Thread: BU-353 Low Signal when starting in underground garage

That has nothing to do with power. Punky explained it correctly. After some time of not receiving a signal from any sats, it will reset to zero knowledge and begin a search which can take quite a long time. One idea would be to unplug it before entering the garage and plug it back in when you leave to make it pick back up with the saved state.

I was going by Nomader....

I guess mine is smart. Without power or without an update, it just retains its last data.
I can't think of anything more sensible.

You could also build a GPS retransmitter. Strictly speaking they're illegal, but I don't think anyone will notice if you use one underground... Just depends on how far away the nearest location is you could put a GPS antenna...

Aside from the antenna, the parts you need are mere pennies... (A bit of #26 AWG, #22 AWG (or thicker), a capacitor and a power supply)...
(See also http://www.bytethebullet.com/geocach...ng_antenna.htm and similar projects, or ready made from outlets like these: http://www.totalpda.co.uk/Carcomm/Ca...enna.7477.html)

Originally Posted by OldSpark

I was going by Nomader....

I guess mine is smart. Without power or without an update, it just retains its last data.
I can't think of anything more sensible.

That's correct as well, as long as it's within probably 1-2 days. That's assuming it's powered off with a lock. When it powers back on, it will see that it's still on Earth in about the same spot that it remembers. But when it's been blinded for several minutes, it will reset and begin acquisition. If powered off before a lock, it won't save it's place so it will start back up at the beginning.

Mines lasted at least a week...

And if it's been moved, it boots with the last live mapping.
Once the signal is received, it's only a few seconds for the new position, then maybe a few seconds of rotations till it figures my direction (it can spin worse than a tanker in a North Sea storm).

But I'd expect that from a cheap GPS.

Like I said - if it has novol then no problems.
But even if not, I can't understand long update delays.

When underground, mine stays at the last known position - hence no benefit for illegal repeaters - not that I understand why you'd bother even if it were practicable.

It sounds like you have the MTK chipset. What model is it?

If it can't hear any sat that it's locked to, it's not going to just stop until it does. It would never work that way. It has to search. They aren't in stationary orbit even in Australia. You probably just don't notice when it happens and have a good view of the sky, which should take 1-2 minutes. 10 minutes would mean the antenna probably has a very narrow or obscured view.

My Tom-Tom has this problem, but the iPad does not. I think it uses AGPS (Assisted) and it is able to get a quick low accuracy estimate of its location, which makes it quicker to lock on.

Mine's a "Go Cruise" model GPS3503 (3.5"). It has inbuilt 850mA LiIon.
I did open it up as soon as I got it, but I don't recall the chips - I have the info or pics somewhere...

I'm not sure what you mean by "it has to search". Yes it does, but until it finds, what does it do?

If mine's been off for a distance, when switched on, it'll be at the last on location. SatLock probably takes less than 5 seconds with the map update maybe another 5 secs (that's an estimate - it's never been a concern; and IMO it's not slow).

When underground, it sits at the last location... That's in car parks etc.
I can't recall what it does in tunnels... (it's only a GPS - no data from wheels etc), but I presume the same freeze and then resume after lock & update....

Until it finds one, it will scan the entire almanac, starting at sat 0 continually. If it can receive even 1 sat in memory on startup it will warm start with the active list, but if there's no reception of even one in memory when powering on it will reset to cold start.

So basically, when you power up with a good view you'll get a fix in seconds, assuming it hasn't been over a week since last run. But when powering up underground, it will wipe all non-transmitting sats and start the scan, and quite possibly be past the valid sats when it hits sky. This would mean it has to run all the way through the list of 32, wrap around and start over before finding the first one and begin to build the active list again. That's a worst case scenerio, but still shouldn't take more than 2-3 minutes and 5 seconds would be impossible. There are a whole lot of time based calculations going on that take... time.

AGPS allows for faster lock because it downloads a current almanac (position of sats) from the net and possibly ephemris (calculations for movement in time) and hot starts with a current location (or close enough).