In my recent post I started a series of posts about working with GPS units on Nikon platform – that post discussed compatibility issues with different Nikon cameras. Now that we figured that out, let’s talk about how to make your GPS unit do its best while geotagging.
First, a little bit of insight into what a GPS generally is. If you know that already, feel free to skip this part. GPS stands for “Global Positioning System”. This initiative has been started as far back as 1972 by US Air Force – initially, strictly for military purposes. Later in 1983 it was declared that GPS system will also be available for civilian use once it is completed, which happened in mid-90s.
GPS positioning works off a number of satellites that are circling around the globe, and are constantly sending signals down to Earth surface. There are 24 to 32 satellites in service at any given time, and each satellite has a different signal that identifies that very satellite. A GPS receiver located somewhere on the Earth receives signals from those satellites, calculates how far each satellite currently is, and then uses that information along with some tricky math to calculate its position on Earth, as well as altitude above/below the sea level. It also receives the current date and time in UTC format, and includes that information into its output. Due to the nature of the satellite signal, here comes the first and the most important rule of improving signal reception:
- GPS receiver needs a clear sky view to do its best.
Modern GPS receivers can also work off reflected signals, but reflected signal always has lower power than a direct one, so reception quality will be lower. Sky obstructions such as tall buildings, mountains, trees, or even thick clouds will all have an impact on GPS signal reception.
Ideally, a GPS receiver would only need to lock on to three satellites to pinpoint a geographic position. However, the signals travel very fast, and slightest inaccuracy in measurements leads to a rather big positioning error. Altitude calculations require even more satellites to obtain a reasonably precise altitude – normally at least four satellite locks are needed to achieve an expected precision on the altitude positioning.
Locking on to more satellites allows to calculate a better position (both horizontally and vertically), and reduce the error margin. The more satellites a GPS receiver is locked on to, the more precise position it will report. Many popular GPS receivers (including our Promote GPS) are based on a SiRF Star III chip that is capable of receiving 20 satellite feeds at the same time. In practice however, a GPS receiver put into average conditions with a clear sky view is eventually locked onto 10 to 12 satellites, which provides about as good a precision as it might get with the current state of technology.
The GPS receiver is first started, it does not know where it is currently located – and consequently, does not know where to look for satellites. This is known as “cold start”. Therefore it has to lock on to at least one satellite first, listen to what it says about where the other satellites are, and then lock on to others. This procedure takes time, and in average requires anywhere from 45 seconds to a few minutes.
Once a GPS receiver has locked to at least 3 satellites, it will normally start reporting a geographical position. However this position will be rather inaccurate until more satellites are found and locked on to. A reasonable degree of precision is obtained once GPS receiver locks on to 5-6 satellites. This may take 2-3 minutes after an initial lock. So here comes the second rule:
- To improve GPS positioning accuracy, let it work for a few minutes after it obtains initial lock upon a cold start.
Now that GPS receiver has locked to quite a few satellites, it’s time to take off and shoot some pictures. But then you turn your camera and your GPS receiver off. What happens then? Most modern GPS receivers have a short term storage they use to memorize the satellite map from the last time it has been turned on. Then, when a GPS receiver is powered back on, it uses that information to quickly find the satellites to lock on to. This procedure is known as “warm start”. A warm start is not a strictly defined term, and it normally applies when you turn your GPS receiver back on within a few minutes of turning it off.
Sometimes the receiver will be able to re-lock to exact same satellites it worked with before being turned off. This is known as “hot start”, and it normally takes 1-2 seconds only.
The more time a GPS receiver spends turned off, the more the satellite visibility will change. Therefore, the longer the GPS receiver is off, the slimmer is the chance of getting a hot or warm start. But here comes an interesting point – the more satellites your GPS was locked on to, the bigger is the chance of relocking to at least a few of them – and as soon as it gets 3-4 locks, we are in business! So here comes the third rule:
- Letting the GPS receiver work for a bit after locking on will also speed up further satellite reacquisition after restart.
Let’s summarize the three rules we found out so far:
1. GPS receiver needs a clear sky view. Make sure it can see the sky for best results.
2. Let the GPS work for a few minutes after obtaining initial lock to improve precision.
3. Do the same if you want GPS to get back online as quickly as possible after cycling its power.
Ideally, a GPS receiver that is always on will report the most precise position – but it will also deplete your batteries faster. From the other side, turning it off means you will need to wait until it locks back before continuing to taking pictures. Of course, there is always a balanced approach that depends on your needs and current environment. In my further posts I will elaborate on how you could balance between power consumption and positioning reliability / accuracy.