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GPS & Packets

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tchrist
776193.  Sun Jan 16, 2011 2:03 pm Reply with quote

Criffer wrote:
In no situation involving UDP or TCP is an accurate clock required.

Thanks for posting that summary of UDP, RTP, and TCP timing issues and non‐issues.

Criffer wrote:
Stephen Fry seemed to be very wrong on how the Internet Protocol works, which is quite disappointing.

Compared with the completely bizarre inversion of who’s the sender and who’s the receiver in GPS, I found the IP confusion only mildly disconcerting. (The things we call GPS units are always receivers, never senders.)

For one thing, Stephen’s a bright enough sort that I’m rather astonished he got the basic mechanics of GPS communication terribly wrong. I’m also quite disappointed that so major a guff went unnoticed by whoever the team was that put together the broadcast show using footage from the live one. That sort of blunder should have been left on the cutting room floor, not preserved to mislead posterity.

It almost makes you wonder whether Stephen’s Elf‐notes might have been the source of the error, and that because the notes and show agreed, nobody caught it during post‐production. Other possible explanations exist, but I still boggle that nobody on staff, be that Stephen or otherwise, spotted the obvious nuttiness of claiming all umpty‐gazillion GPS units sent active signals up to the satellites. It just doesn’t make any sense.

First, that setup could never scale, given how many GPS units there are that would need responding to. And second, it would require two messages per satellite (ground to sky and then back down again) instead of just a single one‐way message broadcast from the satellite to all listeners. Imagine the latency! There’a also the matter of power, given that I’ve seen tiny GPS receivers no bigger than a matchbox. Those little guys would never be able to power the uplink signalling required — they aren’t suitcase‐sized satellite phones!

For another thing, the botched discussion on atomic clocks and GPS was disappointing because it was the perfect occasion for Stephen to point out that we could never measure it accurately enough without taking Einsteinian Relativity into account. It really is Quite Interesting, being a real-world application of relativity, one impossible to get right otherwise. Those don’t come up very often, so it was a missed opportunity.

It’s also QI because it’s both kinds of Einsteinian Relativity. It takes Special Relativity to account for the atomic clocks on the moving satellites ticking 7 µsecs/day slower from the perspective of us here on the ground. And it takes General Relativity to account for the satellites’ clocks appearing to tick 45 µsecs/day faster than ours down here deeper in the more massive Earth’s gravity well, where timespace is necessarily more curved than it is further out.

Alas!

I’m betting Stephen himself would find the whole GPS–Relativity connection quite interesting, provided it were presented to him in an “Isn’t this neat?” sort of way, not as a “Boy were you a blithering idiot!” slap‐down.

--tom

 
Spiritman
776858.  Tue Jan 18, 2011 8:24 am Reply with quote

I think it was in an episode of Country Tracks where they featured a combine harvester, running on auto-pilot, which was accurate to just a few inches. At around £450K, I imagine part of the cost was for a guidance system much more accurate than the common-or-garden GPS we normally use. I'd be quite interested to know if it's the same system the military use.

 
Moosh
776994.  Tue Jan 18, 2011 1:07 pm Reply with quote

Hmm. To be honest my first guess wouldn't be to use GPS for that combine harvester. I'd use a system where the machine could "see" the fence around the field, then it's a fairly simple matter to get it to move around and cover the entire field. Alternatively you could just program it to just follow a set path, "forwards 1 mile, turn round, forwards 1 mile...", which again, doesn't really need GPS.

 
Neotenic
777005.  Tue Jan 18, 2011 1:42 pm Reply with quote

Actually, the GPS-guided combine harvesters were featured in Andrew Marr's 'Britain From Above' series.

 
Jeeves
777225.  Wed Jan 19, 2011 3:07 am Reply with quote

re: GPS navigated combines

As was said above, there are many ways to improve GPS performance. EGNOS / WAAS and DGPS (differential GPS) use reference-stations sited on the ground to correct the incoming GPS signals and give you better accuracy.

These still use the signal in the 'usual' way, by tracking the satellite's access-code modulated onto the carrier signal. The military have their own encrypted signals that work in this way, but with a more accurate access-code, and are good to 1-2m.

Most ultra-high accuracy GPS, used in surveying and also agriculture, uses a sophisticated receiver that can count the number of carrier-frequency cycles (@1.6GHz) so give positioning down to the length of one wavelength (2-3cm or so).

I find the subject itself is Quite Interesting, as almost every aspect of modern physics from Quantum Mechanics to General Relativity is needed to get GPS to work. Shame we've had the 'G' series as there's probably enough in it for an episode...

 
EarlyGrayce
778438.  Fri Jan 21, 2011 11:26 am Reply with quote

I must make mention that the predecessor to GPS was a system as described by Mr Fry although by the time the sattelites were actually put into orbit the interest in them had outweighed their data transfer abilities and therefore there usefulness was quite limited.
The system was quickly replaced with the current system which is used by billions of people daily but unfortunately the US government, last time I heard, had no intention of maintaining it and will slowly degrade as the satellites are destroyed by space junk or altitude loss. The USSR also created a similar system.
I am sure that they would continue using some other system but not advertise it's existence until the public has captured enough data for a company to create new navigation devices.

 
Posital
778533.  Fri Jan 21, 2011 2:22 pm Reply with quote

Don't forget the EU's Galileo Project...

http://news.bbc.co.uk/1/hi/sci/tech/4555276.stm

Quote:
GALILEO WILL HAVE FIVE SERVICES

OPEN ACCESS NAVIGATION This will be 'free to air' and for use by the mass market; Simple timing and positioning down to 1m
COMMERCIAL NAVIGATION Encrypted; High accuracy at the cm scale; Guaranteed service for which service providers will charge fees
SAFETY OF LIFE NAVIGATION Open service; For applications where guaranteed accuracy is essential; Integrity messages will warn of errors
PUBLIC REGULATED NAVIGATION Encrypted; Continuous availability even in time of crisis; Government agencies will be main users
SEARCH AND RESCUE System will pick up distress beacon locations; Feasible to send feedback, confirming help is on its way

 
Tomas
778548.  Fri Jan 21, 2011 2:53 pm Reply with quote

On the subject of the military signal, there is no difference in the accuracy of the two. It was once the case where they had the ability to introduce random deviation into the public signal but this hasn't been used for at least 10 years. See here for information on the subject: http://home.online.no/~sigurdhu/GPS_history.htm (referred to as Selective Availability)

GPS enabled equipment has to comply with regulations stating that it can't be used above a certain altitude or over a certain speed. It's a common issue for people sending high altitude balloon equipment to take pictures of the Earth from near-space.

Also in regard to the USA not supporting the GPS system any more, that is also untrue. There are more satellites planned to enter the constellation this year and in the coming years. See here for list: http://en.wikipedia.org/wiki/List_of_GPS_satellite_launches

 
suze
778658.  Fri Jan 21, 2011 6:40 pm Reply with quote

Tomas wrote:
On the subject of the military signal, there is no difference in the accuracy of the two. It was once the case where they had the ability to introduce random deviation into the public signal but this hasn't been used for at least 10 years.


Now that reminds me of something I once read.

It was claimed in the days of the USSR, their official maps put Moscow in the wrong place. They deliberately drew it about 30 miles away from where it really is, so that anyone relying on their maps to bomb Moscow would miss.

Has anyone heard this claim before, and do we believe it to be true? And does Moscow by now appear in the right place on maps?

 
bobwilson
778699.  Fri Jan 21, 2011 11:50 pm Reply with quote

I think that's one of those myths suze - I've heard similar things related to the location of military installations on maps. I can't imagine it would work for a city - it would be a bit embarrassing if a detente meeting was arranged and the 'plane landed on a field instead of at the airport due to faulty maps wouldn't it?

 
PDR
788372.  Tue Feb 15, 2011 6:20 pm Reply with quote

Spiritman wrote:
I think it was in an episode of Country Tracks where they featured a combine harvester, running on auto-pilot, which was accurate to just a few inches. At around £450K, I imagine part of the cost was for a guidance system much more accurate than the common-or-garden GPS we normally use. I'd be quite interested to know if it's the same system the military use.


It's not "the system the military use" - this requires crypto variables which are changed periodically and are quite highly classified (to the point that when we, as military aircraft manufacturers, need them for flight trials of development aircraft they send someone with the crypto-fill gun to load them for us - we cannot be approved as crypto-custodians).

I suspect the system you saw was a combination of differential GPS (DGPS) for the initial position fix and then a blended inertial/GPS navigation system for the actual navigation - probably using a ring-laser gyro inertial nav. These systems *were* highly classified and very expensive when they were first used in military systems 20+ years ago, but the technology has moved on and become cheaper. The military applications now tend to use nano-structure inertial sensors (aka "MEMS gyros") because they are smaller, lighter, more accurate and much more reliable.

The role of GPS (as originally intended) is generally misunderstood. It was never intended to be used as a real-time navigation sensor, largely because it was though it would be fairly easily jammed and (worse) spoofed. The prefered navigation sensors were inertial - a technology originally developed as part of Germany's V2 balistic missile programme. But the inertial+gyro-based units integrated acceleration to get velocity and position, and this made them prone to noise-based zero-drift. they gyro elements also drifted, but in a more predictable manner. These drifts were definable and tolerable for short-to-medium distances, but made the position accuracy degrade as a function of both time and distance travelled.

The original solution was to use position fixes from landmarks or stellar observations to update the navigation solution and cancel the drift, but this was a manual process that involved techniques which could be dangerous in a military application. To get an idea of the problems involved even relatively recently (1982) read the book "Vulcan 607" about the black-buck raid on the Falklands which involved a precision flight several thousand miles over the open ocean with no landmarks, no radio navigation aids and limited access to astro-navigation due to poor weather. The vulcans had twin carousel-type inertial nav sensors, but they couldn't navigate reliably without position fixes.

So the original idea of GPS was to provide a system which could automatically give an inertial navigation system accurate position fixes every 10-15 minutes or so, to kill the drift. But the actual navigation would be done by the inertial system. The nav system uses a blend of inertial and GPS data processed through a mathematical algorithm called a Kalman Filter to remove distorsions and erorrs, and to allow the system to warn if the GPS position seems to be suspect. This is actually how GPS is still used in commercial aviation, military aviation and most large-ship maritime systems - it allows the GPS to be defined as "non-safety-critical" in the safety case, which is a good thing due to the near impossibility of engineering a GPS receiver to a high enough integrity level for safety-critical applications.

GPS receivers used as sole navigation sources certainly work, but they exhibit anomalies caused by varying error signals as particular satalites drop out of the constellation. I use my handheld Garmin Geko when out walking with the family (usually on 10-mile forced marches to get to cash-dispensers to keep barbados happy), and it frequently shows "peak speed" values of as much as 120mph because a satalite has dropped behind a tree or hill, and the new combination has a combined error term that's 40 feet in the other direction (which the device interprets as a sudden sprint). That's why they can't be used as sole navigation sources on aircraft or ships.

Sorry - far more than anyone wanted to know, I'm sure!

PDR

 
PDR
788376.  Tue Feb 15, 2011 6:34 pm Reply with quote

bobwilson wrote:
I think that's one of those myths suze - I've heard similar things related to the location of military installations on maps. I can't imagine it would work for a city - it would be a bit embarrassing if a detente meeting was arranged and the 'plane landed on a field instead of at the airport due to faulty maps wouldn't it?


It's certainly true - I have seen 1960s russian maps overlaid on satalite photos showing the point. It partly shows the paranoia of the times, but then both the USA and the USSR were planning for contingency hostilities that involved bombers flying long distances at night using intertial and dead-reckoning navigation. If you could persuade these aircraft to end up 50 miles from where they wanted to be they certainly wouldn't be able to search when they got there. Also as the pilots were exclusively male they would never stop and ask for directions. Once both sides accepted that the other had effective reconaisance and mapping satalites they stopped doing it.

It made no difference to road transport, because the roads still went where they said they went. It also made no difference to soviet air traffic because they navigated using radio navigation aids that were also not in the mapped positions, so they still got there (these would have been turned off in times of high tensions or war, of course). And it made no difference to visiting civil aircraft because there were ALL met at the border and escourted by soviet military aircraft, which would lead them along tortuous (and fictitious) "air traffic corridors" before finally bringing them into Moscow.

As I said, it was an interesting illustration of the paranoia of the 50s and 60s.

PDR

 
Posital
788576.  Wed Feb 16, 2011 1:15 pm Reply with quote

PDR wrote:
GPS receivers used as sole navigation sources certainly work, but they exhibit anomalies caused by varying error signals as particular satalites drop out of the constellation. I use my handheld Garmin Geko when out walking with the family (usually on 10-mile forced marches to get to cash-dispensers to keep barbados happy), and it frequently shows "peak speed" values of as much as 120mph because a satalite has dropped behind a tree or hill, and the new combination has a combined error term that's 40 feet in the other direction (which the device interprets as a sudden sprint). That's why they can't be used as sole navigation sources on aircraft or ships.

I contested this last time this was mentioned - I think EGNOS has transitioned to Safety of Life applications: http://www.gsa.europa.eu/go/news/egnos-transition-towards-safety-of-life-service

The last time you mentioned this - you didn't have WAAS/EGNOS available on your Geko - which is why you were getting the crazy readings. IIRC

 
bobwilson
820843.  Tue May 31, 2011 10:54 pm Reply with quote

Quote:
Also as the pilots were exclusively male they would never stop and ask for directions.


Pleased to see at least one part of your post is accurate.

 
PDR
820912.  Wed Jun 01, 2011 7:00 am Reply with quote

As if you could even recognise "accurate", Bob!

PDR

 

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