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.
Operating frequencies
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.
Overall signal structure
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:
$F_1$ | Sync and timing | 1..8 | 9..16 | Sync and timing | 1..8 | 17..24 |
---|---|---|---|---|---|---|
$F_2$ | Sync and timing | 9..16 | 1..8 | Sync and timing | 17..24 | 1..8 |
Chain 1's master provides the $F_1$ sync and timing, while chain 2's master provides the sync and timing for $F_2$.
Sync and timing
This is incomplete.
The SYNC data is transmitted by the chain master and used to allow receivers to synchronise themselves to the transmitted signal.
Function | Settling period | Trigger | Gap | Clock | Gap | Data (TX-TX) | Data (TX-RX) | Settling time | Nav. slots |
---|---|---|---|---|---|---|---|---|---|
Time | 40 ms | 40 ms | 10 ms | 20 ms | 10 ms | 65 ms | 115 ms | 40 ms | 8 x 80ms |
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.
Clock format (as displayed in Mk.II software)
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.
Nav slots
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.