Calibration help

[RHOS ROBIN]

I own a garmin etrex venture hc and whilst out looking for a cache it was counting down 50,40,30 and all off a sudden it started to count upwards,30,40,50 etc etc,so i turned around and walked back,50,40,30 so i stopped at the point where it would of counted up and searched around until i found the cache.There was no tree cover directly over my head but a few trees 5 metres to my side,could these trees have affected my satellite reception?.A friend of mine has a facility on his garmin to calibrate it,do i need or can i calibrate mine?

[Rocketsteve]

I own a garmin etrex venture hc and whilst out looking for a cache it was counting down 50,40,30 and all off a sudden it started to count upwards,30,40,50 etc etc,so i turned around and walked back,50,40,30 so i stopped at the point where it would of counted up and searched around until i found the cache.There was no tree cover directly over my head but a few trees 5 metres to my side,could these trees have affected my satellite reception?.A friend of mine has a facility on his garmin to calibrate it,do i need or can i calibrate mine?

 

DOD (Dept. of Defense) software inputs a random error into the "civilian" positioning signal sent out by all GPS satellites, but WAAS-enabled (Wide Area Augmentation System) GPSr will remove all but 9 feet of the DOD random error, under unobstructed skies. If your Venture HC doesn't have the WAAS enabled, that, in combination with the obstruction of the nearby trees, could have given you the pretty high error that you were reading on the screen.

 

From the "Setup Menu" page, click on "System". The 2nd item is labelled "WAAS/EGNOS" and it should be enabled, but if it's not, click the down arrow, highlight it and click on it. As far as I know, there is no way to manually calibrate your GPSr, but if WAAS is enabled and the problem continues to exist, you might want to contact Garmin for help.

 

From my own experience, most days, my Venture HC works just fine, but on occassion, it will act just like yours has. I'll go back to the same spot a day or two later and the signal locks on tight, and I go right to where I want to go. I suspect it has something to do with the emitted GPS satellite signals, but I can't say for sure. If I were you, I'd go back to the same spot and see if you have the same problem again. Hope this helps.

Edited June 18, 2009 by rocketsteve

[+bittsen]

 

DOD (Dept. of Defense) software inputs a random error into the "civilian" positioning signal sent out by all GPS satellites, but WAAS-enabled (Wide Area Augmentation System) GPSr will remove all but 9 feet of the DOD random error, under unobstructed skies. If your Venture HC doesn't have the WAAS enabled, that, in combination with the obstruction of the nearby trees, could have given you the pretty high error that you were reading on the screen.

 

Do you think this is why my Magellan will sometimes, for no logical reason, all of a sudden put me off my spot by over 200 feet? I can be following it and instantly I will travel hundreds of feet with clear skies and no obstructions.

[Rocketsteve]

 

DOD (Dept. of Defense) software inputs a random error into the "civilian" positioning signal sent out by all GPS satellites, but WAAS-enabled (Wide Area Augmentation System) GPSr will remove all but 9 feet of the DOD random error, under unobstructed skies. If your Venture HC doesn't have the WAAS enabled, that, in combination with the obstruction of the nearby trees, could have given you the pretty high error that you were reading on the screen.

 

Do you think this is why my Magellan will sometimes, for no logical reason, all of a sudden put me off my spot by over 200 feet? I can be following it and instantly I will travel hundreds of feet with clear skies and no obstructions.

 

Absolutely! I still have a Magellan 315 GPS, and occasionally, it will shoot off in another direction for no apparent reason, which is the random error kicking in. In most cases, an older GPSr will be sufficient to get you close to the cache, but when it starts acting goofy, you can wait it out (just a couple of hours) or call it a day.

 

I now have a Garmin Venture HC and without a doubt, a WAAS-enabled GPSr is a nice thing to have, but if you're willing to work a little harder to find the cache, the older GPSr will still get you pretty close.

[+Walts Hunting]

Just tought you might like to know that the random error (also known as Selective Availability) was turned off the first year of Clinton's presidency (1992). The next day the first cache was placed and published on USENET. it is not all that uncommon for GPS's to get disoriented like that. It happened twice today on my Oregon but I will go days. It is usually a result of the software which sometimes seems to do a screen freeze.

[aniyn]

I hope you're not talking about "Selective Availability"

[+Entropy512]

I own a garmin etrex venture hc and whilst out looking for a cache it was counting down 50,40,30 and all off a sudden it started to count upwards,30,40,50 etc etc,so i turned around and walked back,50,40,30 so i stopped at the point where it would of counted up and searched around until i found the cache.There was no tree cover directly over my head but a few trees 5 metres to my side,could these trees have affected my satellite reception?.A friend of mine has a facility on his garmin to calibrate it,do i need or can i calibrate mine?

 

DOD (Dept. of Defense) software inputs a random error into the "civilian" positioning signal sent out by all GPS satellites, but WAAS-enabled (Wide Area Augmentation System) GPSr will remove all but 9 feet of the DOD random error, under unobstructed skies. If your Venture HC doesn't have the WAAS enabled, that, in combination with the obstruction of the nearby trees, could have given you the pretty high error that you were reading on the screen.

 

From the "Setup Menu" page, click on "System". The 2nd item is labelled "WAAS/EGNOS" and it should be enabled, but if it's not, click the down arrow, highlight it and click on it. As far as I know, there is no way to manually calibrate your GPSr, but if WAAS is enabled and the problem continues to exist, you might want to contact Garmin for help.

 

From my own experience, most days, my Venture HC works just fine, but on occassion, it will act just like yours has. I'll go back to the same spot a day or two later and the signal locks on tight, and I go right to where I want to go. I suspect it has something to do with the emitted GPS satellite signals, but I can't say for sure. If I were you, I'd go back to the same spot and see if you have the same problem again. Hope this helps.

SA was turned off in 2000 and hasn't been turned on again except for (possibly) specific regions during combat action, i.e. the DoD has not put any additional random error in the signal since then, especially not within the CONUS. Prior to SA being disabled, position error could be as much as 100 meters.

 

That said, the C/A code that civilians have access to is just fundamentally not as precise as the military P(Y) code (lower code rate), and in addition the civilian C/A code is not broadcast on the second GPS frequency. Dual frequency operation lets a receiver measure ionospheric error, which is one of the largest contributors to GPS error. Fortunately those of us who have WAAS receivers can correct for this error because someone else measured it for us and broadcast it.

 

Also, presence of objects nearby can introduce multipath errors.

 

The calibration procedure present in some Garmin receivers pertains to the electronic compass and barometric altimeter, not to the GPS receiver itself.

[Rocketsteve]

I'd forgotten that SA was turned off during Clinton's reign. I recently read or heard something about the GPS satellite network being in a state of disrepair, and that the DOD wasn't sure how long the network would continue to be stable. If there aren't enough GPS satellites working up to full capacity, because they've passed their servicable time period, I wonder if that could be affecting the quality and strength of the signals being sent back to Earth.

[+tozainamboku]

I own a garmin etrex venture hc and whilst out looking for a cache it was counting down 50,40,30 and all off a sudden it started to count upwards,30,40,50 etc etc,so i turned around and walked back,50,40,30 so i stopped at the point where it would of counted up and searched around until i found the cache.There was no tree cover directly over my head but a few trees 5 metres to my side,could these trees have affected my satellite reception?.A friend of mine has a facility on his garmin to calibrate it,do i need or can i calibrate mine?

My guess is that your GPS is working just fine. Since your unit doesn't have a electronic compass, the arrow on the compass page depends on your moving a certain speed. As you are getting close to the cache you naturally will slow down. You probably walked right past the cache and that is why it started to count up again. The cache was off to your left or right when you got near the minimum distance and had you gone left or right there you would have probably gotten the distance much smaller than 30 meters. Be aware though that you will seldom get to zero because there is a natural random error in the GPS signal caused by many things (atmospheric disturbance, tree cover, tall buildings or cliffs, satellite configuration, clock error, and even how you hold the unit (and point its antenna). It is not caused by the US government putting in random error - as stated this was discontinued in 2000. If you knew the cache was on (or near) the path and the GPS was saying it was 30 meters from the path, remember that the person who placed the cache had some of this random error when they took the coordinates for the cache. While 30 meters difference between two GPS readings would be on the high side, it is not unheard of. As you get practice you will learn not to follow the arrow once you are close to the cache. A lot of people just put away the GPS when they get down to 10~15 meters and start searching. You should be able to get the distance much smaller - usually within about 3 meters - and then look for the cache.

[+Entropy512]

I'd forgotten that SA was turned off during Clinton's reign. I recently read or heard something about the GPS satellite network being in a state of disrepair, and that the DOD wasn't sure how long the network would continue to be stable. If there aren't enough GPS satellites working up to full capacity, because they've passed their servicable time period, I wonder if that could be affecting the quality and strength of the signals being sent back to Earth.

There aren't any current issues, but there is concern due to the fact that the GPS Modernization Program is behind schedule.

 

Many of the current constellation are nearing the end of their service life and due to be replaced, and the replacements' on-time launch is looking like it'll slip schedule. There's a risk of degradation starting in 2010-2011 or so.

 

That said, space-based hardware frequently lasts well beyond its intended service life. Look at the Mars Rovers as one example...

[+bittsen]

 

There aren't any current issues, but there is concern due to the fact that the GPS Modernization Program is behind schedule.

 

Many of the current constellation are nearing the end of their service life and due to be replaced, and the replacements' on-time launch is looking like it'll slip schedule. There's a risk of degradation starting in 2010-2011 or so.

 

That said, space-based hardware frequently lasts well beyond its intended service life. Look at the Mars Rovers as one example...

 

Um, the mars rovers aren't space based hardware. They are land based.

 

[+Prime Suspect]

I'd forgotten that SA was turned off during Clinton's reign. I recently read or heard something about the GPS satellite network being in a state of disrepair, and that the DOD wasn't sure how long the network would continue to be stable. If there aren't enough GPS satellites working up to full capacity, because they've passed their servicable time period, I wonder if that could be affecting the quality and strength of the signals being sent back to Earth.

That report was a load of horse-pucky (to quote Gen. Sherman Potter), and has been roundly discredited.

[knowschad]

 

There aren't any current issues, but there is concern due to the fact that the GPS Modernization Program is behind schedule.

 

Many of the current constellation are nearing the end of their service life and due to be replaced, and the replacements' on-time launch is looking like it'll slip schedule. There's a risk of degradation starting in 2010-2011 or so.

 

That said, space-based hardware frequently lasts well beyond its intended service life. Look at the Mars Rovers as one example...

 

That report was a load of horse-pucky (to quote Gen. Prime Suspect), and has been roundly discredited as well.