Faster GPS fixes

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In recent my post about GPS logs I mentioned that GPS devices can take a while to get a satellite fix when they’re turned on or brought outdoors. With a clear view of the sky and a GPS which has recently been tracking satellites it can sometimes take less than 30 seconds to get a fix, but in areas of poor reception (e.g. under wet trees, in a camera bag, or even in your pocket)  the data connection can get interrupted and gets continually restarted. It can sometimes take a very long time (e.g. 15 minutes or longer) for a location fix to be achieved.

Sometimes there’s data from other GPS units we can use instead (later in Lightroom) but it’s so much more convenient if we can accelerate this initial fix. Some devices support “A-GPS” (assisted GPS) for this.

A-GPS

A-GPS takes two forms. The basic function is a pre-loaded set of satellite orbit data which can reduce the time required for an initial fix to less than 20 seconds. By knowing exactly where the satellites are the GPS can then work out where the unit is. Without A-GPS the unit has to slowly download the current data from the GPS satellites, which can take almost 15 minutes if there are no interruptions to the signal, so already having the data avoids this delay. The data usually needs to be downloaded from the Internet, but will be accurate for several weeks following (while the satellites only transmit data valid for the next four hours).

There’s also an extended form of A-GPS which also mixes in location information from mobile phone (cell) towers and WiFi hotspots, and can be particularly effective in indoor and underground environments. The GPS functions of phones/tablets usually automatically update themselves with data from the network, but the basic mode can also be used in many other GPS devices.

My EOS 7D Mark II doesn’t support A-GPS, but the Olympus TOUGH TG-4 and Sony HDR-AS30V do. In fact the TG-4 prompts you to install A-GPS data as soon as you enable the GPS. I can use software on my iPhone or on my laptop to download the current A-GPS data and install it into the camera’s GPS. Thereafter (until the data expires) the GPS acquisition is usually very fast. A-GPS data can be valid for up to 4 weeks, but some older devices only support 14 days of data. The Olympus documentation mentions both “14 days” and “4 weeks”, but the TG-4 definitely takes a month.

Installing the updated data

The Olympus PC/Mac program needs the camera to be connected via USB for it to then download the data from the net and install it into the camera. The TG-4 can also have its data updated via the Olympus Image Track iOS app (there’s also an Android version). This is done in two stages: first the data is downloaded (via WiFi or via mobile data) and then when the phone/tablet is connected to the camera’s WiFi, the data can be transferred to the camera.

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The Sony PlayMemories Home application will either install the data to a USB-connected Sony camera, or you can install the data onto a memory card which you’ll later put into the camera. The data file (Private/Sony/GPS/assistme.dat) is 270 kB in size. But rather than using a memory card I prefer to know that the camera itself has been updated, and be free to later put whatever card in the camera suits me.

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Have a routine

I have a standing reminder in my calendar to update A-GPS data every 3 weeks. My devices use 4 weeks of data, but it’s better to update sooner rather than later, and I don’t always actually manage to update the cameras when the reminder goes off. If my devices only stored 2 weeks of data I’d probably try to update them every week or so (on the assumption I’d sometimes miss a week). The update needs an Internet connection to download the A-GPS data, so if I’m heading off on an expedition I do the update as late as possible before departure.

Remember, this is all just a convenience. If the A-GPS data is not installed (or is out of date) then the GPS will still work and will in general be just as accurate, it will just take a little longer to get a fix (and thus some images may not have position data). The TG-4 can display (via the menus) the expiration date of the currently-loaded data, but the AS30V doesn’t give any indications.

Mind you as well as updating A-GPS data on my Olympus and Sony devices, at the same time (e.g. every 2-3 weeks or so) I take the opportunity to check the clocks on my non-GPS cameras (the EOS 5D Mark III and M2 cameras). These don’t get automatically updated by GPS, and I want the clocks to all match. This helps later geocoding (see the earlier GPS post) and for just keeping all the photos in the right order when sorted by Capture Time in Lightroom. My laptop’s clock gets set accurately via the Internet, and the EOS Utility software uses that to set the time on the cameras via USB.

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