Kerry.

quote:Originally posted by graveseeker:.... That raises the question as to whether GPSRs can, in fact, be calibrated (more accurately - corrected)? If we do some benchmarking, will we find a consistent N/S and E/W error that we can reliably crank in for correction? For example, if we search for 29 55.9371N by 95 55.823W, could we reliably correct to 29 55.9371N by 95 55.820W (given that +1 and -2 was our predetermined correction factors)? NO, calibration in pure GPS accuracy terms isn't an option. Some of the older units had what was termed a PCF (Position Correction Factor) but that was more to do with correcting for datum issues as they had no datum option (permanent WGS84). Cheers, Kerry. I never get lost everybody keeps telling me where to go

quote:Originally posted by Doug Missy Mickey P:.... 2d and 3d fixes have nothing to do with the number of satellites being tracked... You NEED 4 to get a fix because of the clock/pseudo-range problems mentioned above. 2d or 3d (and your GPS's Estimated Position Error (EPE) or Figure Of Merit (FOM)) are determied by the geometry of the satellites (where are they in the sky). For example, if 2 satellites are very close together, the angle between them is very small and the resulting fix is therefore not very accurate. But accuracy has nothing really to do with the PRINCIPLE of a 2D or 3D fix. You have all the pseudo-range just about right but when the height, altitude, Z can not be determined (not enough sats) it is supplied (as in a 2D fix) then there's only 3 pseudo-ranges allowing 3 unknowns to solve, which are Position (XY) & Time. A 2 dimensional fix has no height as part of the calc (it is assumed) where as a 3D fix has, that's why there can be a 2D and a 3D fix, however these days the 3D fix is generally the norm. 2D or 3D isn't really an issue these days but back during the Gulf war there were only 16 satellites in orbit which provided between 19 to 20 hours of "two-dimensional" (latitude and Longitude) coverage with "three" satellites in view and 15 hours in all of "three dimensional" (including altitude) coverage with four satellites in view. Outside that 2D 3 satellite coverage (~ 4 hours per day) there was simply no GPS capability. 2D and 3D has everything to do with the number of satellites being tracked. Except with a receiver that can manually be locked in 2D mode, which then can in fact be even more accurate than a 3D fix especially if the altiude provided is accurately known. Cheers, Kerry. I never get lost everybody keeps telling me where to go

Assuming you are referring to decimal minutes No, not many can display to 4 decimal minutes but many are now able to output 4 decimal minutes to other software packages. Technically more precise due to the extra significant decimal but not really any more accurate as the accuracy is a function of the overlying system, SPS accuracy specifciations and the limitations of the general recreational receiver. Cheers, Kerry. I never get lost everybody keeps telling me where to go

quote:Originally posted by writer:It takes the fourth satellite to get the additional equation to solve the additional unknown of timing. I believe you are trying to say timing is only available from the fourth but there are some extremely fundamental mathematical principles involved here and the over simplified explanations by some sites are obviously a little confusing. The easiest way to come to grips with this is start with a 3 dimensional fix and most understand that 4 satellites are required to solve for the 4 unknowns. Those unknowns are XYZ & Time with "time" more to do with “timing” as opposed to pure "time". That equation solution could also be solved if ONE of the “unknowns” was supplied/provided and that then requires only 3 values to be solved for and that requires a minimum of 3 equations (hence 3 satellites). In the real world the only value that can really be provided is the Z and this can be provided by the user or the receiver (last occurrence in some instances) and this constitutes a GPS 2D fix in which the values to be solved are XY & Time (Z is provided). To suggest a GPS receiver doesn’t solve for time in a 2D position fix situation (when only 3 satellites are visible) is really showing some misunderstanding in what actually occurs. Just about every receiver made will enter into a 2D position fix with 3 satellites and the assumed value used in the computation is the Z. In the case where the receiver is moving/changing in the vertical (Z) then the position solution can be suspect (very). In this auto type 2D mode some receivers will allow a user defined altitude, where as other receivers can specifically be locked in a 2D mode (with a user provided altitude) and this was used in many marine receiver applications in the past (even the present). These days a 2D fix is not a common occurrence (until specifically configured in receivers with this option) especially considering the state of the constellation. However the possibility certainly exists under differing conditions and receivers allow that 2D option and Time is one of the unknowns that is calculated in a 2D fix, that is for sure. Cheers, Kerry. I never get lost everybody keeps telling me where to go

quote:Originally posted by welch:If a "standard" system is based on 24, then why is the system the Europeans are setting up going to be based on like 30 birds? Will that system have lots of areas of "overlap"? (simple answer please ) Galileo is a 27 satellite (plus 3 active spares) system in a 3 plane 14 hour orbit (10 satellites in each plane), is a simple system to launch, maintain and more suitable for high latitude areas. The aim is still to maintain a minimum 4 satellite world coverage. GPS is designed as a 24 satellite (plus 3 spares) in a 6 plane, approx 12 hour orbit with a total capacity of 32 satellites. Glonass is different again in design and best suits high latitudes (> 50 degrees). Cheers, Kerry. I never get lost everybody keeps telling me where to go

Writer, actually not quite, either. Timing is one of the main components of any fix, 2D or 3D. 3 satellites can solve for the 3 unknowns XY & Time (2d fix), 4 satellites can solve XYZ & Time (3d fix). So in effect the 4th sat gives altitude as Timing is always part of the position solution. It's this altitude that improves the integrity of the position solution as the height has to be assumed (last height available or user input) in a 2D fix as part of the calcs. Cheers, Kerry. I never get lost everybody keeps telling me where to go

To define things properly there's a fair range of variables to consider but in simple terms the standard specs are based on a 24 constellation and the worst case scenario is based on 22 satellites. There's currently 26, down from the long time over populated 28 satellite constellation that has spoilt some users. Next GPS launch is scheduled for Jan 29, 2003 (GPS II-R8) As one can imagine which 2 sats (there's 276 different possible permutations) will have a differing effect, which is part of the control system's responsibility in meeting the defined specs. In some cases removing 2 satellites can have zero effect where as in other cases even removing 1 satellite can be a problem. Even if one can see 4 satellites doesn't mean they are useful as the PDOP (and other things) might be totally off the scale and much more has to be taken into account then just the number of sats. The systems spec's are based on parameters that some receivers might meet or exceed so just because the system provides a certain level of service the user might/might not be able to take full advantage of that service. Cheers, Kerry. I never get lost everybody keeps telling me where to go

Being a Magellan it could be (most likely) related to auto averaging. At a slow pace and with auto averaging kicked in (even though you are still moving) the averaged position will be behind you, then when you do stop (full stop proper) it will gradually catch up to where you really are and be telling you to go back. Auto averaging silly silly idea. Cheers, Kerry. I never get lost everybody keeps telling me where to go

Might find some "relevent" information in these links from Peter Bennet. http://vancouver-webpages.com/peter/gpsant.txt http://vancouver-webpages.com/peter/extant.txt http://vancouver-webpages.com/peter/extant2.txt http://vancouver-webpages.com/peter/gpsptch.txt Downlaod the following ZIP file http://vancouver-webpages.com/peter/gpspatch.zip Cheers, Kerry. I never get lost everybody keeps telling me where to go

The 315 is not WAAS capable. The GPS satellites fit in the PRN# 1 to 32 slots and what is generally displayed on a receiver. These PRN#'s are (but can be the same #'s) generally different to the Satellite Vehicle Number (SVN) and the first of the "extra" satellites (such as WAAS and other systems) start at PRN#33. Cheers, Kerry. I never get lost everybody keeps telling me where to go

Just as well these were "casual" tests wouldn't like to see anybody take them all that seriously. Cheers, Kerry. I never get lost everybody keeps telling me where to go

One of the Continuously Operating Reference stations which forms part of the Australian Regional Geodetic Network. This is just the station mark/antenna. Most of these sites are secure and really not the type of thing one really should be playing with. everybody keeps telling me where to go

Now which way is south and what's the time zone that's an interesting one. Cheers, Kerry. I never get lost everybody keeps telling me where to go

Forgetting about some technical (security) type issues the Standard Positioning Service (SPS) specifications are based on several definitions. One of these is the definition of the "Service Volume", which says the SPS performance supports the terrestrial service volume. The terrestrial service volume covers from the surface of the Earth up to an altitude of 3,000km. So the GPS specifications used on the earth's surface for accuracy, PDOP, reliability, availability, User Range Errors etc are the same out to 3000km. Cheers, Kerry. I never get lost everybody keeps telling me where to go

It's not a theory as GPS as been used in the shuttle for quite some time in conjunction with some "older" type navigation systems. GPS will basically totally replace these older nav systems (for some applications) as these older nav systems are phased out. Cheers, Kerry. I never get lost everybody keeps telling me where to go

Generally averaging in the time available is a waste of time even to the point of being less accurate. One generally spends time averaging without knowing if the result will be better or worse. Basically a 24 hour average has some meaning but is not practical as it traps 95% of the possible errors. 99.99% of all errors is a 30 day average issue. None of these options is practical. Of more importance is the state/strength of the constellation at the time and that is basically more important than averaging, being at a certain location at the best possible time. Cheers, Kerry. I never get lost everybody keeps telling me where to go

The 12XL is getting on in it's life and probably had its last software upgrade in Sept, 2000. Before you make a decision just compare that price with some of the more recent low end models, which for the extra $$'s might/can/does have some advantages. Cheers, Kerry. I never get lost everybody keeps telling me where to go

quote:Originally posted by gurubob:Leica is the best... Have you ever heard of a little navigation company out of Sunny Vale California? Trimble is in my opinion the finest GPS survey equipment maker to walk the face of the earth. /quote] I wouldn't call that an entirely a subjective statement that one. No, I wouldn't call handheld consumer models "very" accurate, not in the overall context anyway. In general it is very close (actually already here) to the time where different manufacturers have had to rationalize there approach and thinking and realize that users of high end receivers want the capability of using whatever receiver for whatever purpose without having to be stuck in the "manufacturers" straight and narrow rut. No longer is it acceptable to be only able to use brand X with brand X especially in a real-time situation as large scale projects have a range of contractors and there's no reason why they have to conform with brand X. Cheers, Kerry. I never get lost everybody keeps telling me where to go

Stunod, the list is probably endless but I'm not all that sure it was really one of those "Military secrets" (as such). The GPS sats (the signals) are very effective and used extensively in weather forecasting (still a new science) and of course the gamma ray detectors on each vehicle effectively reverse engineer the position capabilities by being able to position nuclear detonations almost as they occur. Trailblazer won't like that one Basically GPS became a public utility (the world's first and only global utility) as a result of the shooting down of KAL-007 in Sept 1983. GPS wasn't even 3 years old (with only 7 Block I sats up as part of the initial proving system. Even the implementation of SA didn't come for some time (many years) after this. Unfortuenately I probably won't get to see the program, pity. Cheers, Kerry. I never get lost everybody keeps telling me where to go

Honestly you appear to be totally confused and really at the survey boundary level you should really talk with someone (like a local surveyor) US orientated with regards the way surveys were done and how things have changed over the years, and things have changed both in theory and physically. The "deflection of the vertical" being "7 odd degrees" is totally off this planet. It would appear you are confusing Deflection of the Vertical with probably what we call over here County Arbituary Merdians (CAM Bearings). How it was all implemented in the US I'm not totally up with (and have no real background) but I do know there was an initial meridian adopted based on Washington (passing through the centre of some dome at 24th street and constitution avenue?) and all meridian boundaries were based on that. They were also based on astronomical observations (astronomical parallels) and because of the lower order of accuracy of instrumentation in those early many State lines are not on a true meridian or parrallel. Astronomical latitude and longitude is different to Geodetic lat & long. That difference is basically the Deflection of the Vertical and accross the United States it ranges from about +41 to -38 arcseconds (North/South) and +45 to -43 arcseconds (East/West). You don't mention your approx geodetic location but the deflection of the vertical at say 35N, 75W is -15.83" & 14.96" (based on DEFLEC99) There is absolutely no way it could be anywhere near 7 degrees. It appears you are missing or interpreting something incorrect in all of this? Cheers, Kerry. I never get lost everybody keeps telling me where to go

quote:Originally posted by Trailblazer # 1:.... the once secret tool. Like to expand on your issues (and all this secret society stuff) with GPS. Cheers, Kerry. I never get lost everybody keeps telling me where to go

I'm yet to really understand where your coming from and where your going to (or want to get to). So keeping things simple just explain the following bit, that appears to be missing something. quote:But the Triangulation Station for this is in the front yard yet the same.Refence survey 1906. Then simply explain, what it is you've come across and what is this theoretical question, without all the unnecessary things that really appear to have no relevence. quote:Im trying to do my best to present what I have come across in these fields to try and explain (define) my theoretical question Cheers, Kerry. I never get lost everybody keeps telling me where to go

The "effect" of having WAAS turned ON outside the ground station network. everybody keeps telling me where to go

Distortion models are really the only (practical/accurate) way for this type of conversion as per Alan's NADCOM link. Cheers, Kerry. I never get lost everybody keeps telling me where to go

quote:Originally posted by CCrew:Higher resolution of your position. Provided you can see one of the birds :-) No, not only must one be able to see the WAAS sats but you MUST also be within the WAAS ground network stations (apart from other conditions). Outside that ground network those WAAS sats can be seen from many parts of the world and all the WAAS signal does (outside the ground stations) is decrease accuarcy (big time). Cheers, Kerry. I never get lost everybody keeps telling me where to go