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Posts posted by Kerry.

  1. It's not so much the satellite footprints but the relationship of your location to the correction data broadcast.


    POR is well and truely visible almost right accross Australia but doesn't to a dam thing as far as accuracy goes, actually degrades it.


    LINK to POR (PRN134) and LINK to AOR-W (PRN 122) which interactively will show elevation from any location.


    Elevations much below 10 to 12 degrees not really the best.


    Cheers, Kerry.

  2. Frankly some do need to practice what they preach especially those have have some problems in understanding what's basically a simplistic outline. If some really want a story then asking for it in "english" isn't the way to go about it.


    Hindsight is a wonderfull thing but don't change smart arse remarks after the fact and call them toungue in cheek, don't work.


    And if characters like little leatherman here wanst to drag his irrelvance and crap into things then really that does highlight many things.


    Anyway no point in correcting some of the comments as obviously who gives a rats bt at least some had the incentive to look and think for themselves.


    Cheers, Kerry.

  3. Maybe leatherman would like to translate then :blink:, really quite a basic function of the system but if not then go back to reading the toaster instructions, might be more constructive.


    So do you want to be button pushers or try and understand why the signal you now use can be flicked without affecting the other side. Now that hasn't always been possible but leaterman here is going to tell us why :o


    Cheers, Kerry.

  4. "Come in english" :blink: think it would make a difference :o


    Maybe "appliance" langauge might have been better, much like say the way electric toaster or electric tootbrush instructions are worded.


    Maybe some should be simply limited to pushing buttons, some of the "real" answers might be a shock.


    Cheers, Kerry.

  5. Really something that's not new but this time it's being done on a much larger basis and test like this are needed to determine actual outcomes and issues if ever the system was lost or closed down for all the wrong reasons.


    Basically looking at the affects and deriving continency plans if ever things became real.


    One of the reasons why SA was discontinued was that there is now a capability to regionalize a SA type of inaccuracy. Also in the past SA was required to cloud the C/A code as C/A was required to acquire the P/Y code, so simply turning of C/A (depriving civil users) also basically deprived the military signals. As part of discontinuing SA they worked out a way to acquire the P/Y code without even needing the C/A code.


    Cheers, Kerry.

  6. ....as there is no mathmatical formula that converts between the two that is 100% accurate in all locations.


    From Lat/Long to UTM in the same datum is purely a mathematical function and has nothing to do with locations. We are not talking about changes between datums but changes between coordinates within the SAME datum.


    Why is this point apparently getting mixed up with the process between datum's? this is not the case.


    No way to know in advance of the hunt which it is going to be though which is why you need to test to see the typical range of accuracy.


    Not precisely what the actual accuracy might be but certainly one can tell in advance what is best time to achieve the best possible results/accuracy for a given point on a given day at a given time.


    ....but I rather doubt the innaccuracy is being caused by a defective unit.


    Certainly agree with this one, defective unit? very unlikely, User issue? most likely.


    Cheers, Kerry.

  7. Alan, yes with NADCOM that is based on a distrotion model along the lines of NTv2 and as with any transformations (as opposed to a conversion) there will in many cases be different results depending on the underlying method of the trasnformation. If one looks at say Garmin's implementation there is something like 11 different options/selections from WGS84 to NAD27 depending on which part of the country one is situated.


    However converting WGS84 Lat/Long to WGS84 UTM then there is only and can only be one result and any minor issues with CPU floating point limitations (affecting precision) would not account for 20-30 feet.


    I think we need to know just where these UTM coordinates came from as if the way I read the original post one was using a waypoint in Lat/Long and basically sounded like the fix was perfect (which it simply can't be anyway) then changing to UTM it said it was 20-3- feet different.


    Now if the UTM coordinates were acquired separately and all things being equal then the lat/Long and the UTM positions simply aren't the same physical point on the ground.


    Cheers, Kerry.

  8. Until this question This 20-30 feet difference is compared to what? is answered then Precision, Accuracy with respect to the same datum with regards is irrelevant.


    The question is 20-30 feet difference is compared to what? as if one is referring to cache type coordinates then there is no way that any physical conversion from lat/Long to UTM will differ by 20-30 feet.


    Cheers, Kerry.

  9. With only 3 satellites any receiver will have some changable issues as well as limiting a 2D solution. 3 sats absolute minimum and not the best scenario especially considering t he possible geometry.


    It could have been obstructions (buildings, trees other structures etc) if there were other sats available but not in view (obstructed) or maybe a period where the number of Sats available was low.


    Cheers, Kerry.

  10. I have noticed that no one has mentioned that LONG-LAT goes and starts from East to West.


    That's because it doesn't matter and just in case your not aware what occurs in the U.S isn't the norm worldwide, you things things are different elsewhere.


    .... UTM starts West and goes East....
    all part of false origins and to avoid negative coordinates.


    So yes there will be some difference because of the curvature of the Earth and the Point Started from.(IP) Initial Point of Data sets


    There will be absolutely no difference based on mathematical conversions as in the same datum the values that define a single datum simply have to be the same be it Lat/Long or UTM or ECEF XYZ


    Cheers, Kerry

  11. NADCOM as Alan mentioned is a conversion between "different" datums and this is quite different to the conversion from lat/Long to UTM in the same datum.


    Lat/Long to UTM is a mathematical process and there is really only one possible answer (as such).


    .... I when to a site today, 'stood' right on top the cache based on WGS84 then when to the UTM location and the difference was 20-30+ feet.


    Something in that sounds a little incomplete? So where did the UTM coordinates come from? as if they were simply conversions of Lat/Long (as stored/input in the GPS) then the differences would have effectively been the same.


    This 20-30 feet difference is compared to what?


    Cheers, Kerry.

  12. I just went and found my second cache, GPS off by 99 ft. so for the two I've found it has been off 71 and 99 ft..not even close, this thing is under warranty should I contact Meridian??any suggestions??

    Suggestions? (if not already checked and checked again) check your datum and format settings and then check them again (just to make sure :huh: )


    If this consistency exists/continues then it is probably more likely a user/configuration issue.


    Cheers, Kerry.

  13. Edit: As far as WAAS goes, I believe that any satellite from 33 and above is a WAAS satellite. Maybe someone who knows for sure can confirm that.

    Right now from a GPS perpective the maximum number of GPS satellites that the system allows is 32 relative to the PRN numbers (GPS sats are PRN 1 thru 32). Above this different manufacturers call things a little differently.


    What garmin call 35 (and shows as 35) should really be PRN 122, similar with Garmin's 47, which is PRN 134.


    Sometime in the not all that distant future some receivers will require to be able to determine satellites from several (possibly 3 separate systems), which could total all up around 80 odd satellites.


    But right now with a pure GPS receiver (Garmin at least) anything over 32 is a WAAS or EGNOS or MTSAT geostationary satellite.


    Cheers, Kerry.

  14. The first satellite with L2C (as well as L1M and L2M) isn't due for launch now until early 2005 and actually L2C will be a slightly weaker signal than L1C.


    Today should see IIR-12 launched, all things going well since the last postponement, then there will be IIR-13 latter this year and IIR-14/M-1 will be the first of the next generation (includes L2C, L1M & l2M) sats due for launch on Feb 1 , 2005. IIR-15/M-2 will be around April 2005 and IIR-16/M-3 in June 2005 will make 3 but still not really enough to be usefull.


    Receivers will be backward compatible as nothing is being removed just added. Not only does L2C allow atmospheric type corrections in real-time as opposed to a single frequency modelled correction but allows inproved accuracy due to different ephemeris messages, faster bit rates and forward error correction.


    It's doubtfull that with many receivers it will be straight software upgrade unless the manufacturer has pre-planned for dual freq hardware in advance.


    However one satellite isn't going to be enough to make L2C usefull so it will take some years to launch enough IIR-M's and IIF for the new frequency to become really usefull but every new satellite does provide a slight improvement generally.


    The third civil freq (L5C) won't be available on the IIR-M but the following IIF's.


    Cheers, Kerry.

  15. I would be extremely hesitant, in fact I would not even try and upgrade a 3.xx software version with 4.xx unless you can confirm that this is possibble.


    There are 2 specific software upgrades for the GPS12, 3.53 (April 3 1998) for those with existing versions 3.50 to 3.52 and 4.6 (Aug 4 2003) for those already with 4.00 through 4.58.


    Certanly one couldn't upgrade from 2.x to 3.x but 3.x to 4.x I'm not entirely sure but better to be safe (until you can confirm this) otherwise trying this and if it is not compatible then you will have one stuffed GPS12.


    As for the GPS12? then for sure one can update the software (within reason) and as for accuracy no problem either.


    Cheers, Kerry.

  16. Lets just say that there's nothing wrong with the concept but this string has again raised this dangerous functionality that some receivers have built in, that is the capability to navigate (or think/show they are navigating) when there are no signals to navigate with.


    This function (undesiable capability) has probably seen more rescue helicopters sent in the wrong direction than one would like to think. Basically ground crews arrive, require a chopper, whip out the GPS, turn it on and basically realy coords on the screen as there can in some instances be no indication that the unit is actually navigating, acquired or not.


    Really this little issue with some receivers should be plugged. Might not suit some "concepts" but a receiver that doesn't have a signal really shouldn't show a position at all as it can be extremely misleading and many resuce pilots these days are aware of this.


    Cheers, Kerry.

  17. The GOTO thoughts under water are interesting as any GPS once nav is lost should not show any navigation type information (or capabilities at all) as basically the no nav data then any other functions should be disallowed so as not to cause irrelevant and generally incorrect output.


    A GPS carried at depth will not receive a signal and upon deploying at depth, the diver will not have any idea as to whether the GPS has acquired a signal, how accurate the signal is, or how long the waypoint was averaged. That is a very relevant comment as any receiver that pops the surface won't get an immediate position, how long will it take? well that will/might vary, at times.


    Cheers, Kerry.

  18. It's not the receiver but the base maps etc that will not be of any use.


    Also any receiver used in Oz must have WAAS disabled as the system simply doesn't work there, so even if the software knows this there's really not much point having WAAS enabled in the first place.


    Cheers, Kerry.

  19. Under powered? wouldn't call them underpowered (as such), they do what they are supposed to do but with most things there is a compromise and trade-off. Simply blasting out heaps of power would create all sorts of other problems.


    The GPsIIR-M's in fact will have some increase in transmit power but this has required new design heat dissipation systems. With the IIR-M's and probably anything following the power is now re-allocatable between the different codes.


    Cheers, Kerry.

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