Signal format -- technical details

At the top level, the Datatrak signal consists of sync data and a series of time-multiplexed navigation slots. Each transmitter is assigned one or more slots, which are either master slots (the transmitter generates the slot without a reference) or slave slots (synchronised to a master slot).

The sync data is phase-modulated onto the carrier centre frequency during the sync slot.

Datatrak uses two frequencies, around 10% apart.

• $F_1$: around 146.455 kHz
• $F_2$: around 133.2275 kHz

These are the centre frequencies of the two channels used by the UK system.

The Datatrak signal is based on a 1.68-second cycle. In Interlaced systems, two cycles are chained to produce a pair. This provides 24 navigation slots, and is the “dual-cycle interlaced” scheme mentioned in the introduction.

A cycle takes the following form:

Chain 1's master provides the $F_1$ sync and timing, while chain 2's master provides the sync and timing for $F_2$.

This is incomplete.

The SYNC data is transmitted by the chain master and used to allow receivers to synchronise themselves to the transmitted signal.

These function as follows (names are from the Mk.II Locator serial interface):

• Settling periods – each 40ms. These allow time for the receiver to settle when the frequency has been changed.
• Trigger –
  • This is a phase-modulated signal with a modulation depth of $+\pi, -\pi$ ($+180, -180$ degrees).
  • It encodes one bit of the 64-bit synchronisation sequence using either two cycles of a 50Hz sinusoid (a '0') or 1.5 cycles of 37.5Hz (a '1').
  • This is the origin of the “Datatrak Minute” in the clock shown by the Mk.II Locator.
• Gaps –
  • 10ms unmodulated period to allow frequency switching, or for the trigger and clock to be accurately synchronised against.
• Clock –
  • This is a phase-modulated signal with a modulation depth of (unknown)
  • It is one cycle of a 50Hz sinusoid, with a starting phase of 0, 90, 180 or 270 degrees. This encodes two binary bits per clock cycle.
  • The clock signal encodes a coarse time reference
  • Encoding of the 128 bits per loop is unknown.
• 65ms transmitter-to-transmitter data
  • This is possibly used to send almanac data, or to send commands between transmitters.
  • Modulation format is unknown.
• 115ms transmitter-to-Locator data
  • This is used to send commands to the Locator receivers.

The time format used by Mk.II is: AAAAA:BB:CCCC

• AAAAA is the clock (0 to 65535). This repeats every $65536 \times 64 \times 1.68 \mathrm{seconds}$ – or around 79 days.
• BB is the Gold Code (0 to 63). This refers to the current cycle number; the bit position in the Gold Code sync sequence.
• CCCC is the cycle offset or “Gold Code offset”. This is a value from 0 to 1679 which indicates the number of milliseconds since the start of the cycle.

This is incomplete

During the navigation “slots” two signals are transmitted: first one 40Hz higher than the centre frequency, then one 40Hz below it. These frequencies are known as the “+” (higher frequency) and “-” (lower frequency) signals.