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Everything posted by Kerry.

  1. quote:Originally posted by Cachier:.... The more birds, the better the precision. No Cheers, Kerry. I never get lost everybody keeps telling me where to go
  2. Being a regional SBAS system EGNOS will cover Europe (including the UK) but is currently only provided for testing purposes and not expected to become operational until at least 2004. Wouldn't put to much reliance on it at this point in time. Cheers, Kerry. I never get lost everybody keeps telling me where to go
  3. An hour is certainly enough for things to go from real good to real bad and that's without the effect of obstructions etc. It's a possibility that simply based on the direction of travel and holding the receiver in front of the body could cause the whole satellite reception pattern to be diffderent. It's like facing south (towards the equator) in the northern hemishpere as there's generally (depending on ones latitude to some extent) really no satellites to the north that the body can obstruct but facing north then that's a different situation all together. Can be very dynamic but if you've got a specific date, time and location then it's not difficult to see what what was going on. Obstructions now they're important as well. Cheers, Kerry. I never get lost everybody keeps telling me where to go
  4. quote:Originally posted by Anders:.... Perhaps one can figure out something about this strategy by observing how the unit omits satellites, when WAAS is enabled. If there already are twelve satellites receivable at that instance, then two of them has to go, to leave room for the augmenting sats. With WAAS or Differential that's not a strategy decision as a receiver simply can't use a differential correction if the satellite is not common at BOTH the reference site and the receiver. Differential (dGPS) only transmit corrections for the best 9 satellites if the receiver is in dGPS mode then the decision is easy, if it's not in view then it won't/can't be used. Cheers, Kerry. I never get lost everybody keeps telling me where to go
  5. As I understand it signal strength is used in conjunction with other data like elevation, DOP etc to "decide" just what sats it "should" best use if/when there's a choice. Basically it should use the best geometry from the strongest satellites as the receiver generally doesn't know what's the cause of the weak signal (it can't see). A 12 channel reciever probably doesn't rely on things as much as a say a 5 or 8 channel reciever but the underlying decision principle is probably much the same and also based on other data that's probably not as obvious as signal strength, DOP etc. Lock appears to pick on the easy ones first, starts navigating then sorts out what to do with the rest in decreasing priority relative to configuration parameters (like mask angles), number of channels etc. Cheers, Kerry. I never get lost everybody keeps telling me where to go
  6. Kerry.

    Degrees ?

    WWW, your missing some basic points with coordinate systems (better called formats) in that they are all direct mathematical transformations/conversions based on the underlying cartesian cordinate system, which GPS is based on. Once one starts talking distances then the same "type" of distance (all distance types aren't the same) will be no different based on the same physical locations regardless of the method used to derive that distance (which needs to be specified). The relationship between plane, spheroidal type distances etc are well defined. Cheers, Kerry. I never get lost everybody keeps telling me where to go
  7. Out of the box it will take some minutes to properly start up. It will take a minimum of 12.5 minutes to first download a complete almanac then normal startup will be in that 15-45 second time frame (unless one has moved several hunderd miles with the unit off). That 15-45 seconds will be about "normal" (give or take depending) when the unit has been switched for a period like a few hours +, overnight etc. Startup time will normally be shorter than that if say the unit is switched off then back on within a few minutes. 5 minutes all the time then it has got a problem. Cheers, Kerry. I never get lost everybody keeps telling me where to go
  8. There's not even a remote chance that you've had a constant 12 satellites shown, locked or otherwise. On average 6-9/10 is the norm with 12 (even more if one had more than 12 channels) being possible at times but never always. Cheers, Kerry. I never get lost everybody keeps telling me where to go
  9. The case will not affect reception but being on your belt close to the body certainly will. Cheers, Kerry. I never get lost everybody keeps telling me where to go
  10. Certainly 121.5/243mHz Epirb's have there share of problems with false alerts and it's probably more than 99% but then there's also the 12 oddd thousand people that can directly attrib their existence to the system 121.5 or not. But at least fixing this false alert issue is in progress and hopefully 406's will/might/could solve "most" of the probelm (if registered). Registration is one of the strong points with 406's if it's made a condition when purchasing. Cheers, Kerry. I never get lost everybody keeps telling me where to go
  11. NO, nothing to do with clouds. Being a dynamic system and depending on the time between time in and time out those sats that were there on the way in ain't going to be there (exactly) on the way out. Cheers, Kerry. I never get lost everybody keeps telling me where to go
  12. Bit behind the 8 ball when compared to the rest of the world. Good to see some decisions which should have been made years ago. Cheers, Kerry. I never get lost everybody keeps telling me where to go
  13. quote:Originally posted by Pusher of Clay!:.... I mistakenly thought these units were fair accurate, but up the 30ft off target is a joke for serious off road tracking, or skiing in a white out. ...... Depends on what you think "fairly Accurate" is as up to 30 feet (even 40 feet) is well within the specifications of the system. As with WAAS, EGNOS is not all the answer either and still has some way to go to be fully operation as well. So not real sure what you were "expecting" but sounds like it might be an over expectation. Basically regardless of the consumer grade receiver at this level one still needs to understand and work within the specifications of the system. Cheers, Kerry. I never get lost everybody keeps telling me where to go
  14. Mission Planning Software Really not entirely relevant to the type of equipment or purpose used here but this type of software might help in understanding the relationship between number of satellites, satellite geometry, DOP and to some extent accuracy and a few other things. Probably useful for knowing when not to be at a specific location more so than when one should be there as minimal conditions are generally in the minority these days but probably the most important times to avoid if one wants reliable confident results. In other words when the number of Sats are low and the DOP goes of the top of the graph one really doesn’t want to be expecting good results. Actually there’s more chance of reasonable results when the DOP is low regardless of the number of Sats. Click on the following URL and under Mission Planning click on MP_WIN, which should go to a FTP site and then down load the file MP_WIN.ZIP which is the main software (compressed). http://www.thales-geosolutions.com/skyfix/gpsinfo.html As well as this software one will need to regularly maintain an updated almanac file which can also be downloaded from the same page. Just note the link on this page appears to have an error so use the following FTP link and copy the ALMYY.DOY file (YY=year and this year will be 02, DOY will be something like .290 or whatever the Day Of Year was the almanac was created on) to the directory where the software is installed. If there’s been no system changes then predictions can be made into the immediate future without problems if the almanac is reasonably up to date. ftp://ftp.ashtech.com/almanacs/ The software contains a default list of location files but users can create their own specific locations, which only have to be approximate anyway. Specific settings which will affect the output is masking angle (not all receivers have the same settings) and if necessary obstructions can be added which can change the whole scenario. Setting a mask angle around 5 (or 10 degrees) is best anyway as that probably gives a better indication of real (minimum) conditions anyway. Cheers, Kerry. I never get lost everybody keeps telling me where to go
  15. Alan, different planets yeah some see it like that but we're all using the same "global" system and the world's very first true global utility . As for the DOP check at the following post (I've yet to track down what I'm looking for) and hopefully the software will be useful in helping this discussion, especially with regards the relationship between DOP, satellites and accuracy (to a point). One must remember that simply having X number of satellites in view doesn't dictate what the accuracy will be (totally). Hopefully this software will help compare these things realizing that the geometry of the satellites fixes the DOP value and that DOP value has a major influence (but not all influence) on the expected accuracy of the position. Cheers, Kerry. I never get lost everybody keeps telling me where to go
  16. Trippy1976, I think you've realised that time of day matters and it does as the satellites are in roughly 12 hour geocentric orbits and yes between 8.30am and 11.30 things will certainly have changed, sometimes for the better sometimes for the worse but always generally with the system specifications (95% of the time of course). The following link attempts to outline exactly your thinking/observation http://www.cqnet.com.au/~user/mattk/gps/gps_plan.htm Basically if nothing changes in the system then things should be "similar" from one day to the next however this doesn't take into account other things (like atmospherics etc) that also can change over time. Synchronous orbit? no not really as even if the system isn't touched things will change from day to day. However on most days maintainence is carried out on individual satellites so these are unavailable, which for that period they are unavailable the geometry, accuracy and other things are affected. Sometimes these shutdown warnings are made in advance and can be planned around where as other times one doesn't know a satellite is/was bad until after the event. If the receiever didn't pick up this problem sat (and most stand-alone units can't) then that problem gets used in the position solution. The 95% accuracy of the last graph (Doy 1153 June 2, 2001) was actually 19 feet, which isn't bad at all. The one prior to that (Oct 4/5) was 25 feet and 31 feet, which is still well inside the system spec of 42' @95% SIS. As for 20' well that's a bit more than a few feet (actually a bit more than a few metres) but this general discussion on accuracy focused on the claim (assumption) of 6-8 feet "all the time", "everywhere", which is certainly a myth. Cheers, Kerry. I never get lost everybody keeps telling me where to go
  17. Alan, actually it's the night after the (your) morning before so your just getting into Thursday and here Thursday has just about gone. Things can get more persausive but am I getting any where that's the pertinent question. There's this one thing your missing I don't work in "belief's" or magic or smoke or mirrors and until you have a way of getting the background about some the magic then that's going to make any conclusion to you most difficult. since you've been willing to part with your cash why not find a local charity, I'm sure they'll appreciate it (you can tell them I sent you if you like ) Goodnight. Cheers, Kerry. I never get lost everybody keeps telling me where to go
  18. Seeing you have this fascination with estimated error being the ultimate accuracy indicator maybe you'll see some rather obvious things in this plot. Won't bother with any meaningful numbers as you don't (don't want to ) appear to understand what they mean or imply anyway. So the blue line is the "actual" error relative to the known point and believe me (but that's been a problem so far) this point is extremely well tied down . The Green line now that's the "actual" Error position (maybe read this point again Alan, the "actual" error position) as gets displayed on the screen, you know the one you accept so blindly as being the indicator of when a position is good or not so good. Notice I've left your "less than a few feet line" (the red one) for your reference Now remembering this is "actual" data both the actual position error AND the Estimated error would you like to explain why between 0400 and 0500 as the position error improves the estimated error (the green one remember) goes up . Also a bit strange (don't you think ) that when the "actual" position uncertainly (error) does actually increase the estimated doesn't follow it accordingly everybody keeps telling me where to go
  19. No bluffing tactics here, just up front with your facts, it's that easy. Ok lets explain what this is all about (again ). October 4, 2002 (the blue lines), October 5, 2002 (the green lines), same site, same receiver, 24 hours 1 second data. 95% accuracy 7.7m (25') and 9.4m (31') respectively. There's an 11% and 14% chance respectively of at some time throughout the day seeing a position (for real not imagined) within a "few feet". That translates into approx 2.5 hours on the 4th and 3.4 hours on the 5th out of 24 hours. Would you call that "consistently" less than a few feet and lets try and be truthful about this (for a change). everybody keeps telling me where to go
  20. No Aln I'm talking and your not listening (too anybody). Yeah, it's funny that all tests are the same, doesn't that spell something out to you or are you just going to ignore that fact. They are generally all the same 'cause that's simply the wau it is. And I'll say it again a person without hindsight DOES NOT KNOW what the real reliability of the signal/position is becasue they have no idea where they really are i the first place. The error circle are simply ESTIMATES nothing else, nothing absolute. Lets hope you don't start confusing "Accuracy" with "Precision" but I'm afraid I fell some further confusion (and mis-understanding) coming on. Totally totally and utterly total different to your target shooting. Some users will convince themselves of anything but when one understands what it's all about and even the simple "facts" that there can be 3m (~10 feet) error due to clock stability, 4 odd metres (~13 feet) in the Ephemeris prediction error, between 5 and 7 metres (~16 to 23 feet) due to atmospheric (ionospheric delays etc) plus a few other minor errors then to come up with a statement that GPS accuracy is a "few feet" consistently is totally and utterly rubbish. The answers are all above Alan, the "facts" are as above so you will obviously choose to believe what ever you like. Your obviously going to convince yourself of anything but then so did a few users (now deceased) who also ignored what the real situation is/was all about. However you keep up this "few feet" myth garbage and I'll cut you off at every turn because your giving users a totally wrong and very inapropraite (and to some extent dangerous ) impression of accuracy. You've made some rather ridiculous unsubstaniated claims/statements and raved on about every thing lacking "scientific tests" (how convienient). Time for you to substantiate your claims with some of your own so called "scientific proof" . Cheers, Kerry. I never get lost everybody keeps telling me where to go
  21. Alan, everybody is welcome to their viewpoint but if you want "tests" then it's tests you'll get. Honestly don't have enough web space to have all on line but this should be a start. If you want more "tests" then not a problem. Ok the 5 channel v 12 channel myth based post and pre Selective Availability. Note in the pre SA accuracy figures your claimed "consistent few feet" is also a myth. Oh by the way ALL the following are supposedly "1 second" data and I say "supposedly 1 second" data because even though just about every manufacturer spec's 1 second output that tends to be a bit of a myth in many cases. That's easily "proved" and is fact not fiction. http://www.cqnet.com.au/~user/aitken/gps/gps_5v12.htm Some "facts" on averaging but more importantly in this case ALL 1 second data and don't see any accuracy that "consistently a few feet" http://www.cqnet.com.au/~user/aitken/gps/gps_avg.htm http://www.cqnet.com.au/~user/aitken/gps/gps_obs.htm http://www.cqnet.com.au/~user/aitken/gps/avg_pii.htm Some "real world" data at the time Selective Availability was set to zero and don't let anybody tell you it's "switched off" because technically it's not, just set to zero. Oh all "1 second" data and very "real world" and still don't see accuracy consistent "to a few feet", do you? By the way the 95% accuracy in this case prior to SA being flicked was 47.2m (~155 feet) display EPE was 14m (~46 feet). With SA off 95% accuracy 6.3m (~21 feet) and EPE 7m (~23). Who was kidding who? http://www.cqnet.com.au/~user/aitken/gps/sa_off.htm Another "real world" parallel example of handheld positions compared to the "real position". A bit over 5 metres (17.1 feet) but notice the relative consistency in the "accuracy error". http://www.cqnet.com.au/~user/aitken/gps/1085_1.jpg Another "real world" example highlighting how even the antenna orientation can affect accuracy. More importantly ALL "1 second" data and again very "real world". What's even more interesting with this one is this handheld can actually be recorded to 0.0001' where as most still can't get past the 0.001' barrier which does make the "few feet" rather meaningless with some units since the best a 0.001' unit can technically display with condifence is about 2.6m (~9 feet). http://www.cqnet.com.au/~user/mattk/gps/1104_ant.htm Even more accuracy comparisons on sequential days. Oh the position accuracy scale is 0-20 metres (0 - 65 feet), each horizontal line spans 5 metres (~16 feet). One might notice the importance of the dates and the interest in "actual" accuracy on this day. http://www.cqnet.com.au/~user/mattk/gps/pa_1091.htm Well Alan I don't know about your viewpoints but your starting to sound like a salesperson and some of them are very unscientific. Oh by the way when dealing with accuracy there isn't a great deal of "scientific" difference between supposedly 1 second updates and 30 second updates espeially with SA off. Well Alan it's your call, is that enough "real world tests" or do you need more . More is not a problem and if it takes more to kick this "consistent few feet accuracy" garbage (and garbage is exactly what it is) then so be it. Cheers, Kerry. I never get lost everybody keeps telling me where to go
  22. Anders, at least the DOP's have some underlying common standard that actually relates to the system. Now if every manufacturer displayed some of the DOP values (just the main one or two as there's quite a few of them) then at least those numbers are comparable between different units and also at least the formula's don't change with every second software release. Really the DOP means more than the number of satellites as a low DOP with say only 4 sats is generally a stronger solution than a high DOP from say 8 sats. DOP values are certainly incorporated as "part" of the error estimates. Cheers, Kerry. I never get lost everybody keeps telling me where to go
  23. No, not quite as accuracies might/can/will vary from location to location and to some extent vary during different times of the day. For the same time of day different locations will have different conditions, have different satellites in view, different satellite geometry, will have different atmospheric conditions and a few other differences (that can make a difference). Not all the satellites are equal either and as more of the older sats are replaced with better technology and timing capability things are getting better and better with every new launch. Some of the existing sats are well past their designed life but still going strong but with some being more than 10 years old that is old in satellite technology terms. The 24 hour type graphs are simply based on the computed positions same as what any handheld is producing and using to compute the things it computes. One thing with error cirles, EPE's or whatever is that they are just that an "estimate" as the unit has nothing to compare the positions it's computing to anything absolute. They are an estimate and probably more effective in showing the "possibility" of the position improving relatively over a period of time and not in an absolute sense. Without knowing how error estimates are exactly calculated (and no manufacturer is prepared to outline the methods used for propriety reasons) makes it rather difficult to know what the figures "exactly" mean. However since error estimate routines obviously get changed from time to time it's really appears more a case of massaging the output to suit the current system. If there was any real science in the error estimates then they wouldn't need to change them and there would be a direct comparison regardless of the software version or manufacturer, but there's not. Confidence in an accurate reading (or the best possible accuracy at the time) requires other considerations than simply the estimated error the unit displays. When Selective Availability was set to zero just about all manufacturers were caught out fiddling the error estimates as without SA the real position improved by a general factor of 7 and yet the estimated error only showed reduction of half so obviously somebody was being conned back then. Generally this assumption that accuracies are consistently within feet is a bit of a myth. If you take the position when the error estimate is at a minimum (if one looks at it for long enough) then one is probably best to assume that will be the best position available (at the time). Still no guarantee just how good the position is (in an absolute sense) but "should" be the best at the time. For sure there will be times when the "actual" position is with a few feet but that % will be relatively low, similar with 12 channel receivers, the % of the time there is actually 12 satellites to use is also reasonably low compared to the general average of around 7 to 9 the "majority" of the time. On a world wide average system accuracy is stated (guaranteed ) to be less than 13m 95% of the time and less than 36m 95% of the time on a worse case scenario. Those figures are also Signal-In-Space as the custodians of the system don't have any control over what users do on the ground. They spec what they have direct control over. Even averaging doesn't do all that much (especially these days) as in some cases averaging can effectively decrease the accuracy, not improve it over different time periods. The issue with the scales anology is that with GPS SPS positions one really doesn't know what to throw out, what is right and what is wrong as there's nothing to absolutely compare the data to. All data must be taken into account, which is what the estimated error is based on anyway, if it goes up the position is presumeably getting worse and visa versa. It will fluctuate more so than actually settle. Human interpolation works well in hindsight but at the time the human doesn't really know. I consider EPE's etc more of a relative indication "tool" more so than any absolute indication of accuracy. Cheers, Kerry. I never get lost everybody keeps telling me where to go [This message was edited by Kerry on October 15, 2002 at 11:12 PM.]
  24. a graphic now there's an idea (and a quick one at that). The following is the plot of position distances every 30 seconds (over 24 hours) relative to the average position for that location on that day. The distances are metres (not feet) and based on actual recorded real world data (not opinions) . Same day, same times through the day (every 30 seconds), same GPS system, different locations AND diffferent accuracy (very). Cheers, Kerry. I never get lost everybody keeps telling me where to go
  25. No Alan I'm talking about every day conditions on any day, in the real (and the rest of the) world. If you've forgotten the figures were in response to your following comments as to why the accuracy won't be the same elsewhere. If you think GPS accuracy is 6-12 feet "everywhere" then your blindly mistaken. quote:quote: -------------------------------------------------------------------------------- Just because there was some consistency and that's what it is consistency doesn't mean that if one was to take all those receivers somewhere else that the results would be the same. -------------------------------------------------------------------------------- How do you it won't be as accurate? MY test and trippy's test showed a majority of accuracy readings under 12 feet in two different places. What tests have been done to show this kind of accuracy is not consistant in other areas? Alan Those figures relate to the real world and I'm not all that sure if there's many ways I can say that, the real world on any day, they are real data recorded just as if you were there. I'm not really sure if you understand what accuracy is all about and how accuracy will be different at different times, in different locations for many and varied reasons. Lets try and explain that those accuracy figures are based on real data at those specific locations on Oct 7, 2002 just as if you'd been there yourself with your trusty handheld. On that day at those locations that's the sort of accuracy you would have experienced, recorded, observed or whatever you want to refer to it as. Those figures are not opinions they are real world data and one simply can't get any more factual than that. Those "facts" say that on that day in Kingston one probably would have noted a "consistent" accuracy as the grouping for that location over the time period was very tight (1.9m RMS) but that is entirely different if the same group were standing in Manama as there's simply no way the accuracy would have been the same. There's about 500+ stations around the world that have been/are recording "factual" everyday GPS data 24 hours a day / 7 days a week that would eat your $100 bet without even blinking. Cheers, Kerry. I never get lost everybody keeps telling me where to go
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