Your Bulldog Tracker is designed to check it's location every few minutes and the data received allows you to see your tracker in it's current location.
Some trackers are designed to "sleep" when they have not been moved and there is no vibration detected by the tracker. This feature is designed to preserve battery life of both the tracker and the vehicle the tracker is attached to.
If the GPS Location is showing as Invalid, this just means that it was the last location of the tracker before it went into sleep mode. As soon as your tracker detects any vibration or movement, it will wake up and start reporting live positioning data.