Gps?

[solo_hiker]

There is no magic in the external antenna (its primary advantage is that it allows you to separate the antenna from the display and buttons).
Doug

I beg to differ there: the key is having an ACTIVE external antenna. Both the signal bars and the number of birds seen jump noticeably on my 12XL when my Lowe external antenna is connected, and more than once under deep forest cover I haven't gotten a fix until I've done so. My point is that if you're relying on your GPS under deep forest cover, an active external antenna is a good idea.

It looks like Lowe no longer makes the one I have, but if you wanna see what I'm talking about check this out:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=4668&item=5771591222&rd=1&ssPageName=WD1V

Solo

 

[DougPaul]

I beg to differ there: the key is having an ACTIVE external antenna. Both the signal bars and the number of birds seen jump noticeably on my 12XL when my Lowe external antenna is connected, and more than once under deep forest cover I haven't gotten a fix until I've done so. My point is that if you're relying on your GPS under deep forest cover, an active external antenna is a good idea.

An active antenna is a passive antenna with an amplifier in the same box. Every GPSR with an internal antenna contains a passive antenna and amplifiers. The real issue is the relative performance of the internal and external components.

Whether an external active antenna improves your reception depends on the GPSR and how you use it.

Once you have sufficient front-end gain, additional gain (amplification) will not help. An amplifier adds noise (called the noise temperature or noise figure) to the signal--the signal-to-noise ratio, which is primarily determined by the first amplifier in the system, is the critical determinant of "sensitivity". Additional amplifiers increase the strength of both the signal and the noise (and because they will add their own noise, actually reduce the signal-to-noise ratio slightly). Too much amplification can overload the front end and make things far worse.

One reason for using an amplifier in an external antenna is that the cable has significant loss at the GPS frequency (1575MHz). The amplification sets the noise figure before the cable loss and compensates for the cable loss. Otherwise the cable loss will significantly reduce the signal-to-noise ratio.

The GPSR signal bars indicate signal-to-noise ratio, not signal strength.

(The GPS signal is a 1023 chip spread spectrum signal using different orthogonal spreading codes for each satellite. All satellites transmit on the same frequency. The signals are detected by doppler shifted correlation. When the GPSR is searching for the satellites it is scanning in doppler shift and correlation time to find the signals.)

The 12XL is a rather old design. If an external active antenna helps, it is because:
1) the noise figure of the amplifier in the external antenna is better than that of the 12XL,
2) the front end gain of the 12XL is insufficient,
3) the passive antenna in the external antenna is better than the 12XL passive antenna,
4) you put the external antenna in a better location than you put the 12XL,
and/or
5) the 12XL electronics puts out noise that is picked up by the internal antenna, but not by the external antenna.

It looks like Lowe no longer makes the one I have, but if you wanna see what I'm talking about check this out:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=4668&item=5771591222&rd=1&ssPageName=WD1V

Thanks--I already own 2 (different) active external antennas. Here is a spec sheet for one of them: http://www.pc-mobile.net/ast3spec.htm.

If an external active antenna helps you in your applications with your equipment, by all means use it. But it is not an automatic conclusion that it will help everyone. (There are applications where an external antenna is essential. For instance, when the GPSR is in a shielded location.)

Doug
Electrical Engineer
40 years experience with radio systems.

 

[solo_hiker]

The GPSR signal bars indicate signal-to-noise ratio, not signal strength.

Doug
Electrical Engineer
40 years experience with radio systems.

then you would know the inside of a GPS much better than I do. then it's an improved s/n ratio allowing, at least in my case, that constant fix under thick cover. one of your other points

(its primary advantage is that it allows you to separate the antenna from the display and buttons).
Doug

combined with the above gives a real advantage when bushwhacking on a thick, steep slope: I can pull the GPS out of my pocket and get info at a glance. no waiting and no need to move around.

thanks Doug. one last thing. it may be old, but I love my 12XL/Lowe rig. are the newer units that much better?

Solo

 

[DougPaul]

(Re external antenna)
(its primary advantage is that it allows you to separate the antenna from the display and buttons).

combined with the above gives a real advantage when bushwhacking on a thick, steep slope: I can pull the GPS out of my pocket and get info at a glance. no waiting and no need to move around.

Sounds good to me.

Depending on the usual factors, I can usually keep a half-way reasonable lock while hiking using the internal antenna (on an eTrex Vista). If the lock is lost, I can usually reacquire it within 30 sec when I stop and hold the GPS up--so no biggie. A trade-off between the complexity of several boxes and wires and occasional short waits.

If I were trying to map a trail, I would use the external antenna mounted on my hat, a short pole, or in/on the top of my pack to get the best quality track.

Recently picked up a 60CS--haven't had a chance to test it in the woods yet. Certainly works well in the car--perhaps even slightly better than the Vista. (Obviously many differences between the two units, the antenna is only one.)

BTW, where you carry your external antenna?

thanks Doug. one last thing. it may be old, but I love my 12XL/Lowe rig. are the newer units that much better?

It is the oldest currently availble hiking GPSR in the Garmin line. There must be a reason or two why it is still available. (I know of at least one, but it is not an issue for hiking.) I've never actually used one or even seen one in the "flesh"--but I have read comments from owners (some people are quite happy with them) and just looked at the spec sheet. So what follows is part guesswork on my part.

Let me separate my answer into 2 parts--the basic GPSR (receives the radio signals and produces a location and velocity) and the user interface/feature part. The basic GPSR is a 12 channel unit--basically similar to more recent units. No big updates here, but perhaps some small ones such as lower noise front-ends, faster acquisition, etc. (Technology marches on...) Perhaps you would have less need for the external antenna with a newer unit.

The big difference is in the user interface and features. Newer units include loadable maps (very useful, IMO), more track storage, more waypoint storage, better displays, lighter, WAAS, 2 cells instead of 4, barometric altitude sensors, etc. (Try Garmin's online product comparison if you want a better list for a specific pair of GPSRs.)

So the bottom line is that it depends on what you want (and can afford to spend). Clearly the current rig meets your basic needs. But the new features are nice too and you might decide that you "can't live without them" once you try them. I personally waited for loadable maps before I bought my first--initially got a Legend (8MB) and rapidly exchanged it for a Vista (24MB) to get the larger map memory. Pragmatically, the internal maps allow me to do things that would otherwise fall into the "theoretically possible, but pragmatically too difficult or slow to actually do" category. The internal maps also reduce the need for pre-trip preparation.

Doug

 

[solo_hiker]

Sounds good to me.

BTW, where you carry your external antenna?

Newer units include loadable maps (very useful, IMO), more track storage, more waypoint storage, better displays, lighter, WAAS

Doug

I hold a 1 1/4" washer from Home Depot against the inside of my pack's top pocket and let the Lowe's magnet attach to that. works great. comes off once in a while (under blowdowns), but not often.

is WAAS available in the WMNF, and if so, what can it do for me?

thanks Solo

 

[DougPaul]

I hold a 1 1/4" washer from Home Depot against the inside of my pack's top pocket and let the Lowe's magnet attach to that. works great. comes off once in a while (under blowdowns), but not often.

Sounds good.

Another way is to put the antenna in the top pocket of your pack. (Dry pack fabric is transparent to the signals.) I attach my antenna to a flat plate (cardboard, plastic, metal, etc) to keep it oriented properly.

is WAAS available in the WMNF, and if so, what can it do for me?

Yes--it is theoretically available anywhere in the US, but the signal is sent from geosynchronous satellites which are low on the southern horizon so you will only be able to receive it from high spots. Under ideal conditions, WAAS will reduce the 95% probability error from about 10m to 5m. (It can also make the errors worse.) Receiving the signals may also increase the battery drain. My suggestion is don't bother with it for hiking--battery life is more important than any improvments in accuracy. I generally leave it turned off.

IIRC, the 12XL doesn't support WAAS.

Doug

 

[Rols]

Gps

I have been using the Garmin E Trex (the most basic of models) for 3 years. I do most of my hiking solo...and find it a good companion on the trails. It gives me a good excuse for a break...it helps answer that age old question when traveling to wooded summits "are we there yet"?... and "how much farther"?.

I have found the altitude feature to be quite accurate and very helpful info. (it is much better than my altimeter). Signal is the best in the fall, winter and spring but it is rare that I can't get a signal in the summer even under a dense canopy. I do have to take it off my pack strap and hold it away from me until it picks up the signal.

Since I have the most basic model, it does not have maps loaded. Before a trip I go onto Topozone.com and pick up coordinates along my intended route and load those in as waypoints. I will mark points for trail junctures, river crossings, summits etc. While I truely enjoy having it along, I would not consider going without a good map & compass.

 

[DougPaul]

I have been using the Garmin E Trex (the most basic of models)

I have found the altitude feature to be quite accurate and very helpful info. (it is much better than my altimeter).

That is just the altitude computed by the GPS as part of its normal determination of your position (lat, lon, and alt). Accuracy under ideal conditions is 95% probability of being within about 30m of the true altitude. (GPS altitude error specs are typically about twice the horizontal error spec, 15m for the eTrex.)

Some of the fancier GPSes include a barometric sensor which can be calibrated in any of 3 ways: known (sea level) pressure, known altitude, or automatically from the GPS. The GPS auto-calibration is a nice combination where each part of the system compensates for the weaknesses of the other part yielding a better altitude estimate than either part could alone.

Garmin specs their barometric sensor at 3m "with proper calibration (user and/or automatic calibration)". I typically get within 3m of the published altitude of peaks using the GPS auto-calibration.

Doug