GPS World, February 2014

noise ratio at the input and output of the component and identifies how much additional noise is being added into the signal through the system or its individual components such as the low noise amplifier Good antennas will have a noise figure less than 3dB Why should I be concerned about multipath rejection Positioning errors are caused by multipath which are multiple reflections of the GNSS signal off of nearby or far objects Multipath degrades the positioning accuracy and should be suppressed For optimum multipath rejection an antenna should be mounted as close to the horizontal surface as possible and away from reflecting objects including water antenna masts air conditioners ventilation shafts etc 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 multipathgenerated 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 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 NovAtels GPS 700 series and Antcom G8 series offer superior PCO PCV performance For reference station applications a NovAtel GNSS 750 antenna is ideal How does my application affect my antenna choice Depending upon the application the antenna may have to meet certain environmental mechanical and operational requirements GNSS antennas used for aviation applications should ideally be TSO FAA certified and be rugged enough to handle extreme temperatures and vibration profiles such as the NovAtel Compact GNSS Antenna line and the Antcom G3 G5 G8 product lines Survey rover antennas like the GPS 700 series Pinwheel models should be able to survive rough handling and pole drop The table above highlights some of the important desirable features needed for a GNSS antenna based upon the users application Please go to www novatel com products gnss antennas to learn more Desirable Feature RZ 3UR OH 8OWUD ORZ 3 2 3 9 RZ 3 2 3 9 LJK 9LEUDWLRQ 5XJJHG 6LQJOH UHTXHQF 0XOWL RQVWHOODWLRQ 0XOWL UHTXHQF 57 EDQG IUHTXHQF RUUHFWLRQ 6HUYLFHV 1DUURZ DQGZLGWK HDWKHUSURRI RUURVLRQ 5HVLVWDQW LJK 0XOWLSDWK 6XSSUHVVLRQ 3ROH 0RXQW 0DJQHWLF 6XUIDFH 0RXQW 762 HUWL FDWLRQ WHQGHG 7HPSHUDWXUH 5DQJH 6PDOO RUP IDFWRU LJKWZHLJKW LJK OWLWXGH 2SHUDWLRQ 6XUYH Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ GIS Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ 5HIHUHQFH 6WDWLRQ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ YLDWLRQ HULDO 6XUYH Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ 0DULQH Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ RQVWUXFWLRQ 0LQLQJ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ 3UHFLVLRQ JULFXOWXUH Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ 9HKLFOH 7UDFNLQJ Ɣ Ɣ Ɣ RFN 2SHUDWLRQV Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ 8QPDQQHG LUFUDIW Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ 8QPDQQHG 9HKLFOH Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ Ɣ 7LPLQJ Ɣ Ɣ Ɣ Ɣ Ɣ www gpsworld com February 2013 GPS World 39