Do I Have A Problem....

[Cache Bandicoots]

.... or am I doing something stupid?

 

I've just bought a Garmin Legend to try to get my son outdoors and away from his PC & TV!

 

I've taken a number of readings from the unit, within about 100m of each other and entered these into Google Earth. Google Earth always shows these co-ordinates as being 230m to 260m (in the same direction) away from the points on the Google Earth map where I actually took the readings, despite the Garmin claiming that the readings were accurate to about 34 ft and to having good strong signals from a number of satellites.

 

All of the above was with WAAS disabled but I assume even on this setting, the unit should deliver the accuracy it is claiming to deliver and it clearly isn't.

 

The picture is very different though with WAAS enabled. The actual accuracy is way better, in fact, way better than the 22ft the unit actually claimed!!!! That is absolutely tremendous, despite the fact that the signal strength from the WAAS satellites hardly even regsiters and they never seem to get off the 'horizon ring' and I've never had a 'D' appear at the bottom of the strength meters for any of the other satellites.

 

Is this normal behaviour or does anyone think the unit is faulty in some way, particularly with WAAS disabled?

 

BTW, ,under what circumstances would anyone use the device with WAAS disabled?

 

Many thanks for any help

M

[+zedd10]

To be honest I really don't know what sort of accuracy to expect out of Google Earth, especially when you are trying to benchmark your GPSr. For WAAS you need to download the WAAS satelite almanac since it is not included in the GPS unit. This can be difficult but you only need to do it once (usually). Here is an explanation on how to do this: WAAS. The almanac will download automatically if you are fortunate enough to have good WAAS satelite reception. I'm at 28 degrees N and it can be difficult to get a lock but then the only WAAS satellite visible here is being relocated . Unless the almanac downloaded the GPSr wasn't getting any WAAS benefit and DGPS wasn't being received (no "D"'s) so most likely the improved accuracy was a better lock on the regular GPS satellites. While the WAAS satellites are geostationary the GPS satellites move and it takes some amount of time to fully update the GPS satellite almanac to get the best accuracy.

[ImpalaBob]

I've found Google Maps to be very accurate. Make sure you are using the correct datum in your unit. As always I suggest finding some other experienced cachers to go caching with .... and to compare coordinates. WAAS "I think" is experiencing some maintenance issues right now on #35. As stated you need to let the unit load a new almanac (coarse position chart) after enabling WAAS which takes on average 30 minutes .... unit stationary with clear sky.

 

My wife and I have identical MAP60C units which are sometimes exact .... and sometimes 30 feet different ... Go figure!!!! ImpalaBob

[Cache Bandicoots]

Hi

 

Many thanks for the info on how to acquire the WAAS almanac. Having followed the procedures, the Legend has aquired satelitte 33 and I have seen the 'D' appear on the bars. Geat stuff!

 

I think however that I should also be able to get a lock on 44 (I'm in London). The unit has stopped searching for new WAAS satellites now. How do I get it to start to reload the almanac? I've tried the New Location option but this didn't seem to persuade the unit that it wanted to try again.

 

TIA

[+zedd10]

I don't really know the answer to your question about forcing the unit to download the almanac again. I believe that the almanac will download automatically. In this way changes to satellite position are updated. Unfortunately I am assuming this to be the case. As I understand it the almanac should have information on all WAAS satellites. If 44 is available you should see it.

 

WAAS 35 is the only one I have seen from my location in the southeastern US. This satellite is being moved from of the east coast of the US to somewhere off the west coast. A new sattelite will be positioned later this year. So I get to test my theory about WAAS almanac downloads then I suppose.