Kerry.

The wandering display is basically normal and really these days is nothing compared to what it used be like. Positional accuracy isn't affected if one is moving or stopped. What is affected is things like Course Over Ground, bearings to waypoints etc basicallymore directional issues. These days would have thought 5 mph is rather on the high (max) side for reasonable directional readings. Cheers, Kerry.

No, I suppose there no technical reason why it couldn't be done, why one would even want to do that is beyond me? Unless of course one wants a position solution based on partly corrected data and uncorrected data, just produces part of an unknown position solution that would basically be totally suspect and I mean totally suspect. Well we could go into all this in rather quite some depth but maybe that just might be to tangential for some to follow. In depth now just imagine the "tangential" comment then Cheers, Kerry.

That's exactly what differential type systems must do to maintain integrity and that is they simply can't use satellites for which there is no transmitted corrections. If receivers didn't ignore satellites for which it did not receive data then the accuracy & integrity of the position is simply compromised and uncertain. Satellites MUST be common for dGPS corrections as it is range corrections that are transmitted for each satellite (max 9) not simply a position correction. The receiver then applies what common satellite range corrections it receives to the satellite ranging data that is common. WAAS is slightly different but very similar as there must be a certain number of common satellites and these satellites must be in view from several different ground stations. Data that is not common must be ignored. Cheers, Kerry.

Not specifically on subject but the following outlines (tries to explain) the US railway guage and the relationship to technology. Cheers, Kerry. The US standard railroad gauge (distance between the rails) is 4 feet 8.5 inches. That's an exceedingly odd number. Why was that gauge used? Because that's the way they built them in England, and English expatriates built the US railroads. Why did the English build them like that? Because the first rail lines were built by the same people who built the pre-railroad tramways, and that's the gauge they used. Why did 'they' use that gauge then? Because the people who built the tramways used the same jigs and tools that they used for building wagons, which used that wheel spacing. Okay! Why did the wagons have that particular odd wheel spacing? Well, if they tried to use any other spacing, the wagon wheels would break on some of the old, long distance roads in England, because that's the spacing of the wheel ruts. So who built those old rutted roads? The first long distance roads in Europe (and England) were built by Imperial Rome for their legions. The roads have been used ever since. And the ruts? Roman war chariots first made the initial ruts, which everyone else had to match for fear of destroying their wagon wheels and wagons. Since the chariots were made for, or by Imperial Rome, they were all alike in the matter of wheel spacing. And why was the spacing what it was? The wheels were spaced to accommodate the width of the asses of two horses. Thus, we have the answer to the original question. The United States standard railroad gauge of 4 feet, 8.5 inches derives from the original specification for an Imperial Roman war chariot which was, in turn, based on the width of a horse’s a**. Specifications and bureaucracies live forever. So, the next time you are handed a specification and wonder which horse's rear came up with it, you may be exactly right. Because the Imperial Roman war chariots were made just wide enough to accommodate the back ends of two war-horses. But there's more. When we see a Space Shuttle sitting on its launch pad, there are two big booster rockets attached to the sides of the main fuel tank. These are solid rocket boosters, or SRBs. Thiokol makes, or made, the SRBs at their factory at Utah. The engineers who designed the SRBs might have preferred to make them a bit fatter, but the SRBs had to be shipped by train from the factory to the launch site. The railroad line from the factory had to run through tunnels in the mountains. The SRBs had to fit through those tunnels. At least one tunnel is only slightly wider than the railroad track, and the railroad track, as we know, is about as wide as two horses behinds. So, the major design feature of what is arguably the world's most advanced transportation system was determined by the width of a Horse's rear!

The height a GPS derives from the conversion of cartesian coordinates is generally referred to as an ellipsoidal height, which is based on a mathematical model. The difference between this height and what is generally referred to as sea level varies thoughout the world so to derive sea level heights the GPS generally uses another model (geoid model) to determine separation values to apply to ellipsoidal heights to get sea level heights. Does/can get rather complicated and more involved than most GPS users are interested in. Generally heights are based on mean sea level, which is based over all tide ranges over approx 20 years. However using GPS principles over the past many years, it has been found in some countries that in effect "mean sea level" has been found to be not as level as was assumed. Over large areas the geoid model has inaccuracies but apart from that the initial GPS derived ellipsoid height is about 2-2.5 times less accuarte than the horizontal accuracy. In practice the height derived by a recreational GPS is basically for general interest only, don't place too much critical importance on height. Cheers, Kerry.

Galileo and the politics is rather a long story, been a long time coming and still yet to become an operational system. Essentially some countries that make up the EU don't want to or have this thing about relying on another countries system, which is fair enough I suppose. However just getting ALL the EU countries to agree on the initially funding has been a drama in itself. In essence it also has a lot ot do about commercial issues and obviously if the countries/companies putting in the money weren't going to get something out of it then presumeably Galileo wouldn't be a consideration. European countries have this thing about a military controlled system but then some have an issue with a civilian controlled system and some other issues with Galileo. As for some of the commercial services being muted by Galileo, similar type services have been available for 10 years or more but have been provided by third party providors using GPS as a free platform, where as Galileo will provide the platform as well as provide the services all in one, for a fee of course, just like what the third party providors commmercially market GPS based products. Galileo being basically a "civilian" system some of the questions with regards Galileo and integration with GPS are - The interoperability of a free open system (GPS) with a fee based encrypted system especially with consideration to safety of life applications. - Prevention of misuse. Yes, there's probably more to worry about a civilian controlled system with regards misuse. - Open specifications and standards. GPS has a total open freely available non-propriety signal structure for all civil services. How can an encrypted fee based system be totally open and available to all markets? Cheers, Kerry.

As far as the pure GPS side of things is concerned there's absolutely no difference, GPS is a global utility, the worlds first. Things like mapping, tides, language etc then that's different but that secondary to the system proper. WAAS, EGNOS & MTSAT are all "compatible" Wide Area Satellite Differential systems (WADGPS). One just needs to receive the correct satellite corrections for the supported area. Manufacturers have supposedly now plugged the capability of receiving satellite corrections outside the intended coverage area. 1 metre not unless one is prepared to pay for it, the more accuracy the more it will cost, lets face it Galileo will be a commercially run operation. Civiies can have that type of accuracy now, as long as they are prepared to pay for it. This type of accuracy misconception needs to be quantified as Galileo will be no more accurate than GPS and for statements like this one needs to be comparing similar levels of service. Cheers, Kerry.

True accuracy? is not as good as what many would have one (or like to) believe. Still many misconceptions with regards accuracy in general let alone true accuracy. Cheers, Kerry.

Yes, for sure, to what extent might depend on many factors. Cheers, Kerry.

Why ? Cheers, Kerry.

The uncomplicated (theoritical) answer in GPS distance terms is zero (0). If the GPS was capable of accurate vertical representation then it would show a difference in height of 0.5 miles but the lat/Long would be unchanged (in the true vertical sense). Cheers, Kerry.

so what are they going to do with leap seconds otherwise "time" might pass them by. Cheers, Kerry.

Here in the west of Europe i use WAAS and EGNOS both. In the west of Europe you certanly wouldn't wouldn't be using WAAS. You would however might want to use EGNOS but certainly not both. Both are WADGPS (Wide Area Differential GPS) but WAAS is only applicable (limited) to CONUS and ENGOS to much (most) of Europe. Similar reasons one wouldn't even be interested in trying to use EGNOS in the US, totally not applicable. Most manufacturers have now limited the access of "different" WADGPS in different areas so that users can't use Satellite differential systems that are not relevant to their area and hence give erroneous and invalid results. Cheers, Kerry.

Accuracy with older units "might" differ slightly but will still be less (generally) than the systems accuracy specifications and SA being a system function is discontinued for ALL units. Coords "way off" by 120 feet sounds like another issue/s other than and accuracy issue. Cheers, Kerry.

When Selective Availability was discontinued (it hasn't really been turned off, as such) WAAS was still an infant and then stagnated for many years trying to find out its purpose in life. Gee, if it hadn't been for the Transport department the FAA might have still been deciding if and when to actually declare it partially available for some roles and change the test flag. This only changed in July 2003, something like 3 years after SA was discontinued. Cheers, Kerry.

Lets just say that the FAA started the WAAS thing way before SA was discontinued and for several years since SA was discontinued WAAS really went no where. One can make many assumptions with the delay but one thing is certain WAAS so far has cost in the vicincity of close to 6 billion US and (just) covers the CONUS , GPS since day zero has cost around 12 billion and gives world global coverage. What WADGPS type systems (such as WAAS) do and the portion of the planet they cover in relation to the improvement in accuracy per metre is quite staggering. Well and truely in the (many many) millions per metre but then one has only to look at who drove the recent change in WAAS status for some insight in the politics and it certainly wasn't FAA. Cheers, Kerry.

Accuracy is one thing, Precision is another and for a system spec'd at 13 metres 95% of the time (world average) it's much like the calculator syndrome, see 9 decimals it MUST be precise or accurate or whatever one wants to call it. Recreational differential capability is slightly different but still really only valid to 3 decimals. Generally if the equipment is meant to be precise/accurate then the precision will have been allowed for in the software/hardware. Much like the reasoning behind the limitation of 2 decimal minutes, based on a system spec'd at 100m 95%, what was the point of allowing 3 decimals. No, apart from the methods used by professionals and add on type augmentation systems. But todays satellite is a much improved version, especially with regards timing and increased timing=increased accurracy so really the system is being improved with basically every new satellite launch (especially these days). At this point in time the first of the modified Block IIR's (Block IIR-M) satellites with the second civil freq is due for launch around April 2004. Civil dual freq won't do much for anybody until there's enough of these satellites in orbit. The expected accuracy is around 1.5 metres. A third civil freq will follow. Second freq = new receiver. Much more than technically possible but it won't happen. But if it does you'll have a lot more to worry about than a GPS receiver. Governmental intervention as mentioned is not past the use by date, it could but it won't. Investment in GPSr's will be like most other things, new investment each time you have to replace the last to take advantage of the future. That second civil freq to enjoy the benefits will require a new receiver, the old receiver will continue but one will have to keep up as fancy using an "old" receiver that's inaccuarte With respect some of the other comments. GLONASS has been around almost as as GPS, just the Russians had some funding/priority issues but this has now changed and there should be a few more replenishment sats launched. basically they are launching 3 at a time but then things were getting fairly desperate. GPS+GLONASS receivers have existed for many years just that the Russians couldn't maintain their system so many companies actually never released their receivers either. As for the accuracy being bandied around regarding Galileo there's apparently a few myths starting to float around regarding "claimed" accuracy. There is no difference in accuracy between "Comparative modes" for either GPS or Galileo. The "commercial accuracy" of Galileo is based on encrypted signals, subscription services and access via key in receiver. Really in principle not all that much different to many of the subsription services that's been availabe using GPS for the past 10 years. If one wants accuracy one simply has to pay for the privlidge regardless of it being GPS, Galileo or Glonass. Cheers, Kerry.

Even with good geometry and even over a reasonable period of time (as far as averaging in a practical context is concerned) the position solution is not jumping all around in a random fashion but rather simply wandering/meandering unpredictably along at a generally steady rate. So over the few minutes a person might be averaging for all that period of time the position solution has every possibility of moving further away from the absolute position until a point in time that it tuns around or heads off on a different meandering track. These days (without SA) the position solution has a reasonably steady rate of change over time. Still unpredictable but not random (as such). When Selective Availablility was at its prime a "stationary" receiver would effectively travel (on average) up to 24 kilometres in any 24 hour period but these days is can be as low as 100-200m per hour, which over a few minutes isn't a great distance (on average). This lack of movement over a short period of time also negates any "possible" improvement in the position solution. The following is another example based on what some referred to as "golf ball average". Basically 4 points fixed over a 5 minute period using dual freq post processed coordinates (being true and basically absolute), then the same 4 points located with a handheld over several minutes. Even though there is some slight out of shape movement based on the handheld the general error in position is fairly consistent. The 1m UTM precision stuff is one of those other subjects and if one has ever tried inputing a UTM data set in 1m increments some might have noticed that many units will echo different UTM values (due to internal precision issues) to those entered. Basically input UTM/zone gets converted to Lat/Long then to Cartesian XYZ (for storing) then the whole process is reversed and in the process one doesn't get echoed back what one input. UTM is linked to lat/Long etc and really this 1m UTM isn't any different (accuracy wise) to the Lat/Long it reflects. Cheers, Kerry.

UTM might show figures to whole metres but that's a little different to be accurate to 1 metre. Also in many GPS receivers there is a conversion connection between lat/Long and UTM, with the UTM conversion primarily based on Lat/Long and the underlying coordinate format precision. Effectively both give similar results as far as accuracy is concerned. Cheers, Kerry.

GPS accuracy can be different things to different people depending on the equipment and requirements. Millimetre accuracy (even less) is certainly possible with the right gear used in the right way by the right people but can be dependent on time or distance. With post processing time can be the determining factor and in some cases data can be recorded for hours/days/weeks (even months) to achieve a specific result. With real-time differential then the limitation is distance and the type of differential. Many "different" types of differential also with different accuracy capabilities. For what cachers generally use the Standard Positioning Service (SPS) accuracy is specified at less than 13 metres (~43 feet) 95% of the time, world average. World worst case scenario is spec'd at less than 36 metres (~118 feet) @ 95%. The worst case scenario includes the possible loss of any 2 satellites from a standard 24 satellite constellation. These accuracy figures are Signal In Space (SIS) and don't take into account the possible affect of atmospheric, obstruction or user issues. Then there's also the other unknown 5% Right now the constellation is fairly strong and in general "most" users "probably" get less than 10 metres "most" of the time and probably average less than 7 metres "much" of the time down to less than 5 metres "some" of the time. Cheers, Kerry.

PDOP, have no idea how it's defined or how they might define it in the receiver, but it would probably be tied to the grid point data (maybe?) as transmitted and therefore might be a function of (based on/from) the actual system? Might even look into this one a bit further. There's probably a little more to all this than simply distance as there must also be a certain number of common satellites in view from a number of ground stations (regardless of distance) as well as thr user. Cheers, Kerry.

One needs to differentiate bewteen WAAS reception and WAAS corrections. WASS reception might be good but without valid correction data the whole thing fails. Most recent software upgrades now direct the receiver to ignore received WAAS signals if outside the valid ground footprint as far as being valid correction data is concerned. Cheers, Kerry.

Experimentation now that's the way. It really all comes down to what actually happens in a practical sense. The 50/50 reasoning is simply that, based on actuals and really as GPS accuracy (95% confidence) by definition is based over a 24 hour period then depending on the time frame averaged 50/50 is about the figure especially considering that one doesn't know exactly where one is in the first place. Based on the system spec most days (on average ) will display the following traits in a practical sense, that is if one really knew where one actually was in the first place, which of course is what these results are based on. GPS Averaging - A practical example and GPS Averaging - Part II and GPS Averaging - Affect of obstructions Really to trap ALL errors that might affect the position solution then that's a 30 day exercise and I'm sure nobody has the capacity or time to average 1 second data for 30 days. The other thing that can decieve averaging principles is that mnay receivers might display/output a position every second it can be the same position simply output over several consectutive seconds (sometimes 'can be" up to 5-7 seconds). Averaging principles over enough time will (generally) provide a GPS solution that one would (will) expect to be more reasonable than others. However most users simply don't have the time to adequately do this. Cheers, Kerry.

Averaging for 5 minutes now that SA is discontinued is really not worth the time spent doing it. Averaging can in fact make the position solution worse as there is basically a 50/50 chance of actually getting improved coordinates. Which 50% one might be in, well that's the unknown. GPS accuracy is a statistic and all one can expect is to get the best possible result relevant to the circumstances at the time. At a particular location accuracy will vary throughout the day so there will be times during the day when one should be determining a position (best "possible" soultion) and times that range from not so good to a complete waste of time. It's called planning. Cheers, Kerry.

Inches? in this context is a misconception and incorrect. The Military equiv to the Standard Positioning Service is basically about the same accuracy level. The second freq simply allows for atmospheric corrections to be corrected in real time instead of from a model. In the not too distant future there will also be a civilian second (and third) freq, which will basically bring civilan accuracy down to around 1.5 metres but inches no, not even military without augmentation but then that's a different deal again and something civil users also use anyway. Cheers, Kerry.