GPS-degraded environment

A GPS-degraded environment refers to any situation or location where the Global Positioning System (GPS) signals are unreliable, weak, or completely unavailable. This could be due to several reasons:

1. **Urban Areas**: In cities with high buildings, GPS signals can become degraded due to the "urban canyon" effect. The tall buildings can block direct signals from the satellites or cause multipath errors, where the signals bounce off multiple buildings before reaching the receiver, causing confusion and inaccuracies in positioning.

1. **Indoor Locations**: GPS signals often fail to penetrate buildings, making the system unreliable or unusable indoors. The signals can be blocked or severely weakened by roofs, walls, and other structures.

1. **Forested Areas**: Dense tree cover can also interfere with GPS signals, leading to inaccurate readings.

1. **Underground or Underwater**: Obviously, GPS signals can't reach underground or underwater locations.

1. **Mountainous Terrain**: In mountainous areas, GPS signals can be blocked or reflected by the terrain, causing inaccuracies.

1. **Interference or Jamming**: Sometimes, GPS signals can be interfered with intentionally (like GPS jammers) or unintentionally (such as strong radio, radar, or microwave signals).

In these GPS-degraded or GPS-denied environments, it can be very challenging for autonomous systems like drones or rescue robots to navigate accurately. They must rely on other techniques and sensors, such as inertial navigation systems, lidar, radar, computer vision, or others to maintain awareness of their position and surroundings. These are collectively referred to as "sensor fusion" techniques, and they help ensure accurate navigation when GPS isn't available or reliable.