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The Russian BRAS MF Positioning System

BRAS (Брас in Cyrillic) exists in a number of marks or variants, that being considered here is Bras-3. The method of working is hyberbolic; employing a chain consisting of a master and 2 slave stations transmitting in a timed sequence. In this respect there are similarities to Decca Hi-Fix however the Russian system employs a unique method of ambiguity resolution, using a total of four transmitted RF frequencies with an additional 2 frequencies used for control and synchronisation.

Technical Description

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 26.2626 kHz.

NameFormulaValue
Base FrequencyF26.2626kHz
f164F1680.8064kHz
f465F1797.0690kHz
fsync56F+300kHz1770.7056kHz
f368F1785.8568kHz
fks69F1812.1194kHz
f280F2101.0080kHz

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.

The Ship-Borne Receiver

On switch-on, the receiver goes into search mode. Signals received on fs from master stations are received and compared with transmissions received on the f1channel. This allows the receiver to acquire time synchronisation with the chain. Once time synchronisation is acheived, the receiver then phase locks its reference oscillator to the master station. When complete, It stores the frequencies f1, f2, f3, f4 in memory. The receiver is now ready to start the phase comparison process and give position fixes. From a cold start it takes from 8 to 10 minutes before the receiver is ready to give fixes, after this however they are available at 1 minute intervals.

The receiver determines its position by measuring the relative phase of the master, slave 1 and slave 2 transmissions. Lane ambiguity is resolved by measuring at the difference frequency between two of the main navigation transmissions; this results obviously in very much wider lanes. The process in the receiver for acheiving this is automatic and a very high probability of the ambiguity being resolved successfully is assured.

Kwitok model 3IA receiver

Further Information

Spreadsheet of all possible BRAS frequencies.
Excellent diagram showing the timings for BRAS (and RS-10) together with detail of the GRI pulse and the RF spectrum resulting.

Audio Clip

Here in the UK it is possible to receive BRAS transmissions, I have heard them with varying signal quality around 1810 kHz. Given a quiet location, it should be possible to hear other frequencies too. Referring to the spreadsheet in the link above, transmissions are heard on channels 12, 14, 15 and 17. However not all of the stations in a particular chain can be heard- sometimes two, sometimes just one. For this reason it is difficult to tell which are from which system.

If the player above doesn't function properly please click here to launch the sound clip in your default player. The clip is 30 seconds, 48kbits/s 22kHz mp3 format, 192kB

I recorded this sound clip in April last year at Whitby, North Yorkshire where the RF background is really quiet most of the time. The receiver mode was AM, although I recorded it in USB mode as well. The frequency tuned was 1810 kHz; i.e Ch 14. You can only really hear one station- or can you?



This image is a screenshot from Spectrogram 5.0 which shows the audio spectrum of part of the clip above. You can see that the main component is at 820 Hz and this results from the sidebands mixing together in the AM detector. They are spaced at 820 Hz apart so that's what you get- 820Hz!. Most nearly visble at seconds 10 and 12, there is another burst at a very low level before the station we are hearing clearly. Later on in the recording, this stronger one fades down and the other fades up in level.

The transmission is identified as BRAS-3 because both of the stations we can hear transmit every second. With RS-10 only the master can do this. Furthermore, since with Bras the master transmits twice per second, then we know the stations we are hearing are the two slaves.



My Special thanks to Vaino Lehtoranta OH2LX for providing most of the information used in this article.



Last Modified 04/12/07