GP1020 Datasheet, PDF (10/44 Pages) Zarlink Semiconductor Inc – SIX-CHANNEL PARALLEL CORRELATOR CIRCUIT FOR GPS OR GLONASS RECEIVERS

GP1020
SATELLITE CODE SELECTION
This section describes the code selection for normal GPS and
GLONASS operation; for INMARSAT codes and unusual tech-
niques see full details in DETAILED DESCRIPTION OF REGIS-
TERS section, under CHx_CNTL. The same section gives
details of the other bits of CHx_CNTL.
The satellite code to be used by each channel is set by the
CHx_CNTL registers, which are addressed individually from the
A8-A1 address bus by:
00_H : Read/Write to Channel 1
10_H : Read/Write to Channel 2
20_H : Read/Write to Channel 3
30_H : Read/Write to Channel 4
40_H : Read/Write to Channel 5
50_H : Read/Write to Channel 6
70_H : Write only, to all channels simultaneously
The one GLONASS code may be selected by setting bit 10
HIGH, otherwise this bit should be set LOW and bits 7 to 0 used
to select one of the GPS Gold codes (see Table opposite).
SOFTWARE REQUIREMENTS
The very wide variety of types of GPS or GLONASS receiver
need to operate the correlator in different ways so, to accomodate
this and also to allow dynamic adjustment of loop parameters,
the GP1020 has been designed to use software for as many
functions as possible. This flexibility means that the device
cannot be used without a microprocessor closely linked to it, but
as a processor is always needed to convert the output of the
GP1020 into useful information this is not a significant limitation.
The software associated with the GP1020 can be divided into
two separate modules: one to acquire and track satellite signals
to give pseudoranges and another to process these to give the
navigation solution and format it in a form suitable for the user.
For the Navigation Solution to be possible all of the pseudoranges
must have exactly the same clock error, which can then be
removed iteratively to give real ranges if sufficient satellites are
tracked (3 if the height is known, otherwise 4). This need for exact
matching of timing errors explains the need for all of the compli-
cated synchronisation between all channels and between mas-
ter and slaves.
The following relates only to the signal processing aspects of
the software, to acquire and track signals from up to six satellites
per GP1020 and to obtain the pseudoranges and the navigation
message. The operation of the navigation software is not
dependent on the details of the correlator, and so does not need
to be included in this data sheet.
An on-chip interrupt time base INT OUT is provided to help
implement a data transfer protocol between the microprocessor
and the GP1020 at fixed time intervals, otherwise a software
based polling scheme will be needed – the choice is set by the
application. If INT OUT is used, and perhaps also if polling is
used, the data transfer rate is about twice the correlation result
rate for each channel, so many transfers will not give new data.
Bus use can be reduced by examining the status registers before
each transfer to see if new data is available and then only reading
the data if it useful.
It is important to note that the timing of each of the correlator
channels will be locked to its own incoming signal and not to each
other or to the microprocessor interrupts, so new data is gener-
ated asynchronously. The sampling instant of measurement
data of all channels, however, is common to give a consistent
navigation solution.
In order to acquire lock to the satellites as quickly as possible,
the data from the last fix should be stored as a starting point for
the next fix. It is also useful to have a real-time clock built into the
receiver to give a good estimate of GPS time for the next fix; the
navigation solution can be used to measure clock drift and
calculate a correction for the clock to overcome ageing. The
10
Bit setting
7654 3210
GPS PRN
reference
number
Selected taps
to be EXORed
together
0001 0101
0010 0110
0011 0111
0100 1xx0
0000 1xx0
0001 1xx1
0000 0111
0001 1xx0
0010 1xx1
0001 0010
0010 0011
0100 0101
0101 0110
0110 0111
0111 1xx0
1xx0 1xx1
0000 0011
0001 0100
0010 0101
0011 0110
0100 0111
0101 1xx0
0000 0010
0011 0101
0100 0110
0101 0111
0110 1xx0
0111 1xx1
0000 0101
0001 0110
0010 0111
0011 1xx0
0100 1xx1
0011 1xx1
0000 0110
0001 0111
0011 1xx1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33*
34* = 37
35*
36*
37* = 34
26
37
48
59
19
2 10
18
29
3 10
23
34
56
67
78
89
9 10
14
25
36
47
58
69
13
46
57
68
79
8 10
16
27
38
49
5 10
4 10
17
28
4 10
GPS Gold codes. *Note that these codes, 33 to 37, are
reserved for non-satellite use only.
user’s location (or a good estimate of it) along with the Almanac
and the correct time will indicate which satellites should be
searched for and may be used to find an estimate of Doppler
effects, while the previous clock error is the best available
estimate of the present clock error. If this information is not
available then the receiver must scan a much wider range of
values, which will greatly increase the time to lock. The satellite
Clock Correction and Ephemeris are needed for the navigation
solution, so if a recent set is held in memory the calculations may
begin as soon as lock is achieved and not need to wait for the re-
transmission (18 to 36 seconds).
This description applies to just one tracking channel but is the
samd this is not necessarily the same as the other channels.
The GP1020 contains four different types of registers:
• ^{Control Registers which are used to program functions of}
• ^{the device.}
Status Registers which provide a status indication of the
process taking place in the device.
• Accumulated Data Registers which provide the results
of correlation with the C/A code every millisecond. This is the
• raw data used to acquire and track satellite signals.
Measurement Data Registers which latch the carrier DCO
phase, carrier cycle count, code DCO phase, 1 millisecond