GPS World, February 2016

sponsored by Plot of Good and Poor Antenna Phase Center Offset over Elevation Angle L band frequency Correction Services Narrow Bandwidth Weatherproof Corrosion Resistant High Multipath Temperature Range Small Form factor FEBRUARY 2016 WWW GPSWORLD COM GPS WORLD S3 Choke ring antennas offer high multipath rejection due to their unique concentric ring design that block out multipath signal reflections to the antenna element The Axial Ratio also defines the antennas ability to reject multipath generated replicas of the original GPS signal A good GNSS antenna provides a low axial ratio not only at each of the GNSS frequencies but also over all elevations angles of the satellites in view NovAtels GPS 700 series of antennas maintains an axial ratio as low as 3dB at azimuth or horizon What is phase center stability and why is it important The phase center of the antenna is the point where the signals transmitted from satellites are collected When your receiver reports a location fix that location is essentially the phase centre of the antenna If you are specifying an antenna for use with geodetic survey or a reference station application phase center offset PCO and phase centre stability or variation PCV are important The electrical phase center of any antenna will vary with the position of the transmitting signal it is receiving by as much as a few millimetres As GNSS satellites move across the sky the electrical phase center of the signal received will typically move with the satellite position unless the antenna has been carefully designed to minimize PCO PCV The Phase Center Offset with respect to the antenna reference point ARP is the difference between the mechanical center of antenna rotation and electrical phase center location The PCO is also frequency dependent which means that there can be a different offset for each signal frequency The Phase Center Variation identifies how much the phase center moves with respect to the satellite elevation angles Many users can accept accuracies of less than a metre so these small phase center variations cause a negligible amount of position error But if you require high precision Real Time Kinematic RTK receivers can achieve position accuracies of 2 4 cm and a few millimetres of phase center error translates to a 10 15 error in reported position For RTK survey applications geodetic grade antennas such as the NovAtel GPS 70x and Antcom G8 series offer superior PCO PCV performance For reference station applications a NovAtel GNSS 750 antenna is ideal What are the requirements for my application Depending on the application an antenna may have to meet certain environmental mechanical and operational requirements GNSS Antenna is characterized by several metrics gain gain roll off multipath rejection axial ratio bandwidth PCO PCV Gain is not necessarily the most important metric various antenna parameters can impact positioning accuracy Survey PPP positioning using L band delivered corrections and Heading applications are shown to be impacted by antenna selection A well designed antenna is a critical part of a GNSS System The table below highlights some of the important desirable features needed for a GNSS antenna based upon the users application Go to novatel com antennas to learn more Plot of Good and Poor Antenna Phase Center Variation over Elevation Angle Desirable Feature Low Profile Ultra low PCO PCV Low PCO PCV High Vibration Rugged Single Frequency Multi Constellation Multi Frequency RTK Suppression Pole Mount Magnetic Surface Mount TSO FAA Certification Extended Lightweight High Altitude Operation Survey GIS Reference Station Aviation Aerial Survey Marine Construction Mining Precision Agriculture Vehicle Tracking Dock Operations Unmanned Aircraft Unmanned Vehicle Timing