GPS use for NAV when coded not in WGS84

[Boroda]

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

[galaxy flyer]

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

[Boroda]

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?

[rudderrudderrat]

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.

[Boroda]

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.

[galaxy flyer]

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

[rudderrudderrat]

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.

[Boroda]

RF:
no use petionining to state

RRR:
good advice, thanks.

[galaxy flyer]

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

[Boroda]

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.

[777AV8R]

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.

[galaxy flyer]

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

[PantLoad]

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

[galaxy flyer]

PantLoad

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

GF

[manuel ortiz]

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

WGS-84 Updates - Jeppesen (http://74.125.47.132/search?q=cache:Ji5fYhtHwLEJ:www.jeppesen.com/company/publications/wgs-84.jsp+wgs+84+compliant+states&cd=1&hl=es&ct=clnk&gl=gt)

[Boroda]

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:

[Denti]

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.

[PantLoad]

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

Thanks,


PantLoad

[ahramin]

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

[galaxy flyer]

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

GF

[galaxy flyer]

Doing some late night research on Wiki I found this:

GLONASS uses a coordinate datum named "PZ-90" (Earth Parameters 1990 - Parametry Zemli 1990), in which the precise location of the North Pole is given as an average of its position from 1900 to 1905. This is in contrast to the GPS's coordinate datum, WGS 84, which uses the location of the North Pole in 1984. As of September 17, 2007 the PZ-90 datum has been updated to differ from WGS 84 by less than 40 cm (16 in) in any given direction.

I could not find any primary reference to quote, but it makes me wonder why in hell the Russians don't just use WGS-84. In any case, WGS-84 is the standard, by ICAO agreement.

GF

[777AV8R]

Any chance they have this set up for 'their' tactical system instead?

[galaxy flyer]

Probably, but I don't see the point. Everywhere in the RF and China has been extensively mapped and put into WGS-84 by us, so they aren't hiding anything, anymore. If they want to use different datums, just have the equipment set to change datums. We could do it in the A-10 in 1986, it's nothing dramatic.

GF

[rudderrudderrat]

but it makes me wonder why in hell the Russians don't just use WGS-84.

Probably for similar reasons we have Microsoft & Apple, (or had Betamax and VHS). Why is a system based on where the Poles where in 1900 or 1984 any better than a modern system?

Europe is building its own satellite navigation system called Galileo. It will consist of 30 satellites and the first should be launched in 2010, with the system operational at the end of 2013. Total cost about 4.9 bn EUR.

From NewScientist 20May 2009 : "
DON'T take your satnav for granted. Existing satellites are ageing, and replacements are behind schedule and over budget, according to a report from the US Government Accountability Office (GAO).

Satnavs and other GPS devices calculate their position by comparing time signals from at least four satellites. To keep that many within range at all times requires a fleet of at least 24. For now there are 31 operating, but 13 of them are more than four years past their design lifetime.

The first replacement "block IIF" satellites are not due to launch till November, three years behind schedule, and the GAO predicts a 20 per cent chance that the fleet will drop below 24 at times in 2011 and 2012. That wouldn't cause GPS to shut down, but its accuracy would drop unpredictably.

Plans by the US air force for the next generation of improved "block IIIA" satellites could also fall behind. The GAO calculates that if they slip by just two years, there is a 90 per cent chance that the fleet will drop below 24 in 2018."

[Thridle Op Des]

LIDO Route Manual RAR Appendix G

RUSSIA
is using “The Parameters of the Earth - 1990 (PZ-90) which is practically identical to WGS-84.

Regards

TOD

[galaxy flyer]

TOD

For those of us without LIDO, can you post the complete quote or a link?

This could generate a big change in aircraft limitations.

GF

[Boroda]

GF,
why do we have 2 or even more, for example, hydraulic system in aircraft? and so on

[galaxy flyer]

Because an hydraulic system can fail and we need a back-up. Once the Earth is surveyed, how would the coordinate system fail? The satellites might not be there or fail, but lat/longs are just there.

Perhaps, the RF should just roll into the ICAO and use WGS-84, there is no advantage to holding onto a datum that is just a duplicate of it.

BTW, I quite agree that the three systems are needed and should seamlessly integrate because GPS was never intended to be used as it is now being used. And that is one of the reasons for objections to it.

GF

[rudderrudderrat]

Perhaps, the RF should just roll into the ICAO and use WGS-84, there is no advantage to holding onto a datum that is just a duplicate of it.

Like temperature in degs C and F, or pressure in mbs and inches of Hg, or mass in Kgs and Lbs.

I'm really looking forward to the day when there is just one agreed datum - but I won't be holding my breath.

[galaxy flyer]

Rudderrat

Technically, the ICAO is standardized on Hpa and Mbs is a UK exception. And so is inches in the US, an exception. I still call it millibars, though. Kgs is a unit of mass, while Lbs is a unit of Force. 1 kg would be the same here or on the moon, while a pound here is 1/6th of a pound on the moon.

GF

[john_tullamarine]

.. some of us still think in slugs and poundals, not to mention forces due to the flickering of a newt's whiskers, ergs, and so forth.

If disciplines ever end up with uniform units there will be no remaining mysteries nor any useful things to pontificate over a beer at the bar on Saturday night for old chaps.

Younger chaps can always find merit in the appearance of the young lady at the other end of the bar, of course ...

[OzExpat]

While I can't be sure about the Russian Federation standard datum, I know that the Papua New Guinea WGS-84 survey was locally referred to as PNG-94 (the year in which the survey was initiated). Perhaps the RF did something similar for local consumption while, at the same time, complying with the ICAO standard of WGS-84.

[Thridle Op Des]

Hi GF, as requested the entire page with the reference

http://i197.photobucket.com/albums/aa266/thridleopdes/LIDORAR.jpg

[rudderrudderrat]

Hi GF,

1 kg would be the same here or on the moon, while a pound here is 1/6th of a pound on the moon.

A mass of one pound will still have the same mass wherever it is (earth, moon, in orbit in the space station etc.) However it will only weigh 1/6 of it's weight on the moon as compared to earth, (Likewise 1 kg will only weigh 1/6 on the moon). Both will be weightless in the space station.

[Boroda]

GF,
The satellites might not be there or fail, but lat/longs are just there.

We will roll in but Is WGS84 ICAO standart or recommedation? I think the last one.

I asked about hydraulic meaning not system redundancy. It is something more important, something with world monopoly. When monopolist after some time starts thinking that his democracy is the most democratic in the world, his english is englisher than in England he tend to think that oil in another country is their oil because this part of the world is just coordinated in WGS84.

Don't take these words seriously - we are not soviet militarists anymore:), I just want to show that alternatives are very important sometimes despite he fact their are stupid.

[galaxy flyer]

Thanks very much, TOD

Boroda

But, WGS-84 is not a monopoly, it is not even a tradable good, it just IS. ICAO cannot dictate anything, but as a Standard and Recommended Practice, the ICAO said aviation will use WGS-84. Hell, we could use Argentine 1925, just as long as everyone's equipment is compatible and we are all using the same thing.

Considering PZ-90 has been modified to nearly conform with WGS-84, the only alternative we are talking about here is a name. Let's just call it "Fred's Lat/Long System".

GF

[777AV8R]

Galaxy...have you got some kind of 'backroom' deal going with Fred. I know that Dave has a good system, too!:)

[galaxy flyer]

777AV8R

I picked FRED, well my handle should explain it all. Check your PMs, I sent one.

GF

[manuel ortiz]

Borda,

ICAO specifies the World Geodetic System 1984 (WGS-84) - or equivalent - as the geodetic reference datum Standard for air navigation latitude/longitude coordinates

[rsiano]

Hi GF,
What is the source for suggesting NAD-83 is the same a WGS-84?
Thanks!
Dick Siano

[FE Hoppy]

Ejet procedures for non WGS84 or NAD83 is to deselect GPS position determination below transition alt.

[galaxy flyer]

Rsiano

I can't find the definitive reference for that comment--the nearest, I got was a two meter bias between the two datums. But, I'll check the Jepps, nixt time I'm in the office. That comment might also only applicable in the US.

GF