GPS Question..Please help
During my ATPL viva i was asked this question
Whats the difference between the aircraft GPS reciever and the Gps in the cellphone Why cant we use Cellphone for gps in flight ? i was confused :\ i answered about the integrity of the GPS with the FMGS they were not looking for that but were looking for a technical answer .. please shed some light and help me find a tactical answer |
Airbourn gps aims to acquire 4 satelites to triangulate lat, long positions and also altitude. Ground based gps such as cell phones obviously dont require altitude hence will acquire 3 satelites.
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found it .................Assisted GPS is the answer
Thanks chuzwuza for taking time to answer my question.:ok::):) |
Airbourne GPS must have a RAIM function too.
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Dunno, does it have to? At least there is no way to check RAIM status on the 737NG, there is simply no RAIM indication. It might still use it internally, or not, who knows, after GPS is not necessary to fly GPS approaches in that type ;)
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GPS is certainly required for RNAP AR in the NG. And, you very well may not pass muster for "regular" RNAV IAPs without GPS.
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I thought the Indian DGCA had started to grow up.. And refrain from asking silly questions?
Anyway, most probably the answer he was getting at is that a cellular telephone device *can* use assisted GPS where it triangulates it's position from the cell towers. This is used to speed up the acquisition of positional information. Most cell phone handsets will try to acquire as many GPS satellites as possible, so the answer about it not computing a 3D position is incorrect. To be honest the question is stupid, as there is no right answer unless he specifies a make, model and version number of a particular handset. Even then there can be significant differences, as India found out to its detriment with the Blackberry, you can buy a version with security services standard end to end encryption. The GOI were far from happy about that! |
@aterpster, i wasn't talking about RNP AR, just GPS/GNSS approaches. And yes, we do not need any GPS for those, nor for any RNAV approaches with more than RNP 0.10. You are correct however, for any RNP AR with RNP 0.1 or lower we do need GPS, if the RNP is higher than that and if DME/DME coverage is sufficient we don't need it again. Quite academic though as i haven't seen any complete GPS outage yet and since the NG does not have any RAIM requirement nor indication we do not need to care about that either.
RNP AR is pretty uncommon still in most parts of europe. Within our route network we have only one airport that has an RNP AR (soon two, we fly both currently, the second one as trial), and that is innsbruck. However, i have to admit that our route network is pretty small with only around 170 destinations. @PT6A, assisted GPS does not only speed up position updating by triangulating via cell towers, most assisted GPS installations can do a far more first position update by checking for known WLAN installations, which helps a lot especially in cities. And they do download the current almanach data via the cellular data network which helps the GPS receiver to get a faster fix on satellites. |
Denti,
Afirm on what you said..Also depending on the specific device, will depend if the device needs the network for the GPS to work. Some devices outsource their processing to the network, while others retain the ability to function as a standalone GPS when not connected or outside of the network coverage. |
Denti:
for any RNP AR with RNP 0.1 or lower we do need GPS, if the RNP is higher than that and if DME/DME coverage is sufficient we don't need it again. In addition, in the U.S. for RNAV IAPs DME/DME is specifically prohibited on the chart. DME/DME is permitted on some SIDs and STARs, where the DME geometry has passed flight inspection. |
Why cant we use Cellphone for gps in flight ? Sorry could not resist... It's as good an answer as any to a vague "trick" question. BTW: One other advantage cell phone GPS has is that the cellpone already has a fairly precise time of day - this also makes satellite aquisition faster. |
Whats the difference between the aircraft GPS reciever and the Gps in the cellphone |
@aterpster, i wasn't talking about RNP AR, just GPS/GNSS approaches. And yes, we do not need any GPS for those, nor for any RNAV approaches with more than RNP 0.10. You are correct however, for any RNP AR with RNP 0.1 or lower we do need GPS, if the RNP is higher than that and if DME/DME coverage is sufficient we don't need it again. Quite academic though as i haven't seen any complete GPS outage yet and since the NG does not have any RAIM requirement nor indication we do not need to care about that either. RNP AR starts at RNP .3and requires operator specific certification to go below that to a minima of .11 for the B737CL and .10 for the A320. Dual GPS and Dual FMC mandatory for RNP AR RNAV (GNSS): Ends at RNP .3 and can be Single GPS and Single FMC minimum with alternate ground navigation capability available for missed approach. With a Dual GPS/FMC ground aid not required. RNAV: DME/DME availability required to be demonstrated as able to supply an acceptable geometric update for FMC A stand alone aircraft GPS unit does need RAIM however with a GPS as a "sensor input" to an FMC equipped aircraft the FMC will calculate an ANP (Boeing) or EPE (Airbus) which will indicate the confidence in the calculation of a position. |
Wow, now I have a headache this early on this one!
but here goes, RAIM is built into the GPS system on the aircraft, and is part of what is balancing all the the different sat connections and detecting faults the pseudorange measurements. When you look at RAIM predictions,they are configured with 3 modes, Fault Detection, FD(5 sats req'd), Fault detection and exclusion, FDE (6 sats req'd), and if Selective Avail, SA is turned on. For 3D navigation, you need a min of 4 sats, but will have no RAIM fault detection. The RAIM function that you will see in the box is represented with the HIL number. Depending on your box, and RNP level set, the box will alarm when the HIL number reaches a certain threshold, the HAL, telling you that you horizontal integrity is getting near the limit Horizontal Alarm Limit of the RNP level. The HIL, HAL, and HPL numbers are some voodo combination of the secret sauce inside each box, and given the potential combinations of algorithms and factors, I have no idea how alarm limits are decided. As an example, during flight validation testing, with the Smiths box, RNP level 0.3, I had the box alarm at HIL of .51 in a 737, while in a 320,with a Honeywell box on RNP 0.1 it alarmed at .67.... Hope that helps! RAIM prediction tools |
Whats the difference between the aircraft GPS reciever and the Gps in the cellphone Why cant we use Cellphone for gps in flight ? Airbourn gps aims to acquire 4 satelites to triangulate lat, long positions and also altitude. |
Triangulate is not correct. GPS uses trilateration Trilateration is a fallback when only 3 satellites are visible and, as a previous poster pointed out, won't give you an altitude. |
Difference between GPS etc
Does airborne GPS not also use WAAS (Wide Area Augmentation System) to achieve greater accuracy for ILS procedures ?
This would not be a feature of cellphone GPS use. Interestingly, Garmin uses this (WAAS) extensively on their products including the small Etrex Vista unit that has served me well in gliding for many years. |
Not all aviation GPS units are WAAS capable. Airlines don't typically have WAAS, while it's commonplace in the GA / flight training world.
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Actually, cell phones do use WAAS if they receive the signal (depending on cell phone model of course), however it is not a big marketing thing as there is no real additional use for groundbased ops as there is in airborne application. The only thing is an increased accuracy.
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GPS on phones and iPads and things.
The poblem is, as I see it, is: Despite the accuracy questions or whether it has RAIMS capability or Vertical Nav (Elevation) or not - the main probs are whetther or not you are receiving a phone network coverage signal = for, if there is no signal due to altitutde, coverage or whatever, then you won`t get your GPS working, or am I wrong?
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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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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. |
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 :D
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. |
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:
This may help What is LPV. |
Installation is currently being done by STC and requires: dual WAAS approved GPS receivers It's possible that the dual WAAS receiver requirement is for commercial operations? |
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 http://operationsbasednavigation.com...8311627288.png 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? |
Hi FlightPathOBN,
I am wondering what is checked or how this is accomplished? |
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. :ugh: |
FlightPath... I might state some things you already know, sorry!
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? 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... 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. 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 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. |
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. http://operationsbasednavigation.com...2/05/SC-04.jpg 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.... |
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.
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. I should clarify that RAIM calculates the Navigation System Error and won't give you a TSE. 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. 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? |
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... |
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. |
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.
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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. |
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. 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. 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 |
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. Read slide 17 and 20 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... |
FlightPath...
The unit or system, is giving you a position based on many factors. Many of these terms relate to sets, subsets, version, and couplings. |
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... |
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. 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. ATCast |
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