Navaids

[Boslandew]

I've been out of the loop for some years now. On long-haul over-water flights is the main navaid still inertial or has GPS now officially taken over?

Thanks in advance

[autoflight]

Older A320s that I previously flew had tripple Inertial Reference System (IRS) but the newer A321 had that plus dual GPS. The GPS automatically updated the IRS position, unlike the older system that relied on a passing DME for updating. Single DME updating was used on the very much older INS often installed in B707, B727 and probably earlier B747 and original B52 etc.
I believe the modern civil standards will be similar to the A321 system described.

[Gas Bags]

The newer systems still use inertial as primary. As autoflight stated the newer inertial navigational system will position update via GPS, as well as DME inputs, when available. This corrects for position error throughout the flight.

The IRS will also continually monitor its position error rate (which should not be more than around 1-2 nm per hour) and on landing calculate a summed error rate over the flight and correct itself to counter this error rate. This happens on a continual basis and will take into account the wear rate of all components in the system, to allow a 'component lifetime' of accurate use within error rate limits.

The older INS system (obsolete now except for older models still in operation) utilized a gimbal stabilized platform to navigate. The latest IRS units are laser ring gyro's (strapdown) and the very latest utilize fibre optic cable in place of the laser ring system for even greater accuracy, which I believe can be as low as 0.5 nm per hour which is a perfectly acceptable position error rate in any accurate navigation system.

I am not sure if GPS will take over as the primary navigation reference system as it is dependent on signals external to the aircraft for reference. The inertial systems are entirely self contained and extremely accurate.

[172_driver]

The newer systems still use inertial as primary. As autoflight stated the newer inertial navigational system will position update via GPS, as well as DME inputs, when available. This corrects for position error throughout the flight.

Isn't that saying that GPS is indeed primary for meter-precision at north 60?
Not sure about other planes, but in the new Boeings the FMC calculates the most probable position and is more or less identical to what the GPSes say. The IRS position quickly strays as a function of time.

I am quite sure you can get unlimited MNPS approval with dual GNSS of correct type & installation. Smaller airliners and bizjets are normally not equipped with inertial navigation.

[VinRouge]

We are allowed to dispatch transatlantic if all 4 (2 mil, 2 civvie) of our gps's are unserviceable based on radio update on our 4 remaining ring laser gyros....

Talk about redundancy!

[c100driver]

To be technically correct, the GPS does not update the IRS. The IRS provides attitude information to the flight instrumentation and position information to the FMC.

That is, the IRS is a sensor input to the FMC, the GPS is also a sensor input to the FMC. It is the FMC that navigates the aircraft.

[nitpicker330]

"Small Airliners and biz jets are not equipped with Inertial...."

Huh??? All new build Biz jets would be made with inertial and GPS and probably a full FMS too. A lot come with the Garmin G1000 or similar now..

[172_driver]

You or me must have gotten the terms mixed up then..

My understanding of inertial navigation is measuring accelerations in different directions and calculate current position from a known starting point, i.e. INS/IRS.

[Boslandew]

Many thanks for your comments and information. I had assumed that GPS was now king and the standard longhaul navaid. From your comments, it would seem that the standard is INS/IRS updated by GPS (and/or DME)

However, it would also seem that both INS/IRS and twin GPSS are sufficient on their own. From your further comments, it would appear that aircraft may legally fly long-haul over water equipped with either of the two but ideally with both, one updating the other.

Thank you again for the information

[nitpicker330]

172.....I'm not confused and stand by my comment that most new build biz jets and small airliners come fitted with an inertial Nav system...:ok:

Maybe I'm wrong then??

Certainly some have a basic AHRS system fitted if they don't have an INS/IRS.

Even a PA28 comes with a AHRS fitted now.

[172_driver]

Have indeed flown lots of AHRS.. in my.. 172 ;)

Wouldn't call it inertial navigation.. but don't really care as long as I know what it can do.. and can't

But then I have seen that pilots and engineers usually argue over what's an INS and what's an IRS, too :ok:

[FE Hoppy]

The moment GPS updates an IRS position it is no longer an independent nav sensor!

The FMS manages the aircraft position based on all available nav sensors with weighting for the most accurate.

The FMS position will therefore be a blended position based on either GPS or IRS or an other depending on where you are and what's available.

Some FMS monitor the IRS error from to GPS position and apply this correction to the raw IRS position if GPS is lost. Thus the FMS position is a corrected IRS position but the IRS position remains uncorrected.

[nitpicker330]

I never said the AHRS was an Inertial system.