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Old 27th May 2012, 17:01   #21 (permalink)
 
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It will still work and eventually get a position fix. Basic GPS does work, but of course it is slower, especially if it didn't get a fix in the approximate area recently. One thing of note is that the GPS in iPhones is part of the cellphone chipset and therefore doesn't work in flight mode. It might be similar for other cell phones as well, but I don't know that.
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Old 27th May 2012, 18:16   #22 (permalink)
 
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That is correct, the AC must be WAAS enabled. WAAS never gained any traction in the US, and airlines never saw the benefit to spend the money to enable it.

You also have to remember that WAAS is an augmentation, rebroadcast of the correction factor, that a WAAS enabled receiver can use.
GBAS is far more useful as an augmentation, and can also broadcast the Approach for the AC to use.

While the cell phone may be able to receive WAAS, as most GPS antennae can, very few, if any can use it.
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Old 27th May 2012, 21:08   #23 (permalink)
 
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Correct me if I'm wrong - from the reading I have done WAAS is a requirement in the States for LPV approaches, isn't it? Never mind that the larger airports still have ILS the FAA recognized the potential to establish instrument procedures at a lot of underdeveloped airports without costly ground based aids, something that is completely unheard of in Europe

Regardless, GBAS is currently only part of the GLS implementation (where at least in the 737 you would even manually select the GBAS station on the MMR) at this stage, enroute ANPs of 0.05 even without WAAS are usually not a problem anyway.

Last edited by STBYRUD; 27th May 2012 at 21:10.
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Old 28th May 2012, 18:51   #24 (permalink)
 
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FAA WAAS LPV procedures can be used with a certified WAAS GPS system only.

You cannot upgrade your existing system system to WAAS capability. Current systems are certified under TSO C129, a completely different criteria.
TSO C145 and 146 certifies the unit as a standalone receiver.

The antenna’s are different from the TSO 129 box to what’s certified on a 145/146.
Installation is currently being done by STC and requires:
  • dual WAAS approved GPS receivers,
  • other equipment mods, such as the scaling and autopilot,
  • annunciation, whether it’s external or on an EFIS system,
  • and a flight test procedure are all required.
WAAS-capable avionics do not automatically mean that you can fly to an LPV minimum. To accomplish the LPV minimums, you need dual WAAS receivers, and the ac must be certified under TSO 145/146.

This may help What is LPV.

Last edited by FlightPathOBN; 28th May 2012 at 18:55.
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Old 29th May 2012, 02:55   #25 (permalink)
 
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Quote:
Installation is currently being done by STC and requires:
dual WAAS approved GPS receivers
I'm pretty sure you don't need dual WAAS receivers to do LPV. In Canada we pretty much copy word for word the US regulations and I can guarantee that you don't need dual WAAS receivers to do a LPV approach in Canada. We now have approval to go down to 200' with LPV in Canada (according to TP 308 Change 5.3). We currently don't have approaches published down to 200' but I'm sure that will be coming soon - all with one receiver.

It's possible that the dual WAAS receiver requirement is for commercial operations?
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Old 29th May 2012, 16:49   #26 (permalink)
 
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italia,

I only work with commercial ac, so I am not aware of the requirements for private ac.

Given the technical and other requirements, IFR with WAAS seems to be quite an adventure for a private ac. Just as the RAIM predictions must be checked, so must the WAAS predictions, and the procedure must be in your nav database to select. (looks like Canada gets a buy on this one!)
FAA Real time plots

LPV 200 requires a level of VAL and VPL accuracy of less than 35m. While the predicted accuracy of the stations is around 12 to 15m, the real time accuracy of the receiver on the ac is almost always more than 35m.
There is also a significant difference between the potential and actual WAAS service.
Autoland requires maximums of 12m, so the VAL of 35m, with the inability of WAAS to guarantee actual vertical error performance, autocoupling is restricted to altitudes above 200 feet in the LPV 200 rules.

Other notes:

I am curious in reading through the thread, about the RAIM requirement or indication on the ac. The pilot is required to verify RAIM prior to departure, at least at midpoint of en-route, and before approach before considering the procedure, or which procedure to use.
I am wondering what is checked or how this is accomplished?

Last edited by FlightPathOBN; 29th May 2012 at 17:14.
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Old 29th May 2012, 17:15   #27 (permalink)
 
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Hi FlightPathOBN,
Quote:
I am wondering what is checked or how this is accomplished?
We have a company link to "AUGUR GPS RAIM Prediction Tool - Terminal/Approach Tool" which allows you to specify up to 10 airports, and the result gives the predicted RAIM unavailable / available in a simple colour coded bar chart for 3 days (-12 hours to +60 hours) with mask angle 5.00 Algorithm, Fault Detection only (FD), Mode Approach (my selection), Active NANUS.
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Old 29th May 2012, 18:20   #28 (permalink)
 
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rudder,

Yes, that is a great tool for dispatch pre-flight check requirements.

I was referring to in-flight, when the pilot is required to verify real time RAIM.
Depending on the model, variant of the FMS, different things can happen. On the MCDU PROG page, you can see the predictive GPS req'd, estimated, and accur values. Checking the 'estimated' value enroute will let one know what may be expected on approach. If the value is not enough the unit will have issues going to approach mode.

The different boxes have different features with a hybridized HIL that estimates when GPS integrity falls below certain limits, creating an 'equivalent' HIL, but for short amounts of time, which differs depending on equipment.

What I was also looking at was some of the rest of the posts that were talking about "PRIMARY LOST" or "GPS UNAVAIL", if the pilot was looking at the page for the explanation.

umm... I will leave this up if it helps, but I just realized the issue, there are 2 very similar threads on this in Tech Log, and I was responding to the wrong thread..sorry about that.

Last edited by FlightPathOBN; 29th May 2012 at 19:20.
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Old 30th May 2012, 03:34   #29 (permalink)
 
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FlightPath... I might state some things you already know, sorry!

Quote:
I am curious in reading through the thread, about the RAIM requirement or indication on the ac. The pilot is required to verify RAIM prior to departure, at least at midpoint of en-route, and before approach before considering the procedure, or which procedure to use.
I am wondering what is checked or how this is accomplished?
For C129 there is a requirement for the pilot to check approach level RAIM (0.3NM) before commencing an approach. There should be an approach level RAIM prediction feature in the GPS unit itself. Under the TSO, the GPS unit will not go into the ACTV (Active) mode unless there is approach level RAIM available from the FAF to the MAP - RAIM is auto calculated at 2.0NM from the FAF. So it basically won't work if you don't have approach level RAIM at the FAF and MAP but you're still required to check approach level RAIM before commencing an approach.

If you're C145/146 you don't have a requirement to check approach level RAIM, as long as you have WAAS available at the time. If there are no WAAS NOTAMs and you currently have WAAS while in the air, you don't need to check approach level RAIM before commencing an approach.

RRR... I have used that AUGUR tool a number of times but I just noticed that it will be restricted to ECAC airspace starting 1 July 2012.

FlightPath...

Quote:
The different boxes have different features with a hybridized HIL that estimates when GPS integrity falls below certain limits, creating an 'equivalent' HIL, but for short amounts of time, which differs depending on equipment.
From what I've seen, Horizontal Figure of Merit (HFOM) and Horizontal Integrity Limit (HIL) both related to the Total System Error (as calculated by RAIM - RAIM calculates Navigation System Error and Flight Technical Error; assuming that the Path Definition Error is negligible, which it generally is, the NSE and FTE add up to the TSE) for the aircraft. Garmin seems to use HFOM for the 95% requirement and from what I've seen HIL relates to the 99.999% requirement.

PDE - Desired path vs defined path
FTE - Estimated position vs defined path
NSE - True position vs estimated position
TSE - True position vs desired path

During all phases of flight, the HFOM must be equal to or less than the level required for the airspace you're flying in. If it's enroute airspace (2.0NM) then as long as the HFOM is equal to or less than 2.0NM, you will "have RAIM". If your HFOM is 1.5NM and you enter Terminal airspace (1.0NM) you will receive a RAIM alert. The GPS will still provide a position solution but you have to discontinue IFR use. But HFOM is not the same as checking approach level RAIM. HFOM is current and RAIM prediction is used for the future. Also, the RAIM prediction is for approach level RAIM so you might find that when you're enroute or in terminal airspace, if you calculated RAIM for your current position, it might say that you don't have RAIM but your GPS wouldn't alert you because you still have either enroute or terminal level RAIM.

The airspace designated as RNAV 1.0 is virtually the same as airspace designated as RNP 1.0. The difference is that for RNP airspace, the equipment must have on-board performance monitoring and alerting. The whole concept behind RAIM satisfies this RNP requirement and that's why you'll see GPS being used to help satisfy the requirements for RNP airspace. However, there are different types of RNP and certification is somewhat complicated so you can't just fly a RNP procedure because you've got approach level RAIM on your GPS. The number is the accuracy required and means that the TSE must not exceed 1.0NM in this case, 95% of the time. The containment limit is 2x the RNAV or RNP - 2.0NM in this case and the TSE must not exceed the 2.0NM limit 99.999% of the time. ICAO Doc 9613 - PBN Manual (Implementing RNAV and RNP) provides some good information on this.

There are so many requirements and they differ (especially with commercial ops) that the best way to figure this out is to read the manual for the system in question. For example, here is a manual I found online for Universal Avionics - HORIZONTAL INTEGRITY LIMIT (HIL) for Universal FMS Training Manual

Quote:
What I was also looking at was some of the rest of the posts that were talking about "PRIMARY LOST" or "GPS UNAVAIL", if the pilot was looking at the page for the explanation.
All those messages will be explained in the GPS manual for the GPS system in use.
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Old 31st May 2012, 16:16   #30 (permalink)
 
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italia,

This is a great discussion, I am hoping we have not strayed too deep, or too far from the original question.

My original thought, after reading through the posts, was the confusion about RAIM, RAIM prediction, and what it means.

Yes, the unit will do an automatic RAIM check at 2nm from the FAF, to let you go to approach mode, in reality, the unit is checking RAIM all of the time. The check at 2nm is to verify that there is sufficient sat coverage, and HIL (or HPL), required for approach. The approach requirements vary, RNP level, APV, LPV, for example, have different required accuracy limits for these modes.

The tools mentioned, such as AUGER, are pre-flight checks, and conditions can change during the actual flight, but what really matters is what the ac has real-time.
You mentioned NOTAMS, the WAAS system is CONUS only, so the WAAS NOTAM system is not supported outside that area, and I dont believe NavCanada has a similar system.

The RAIM level relates to the number of sats the unit can see. From before, you need at least 4 sats for 3d flight, 5 sats for Fault Detection, and 6 sats for Fault detection and exclusion. RAIM prediction tools will simply show you the predicted coverage, ie number of sats, based on a 5 minute outage.

I would have to disagree on some points, RAIM is simply a part of the system, and provides error checking and error correction of the sat signals. It does nothing else but monitor the integrity of the sat signals, and produces the HIL. HIL and HPL are the same term.
The HIL is generated by the RAIM system, creating a sphere to estimate to true vs calc position of the ac. The size of the sphere in m is the HIL. One alarm is when HIL>HAL, and HAL varies between units and modes.

The alarm timeframes differ as well. When GPS integrity is lost, the unit will create an artificial or equivalent HIL based on internal IRS drift models.

Under ACCUR, you see the required value and the estimated value, or EPE. This is not HIL, but EPE uses HFOM, latency of the FMC, ground speed, and bleed error. HFOM is a function of the IRU.


TSE is a much different animal...

HCE is the horizontal coupling error (along track error on descent)
FTE is the flight technical error (perf of system to control ac on vertical path)
ASE is the altimetry system error.

RNP, well...that is for another day....

Last edited by FlightPathOBN; 31st May 2012 at 18:16.
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Old 3rd Jun 2012, 07:16   #31 (permalink)
 
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FlightPath... Sorry for the late reply, I'm taking a course this weekend so I don't have too much spare time! After writing this I'm still not satisfied with my understanding of the subject - I really wish there was a resource that would explain all of this in detail. I still have many questions myself. Hopefully what I say below doesn't confuse people.

Quote:
You mentioned NOTAMS, the WAAS system is CONUS only, so the WAAS NOTAM system is not supported outside that area, and I dont believe NavCanada has a similar system.
They do have a system and it's described in the Transport Canada AIM - COM 3.15.6.2.

Quote:
The RAIM level relates to the number of sats the unit can see. From before, you need at least 4 sats for 3d flight, 5 sats for Fault Detection, and 6 sats for Fault detection and exclusion. RAIM prediction tools will simply show you the predicted coverage, ie number of sats, based on a 5 minute outage.
RAIM is not only related to the number of satellites but also their geometry relative to the receiver, ephemeris data, almanac data, etc. If you're picking up satellites from around the same place in the celestial sphere then you'll have poor geometry and your HIL/HPL will possibly exceed the alert limit. The RAIM prediction tools on a PC will show you if RAIM is available in the area of interest and will depend on what level of HIL/HPL you need. It's the number of satellites in view and their geometry at that time. With PC software you can plug in the satellites you know will be down for a specified duration (by looking at the KGPS NOTAM file) and get a more accurate RAIM prediction since the onboard RAIM prediction only uses current info broadcast by the satellites.

Quote:
I would have to disagree on some points, RAIM is simply a part of the system, and provides error checking and error correction of the sat signals. It does nothing else but monitor the integrity of the sat signals, and produces the HIL.
Yes, it's a integrity monitoring system (error checking) but I don't agree that it provides error correction - differential corrections would do that, ie: WAAS and LAAS.

Quote:
HIL and HPL are the same term.
It appears that way but here is an interesting quote from FAA AC 20-138C: "GPS-based sensors output an HPL (see RTCA/DO-229D for definition) that is sometimes referred to as horizontal integrity limit (HIL). The HPL is a measure of the position estimation error assuming a latent failure is present. In lieu of a detailed analysis of the effects of latent failures on the total system error, an acceptable means of compliance for GPS-based systems is to ensure the HPL remains less than twice the RNP value, minus the FTE 95%, (i.e. HPL < ((2*RNP) - FTE 95%)) during the RNP AR approach operation."

Quote:
The HIL is generated by the RAIM system, creating a sphere to estimate to true vs calc position of the ac. The size of the sphere in m is the HIL. One alarm is when HIL>HAL, and HAL varies between units and modes.
Agreed. The size of the HIL/HPL is a measure of accuracy which is compared to the 1xRNAV area (HAL). Accuracy is defined as the "measure of position error, which is the difference between the estimated and the actual position."

I should clarify that RAIM calculates the Navigation System Error and won't give you a TSE.

Quote:
The alarm timeframes differ as well. When GPS integrity is lost, the unit will create an artificial or equivalent HIL based on internal IRS drift models.
Integrity and accuracy isn't really 'lost', it's just degraded. You don't need any other info to plug in to get an HIL. My understanding is that you'll get a HIL all the time, as long as you have RAIM. It's only when it gets to the HAL that you get an alert but it still keeps calculating the HIL based on the number of satellites, geometry, ephemeris/almanac data, etc.


As far as I know they've stopped using EPE for aviation and now there is a more strict definition called Estimated Position Uncertainty. EPE isn't always a 95% probability or confidence level. This is a quote from Garmin:

"The accuracy of the aircraft’s GPS fix is calculated using Estimated Position Uncertainty (EPU), Dilution of Precision (DOP), and horizontal and vertical figures of merit (HFOM and VFOM). EPU is the radius of a circle centered on an estimated horizontal position in which actual position has 95% probability of laying. EPU is a statistical error indication and not an actual error measurement.

DOP measures satellite geometry quality (i.e., number of satellites received and where they are relative to each other) on a range from 0.0 to 9.9, with lower numbers denoting better accuracy. HFOM and VFOM, measures of horizontal and vertical position uncertainty, are the current 95% confidence horizontal and vertical accuracy values reported by the GPS receiver."


It seems that HFOM relates to the geometry of the satellites - this is a quote from IATA: "GNSS receivers also output a parameter termed Horizontal Figure of Merit (HFOM), which is the expected accuracy of the position data assuming that all satellites in view are operating correctly. Note that HFOM does not protect from satellite ranging errors."

Your TSE picture is different from the one I was talking about in my post - https://www.box.com/s/c2bff47a372d7cb20ce4

Here are some good references:

http://www.gps.gov/technical/ps/2008...e-standard.pdf
NPS GPS Support Facility - What is EPE?

Last edited by italia458; 3rd Jun 2012 at 07:48.
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Old 3rd Jun 2012, 20:56   #32 (permalink)
 
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Italia,

Again, I dont know about non-commercial units, nor the Garmin.
Most of the data that I have provided is either from the Honeywell or Smiths manuals.
As for coverage in Canada, I dont know if Canada has full implementation of WAAS yet, or even how many WAAS procedures there are out there.

As for the augmentation, this is very important to remember, it is simply a correction factor for the particular sat, defining it for the ac receiver to use. It is not a rebroadcast of a GPS signal, or a stand alone navigation signal, it is simply a correction factor.
The sat is happy to orbit broadcasting its signal. The WAAS computers are looking at each sat, the signal, position, geodetic position, atmospheric conditions, etc, and broadcasting this correction factor through the WAAS sat, for your receiver to use.


Perhaps this will help on RAIM. Receiver Autonomous Integrity Monitoring - Wikipedia, the free encyclopedia

Note that online RAIM prediction tools are dispatch level tools, just to give you an idea of the potential coverage. As I stated before, there are many GPS only arrival/departure procedures, that if the RAIM prediction is below a certain level, the ac cannot depart.

RAIM is an internal function of the GPS receiver on the ac or ABAS, meaning it is an aircraft based augmentation system, WAAS is sat based augmentation, and GBAS is ground based. These are an augmentation that further define the internal algorithms for higher accuracy values. This is why you must have a specific WAAS or GBAS capability. The ac is still required to correct and balance the GPS for nav, even without external augmentation.


How each system uses data, couples with other data such as the IRU, is specific to the manufacturer.
One has to remember that these are real time measurements, with the unit having different latency values, depending on the unit and equipage. In flight the unit is polling all of the other units and returning the estimated position. While this happens very quickly, the ac is also moving very quickly. While a latency value of 0.74 seconds to poll appears a very short time, the ac has gone how far in that amount of time? When you look at accuracy down to 10 m, this is quite an accomplishment.

When one views the estimated position vs required, the estimated position is the EPE as currently defined in the criteria. While the terminology may be evolving or be proprietary, it is understood what the foundation of the calculations is based on.

It appears that your Garmin is saying the same as I stated for EPE. In your example, that "The accuracy of the aircraft’s GPS fix is calculated using Estimated Position Uncertainty (EPU), Dilution of Precision (DOP), and horizontal and vertical figures of merit (HFOM and VFOM). EPU is a statistical error indication and not an actual error measurement.
Note Accuracy, ie EPE is calc'd USING....it also states EPU is NOT the actual error measurement.

We are talking about the same value for HFOM and VFOM, these are values that talk into account the IRU.

The example that I provided for TSE was for vertical navigation position, your example is for horizontal navigation.

HAL is certainly a function of the unit, options, and coupled equipment. In my example, the units will go to a hybrid HIL, depending on configuration, and provide an 'equivalent' HIL. Depending on the IRU updating capability, the timeframe on on the equivalent will vary.
While the HAL is based on 2 times the selected level, how the units derive this estimate varies. In my example, the Smiths box on the 737-700 went to HAL at 0.51, with .3RNP selected, not 0.60.

Here is something else that we must consider in GPS navigation, the latency to config between levels and/or phases. The ac must calc and configure all of the variables prior to use. This becomes an issues sometimes, especially when going from RNP 1.0 to RNP .3 and especially RNP .1. If the procedure at waypoint A is RNP .1, then the ac must be configured prior to crossing waypoint A. All systems must be on board, and this takes some time.

When the unit is doing a check 2nm from FAF, that is a final check, and everything is configured for the final segment, prior to the FAF, not beginning at the FAF. Depending on the change in RNP or required levels, the unit must config further back than 2nm...

Last edited by FlightPathOBN; 3rd Jun 2012 at 21:01.
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Old 4th Jun 2012, 14:23   #33 (permalink)
 
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A lot of the confusion in discussions such as this is the understanding of how each aircraft is configured.

In most airline original fit such a 'glass Boeings and Airbus' the GPS is just a sensor input to the FMS. It is the FMS that does the navigating not the GPS. As such it is the FMS accuracy and performance using all available sensor imports that is being measured and displayed to the pilot. The GPS derived position is added to the mix IRS, DME/DME, VOR/DME etc at a differing level depending the aircraft FMS design.

Another misunderstanding is that EPE, ANP etc is an error calculation. The question to ask would be if it knows the error why can it not correct the position! So all that the units can do is predict how accurate, based on a number of technical measurements and some statistical calculations the FMC is able to have "confidence" of it's Nav capability.

In GA world it is the GPS that is the heart of the navigation, so the performance of the system is totally dependent on the health of the GNSS system.
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Old 6th Jun 2012, 04:20   #34 (permalink)
 
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Quote:
Originally Posted by chuzwuza
Airbourn gps aims to acquire 4 satellites to triangulate lat, long positions and also altitude. Ground based gps such as cell phones obviously dont require altitude hence will acquire 3 satelites.
Ground based receivers do require 4 satellites to compute a position. In order to compute a position with only 3 satellites, the receiver elevation must be known to the receiver. Being "ground based" is not the same as at a known elevation. A "ground based" receiver can have an elevation anywhere from -400m to +8800m. That's not close enough to calculate an accurate horizontal position. Now, in situations as on a boat on the ocean, where you know that your elevation will be sea level (or close enough, anyway) that information can be entered into the receiver, and it is then able to compute a position based on only 3 satellites. Now, it may be that there is some trickery going on in a cell phone that allows the gps receiver within to know it's approximate elevation based on the network cell it's in (I'm locked to cell XX56Mx, therefore, my elevation is approximately 85 m above sea level) IN that case a receiver could determine a position using only 3 satellites. But it is not generally true that a "ground based" receiver only needs 3 satellites for a position.
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Old 6th Jun 2012, 20:13   #35 (permalink)
 
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The units still base the elevation from the WGS84 ellipsoid. Different units do this differently.

For aircraft, the altitude is still based from the WGS84 ellipsoid, and again, depending on the box, calc it differently. Smiths uses a grid over the ellipsoid with a lookup function. Honeywell interprets the ellipsoid.
Because the shape of the Earth is not symmetrical, the ellipsoid is an approximation, and the altitude generated by the box, needs to be further corrected.

That is why, when you look at the diagram of the ASBL, it looks so odd.

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Old 9th Jun 2012, 17:38   #36 (permalink)
 
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FlightPath...

I don't think we understand each other. I completely understand everything you've said about RAIM, WAAS, how GPS works, etc.

What I meant when I said I didn't understand this stuff is below.

Quote:
It appears that your Garmin is saying the same as I stated for EPE. In your example, that "The accuracy of the aircraft’s GPS fix is calculated using Estimated Position Uncertainty (EPU), Dilution of Precision (DOP), and horizontal and vertical figures of merit (HFOM and VFOM). EPU is a statistical error indication and not an actual error measurement.
Note Accuracy, ie EPE is calc'd USING....it also states EPU is NOT the actual error measurement.
The quote says that the accuracy is calculated using EPU and HFOM... so EPU and HFOM shouldn't be the same, correct? Well take a look at this: http://adsb.tc.faa.gov/WG6_Meetings/...20briefing.PDF

It says, "HFOM is also known as EPU". This is what I mean when I say I don't understand this stuff! I'm getting conflicting information. I'm hoping you can see what I mean here!

According to this (Aero 12 - Required Navigation Performance), EPU is the same as ANP.

Here is a definition of EPU: http://www.honeywell.com/sites/servl...B-7FF07F662003

Another definition of EPU: http://www.mitre.org/work/tech_paper...93/09_4093.pdf

OK, disregarding what I wrote above, this is the way I see it now: HFOM is basically the calculated accuracy of the GPS position with a 95% confidence level in the calculation. DOP takes into account satellite ranging errors and geometry. EPU is the 95% confidence that you lie within a circle, with a radius in NM equal to the EPU, centered on the estimated GPS position. So, take that estimated GPS position (one pin-point position) and apply the 95% confidence in the accuracy (HFOM) and then take into account the geometry and satellite ranging errors (DOP) and after all that you will get the EPU. There are more components involved but right now that's the way I see those terms related to each other.

But nothing has shown me this relationship clearly so I don't know if my view is correct. It's not a matter of understanding something complicated here, it has to do with getting clear and accurate definitions and some explanation that ties them all together! This is not rocket science.

EDIT: Read Note 3 under 3.2.11 -- http://adsb.tc.faa.gov/WG6_Meetings/..._MASPS--V2.pdf

Read slide 17 and 20 -- http://rms.ion.org/wp-content/upload...g-20091210.pdf

Last edited by italia458; 9th Jun 2012 at 17:57.
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Old 9th Jun 2012, 20:22   #37 (permalink)
 
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Italia,

Okay, perhaps this may help. The unit or system, is giving you a position based on many factors. Many of these terms relate to sets, subsets, version, and couplings.
My explanation was given for a commercial application, that basically has every single couple. By couples I mean, multiple IRU's, antennae, and capability.
As another poster already mentioned, the Garmin is stand alone, and provides the navigation for the aircraft. On a commercial aircraft, the FMS does, coupled with many other instruments.

When the unit is coupled with the IRU, the HFOM used by the FMS comes from that couple, combined with GPS/MMR/etc.

I assume that your Garmin is a stand alone unit, therefore there are no couples that become part of the calculations. Different manufacturers look at the calculations differently, and make certain assumptions, based on capability.

Quote:
Read slide 17 and 20
Note on 18 that RAIM algorithm depends on the manufacturer...

This is exactly why ADSB is in its current 'state' of development.
They have spent hundreds of millions to study and define ADSB, but it doesnt match, or needs to be translated from the terminmology and calculations used on the ac.
ADSB out!!, these guys crack me up!
What I find humourous, that in the same document, they mention rollout beginning in 2008, most in 2010, and the rest by 2013...then later in the document, they say that FAA will define the strategy for ADSB by 2012, with full implementation by 2020! (This doc was from 2009) Now the deadline is to first tier ADSB by 2020, with ADSB out by 2030....
Put this right along side of the microwave landing systems, byt the time it gets sorted out, the technology is hopelessly outdated and wont be used...

plus, dont confuse the ADSB broadcast terminology with any known reference to the real world...

Last edited by FlightPathOBN; 9th Jun 2012 at 20:29.
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Old 9th Jun 2012, 21:37   #38 (permalink)
 
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FlightPath...

Quote:
The unit or system, is giving you a position based on many factors. Many of these terms relate to sets, subsets, version, and couplings.
I understand all that. I know you're trying to help me but you haven't clarified how all the terms relate to each other and what the differences actually are between them. That's all I'm looking for here.
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Old 9th Jun 2012, 23:19   #39 (permalink)
 
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How about this, there is the result, and the path to get there.

Just as RAIM is a defined term, RAIM calculations are different per manufacturer.

I deal with Smiths, Honeywell, and Thales all the time, and the calculation methods, and in many times, the effect to the flight controls, are all different.

The manufacturers have proprietary calcs, means and methods, and I am sure there are patented processes that the others need to avoid.

In the end, the process of your unit meets a certain spec, and has a certain path to get there.

The end justifies the means.

Sorry, but in aviation, terminology is but a snapshot in time, and/or purposely vague...

Last edited by FlightPathOBN; 9th Jun 2012 at 23:25.
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Old 10th Jun 2012, 10:54   #40 (permalink)
 
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Quote:
Quote:
It appears that your Garmin is saying the same as I stated for EPE. In your example, that "The accuracy of the aircraft’s GPS fix is calculated using Estimated Position Uncertainty (EPU), Dilution of Precision (DOP), and horizontal and vertical figures of merit (HFOM and VFOM). EPU is a statistical error indication and not an actual error measurement.
Note Accuracy, ie EPE is calc'd USING....it also states EPU is NOT the actual error measurement.
The quote says that the accuracy is calculated using EPU and HFOM... so EPU and HFOM shouldn't be the same, correct? Well take a look at this: http://adsb.tc.faa.gov/WG6_Meetings/...20briefing.PDF
HFOM, Horizontal Figure of Merit, is for GPS the 95% horizontal position accuracy bound, also known as EPU (Estimated Position Uncertainty).

For GPS, HFOM is calculated by:

HFOM = 2 * UERE * HDOP

With
UERE = User Equivalent Range Error (1 sigma value)
HDOP = Horizontal Dillution of Precision, a factor depending on satellite geometry.
The 2 is in there because, in a normal distribution, 2 sigma deviation from the mean covers 95%.

The UERE is an estimation of the Pseudo Ranging accuracy, i.e. an estimation of how accurate the distance between the GPS receiver and a satellite can be measured.
It is a combination of signal in space errors (e.g. satellite clock noise, ionospheric/tropospheric delay, ephemeris data error) and user equipment errors (e.g. user clock noise, multipath, radio interference). UERE is given as the (estimated) standard deviation of the range error.

Until the year 2000 the UERE was dominated by satellite clock noise that was added by the US military on purpose to the civil GPS signal to lower the accuracy in order to prevent enemy troops to use GPS to their advantage. This is called Selective Availability (SA).
In the year 2000 SA was disabled permanently so from then on the Pseudo Range errors was much smaller.
Old generation GPS receivers have the assumption that SA is turned on still hard coded into their system. Therefore these receivers largely over estimate the UERE and as a result the HFOM that they report is nowhere near an accurate estimation of their position uncertainty. Newer GPS receivers are aware of the SA state so their HFOM is, under the same circumstances, much smaller. But even the HFOM of the SA aware receivers is very conservative.

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