Same Units - Side By Side In Real World

[jackbear]

I was just reading some old discussions about satellite lock between unit x and unit y (etrex vs. explorist, quad vs. patch, etc.)

 

My question is has anyone tried a side by side comparision of the same units (Etrex Legend vs. Etrex Legend, or Geko vs. Geko) as a control experiment?

 

Lot of discussion in the past about "jumping around" or "off by". Will two units of the same make and model perfrom the same? We assume so, but who has tried it?

[+jacques0]

Excellent question, Jackbear. This will be a fun thread to watch.

[Neo_Geo]

It would be worthy to note the firmware versions of the two test subjects, and would be interesting to see how two of the same models compare with and without having the same version.

[+Rubberhead]

I have been able to compare an eTrex Vista vs. an eTrex Legend, and an eTrex Legend vs. an eTrex Venture.

 

There were noticable difference in satellite reception.

 

I could be just coincidence but it seemed like the more advanced the unit the weaker the signal. The Venture was stronger than the Legend and the Legend seemed stronger than the Vista.

[peter]

Considering all the analog components in the frontend of a receiver I'd actually be more surprised if all samples of a given model performed the same.

 

I've been on bicycle rides in wooded and hilly terrain where there were multiple eTrex (basic yellow) units all running the same firmware and mounted in similar positions on handlebars. A couple samples recorded almost a complete uninterrupted tracklog while another had numerous gaps where satellite lock had been lost for over 30 seconds.

[jackbear]

Now that might also lead me to think to consider battery power as well...both units would have to have same brand of fresh batteries. I think a test like this would be educational to learn the limitations of a model of GPS, and gain some facts besides..."Garmin is better than xyz". How does one model of GPS differ between units.

[peter]

I have tested my own Garmin receivers (12, III+, eMap) for any effects related to the battery charge level. I alternated between fresh alkalines (1.6 V/cell) and nearly discharged NiMHs (0.95 V/cell) and checked for number and strength (SNR) of received satellites and stability of reported position. I didn't see any reproducible effect of battery voltage on either reception or position reporting. I believe the units regulate the voltage internally so the battery voltage has no effect on performance as long as it's sufficient to avoid a shutdown.

[+the hermit crabs]

We have frequently compared two eTrex Vistas side by side. For months, they gave identical readings. Then one of them went a little wacky (we used a Venture as a tiebreaker, and it agreed with the "good" Vista).

 

The only thing that we had done differently with the one that went a little haywire was to try using disposable lithium batteries in it. The batteries lasted a long time, but the unit's operation gradually degraded. It started taking longer and longer to acquire satellites, and it never got the same ones that the good Vista and the Venture did. Eventually it was reading up to 100 feet off (this was true even after ditching the lithiums and going back to either alkalines or rechargeables).

 

I emailed Garmin, and they had me do a master reset (power off, then press and hold the Page, Find, Click-Stick, and Power buttons all at once for five seconds). This clears everything from the unit except for maps, and you have to sit it outside for 20 minutes to let it re-acquire the full satellite almanac. We did that, and it did fix it; now the two Vistas are pretty much identical again.

 

But we never use lithium batteries anymore (even though Garmin said lithiums probably didn't cause the problem -- it was too much of a coincidence for me).

[+sbell111]

...I could be just coincidence but it seemed like the more advanced the unit the weaker the signal. The Venture was stronger than the Legend and the Legend seemed stronger than the Vista.

Interesting. I wonder if the fancier units aren't forced to dedicate a larger amount of juice to all the extras, thereby leaving less for elementary processes.

[+Night Stalker]

I have compared my 60CS with another 60CS and they were considerably different in their results. Mine was far enough off that I returned it to Garmin and they replaced it under warranty, so yes there is a difference unit to unit of the same model even.

[+Sputnik 57]

If the are actually side-by-side, they may generate interfere that makes them both off, at least according to this normally reliable source.

[GeoBobC]

Very good topic. I've often wondered why different units perform differently. I've had two different Garmin units (of the same exact make: two 60c's, for example) side by side, and the specific satellites that each acquires are different. GPS A may pick up satellite 8 while GPS B does not, yet GPS B will acquire satellite 10, and GPS A will not. You would think that two units from the same manufacturer would acquire the same satellites.

 

Garmin swears there is little or no difference, unit to unit. If that's true (I don't believe it is), then other factors contribute to the difference. Satellite configuration can. Even moving a unit a few inches (when there are obstacles to clear view) can change the reception significantly. I've not experienced a change due to battery power; others may have.

 

It may be that the almanac data each unit has is different enough to contribute to the differences as well. I've heard that units "learn" where satellites were (when last powered on) and look for them in that location first. If two units look in different locations that could be a factor.

 

It almost seems random at times but it cannot be.

[+S&G.Davison]

we both have Garmin V's ... Exact same FW release in both .... and we spend many a cache stood 50 foot from each other saying "I'm on it" .." No I'm on it"

 

No two (commercial) GPS units will ever record the "exact" same position - As we all know (very simply) GPS's units work by comparing the received data from the sats - this data contains the sat position and a time stamp .. they calculate how long the signal took to arrive at the unit (and as the signal travelled at a know spped ie speed of light) how far away from the sat the unit is ... to do this they need an internal timing reference which is a fairly simple crystal controlled oscillator - thats why the debug screen" often show unit temp as temp affects oscilator "speed"...

 

Interesting Link re lockon and running

Edited June 3, 2005 by S&G.Davison

[+briansnat]

According to GPSinfo.net, when comparing two units side by side one might be affected by the other. So you should separate them by 4 or 5 feet.

[+wickedsprint]

My 96C when side by side with my Garmin V I make sure to tilt the 96C into the vertical to make best use of its antenna, it gets the same number of birds with higher signal strength. This was conducted in the passenger seat of a car on my lap.

Edited June 3, 2005 by wickedsprint