Why is my AIS receiver not detecting all vessels correctly?

This issue is commonly reported and is a fundamental limitation of AIS receivers rather than a fault with the device itself. The performance of an AIS receiver is affected by radio transmission physics, meaning there is always a trade-off between maximum and minimum operational range.

If the receiver is too close to several AIS transponders, the device can become “swamped” with signals, making it difficult to process the information correctly. Conversely, if a vessel is too far away, the AIS message integrity deteriorates, reducing the chance of accurate reception.

To understand this better, think of two people shouting at each other:

• If they are too far apart, they can barely hear each other and struggle to understand.
• If they shout directly into each other’s ears, the sound is overwhelming and unclear.

Since AIS transponders do not adjust their transmission power, they are always broadcasting at the same level, aiming to reach as many receivers as possible.

Solutions to Improve AIS Reception

1. Increase the distance between the receiver and the transponders.
   • If possible, move the receiver further from the harbour to improve reception of vessels within the port.
   • This will reduce signal overload but may also slightly reduce offshore reception.
2. Reduce the received signal strength.
   • Use a lower-gain antenna (a shorter antenna).
   • Introduce a small attenuator in the antenna path to reduce signal amplification.
3. Hardware modifications (not recommended).
   • The receiver is designed with high sensitivity to maximise range.
   • Reducing amplification would require redesign and regulatory re-certification (EMC, RED/FCC compliance).
   • This option is not economically feasible unless significant demand exists.

Recommended Approach

The simplest and most cost-effective solution is Option 1, which involves repositioning the receiver to balance range and accuracy. Option 2 can also be effective if repositioning is not viable, allowing the use of an additional receiver with a lower-gain antenna for close-range monitoring.

Thousands of these receivers are used globally with these configurations, and this limitation is well understood within the industry. Let us know if you need further assistance.