Real-Time Centimeter-Level GPS Positioning in the Works to Aid Vehicle Safety Systems
IVsource.net
8 June 2002

The US Federal Highway Administration, a pioneer in making differential GPS available for ITS applications, is now experimenting with centimeter-level accuracy.  Such precise positioning could become a key element in active safety systems and vehicle-highway automation.  IVsource takes a look.

The US Federal Highway Administration (FHWA), a pioneer in making differential GPS available for ITS applications, is now experimenting with centimeter-level accuracy.  Such precise positioning could become a key element in active safety systems and vehicle-highway automation – coupled with high accuracy digital maps, reliable positioning at this level could support inter-vehicle maneuver coordination, obstacle avoidance, and lane keeping.

FHWA, in association with the US Coast Guard, National Geodetic Survey, and several other government agencies, is implementing a High Accuracy Nationwide Differential Global Positioning System (HANDGPS) capability for test purposes at the Hagerstown, MD, Nationwide Differential Global Positioning System (NDGPS) facility.  The work is led by Jim Arnold of  FHWA's Turner Fairbank Research Center in Northern Virginia.

The accuracy of this broadcast correction signal is expected to enable 10-centimeter (95%) dynamic accuracy within 200 miles of the broadcast facility. The signal will be available for approximately one year and can be received by anyone with appropriate receiver equipment.  The goals of the program are to enable dynamic positioning at the centimeter level throughout the NDGPS Network, coexist with the current NDGPS Broadcast, and minimize the incremental cost of adding High Accuracy DGPS to NDGPS sites.  The approach to a low cost system includes utilizing existing broadcast antennas, reference stations, and site infrastructure; the incremental cost is expected to be around $75K for each existing site (of which there are several dozen in the US).

Background

Using the US Coast Guard's Maritime Differential GPS service as its base, a seven agency group began deploying the NDGPS service in 1998.  Since that time, roughly 80% of the country has been covered with this low frequency broadcast of differential corrections.  This service offers a 1-3 meter accuracy, 5 second integrity, and 99.97% availability.  This level of service has enabled many surface based applications, but many more applications actually require higher accuracy with equivalent integrity and availability.  According to FHWA, it is the goal of the High Accuracy NDGPS program to develop a long-range method of providing a higher accuracy radionavigation solution in order to promote the development of applications that would not otherwise be available in the foreseeable future.

HANDGPS Configuration

The current planned configuration will broadcast carrier phase data on a second low frequency, while maintaining the existing NDGPS broadcast at 307kHz.  To achieve this, carrier phase information will be collected from one of the existing NDGPS reference receivers and packaged appropriately.  This data stream will be provided to a new modulator, with this modulated signal fed to a new transmitter.  This signal will then be diplexed onto the existing NDGPS tower and broadcast at a frequency of 454 kHz.

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For More Information ...

... contact Jim Arnold, FHWA, (202)493-3265, james.a.amold@fhwa.dot.gov. 

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