Why are coordinates marked at parking bays at civil airports?


These checkpoint coordinates are used to verify the current location of the aircraft for navigation purposes. The coordinates can be used to check, input or calibrate the navigation system's current location. It can be used to input or calibrate the aircraft's Inertial Reference System (IRS) during initial alignment by providing a precise Latitude/Longitude location the pilot can input into the system when the aircraft is powered up.

The IRS uses 3 laser gyros to sense the forces created when the aircraft moves from this starting position, updating the IRS's position accordingly.

As you'd expect, due to drift, the IRS position will not remain accurate after crossing the Pacific Ocean. Once within range of ground-based navigation facilities, the aircraft's location is updated by the Flight Management System ("FMS" - the airplane's navigation computer) utilizing data from these facilities, such as VORs (many of which provide distance information via DME) which the computer uses to triangulate a position.

Information from ground-based navigation facilities is not available until airborne, so you need the Lat/Long gate location for INS initial alignment at the gate (especially if you were flying in 1990 or are currently flying an aircraft from the 1980s or early 1990s that is not yet updated with GPS capabilities).

The Flight Management System (FMS) navigation database of a modern airplane will typically contain the Lat/Long coordinates for various gate locations at major airports.

GPS (Global Positioning System) satellite position information can also be used by the FMS to update the aircraft's position.

On modern aircraft GPS provides an accurate Latitude Longitude position to your navigation system no matter where you are (including sitting at the gate). While these Lat/Longs at the gate are likely a bit of a holdover from the 1980s/1990s when you needed a Lat/Long position for your INS and could not get it from GPS, these coordinates at the gate are still valuable, giving you the ability to check the accuracy of your indicated GPS location or verify the gate position shown in the FMS navigation database (in addition to giving your IRS a current location for initial alignment).

These gate Lat/Long coordinates are also indicated on some Jeppesen airport diagram charts which show zoomed in gate details.

I just did a search to see if someone already has explained the IRS system here, and found an extremely thorough and detailed description. Check it out: Why are there two IRS switches on the 737NG?

  • 2
    $\begingroup$ Mentour Pilot covered IRS (from 07 min 44 secs) and the other kinds of navigation in great detail in How do pilots actually navigate the skies?. The approx. 10 minutes required for alignment of IRS after aircraft startup at the airport is explained from 14 min 25 secs. It didn't include anything about the signs (except perhaps hinted by "input position"). $\endgroup$ Oct 21 '21 at 14:13
  • $\begingroup$ You should also mention that DME/DME systems may not be able to provide a fix to the INS until airborne, so they need a precise fix at startup time. Even GPS systems may need several minutes, depending on how long they were shut down (or if they don’t cache ephemera). $\endgroup$
    – StephenS
    Oct 21 '21 at 15:13
  • $\begingroup$ Doesn't the FMS know the position of each gate already? $\endgroup$
    – mins
    Oct 21 '21 at 17:28
  • $\begingroup$ Even with BRNAV (Like INS only with RNP 5) you can do a usual RNAV SID and STAR if it does not require PRNAV or RNP1 or equivalent. $\endgroup$
    – Vladimir F
    Oct 21 '21 at 18:15
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    $\begingroup$ @mins If you have a FMS with a database of them, then sure. If you don't, you can use the sign. $\endgroup$
    – Vladimir F
    Oct 21 '21 at 18:16

If you are referring to GPS coordinates shown at the gates, as in this image:

Image showing two airliner parking spots ("95B" and "94") at Calgary International Airport, Alberta, Canada. On the spot signs are GPS coordinates.
Source: ADB Safegate

the reason is: to allow the pilots to properly prime their inertial navigation system with a known location before the flight begins.

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    $\begingroup$ With that much precision given, I am curious if that is the latlong position of the sign, or of the approximate position a cockpit would be when stopped at that stand. When the position is that of the sign, are there procedures to convert that to the cockpit position? $\endgroup$
    – ontrack
    Oct 21 '21 at 13:13
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    $\begingroup$ Looking at google maps it's probably the cockpit position $\endgroup$
    – Ian
    Oct 21 '21 at 16:02
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    $\begingroup$ @Ian I suspect there'd be some variation in that depending on the model of the plane, but it'd probably need a few more decimal places to be accurate enough for that to matter? $\endgroup$ Oct 21 '21 at 17:13
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    $\begingroup$ Those are not "GPS coordinates", they were used there well before GPS was invented. The OP's term "coordinates" is entirely sufficient. $\endgroup$
    – Vladimir F
    Oct 21 '21 at 18:08
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    $\begingroup$ @mins There are aircraft without GPS. Even with FMS but without GPS (but becoming rarer with every year). The INS needs a starting point and then tracks the movement. The drift can be corrected using DME triangulation, but for the starting point the coordinates written there are much more accurate than DME. The FMS certainly can have the posittion in the database. Or it might not. $\endgroup$
    – Vladimir F
    Oct 21 '21 at 18:17

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