Basic things you need to know about GPS (Global Positioning System)*

GPS (Global Positioning System)
It is a system of 24 satellites (launched and maintain by US Department of Defense) orbiting the earth on exactly defined tracks that provide the whereabouts (longitude, latitude, altitude, speed, etc) to a user using the GPS receiver. At any point in time, a GPS receiver can only receiver up to 12 satellite signals because the other 12 are under the horizon (i.e. on the other side of the Earth)
GPS receivers must maintain line-of-sight with the satellites. GPS will not work indoor and its accuracy tends to degrade significantly when the receiver is located under sheltered area or near high-rise building structure or terrain.

2D Positioning Fix
A satellite lock that allows GPS users to know the longitude and latitude of their whereabouts.

3D Positioning Fix
A satellite lock that allows GPS users to know the longitude, latitude AND altitude of their whereabouts.

A-GPS
Aided GPS describes a system where outside sources, such as an assistance server (by providing aid to the Almanac or Ephemeris data) help a GPS receiver perform the tasksrequired to make range measurements and position solutions. With such assistance from the network, the receiver can operate more quickly and efficiently than it would unassisted, because a set of tasks that it would normally handle is shared with the assistance server.

Almanac Data
When the satellites try to acquire a lock they need to rely on both Almanac and Ephemeris data. Almanac data is course orbital parameters for all satellites in the GPS constellation which isn't very accurate information but is usually current for up to several months.

Ephemeris Data
When the satellites try to acquire a lock they need to rely on both Almanac and Ephemeris data. Ephemeris data which is very precise orbital and clock correction for each satellite and is required for precise positioning, eg 3D fix. Each satellite broadcasts only it's Ephemeris data which has a life span of approx 5 hours per satellite. Each satellite will broadcast the Ephemeris data for a 30 second period, and then re-transmit, so if the GPS receiver loses track of the data part way through the 30 second cycle, you will have to start again at the next 30 second cycle.

Time To First Fix (TTFF)
(also known as factory start)
If you have not used your GPS unit for several days, the almanac data for the satellites may be out of date. The unit is capable of recollecting this information on its own, but the process can take several minutes up to fifteen minutes.
Time to First Fix (TTFF) is the time it takes a GPS receiver to find satellites after the user first turns it on (from factory default), when the GPS receiver has lost memory, or has been moved over 300 miles from its last location.

Cold Start
Occurs after 4hrs from the last GPS usage, the GPS receiver has some knowledge of Almanac data but no Ephemeris data. Almanac data is not precise, but current for several months. When a cold start is required, the receiver has to download a full set of Ephemeris data. The time for the receiver to get its fix from a cold start can be from 40 seconds to 3 minutes on average.

Warm Start
Occurs within 4hrs from the last GPS usage, the receiver has some Ephemeris data. Time for receiver to get its fix from warm start can be from 20 seconds to 1 minute on average.

Hot Start
Occurs within a short period from the last GPS usage. The receiver has almost a complete set of the Ephemeris data. Time for the receiver to get its fix can be from 1 second to 20 seconds on average.

* Note that some of the terms are used interchangeably, e.g. TTFF is sometimes used to describe the time for the GPS receiver to get retrieve its coordinates from cold start or warm start.

The above terms are compiled by users at Trends Mobile based on our own experience as well as other sources relating to the topic of GPS.