Innovation: GNSS Antennas -- An Introduction to Bandwidth, Gain Pattern, Polarization, and All That

Innovation: GNSS Antennas
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An Introduction to Bandwidth, Gain Pattern, Polarization, and All That

Feb 1, 2009
By: Daniel Orban, Gerald J.K. Moernaut
GPS World

INNOVATION INSIGHTS with Richard Langley

The antenna is a critical component of a GNSS receiver setup. An antenna's job
is to
capture some of the power in the electromagnetic waves it receives and to
convert it into
an electrical current that can be processed by the receiver. With very strong
signals at
lower frequencies, almost any kind of antenna will do. Those of us of a certain
age will
remember using a coat hanger as an emergency replacement for a broken
AM-car-radio
antenna. Or using a random length of wire to receive shortwave radio broadcasts
over a
wide range of frequencies. Yes, the higher and longer the wire was the better,
but the
length and even the orientation weren't usually critical for getting a decent
signal.

Not so at higher frequencies, and not so for weak signals. In general, an
antenna must be
designed for the particular signals to be intercepted, with the center
frequency,
bandwidth, and polarization of the signals being important parameters in the
design. This
is no truer than in the design of an antenna for a GNSS receiver.

The signals received from GNSS satellites are notoriously weak. And they can
arrive from
virtually any direction with signals from different satellites arriving
simultaneously. So
we don't have the luxury of using a high-gain dish antenna to collect the weak
signals as
we do with direct-to-home satellite TV.

Of course, we get away with weak GNSS signals (most of the time) by replacing
antenna gain
with receiver-processing gain, thanks to our knowledge of the pseudorandom noise
spreading
codes used to transmit the signals. Nevertheless, a well-designed antenna is
still
important for reliable GNSS signal reception (as is a low-noise receiver front
end). And
as the required receiver position fix accuracy approaches centimeter and even
sub-centimeter levels, the demands on the antenna increase, with multipath
suppression and
phase-center stability becoming important characteristics.

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