Real-world Receiver Sensitivities

[+mochalatte]

I've got a philosophical question: If a GPS receiver in the middle of a forest doesn't receive, does it really exist?

 

I’ve been reading this forum this week gathering information for picking a GPS to buy. I’ll mostly be using it on mountain bike rides in the thick NC woods but it will also be used for taking my kids geocaching and for navigating in the car. So far, it looks like the Garmin GPSMAP 60CS is going to be the perfect tool for all of this, but I’m a little concerned about how good of a job it will do holding onto satellite lock with all the kudzu-covered trees we have around here. Based on everything I’ve read, I really prefer the Garmin, but the one thing that Magellan handhelds seem to do better is receive.

 

I know the patch antenna in the Garmin eTrex series traded off receiver sensitivity for smaller unit size & weight. My only first-hand experience with GPS is on my brother’s Legend and a friend’s Vista out in Colorado over Thanksgiving. Sitting in a car under a wide-open sky, the Vista had problems. I had to keep it on the dash in order to hold lock. If I held it where I could read it, it lost lock. The Legend did better, so maybe it was just a problem with that particular Vista. Both were powered with rechargeable batteries.

 

So the question is how good do the Garmin units with a quad helix antenna (like the 60CS will have) compared to their patch antenna units? And do the Garmin units with a quad helix antenna (like the Rino and GPSMAP 76S) perform as well as the Magellan models in real world-conditions under a canopy of trees? My heart is set on the 60CS, but it will break my heart if it doesn’t accurately track my bike rides.

 

Thanks!

[+Cornix]

I think there's a reason why the antenna sticks out so clearly on the new Garmin 60 units.

 

Cornix

[+Desert_Warrior]

There are a lot of factors. It isn't just the patch antenna being the problem. I assure you, I can design a patch that will outperform any of the current crop of quad-helix antennas being built. That part is too simple. BUT... no way it will fit in a reasonable GPSR. I use a 1.2 GHz home-made patch on a 3 ft dish to listen to a satellite 60,000 km in space. I hear it just fine.

 

Orientation of the antenna is critical. Quad Helix likes to point up. Patch likes to lay flat.

 

Capture area is another part. Larger antennas capture more signal. Smaller does not.

 

Gain is yet a third problem. Antenna gain is a function of antenna pattern. Like comparing a lantern to a spotlight. Same bulb, same battery, same output. Lantern covers a large area with a little light. Spotlight concentrates all the light into a small area. Get too much gain in a GPSR antenna, and you start excluding satellites outside the antenna pattern "spot". Too little gain and you cannot hear the satellites.

 

Keeping lock in any vehicle is a hit/miss proposition. Unless it is a convertable.

 

No word yet on the 60. I am interested in it too, although my VISTA is good enough for me right now.

 

It don't help much. but hope you find it interesting.

[+Brenin Tegeingl]

Looks like the GPS 60 is going to use the same quad helix aerial as the Rhino series. Any with a Rhino care to comment on its ability to maintain a lock. Dave

[+GeckoGeek]

From what I've gathered here, an external antenna will beat the heck out of anyone's built-in antenna. If you can find a good way to mount it on your bike (someplace where you don't block it's view to the sky as you stand up and strain to go uphill) or backpack it would be livable.

[+Renegade Knight]

From what I've gathered here, an external antenna will beat the heck out of anyone's built-in antenna. If you can find a good way to mount it on your bike (someplace where you don't block it's view to the sky as you stand up and strain to go uphill) or backpack it would be livable.

You can tell the differnce betwene a patch and quad in real world use. I've seen advantages to both and in my kind of caching and terrain their is no real advantage.

 

However the difference isn't enough to really matter.

 

If you really need a better antenea buy an external. Don't let the patch vs. quad debate steer you away from the features you want.

[Grimmy]

One of the reasons I bought the Unit I have (GPSV) is the antenna jack. THe signal is not going to go through that sheet metal roof. This system is line of site and if anything conductive it can't pennetrate. It won't go through the glass on most sunroofs because of the UV tint. Being under the roof is only going to alow the receiver to get 50% of the available sky.

 

I run a Mag mount so it can go car to car. With it I don't have any issue of not staying locked. I can put the unit where it's easy for me to read. It on my dash about 6 inches from my right hand. That lets me look at it without having to take my eyes to far off the road.

 

I am really happy with my GPSV. It has performed well for the way I use it. It's great for my directionaly challanged wife. I can program her destination and hand it to her so she doesn't get lost. It will tell her turn by turn how to get where she is going. It's great in the car and I really like the Mapsource CD it came with. Loaded the areas I travel into it and not only does it show thwe street I'm on it shows things like resturants, gas, local attraction etc.

 

It might be a little heavy and combersome for bike riding and I don't know if there is a mount for that application.

[+mochalatte]

One of the reasons I bought the Unit I have (GPSV) is the antenna jack.

 

<snip>

 

It might be a little heavy and combersome for bike riding and I don't know if there is a mount for that application.

I thought about the GPSV, but the 60CS will have so many advantages over it: color, 35 MB more memory, smaller, lighter, battery life, sensors. About the only thing going for the V is the ability to switch the display between vertical and horizontal.

 

What is the external antenna you are using?

[Vlad]

garmin's quad helix antennas work as well as any of the magellans. don't get too caught up in the antenna wars - there are pros and cons for both. i think the 60 will be outstanding and considering it's designed for essentially all purposes i think you're going to be very happy with this unit - as long as it does (and is) what garmin's website says.

 

re etrex reception issue - gps is essentially line of sight - if you can see the sky, so can your gps - if you can't, neither can the gps. when i say that please understand that i am primarily referring to metal and water obstructions... like a car's roof.

 

no gps will perform well when you've got it positioned between the 2 front seats on the transmission hump and while it may work it's not going to be optimal positioning. better to have a gps on the dash where it has an essentially unobstructed view of the sky. how do you get around the small display if it's located so far away?

 

well on a legend or vista you switch it to "big numbers" mode. the digits are larger than anything you'll find on a meridian or even the 76 family from garmin (at least where i've got 4 fields of data onscreen at one time). little known secret with these models is you can have one of these fields being the "pointer" - the same directional indication as you would have if you were to switch the unit to the compass screen on a goto - very cool!

 

secondly nobody should be watching the map screen of their gps while driving - that is a poor implementation that leads to traffic accidents - you want a unit that prompts you with turn info and allows you to concentrate on the road - not the map screen. garmins implementation on the gps v, streetpilot iii and 2610 and ique is very well done - a screen pops up with large text and a very simplified display showing you either a right or left turn coming up - nothing else! i believe the 60 will approximate this implementation as well although i've yet to see this sort of detail - eagerly awaiting garmin's release of the user manual.

 

wait for the 60 - it looks excellent in all respects!

[+Bloencustoms]

On the antenna thing, I have been caching with several friends who use magellan receivers, and I use a Vista myself. Just to illustrate how unimportant your choice of antenna is, I can say with complete confidence that not a single one of the many caches I did not find the first attempt was due to lack of reception. If you purchase a good unit from a reputable manufacturer, it will get you to the cache. Finding it once you get there is up to you.

[+nincehelser]

secondly nobody should be watching the map screen of their gps while driving - that is a poor implementation that leads to traffic accidents

Once with my eTrex on the dash, I noticed it was reflecting in the windshield....like a "heads up" display.

 

Of course, the image was reversed, and conditions had to be "just right", but it made me wish for a GPS purposely designed for "heads up" display.

 

It would probably be a battery eater, though

 

George

[+Rich in NEPA]

 

 

I've found this setup to work extremely well for creating tracklogs while mountain biking. The Garmin GA27C antenna is attached with Velcro, and the GPS unit goes into my cycling jersey back pocket or in a fanny pack. In addition I installed a safety connection in the cabling so that if (when!) the antenna gets snagged by a branch it all comes apart easily with no damage. As for quad helix v. patch, they both seem to work fairly well in heavy tree cover but I'd have to give the edge to the patch design for maintaining signal lock better due to its overhead reception pattern. The main issue with the quad helix design is keeping the unit in a vertical orientation, especially when walking or riding. A handlebar mount might help, only I don't think I'd want to subject my receiver to the constant shock and jarring of riding rough trails.

 

The GA27C also comes in really handy when I'm taking benchmark photos with the GPSmap76 positioned flat on the ground next to the mark. Typically three out of four times at high-order horizontal control stations (such as triangulation stations) and with WAAS enabled I record coordinates that are an exact match to a thousandth of a minute with the NGS coordinates.

 

Cheers ...

[Squonkpups]

My GPS's are off as much as 300 feet just sitting in the same spot due to drift. I thought the government disabled the inaccuracy bug. Good luck. GPS's aren't what they claim to be. I want to pinpoint within 30 feet like the government can.