the phone gps usually works, but sometimes it seems to struggle and you can see gaps in the route when i get home. It isn’t tree cover, buildings, etc because on the same route one day it is fine the next not so.

I know a phone is not a dedicated gps and as such compromises are probably made, but is there anything that can be done to improve performance?

Hold it above your head, so that its closer to the satellites.

cloudy days?

Where do you have it when you are riding? It’s possible that you are blocking the signal with your body.

Wet trees block the signal really well, dry ones not so much. So, even on the same route, you can get different results.

i usually put it in a jersey pocket. The tree thing might explain sometimes, certainly not a universal answer though.

cloudy days?

Is there another kind of day?

Bluetooth GPS receiver sited where it can get a clear view of the sky

Is there another kind of day?

Very cloudy days?
Do you lose signal at the same places?

Do you lose signal at the same places?

No, if you look on Garmin/Endomondo/Strava you see that sections of the route are sometimes missed. It might be a few tens of metres or it could be 100’s of metres. Curious as much as anything, is it the phone, the gps, the OS, etc.

Which handset?

What phone? I know that having wifi turned on on the iPhone is supposed to help GPS accuracy.

I use a bar mount and get really good traces, even under tree cover, so letting it “see” the satellites definitely helps.

samsung galaxy s running CM9,

The Galaxy S had notoriously bad GPS on launch. Last I looked, a long while ago, it was being fixed / improved in firmware; I’d suggest making sure you’re on the latest ROM, but if you’re running a CM then I’ve no idea (unless it’s a really old version I suppose).

I had heard the GS wasn’t brilliant but had been fixed. It is a relatively new phone so would suspect firmware was ok.

Just curious why one day it will be spot on and the next not?

It is a relatively new phone

It’s two years old and superseded by the S2 and S3, that’s not particularly new. Regardless, you’re running CM9 which will dictate the age of the firmware. Are you on the latest release?