How Can the Effects of Reflected GPS Multipath Signals be Eliminated on an Autonomous Mobile Vehicle?

S. Elder and M. Elder

Abstract:	My science project was about my membership on the Team CalTech terrain team and I was primarily involved in the GPS navigation of 'Alice'. Alice is the name of the Ford Van that was the Team CalTech DARPA Grand Challenge entry. My science project is about how the effects of GPS multipath on Alice were minimized by the physical installation of my multipath blocking fixture for the two GPS antennas. The DARPA Grand Challenge 2005 is a competition of robotic ground vehicles with the intent of advancing autonomous vehicle technology. The vehicles must travel approximately 150 miles over rugged desert roads using only onboard sensors and navigation equipment to find and follow a predetermined route while at the same time avoiding obstacles. DARPA awarded $2 million to the team whose autonomous vehicle successfully completed the 2005 route the fastest within a 10-hour period. My hypothesis for my science project was 'If I can define the multipath Fresnel zones for the vehicles GPS antennas, then I can disrupt the reflection and eliminate the effect of the multipath signal.' This project is an extension of my 2004 science project that determined Brewster's angle at the GPS frequency. In 2004, I created a controlled multipath environment by mounting a GPS antenna on a test fixture and pointed the antenna towards the WAAS POR satellite. The WAAS POR satellite supplies a GPS signal as well as differential corrections. The benefit of using this satellite is that it is a GEO satellite so its position in the sky stays the same. Also, this signal is lower power than GPS signals so it will react more to multipath. With this test fixture, I was able to create a consistent multipath condition by adjusting the look angle of the antenna to the GEO satellite. For this experiment, the multipath condition caused a loss of receiver tracking for the GEO satellite. My 2005 science experiment isa continuation of my 2004 project and supported my work with Team CalTech. When a GPS signal is reflected on a material, it will change from a right hand circular polarized to a left hand circular polarized signal at Brewster's angle. If the reflected signal reaches a GPS antenna, it can cancel out the real or direct GPS signal. This is called multipath and it can cause a position error in a GPS receiver. I first created a controlled multipath environment by mounting my GPS antenna on a board and pointed the antenna towards the WAAS POR satellite. Once I determined there was multipath, I placed various materials in the reflection path to eliminate the multipath effect. After I had discovered the best way to disrupt the multipath signal on my test fixture, I then analyzed Alice for the location of multipath Fresnel Zones and reflection points. Alice has two GPS antennas placed at opposite ends of the roof. Alice's primary navigation is from GPS waypoint to GPS waypoint and the GPS multipath signals had to be eliminated. Another consideration is that the fixture had to be rugged enough to withstand the DARPA Grand Challenge race course and also be able to be mounted on the roof of Alice. Elimination of the multipath signal required the use of the Raleigh equation to determine what configuration of material needed to be used to disrupt the reflection spot. There were a variety of issues to consider when designing the multipath disruption fixture. One of the key considerations was that the GPS satellites could be in any position in the sky at any time. The fixture had to reduce GPS multipath signals while at the same time not add a new multipath condition. Another consideration was with the tracking of the differential satellites. These GEO satellites were at a low elevation and their differential corrections were critical to the navigation of Alice in case there was any ionospheric activity during the race. My hypothesis was proven valid and I was able to design and mount multipath blocking fixtures on each of Alice's GPS antennas to block GPS multipath signals. Multipath overview Multipath is the result of a radio signal being reflected off an object. Multipath is what causes 'ghost' images on a television set. With GPS, multipath occurs when the signal bounces off a building or terrain before reaching the GPS receiver's antenna, meaning that the signal takes longer to reach the receiver . This added time makes the GPS receiver think the satellite is farther away than it really is, which adds error to the overall position determination.
Published in:	Proceedings of the 2006 National Technical Meeting of The Institute of Navigation January 18 - 20, 2006 Hyatt Regency Hotel Monterey, CA
Pages:	798 - 802
Cite this article:	Elder, S., Elder, M., "How Can the Effects of Reflected GPS Multipath Signals be Eliminated on an Autonomous Mobile Vehicle?," Proceedings of the 2006 National Technical Meeting of The Institute of Navigation, Monterey, CA, January 2006, pp. 798-802.
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