Agnosto:

You are the poster child! If you currently have 5.1 and access the override, could you please report what they replace it with (version) and if the override still functions as it did before the disk change, or if they have disabled it (as you are hoping for 6.1, I am guessing that you do not utilize the override, but if you could still please check it out we would all be appreciative). I recommend that you make a copy of your current dvd, if possible, as you may want the option to switch back...

Thanks

 

It would seem that the further north we are from the equator, the fewer satellites can be seen. My Garmin GPS (dash mount) is actually only showing 5 and sometimes as few as 3 satellites these days when it would consistently show 7 this last summer. My friend in Texas tells me that his Garmin will typically show 9 satellites or more.

So I was wondering if there could be some correlation to our position on the globe or the time of the year. Apparently not as it's not mentioned in the reference material I could find - so it remains a curiosity.
Although the receiver clock error may have something to do with it..

Here's some of the info I found on Garmin's site:

Sources of GPS signal errors

• Factors that can degrade the GPS signal and thus affect accuracy include the following:
  • Ionosphere and troposphere delays — The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.<
  • Signal multipath — This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors.<
  • Receiver clock errors — A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.<
  • Orbital errors — Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.<
  • Number of satellites visible — The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.<
  • Satellite geometry/shading — This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.<
  • Intentional degradation of the satellite signal — Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.<
  <

 

Not to suggest Garmin is not expert at GPS navigation, but the marketing guy who put this list together didn't get it all exactly right!

The signal slows and bends. The ionospheric errors are a function of how much of the signal path is in sun light (which causes higher electron activity). The satellites broadcast their signals on two separate frequencies, and the signal delay is a function of the frequency. By measuring the difference between the two signals (for each satellite!), and by knowing the position of the sun and earth relative to the signal paths, these delays can be computed fairly accurately.

Correct.

Mostly irrelevant! The very precise time needed to navigate is computed from the satellite signals. The built in clock's error will only cause a slight delay for initialization of the unit in acquiring the satellites and beginning navigation.

The ephemeris data is updated daily by the satellite ground controllers, and that data is broadcast continuously by the satellites down to the receivers. There is virtually no errors in navigation from this once the updated ephemeral data is received.

Very true. Interestingly, if an extremely accurate clock is in the nav unit, then one less satellite is needed. Also, for ships, which are always at sea level (0 ft. altitude, basically), accurate navigation can also work with one less satellite visible. Otherwise, a minimum of 4 well spaced (next point below) satellites are needed.

Correct.

Also correct.

They left out some other compensated errors, including the most interesting one of all: The satellites are at half geo-syncronous orbits, about 13 thousand miles high. The gravitational field at that altitude is quite a bit lower, and that combined (to a much lessor extent) with the high velocities of the satellites (orbital velocity) puts them in a different relativistic time frame than on the earth surface. This relativistic effect causing the correct time onboard the satellites being different than the correct time at your receiver requires a timing compensation (as predicted by Einstein). There is also doppler compensation for whether the satellite is moving towards you or away from you, and other fun calculations like these that some engineer had to calculate and program into your GPS navigator.

Note that all my info is from memory, having designed a GPS receiver for Sperry (now Honeywell) in 1985 when the satellites were just being launched (there were only six in the sky at the time, and I could only test my system for about 2 hours per day in Phoenix). Fond memories, except for my work schedule rotating completely around the clock to follow the availability of the few satellites, which advanced 4 minutes per day, advancing 24 hours per year!

-- Alan

 

Wow Alan - That was some GREAT info!!!


So - tell me (well okay - US) - does your geographical location in relationship to the equator make any difference in the accuracy of GPS? Time of day make any difference?

Thanks!

 

If you have DVD NAV v.05.1 and you insert a DVD NAV v.06.1 your override is gone and you can not get it back if you insert the original v.05.1 (maybe there is a way, but do not know for sure) as I think it checks whether the version is the most current one and does the firmware flash...

 

You can get it back if you put the 5.1 DVD back in and disconnect the 12V battery for a few minutes and then reconnect it. Obviously, you get the older maps then, too.

 

OK, so how do I get a copy of the 5.1 disk?

Anybody upgarde, and want to sell their old disk?

 

Now that the constelation of satellites is complete (for a dozen or so years now!) there is virtually worldwide 24 hr/day complete coverage. Well, there are some transient holes, but nothing you need to worry about most places you are likley to find yourself. There is an accuracy issue with longitude convergence, and other geographical issues that are not really GPS related issues (e.g., paper maps also have many of these problems!). Your biggest concern for accuracy is signal blockage from things like buildings and mountains and near by structures.

-- Alan

 

alan_in_tempe — I think you meant that the gravitational field at the height of the GPS satellites is greater (not less) than that at geosynchronous height. Their orbits are closer to the earth, aren't they?

Stan

 

Hmm why live with the non updated maps.

But on to the real question...anyone get this yet? Did you loose the ability to do the override?

Here is another thought...wait until the next generation of the DVD to call this tidbit in. Since I was able to get the hybrid DVD I don't need the 06.1 maps so upgrading now isn't worth it...BUT...when the next generation of maps comes out I can see calling this card in to get the next version of maps...and then of course do the hybrid thing again so I don't have to loose the override.