Ensures optimal GPS reception for your TomTom: Some athermic heat-reflecting windshields and built-in windshield heaters block GPS signal reception inside the car. The magnetic, waterproof External Antenna overcomes this and ensures optimal GPS reception, should you place it inside or outside the car.CompatibilityTomTom GO 720, GO 920, GO 920 T, ONE, ONE XL, ONE XL*S TomTom GO 510, GO 910