Sunday, March 8, 2009

PN-40 Review - Signal

Your impression of the PN-40's accuracy and speed of signal aquisition will depend heavily on what GPS units you have used previously. If you are a few generations back, the PN-40 will probably impress you greatly. If you've owned a Garmin 60CSx or newer, then it will be about on par, maybe a little better, maybe a little worse, than what you are used to. My last GPS was a Magellan Meridian Gold, so the PN-40 represents a MAJOR improvement. As a note, all of my observations are based on the current DeLorme firmware, version 2.5 beta 3. Prior to that version there were some WAAS issues and some minor accuracy issues that made me hold off on this review, but the situation has improved dramatically.

Signal aquisition is pretty darned fast on the PN-40. Double digit seconds range for a cold start, and single digits if you've used the unit a few minutes previously. You are up and navigating with a 3-D lock fast enough that I usually don't have my shoes re-tied before it finished. If you turn the unit off when not navigating to conserve power, like I do, this fast signal aquisition is key.
Accuracy is frequently in the single digit feet range, and almost always in the low teens or better. I've tested the unit on flat terrain, on gently and steeply sloping terrain, and on terrain where large portions of the horizon are obscured by rocks or a whole mountain, and I'm more than satisfied with the accuracy of the unit, and its ability to hold a signal lock sufficient for navigation.
The WAAS implementation was a little squirrely prior to the beta firmware. DeLorme has indicated that the problem was with the Cartesio chipset's firmware from STM. Prior to the firmware, getting a WAAS lock with the PN-40 was downright difficult, and impossible for some users. I can now get a WAAS lock fairly quickly, and maintain WAAS correction data for most of a hike unless the terrain prevents a view to the south west. However, I don't really feel that WAAS is an essential feature for the bushwhacker anyway, since it only provides a small increase in accuracy (sometimes none), and most bushwhackers are trying to find objects that are somewhat easier to find than the film canister a geocacher might be seeking. ;)
As part of the beta firmware release, DeLorme added a new feature to the PN-40 called "Predictive Ephemeris" that provided a dramatic increase in signal aquisition speed. I won't go into the details, but basically the unit makes an educated guess as to the individual satellites locations, sacrificing accuracy for speed. Previously, the PN-40 had to wait for emphemeris data to download for each satellite it was tracking for that satellite was used to calclulation position. Now, while the PN-40 downloads new emphemeris data, it estimates what the data will be, and runs with the estimate until the new data arrives. Again, if you turn your unit off a lot while hiking, this is good for you.

Overall, I am very pleased with the accuracy of the DeLorme PN-40, and consider it more than capable of being used on a bushwhacking trip. I have not been able to test the unit under decidious foliage as it is winter, so keep that in mind, and I'll update when I can.

Next: DeLorme PN-40 Review - Maps
Back: DeLorme PN-40 Review - Field Operations

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