It likely will not come as a surprise to everyone, but I suspect that many people don’t understand that the GPS System’s primary customer is the Department of Defense. That means, among other things, that civilian users aren’t guaranteed access to the system under all conditions.
I was curious as I read the article about PTC (November 2014) regarding the extent to which the railroads have taken into account the possible loss of GPS access. In other words, are there backup systems that would continue to provide the railroad safety functions even in the absence of GPS?
Not quite sure under what ‘conditions’ you think railroads, as a strategic national asset, would no longer be given access to the GPS constellation.
It has been a very long time since ‘military precision’ has been encrypted – and in any case it doesn’t matter; the current NDGPS methods using assets like state GIS beacon networks and a large number of ‘enabled’ cell phones with appropriate chip support have made it cheap and, in many places, ubiquitous to have practical accuracy far better than that provided by the ICO constellation alone. (IIRC one issue in the past was that even at military precision it was difficult to resolve lateral location on multiple track… not a problem with modern methods, even the ‘ad hoc’ kind.) With the addition of some additional primary time standards in these networks, it is reasonably possible for modern ground-based ‘networks’ to provide adequate functionality for PTC without an actual satellite constellation – requiring little more than the establishment of additional beacons 40-50 miles apart where existing ground-based coverage doesn’t exist.
If you are concerned about some sort of hack or space-based attack taking down access to the GPS satellite constellation == even if the ground-based alternative was not fully in place at the time, there are other constellations (Galileo and GLONASS) that could be used.
Finally, even ‘if all else fails’ it would be possible to use an adaptation of the Canadian method which uses beacons on the locomotives and receivers that indicate mutual proximity of locomotives. That would at least preserve safety against collision between consists…
So the military would not let the railroads, who are moving all of their equipment, food, fuel and ammunition, plus supplying the national industrial base have access to GPS? Hmmmmmm
What they have now that has been working for the last 150 years.
I am confused by the notion the the mititary could deny access to GPS. The GPS sattelites just broadcast the time - continuously. All the smarts is in the receiver. It compares the difference in the time it receives from the various satellites and triangulates to figure out location. That’s it. I suppose someone could shoot down the sattelites.
The backup is the fixed signal system plus dead reconning. If GPS went away long term, you could use track transponders with antenae on the locomotives.
A key point will be the relative ease with which the GPS-based PTC can be dialed back or disabled (requiring a return to current methods of traffic control).
While I wouldn’t want crews to be able to disable the system, I would hope that PTC would not be so deeply interlocked with the locomotive that the engine can’t be moved unless PTC says so.
(1) The “P-Code” limiter has been turned-off for years .
(2) There are more “birds” in orbit now than ever before. There is less moving around of birds to suit the military at the exopense of the civilian user, Then there are the Russians (GLONAS), the Europeans (Galileo) and the Chinese (BeiDou) with their own constellations. The big deal challenge will always be relative precision. The human computer will still be called on to sort out the always present inconsistencies.
(3) It’s more than GPS alone. Autonomous GPS is notoriously squirrely* and the HARN network plus the repeaters and VRS were never a sure fire stand-alone network. The various different systems use similar, but different applications to help build redundancy (or so we are told)…The amount of computer power to process gobs of recorded epochs in real time to get decent reliable results is huge. Ten years ago, surveyors couldn’t get the precision they get now in real time and their equipment is far more involved than what PTC is at now on the PTC equipped trains. If it were GPS alone, every meet would be an alarm, especially where 13 foot or less track centers still exist.
(*) UP runs PMV cars over their lines plus CSX and others multiple times a year. Precision from one run to the next are not that great, but over time they have something they can live with for their purposes. Surveyors can tell you the difference between accuracy and precision - most others can’t. That works against the PTC doomsayers and, oddly enough, proponents of black box technology. For all the money thrown at PTC, don’t expect it to work to the expectations being touted. At least at first. This will be one VERY long process with lots of waivers until system reliability gets to a multiple sigma comfort level.
Most of the consumer GPS systems have a built in error factor…if you are willing to pay more, and pass the security clearance, you may purchase a more accurate version, close but not exactly identical to the military version.
Ask Mudchicken, or Paul North what their companies pay for the GPS service they use in their jobs, and how much more accurate they are.
If you think about it, having super accurate GPS for civilian use could be dangerous…I know a few computer geeks that would have no problem programing a throw away burn cell phone to guide a homemade missile or remote control small plane right through the Pentagons front entrance if they have access to the right GPS system.
Not that they would, but they could…I am not all that sure I want Tommy the Terrorist having that capability.
I can think of two or three reasons the DOD or NSA would want to close access to the system for civilians, such as a repeat of 9/11….and if you think that can’t happen again, you have had your eyes closed way to long.
One of this forums members, who has been quite for a while, used to be Deputy Director, Nuclear Weapons Placement and Deployment, USAF…some of the stuff he and I discussed is beyond scary, and the ease of access to some of the hardware and programming outside the US is just as scary.
You can lauch a Minuteman III missle form Minot AFB in North Dakota, send it ten thousand mils and it wil ldeliver up to 12 W88 warheads within 300 feet o fits target. AT 400 kilotons each close does not matter, but one would drop within 300 feet of the very center of center Kremlin. They use both inersiail and GPS guidancesystems. That is pretty darn accurate.
YES! Lots and lots of issues - like “how do I know the information I’m feeding the braking algorithm is any good?” PTC depends on lots of “commercial grade” data for safety critical work.
In the November 2014 trains magazine on page 29 is a box article stating the PTC systems may be subject to hacking. That i a worry some prospect. IMO the ACSES of Amtrak’s NEC is a much better approach as the control uses additional track circuits overlaying the cab signal system. That is important due to planned speeds and multiple tracks with a fairly easy ability to add tracks.
In my opinion, ACSES is goofy. They took the PRR 100 Hz cab signal and added a 250Hz cab signal overlay so they could get some higher speed signal aspects and still retain backward compatiblity with the original PRR system.
That part’s not so bad.
But, it doesn’t have predictive braking enforcement - particularly at absolute stop locations. Nor does it know about curves, bridges, etc, with speed restrictions, so, they added transponders screwed down to the track. But, wait! What about temporary speed restrictions and work zones. Let’s add 220 MHz data radios (but don’t use PTC data messaging protocol!)
So, now they have two sets of cab signals, transponders and readers, and data radio communication - and it’s all integrated into the goofiest display ever. It shows governing cab signal speed and governing transponder and data radio speed, but the engineer has to look at both to see which one is lower. And, it doesn’t show him at all where he is and what the upcoming restrictions are. He has to know and react accordingly.
Worst of all, the transponder stuff isn’t in use except in a few select areas on the NEC (150 mph territory). The rest of the NEC is not PTC complilant.
For any Amtrak locomotives that might want to operate on the NEC and in I-ETMS territory - they’ll need two sets of equipment. NS, CSX and Conrail are hoping Amtrak can use those data radios to sent I-ETMS messages to allow PTC operation on needed portions of the NEC w/o having install all sorts of stuff on a captive fleet.
Ed, your first sentence, “Most of the consumer GPS systems have a built in error factor…” gives me an explanation as to how the GPS system two different people were using to find my house before I moved told each one to turn west instead of east when they came to the street I then lived on. My address was 4778 West West Point Drive (it should have been named “Military Academy Drive”). They were driving on 4800 West Street (that IS is the name of the street); they had to call me and ask how to get to my house.
I also wonder if some map designers use an inaccurate GPS to show the location of house numbers. My house number is 584, and at least two maps put my house east of the street that divides the 500’s and the 600’s.
If it (GIS Data) screws up finding your house, then it’s a slam dunk that it can’t tell you a thing about the railroads in the area. Even the FRA’s Tiger/ ArcVue data is a screwed-up mess (calling it a cartoon would be an improvement)
What I have found is the navigation system in my car, provided by Sirius, has an error factor between 100 to 500 feet…it can get you close, but not spot on a street address and it gets confused on left or right side of the street…
It also fails to show most streets newer than last year, or changes in the IH 10 corridor here, which should be a simple fix one would assume.
Funny, my Dad was a Quartermaster in the Navy….with a sextant, good compass, accurate time piece and good charts he could take you from San Diego and put you in the harbor entrance of any port in the Pacific…he said once you had the math worked o
As a pilot we almost always use GPS. However there are many restrictions. Most of my cohorts always back it up for several reasons.
There has to be at least 5 uninterrupted CPS satellites avaiilable the full trip time + two hours after. That has delayed a few trips. Aircraf that have inertial reference systems have much relaxed rules that are not known.
Selective availability is the defense department’s ability to degrade accuracy to any error they desire. Supose a missle is coming toward the USA guess what will happen ?
During desert storm accuracy was actually improved because so many of the troops had only commercial units. Knew surveyors who worked fevershily during that time.
The unlikely problem of a coronal mass ejection ( CME ) aimed directly at earth being strong enough to permanently disable some or all satelites should be a concern.
Does anyone know if PTC systems will allow immediate default to present day traffic contol systems ?
That is your answer right there. Change the time signal and accuracy is changed. The more you change the time the less accurate. Also GPS units do transmit position.
Every day at 0700 mountain time the GPS atomic clock and position is updated from Falon air force base ( almost forgot which one as it may have changed ) . Remember these clocks run at a different speed due to relativity factors. Some satellites are not in circular orbits and some polar orbits as well. complicating relativity. Time differences from different units establishes position. The math must have been wild ! It is a RHO - RHO - RHO slution much like loran.
Pictures & detailed drawings seen show a bank of corkscrew like antennas pointing at earth. Have no idea how directional they are. i
Last heard ( long time in past ) there are about 35 operational units and a couple older ones in a parking orbit as spares ?