[Drake]

Originally Posted by Vigil

Great idea to use a Pocket PC instead.

I would believe they offer more fuctions and faster access to those functions with your touch screen.

One question though, how fast does a pocket pc secure you position and how well does it work under foliage cover?

I know they work in the city so I would assume that they are pretty capable of acquiring and holding satellite signal, but if you could confirm that, I will definitely consider switching to a pocket pc setup.

The HP iPAQs are built really ruggedly; they're bulkier and a bit heavier than other PPCs/PALMs but with their rubberized finished and very solid construction they feel like a man's hand held.

The touch screen is very handy (a lot of nav software have oversized buttons so you can press them with your fingers instead of having to use the stylus; very useful when driving, for example) and the flip up visor keeps your screen safe.

As for it getting a fix on my position, since it's a bluetooth GPS receiver, everything happens on the HOLUX 236's end, which works completely independently from the Pocket PC (which can even be switched off).

That being said, it's important to know there are several types of information being transmitted by GPS satellites, which can affect TTFF (Time To First Fix); in addition to the regular GPS signal broadcast used to calculate positions, a GPS receivers needs other data, specifically an almanac and ephimeris. These contain information about where in the sky satellites will be in relation to a geographic position at a given time, and specific information about each satellites orbit (the almanac contains very imprecise locations whereas the ephemeris contains very precise orbital data and clock corrections).

Almanac data remains valid for several months, however ephemeris data degrades after 3 hours (it's generally only considered valid for 4-5 hours).

So when powering up, before a GPS receiver can acquire it's first fix, it must check that its data tables are up to date, and update them if they aren't. This affects the type of startup the receiver will do, and each startup takes a different amount of time:

A "factory startup" occurs on new receivers, or receivers that have been reset, had their firmware upgraded, etc. and also on receivers that haven't been used for an extended period of time (several months). Almanac and ephemeris tables are blank and all data must be downloaded. GPS satellites transmit at about 50 bps (bits per second -- 50 bps works out to 0.006 KB/s, REALLY slow even for a 1 KB textfile). A complete almanac takes 12.5 minutes to download in optimal conditions. Ephemeris data must also be downloaded from each satellite "in view." It's generally accepted that under real world conditions, this type of startup will take about 20 minutes.

A "cold start" is what will usually be experienced if the unit hasn't been used for some time, but the almanac is still valid. In this case, a full set of ephemeris data must be downloaded. This generally takes 30 seconds to a minute under normal conditions.

If the receiver was used recently, only a "warm start" is required, which involves minor updates of the ephemeris. This will take less than 30 seconds. An up-to-date receiver (used within the past several hours) can perform a "hot start"; in the case of the 236, it can get a position fix almost instantly.

As far as foliage cover, it depends on your receiver. The 236 uses SiRF Star III architecture, which is now common in most new GPS devices; they're able to operate with very weak/degraded signals, and can compute a 2D position with as few as 2 satellite signals.

Also, keep in mind that while accuracy can be as good as "within 3 meters", when factoring in all errors, accuracy for GPS is only accurate to within 15 meters (i.e., even if you do get 3 meter accuracy, you always have to expect it can potentially vary as much as 15 meters).

Given how slowly you'd progress on foot, over terrain with heavy tree cover, and the inherent inaccuracy of GPS, it's not a big problem. From experience, even if you have "coverage gaps", between last known positions and the on-screen map, you can still figure out where you are (unless you were out on the water or in the desert, for example, where there are no immediate references).

Another thing you want to do, for this type of application, is use SIRF programming software to disable "Static Navigation" (if its enabled on your unit), since having it on will muck up slow-speed navigation (such as hiking).