GPS World, February 2014

Selecting the Right GNSS Antenna We recently met with Gordon Ryley of NovAtel to provide additional information on how to select the ideal GNSS antenna for your application Where do you start The antenna you choose must match your receivers capabilities and specifications Since the antenna behaves both as a spatial and a frequency filter it will have significant impact on the performance of the GNSS receiver Your application will also help determine form factor and antenna performance What GNSS constellations and signals do I want to receive Active GNSS constellations include GPS GLONASS and BeiDou Galileo and QZSS will soon grow to full operation Position correction services known as Space Based Augmentation Services SBAS transmit on geostationary satellites in orbit around the earth Each of the constellations and SBAS have their own signal frequencies and bandwidths Some of these frequencies are shared e g GPS L1 WAAS SBAS Galileo E1 Choose an antenna that covers the signal frequencies transmitted by the constellation and bandwidth supported by your GNSS receiver Multiple frequency antennas are used to help correct for ionospheric errors in Real Time Kinematic RTK applications The signal bandwidth of your antenna is important as many GNSS receivers take advantage of the full signal power transmitted NovAtels GNSS receivers utilize a patented Pulse Aperture Correlator technology PAC which uses a wider signal bandwidth to mitigate position errors caused by multipath and can support precision carrier phase RTK algorithms with centimetre accuracy Many basic GNSS receivers only look at the constellations coarse acquisition C A code and only require a narrow bandwidth of a few MHz and as such are restricted to positional accuracies of a few metres What amount of antenna gain do I need Gain is a key performance indicator of a GNSS antenna A minimum gain is required to achieve a minimum C N 0 carrier to noise ratio to track the GNSS satellites The antenna gain is directly related to the overall C N 0 of the navigation GNSS receivers Hence antenna gain helps define the tracking ability of the system GNSS receiver manufacturers will typically specify the expected RF input level required for optimum performance This level is important to avoid under driving causing poor C N 0 or overdriving causing limiting or distortion the front end of the receiver Nearly all receivers have automatic gain control but there is only so much dynamic range in the system and the antenna gain including cable losses needs to be considered carefully Why is element gain so important The element gain defines how efficient the antenna element is at receiving the signals In any signal chain you are only as good as the weakest link so an antenna with low element gain might be compensated by an increased low noise amplifier gain but the signal to noise ratio or C N 0 is degraded How important is antenna beamwidth and gain roll off Very important Gain roll off is a factor of beamwidth and specifies how much the gain changes over the elevation angle of the antenna From the antennas point of view the satellites rise from the horizon towards zenith and fall back to the horizon The variation in gain between zenith directly overhead and the horizon is known as the gain roll off Different antenna technologies have different gain roll off characteristics Patch antennas can track down to low elevation angles such as 10 degrees Most choke ring antennas will typically not receive signals below 30 degrees with the exception of NovAtels GNSS 750 which can track down to and below the horizon What is Noise Figure Noise figure directly impacts the C N 0 of the GNSS receiver The lower the overall noise figure the better the C N 0 and tracking capability The noise figure is the difference between the signal to GORDON RYLEY Antenna Smart Antennas Product Manager Plot of Good and Poor Antenna Phase Centre Variation over Elevation Angle GPS World February 2013 www gpsworld com 38