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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.

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I've seen something similar with Garmins - but all the standalone GPSs I've used have been Garmin - where it seems as though getting a lock can get stuck, giving the impression that it's trying too hard to lock on to one satellite, when it should give up on that and try for another. –  Chris H Sep 11 '13 at 12:13

3 Answers 3

up vote 4 down vote accepted

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.

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1  
Hey, nice citation! I'm going to edit your answer to add an interesting tidbit about ephemiris data. –  Mac Sep 13 '13 at 6:57
    
Marked this as accepted as the info on Wikipedia has led me to what has been the most successful solution. –  Mac Nov 9 '13 at 21:59

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.

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+1 I have an Edge 800 and would go along with this. I always wait until I have a lock before riding, and it just so happens that outside my home (which is where 95% of my rides start) there is a clear view of the sky. If I were to estimate how long it takes to get a lock, I'd say less than a minute. Agree also with the second point - locking takes longer for e.g. first ride on holiday or first ride back home. The unit obviously stores where to look for satellites and will use these values as a first resort. –  PeteH Sep 11 '13 at 14:21
    
Yeah, I lost signal once on a train trip, and didn't get a GPS lock until pretty close to my destination. Apparently it's pretty hard to get a lock on the satellite when you're moving at 160 km/h. Even at the stops it didn't have enough time get the signal before the train was moving again. With my GPS, I find I often have a signal less than 5 seconds after it has "booted" which only takes about 30 seconds. I picked the one I did because I was unimpressed with the feature-set of "cycling" GPS Units for the price they were asking. –  Kibbee Sep 11 '13 at 16:03
    
yes they don't come cheap. I started off with a 500 but upgraded to an 800 for the maps - I find these invaluable when I'm riding unknown routes and have an array of OSM maps which come for free (in fact I reckon a large part of my boot-up time is loading these maps, around 4GB worth). Also one of the reasons I cycle is for weight maintenance and I can get the 800 to boogie with a Body Composition Monitor which tells me things like body fat etc. So I figure I'm squeezing as much as I can out of it. But yeah, in large part a toy, got it because I could. –  PeteH Sep 11 '13 at 16:17

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.

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My unit doesn't show how many satellites it can see... there's pretty much no progress indicator which is part of the reason it's frustrating me. –  Mac Sep 10 '13 at 3:19
    
I was using an iPhone but the big downsides are: limited battery life, I broke one when it fell off my handlebars into a river (in a biologic case), and the inconvenience of having to start it, put it in my backpack, ride, stop, open my backpack to stop etc –  Mac Sep 10 '13 at 3:21
    
Sorry, I thought yours is 205. I have 305 which is basically 205 with HRM. 200 is a completely different model. Battery life is an issue, i though it's ok for commuting. Still, you shouldn't need to touch it if you only want to log rides because the apps have auto-start/stop. –  imel96 Sep 10 '13 at 6:48
    
Personally, I've the exact opposite experience. My phone, which has the ability to use the cellular network takes 3-5 minutes to acquire a signal. I've talked with other people, and they think that's a normal timeframe for acquiring your position. My GPS on the other hand, gets the signal within 30 seconds, almost every time. Often within a few seconds of booting. –  Kibbee Sep 11 '13 at 19:12
    
@Kibbee that could be the case too. You probably have a newer GPS than my edge 305. Newer chips are much faster. Cellular can be slow when it covers large area. Wifi is the fastest, you turn it on and it know exactly than your within 20 m from the hotspot. Another factor is cellular & wifi rely on data communication to know the locations. –  imel96 Sep 12 '13 at 0:59

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