Inertial Navigation - How Accurate/How to Reset/Update?

Factual Questions
#1

I’ve never had any exposure to INS, but am aware that they are/were standard on large aircraft.

I take they “drift” with time/distance, so the question becomes:

I land/dock/splash down at a point for which I have exact Lat/Long coordinates.

Is there a way to enter the coordinates manually? A button to press if you agree with the numbers the GPS/Loran/Other is showing?
Do the airports/docks have magic boxes to provide the updates?

If I fly from Boeing Field to Sydney, can I use the Sydney coordinates to update the INS?

Or, for that matter: has INS gone the way of Loran?

Yes, I am referring to DPRK’s playing with a GPS ‘jammer’ (or at least what they think is a GPS jammer and nobody is telling them about the frequency changes) :wink:

#2

I was an aircraft mechanic on USAF F-111s in the mid-70s. The INS could be very accurate often showing the aircraft within a few feet of it’s parking spot after flying a 3 hour mission somethings at over Mach 1 speeds. This was with old analog computers.

On the other hand the INS would fail and show the aircraft in Canada when it was actually in New Mexico. The Canada thing must have been some kind of default as it happened frequently.

#3

Was there a way to update its ‘current location’ and get it to believe you instead of its internal calculations?

I’m guessing the Air Force was a bit touchy about the navigation of its toys.

#4

Short version:
In general the INS* is cold-started before each flight and told the local position as part of the start-up process.

By the end of a flight it will have drifted some. So it’s shut off while the airplane is parked between flights.

And then reinitialized with a fresh cold-start and a fresh position as part of the prep for the next flight.

The local position used to initialize the system comes from either pre-surveyed paper charts, tables, electronic equivalents or a GPS receiver.

As to accuracy …

After several hours of flight the IRS (see below) will generally be drifting at a rate of a knot or so. So left to its own devices a non-redundant system would be a couple to a few miles off reality after a long flight. By having three independent gyro “platforms”, odds are they’re each drifting in a different direction and the average position of the three is much closer to the real position than is any individual gyro’s.

  • The term “INS” is obsolete, at least in aerial nav. It refers to a complete *navigation *system using solely inertial sources.

In modern parlance we have an “IRS” = Inertial Reference System which is one of several inputs into a combined navigation system.

The combined navigation system takes IRS and other inputs to derive a position and velocity vector. And merges that with other nav-relevant data such as locations of airports, pre-surveyed fixes, planned routes, etc. to provide a complete solution to the navigation problem. Which problem is defined as “Where am I? Where am I going? How do I get there?”

I don’t have time for more now, but if you want more, ask away and I’ll get to it sometime today.

#5

I can speak to INS’s in civilian aircraft and I assume they work similar in other platforms (military may be significantly different–I have no idea).

The INS is initialized on the ground and during that process the most accurate location coordinates you have are entered. Nowadays this is the GPS position, however it could be coordinates of

  1. The Gate (lat/longs of gates at major airports are printed on charts)
  2. The Airport (Not as accurate, but can be updated later)
  3. The last known INS position if the aircraft has not been moved (least accurate for obvious reasons)

If the position entered varies significantly from the last known position the system will question you to verify the entries. Some aircraft have the lat/longs of the departure runways in the data base and can be updated with the push of a button when the aircraft pulls on the runway.

These are a good starting position, but as you are aware INS can drift significantly. All the new systems incorporate GPS and ground based navaid inflight updating. It will use GPS if at all possible, if it is not or the system detects a degraded or incorrect position it will automatically disregard that input. At that point it has a priority system of ground based navaids within rage and computes position off of DME/DME, VOR/DME, or VOR/VOR triangulation. Once out of range of ground based systems and GPS is not available it then reverts to full INS nav until one of the updating systems becomes available again.

Although most aircraft have a dedicated INS entry keypad and (limited) display, almost all entries are done through the FMS’s and displayed on a moving map. You can bring up the nav page on the FMS and see what source of data is being used for the current position and the computed position of the other sources. Operation is mostly transparent although some irregularities such as degraded gps signals will generate a message to the crew.