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GNSS
Design methods for RF part
of GNSS software receivers
The paper focuses on the construction of RF front end for experimental and software GNSS receivers
PAVEL KOVÁR, PAVEL PURICER, JOSEF ŠPACEK
Many researchers and R&D
laboratories in the world
deal with the design of
the software-based or
experimental GNSS receivers using
digital signal processing for work and
experiments with received navigational
signals. These concepts have one in
common: the necessity of use of some
analogue RF part before conversion of
the signal to the digital domain. The
problem of the RF front end design of
the experimental or special purpose
navigation receivers has to be solved.
It is not an easy task as can be seen
from many papers and conference
contributions. The aim of this paper is
analysis of the possibility of such GNSS
RF front end design. We will discuss
the following three main approaches:
• Possibility of the use of the lot
manufacture receiver RF front end,
• Application of RF front
end GPS/GNSS ASICs,
• RF front end design from
universal RF integrated
circuits and components.
Technical requirements and
characteristics of the experimental
receiver RF front end come
from the research aims. In this
design phase the designer has
to define the parameters like
• Number of RF channels,
• Processing frequencies,
• Bandwidth of each channel,
• Channel selectivity,
• Local oscillator frequency stability,
• Output signal specification,
• Dynamic range
Experimental versus lot
manufacture receivers
The requirements on RF front end of
the experimental GNSS receiver are
rather different than requirements
on RF front end of the commonly
manufactured GNSS receivers. The
lot manufacture receiver processes
navigation frequencies in which
the navigation service is routinely
provided. Most simple receivers
process only GPS L1 frequency.
Some present high performance
receivers process also GLONASS
L1 and GPS L2 signals. On the
other hand the experimental GNSS
receivers should process all known
navigation signals in extreme case.
The bandwidth of the common
production receiver depends on
its type. Low cost low power
consumption GPS receivers use low
sampling frequencies and bandwidth
of about 2 MHz. These receivers
usually have not implemented any
multipath mitigation algorithms. The
bandwidth of the high performance lot
manufacture GPS receivers is much
wider, i.e. 8 – 16 MHz. The bandwidth
of the experimental receivers depends
on the receiver mission. Typically, the
bandwidth of such receiver should be
adjustable in wide range to support
broad spectrum of experiments.
The frequency plan of the lot
manufacture receivers is optimized
from many aspects. The all required
frequencies used for frequency
conversion, signal sampling, and
processing should be derived from
one low cost high stability frequency
standard. The power consumption
of this frequency source should
be kept low. Therefore most RF
ASICs use only one synthesizer,
which directly generates signals
of the first local oscillator. The
remaining frequencies are derived
from this signal by dividers. This
approach leads for utilization of
the non standard intermediated
frequencies. It is not problem for
mass market products, where the
special IF filters and frequency
standard of required frequency are
developed and produced for particular
chip sets. The filters are usually
manufactured by lot manufacture
low cost SAW technology.
In the case of experimental GNSS
receivers, the development and
production of the specialized
components in the same manner
as in the case of lot manufacture
is very expensive. Therefore the
standard RF components and
integrated circuits should be used.
The power consumption of the
experimental receiver, cost and count
of the components are not critical
measures. It is appropriate to use
common frequency standard and
standard intermediated frequency.
Nevertheless, the design of the
synthesizers for experimental
receivers is more complicated.
Lot manufacture
receiver front end
The easiest and cheapest way how
to develop RF front end for GPS/
GNSS signal experimentation is an
utilization of the RF front end of the
lot manufacture GPS receiver. The
most modern GPS receivers consist
of at least two chips or blocks, RF
ASIC and IF DSP ASIC. The signal
on the RF ASIC output can be easily
split and utilized for the experiments.
For example in fig. 1 is shown
archaic GPS receiver Magellan
Meridian XL artificially equipped
34 April 2006