Bulldog Trackers can provide location data based on the mobile phone networks as well as the global positioning satellites (GPS). The mobile phone network positioning works by calculating an approximate location based on the mobile phone masts (GSM) that your tracker is communicating with and accuracy varies depending on how many masts are in your area. Whereas, GPS data is a lot more accurate.
You can set your phone app to display the last known location of your tracker based on GSM or GPS. We recommend you use GPS. The instructions provided with your tracker give fulls detail on how to change the setting.
Tap Menu, Setting then under GPS Invalid tap the option and select "Show last GPS Location"
Details of why GPS may be showing invalid can be found here.