BaltACDWhen in the Triumph the starting point and the ending point are the same - however the displayed 'altitude' will vary by 10 to 15 feet.
When I was preparing my last post, my phone app GPS was sitting in front of me (and under a metal roof) whilst the displayed altitude varied some 40-50 feet up and down...
I happen to know that my altitude above sea level here at the house is about 200 feet. There's a benchmark not far from the house.
Larry Resident Microferroequinologist (at least at my house) Everyone goes home; Safety begins with you My Opinion. Standard Disclaimers Apply. No Expiration Date Come ride the rails with me! There's one thing about humility - the moment you think you've got it, you've lost it...
NittanyLionI'm skeptical there's a major deviation in altitude readings. I used to work as an FAA contractor that was involved in ATC systems and it literally never came up. ADS-B wouldn't even work if that was an issue.
I used to work as an FAA contractor that was involved in ATC systems and it literally never came up. ADS-B wouldn't even work if that was an issue.
I have some Garmin VIRB action cameras for recording my on track activities in the race car. I also can mount them to my Triumph TR-7 to record pleasure drives.
When in the Triumph the starting point and the ending point are the same - however the displayed 'altitude' will vary by 10 to 15 feet. The camera get their data by recording GPS data.
Never too old to have a happy childhood!
I'm skeptical there's a major deviation in altitude readings.
tree68GPS gives altitude with relation to sea level. Right now my phone's GPS app shows my house at ~200'... Better be careful when I step outside.
blue streak 1There is a further problem in using GPS for air navigation. Airplanes have to use two different altitudes. One is indicated altitude which all airplanes fly at.
Aircraft barometric altimeters are corrected to sea level, as I recall.
GPS gives altitude with relation to sea level. Right now my phone's GPS app shows my house at ~200'... Better be careful when I step outside.
blue streak 1There is a further problem in using GPS for air navigation. Airplanes have to use two different altitudes. One is indicated altitude which all airplanes fly at. Then there is true altitude which is applied to all terrain warning systems.
Using the result for either ATC or TCAS might actually be workable if flight levels are 1000' apart as in United States practice. Note again that at any level above terrain, the GPS z is an absolute intercomparable reading between aircraft.
There is a further problem in using GPS for air navigation. Airplanes have to use two different altitudes. One is indicated altitude which all airplanes fly at. Then there is true altitude which is applied to all terrain warning systems. Is there a difference ? You bet! I have been over the Amazon which my portable GPS showed a true altitude over 4000 feet higher than indicated altitude. On all aircraft have been on true altitude is only available on terrain warning systems. On the other hand have been in Canada where indicated altitude was 1200 feet lower than true altitude.
This used to be a real problem when flying into very cold locations from warmer ones. Flying IFR you could fly too low. The fix is to fly higher that the published minimum enroute altitude on non precision instrument approaches and enroure operations. So, true altitudes can be used if qualified allow for non precision approaches to published values. The above procedures do require at least 5 satelites in view as well as at least 1 or 2 hours before and after planned times at any point. For true altitudes of aircraft the gate height of antenna(s) is a hard entry of the altitude into the computer after initiation of land coordinates.
Now the data bank in locos should have those altitudes which can reduce needed satelites to 4??? Someone who knows can speak to the altitude protocols of loco GPS.
Altitude reporting by aircraft for ATC and TCAS would require a whole another post.
BaltACDAt present Civil GPS does not have sufficient precision for railroads to rely upon it for the exact location of trains. Remember the centerline between tracks is nominally 15 feet or less.
There is a bit of a way to sidestep this. I'm not arguing it for a matter of control but something akin to ADS-B or AIS where you can gather positional data and motion data for monitoring purposes.
The north/south inaccuracy is quite readily solved. The five meter variance is entirely inside the boundaries of the locomotive for a sensor mounted roughly in the middle of the long hood. You've got a 10 meter range, which is smaller than the typical locomotive. If you did want to use it for control purposes, you would need to pad out locations to account for a train being 15 feet further down the track than it is in reality, which I don't believe is a major issue.
East-West does present an issue for the exact reason you describe: a parallel track is just close enough that as extreme deviation in the reading can make a train appear like it is on the parallel track. But, I suspect there is a way around this issue. It should be possible to error correct based on known positional data from previous locations combined with the physical geometry of the track. That is to say, you should be able to tell when it is impossible for Train 29 to be on Track 2 because the positional data before and after the last possible opportunity to cross over to Track 1.
But, that's fixing a problem that you may not need to fix anyhow. High grade receivers have CEPs more around 2 meters, putting you well within the bounding box of the locomotive. High quality dual frequency units that beat the pants off the units in our phones have 95 percent accuracy at telling you where the unit is inside the cab of the locomotive.
MidlandMike Yes and no. A local time can be used in the calculation when only 3 satellites are in view but any time standard used in portable devices will be considerably less accurate. jmonier Most of the inaccuracies are NOT due to the accuracy of the time standard in the receiver. As long as four satellites are in view, the position calculated in the receiver from the GPS satellites includes very accurate time. With only 3 satellites in view, the accuracy becomes degraded slightly more and more for the duration of the limited view. With only 2 satellites in view, position can no longer be calculated. My recollection from GPS training is that you need a minimum of 4 satellites just to solve the solid geometry problem that is the unique location.
Yes and no. A local time can be used in the calculation when only 3 satellites are in view but any time standard used in portable devices will be considerably less accurate.
jmonier Most of the inaccuracies are NOT due to the accuracy of the time standard in the receiver. As long as four satellites are in view, the position calculated in the receiver from the GPS satellites includes very accurate time. With only 3 satellites in view, the accuracy becomes degraded slightly more and more for the duration of the limited view. With only 2 satellites in view, position can no longer be calculated.
Most of the inaccuracies are NOT due to the accuracy of the time standard in the receiver. As long as four satellites are in view, the position calculated in the receiver from the GPS satellites includes very accurate time. With only 3 satellites in view, the accuracy becomes degraded slightly more and more for the duration of the limited view. With only 2 satellites in view, position can no longer be calculated.
My recollection from GPS training is that you need a minimum of 4 satellites just to solve the solid geometry problem that is the unique location.
The advantage of having an accurate time reference is the ability to maintain a semblance of accuracy when only three satellites are in view. With an accurate timebase, determining the position comes from finding the intersection of three spheres as opposed to finding the intersection of three hyperboloids. One caveat with determing position with three satellits in view with an accurate timebase is that the accuracy is degraded when the three satellites are roughly in the same general direction (which would be the case for restricted views) as oposed to the ideal case where they are at right angles with respect to each other.
One of the more bizarre facts about GPS is that the raw signal levels are below the thermal noise levels of most receivers and that reception is made possible by "processing gain", much the same way where an FM station can come in clearly when only somewhat above the noise level for the raw signal.
MC's comments about GPS come across as someone who has had a lot of experience with using GPS. Oe thing to keep in mind is that the speed of propagation through the atmosphere (which includes the ionosphere) is emphatically not a constant as it is in a perfect vacuum.
Cotton Belt MP104... Another inspector story. A local industry loaded gondolas with small chips of steel. They noticed the car was resting on the wheels of the car. Inspector shows up. Announced car is overloaded. A local backhoe was called to unload "the excess" weight. Inspector went to lunch. Supervisor asked backhoe to unload the whole car. Yep, Inspector returns and announces, NOT ENOUGH removed. He then was invited to climb aboard an view the EMPTY car that had weak springs in its trucks. endmrw0414231452
Another inspector story. A local industry loaded gondolas with small chips of steel. They noticed the car was resting on the wheels of the car. Inspector shows up. Announced car is overloaded. A local backhoe was called to unload "the excess" weight. Inspector went to lunch. Supervisor asked backhoe to unload the whole car. Yep, Inspector returns and announces, NOT ENOUGH removed. He then was invited to climb aboard an view the EMPTY car that had weak springs in its trucks.
endmrw0414231452
In reality, the Inspector at the serving yard to the industry is the one that failed, as the ride height of the empty car should have been noted and the car sent to the shop track at the serving yard and the car never should have been sent to the industry for loading in its condition.
Mud Chick: You made two comments, personal to me.
1. "poor precision and awful accuracy" . As a HS physics and chemistry teacher it was fun to point out the difference. Since most people see them as the same. A piston range target with ALL bullets striking high left of bulls eye. PRECISE but not accurate. Whereas a target that has bullet holes all around the bulls eye and it is forming a nice circle of the bulls eye. ACCURATE but certainly NOT precise.
2. During past summers long ago,I was a surveyors helper. He told me surveying at that time was an "art". i.e. If in court, give me the way the opposing surveyor is working. I will start at another reference point and go to the land in question with a different desceiption that is opposite. A recent visit with my friend, and I repeated his quote. Not now he said. With GPS it has become a "science" and not "art" anymore.
BTW to keep this RR related, comment has been made, how rough it is riding the rails. A local industry manufactured lifts that took total 18 wheeler (tractor included) raised the rig high enough to gravity out the contents. i.e. grain, woodchips etc. The rig came in longitudinal halfs. A crane loaded each half a TOFC and then WELDED it to the car. DONE MANY times until an RR inspector happened by. OH NO you can't do that. CHAIN BOOMER it down. The welder said, "Ok, when it leaves here, it's your baby." Of couse few realise with loads that heavy and the rough ride, things happen. Sure "nuff" it came loose somewhere down the line. Inspector never showed up again and welding continued.
mudchicken The alleged shielded main generators on the locomotives frequently aren't and the multipath errors around locomotives are a major problem.
The alleged shielded main generators on the locomotives frequently aren't and the multipath errors around locomotives are a major problem.
I've wondered if making a GPS receiver designed to use several antennas (diversity reception) would reduce the multipath problem. The multiple antennas would act as a phased array to attenuate signals (and noise) not coming from the direction of the satellite. The technology to do this isn't much different than MIMO (multiple input, multiple output) for WiFi which makes use of multipath to increase channel capacity. The downside is that the receiver would need IC's specifically designed for this use.
I would be very surprised if multipath was NOT an issue on railroads.
There's a move with EV manufacturers to eliminate AM radio receivers as the traction electronics causes a lot of intereference to the AM band.
rdamon Nice thing about rails is that the don't move (or at least should not) a transponder between or above the rails can give an exact reading to identify track number.
Nice thing about rails is that the don't move (or at least should not) a transponder between or above the rails can give an exact reading to identify track number.
Perry BabinIt was previously stated that the accuracy of civilian GPS was only about 15 feet and not good enough for train location. If that's true, how accurate can GPS for a race car be? How common is it to have 8 or more satellites available to the datalogger?
How common is it to have 8 or more satellites available to the datalogger?
GPS in the race car is only logging data points - in the case of my equipment at 10hz. GPS does not have active control of anything.
As mudchicken postulates, the race car has full open access to the sky in all areas of the track
Right now you have a surplus of birds. Ten years ago you still were fighting to get a minimum constellation of 5 that stayed above the horizon. You had to carefully plan your work then. With GLONASS and the others now in the mix (plus servicable older birds still working), redundancy is pretty good. You still have to have the processing power to make real-time use practical.
....and then there is the issue of trees, tunnels, buildings and inertial guidance systems.....GPS is NEVER going to be the solution for everything, the real world is not that simple.
....and then there is the issue of trees, tunnels, buildings, p-code flutters* and inertial guidance systems.....GPS is NEVER going to be the solution for everything, the real world is not that simple.
(*) p-code is still messed-with by the military. Surveyors often notice something is about to happen before everybody else
Perry BabinHow common is it to have 8 or more satellites available to the datalogger?
Here is one of the original papers on the SA.45s CSAC when it was being marketed by Symmetricom. (The current version has a 1PPS input that lets the clock be further disciplined to ~1ns in phase and 0.5e-12 accuracy.)
https://www.microsemi.com/document-portal/doc_download/133185-the-sa-45s-chip-scale-atomic-clock-early-production-statistics
It was previously stated that the accuracy of civilian GPS was only about 15 feet and not good enough for train location. If that's true, how accurate can GPS for a race car be?
mudchicken4 "birds" give you poor precision and awful accuracy . You really need 5 and a computer the size of a small locomotive to process in real time to handle the level of accuracy the OP expects in kinematic mode. The alleged shielded main generators on the locomotives frequently aren't and the multipath errors around locomotives are a major problem. There are not enough CORRS stations around, especially in the stix and we still have issues with clueless engineers (they have a coordinate and no idea of how good it is ... Brian probably saw that with discouraging frequency ... we still do)
There are not enough CORRS stations around, especially in the stix and we still have issues with clueless engineers (they have a coordinate and no idea of how good it is ... Brian probably saw that with discouraging frequency ... we still do)
My race car has a GPS driven Data Logger. One of the things the logger measures is the number of GPS Satellites the device is in contact with during the recorded lap. One lap that I looked at had the logger in contact with between 8 and 11 GPS Satellites during a lap around Virginia Internationa Raceway. As Info.
4 "birds" give you poor precision and awful accuracy . You really need 5 and a computer the size of a small locomotive to process in real time to handle the level of accuracy the OP expects in kinematic mode.
TCXO's? OCXO's? How about a Rubidium frequency standard? These are getting down to about 1 cu in and a not outrageous price. These could be of help when only a couple of satellites are in view.
//raises hand
I was, and I do know about them. One of the last projects I was a Senior Systems Engineer on had an extremely accurate TCXO (temperature-controlled crystal oscillator) built into a data transmitter/receiver.
Brian (IA) http://blhanel.rrpicturearchives.net.
"A really good receiver will have an 'oven' around the time crystal"
Were you an engineer (not the railroad type)? Not many people know about ovens and oscillators.
jeffhergertI've noticed when stopped with PTC engaged at a Stop signal, the distance to that signal will change while stopped. It's only a matter of a few feet, I think 10 feet is about the most I've noticed.
The clock in the receiver is what allows the receiver to distinguish between speed-of-light transmissions from satellites only a few thousand miles at most 'differentially' distant from the receiver, to an accuracy capable of giving the distance measurements seen.
A really good receiver will have an 'oven' around the time crystal, and the crystal will be cut to run at reference frequency at that temperature. But many receivers don't have this much precision in their construction -- iPhones and the like are notorious for many feet of random drift when the phone comes out of a pocket, or sunlight falls on it, or someone holds it wrong. What is pernicious is that the guidance instructions may instantly change to reflect this wack position... I think everyone has had the 'turn left' ... 'turn right' (in exactly the same tone of voice) or been directed down side streets to turn around to get somewhere you passed.
Our community is FREE to join. To participate you must either login or register for an account.