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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: NTP, possible solutions, and best implementation
On Fri, 03 Oct 2003 09:59:59 -0700 Owen DeLong <[email protected]> wrote: > I used to work with GPS navigation / calibration. The entire system is designed to "free wheel" for at least a month, and probably many months, giving adequate performance even if all the ground control stations were destroyed. The only thing I would worry about (besides failures of my own equipment) would be that roof access might be blocked (say if debris fell on the roof), and thus the signal could not be acquired, for some period of time. Selective availability (SA, the jittering of the clocks on the public signal) introduced timing errors only at the level of 100 nanoseconds. If you need timing better than that, you should worry (a little) about having a backup time source, in case SA gets turned back on in a dire national emergency. Regards Marshall Eubanks > >> Derating GPS wouldn't affect the time reference functionality. Turning > >> off GPS entirely would seriously affect military aviation operations. > > > > Not so: > > > > "Selective Availability (SA) is the deliberate introduction of error by > > either altering the precise timekeeping of GPS satellites or the position > > of the satellites in space, through the on-board software, thereby > > reducing both positioning and timing accuracy for civilian users." > > > > GPS accuracy is generally reduced by adding noise to the timing. Now you > > would have to derate GPS pretty significantly before timing accuracy would > > be significantly affected. But it's possible that some time references > > would refuse to lock on at all with sufficient derating. The affects of > > more extreme derating than SA are, at least to some extent, unknown. > > > While this is true, the derating in common practice for SA when it was > turned > on actually turned out to be somewhat less inaccurate than the combination > of atmospheric error and other issues in most GPS-based time sources. For > NTP, network jitter would exceed SA jitter in most implementations. > > >> > Aviators try very, very hard not to trust their lives to GPS. > > > >> As opposed to LORAN ? > > > > Generally, aviators don't like SPOFs. So they try very hard not to trust > > their life to any one thing. GPS is used in conjunction with VORs, > > pilotage (navigation by reference to fixed objects), and dead reckoning. > > > Pilotage is _VERY_ difficult in IMC. Most IFR pilots don't rely much on > pilotage most of the time, and almost never attempt pilotage in IMC. > It is true that most of them use VORs and RADAR as their primary > navigational > backups under IFR in IMC. > > > GPS is used for instrument approaches, but only under extremely > > controlled conditions by very experienced pilots. A significant fraction > > of instrument training is how to cross-check instruments and detect > > failures. GPS approaches are individually approved by the FAA and factors > > such as runway lighting are critical. FAA approved GPS units must be used > > and one of the things these GPS units must do is monitor signal integrity > > (RAIM). From time to time, you will read FAA accident reports of people > > who attempted to perform GPS approaches with just a handheld GPS. > > > Excuse me? GPS is used for instrument approaches by virtually any > instrument rated pilot. A pilot can conduct a GPS approach solo with > as little as 75 hours of PIC experience (35 hours part 141 private > course and 40 hours instrument training) (14CFR parts 61 and 141). > I would not consider a pilot with 75 hours or even 100 hours "very > experienced". Heck, I have over 650 hours and I don't consider myself > "very experienced". I haven't looked back at my logbook to be sure, but, > if memory serves, I got my instrument rating at about 225 hours, and, > shot my first solo GPS approach with around 250 hours of PIC experience. > > You are right that a significant portion of instrument training is how > to cross-check instruments and detect failures. Mostly, however, this > focuses on failures of instruments related to keeping the airplane > right-side up. Some cursory coverage is given to detecting navigational > failurres, but, as much as I try to behave differently, and, as much > as I wish this weren't true, the primary mode of navigational failure > detection employed by most IFR pilots I've met is when the controller > says "Where the heck are you going?" (no, this isn't from the Pilot > Controller glossary, nor is it how they usually convey that message). > > It is true that to begin a GPS approach, you must have an approach > certified (TSO'd) unit in an installation that the FAA FSDO has > signed off as an approach capable installation. It's also true that > you need RAIM, and, RAIM provides a certain amount of integrity more > than standard GPS and more than ILS. (Actually ILS glide-slope only > failues are the ones that scare me the most as an IFR pilot). > > I'm not saying the system is unsafe. I think it's very safe. I also > agree about the accident reports regarding handhelds, however, I will > say that with a safety pilot on board, I occasionally do make sure that > I can do a panel-out (yes, that means put the sectional over the entire > panel) approach using my Garmin 195. I would never do this in actual > IMC, and, I would never do it without a safety pilot looking out the > window and watching what I was doing. However, I feel safer knowing > that I can, if evertyhing else goes to heck, get the plane down a > GPS approach using the handheld. It is a _VERY_ challenging approach. > > > Owen >
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