Thunderstorms and Clouds

[9mmCaching]

Is the accuracy of my Garmin III+ affected by thunderstorms and heavy cloud cover? I was in a storm yesterday and the coordinates were not matching with the cache...doing some troubleshooting.

[+MrGigabyte]

The GPS system was designed such that it is NOT effected by weather.

 

There is a "rain window" about 1.5Ghz and that is one of the reasons GPS is there. There is almost no attenuation even in the heaviest downpour. (Just a db or two). On the other hand, just a few millimeters of standing water will absorb/reflect 1.5Ghz waves.

 

[solohiker]

I have had problems on overcast days. I have also had overcast days with no problems.

 

I have also found that if I get a good signal on clear flat land and then walk into heavy tree cover/steep ravines, my positioning is relatively stable. If I turn on my gps in the same deep cover, I may never get a position reading.

 

I am no gps expert, but I believe the gps must recieve a large packet of data to 'lock on'. Once the data packet has been downloaded from the satelite(s) the data required to maintain the location signal is much less.

 

My observations are based upon experience, not expertise. I may be wrong.

[+rdw]

The GPS needs two sets of data to work.

 

First is the almanac data. This is rough position data for all of the satellites so the GPS can calculate what sats will be visible at a particular time and place. This data is good for many weeks once it is received.

 

Second is ephemeris data. This is the precise position of the satellites that is used for the position calculation. Once received, this data is good for 20-30 minutes.

 

If you turn on the GPS in a canyon after being off for a while, it can't get the ephemeris data and therefore can't calculate your position. If you get a lock outside of the canyon, it has received the data, and can keep a lock down in the canyon.

 

rdw