GPS World, September 2013

INNOVATION Algorithms Methods O ver the past few years we have been developing new GNSS receivers and antennas based on an innovative signal processing VFKHPH WR VLJQL FDQWO LPSURYH 166 tracking reliability and accuracy under degraded signal conditions It is based on the principles of synthetic aperture radar Like in a multi antenna phasedarray receiver GNSS signals from different spatial locations are combined coherently forming an optimized synthetic antenna gain pattern Thereby multipath signals can be rejected and the line of sight received signal power is PD LPL HG 7KLV LV HVSHFLDOO EHQH FLDO in forests and in other degraded environments The method is implemented in a real time PC based software receiver and works with GPS GLONASS and Galileo signals Multiple frequencies are generally supported The idea of synthetic aperture processing is realized as a coherent summation of correlation values of each satellite over the so called beamforming interval Each correlation value is multiplied with a phase factor For example the phase factor can be chosen to compensate for the relative antenna motion over the beam forming interval and the resulting sum of the scaled correlation values represents a coherent correlation value maximizing the lineof sight signal power Simultaneously signals arriving from other directions are partly eliminated Two main difficulties arise in the synthetic aperture processing First the clock jitter during the beam forming interval must be precisely known It can either be estimated based on data from all signals or a stable oscillator can be used In one of our setups a modern oven controlled crystal oscillator with an Allan variance of 05 10 13 at an averaging period of 1 second is used Under Cover Synthetic Aperture GNSS Signal Processing Thomas Pany Nico Falk Bernhard Riedl Carsten Stöber Jón O Winkel and Franz Josef Schimpl A SYNTHETIC APERTURE WHATS THAT Well an aperture in optics is just a hole or opening through which light travels Those of us into photography know that the amount of light reaching the cameras imaging sensor is controlled by the shutter speed and the size of the lens opening or aperture called the f stop And a correct combination of the aperture setting and shutter speed results in a correct exposure For an optical telescope its aperture is the diameter of its main light gathering lens or mirror A larger aperture gives a sharper and brighter view or image In the radio part of the electromagnetic spectrum the term aperture refers to the effective collecting or transmitting area of an antenna The gain of the antenna is proportional to its aperture and its beamwidth or resolution is inversely proportional to it Astronomers whether using optical or radio telescopes often seek higher and higher resolutions to see more detail in the objects they are investigating Conventionally that means larger and larger telescopes However there are limits to how large a single telescope can be constructed But by combining the light or radio signals from two or more individual telescopes one can synthesize a telescope with a diameter equal to the baseline s INNOVATION INSIGHTS with Richard Langley The technique improves GNSS observations in degraded environments connecting those telescopes The approach is known as interferometry It was first tried in the optical domain by the American physicist Albert Michelson who used the technique to measure the diameter of the star Betelgeuse Radio astronomers developed cable and microwave connected interferometers and subsequently they invented the technique of very long baseline interferometry VLBI where atomic clock stabilized radio signals are recorded on magnetic tape and played back through specially designed correlators to form an image VLBI has also been used by geodesists to precisely determine the baselines between pairs of radio telescopes even if they are on separate continents A similar approach is used in synthetic aperture radar SAR Mounted on an aircraft or satellite the SAR beam forming antenna emits pulses of radio waves that are reflected from a target and then coherently combined The different positions of the SAR as it moves synthesize an elongated aperture resulting in finer spatial resolution than would be obtained by a conventional antenna But what has all of this got to do with GNSS In this months column we take a look at a novel GNSS signal processing technique which uses the principles of SAR to improve code and carrier phase observations in degraded environments such as under forest canopy The technique can simultaneously reject multipath signals while maximizing the direct line of sight signal power from a satellite Along with a specially programmed software receiver it uses either a single conventional antenna mounted say on a pedestrians backpack for GIS applications or a special rotating antenna for high accuracy surveying Want to learn more Read on Innovation is a regular feature that discusses advances in GPS technology andits applications as well as the fundamentals of GPS positioning The column is coordinated by Richard Langley of the Department of Geodesy and Geomatics Engineering University of New Brunswick He welcomes comments and topic ideas To contact him see the Contributing Editors section on page 4 GPS World September 2013 www gpsworld com 42