GPS is now the reference for measuring distance and location on earth... which invites the question: how was it originally tested and calibrated? I mean, you throw those satellites up there, power on, get some data... but how do you evaluate the data? GIGO is reality, and we must pay homage to the great god Murphy...
I'm looking at it as an engineering problem - how was the system debugged, calibrated, and how do they do ongoing verification?
-- Rich
GPS provides Postition, Velocity and Time (PVT)
which invites the question: how was it originally tested
The figure of the earth for gps is modeled by WGS-84 datum.
---------------- and more abstractly talking
it was done by **trial and error!!**
as most practival science and technology is done !! while the precision is a result of that long process of trial and error
GPS is too heavily dependant on too may factors to be 'predicted' precisely -by any theory
ATB Y.Porat
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The information you want is readily available for a high level of understandingexcruciatingly difficult to understand the details.
I believe that the accuraccy ( the constant updating of the ephemerus data) is gathered from constant measurements made by 5 stattions setup around the world. These stations are in Colorodo, Hawaii, Ascension Islands, Diego Garcia and Kawajalein. The coordination of all the data is done in Colorado.
Before and during Navstar/GPS, laser ranging sats with retro reflectors and ground based lasers were used to map out orbits relative to WGS-84, as well as other means.
Steve
Details on the developmental history of GPS are extremely scanty on the internet, but it appears that design work began in 1973. In 1978 the DoD sent up the first of several engineering prototypes, at first to establish proof-of-concept, then to test the evolving hardware/software designs and finally integration test of partial and then complete systems.
Jerry
-------------- so as i said a trial and error system nothing to do with the crooks (or suckers )of GR!!
Y.Porat
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There were prototypes:
I used to sell time on a PDP-8 to Offshore Navigation Inc, to analyze Transit data... paper tape in and out, hardly realtime navigation. I also cobbled together a custom version of the FOCAL language, for the PDP-11, to do satnav calcs for another outfit. Both were sort of test applications, before the GPS constellation was launched. A lot of people were interested early-on.
ONI used the Raydist (Loran-like) and SHORAN (uhf pulsed thing) systems for navigation to drilling rigs and well heads in the Gulf of Mexico. Their service was expensive, and included an operator with every system. I bet GPS sure whacked them. Luckily, they had another division, OHI, Offshore Helicopters Inc.
I think there are still a few LORAN nets working.
John
Rich, thanks for posting one of the more thought provoking question that I've read in recent years on sci.physics.
As Sam has pointed out, I believe that GPS relies rather heavily on WGS-84 datum, which also is the datum used on the current generation of navigational charts in use, and the delineation of state, national, and international boundaries. Lat-Lon reference points or benchmarks exist at a number of major observatories both in the US and distributed througout the globe.
The calibration basis for WGS-84 is interesting in itself, and is based upon an update of WGS-60 data supplied using very high precision surveying methodology that replaced the use of physical survey chain measurement with electronic chains. (This was prior to the availability of laser based survey equipment.)
Some of us are old enough to remember seeing being done in the areas where we lived, although the purpose (other than to precisely fix international borders) was somewhat murky. I remember that it involved building a number of large, steel, towers that were later removed and relocated elsewhere. I have no idea how the measurements were made, but they either employed optical or electronic means. What later was revealed to the public related only to a comparision to the Lewis and Clark survey and how amazingly accurate the Lewis and Clark datum was, their benchmarks often being off only by a few yards, or in some cases only a few feet!
Your question is also interesting for a totally different reason, and it points to the fact that you are likely either an engineer, physical scientist, or a student in the field. You evidently realize the difference between "precision" an "accuracy", while very few laymen do. No one doubts that GPS is capable of a great deal of accuracy, and can deliver location coordinates to almost any called for number of decimal places. It is a system capable of great precision, and most people understand this. What you are questioning is it "accuracy", and how that accuracy is obtained.
With GPS, accuracy is determine largely based upon the primary reference benchmarks, and is therefore has a maximum accuracy that can be no greater than the positional accuracy the benchmarks upon which it is based or calibrated. All GPS locations, other than at the refence benchmarks, are determined interpolation, hence can be very precise, yet still slightly inaccurate. The question becomes, how inaccurate is GPS; how is this inaccuracy quantified; and how is the accuracy improved?
Harry C.
Whoops, that post contained some wording that can lead readers a very incorrect picture of the GPS system.
It should be pointed out that. in essence, GPS satellites transmit only timing information with which your GPS reader determines your your position on earth, latitude, longitude, and elevation. GPS satellites transmit no cartography data at all. only very accurate and precise time checks. Based on a reception of the timing information received from multiple GPS satellites, it computes its position on earth, and processes all of the corrrections, interpolations, or user inputs that it has been programmed from its manufacturer to perform.
Rember that GPS is a collective term for the entire system, which includes a constellation of orbital satellites that provide timing data, and your reciever which performs all location computations.
This raises still another potentially interesting question. How and from where do GPS receiver manufacturers obtain their software algorithms? Is there a softare design document distributed by the government? Does Garmin or Magellin market this information, and if so, where and how did they get it? Is it a product of US government funded R&D, and because of this in the public domain?
Harry C.
With expensive enough instruments you can pinpoint where the satellite is, you should know just how much time it'll take for its signal to get to you, and how precise its clock is.
From this, you can work out whether it's working within spec on a satellite-by-satellite basis.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html
The idea is simple. You decode phase-shifted timing data from three to four geosynchronized satellites (known locations). Any good second year engineering student can do it. The difficult part is building low power and low cost components.
Yes.
These days, I imagine they just check out one of the many books on the topic from Amazon. :-) (I expect something like this;
Note that these days a lot of the "consumer" GPS receivers by Garmin and Magellan use off-the-shelf GPS modules containing chipsets from, e.g., SiRF Technology, Atmel, etc.
---Joel
Joel Koltner schrieb:
Or download ICD 200: http://206.65.196.29/pubs/gps/icd200/ICD200Cw1234.pdf
Falk
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This is incorrect. GPS satellites transmit both the time and the location of the satellite.
Dave
Well, that's the problem with GPS though. When you transmit both the "time" and "location" from GPS, you are MAXIMALLY redundant in the information theory sense. Which is why Cruise Missiles, AAVs, AUVs, AHVs, AVVs, HDTV[], Post Ford Batteries, Post AT&T Fiber Optics, RISC++, Post GM Robotics, XML, and USB work so well in sinking submarines.
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The US Navy Observatory freequently sends calibration signals to the satellites to keep the system operating in spec.
They also check the prevailing attitude on sci.physics to see whether or not to apply the relativistic corrections.
They also check the prevailing attitude on sci.physics to see whether or not to apply the relativistic corrections.
Love it!. Just my thing. First time today I've laughed out loud.
The US Navy Observatory freequently sends calibration signals to the satellites to keep the system operating in spec.
They also check the prevailing attitude on sci.physics to see whether or not to apply the relativistic corrections.
========================================= Hahaha! And since the heart of the system is in the receiver, absolutely and totally independent of the US Naval Observatory, Tom-Tom and Garmin also check with sci.physics.relativity on how to design the software in their product -- just in case the clock is 38 nanoseconds off local time. Apparently being an hour off when we set the clocks back at the end of daylight savings time places us somewhere near the orbit of Saturn.
Me too!
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