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.
They do have a system and it's described in the Transport Canada AIM - COM 3.15.6.2.
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.
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.
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."
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.
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?