Kerry.

Looking at the age of what's currently up there, 2 Block II's, 16 Block IIA's all more than 7.5 years old (oldest June 1989), 2 Block IIA's just under 7.5 years old and 8 Block IIR's (oldest one of which is 6+ years) plus the 1 yet to be brought on line, that makes 28+1 "a half dozen more" with room for only 32 (actually 30), won't help ya much at all. PRN11 these days is a Block IIR (IIR-3) launched Oct 99 Cheers, Kerry.

Compared to the existing Block II's & IIA's, the oldest of which is now around 13 odd years, the IIR's are better "technology", better clocks, better everything and improved timing (new Rb standard) does translate into improved accuracy. Even though the system spec is a constellation of 24 + 3 spares a GPS satellite's life span is not increased by being "off-line" so all are available but if the odd one fails then the redunancy reduces towards what the specifications are actually framed around. With every new launch of a IIR the system is improved in many ways, design life of a IIR is 10 years, the current II/IIA's is 7.5 years, yet many are way past this mile stone so many of these are in affect past there use by date. With this one makes 9 IIR's (IIR-1 was destroyed during launch in Jan 1997) and with every new IIR the system becomes that little bit more up to date. IIR-14 will be the start of something completely different, starting maybe sometime next year (at this rate). Cheers, Kerry.

"best", well best of luck. Not sure what you refer to as "3rd party software" as in this area it's not one of those favourite manufacturer things to do, to allow software/mapping in the receiver itself other than what the manufacturer dictates. Cheers, Kerry.

It is outdated but still certainly good enough. Like all GPS receivers it will/might/could have issues under trees but don't all rapped up in this thinking that because a receiver has more channels it's more accurate, more satellites don't necessarily mean better accuracy. Same precision display capability as any other recreational out there today and really the pure accuracy difference between this unit and 12 channel units can be a bit academic. Cheers, Kerry.

IIR-10 had been slated back in June for an early October launch then it got moved to late Oct then further delayed and still not off the deck. Actually GPSIIR-11 was originally set for Dec 19 launch and now being put further behind probably makes the first IIR-M launch (originally set for April 04) even further down the track. The IIR-M's will be the real start of something new. Cheers, Kerry.

"Not found" unless one can "prove" beyond all reasonable doubt that it is actually has been in fact destroyed. Cheers, Kerry.

"Even with WAAS off" Where did this idea come from? Cheers, Kerry.

Funny that manufacturers never really liked the 95% figure eventhough it's what system accuracy is generally based on. CEP (that 50% figure) was obviously one of those little invented specifications to confuse accuracy. Cheers, Kerry.

Trying to compare EPE's between different makes/models is really a lost cause, means very little if anything at all. For a particular unit the relative movement is probably a more meanifull indication of the position improving or getting worse, but in absolute terms don't mean all that much Cheers, Kerry.

I do recall seeing the first GPS/WAAS certified avionics receiver announced but this was only in the last few months. Not sure if that was both horizontal or vertical certification but more likely only horizontal as vertical didn't have certification the last time I checked. The very first WAAS completed (proving) commercial flight was in March 2003 and done using a specially programmed and modified receiver that ignored the type 0 (zero) message. One of those just making sure they could do it exercises. Really until there is at least a third WAAS Geo then without the availability and reliability one would be putting an awfull lot of confidence on basically a single transmission source as certainly the luxury of receiving both the existing Geo's is minimal. Cheers, Kerry.

It takes some time to upgrade maps and generally much of the mapping used with GPS is compiled from older survey "methods". Apart from the scale of the map one should also look at the resolution of the map as for example a 1:250,000 map the pixel resolution could be around 30 metres and yet one is using a device probably capable of less than 10 metres. Really until maps can be "viewed" at 1:1 rather than zooming a map designed for hard copy production at a set scale then there will always be some differences as depending on the scale of the map features are sometimes distorted (displaced relative) for definition of the printed product. Cheers, Kerry.

Since on a world wide basis ellipsoid-geoid separation can vary up to 50 metres it's certainly a very simplictic model that only interpolates to 100 feet (~33 metres). At 100 odd feet difference it's really a wonder why they would even bother at all as really the height based on their "simplictic" model isn't worth much at all. Why bother at all, why not just provide ellipsoid height and be done with it as then at least ellipsoid height is something that has some meaning albeit not that reliable in the first place. "Anomolies" really isn't the best description for ellipsoid-geoid separation distances but for sure these differences can range up to a little more than 50 metres (in the US between about 10 to 50 metres +/- a few) so meaningfull heights (as per maps etc) from GPS do rely on the accuracy of the geoid model. Primary elevations derived using GPS relate to the ellipsoid of the datum reference being used. These are not inaccurate due to the geoid, simply inaccurate being GPS derived heights, inaccurate is depending on equip/methods used of course. The accuracy of the geoid model then has an additional affect on accuracy of the orthometric height but probably not as much as the initial accuracy (inaccuracy) of the GPS ellipsoid height. Elliposid-Geoid models are really much better at giving relative height differences (depending on distance etc) much more so than an absolute height as there are simply to many unknowns to absolutely take all factors into account. But then as per Stunod's example the final height can never be any better (probably worse) then the inaccuracy of the ellipsoid height no matter how good (or bad) the geoid model is. Cheers, Kerry.

I understand FAA are in the process of reviewing the use of "electronic devices" on aircraft. These are the normal run of laptops, PDA's, mobile phones etc and if these devices are allowed in the future then there should also be no obvious reason why a GPS receiver wouldn't be allowed based on pure technical, freq and interference issues. Cheers, Kerry.

Many variables with planning and time of day but Some comments on Time of day Cheers, Kerry.

No not so there's a precession factor of about 4 minutes, each and every day. Cheers, Kerry.

Averaging is really a feel good thing and if it feels good using it then by all means, now what it means now that's different. Still better means to ensuring a Wp has the best confidence and integrity possible. Cheers, Kerry.

Being a dynamic system things can be quite variable during the day and from day to day depending on the status of the constellation for a particular time at a particular place. There can be quite a bit of difference in the "best" time even with one satellite shutdown for some reason. Also for a particular place/time obstructions can come into affect, even briefly, that might affect the results. There is planning software freely available, which will give an "indication" of when is the better time to be at a certain location. Cheers, Kerry.

Big wet trees can cause issues with multipath but it takes some analysis to prove it and no handheld can really do that. Space weather is much more imposing and variable but the earth weather really isn't an issue. GPS is a very dynamic system and conditions change from day to day, moment to moment, location to location etc etc. Many of these things are taken into account with regards accuracy spec but many things are also beyond the control of the system custodians and must be managed by the user. Cheers, Kerry.

That really should be "the position accuracy CAN/MIGHT/COULD get better the longer you stay there." THe other half of the time averaging makes absolutely no improvment or even decreased accuracy. Just depends how long one is prepared to stay and these days without SA most aren't prepared to spend the amount of time that is really required for averaging to be usefull have have some integrity. Cheers, Kerry.

Bit of a misconception and whoever told you that one forgot the rest of the infomation. 300 feet well that's possible as well. As for "Position Pinning" that rather different to averaging as it basically stops/locks the position until one has moved a short distance or moving at a slow speed. This was just to prevent the wandering affect of SA and gave this disallusion that the position was steady, which of course it wasn't. While Position Pinning is ON the unit is basically not navigating and simply displaying the last available position before the unit stopped. Much more important than averaging, which can be rather deceiving is being in the required location at the best possible time and take advantage when the best conditions exist. Cheers, Kerry.

Many of the changes to firmware has to do with interference between different WADGPS operations, especially now that EGNOS has stepped up testing and simply because one shouldn't be using it outside the coverage area. It was even more disturbing that some manufacturers were advertising WAAS "with pin point accuracy" in countries it never had a hope in hell of working. WAAS's correction principles are a bit more complex and involved than stating there has to be X reference stations with Y satellites in view that are common to user Z. Spatial de-correlation principles are basically the difference between WAAS and dGPS corrections, dGPS corrections are affected by distance, WAAS basically aren't. As for what the satellite display is showing, I believe there needs to be some differentation with what the display is actually trying to tell the user based on how the user has the receiver configured. In affect NMEA data is a very good indicator of what is going on with respect type of fix, validity of fix, satellites in fix, satellites in view, fix quality, satellites being tracked etc. As far as Fix quality there's no such thing as "half" a fix (GPS/dGPS/WAAS etc), it's either one or the other, there is nothing in between. Configurations differ between receivers but most have this generic automatic line that basically always have a lowest common denominator. 2D v 3D fix can be dependent on how the unit is set. Where "3D only" is an option then that's exactly the conditions one must have to have a valid fix. Similar with differential settings, Differential mode can be say Automatic or Differential exclusively as well as DGPS modes of say Automatic, WAAS Only, RTCM Only or None etc. How a user applies these options is specific to a users purpose. With all auto then one will get a 2D fix if insufficient satellites, with other conditions in hierachical order depending on user settings. If configured to "WAAS only" then no WAAS corrected position No position/navigation, period, regardless of what is available as far as straight GPS is concerened. Cheers, Kerry.

WGS84 was the fourth generation of an Earth-centred reference frame developed by the US Defense Mapping Agency. WGS84 basically superceded WGS72, WGS66 and WGS60 all of which were predecessors to WGS84, much the same as WGS84 has been redefined several times since WGS84 was first adopted using techniques including GPS, laser measurements and other techniques. WGS84 is based on GRS80 (Geodetic Reference System 1980), which are basically identical except for one minor exception. Until Jan 27, 1987 GPS still used WGS72. WGS72 was also used for the Transit Doppler System until Jan 1989, which was operational long before GPS was launched. At the time it was the best (earth-centred reference) there was. As more and more data is collected WGS84 will probably be redefined again but any changes these days won't be as marked as those in the past, not that many users would have even noticed changes made to the GPS reference in recent years. Defining WGS84 (as it became to be known) was basically in progress before GPS was launched. Cheers, Kerry.

That should be "limited" equipment and technology .... About the best one could do with a handheld to get the "best" results with some confidence would be about 30 days logging ALL position solutions. 30 days will generally account for 99.9% of all errors/anomolies and give some confidence to the final result even though the final result would still be limited by precision issues. Depending on the receiver make/model it is possible to record carrier phase from some recreational handhelds and post process this against the closest CORS site. Still has some limitations (accuracy wise) including precision. Cheers, Kerry.

Would that be the same as saying a fix with WAAS ON and "some" corrections is not suspect, cause this is what receiving invalid WAAS corrections does to the position solution. Affect of WAAS "corrections" on accuracy outside the coverage area Realize this is a long way outside the coverage area but similar things could be expected around the edges of WAAS limits. Probably one of the first things in understanding this issue is to understand exactly what an AUGMENTATION is, what it does, why and how it works to achieve the purpose of an augmentation system. Don't want to get too tangential (do we ) but in reality WAAS corrections have basically a different approach than LAAS or Differential corrections. WAAS puts errors into categories and creates models for these categories, which is applied to the GPS observed ranges at the user end as a vector correction. WAAS in this regard is a little different to differential that actually transmits a scalar correction (applied to the pseudo-range) for each and every individual satellite in view (max of 9 in the case of differential). WAAS modelling of errors (basically 4 different error models) is one of the reasons that accuracy is maintained over large areas, where as the further one gets away from a LAAS reference station or USCG type differential site the accuracy can tend to degrade. So this talk about minumum common satellites and multiple reference stations is not really the same thing as conventional differential requiring exact common satellites. Now to improve the accuracy of a fix one must have some basis for that to occur. The thing is one has to KNOW that whatever information is being used is correct and without additional verification then that information is useless to be incorporated into a system solution like WAAS. The whole purpose of WAAS is for airbourne navigation with a CERTIFIED level of integrity in REAL-TIME. This simply can't be achieved from a base system for which is was never designed or intended. To assume recreational receivers are somehow going to be different and implement what amounts to unknown error sources is simply a fallacy. Cheers, Kerry.

These "facts" can indeed be checked, validated or whatever (I think anyway) in a practical sense. It will however take a few days to log enough relevent data to make some assumptions. It's an interesting theory and as always one must not jump to conclusions as just because a Garmin "might" do this doesn't mean any other or all other manufacturers implement things the same. As usual one generally has to keep an open mind on many of these things as sometimes many things disguise the real facts. Cheers, Kerry.