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ARTICLE: SatNav for beginners


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Location : Sheffield, UK

ARTICLE: SatNav for beginners

Post by spitonyourgrave22 on Sat 30 Oct 2010, 6:28 am

Submitted by Fr33flow

SatNav, or GPS, is widely available for bikes as well as cars now, and waterproof units [essential for UK riding] are retailing around the £200 mark. The well-publicised errors in directions that units can give certainly happen, but this is to do with the programmers of the direction finding software rather than the navigation system itself.

The receiver in modern Sat Nav systems measures 15 x 17mm

So, how does the system work? The unit fitted to a vehicle doesn’t actually transmit anything to the satellite system, but just listens for the positioning data. The data the satellites transmit consists of three parts that take 30 seconds to send, and are continually repeated. The first part is a time signal from the highly accurate atomic clock onboard each satellite and takes six seconds to transmit.

This alone cannot locate the user, so the second part of the signal, taking 12 seconds to transmit is the ephermeris, which details the satellites orbit, and hence distance from the receiver.

The final 12 seconds broadcast an almanac, which tells the receiving unit which satellites will be above the horizon in the future. It is the almanac of the signal that older units couldn’t detect leading to long detection times on starting up the unit as it searched for signals from the orbiting satellites.

The receiver needs signals from three satellites to accurately find its location. Why three and not just one, if each satellite broadcasts a time signal and it’s position in the sky? From one satellite, the time signal and distance information will place the receiver somewhere on a circle on the Earth’s surface. This is because the signal will reach the same point on the circle from an orbiting satellite at exactly the same time, as shown in figure 1.

Fig 1, single satellite transmission describes a circle on the Earth’s surface.

With a second satellite giving the same time and position data, another circle can be drawn that will locate the user to one of two points where the circles overlap, as can be seen in figure 2, the user could be at either of the points arrowed.

Figure 2, with two satellites the user can be in one of two positions.

When the third satellite that the receiver looks for is added to the existing two signals a third circle is produced that intersects one of the two overlaps from the first two satellites, as can be seen in fig 3.

Fig 3, a three satellite system.

It can be seen that the three circles do not overlap neatly. This is the cause of the inaccuracies within the GPS system. The reason that the circles don’t meet at an exact point is that the radio signals from the satellite system are subject to interference from atmospheric effects, the environment where the user is that may block a view of the sky or bounce signals sent from the satellite off adjacent buildings, and even the speed of the satellite relative to the receiving station.

Most newer GPS systems will look for signals from at least four satellites, and more if they are above the horizon. ‘Horizon’ in this case does not just mean the Earth’s own horizon, but takes into account structures and trees that can block the signals. Accuracy for GPS is usually within 10m, and on a good day can be to the nearest foot.

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