There’s also the matter of MEO being ten times the size of LEO, so there’s just plain more volume to cram all those satellites in.
PTC is operative in an interim phase in my area. On the portions equipped with either cab signals or automatic train control, if the locomotive is equipped with PTC we run with that engaged. The CCS or ATC is cut out. If the engine isn’t equipped with PTC or it fails, the CCS/ATC needs to be cut in.
When setting up the PTC in locations of multiple possible tracks, it asks which track you are currently on. If not on a PTC track, such as in a yard, it will give an “unmapped track” option. This allows actual selection to be made after a PTC track is occupied. Once when entering from an unmapped section onto main track one, I selected track one and confirmed it. Still the PTC system put us on track two, and started counting down to a penalty brake application because the next signal was at stop. I was able to make a normal, non-penalty stop and then reselect location. This time it took and we continued on. From this experience, I don’t think PTC currently relies completely on GPS for train location. I think it may use track circuits and/or logic to help place a train on the map on the correct track. I’ve noticed when comparing the mapped location to actual location it can still be off by 100-150 feet at times.
So far, most (including me) seem to like PTC. You can “see” about 5 miles ahead and can tell if you’re lined up straight or to crossover at control points. One thing I don’t like is at one crew change point, the pad is about 150 feet from an absolute. If the dispatcher can’t give you a signal you have to crawl up to it to avoid the penalty brake application. Was fun with a 20000 ton sand train last week moving at 1 mph.
Jeff
GPS has at least 35 operating satelites at last count. As well there are several older less sophiscated that are in standby orbit. Note there are 3 distinct orbit altitudes as well as some polar orbits.
As far a penalty braking is concerened at least 1 out of 10 south bound CSX trains here take a penalty slow when traversing our local siding. They either have to stop or slow to less than 5 (?) MPH. That is even with clear signals at both ends of sidings.
GPS implemented at present is in a ‘production testing’ enviornment. Put it in place, see what problems crop up and fix the problems. Can’t speak for the other carriers, CSX had (before EHH) a entire department following up on the reported PTC issues and working to correct them going forward. In December 2016 when I retired PTC had been implemented on 29 Subdivisions of all kinds of operating characteristics. I don’t know how many more subdivisions have been implemented since I retired - but I do know that more have been implemented.
My recollection (perhaps mistaken) was that the GPS satellites orbited in three distinct orbital planes and all orbits were of the same radius. The intent was that every spot on earth would have at least four satellites in view at all times, preferably more. To get a good fix, three of the satellites need to reasonably orthogonal to each other. Some of the requirements can be relaxed if the receiver has a good timebase (i.e. atomic clock), which would allow a 3D fix with only three reasonable placed satellites. Keep in mind that a 10E-10 error (gain/loss of 1 second every 300 years) will cause a position drift of up to 1 foot every 10 seconds.
Erikem is correct. One item that keeps GPS accurate is at 0700 mountain time Fallon Air Force base initiates an update to the atomic clocks in the sattelites. Since they are in different orbital planes relativity comes into play due to their different oribital speeds the clocks need resetting. Wonder how that is mitigated when universal time has the occasional second added ?
GPS positioning for aircraft has the additional problem in that aircraft fly pressure altitude not absolute altitude. The GPS receivers there for have to take a reading from aircraft air data computer to make required adjustments. pressure altitude is affected by outside air temperature so when air is hotter than standard the aircraft will be higher than absolute altitude.
They fly at 12,550 miles, and there are currently seven operational spares flying as well.
I use an app on my phone called “GPS Test” quite often, and can usually see at least 7 birds.
If you have anything less than 5 “birds”, the precision (and by default, the accuracy) goes down the rabbit hole. If your constellation of 5 or fewer birds is out on the horizon, your actual position really blows up when trying to check your relationship to something else nearby. It wasn’t that long ago, after the Challenger tragedy, GPS got to be very dicey to work with - it still isn’t 100% useful. For whatever reason, if you don’t have the spares and your older birds start to fail, you find yourself to be working at 2AM and checking an ephemeris constantly to make sure you have good data - a serious threat to PTC’s reliability. The old dinosaur systems become the backbone of what keeps the system fluid which can’t run only at certain hours.
MC: Thanks for reminding about the less than 5 birds in view good view. Aircraft GPS navigation is not solely permitted when that restriction is in effect. Often that restriction will only be for a couple hours or less. That is why now most modern aircraft ( B-757 and later + older aircraft refits) have inertial reference units installed ( IRS, IRU ) These units are relatively inexpensive compared to older inertial units costing 1975 as $250,000 per unit. ( 3 in early B-747s ).
Now days these newer units are required for many over ocean and non radar routes because of the potential of not enough birds in view 2 hours before and after a flight. .
Now what MC stated about using GPS at 0200 is what this poster worries about putting too many eggs in the GPS basket.
I worked for a satellite terminal provider that tried to get one of the Class 1’s interested in using their VSAT terminals. The subject of rain fade came up. These days both the uplink facility and the VSAT terminals power up and down to compensate for rain fade but there will be times when the service is disconnected.
When that happens, all signals in the block(s) served by that terminal are to drop to danger immediately.
The Air Force publishes scheduled GPS outages. A friend of mine who was a hobbyist pilot once showed me a web site that mapped and animated the scheduled outages for pilots to know when GPS is degraded. It’s a real thing.
But what people don’t seem to remember is that trains frequently travel in tunnels that have no GPS coverage at all.
There is a new threat to GPS. “IF” a certain country does an atmosphere or above atmosphe test of a neculear explaosion depending on the explosion nature ( possible EMP ) it could have anywhere from minimal to serious damage to the satellites in view. At least Fallon might initiate a temporary shutdown of any satellites that would be in view ?
Other GPS type satellites might also be vulnerable as wel as many other types of satellites.