GP2021
Carrier Cycle Counter
Although the GP2021 contains a C/A code generator which
can be used to demodulate GLONASS signals (selected
by setting the GPS_NGLON bit in CHx_SATCNTL to 0),
the GP2021 does not have a sufficiently wide Doppler-
offset compensation range to allow it to be used effectively
for GLONASS.The total frequency offset, which the carrier
DCO can cope with, is 2·857MHz. This means that the
GP2021 will not be able to deal with the complete range of
GLONASS signals, unless they are mixed separately down
to a digital IF of approximately 1·4MHz.
The Carrier Cycle Counter is 20 bits long, and keeps a
count of the number of cycles of the Carrier DCO between
TlCs. This is not needed for a basic navigation system but
may be used to measure the range change (delta-range)
to each satellite between TlCs. The delta ranges can be
used to smooth the code pseudo-ranges. For finer detail
the Carrier DCO phase may also be read at each TIC to
give the fractional part of the cycle count or delta-range.
C/A Code Generator
The C/A Code Generator generates the selected Gold code
for a GPS satellite (1 to 32), a ground transmitter
(pseudolite, 33 to 37), an INMARSAT-GIC satellite (201 to
211) or a GLONASS satellite. A Gold code is selected by
writing a specific pattern of 10 bits, as listed in the Detailed
Description of Registers section, to the CHx_SATCNTL
register, or by setting the GPS_NGLON bit to Low for the
GLONASS code. Two outputs are generated to give both
a PROMPT and a TRACKING signal. The TRACKING
signal can be set to one of four modes: EARLY (one half
chip before the PROMPT signal), LATE (one half chip
behind), DITHERED (toggled between EARLY and LATE
every 20ms) or EARLY–MINUS–LATE (the signed
difference).
The GP1020, also available from Zarlink Semiconductor , has
10 separate sets of SIGN/MAG inputs from RF front end
devices, which can be configured to connect independent
to any of 6 correlator channels, making this device more
suitable for GLONASS applications. Contact your regional
Zarlink sales office for more information.
Source Selector
In Real_lnput mode the Source Selector selects which input
signal pair to use (SIGN0/MAG0 or SIGN1/MAG1). In
Complex_lnput mode SIGN0/MAG0 are passed to the
In-phase arm and SIGN1/MAG1 to the Quadrature arm.
The data is treated as having the values shown in Table 5
(in both modes).
The output code is a sequence of 1s and 1s for all
code type sexcept EARLY–MINUS–LATE where the result
can also be a 0. To avoid having an unused LSB in the
accumulators, the values in EARLY–MINUS–LATE mode
are halved from the 2, 0, 2 that results from the
calculation ( 1 or 1 ) ( 1 or 1) to 1, 0, 1. This
must be considered when choosing thresholds in the
software, as the correlation results will be exactly half of
the values otherwise expected.
SIGN
MAG
Value
0
0
1
1
1
0
0
1
3
1
1
3
Table 5 SIGN/MAG values
Carrier Mixers
The Carrier Mixers multiply the digital input signal by the
Carrier DCO digital local oscillator to generate a signal at
baseband. In Real_lnput mode both I and Q Carrier DCO
phases are directed to the appropriate mixers. In
Complex_lnput mode a single In-Phase Carrier DCO
output is used in both mixers since the input signal is
already in I and Q form. The mixing of the Carrier DCO
outputs with the input signal produces a baseband signal
which can have the values 1, 2, 3 and 6.
At the end of every code sequence (1023 chips in GPS
mode or 511 chips in GLONASS mode) a DUMP signal is
generated to latch the accumulated data for use by the
signal tracking software. Each channel is latched
separately, because the satellite signals are not received
in phase with each other.
The nature of GLONASS signals is that they are modulated
with the same PRN Gold Code, but are separated in the
frequency domain (1597MHz to 1617MHz). Navstar GPS
signals are modulated with different PRN Gold Codes, but
are transmitted on the same frequency (L1 = 1575.42MHz).
Code Mixers
The Code Mixers multiply the I and Q baseband signals
from the Carrier Mixers with both the PROMPT and
TRACKING local replica codes to produce four separate
correlation results. The correlation results are passed to
the Accumulate and Dump blocks for integration.
For the GP2021 to effectively demodulate GLONASS
signals, it would ideally need to have a separate set of RF
signal inputs for each correlator channel, in order for it to
differentiate between the different frequencies used by
each GLONASS satellite. Since this facility is not available,
the GP2021 cannot be used effectively to decode a
constellation of GLONASS signals.
Accumulate and Dump
The Accumulate and Dump blocks integrate the Mixer
outputs over a complete code period of nominally 1ms.
There are 4 separate 16-bit accumulators for each channel.
10
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