GAP & Advertising DF

Direction Finding

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Bluetooth 5.1 feature using Angle of Arrival (AoA) or Angle of Departure (AoD) with antenna arrays for direction estimation.

    


    
    
    

      Also known as:
      
      AoA
      
      AoD
      
      Angle of Arrival
      
      Angle of Departure
      
    

    
  


  




  
  

    
Direction Finding


Advertising">Direction Finding (DF) is a Bluetooth 5.1 feature that enables BLE devices to determine the direction of a received signal using antenna arrays. It provides two complementary methods -- Angle of Arrival (AoA) and Angle of Departure (AoD) -- that can determine the angular position of a transmitter or receiver with sub-degree accuracy.


Angle of Arrival (AoA)


In AoA mode, the transmitting device sends a signal using a single antenna, while the receiving device uses an array of antennas. The receiver samples the incoming signal across its antenna elements in rapid succession during a Constant Tone Extension (CTE) appended to BLE packets. By measuring the phase difference between antenna elements, the receiver calculates the angle from which the signal arrived.


AoA is typically used when the locating infrastructure has antenna arrays (e.g., ceiling-mounted locators in a warehouse) and the tracked device is a simple BLE tag with a single antenna. The infrastructure calculates the tag's position based on the angle measurements from multiple locators, keeping the tracked device simple and inexpensive.


Angle of Departure (AoD)


In AoD mode, the relationship is reversed -- the transmitting device has an antenna array and switches between elements during the CTE period, while the receiver uses a single antenna. The receiver measures the phase shifts caused by the antenna switching pattern and computes the departure angle.


AoD is used when the mobile device (e.g., a smartphone) performs the position calculation. BLE beacons with antenna arrays transmit AoD signals, and the phone computes its own position based on the known beacon locations and measured angles. This approach keeps the computation on the mobile device and scales to any number of simultaneous users.


Accuracy and Infrastructure


Direction Finding typically achieves angular accuracy of 1--5 degrees, translating to positional accuracy of 0.5--1 meter at typical indoor distances. Accuracy depends on the number of antenna elements, array geometry, multipath environment, and calibration quality.


For higher precision, Direction Finding can be combined with Channel Sounding (Bluetooth 6.0), which provides accurate distance measurement. Together, angle plus distance gives full 3D positioning for next-generation indoor positioning systems. Hardware support is available on SoCs like the nRF52833 and nRF5340.

  

  

  
  
  




  
  
Related Terms

  

  

    
    RSSI
    
    Channel Sounding
    
    Indoor Positioning System
    
  





  

  
  

  
  

  
  
  

    
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Frequently Asked Questions


  

    
    

      
      

        
Our glossary covers 90+ BLE technical terms organized by category. Each term includes a definition, related terms, and links to relevant chips and guides.