Posts Tagged ‘global positioning’

Adaptive Cruise Control Goes Mainstream

Tuesday, August 4th, 2009

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Heading south on the New Jersey Turnpike, Ford Motor Company engineer Jerry Engelman swings his 2010 Taurus into the left lane to pass a semi. The Taurus hesitates, slowing down, and then Engelman adjusts his heading. The car takes off. “Larry,” he calls to his colleague in the back seat, “write that down!”

Engelman is driving, but just barely. The Taurus has a radar-based adaptive cruise-control system that lets him set a top speed and then simply steer while the car adjusts its velocity according to traffic. He’s been weaving and changing lanes, doing between 45 and 70 mph—and hasn’t touched a pedal in an hour. Over the past few years, Ford engineers have driven 60,000 miles to test, tweak, and optimize this system, which also provides collision warning alerts. They’ve been focused on the future in a financially dismal present—analysts actually praised Ford for losing only $1.4 billion in the first quarter. “It’s a tough market,” says Derrick Kuzak, VP of global product development, noting that the Taurus is important for “reestablishing us in the large-sedan market.” That’s executive-speak for “This car really needs to sell.”

source: wired.com

Sources of Errors in GPS

Wednesday, July 2nd, 2008

Selective Availability

The most relevant factor for the inaccuracy of the GPS system is no longer an issue. On May 2, 2000 5:05 am (MEZ) the so-called selective availability (SA) was turned off. Selective availability is an artificial falsification of the time in the L1 signal transmitted by the satellite. For civil GPS receivers that leads to a less accurate position determination (fluctuation of about 50 m during a few minutes). Additionally the ephemeris data are transmitted with lower accuracy, meaning that the transmitted satellite positions do not comply with the actual positions. In this way an inaccuracy of the position of 50 – 150 m can be achieved for several hours. While in times of selective availability the position determination with civil receivers had an accuracy of approximately 10 m, nowadays 20 m or even less is usual. Especially the determination of heights has improved considerably from the deactivation of SA (having been more or less useless before).

The reasons for SA were safety concerns. For example terrorists should not be provided with the possibility of locating important buildings with homemade remote control weapons. Paradoxically, during the first gulf war in 1990, SA had to be deactivated partially, as not enough military receivers were available for the American troops. 10000 civil receivers were acquired (Magellan and Trimble instruments), making a very precise orientation possible in a desert with no landmarks.

Meanwhile SA is permanently deactivated due to the broad distribution and world wide use of the GPS system.

The following two graphs show the improvement of position determination after deactivation of SA. The edge length of the diagrams is 200 m, the data were collected on May 1, 2000 and May 3, 2000 over a period of 24 h each. While with SA 95 % of all points are located within a radius of 45 m, without SA 95 % of all points are within a radius of 6.3 m.

Plot of the position determination with and without SA
(Diagram from http://www.igeb.gov/sa/diagram.shtml (page no longer available)
With friendly permission of Dr. Milbert (NOAA))
Plot of the position determination with SAPlot of the position determination without SA

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