Should I wait for clear views of the sky before turning on my Garmin Edge GPS?

Question

I have a Garmin Edge 200 GPS unit. I commute from the center of Melbourne CBD, where there are many tall buildings and not a good line of sight to satellites. I park in an underground garage, so there isn't an option to turn on the GPS for a few minutes before I set off.

I've found through experimentation that if I turn on the GPS as soon as I start riding, it takes longer to get a satellite lock than if I wait until I have clearer views of the sky and then turn it on.

So if you consider the first 5 kilometers of the ride, if I turn it on at kilometer 0, it can be 5 kilometers (or even longer) before it gets a lock. But if I turn it on at kilometer 2 (when I'm clear of the tallest buildings), it will get a lock almost immediately.

Is there any basis to my experience? Am I better off waiting until I'm a bit clearer, or is there another strategy I can use to try to capture my entire ride?

Update:

I've experimented with a few things, and have found that the most reliable method seems to be to turn it on and then not move until I get a fix. This works quickly regardless of the buildings around me. It seems that temporarily blocking a satellite by moving makes it take a lot longer.

Answer 1

It is probably worth switching on the device as soon as you start heading home, even if you are inside a building.

While it may not acquire a lock inside, the signal might be strong enough to download some of the data the device has to acquire, meaning it locks quicker once you reach clearer skies.

It's also worth bearing in mind that acquiring a GPS signal involves quite a lot of complicated steps, many of which might impact "lock time" in seemingly random manners.. The GPS signals page on Wikipedia gives a good overview (or, shows how complicated the system is)

From wikipedia:

If the signal from a satellite is lost while its ephemeris data is being acquired, the receiver must discard that data and start again.

This may be causing some of the slowdown if buildings briefly occlude the satellite and it needs to start downloading the ephemeris data from scratch.

Answer 2

I don't have any experience with that particular unit, but I do have a Garmin Oregon 450, which is a "hiking" GPS, but I find it works great for cycling. I've found there's 2 important things to getting a lock on satellites. First, as you mention, is a clear view of the sky. The second important thing is don't move. The faster you are moving the more difficult it is for the unit to lock onto a satellite.

The best advice is to turn it on before you ride and wait until it has a lock before you start riding. I've found that I don't really need a view of the sky, as I can turn it on in my house or in my garage, and get a signal no problem. I can't however, turn it on in my office, where there's quite a bit of concrete around me.

I also find it helps if you turn it on in the last place you turned it off. I think the GPS assumes that it's in the same spot it was last time, and tries to lock onto the same satellites. I find if I travel with the unit turned off, it takes longer to get a signal when I do turn it on. I don't know if this is how it actually works, but this is how it seems to work for me.

Signal is also affected by where you are on the earth. It works better when you're closer to the equator. You're in Melbourne, which isn't close to the equator (-38 degrees latitude, but a little closer than I am up in Canada (45 degrees latitude). I don't think you're far enough that it should be a serious problem.

Answer 3

It's a common problem with GPS that has no wifi or cellular network assistance. In any case you should wait for the GPS to have a lock before you ride. What I usually do is get out on the road, stop, wait for lock then continue riding.

You obviously don't want to stop and wait forever. Good thing is while it's trying lock your position, you can see on the display how many satellites it can see and how strong their signals are. So, if just outside your office you see that only there are only three satellites the GPS can see, better to go to a near open space (e.g. the federation square) and try to get a lock there.

Other option is to use a mobile that has a GPS. Mobiles can use wifi and cellular network (which are plenty in CBDs) to lock position quickly. Possible down side is less accuracy on cheap mobiles, but I don't think that's a big issue for commuting where the roads around cities are straight. Mobiles should also be more accurate with tall buildings around where satellites signal are patchy and completely blanketed by wifi and cellular networks.