Television broadcasts increasingly use technology to enrich the experience of sporting events. Such technology, along with its associated graphics and statistics, not only entertains viewers but also enables them to gain insight into the performance of the athletes and, in the case of motor sports, their machines.

Founded in 1998 as a privately held company headquartered in New York City, Sportvision develops technology-based enhancements for the Internet, sports television, and new media platforms. Among its most recent innovations is RaceFX, a system that incorporates GPS and other technologies to enable real-time tracking and display of the location of all cars throughout a racing event. The custom GPS engineering described in this article was developed in conjunction with NovAtel Inc., a GPS technology supplier based in Calgary, Alberta, Canada.

Figure 1 Screen captures of typical RaceFX graphics

RaceFX gives the broadcaster new tools to tell the story of the race and increase the enjoyment of racing fans. The National Association of Stock Car Automotive Racing (NASCAR) introduced RaceFX in television broadcasts at the Daytona 500 in February of this year. NASCAR uses the system to generate graphics, calculate speeds, and compute other performance-related parameters of interest to the racing fan.

To achieve this capability, RaceFX uses compact, high-performance GPS receivers to provide real-time measurement of racecar positions. Coupled with other data inputs, the system generates graphical effects relevant to the current view of a race camera by directly coupling real-time vehicle positions to the broadcast screen images. Accomplishing this requires precise information of the target car's position and how each pixel of the video display maps to the real-world coordinates of the track.

This article describes the design and development of RaceFX and its implementation in the challenging environment of high-speed racing dynamics.

The Flag Goes Down 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 GPS data, however, allows creation of "virtual" lines by defining positions along the track. Comparison of lap times using the GPS sensors and lap times measured with inductive sensors agrees to within 0.005 seconds. Virtual lines are used in the broadcast to show instantaneous speeds at arbitrary points on the track and average speed between points along the track, such as turns and straightaways.

Figure 2 The RaceFX sub-systems

Precisely locating a racecar in the video frame enables a variety of graphical enhancements. For example, a graphic could be created to point at a specific racecar. This graphic could show the name of the driver and related performance information, such as speed or time behind the leader (see Figure 1). This would help the viewer understand important aspects of the race more clearly. With knowledge of where every race car is at all times during the race, the system can generate rich statistical information, such as what racing "line" or path the fastest cars drive on the race track.

These performance objectives for RaceFX posed a complex set of challenges to system developers. Accurate vehicle positions needed to be obtained, calculated, and transmitted during a high-dynamic operations under racing conditions in which GPS satellite signals are frequently blocked or reflected (multipath). The derived positions then need to be interpolated and linked to screen images of the vehicles and their associated graphics in real-time.

System Description The Sportvision RaceFX system consists of four subsystems (see Figure 2) GPS, telemetry, time synchronization, and video overlay. Each racecar has a GPS receiver and a 900 MHz transceiver. Mating 900 MHz transceivers are remotely located around the track and send data over DSL modems to the control center in the on-site television broadcast center where the communications controller, time synchronization, and video overlay system reside.

A racetrack has numerous features that can obstruct satellite signals, as seen in the photo at top.

RaceFX employs a sophisticated telemetry system that transfers position and other vehicle information from all race vehicles to a central processor at the rate of five times per second. Differential GPS (DGPS) pseudorange and carrier phase tracking techniques generate vehicle coordinates accurate to 50 centimeters (1 sigma). The telemetry conveys differential messages from GPS base stations to the racecar rover units at 0.5 Hertz and racecar rover information to the video subsystem at 5 Hertz.

RaceFX processes the five-Hertz DGPS positioning data to derive many useful parameters. The instantaneous position and time data are used to place a virtual object in the video stream. Because the data packet from each rover must be kept small, speed and heading are calculated from the computed velocity vector between sequential GPS positions. Lateral and along-track acceleration are also calculated using the velocity data.

Six broadcast cameras are instrumented to measure their pan, tilt, zoom, and focus 30 times per second or once per video frame. RaceFX interpolates racecar position information to correspond with the camera orientation in each video frame. High-speed computers combine this data to appropriately juxtapose the car and data in the video frame.

Figure 3 This color-coded map shows the number of GPS satellites visible on different portions of a NASCAR racetrack.

The video overlay accepts information from all rovers and reformats it for individual video frames of the particular camera used in the broadcast, while the time synchronization subsystem tags each video frame with a GPS time stamp. The racecars travel at speeds up to 90 meters/second. Relative timing between the video, and GPS must be accurate to one millisecond to keep time-induced errors below 10 centimeters. The GPS-based system maintains the timing to about 10 microseconds, or 100 times better than the minimum requirement.

A key element in RaceFX is the creation of an accurate digitized model of each racetrack. This model supports RaceFX's video display of real-time data and is also used in the signal-processing method to improve the real-time positioning accuracy of the racecars. We will discuss the creation and use of these track models in a separate section.

GPS on Board Tracking satellites and computing an accurate position in a racecar does stress a conventional GPS system. The greatest challenge to maintaining continuous, accurate GPS position fixes on a racetrack is obtaining visibility of the minimum number of GPS satellites required for position determination.

Although four satellites is the minimum number required for three-dimensional positioning, in a constrained environment such as a racetrack four satellites often do not have the necessary "geometry" to obtain the accuracy required for the RaceFX application. Satellite geometry translates into the so-called GPS dilution of position (DOP), which acts as a multiplier of the cumulative error generated by other factors, such as receiver electronics, ionospheric effects, and multipath.