Controlling gps on/off?

**Elder** · 09-12-2006, 12:21 AM

I've been usings bu-303 units and they've been going fine, except now I have the need to switch off my GPS units for short periods of time (I'm talking 1 second at most, just enough time to do a scan with another antenna next to the GPS unit without the GPS interfering). The GPS unit then needs to find satellites again within a few seconds, I believe it only takes a few seconds to find satellites again after a very short turn off period but if this isn't true please let me know!

I've only really started to look into this, but from what I've read it seems I can't disable power to individual usb ports, so I cant turn the GPS on/off by usb control :( If that is so, I need to either find a GPS unit that I can send an on/off command to, or I need to cut the usb power line and add a software controlled switch (I really dont want to have to do the last option!). Does anyone know of GPS units with on/off command support?

**Wiredwrx** · 09-12-2006, 01:24 AM

Originally Posted by Elder

I've been usings bu-303 units and they've been going fine, except now I have the need to switch off my GPS units for short periods of time (I'm talking 1 second at most, just enough time to do a scan with another antenna next to the GPS unit without the GPS interfering). The GPS unit then needs to find satellites again within a few seconds, I believe it only takes a few seconds to find satellites again after a very short turn off period but if this isn't true please let me know!

I've only really started to look into this, but from what I've read it seems I can't disable power to individual usb ports, so I cant turn the GPS on/off by usb control

If that is so, I need to either find a GPS unit that I can send an on/off command to, or I need to cut the usb power line and add a software controlled switch (I really dont want to have to do the last option!). Does anyone know of GPS units with on/off command support?

Well, a couple of things.

First, you can interupt the power wires of the GPS cable, to turn off the unit. You can do this manually, or you can use some sort of relay controlled by a computer controller.

The other thing is, GPS units are RECEIVERS, and do NOT transmit. While it is true there is an oscilator in the unit, and that can cause some very low level RF, it shouldn't cause any interference to other communications devices. What frequency do you think is being affected. The EASIER solution is to seperate the antennas by at least a wavelength. Since GPS is in the 2.4 gig range, you can get significant isolation with just 5 inches of seperation, though the more, the better.

Michael

**Jahntassa** · 09-12-2006, 02:25 AM

Just as factual correction, GPS currently uses three signals, on 1575.42 MHz, 1227.60 MHz, and 1381.05 MHz. There are two additional signals being studied, 1810.40 MHz, and 1176.45 MHz.

While I haven't done the math, I doubt even the harmonics would be enough to interfere with anything. And unless there's some sort of battery keeping the logic in the GPS running (which, of course, would also keep the reciever circuit working) you're probably looking at at least 30-60 seconds of 'warm' reacquisition time.

**Elder** · 09-12-2006, 03:46 AM

Thanks guys.

The other receiver is a broadband VLF (1khz - 30khz) receiver that is very sensitive. I am not 100% sure the GPS is going to create noise, but I've been told that similar units have had GPS interference problems in the past, this could have been a hand held unit with all the bells and whistles (lcd etc). But still the easiest way to avoid trouble is to just turn it off. I was thinking a transistor switch rather than a relay, these things are pretty low power...either way its not really that hard its just one of those messy things I'd rather avoid.

As far as 'reaquisition' goes. Some GPS specs have a 'Snap start' value, usually around 1 second or 2 seconds or so. I'm pretty sure I read this is the time for reaquisition at the same spot for short turn offs....I think the document I read used the example of starting your car shortly after stopping it as an example of a 'snap start'. I hope that is the real meaning of snap start

Edit: can't find a definition of snap start, but one of the gps units had this written for snap start: (looks promising)
Snap Start: < 3 sec (at < 25 minutes off period)