What is a "connected PND"

This week I learned TeleNav launched the Shotgun, a connected PND. The advantage of a connected PND is that you can get real time POI updates, traffic, and weather feeds. But more importantly you can get a faster initial fix. This is why:

As you know, gps systems work with a constellation of satellites that send time stamps to a receiver. By calculating the time of arrival of those time stamps and knowing the location of the satellites you can calculate the location of the receiver, aka get a fix (that is a geographical fix, not the other kind). This process is called triangulation.

In the triangulation process I described, besides the timestamps, the recevier needs to know the position of the satellites and then find at least 4 on site. It typically takes several minutes to find 4 satellites, but if you knew where they are, you'll find them faster. Satellites follow orbits that are roughly known. The long term orbital postions of the satellites is stored in a file called the Almanac and short term variations are called ephemeris. Typically PNDs download the ephemeris from the satellites at a whoping speed of 50 bps (that is bits per second) so it is sloooow. This file is valid for several hours and it is typically downloaded when the device is first turned on during a 24 hour period or so. This can take up to several minutes. If you had a faster connection available to download the ephemeris you could get a much faster location of the satellites and a faster first fix. A connected PND such as the Shotgun or the defunct Dash has a cellphone like connection that can be at least 1000 times faster than the satellite. Hence, a connected PND can get the first fix in seconds. Good stuff.

Is that worth a $10 / $12 monthly fee?

If you have a TomTom you can get the ephemeris file from your PC via the connection utility before you leave home. It is valid for a bit more than 24 hours so you get the same effect. If you remember to plug it in every night after you come home, and take it back to your car when before you leave you will get the same effect for free. I never remember so I have learned to get a slow fix.

The traffic and weather feeds do require a monthy fee after the first 6 months. I tried them and found them not that useful. Granted my commute is 20 miles boring and usually traffic free.

It is not clear to me whether you can use the Shotgun overseas as a connected device and I belive it does not include maps outside the US or Canada. So as a traveling companion I'm not sure of its value.

Enjoy Tech and Trips