Arm Enterprises GP4020 Manuale Utente
7: 12-Channel Correlator
58
GP4020 GPS Baseband Processor Design Manual
CHX_CODE_INCR_LO / _HI registers to steer the Code DCO and gradually bring the gold code phase to the right
value.
value.
7.3.2 Signal
Tracking
The incoming GPS signal will exhibit a Doppler shift that varies with time due to the non-uniform motion of the
satellite relative to the receiver, and the user clock bias is likely to also vary with time. The net result is that unless
dynamic corrections are applied to the code and carrier DCOs, the GPS signal will be lost. This leads to two servo
loops being required: one to maintain lock on the Gold code phase and a second to maintain lock on the carrier.
This can be implemented with the following.
satellite relative to the receiver, and the user clock bias is likely to also vary with time. The net result is that unless
dynamic corrections are applied to the code and carrier DCOs, the GPS signal will be lost. This leads to two servo
loops being required: one to maintain lock on the Gold code phase and a second to maintain lock on the carrier.
This can be implemented with the following.
The raw data used to steer the two servo loops is the Accumulated Data, which is output by the tracking channel, at
the rate of once per millisecond. The tracking arm Accumulated Data is used for the Gold code loop. Some
approaches use an 'early minus late' Gold code to implement a null steering loop. Others use a dithering code that
alternates between a code one-half chip late and a code one-half chip early. In the GP4020 12-channel correlator,
the dithering rate is 20 ms (20 code epochs) each way, starting with Early after a reset, when this type of code is
selected through the CHx_CNTL register. The Gold code loop is closed by regularly updating the code DCO
frequency using the CHx_CODE_INCR_LO / _HI registers.
the rate of once per millisecond. The tracking arm Accumulated Data is used for the Gold code loop. Some
approaches use an 'early minus late' Gold code to implement a null steering loop. Others use a dithering code that
alternates between a code one-half chip late and a code one-half chip early. In the GP4020 12-channel correlator,
the dithering rate is 20 ms (20 code epochs) each way, starting with Early after a reset, when this type of code is
selected through the CHx_CNTL register. The Gold code loop is closed by regularly updating the code DCO
frequency using the CHx_CODE_INCR_LO / _HI registers.
The prompt arm Accumulated Data is used for the carrier phase loop (although the dithering mode in the tracking
arm may also be used). One approach consists of varying the carrier DCO phase in order to maintain all the
correlation energy in the in-phase correlator arm and none in the phase-quadrature correlator arm. The carrier
phase loop is closed by regularly updating the carrier DCO frequency using the CHx_CARR_INCR_LO / _HI
registers.
arm may also be used). One approach consists of varying the carrier DCO phase in order to maintain all the
correlation energy in the in-phase correlator arm and none in the phase-quadrature correlator arm. The carrier
phase loop is closed by regularly updating the carrier DCO frequency using the CHx_CARR_INCR_LO / _HI
registers.
7.3.3 Data
Demodulation
The C/A code is modulated with Space Vehicle (SV) data at 50 Baud to give the navigation message. This
modulation is an exclusive-OR function of the C/A code with the SV data. This means that every 20 milliseconds,
(which is every 20 C/A code epochs); the C/A code phase will be reversed (shifted by 180 degrees) if the new data
bit is different from the previous one. On the prompt arm, once the signal is being correctly tracked, such a data bit
transition will change the sign of the accumulated data. Data demodulation can then be achieved in two stages:
modulation is an exclusive-OR function of the C/A code with the SV data. This means that every 20 milliseconds,
(which is every 20 C/A code epochs); the C/A code phase will be reversed (shifted by 180 degrees) if the new data
bit is different from the previous one. On the prompt arm, once the signal is being correctly tracked, such a data bit
transition will change the sign of the accumulated data. Data demodulation can then be achieved in two stages:
1. Locate the instants of data bit transitions to identify which C/A code epoch corresponds to the beginning of a
new data bit. This will allow initialisation of the 12-channel correlator epoch counters by the signal processing
software (through the CHx_1MS_EPOCH and CHx_20MS_EPOCH registers) to count code epochs from 0 to
19 in phase with data bits. At each new cycle of the 1 ms epoch counter, the 20 ms epoch counter will
increment.
software (through the CHx_1MS_EPOCH and CHx_20MS_EPOCH registers) to count code epochs from 0 to
19 in phase with data bits. At each new cycle of the 1 ms epoch counter, the 20 ms epoch counter will
increment.
2. Record the sign of accumulated data on the prompt arm for each data bit period of 20 ms, with filtering to
reduce the effect of noise on the signal. Note that there is a sign ambiguity in the demodulation process in that
it is not possible to tell which data bits are '0's and which are '1's from the signal itself. This ambiguity will be
resolved at a later stage when the full Navigation Message is interpreted.
it is not possible to tell which data bits are '0's and which are '1's from the signal itself. This ambiguity will be
resolved at a later stage when the full Navigation Message is interpreted.
7.3.4 Pseudorange
Measurement
The measurement data registers provide the raw data necessary to compute the pseudorange. This raw data is a
sample, at a given instant set by the TIC signal, of the 20 ms and 1 ms epoch counters, the C/A code phase
counter and the code DCO phase. By definition, the pseudo-range is expressed in time units and is equal to the
satellite-to-receiver propagation delay plus the user clock bias. The user clock bias is first estimated (blind guessed
is more likely with a cold start, but iteration then takes longer) and then obtained as a by-product of the navigation
solution. The pseudorange is equal to the user's apparent local time of reception of the signal (t
sample, at a given instant set by the TIC signal, of the 20 ms and 1 ms epoch counters, the C/A code phase
counter and the code DCO phase. By definition, the pseudo-range is expressed in time units and is equal to the
satellite-to-receiver propagation delay plus the user clock bias. The user clock bias is first estimated (blind guessed
is more likely with a cold start, but iteration then takes longer) and then obtained as a by-product of the navigation
solution. The pseudorange is equal to the user's apparent local time of reception of the signal (t
1
) minus the GPS
real time of transmission (t
2
).
With the demodulated data, the software has access to the Space Vehicle Navigation Message, which contains
information on the GPS system time for the transmission of the current sub-frame; this is equal to term t
information on the GPS system time for the transmission of the current sub-frame; this is equal to term t
2
.
The time information in the navigation message allows the receiver time to be initialised with a resolution of 20
milliseconds (one data bit period) but with knowledge of the precision to much better than one C/A code chip; a little
less than 1 microsecond. As the time-of-flight from the satellite to the receiver is in the region of 60 to 80
milliseconds (one data bit period) but with knowledge of the precision to much better than one C/A code chip; a little
less than 1 microsecond. As the time-of-flight from the satellite to the receiver is in the region of 60 to 80