GPS Based Navigation Accuracy Question
 

The Wide Area Augmentation System (WAAS) claims accuuracy of a few meters vertically and laterally. Are airspeed and aerodynamic/atmospheric factors known to alter GPS based navigation accuracy to a large degree?

Thanx.

:ok:

Greetings

No, airspeed and/or aerodynamic dont affect the WAAS, LAAS, DGPS, and GPS. :ok:

Yep, generally the facts.

Now, lets look at one specific GPS navigation unit installation, the King KLN-89B, installed in my private aeroplane.
Enroute navigatiuon accuracy, RNP 5.
Terminal, RNP 1.
Approach accuracy, +/- nine meters, laterally.

Now, this unit is eight years old, new(er) ones do better.
Quite a bit better.

Fact.

The only reason that the Europeans have not embraced GPS is the old familiar tune....'not invented here'.

So if an autopilot flight plan intends for a plane to be at a certain location vertically and laterally at a given moment during enroute flight somewhere within WAAS coverage, one can expect that their aircraft will acheive this placement within the stated WAAS accuracy?

Thanx.

:ok:

ionospheric refraction is the lagest contributing error factor to GPS - and augmentation, such as WAAS etc, exists to help correct this. For aviation use, enhanced integrity is just as important, and augmentation such as WAAS etc, exists to provide this.

Greetings
Lets not forget that GPS was created and deployed for and by the US military during the cold war mainly to guide US missiles over Moscow, all paid by tax money, is there any ROI or CBA when military is involved?
Whereas Galileo is a civilian project and unfortunately civilians have bad habits such as ROI and CBA.:}

No, that's not a reasonable expectation. The WAAS accuracy relates only to the accuracy of the position solution out of the WAAS box, known as Navigation System Error (NSE). You then have to add the Flight Technical Errors (FTE) which relate to the ability of the aircraft to follow the WAAS guidance. Generally for WAAS, FTE dominates NSE.

GPS accuracy is based on a number of factors not the least of which is the refraction of the radio propogation through the ionosphere.
Fortunately, the corrections supplied by the various services (WAAS and the like) do a pretty good job of taking out the effects partly because it is assumed that the ionosphere is largely homogenic and what affects the receiver will also be similar at the base stations that provide the reference signals for the correction.
Fortunately, the whole GNS system has had some remarkably quiet ionospheric conditions over the last couple of years to optimise the models and refine the positions of the Ground station network on which the whole thing depends.
When and if the sunspot activity increases again, the disturbance of the ionosphere will degrade the accuracy of the positioning calcs and you may well see some "outliers" that exceed the limits that the equipment specification provides.
Therefore, be aware that the current accuracy that we now enjoy, may not necessarily be provided in the future.

Actually, it was Mr. Reagan.
Mr. Clinton authorized dropping of the scrambled, extended accuracy portion of the signal. Especially useful if you are in an area that doesn't have access to WAAS and the like.

I believe the position error of say 5 meters X/Z and 9 meters Y stated above is a statistical model in which the position is calculated from the various solutions.

One way of looking at it in a stationary unit is like a shooting target. With a stationary shooter and one shot on the target, you can not estimate the centre, with 10 you can make a very rough guess, with a thousand you can be quite close, and with an infinite set of shots you can nail down the location of the centre of the target to the point where Mr Heisenberg gets involved.

Now, with GPS add to this that the satilites are orbiting, and the plane is flying and you get an idea of how difficult things can be and how your accuracy is always limited.

This gets at one reason that GPS units can measure the rise of a volcanic dome in milimeters, get only give it in meters on an airplane.

Now for the original question, GPS works based on the time stamps of the signal, based on an atomic clock on the satilite, sent by the satilite. If that signal for some reason took more or less time to reach the reciever then the accuracy would be altered.

If that was a constant situation then the actual result would be constantly wrong, if it was transient, then the situation would be transient.

As c is different in different mediums, I would not be surprised to learn that results varied based on the atmospheric density, but I would expect that to be in mm over a day and not more. Ionospheric refraction, as noted above, would cause the signal to take a longer route to the reciever and thus result in a differnet solution.

Hope that give some background. And to those who know more, sorry if I am a bit hazy about some parts of it, I sold them about 10 years ago for Garmin, but things have cahnged and I have forgotten much.

My understanding (and memory) is that Ronnie allowed restricted use of GPS to civilians, but Bill signed a Presidential decree allowing full usage to all and sundry (circa 1995?).

Do we really need WAAS any more? Surely with SA switched off you're getting enough accuracy to do a precision approach?

It's the integrity monitoring that's equally, if not more important, than the accuracy for precision approaches.

Timeline as I remember...
ok, ok... it's mostly pasted from Wikipedia:
# In 1978 the first experimental Block-I GPS satellite was launched.
# In 1983, after Soviet interceptor aircraft shot down the civilian airliner KAL 007 that strayed into restricted Soviet airspace due to navigational errors, killing all 269 people on board, U.S. President Ronald Reagan announced that the GPS would be made available for civilian uses once it was completed.
# By 1985, ten more experimental Block-I satellites had been launched to validate the concept.
# On February 14, 1989, the first modern Block-II satellite was launched.
# In 1991 Gulf War 1, soldiers were asking families to send them commercial handheld GPS receivers, since the Army wouldn't/couldn't give them to all who wanted.
# In 1992, the 2nd Space Wing, which originally managed the system, was de-activated and replaced by the 50th Space Wing.
# By December 1993 the GPS achieved initial operational capability.[71]
# By January 17, 1994 a complete constellation of 24 satellites was in orbit.
# Full Operational Capability was declared by NAVSTAR in April 1995.
# In 1996, recognizing the importance of GPS to civilian users as well as military users, U.S. President Bill Clinton issued a policy directive[72] declaring GPS to be a dual-use system and establishing an Interagency GPS Executive Board to manage it as a national asset.
# In 1997, my humble self bought his 1st receiver.
# In 1998, U.S. Vice President Al Gore announced plans to upgrade GPS with two new civilian signals for enhanced user accuracy and reliability, particularly with respect to aviation safety.
# On May 2, 2000 "Selective Availability" was discontinued as a result of the 1996 executive order, allowing users to receive a non-degraded signal globally.

Up until full elimination of scrambling on May 2, 2000, the
instantaneous horizontal plane position error was expected to be less than 100 meters 95% of the time. After that date, we're down to 20 meters. There are very few applications where this raw error cannot be reduced by some form of averaging.

GPS and ATC Altitude
 

GPS altitude is close to true. ATC at flight levels is based on barometric altimeters set to arbitrary (760 mm 29.92 in) reference pressure. This puts all aircraft on the same basis.

Thus GPS reported altitude should not be used to control FL, in the US or anywhere else.

There are very few applications where this raw error cannot be reduced by some form of averaging.

Considering that much surveying is now performed with DGPS installations ... with claimed accuracies down to a centimetre or so ... the system's capabilities are not to be sneezed at ...

Thank you John, and for all the folks who scoff at GPS, and it's accuracy...well, it is very very good, and with my private airplane, is superb, in every respect.
WAAS...or no.
All courtesy of the US taxpayer.

I would say, thanks are appreciated, and you all are welcome.
The USA leading the way...as usual, for precision navigation.
Not forgeting, of course, the Brits, who provided fully automatic landings, so long ago.
And RADAR.
And no, not forgetting the French, with the autoland capabilities on the Caravelle, with Lear-designed autopilots.
The latter lost in history, but 'tis a fact.

On the subject of GPS indicated true altitude: - for a given indicated flight level, can anyone tell me how this GPS indicated true altitude increases or decreases with change in latitude. :confused:

gps alt
 

Here is how I understand it.GPS sattelites orbit with reference to the center of the earth.that is at a fixed orbital distance.Thus they can pinpoint an a/c position quite accurately in space.But when it comes to the same a/c ht above the earths surface,its only as good as the model of the earth stored in its memory.That happens to be WGS84.Earth not being a perfect sphere does not help either.terestrial gps recievers as used in a car will have a local map of the city stored and position is super imposed on that map.
:)
hence the alttitude discripency.No relevance to change of lat/long

I take it that as latitude increases, the WGS84 ellipsoid model will cause a gradual increase in GPS indicated true altitude at a constant flight level, or have I got it the wrong way round ???

Greetings

GPS altitude is equal to true altitude +/- geodic correction, you can find some software computing the geodic for a given Lat Long.

And you can learn more than you actually want to know about WGS84 here.

Essentially, GPS altitudes are above a shape which is close to sea level, the errors from real sea level are small. Aircraft normally use pressure altitudes, and the daily variations from Interantional Standard Atmosphere (which a perfect altimeter would indicate) are considerable.

To avoid hitting a lump of cumulo granitus GPS altitude is more useful. To avoid hitting another airborne moving object pressure altitude is what you need.

We all know what Mr Clinton emraced and it wasn't always Hilary

Actually GPS height is equally good for terrain avoidance and aircraft separation - assuming everyone is working to a common system. It has in fact been mooted that aviation could, in time, move completely to using geometric height. Not only would this remove the complication of transition altitudes, but could save fuel by avoiding the invisible climbs and descents that are an inevitable consequence of flying through pressure systems. However many aspects of aircraft performance relate to pressure altitude. One could envisage the bizarre situation of aircraft having to request a descent simply as a result of flying into a region of low pressure.

Also, the algorithm used for the Z measurement (height) is different from that for X-Y (Lat-Long) because of the geometry of the system, and that it is optimised for land/sea use. The GPS altitude is subject to larger errors, and variation dependant on the satellite constellation being used.

I suspect we are stuck with pressure altitude as the standard for some years to come, in spite of the inherent inaccuracies.

No,the shape of the earth is known from WGS84, and the height of runway is known relaitive to this datum. Even without augmentation9e.g WAAS) GPS altitude is better than baro

Iforgot to say that vertical error is not much greater than horizontal, usually a factor around 1.5. It is simply geometry. In vertical , satellites are spread over 90 degrees, horizontal is 360degrees

Greetings

Vertical accuracy can be enhanced (for NPA, RNP...) by increasing the lateral accuracy (at that position, I should be at that height on that FPA) and through hybridation of GPS/accelerometers.
GPS is long term very precise, and IRS short term very precise, so we would have to verify if the acceleration measured by the IRS matches the change in position as given by the GPS.
:}

Ladies and gentlemen, we must not forget the effects of jamming on GPS. Because of the very low signal strength(transmitter is 10k miles away) GPS is dead easy to jam with amateur -made jammers--THIS MEANS SOME FORM OF BACK-UP

Any news on how LORAN-C is shaping as a back-up to GNSS?

Greetings

Ladies and gentlemen, we must not forget the effects of jamming on GPS. Because of the very low signal strength(transmitter is 10k miles away) GPS is dead easy to jam with amateur -made jammers--THIS MEANS SOME FORM OF BACK-UP

In fact it is a legacy from the cold war, to achieve the greatest precision the US located most of the GPS ephemeris over Moscow(In fact when we study cases of best GPS reception we call it the "Moscow case") So the Russians being very upset about the gps constellation invented hand held jammers, the story priced these jammers 150 US$ in the streets of Moscow.

The majority of the ephemeris may or may not be over Moscow, I don't know and I've not seen an authoritative cite that they were.

What I do know is that no US ICBM does or did use GPS for primary guidance. First of all, it is not needed. Inertial guidance with terminal phase radar guidance is extremely accurate. Secondly, as you said, GPS signals can be jammed with a USD 150 device and no ICBM would be risked by such a trivially defeated system.

Quote -

GPS guided weapons are provided with an integral multi-channel GPS receiver and Inertial Measurement Unit (IMU) which monitors the weapon's locations and attitude to adjust its flight path to accurately impact on the target. In low cost un-powered weapons, the guidance system adjusts the weapon's free fall to hit a pre-selected point fed into the weapon prior to takeoff. GPS is also used in guided missiles and cruise missiles, for mid-course navigation.

Further info from -
GPS/INS Precision Guidance System

Good news for once! More readable and up to date than the official sites:

International Loran Association

GPS Altitude
 

GPS Altitude
Gearpins is pretty close. GPS uses a spheroidal model of the earth (WGS 84) as a reference which is based on an equipotential surface worldwide. This averages all the peaks and troughs of the real earth's surface in terms of their gravitational effect. The altitude calculated by a GPS receiver will be the distance above this theoretical surface and may disagree with your real altitude above mean sea level. Neither can be equated to the reading on an altimeter (set to QNH) but it is probably close enough for most purposes. Just don't be surprised when you ditch in the Pacific and find your GPS reads -180ft even at high tide.
As far as GPS altitude variation with latitude is concerned, the satellites are not polar orbiting so there may be some reduction of accuracy (increased dilution of precision) in altitude near the poles. That is why you have triple IRS to iron out the wrinkles.
Hope that isn't too much information.
Cheers Chart8R

411A: I am a bit bemused by your statement:

On every aeroplane that I have flown for the last 10 years or so, and they have been both Airbus and Boeing, they have had GPS installed. I fly for a European airline, and I am European. How have we/ I not embraced it?

It is a wonderful tool, and the greatest single improvement to safety in my 34 year flying career.

No ICBM's in the USAF inventory use GPS for guidance, too easy to jam as posted above. Some aircraft dropped bombs/missiles use it for guidance though.

I am not a tin-foil hat wearing person but I believe the Euros really didn't like the fact that the USA could deny access to the system with the push of a button, one of the reasons for Galileo.

Greetings

I think we should stay on the main subject, and avoid unpleasant and useless remarks.:}
GPS was the key enabler for FANS, so whoever invented it doesnot matter, for the good of the entire industry we all have to be able to use it.:ok:

I'm not sure if where you are born affects the conclusion that the system is prone to interference in the EMI sense, perhaps the political control one.

It's an impressive implementation of a simple idea, but if you rely on it as the sole means of navigation and you'll get what you deserve one day! I think the civilian gps is flawed in security terms due to lack of signing of the data, but i welcome corrections on that. I imagine truly malicious practical exploits would be difficult. I've just read there are some simple software based approaches to anti-spoofing - it would be interesting if these criteria are required for aviation.

More so than ACAS/TCAS?