Understanding TETRA
VectorAlert is built around a deep understanding of the radio technology used by UK emergency services. Here is how it works.
What is TETRA?
TETRA — Terrestrial Trunked Radio — is a professional digital radio standard designed specifically for emergency services and critical communications. In the UK it forms the backbone of the Airwave network, used by police, fire, ambulance and other authorised organisations across England, Wales and Scotland. Northern Ireland operates a similar system called Barracuda.
TETRA replaced older analogue radio systems because it offers encrypted communications, much greater range, better in-building penetration and the ability to support both individual and group calls simultaneously.
How TETRA works
TETRA uses Time Division Multiple Access — TDMA — to carry communications. Rather than transmitting a continuous signal, TETRA radios transmit in short, rapid bursts, with four separate users sharing a single 25 kHz radio channel by taking turns in precisely timed slots. This makes extremely efficient use of the available radio spectrum.
Every TETRA radio — whether carried by an officer on foot or fitted to a vehicle — continuously transmits short periodic registration bursts to maintain its connection to the network. These bursts occur even when no voice communication is taking place, and even when the radio appears to be idle. This is the signal that VectorAlert detects.
UK frequency allocation
In the UK, emergency services TETRA operates in the 380–400 MHz frequency band. Specifically:
— 380–385 MHz is allocated for mobile uplink transmissions — signals from handheld and vehicle radios to the network — 390–395 MHz is allocated for base station downlink transmissions — signals from network masts to the radios
VectorAlert monitors both bands simultaneously, providing comprehensive coverage of all emergency service TETRA activity in your vicinity.
Transmission power
TETRA radios operate at varying power levels depending on their type. Handheld radios carried by officers typically transmit at 1 to 1.8 Watts — enough for reliable communication with the nearest base station while remaining practical for everyday carry. Vehicle-mounted mobile units transmit at significantly higher power, typically up to 10 Watts, with some installations capable of up to 30 Watts.
Network power control dynamically adjusts transmission power based on signal conditions, meaning a radio close to a base station will transmit at lower power than one at the edge of coverage. VectorAlert is sensitive enough to detect both handheld and vehicle-mounted units at useful distances.
Why TETRA is detectable
Because every TETRA radio must continuously maintain its network registration, it transmits periodic bursts regardless of whether it is actively being used for voice communications. A police officer sitting in an unmarked car, making no radio calls whatsoever, is still generating detectable TETRA activity as their radio maintains its network connection.
This is the fundamental principle behind VectorAlert. No communications are decoded or intercepted — VectorAlert simply recognises the characteristic pattern of TETRA radio activity and alerts you to its presence.
The future — Emergency Services Network
The UK government has been developing a replacement for the Airwave TETRA network called the Emergency Services Network — ESN — based on 4G LTE technology. The transition has faced significant delays and TETRA remains the operational standard for UK emergency services. VectorAlert monitors the established TETRA bands and will be updated via free firmware upgrade to support any future changes as the network evolves.
VectorAlert does not decode, intercept or record any TETRA communications. It detects only the presence of radio activity. Legal to use in the UK.