The Hi-Fix system was engineered to fulfil a number of roles, varying form temporary deployments to fixed chains. To enable this, the equipment comprised a series of units each built on a 19-inch chassis 4U high (usually) and housed in a light alloy military-style Creeth transit case. The cases could be fastened together to form a rack, or be free standing.

The Hi-Fix system comprised the following main components. The links in the table below will take you to a full description of the unit.

The HiFix system required 3 transmitter stations. One of these, the Master, was located centrally between two slaves, and at the intersection of the baseline from Master to Slave 1, and Master and Slave 2. The sites transmitted in sequence on the same RF frequency, as shown in the diagram below:

The main frequency (referred to as the 'pattern frequency') could be in the range 1700 to 2000 kHz, but any one chain and its associated user receiver would be set up to operate on one of 4 channels within a 25 kHz switching bandwidth. The three sites all radiated a 200mS pulse within a 300mS 'slot', which allows for a 'guard time' to allow for the operation of the various switching circuits. In addition to the main pulse on the pattern frequency, the master station also transmitted a trigger pulse on the 'signalling frequency'- a frequency 60Hz lower than the nominal pattern frequency.

Click play on the player to listen to a simulation of a Hi-Fix chain in operation.

If the player does or appear or operate correctly, please click here to play the simulation in your default wav player.

The master station consisted of a Master Drive Unit (MDU) and a Transmitter unit. The MDU contained a highly stable crystal oscillator from which all frequencies used in the chain are ultimately derived. The MDU provided RF drive to the transmitter unit, which contained an RF power amplifier.

The slave station consisted of a receiver and a transmitter. The receiver picked up the transmission from the master station, and kept an oscillator in phase lock with it. It also used the trigger pulse to synchronise a timer. At the appropriate time, the slave's transmitter was keyed and radiated the slave pulse.

The same receiver unit could be used either on board ship or vehicle mobile. It picked up the transmissions from the chain transmitters, and gave a continuos readout of position relative to the transmitter sites on decimal digital counters. As an option, the receiver could be connected to a chart recorder to give a historical trace of location.

The system described above is referred to as Type A, or Lane Integration working. This type cannot resolve lane ambiguity and so it is necessary to know where you are are starting from, and then the HiFix receiver will count the lanes as they are crossed, hence 'integration'. Loss of signal (which can happen due to lightning noise, for example) means that you must needs back track to your last known position- a marker bouy for example.

HiFix type B, or Lane Identification working, uses a second set of transmitters working on a different RF frequency, but located at the same Master and Slave sites. This allows the lane to be positively identified. The additional RF frequency needs to be about 10% lower than the main one. This is how it works

Consider an area where, on the main channel of 1900kHz, there are 10 lanes. At the lower frequency, 1710kHz, there will be only 9 lanes (because of the longer wavelength). So when the lane counter on the HF receiver reads 1.0, that on the LF one will read 0.9, and so on. If we subtract the LF reading from the HF reading, we find that the difference is always one tenth of the HF reading. If we move the decimal point one place to the right, we have the full lane reading of the HF pattern. Here is an example: HF lanes: 4.50, LF lanes, 4.05. Difference is 0.45. Moving the DP one place to the right results in 4.5.

In this way we can resolve the ambiguity to 10 lanes- we don't know whether the lane number in the above example is 4.5, 14.5, 24.5 so we have to be able to determine this by other means. However, we are still 10 times better off than with Type A!

Last modified 16/11/07