The BRAS system perates in the frequency band 1660 to 2115 kHz. As stated above a chain consists of a master and two slave stations. The working cycle of a chain is just over one second. The cycle is split into 4 periods of approximately 270 mS; the master station transmits during slots 1 and 3 and the first slave during slot 2, and the second slave during slot 4. Each transmission is actually about 230 mS long, and is followed by a 40mS silence period before the transmission in the next time slot begins.
The RF frequencies radiated by the chain are all multiples of a base frequency, and the pulse timings are also derived from the base frequency. 22 different base frequencies in the range 25.8 to 26.4 kHz therefore provide sufficient 'channels' for different chains to operate in.
An example of how the frequencies are derived is given here, for a base frequency of
At each station is a reference oscillator which runs at the base frequency. From this are derived 6 different RF frequencies, 4 of which (f1, f2, f3, f4) are used for navigation purposes. To synchronise the reference oscillators at the slave sites, the master station transmits a pulse with a duration of 300μS at fsync. Also, all stations transmit a 'command and control' pulse on fks; again with a duration of 300μS.
A transmission from a master or slave station consists of 180 bursts, each around 1.2mS long; hence the transmission lasts 200mS.
Each of the 1.2mS bursts (called GRI's) consists of 6 pulses each of 200μS duration in the sequence f1-f3-fsync-f4-f2-fks for a master station, and f1-f3-fks-f4-f2-fks for a slave station.
The two slave sites are normally installed with the master located in between them, the base lines can be any length up to 150km. The maximum range of the system, at night, is 200km from the outermost station however it will be more than this during the day. The transmitter output is 15 watts and the receiver requires a minimum field strength of 70μV/m; this being equivalent to a signal-to-noise ratio of 10dB.
Accuracy is 12m in two-range mode or 12 to 60m in hyperbolic mode.