What is GPS?
GPS stands for Global Positioning System. It refers to a technology for determining locations using signals broadcast from a network of satellites. The technology allows a GPS receiver to determine its location (latitude, longitude and elevation) by comparing signals transmitted by GPS satellites that orbit the Earth. A GPS satellite is a device that travels around the Earth in a low orbit, transmitting data that can be used by GPS receivers to determine their location. GPS works pretty much anywhere on the planet, including remote locations. All that is needed is a GPS receiver and a clear view of the sky to receive signals from at least three or four GPS satellites. Most Android phones have a GPS receiver and Impel's Impel Touch solution leverages that to report locations, where needed.
What is GPRS?
GPRS stands for General Packet Radio Service. GPRS provides the mobile data service on 2G and 3G cellular communication systems. GPRS provides SMS broadcasting and messaging, "Always On" Internet access, Multimedia Messaging, Push to Talk and other services. Data from the device is synced back to servers via GPRS.
GPS / GPRS usage in Impel Touch
Handset locations are tracked by Impel Touch using the GPS signals. GPS locations are stored locally on the handset, if GPRS is not available. Whenever the handset gets back into a GPRS-available area, data stored on the handset is synchronized with the Impel servers on the Cloud.
GPS on Smartphones
Most smart phones including Android, Symbian, Windows Mobile, Blackberry OS and iOS phones are equipped with handset based software that helps to determine the handset's location. The Android operating system provides a straightforward mechanism to access location information derived from the GPS hardware along with other, less accurate sources like Wi-fi and cellular network connection. Impel Touch uses a combination of these techniques to figure out where the current device is.
GPS accuracy depends on a large number of factors coming together at a particular location and time. Ideal conditions for GPS accuracy are a clear view of the sky with no obstructions from 5 degrees elevation and up.
GPS requires a direct line of sight between the receiver (i.e. your phone/tablet, in the case of Impel Touch) and the satellite. When an object lies within the direct path, accuracy suffers due to reflections and weakening of signals. This is a particular problem in urban environments, within valleys and on mountain slopes. In these cases, the objects (buildings and the Earth itself) are large enough to completely block the GPS signals. When weak signals are received, they may be those reflected from buildings or surrounding landscape. Impel Touch records these problems and reports them back to the Cloud. Such reporting makes sure that, if the distances that Impel Touch records are not accurate due to environmental reasons, the user at least knows the reasons clearly.
Even when the obstructing object is not as large (tree cover, car roof or your own body or clothing), reflection and weakening of signals may still occur. When carrying a GPS device (including a phone), the higher the device is fixed, the better the reception. Good positions include the shoulder strap or the top pocket of a backpack. You may have noticed runners and walkers strap their phones onto their arms or on wristbands to ensure good GPS reception. Putting your phone in your pocket is definitely not a good idea.
Being in an enclosed space such as a steep sided valley or a high-rise building reduces the area of sky visible to the GPS receiver in your phone. First, this reduces the number of satellites that are in direct line of sight to the receiver. Second, it prevents the GPS receiver from receiving signals from a wide set of satellites. Signals from a closely clustered set of satellites can result in large positional errors. So keep an eye on your GPS device periodically, so that you are aware when the signal quality drops.
If you are travelling in a car or other vehicle, make sure that you get good GPS fix before you enter it. The "fix" is usually retained even if you momentarily lose direct line-of-sight contact with a GPS satellite.
Impel Touch and GPS/GPRS
Impel Touch uses a combination of GPS and GPRS signals to figure out where you are and how far you have travelled. When GPS is turned on in your device, Impel Touch uses it to make its calculations. But if GPS is turned on and you don't have a GPS signal (because you are in a covered area, for example, or because the phone is in your pocket), Impel Touch reports that the signal is weak, but can't do much else. If, on the other hand, you turn off GPS, Impel Touch uses GPRS to track your location. GPRS is usually more reliable than GPS in covered areas, since it does not depend on line-of-sight, but it not at all as accurate. While GPS can be accurate to a few meters, GPRS is typically accurate to a few hundreds of meters, particularly in rural areas where cell towers are not as dense as in urban areas. So distances calculated using GPRS can be quite inaccurate - to factor of being 20% off, sometimes. And there are strange situations where, even if you moved your phone just a few feet in your office/home, the GPRS locator may show you as having moved by 200 meters, since it lost connection with one cell tower and connected to another. In such cases, you can tell that the accuracy is wrong by looking at the Accuracy report in Impel - that tells you clearly how good (or bad!) the calculation can be.
Of course, Impel Touch figures out and reports the phone's location only when you are logged into it, so you can safely ensure your privacy by logging out of Impel Touch when you are not at work.
For more information on GPS tracking and accuracy, you can read some of these articles. Some of these will give you a good laugh at the strange problems that technology brings to our lives.
Wiki Open Street Map - Accuracy of GPS Data
GPS Basics = Factors Affecting GPS Accuracy
Earth Measurement - GPS Accuracy
Wikipedia - Mobile Phone tracking
Ask Me Help Desk - Phone GPS tracking