Hi Guys,

My wife and I would like to have a GPS for when we go out trailing, Oh wait I can't use that term, I mean 'wheeling'...anyway any suggestions? I have often looked at the Garmin Rino 530, but I have to say even though I am a Tech geek I know little about GPS's.

Chris,
Garmin is a real solid product, what I have found when hiking is that battery life becomes an isseue so always get the external power chord for your truck. Also you will need to concider how much info you want. The higher end Garmins' have pages and pages of info to sort through.
Did you really post this at 12:07 this morning?

If you're leaving it in the car, something newer like the 276C is pretty cool
http://www.garmin.com/products/gpsmap276c/

While the 530 does look cool, I don't see that it comes with a DC kit to plug in to the truck, as Rich mentioned is important.

If you're using it around town as well something like the Streetpilot might be cool
http://www.garmin.com/products/sp2720/

Or this big mother.....
http://www.garmin.com/products/sp7500/

Ok, maybe a bit more than you need for trails and such. The 276C and the ones that came before it are fairly popular and I've always liked Garmin GPS products.

pwc

i have an eTrex Vista and it's been a nice multi-use unit. BUT! it doesn't work worth a damn inside a rover. Get something like the 276 where you can connect an external antenna and a DC power source.

the biggest issue i have with GPS is that the signal diminishes so quickly under tree cover it becomes questionable value. while out hiking I can only get signal maybe half the time on many of the "greener" trails. inside the car is pointless for anything except the freeway and paved roads, and mine only picks up signal when it sits right up next to the windshield on the dash, or when it's held by the armrest close to a side window.

there are a couple companies that make a "re-radiating" antennas - i can post more about those if interested.

really though you'll want something with an external antenna input, which knocks out most of the portables like eTrex and Rhino, despite them being pretty solid units.

cheers,
-isaac

The 276c is what I have. You can use both the routing street maps and Topo maps. Also can use charts if you are a boater. It s avery sold unit.

To echo Issac, losing the horizon (in a canyon or narrow valley) the aperture narrows and finding 3 satellites becomes a problem.
Nature of the beast.

Being a Geo geek... you actually need 4 sattelites to get a position since it is a 3-D position... :-)

I prefer Trimble GPS units hands down over everything else. That said, unless I am surveying the price is prohibitive. Trimble is still able to over-price their products do to the military, surveying, and GIS applications. Most of the additional features of a trimble aren't really of any use to consumer anyway.

Also, even when you are getting 4 signals in a canyon, the signal may be bouncing off the canyon wall making your position less accurate.

For consumer use, I like the Garmin products, OR a ruggedized laptop/tablet. The new origami form factor looks really sweet for offroad navigation. Amrel and Panasonic make some pretty awesome ruggedized gear. If you go this route, then there is a plethora of software available.

--Craig

Huh, I thought it was 3 to nav and 4 to position, but you always need 4? Thanks for the insite.

What do you mean by nav?

--Craig

Warning... geek content ahead.

GPS measurements yield distances instead of angles. It actually does trilateration instead of triangulation. Distance is measured as c x dt (speed of light x delta time). The satellite sends a time-stamp, and delta time is the difference in time between when the time-stamp is sent and when it is recieved. Because there is huge error in dt it *has* to be removed to accurately calculate your position. The fourth satellite is used for this.

In GPS, time is the fourth dimension. It is the reason we need at least four satellites, instead of three, to locate the receiver. The four coordinates to be computed are x, y, z, and c dt. c dt has the units of distance then.

The goal is to get a fix on the reciever's position. Suppose there were no clock errors (which is false). Then the distances from three satellites would provide a fix. Around each satellite, the known distances determins a sphere. The first two spheres intersect in a circle. Assuming that the two satellites do not lie on a straight line, the third sphere will normally cut this circle at two points. One point is the correct receiver position, the other point is somewhere out in space. So three satellites are sufficient if all clocks are correct and all ranges are measured precisely.

In reality the receiver clock is typically inexpensive and innaccurate. When the clock error is dt, every range measured at that instant will be wrong by a distnace of c dt. we are measuring the arrival time of a signal that contains its own departure time. The incorrect range, which includes c dt from the unknown clock error, is called a pseudorange.

From two satellites we have two pseudoranges p1 and p2. Their difference d12 = p1 - p2 has no erro c dt from the recievers clock. The receiver must lie on the hyperbola of revolution, with the two satellites as the foci. This is the graph of all points in space whose distances from the satellites differ by d12.

The third pseudorange locates the reciever on another hyperbola of revolution (a hyperboloid). It intersects the first in a curve. The fourth pseudorange contributes a third independent hyperboloid, which cuts the curve (normally twice). Provided the four satellites are not coplanar, we again get two possible locations for the reciever: the correct fix, and a second point in space that is far from correct and readily discarded.

I hope that helps more than it hinders, and hopefully I didn't make any mistakes. Its been 4 years 6 years now since grad school and I don't work with this stuff in detail everyday.

--Craig

3 to navagate across a surface, 4 for altitude.
But, I think I have brought clarity to my confusion, Trimble has a nice little tutorial that explains the 4 satellite. 3 satellites reduce your position to two possibilities, the fourth satellite gives altitude which eliminates the wrong position, yes?
Sounds like I replace altitude with "time".

To be honest Rich, I'm not sure what the answer is because there really isn't such a thing as 2-D in GPS. Satellites are in orbit around the WGS84 Datum (center of gravity of the earth). That's why locations are reported as lat/long (angles from the center of the WGS84 Ellipsoid) and a distance (z) away from that point (or as elevation above the ellipsoid/mean sea level). Lat/Lon without a height means you could be anywhere from the center of the earth to infinity distance out in the cosmos.

I suppose that if you assume the location being calculated is on the WGS84 Ellipsoidal surface (reasonable assumption), or mean sea level ellipsoidal surface then the z-value could be replaced with that surface. This would remove one of the variables in the equation (x,y,t) and make it possible to correctly compute the location. I'm just speculating here though. I'd have to look it up in my GPS text book (gasp) or ask a geodesist.

If I get some time, I might look into it further. My book is pretty dense with linear algebra though, so I'll have to be in a serious learning mood to do it. :-D

--Craig

Since my brain is on this stuff now....

In case anyone cares, some of the sources of error in a GPS signal are:

Ionosphere, Troposphere, accuracy of the recievers clock, signal bounce (off canyon walls for example). Some of these are accounted for in the error correction routines in the receiver, others like canyon walls can't be corrected for even with a differential correction using a second GPS receiver.

DGPS is the way to go if you can swing it. Either live via Coast Guard signals, or post-processing. DGPS requires that you have another GPS at a known location (surveyed in), and a GPS that logs all of the timing information that the satellites send. You then use software on a computer to correct the signal for the troposphere, ionosphere, etc error that has been introduced. This is the primary difference between consumer grade and survey grade GPS systems. Literally, the consumer grade systems just don't log the extra data necessary to post process the data. Otherwise they are practically identical (lame IMHO).

If consumer grade GPSs recorded this information, then I could take the GPS tracks that you guys upload and correct them to improve the accuracy by several meters.

Many consumer grade GPSs now support DGPS by receiving the signals via an antenna and applying the correction in real-time. As long as you have a signal, this is just as good. When you are looking for a GPS get one that supports DGPS correction.

--Craig

www.mightygps.com is a good place to look for GPS gear. If you want to use a GPS with a laptop this is a good GPS. http://mightygps.com/WeatherproofGPSReceiver.htm. It mounts externally, which will get much better reception than anything in the cabin; especially if you have a heated front windshield.

--Craig

Most systems eliminate the possibility of being outside the contellation or inside of the earth for the purposes of expediting calculation.

Rich,

That Trimble tutorial sure was a lot easier to understand than my mumbo jumbo. Good find.

http://www.trimble.com/gps/how.html

--Craig

dang Troposphere. we should get rid of it

This is all great info, thanks! Craig you are a total Geo geek and it shows ;-) . Man I feel like I should get a certificate of achievment after reading this entire thread :-)

Chris,

I've been checking out GPS units a little bit lately. I really like the Garmin GPSMap 60Cx.

- SIRF Star III Chipset - Works amazingly well. I have this in my new vehicle GPS and it was able to not only get an accurate position fix under very heavy tree cover, but do it while sitting on my dash below my heated windscreen. This is the first time I've ever been able to to get any signal through the heated windscreen, and this chipset gets the same number of satellite locks on the dash or out on the roof (wire through sunroof). Whatever you get, get a GPS with a SIRF STAR II Chipset
- WAAS - Improves positional accuracy to 2m 95% of the time.
- Helical Antenna: Also helps get good reception under tree cover
- Color screen
- Mini SD expandable so you can download all the maps you want
- Supports routing, so it is useful for vehicle nav as well
- Looks pretty rugged

http://www.rei.com/online/store/ProductDisplay?productId=48008915&storeId=8000&catalogId=40000008000&langId=-1

For me, this is the low end that I'd enter the GPS market at. The features will just work, and it has enough storage capacity to keep you happy for years. Anything more is likely bling. Anything less won't work in the trees (E.g. this is the cheapest color screen, SD expandable, SIRF III, WAAS enabled GPS I could find).

--Craig