Global Positioning System Reference
In-Depth Information
4.5.2 Galileo E5 wideband
In this case both the E5a and E5b signals are processed together as a single wideband signal
(with a bandwidth of at least 51.15 MHz). The local code has to be generated individually
for all the four components (E5a-pilot, E5a-data, E5b-pilot and E5b-data) of the signal and
the generators require 14-bit shift registers. However, because the four signal components
and the complex modulation, the local reference mixer is computationally intensive (more
LUTs). In addition, a quadruple number of accumulators are required. As mentioned earlier,
independent correlation for all the four signals is performed to allow design freedom for
the subsequent stages in combining these four components. As a result of a very high
operating frequency and drastic increase in the resource requirements compared to GPS L1
C/A correlator, the power consumption is expected to be very high.
4.6 Baseband architecture overview for the GPS and Galileo OS signals
Fig. 4 shows the baseband architecture assuming that the correlator processes GPA L1 C/A,
L2C, L5 and Galileo E1, E5 signals. The computation block is marked separately to the local
signal (local carrier and replica reference signal) generation block. This sort of grouping
the blocks helps the design because different signals (and different channels in some cases)
share common parameters and optimising the hardware becomes easier. In this architecture,
it is also assumed that the baseband is commanded (assigned PRNs, Doppler and delay
parameters etc) to operate from an external processor. The correlation values of different
signals are read and processed in the succeeding decision feedback and control stage.
5. Resource requirements for the new signals and recommendations
5.1 Core resource requirements using a straightforward extension of the GPS L1 C/A
design
In order to gauge the resource requirements in terms of the number of registers and
combinational logic, the core correlators for the GPS and Galileo open service signals have
been implemented on the Altera Cyclone-III family device EP3C120F780C8. The FPGA
resource utilisation parameters are listed in Table 2.
The resource and the power consumption values closely match the expected outcomes
mentioned in the previous section. While the Galileo E1b or E1c core requires almost the same
resources as that of GPS L1 C/A, the power consumption is higher. The power consumption
for the single component Galileo E1 is 0.8mW more than that of the GPS L1 C/A. This is due
to the presence of the memory block, increased accumulator width and increased operating
frequency. The Galileo E1 correlator where both the E1b and E1c signals are processed
together has a power estimate of 2.24mW, only about 0.6mWmore than the single component.
This is because some of the blocks such as the carrier NCO and the carrier mixer are common
for both the signal components.
The resource for the GPS L5 signal is increased to 701 registers and 204 combinational units
which is due to the increase in the accumulator width and also due to the presence of two
signal components. The power consumption estimate of the GPS L5 signal is about 11 times
that of the GPS L1 C/A signal and is attributed mainly to the operating frequency. The
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