GPS World, January 2017

JANUARY 2017 WWW GPSWORLD COM GPS WORLD 55 WHICH IS MORE IMPORTANT for GNSS equipment the antenna or the receiver Of course answering this question is a mugs game as both are vitally important and one is useless without the other It is true that the development of sensitive receivers has permitted the use of inexpensive linearly polarized wire or chip antennas in consumer electronics such as mobile phones But demanding applications such as geodetic surveying timing and machine control require a proper right hand circularly polarized antenna However regardless of the application whether low accuracy or high the antenna must be omnidirectional So GNSS antennas typically have a broad gain pattern allowing reception of signals arriving at any azimuth and elevation angle Many simple antennas such as a microstrip patch on a small ground plane may even have significant sensitivity to signals arriving from below that is ground bounce multipath The multipath signals whether coming from the ground or nearby structures once passed to the receiver interfere with the direct line of sight signals and can be a real pest degrading the pseudorange and carrierphase measurements and limiting the resulting position velocity and timing accuracy of the equipment Advanced correlator techniques and clever broadpattern antenna designs can mitigate some forms of multipath The multipath estimating delay lock loop is an example of the former while the choke ring antenna and the novel antenna design discussed in this column a few months ago are examples of the latter Ideally a GNSS antenna should only receive line of sight signals from the satellites except for some scientific applications like snow depth monitoring or water level measurement or when some line of sight signals are blocked such as in concrete canyons and a reflected signal is better than nothing That could be arranged by using a narrow beam antenna such as a small parabolic dish In fact such an antenna was used by the Jet Propulsion Laboratory for one of the first codeless GPS receivers Called SERIES for Satellite Emission Range Inferred Earth Surveying it used a 15 meter diameter dish antenna mounted on a trailer It would cycle through the visible satellites repointing the dish and spending several minutes on each satellite to determine the antennas position Additionally by using a pair of terminals and taking data over an hour or so the baseline between the terminals could be determined to a few centimeters SERIES was an outgrowth of JPLs work in very long baseline interferometry In interferometry a very narrow antenna beam is synthesized by combining the measurements made by the two or more antennas and receivers The beam width is proportional to the wavelength of the received signals and inversely proportional to the baseline length While VLBI observations of quasars and other esoteric celestial objects have provided some of our best knowledge of plate tectonics and the Earths rotation and establish the link between the terrestrial and celestial reference frames interferometry using slewing dishes was not a practical approach for GPS positioning and JPL moved to more conventional antennas for its SERIES receivers JPLs use of interferometry for GPS positioning also pioneered by the Massachusetts Institute of Technology with its Macrometer receiver led to the common carrier phase double differencing technique widely used today for high accuracy GNSS positioning But the concept of a narrow antenna beam for GNSS signal reception would be practical if the beam could be rapidly directed in sequence towards each of the visible satellites This could be done with a pair of adjacent antenna elements by adjusting under software control the relative phase of the signals provided by each element A more efficient approach would be to use multiple elements Such beamforming antennas have actually been constructed and are commercially available Not only do these antennas provide enhanced multipath rejection they can be configured to produce a null in the combined gain pattern in the direction of an interference source an important antenna characteristic for military applications As you might expect these beamforming antennas and their associated electronics are large and heavy and consume a fair bit of power and so are not wellsuited for general purpose positioning However a novel approach to beamforming without these shortcomings and which was commercially developed for use in the 24 GHz band has been adapted for GNSS use In this months column a team of researchers at the U S Air Force Institute of Technology discuss how they implemented the approach termed correlator beamforming and tested it with live GPS signals with excellent results INNOVATION INSIGHTS BY RICHARD B LANGLEY