

>To get back to my original question :-), what will >eventually drift? >Position? Groundspeed indication? Everything will drift in time (simply through the buildup of errors).Here's a shot of the Captain's PFD on 747-400 parked at the gate.... Some errors are bigger than others ;-)http://members.ozemail.com.au/~b744er/PFDGroundspeed.jpgCheers.Ian.

Okay guys, I've just read some right things but also lots of things quite wrong. An IRS comprises basically laser gyros and accelerometers.1) Laser Gyros. They are used to measure rotational acceleration around a center of rotation (by using the properties of the light having constant speed even if it's source is in movement and by measuring a phase shift in an amount of time to find that rotational acceleration). This integrated once gives you a rotational velocity, integrate it a second time and you'll get an angle of rotation. Assume you're on the runway, taking off. The IRS is aligned and everything, so his reference is let's say 0

Aha, this explains the sometimes erroneous GS readouts on the PSS Boeings and Airbus! ;-)Martin767 fetishistIt's a lot like life and that's what's appealing

Hi Andr

Mark,Drift doesn't depend on if the groundspeed is higher on one IRS than on others. It depends on how much different the computed groundspeed is different from the real groundspeed of the aircraft. E.g. if IRS 1 computes a GS slightly higher than the real GS, while IRS 2 & 3 compute a speed quite a lot slower than the real one, then IRS 2 & 3 will drift quicker than IRS 1. But you are right on saying that speed is the integer of acceleration and traveled distance is the integer of speed, both over time. Again, you're right when you say that the position computed by an IRS is the one you entered modified for all the accelerations integretaed twice over time that the accelerometers sensed. The IRS knows were it is when you give it the position, then updates that position by moving it according to the accelerations sensed and integrated into distances travelled...I've thought again about the groundspeed and what I said about it not drifting is actually incorrect. Since it is integrated over time, this means that is it the sum of an infinite small speed measurements, which all can be a little off, which means that the total sum can be off as well. However, as for the aircraft position, the amount of error doesn't necessarly increase, it can decrease, then increase in the opposite way, I think they found some kind of a slow sinuso

"I seem to recall Ian or someone else here saying that the IRS errors are logged after every flight to make sure they are within spec."That's correct. Each company will have a procedure for monitoring IRUs accuracy. However, the following is typical of such procedures.After engine shutdown and completion of the Parking Checklist, record the following information in the Airplane Flight log:A - The left, Center and Right IRS positions from the POS REF 2/2 or the drift rate for each IRS from the IRS MONITOR page.B - The residual ground speed for the Left, Center and Right IRSs from the POS REF 2/2 after a slash following the IRS Positions.C - The Block time. D - The Gate.Example:L N3356.3 W11824.1/02C N3356.9 W11823.8/12R N3356.6 W11824.6/0114:07C17orL 1.0nmC 0.9nmR 0.4nm14:07C17Best,Bruno Francescoli.

Hi Andr

>Okay guys, I've just read some right things but also lots of >things quite wrong.... True, but we're getting closer to the truth with each message :-) >I don't know for Boeing, but on Airbus and Avro-RJ the IVSI is >an IRS output through the Air Data Computers (or ADIRS for >the airbus).On Boeing, it's the other way around. The Boeing ADC's feed baro-derived V/S into the IRS's and the IRS's mix this with it's own vertical rate data, then send this off to the systems that require it (V/S indicators, etc). >3) As it has been described in another >reply, the gravity and earth rotational apparent forces are >used to determine where the true North is pointing to. But >it also uses the flux valve situated in some place of the >aircaft not submitted to electro-magnetic disturbances, >which senses the magnetic North.There are no IRS-based navigation systems with flux gates. These went out with the old INS systems. IRS's don't need fluxgates.>4) Drifting over time. The only error that can accumulate >with time is the aircraft's position derived by the IRS. GS >can be slightly wrong in reading, but it doesn't get worse >over time.Perhaps we have a different definition of drifting? According to my 747-400 training notes:"The FMC position reference (POS REF) page provides latitude, longitude and ground speed for each IRS and GPS. Note that when on the ground and parked, and with an IRU in NAV, ground speed is IRU drift rate (residual groundspeed).">The preference is given to VOR/DME stations.Unless GPS and LOC is available ;-) DME-only stations are quite acceptable also.Hope this helps.Cheers.Ian.

Ill give a quick example of how we check drift in the plane i fly.It will give you an idea of drift..We use the old fms800 system. It uses dual gps and tripple inu. Our inu system will not really annunciate if there is a drift problem so we enter correct lat long in the alignment and check them after each flight. But you can update them if they drift too far from the gps. During the before leaving aircraft check. The easy way for us is not to update. If no updates were done you can get the residual groundspeed by selecting inu1(pilot)/inu2(copilot) as the steering source and checking the gspeed in the ehsi.For inu3 we select captain on aux and select it for steering. From there we go to the designated pilots lateral steering page and make the nav solution the gps. then on the inav page select inu 1,2, n 3. We divide the diff distance by the flight time to get a drift rate...our tolerance is a max of 3nm per hour or a max residual of 15 knots..

>On Boeing, it's the other way around. The Boeing ADC's feed >baro-derived V/S into the IRS's and the IRS's mix this with >it's own vertical rate data, then send this off to the >systems that require it (V/S indicators, etc). Cool! Thanks for the info!>There are no IRS-based navigation systems with flux gates. >These went out with the old INS systems. IRS's don't need >fluxgates. Really? I don't really see why IRS wouldn't need it just because it's IRS and not INS...How can the aircraft's heading stay accurate over time without drifting (from the initially true north that was found) if it wasn't updated with a fluxvalve, like the way it is done for small aircraft ?>>4) Drifting over time. The only error that can accumulate >>with time is the aircraft's position derived by the IRS. GS >>can be slightly wrong in reading, but it doesn't get worse >>over time.>>Perhaps we have a different definition of drifting? >According to my 747-400 training notes: >>"The FMC position reference (POS REF) page provides >latitude, longitude and ground speed for each IRS and GPS. >Note that when on the ground and parked, and with an IRU in >NAV, ground speed is IRU drift rate (residual groundspeed)." That's right, I've already corrected myself in a previous reply...sorry for the misunderstanding.>>The preference is given to VOR/DME stations.>>Unless GPS and LOC is available ;-) DME-only stations are >quite acceptable also. Well this depends on the type of IRS probably since in the Avro-RJ preference is given to DME/DME fixes and VOR/DME over GPS. In fact, in the Avro-RJ at least the VHF stations tuning are separated and called VPU's. VPU's have priority over GPS unless it's quality is too bad (VOR/VOR only etc) And conerning LOC, I really don't see how this could work at 35.000 feet ? You can't receive the signal! I guess this is only used on approach ?And of course DME only stations are just the same as if there was a VOR with it, as long as we speek about DME/DME fixes...Regards,Andr

">There are no IRS-based navigation systems with flux gates. >These went out with the old INS systems. IRS's don't need >fluxgates. Really? I don't really see why IRS wouldn't need it just because it's IRS and not INS...How can the aircraft's heading stay accurate over time without drifting (from the initially true north that was found) if it wasn't updated with a fluxvalve, like the way it is done for small aircraft ?"Hi, Andr