reply to post by shmo5
NO! the airplanes DID NOT have GPS capability.
shmo5....do you own a car-based navigation system?
If you do, then this is where you have a problem, in understanding how airplane navigation systems worked, before GPS was incorporated.
See, in 2001....the FAA had not approved the GPS Updating for commercial airliners, yet.
The tech existed, but wasn't approved....although business jets had it.
The early days of the platforms, regarding what was then called 'INS', for 'Inertial Refercence System' involved actual, physical
gyroscopes....three, in three different axial directions, along with electronics that would measure the precession of each gyro, as it was moved in
direction, thus imputing the direction of the airplane. Based on the 'initialization' of the system, telling it, by Pilot inputs, the Latitude and
Longitude, the system could then calculate, via the gyroscopes, where it was...up to a point.
Problem is, there is this nagging problem called 'friction'...and, also, 'precession'.
So the early INS's had errors, that could accumulate. That is why for overwater Operations THREE were required. They could check each other, for
errors....and if one went foul, it would get 'thrown out', and pilots notified.
Then, we had the 'IRS's....'Inertial Reference Systems'
These used 'laser gyros'. So, instead of mechanically spinning gyros, they used light instead....far more accurate....PLUS now they incorporated
something caled 'Radio Updating'....into the software. This means, you still initialize the IRS at your gate, on the ground....(and it remembers
where it was last shut down, so you can't fool it, or it will ALERT!)
BUT, now it receives other inputs, when within VHF range of a VOR or DME, to help it verify, or even update its position (where it thinks it
is)...(again, 3 of them, to check each other, and throw out one that goes wrong, then notifying the Flight Crew)
NOW, most US jets also have GPS updating.....far, far more accurate.
Same triple redundancy still applies....We can access the positions, of each IRS, on a screen....the three displayed, along with the 'computed'
position, as a compilation of all three, if they are still being accessed....that is what is called the 'FMS' position.
BUT, if on a 'precision' approach, we still use standard VHF and UHF radio signals. There is a process in the works for more 'GPS' appraoches,
but progress is slow. Safety is paramount.
I've given a lot of detail, but only to summarize.
The onboard computers had a lot of 'waypoints' in the database....think of the GPS in your car, for instance. It is not difficult to access the
database, and display it in a way that any pilot, even with minimal skills, could navigate to. Once HE or SHE knew how to access the data.
Then, turn and steer, follow the purple (in Boeings) path, as displayed on the screen......(or white, until you press the 'EXEC' button.....)