Sat-nav indicated I wasn't speeding, but...
Discussion
Sounds like the answer here is a dodgy camera but be aware that Sat Nav systems of the type typically used in cars are great for accurate speed display only when cruising and with a good view of the satellites.
If you are accelerating or decellerating the display will lag the actual speed by a variable margin which needs to be taken into consideration. A car with some performance could be going much faster (under accelleration) or much slower (under braking) than the sat nav shows at any instant. I'm sure you are all aware of this.
The less obvious problem, typically, is a weak satellite view or temporary signal loss or recovery from that sort of situation. Most units will think they still have some form of signal for a while and so continue to calculate the speed (really a very short timescale average) which can result in some very wrong figures displaying for several seconds. For example I had 8mph showing on my Road Angel the other day for several seconds after coming to a stop.
More worrying is the intermittent signal issue where the speed calc will often give a low number as it tries to work out what is going on.
Providing the GPS system is itself OK, and I have had one situation where it seemed not to be working at all, motorways and open roads should be no problem. City and town canyons and roads in deep valleys or with a lot of trees may give misleading readings and, in the case of sat nav, even suggest that you are somewhere completely different until they regain their bearings.
These situations seem to be rare in general but can consistently be a problem on certain roads.
HTH
If you are accelerating or decellerating the display will lag the actual speed by a variable margin which needs to be taken into consideration. A car with some performance could be going much faster (under accelleration) or much slower (under braking) than the sat nav shows at any instant. I'm sure you are all aware of this.
The less obvious problem, typically, is a weak satellite view or temporary signal loss or recovery from that sort of situation. Most units will think they still have some form of signal for a while and so continue to calculate the speed (really a very short timescale average) which can result in some very wrong figures displaying for several seconds. For example I had 8mph showing on my Road Angel the other day for several seconds after coming to a stop.
More worrying is the intermittent signal issue where the speed calc will often give a low number as it tries to work out what is going on.
Providing the GPS system is itself OK, and I have had one situation where it seemed not to be working at all, motorways and open roads should be no problem. City and town canyons and roads in deep valleys or with a lot of trees may give misleading readings and, in the case of sat nav, even suggest that you are somewhere completely different until they regain their bearings.
These situations seem to be rare in general but can consistently be a problem on certain roads.
HTH
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