GPS World, January 2017

Correlator Beamforming for Low Cost Multipath Mitigation 54 GPS WORLD WWW GPSWORLD COM JANUARY 2017 WITH RICHARD B LANGLEY GNSS PEST CONTROL By Sanjeev Gunawardena John Raquet and Mark Carroll M ultipath is the single largest naturally occurring un modeled error source that affects high accuracy and differential GNSS applications Even though decades of research and development on advanced multipath mitigating antennas and correlator gating techniques have contributed significantly to reducing the effects of this error source short delay higher elevation angle and carrier multipath continue to be a problem It is well known that antenna array based beamforming is particularly effective against these types of multipath However traditional antenna array and related beamforming processing technology is large heavy power hungry and costly in many applications A new alternative solution called correlator beamforming employs simple radio frequency RF signal switching and a single front end to reduce complexity power consumption and cost This technology is privately patented and is already commercially available in devices that run in the 24 GHz industrial scientific and medical ISM frequency band These systems have been leveraged into heavy industrial environments where precision position navigation and time PNT is critically important to drive operations especially for a large number of vehicle and fleet automation systems under development These new unmanned aerial vehicle UAV machine automation and fleet management systems must have a level of continuous reliability which cannot be guaranteed by satellite based systems in difficult high multipath environments such as mines ports warehouses and urban canyons Correlator beamforming has been shown to be effective at mitigating multipath for these non GNSS terrestrial and challenging indoor applications Intrigued by the technologys demonstrated accuracy in multipath plagued environments the Air Force Institute of Technologys AFITs Autonomy and Navigation Technology ANT Center initiated a collaborative research and development agreement CRADA with Locata Corporation to investigate the feasibility of applying the correlator beamforming techniques to standard GNSS The AFIT results show that a GPS receiver employing correlator beamforming technology is nearly as effective as a traditional beamforming receiver at rejecting multipath BACKGROUND Often considered the bane of precision navigation for indoor or urban applications using RF signals multipath continues to be one of the major error sources of GNSS The presence of reflected signals in these environments often degrades the accuracy and reliability of such PNT systems a problem that GPS engineers have struggled with since GPS signals were first broadcast Fortunately the industry has been able to implement multipath mitigation approaches albeit with varying levels of success and technical tradeoffs Nevertheless there is a clear understanding today that future autonomous mobile and personal applications require a level of accuracy and reliability that demand better multipath mitigation solutions There are two prevalent techniques apart from modern GNSS signal structures that have anti multipath features by design that are used to mitigate multipath antenna gain pattern shaping and receiver correlator gating The first technique limits the effect of ground multipath by reducing antenna gain at low elevation N element CRPA Full beamforming channel Full beamforming channel Full beamforming channel Σ 20 ms 100 ms Prompt Early late Σ Σ Carrier aiding Beamforming N digital sample streams Phase Shifters Carrier Typical channel processing NCO FIGURE 1 Traditional beamforming receiver architecture PLL DLL Code NCO φ 1 φ 2 φ 3 RFFE RFFE RFFE RFFE sign Q