Time-keeping query on School Zone Speed Alert
With reference to the School Zone Speed Alert project, the worst-case crystal accuracy of 40 ppm will produce a time error of approximately five minutes over a school term. I do not know how much leniency is applied regarding the starting and finishing times for school zones but a 5-minute error may result in one incurring a fine and loss of points.
Maybe your readers would be wise to check the Speed Alert’s time at the beginning of and halfway through each school term.
Col Hodgson,
Mount Elliot, NSW.
Comment: there is a reference to “trimming the crystal” on page 41 of the article. This covers your point.
Not all GPS satnavs need constant 3-satellite reception
In-car navigation comes in two versions: satellite reception only or satnav with GPS and speed data comparison. Both need about three satellites for triangulation initially to determine the vehicle’s position. The in-car GPS receivers first introduced into Australia by Philips are not blacked out by satellite network shadow areas like city buildings, car parks, valleys and under thunderstorms. They continue to operate well, utilising a microprocessor computer in the unit, which processes extra data from a simple gyro and electrical pulses from the speedo.
Advanced GPS units like this process all this information to know the direction and speed of the car, comparing it with the satellite data and inbuilt maps for more accurate indication. In practice, I did find some inaccuracy once in a heavy rainstorm but generally it was unstoppable.
The Philips system worked so well there was an instance when the antenna plug was accidentally disconnected and the unit continued to work for a few days with no satellite reception. Of course, units with this technology are superior but more expensive.
Kevin Poulter,
Dingley, Vic.
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