GP2021 Datasheet, PDF (22/62 Pages) Mitel Networks Corporation – GPS 12 channel Correlator Advance Information

GP2021
CONTROLLING THE GP2021
The following section describes typical methods for
controlling the GP2021. These include: signal acquisition and
tracking, carrier phase measurement and timemark
generation.
Search Operation
To perform signal acquistion, the carrier frequency and
code phase space needs to be searched until the signal is
detected. The maximum carrier frequency excursion from its
nominal value is defined by the maximum carrier Doppler shift
plus the maximum receiver clock error. The maximum code
phase is defined by the (fixed) code length. Typically, all code
phases will be searched at a given carrier frequency before
advancing to the next carrier frequency bin and repeating the
code phase search.
Carrier DCO Programming
The following registers:
CHx_CARRIER_DCO_INCR_HIGH
(or X_DCO _INCR_HIGH),and
CHx_CARRIER_DCO_INCR_LOW are programmed in
sequence with the relevant data according to the frequency
bin being searched. It is always necessary to write to both the
_HIGH and _LOW registers. Carrier DCO programming will
become effective as soon as the channel is released (made
active). If the channel is already active, writes to
CHx_CARRIER_DCO_INCR_LOW are effective
immediately. (A small delay of up to 175ns will occur, to allow
synchronisation of the processor write operation to the chip
operation.)
Code DCO Programming
The CHx_CODE_DCO_INCR_HIGH
(or X_DCO_INCR_HIGH)
and the CHx_CODE_DCO_INCR_LOW registers are
programmed in sequence with the relevant data according to
the estimated code frequency offset. It is always necessary to
write to both _HIGH and _LOW registers. Code DCO
programming will become effective as soon as the channel is
released (made active). If the channel is already active, writes
to CHx_CODE_DCO_INCR_LOW are effective immediately.
(A small delay of up to 175ns will occur to allow
synchronisation of the processor write operation to the chip
operation).
Code Generator Programming
For each channel, the CHx_SATCNTL register is
programmed as follows:
(i) Set the SOURCESEL bit to select the input signal source.
(ii) Set the TRACK_SEL bits to set the Tracking arm code to
either early or late (with respect to the Prompt arm).
(iii) Set the G2_LOAD bits to select the required PRN code.
(iv) Program the CHx_CODE_SLEW register with the
desired code phase offset. The slew operation will
become effective upon CHx_RSTB release. The first
DUMP will generate accumulated data for the channel
and set the associated CHx_NEW_ACCUM_DATA
status bit.
(v) Release the relevant CHx_RSTB bits of the
RESET_CONTROL register to make the channel active.
When the code clock is inhibited (to slew the code phase)
the Integrate and Dump module is held reset. It will start to
accumulate correlation results only after the slew operation is
completed.
22
A search for a satellite on more than one channel may be
performed using the MULTI channel addresses and different
code slew values as appropriate.
Reading the Accumulated Data
At each DUMP the corresponding
CHx_NEW_ACCUM_DATA status bit is set in the
ACCUM_STATUS_A register. The status register, together
with all accumulation registers (CHx_I_TRACK,
CHx_Q_TRACK, CHx_I_PROMPT, CHx_Q_PROMPT) are
mapped into consecutive addresses. These can be read as a
consecutive block, if required, after every ACCUM_INT
interrupt. Alternatively, the Status Registers may be polled.
The Accumulation registers are not overwrite protected,
therefore the system must respond quickly when new data
becomes available. Whether or not it is necessary to process
the accumulation at every DUMP is dependent upon the
application. The order of reading them is optional, but ideally
the CHx_Q_PROMPT register should be read last, because
this resets the CHx_NEW_ACCUM_DATA bit.
The CHx_MISSED_ACCUM bits in the
ACCUM_STATUS_B register indicate that new accumulated
data has been missed. These can only be cleared by a write
to CHx_ACCUM_RESET or by deactivating the channel.
Search on Other Code Phases
When it is desired to correlate on the next code phase,
such as one whole chip later, the CODE_SLEW has to be
programmed with a value of 2 (the units are half code chips).
The slew will occur on the next DUMP. The effect of
CODE_SLEW is relative to the current code phase. To repeat
a CODE_SLEW, the register needs to be written to again even
if the same size slew is required.
Once the signal has been detected (correlation threshold
exceeded), the code and carrier tracking loops can be closed.
The tracking loop parameters must be tailored in the software
to suit the application.
Data Bit Synchronisation
The data bit synchronisation algorithm should find the
data bit transition instant. The processor calculates the
present one millisecond epoch and programs this value into
the 1MS_EPOCH counter. Ideally, epoch counter accesses
should occur following the reading of the accumulation
register at each DUMP.
Alternatively, the epoch counters can be left free–running
and the offset can be added by the software each time it reads
the epoch registers. Note that if the integration is performed
across bit boundaries, the integration result can be very small.
Reading the Measurement Data
At each TIC, the measurement data is latched in the
Measurement Data registers
(CHx_EPOCH,
CHx_CODE_PHASE,
CHx_CARRIER_DCO_PHASE,
CHx_CARRIER_CYCLE_HIGH,
CHx_CARRIER_CYCLE_LOW,
CHx_CODE_DCO_PHASE ).
The ACCUM_STATUS_B or MEAS_STATUS_A
register must be polled at a rate greater than the TIC rate (to
see if a TIC has occurred), otherwise measurement data will
be lost. The ACCUM_INT or MEAS_INT events can be used