GPS use for NAV when coded not in WGS84
 

A320 Limitations FCOM 3.01.22

GPS can be used only when difference with coded in another geodesic system flight plan is negligible. One of the methods to asses this shift is "on ground perfoming a GPS survey of the procedure waypoints".

How do you understand this survey?

Spasibo

I'll take a stab at it--generally GPS position cannot be used in non-WGS-84 locations for terminal or approach applications. There are too many differences between WGS-84 and other datums to be safe. The ground-based navaids and ground features will not be where the FMS, used GPS positional information will not be where they are are. In the early days of GPS ops, there were numerous cases of airliners nearly hitting terrain due to these positional differences, esp. in South America. The Russian Federation is very definitely non-WGS-84, which is a problem with the future use of GLONASS.

Now, EGPWS data bases are compliant with WGS-84, so EGPWS can be used regardless of local datum.

How an operator would survey for these differences is beyond my knowledge. I suspect it would require a FMS system that would allow for the entry of different datums. This is feasible, as Collins did it for FMS-800 used by the US military. We had about 40 different datums and could program the FMS to use any of them. No civil system is equipped this way to my knowledge; it would create the possibility for all kinds of errors.

GF

We fly throughout the Russia despite another own geosystem, no problem actually has occured untill now. But the fact is people don't even evaluate possibility of GPS using, they got used to using it always, everywhere. So how do we have to asses, survey accuracy on ground before take off?

Hi,

If you aligned the FMGCs & IRS on stand using GPS, and then compared the received VOR Radial & DME distance with the "PROG" page calculation to the same VOR, it would show the MAP shift error to the VOR before you start. If possible I would suggest you did this to two VORs at 90 degs relative to each other from your stand / taxi position.

Thanks for the direct answer to the question. One more do I have following the suggested check:
- what if there are no navaid nearby - it is Russia; moreover FANS concept has the aim to fly without ground navaid structure - so there will no VORs in your side as well - I mean what if no aid to check accuracy. Moreover I strongly recommend you not to use navaids reading as a last source in my country.

rudderrat

Finding a map shift "bias" as you described is true, but that "bias" may not always be the same throughout the datum area, hence applying that "bias" to all nav solutions is not safe or possible.

The answer is, I submit, to use the FMS with WGS-84 datum in the enroute structure, but cannot be used in terminal/approach structure. This is how to deal with different datums. Or use DME/DME RNAV, if the 'Bus is capable of it. Or, as I mentioned earlier, have the appropriate datum entered into the FMS. If you are not in WGS-84 airspace, you must use the ground-based navaids. All GPS positioning is based on WGS-84, so the plane will be calculating its position in reference to WGS-84 and assume lat/long positions are WGS-84. Where a different datum is used the plane will not be where it FMS believes it to be. Very unsafe.

Perhaps a petition to the Russian Directorate of Civil Aviation would help overcome their reluctance to survey Russia to WGS-84 datum.

GF

Hi

GF, My suggestion was just to enable an estimate the error difference between GPS WGS - 84 & the Russian co-ordinates for the departure way points - not to apply any bias.

Boroda, If there are no ground aids, then all I can suggest is to check the FMGC tracks and distances to your departure way points, and compare them with the published figures. When you are at the holding point, check the track and distance to your first way point (against the published data), and if it all agrees to within the accepted tolerance (e.g. <1 nm for PRNAV) then the two co-ordinate systems are close enough.

RF:
no use petionining to state

RRR:
good advice, thanks.

And I will reiterate, GPS cannot be used for navigation where positional accuracy is critical (terminal and approach phases)in a non-WGS-84 environment. In those flight phases, ground-based navaids must be used, i.e. DME/DME or VOR/DME RNAV or raw data unless using the correct datum.

There have been some real close calls with map shifts in the early days of GPS and WGS-84 involving terrain. Which is why ICAO made WGS-84 the international standard, it wasn't without reason. I have seen some colossal track and distance errors between what was "out the window" and what the EGPWS, using GPS-derived position, said.


GF

GF, may be you are right logically, but a320 limitations do allow such operation, we just have to check the difference is negligeble. Though no precise figures. To win the market they have to trade safity for.

Agreed...
 

Yes, Galaxy...totally agree. The GPS needs to be turned off. I operated out of Shanghai-Pudong with the 777. The airport is not WGS84 compliant. If we used the GPSs on departure, when we selected the TOGA buttons for takeoff, we would receive a master caution and the FMC Runway Disagree EICAS message would appear. There was also a very large map-shift. Disabling the GPS and letting the inertial look after the system kept this message from coming back and there was never any more map shift errors.
The majority of the approach from the east was on vectors anyways and there was never any non-precision approaches in there that we required our GPS to be operational.

777AV8R

Thanks!

Boroda

I just looked up the Airbus 320 limitations on Smart Cockpit. It looks just like my plane--if GPS (FMS) in non-WGS-84 airspace, is used, the raw data, ground-based navaid must displayed or GPS must be deselected, in other words, ground-based navaids are necessary. That limitation applies for in-flight purposes, the other choice is to survey the approach fixes, that is, take a GPS receiver out to the physical location of each fix and cross-check the GPS-derived position against the charted position. Not very practical. RNAV (GPS) or RNP approaches require using WGS-84, no exceptions.

GF

Correct....
 

I can't remember exactly, but the limitation applies to takeoff....where the local airport is not WGS84 standard and the difference is not negligible, the GPS must be deselected.....and cannot be re-selected until after takeoff and a safe altitude is reached. (However that's defined....)

The issue, in my mind at least, is what defines 'negligible'. Unless your airline specifically states the difference is definitely negligible, then I'd say you are mandated to follow the limitation.

What airports are not WGS84? Don't know of any personally.....


Fly safe,

PantLoad

PantLoad

How about all of the airports in the Russian Federation and in China? Everyone of them.

GF

Here's a list of Countries and its WGS84 status.

WGS-84 Updates - Jeppesen

GF,
that is, take a GPS receiver out to the physical location of each fix and cross-check the GPS-derived position against the charted position. Not very practical.

It's not very practical for you in USA, but for me it's everyday routine:ok:

Ask your navigation map provider if he can provide maps that have been recalculated into the WGS 84 format even if the state provided format differs. Recalculation is possible and in fact had to be done by most countries when they changed to WGS 84.

Galaxy Flyer
 

Sorry, don't know about this....never been there.

Thanks,


PantLoad

The Jeppesen list says WGS-84 or equivalent. Is NAD-83 equivalent? What other datums are equivalent and what is the difference then?

NAD-83 is the only equivalent to WGS-84.

GF