Adventures in GPS

As well as a chance to use my new (used) Garmin GPSMAP-296 in flight for the first time, I also got a chance to use it in auto mode to navigate around a strange city.

Before we left, I made sure I’d loaded the detail maps for Pittsburgh and surrounds into the data card. (BTW: can somebody please tell me why Garmin used a proprietary data card instead of CF or SMC? Oh yeah, so they could force you to buy it from them, right.) I also entered the addresses for Laura’s dorm and our hotel as user waypoints.

I brought along all the car stuff – the special charger adaptor with the speaker so the GPS can tell you verbally where to turn, the bean-bag holder to put it on the dashboard.

When driving, the GPS was great some times – it got us from the airport to Laura’s dorm, and from there to the hotel perfectly. But other times, it was both the cause of, and solution to, all the problems we had.

First problem – we decided after dinner Saturday night to make a run out to the airport to load the stuff that we’d gotten from Laura’s room to load it into the plane so we’d have more room in the car on Sunday. The route it gave us seemed like it took every narrow stop-sign laden low-speed-limit street through scary neighbourhoods. There wasn’t a single leg over 2 miles long.

The second problem was the way back. The route looked way better – long legs with instructions like “take the ramp to the left”. The problem was the execution – Pittsburgh has a lot of intersections where two or more streets go off on the same side in very short order. And it seems like the GPS wasn’t very good at differentiating, plus I wasn’t very good at estimating distances when it said things like “turn left in 400 feet”. After we got off course, the other problem surfaced – the accuracy wasn’t all that good. So I’d be on a road that was parallel to the one I was supposed to be on the GPS wouldn’t know until I was way off. I never saw the GPS pick up WAAS while I was on the ground, which is too bad because that’s where the extra accuracy would have really come in handly. And then I got into the high buildings, and the GPS accuracy got even worse. At one point it totally lost signal and just kept extrapolating the last direction it had seen me on, which indicated that I was driving through the middle of a park. Eventually we got back to the hotel.

Now in flight, I did pick up WAAS. So when I’m following an airway that I need to be within 4 miles of the center line, I had 5 foot accuracy available, but when I was trying to tell whether I’m on one of two parallel streets that are 30 feet apart, I had 100 feet accuracy. Such is life. I have to wonder if the inability to pick up WAAS in the car was due to using the small stick antenna instead of an external one? I’ll have to look into that.

In flight, I was using the arc mode, which isn’t like the HSI I’m used to, but it had the information I needed to follow the route, without obscuring the information below. And the information below was mostly there for information and entertainment – town, highway, and river names and the like, although it was good to keep track of where the nearest airports where. At 8,000 feet in the flat terrain of New York and Pennsylvania, the terrain page is pretty useless. The “panel” page is nifty. I can see how useful it would be in IMC to check the built in instruments, or even as an emergency backup if they fail.

3 thoughts on “Adventures in GPS”

  1. Everyone gets lost trying to leave Pittsburgh. There is something about the city that makes maps look nothing like the actual roads you travel. I’m somewhat glad to hear that Pittsburgh also has the ability to interfere with GPS—it just confirms my suspicions about its Hotel California-like properties.

  2. A powered antenna for these GPSs makes a big difference – it doesn’t have to be external, just placed better.

    Another factor to consider is that some cars have laminated windshields that include a layer that weakens the GPS signal. In my Aztek, the powered puck antenna is stuck to the top of the windshield, just outside the area of that signal-blocking film.

  3. (There is “external” as in separate from the GPS unit, vs. “external” as in mounted on the outside of the vehicle. The first “external” is of course easier and gives perhaps 90% of the benefit.)

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