Your Garmin GPS navigation device must lock onto a minimum of three satellite signals to determine its latitude and longitude and four signals to calculate altitude as well. The device also may be able to determine its direction and speed by utilising satellite signals.
Your Garmin must have a good view of the sky to lock onto satellite signals. The more visible sky there is available, the quicker and easier it will be for the unit to acquire signals. Overhanging tree canopies, tall buildings and other obstructions can slow the process.
If you travel more than 500 miles with your Garmin device turned off, it will take longer for the unit to acquire satellite signals the next time you turn it on. Many of the Garmin units allow you to set the location under settings, which helps the GPS device lock onto signals more efficiently.
Most Garmin GPS navigation units cannot acquire satellites while they are inside a building or parking garage. Take your Garmin outside to lock onto signals.
Some car windows have special coatings that reflect heat or have heating devices embedded in them to melt snow and ice. These types of windows prohibit a GPS unit from effectively acquiring satellite signals. You might have to install an external antenna.
More Satellite Signals.
The more signals that the unit can acquire, the longer it will work accurately in obstructed areas. Allow your Garmin to have a clear view of the sky for at least five minutes and up to 20 minutes before you go into densely wooded areas, caves or other places with poor signal reception.