Beacon Density & Geometry
The Geometry of Precision (GDOP)
Accuracy is driven by Angle of Arrival, mathematically described as Geometric Dilution of Precision (GDOP). If your APs are too close to each other or arranged in a straight line, the intersection of their range "circles" creates a massive area of uncertainty.
The Fix: Always stagger your APs in a Diamond or Staggered Grid configuration.
The Z-Axis Instability Trap
Most indoor navigation failures occur in the vertical domain. This is due to two physical factors:
1. High VDOP (The Pancake Effect)
If all your Access Points are placed at the same height (e.g., on tripods at field level), the vertical separation is zero. The Vertical Dilution of Precision (VDOP) is essentially infinite. The drone will know its latitude/longitude, but its altitude will jump wildly between 1m and 15m.
2. Ground Reflections (Multipath)
Radio waves reflect off concrete floors and stadium seating.
- Constructive/Destructive Interference: The direct wave and the reflected wave can cancel each other out at specific altitudes (nulls).
- The Reaction: If you use BLE beacons for altitude, the drone might fly into a signal null, interpret it as a sudden 10-meter climb, and cut motor power to descend—resulting in a hard crash.
Deployment Strategy
To ensure 3D stability, you must create Vertical Diversity:
- 50% Low: Mount anchors at 2m (tripods/hoardings).
- 50% High: Mount anchors at 8m-12m (stands/rigging).
- Avoid 90-degree Zenith: Do not fly directly underneath an AP. RTT accuracy is best when the angle of incidence is shallow.
Layout Density
- Optimal Spacing: 20-30 meters.
- Stadium Coverage: A standard pitch requires 8 to 12 APs for robust coverage. A 4-corner setup is insufficient as it leaves a massive dead zone in the center.
Bluetooth (BLE): The Secondary Constraint
While BLE beacons (iBeacon/Eddystone) are too jumpy for primary flight control (especially on the Z-axis), they are extremely valuable as coarse logical constraints.
1. Zone Detection
Stick a BLE beacon to a wall or near an expensive scoreboard. Your app can use the signal strength to define a "Hard No-Fly Zone." Even if the Wi-Fi RTT fix drifts, a high RSSI from a specific BLE ID can trigger an immediate software-level override to prevent a collision.
2. Waypoint Passing (Gate Correction)
In a racing or survey scenario, place a beacon on each gate or corner. When the drone passes through a gate, the app "hears" the beacon and can reset its accumulated drift error for that specific coordinate.
The Rule: Use BLE for "Am I near here?" logic. Never use it for "How high am I?" flight control.