Applying GPS Tracking Devices: Real-time RaceFX

Tom Ford is a GPS specialist at NovAtel Inc. He became involved with inertial navigation systems (INS) and GPS technologies at Sheltech and Nortech surveys in 1981. He is a member of the original group of GPS receiver developers at NovAtel Inc., where he has helped develop many of the core tracking, positioning and attitude determination technologies used there. His current focus is the integration of GPS with other supplementary systems, especially INS for applications similar to the RaceFX case.

Special effects (FX) have long been a broadcasting mainstay. Now a New York company specializing in enhancements for televised sports has incorporated GPS tracking devices into its toolkit. The result is a system that tracks and displays the real-time location of all cars during a racing event. Founded in 1998, Sportvision develops technology based enhancements for the Internet, television, and new media platforms. Among its most recent innovations is RaceFX, a system that incorporates GPS tracking devices and other technologies to enhance the viewer's experience of a race.

Auto racing has traditionally used inductive sensors similar to automated traffic sensors in the track to measure lap times and speeds by detecting when a racecar crosses the start/finish line. Using high accuracy data from GPS tracking devices, however, allows creation of "virtual" lines by defining positions along the track. Comparison of lap times using the GPS tracking devices and those measured with inductive sensors agrees to within 0.005 seconds. The performance objectives for RaceFX posed a complex set of challenges to system developers. Accurate vehicle positions needed to be obtained, calculated, and transmitted under conditions in which GPS satellite signals are frequently blocked or reflected.

To overcome this, RaceFX employs a sophisticated telemetry system that transfers position and other GPS vehicle tracking information from all racecars to a central processor. Differential GPS, pseudo-range and carrier phase tracking techniques generate GPS vehicle tracking coordinates accurate to 50 centimeters. The telemetry conveys differential messages from GPS base stations to the racecar rover units at 0.5 Hertz and racecar rover information to a video subsystem at 5 Hertz.

NASCAR authorities mandate that an electronics package cannot affect the aerodynamic or mechanical performance of racecars. Installing the GPS vehicle tracking system without affecting the aerodynamics of a car traveling 200 mph and yet maintaining good satellite visibility requires an innovative design. The RaceFX installation employs a metallic "soap dish" containing the GPS antenna that fits under the car's roof. A hole cut in the roof and covered with a fiberglass plate forms an RF window. Once installed, the GPS antenna is invisible from the outside of the car and does not affect the aerodynamics. However, this recessed design reduces the GPS antenna gain for signals from satellites at low angles.

Based on the measured distance between the GPS antenna and the ground in each vehicle, a precise location value can be defined for each vehicle's antenna with respect to its position on the track. As long as the vehicle operates on the track, the distance to the GPS antenna in a direction perpendicular to each "virtual" line (see second paragraph) is known. Once a vehicle is identified with an appropriate line, this information provides an observation equivalent to a very accurate measurement from an additional satellite.

RaceFX has demonstrated itself under commercial operational conditions as a system capable of displaying video annotation of NASCAR races to a television audience in real time. The core of the GPS vehicle tracking system is a set of modified receivers capable of operating in pseudo-range or differential mode with the capability of incorporating a digital track model into either of the positioning solutions. Although this GPS vehicle tracking technology has been tailored to a particular racing application, there is no reason why it could not be applied to other navigation problems. Areas that come to mind are precision farming in which satellite signal coverage is occasionally restricted, vehicle navigation in urban canyons with similarly limited coverage, or other broadcast sports applications such as sailing and aircraft racing.

(Adapted from the GPSWorld article by by Ken Milnes and Tom Ford)

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