Two sensitivity values can be specified for GPS receivers – acquisition sensitivity and signal tracking sensitivity. Acquisition sensitivity is the lowest power level at which a receiver is able to achieve a position fix which requires a lock on four GPS satellites to achieve. Signal tracking sensitivity (commonly referred to simply as tracking sensitivity) is the lowest power level at which a receiver is able to track an individual satellite. GPS receivers with a lower tracking sensitivity are more capable of detecting very weak GPS satellite signals which are most likely to occur in urban canyons due to the blocking and attenuating influence of buildings or other structures.
The tracking sensitivity of a GPS receiver does not refer to tracking of the GPS receiver as is the case for GPS enabled tracking devices. The GPS satellite system is what is referred to as ‘open loop’ as it does not require any feedback from GPS receivers to function. It is simply the broadcast of a specialized radio signal from each of the GPS satellites. A GPS enabled tracking device requires either a 3G mobile network connection or a communication satellite connection to report back the position determined from the GPS satellite signals.
For further information on how GPS’s work, have a look at the following article