HyperFix is a third generation small position-fixing system developed to operate in the frequency band 1.6-3.4 MHz. HyperFix supercedes the earlier Hi-Fix system, development of which began in 1960. The fundamental operating principle is that of phase difference measurement in the receiver. The operating range over sea in full daylight conditions is approximately 700 km and during the night approximately 250 km, depending on the antennas used and prevailing propagation conditions. Over land and inland water, and during difficult atmospheric conditions, the range is correspondingly less. The accuracy obtainable depends on a number of factors, but under ideal conditions it is around 0.5 metres for a system operating at 3.4 MHz and 1 metre for a system operating at 1.6 MHz. HyperFix was designed to be a highly flexible system, meeting the needs of a wide variety of users. It is capable of being used with both temporary and permanent shore-based chains, and microprocessor control is used to enable the main system features to be programmable. It also enables a high level of flexibility automation of functions such as line identification and ambiguity resolution. Although single frequency operation is possible, normally two frequencies are always used, and this does not require any additional equipment. Frequency synthesis is used to generate the transmitter drive signal and phase comparison and control is carried out by digital signal processing.

HyperFix can operated in three ways, in Hyperbolic, Circular and Combined mode.

A minimum of 3 shore stations are required, a master and two slaves. The patterns produced are the classic hyperbolic lines, with the stations located at the focus of the hyperbolae. The accuracy depends on the 'angle of cut' of the hyperbolae, which itself is dictated by the geometry of the chain. The chain may be extended by the addition of more slave stations, such that continuous coverage of an area of coastline is achieved. The user may select 3 pairs of stations on the receiver for display of the lane information.

Also known as range-range operation, this mode requires a minimum of two shore based stations. One or more stations are carried aboard the user's vessels. The positional lines in this case form a series of concentric circles centred on the shore stations, and the fix is obtained by measuring the distance to each shore station. As the distance from the shore increases, the accuracy of hyperbolic mode degrades and this is when circular mode is the most useful.

HyperFix enables the simultaneous provision of both hyperbolic and circular operation. In this way, the best use can be made of hyperbolic operation close to the shore, and circular operation further out to sea.

The flexibility of microprocessor control means that the system can be set up to fulfil a number of different tasks and to achieve this Racal have defined a number of 'Modes' of operation for HyperFix chains. In the timing diagrams that follow, the following key applies:

• T - Trigger pulse transmitted by the master station at the start of each cycle in order to achieve timing synchronisation.
• D - Data slot, used for signaling between stations when remote control is in use
• S - Slot: a station transmits in one or more slots (1 to 6 in a Mode 1 cycle).
• U - Unit: In mode 2, each station transmits in a pre-determined slot, so each slot is related to a unit number.
• G - Guard period (transmission break)
• A - Used by the Auto-aliasing lane identification error routine

The transmission sequences are divided into a number of slots. Except for guard periods, a slot may contain a transmission from a single station. Each slot is 37.5 mS wide, and the RF pulse would be of 36 mS duration.

This mode is primarily intended for hyperbolic positioning with up to 6 shore stations in the chain. One of the 6 stations may be located on board a vessel and used for circular operation, however. The duration of the timing cycle is 600 mS, and is shown in the diagram below.

In the soundclip below are three small chains all running in Mode 1. Or click here if the player doesn't work properly

This mode is principally intended for circular operation, by siting one of the stations on board a ship. Two ship stations may use up to 4 shore stations in this way. Interestingly, it can also be used for joint hyperbolic and circular operation by siting one of the ship stations ashore. The diagram below shows the timing diagram for this mode, which has a cycle time of 620 S.

Mode 3 is a combination of the facilities available in modes 1 and 2. Up to three mode 1 or mode 2 timing cycles may be used, in any combination. For this reason, a mode 3 cycle is said to consist of up to three sequences. The advantage of this approach is that the chain can be configured to meet what would otherwise be conflicting requirements. The cycle length varies from 760mS for a single sequence up to 2040 mS for a full 3-sequence cycle. Extra slots are provided for a remote control data link and a lane identification error correction routine. The diagram below shows examples of Mode 3 cycles, based on combinations of Modes 1-2-1, Mode 1 only, and Modes 2-1.

The sound clip below is a simulation of a large HyperFix chain running a 121 sequence in Mode 3. It is a very accurate simulation of how the UK East Coast chains sounded before they were decommissioned in around 1995.

Last Modified 16/11/07