Weather radar shows the past instead of the future

[Peobody]

This is an interesting discussion. I never thought the weather radar on my XT was, or could be, future. I have three weather apps on my phone. Only one is capable of showing future, but that is by specific request. The others as always past. Even when the word "current" is used there is a timestamp showing some minutes in the past, and updates occur only every x minutes. Oddly, I think I am more comfortable with extrapolating from the past trend rather than looking to a prediction of the future. I'm talking in the very short term here. If I see dark clouds ahead, the size and movement obtained from the weather radar helps with the decision to suit up or not. That's the extent of my use of it. Like everyone else, I hate having to stop to view the trend.

[jfheath]

GPS units by definition live in the past. For example, your actual position is only updated roughly every other second. This is why the MPH display is most accurate when you travel at a steady speed. During rapid accel or decel it "misses" some readings and displays "old" information.

Yes, spot on. I reckon (and I don't know why) that the speed you are travelling is calculated by a rolling data stream of positional fixes 5 seconds apart - from which it calculates the distance, divides it by the time in order to calculate your speed.

So if you accelerate hard or brake quickly, it takes about that long to catch up with you actual speed. I've said many times that satnav do not show how fast you are going, they show how fast you have been going.

There's a nice little test you can play. Go into a tunnel slow and then speed up to the limit in the tunnel - say speed up from 30 to 40 mph. Maintain 40mph throughout the journey. It used to be that the Zumos would say - no satellite signal and wouldn't show you anything. The newer ones realise that you are on a road - the only road, so they assume that you are traveling at the same speed that you were before you entered it. 30 mph. And they plot your progress through the tunnel at that speed. When you emerge, watch the speed on the satnav. As soon as it gets a satellite fix of your current position it realises that you are a long way ahead of where it thought you were. And for a couple of seconds, it is calculating the speed from where it thought you were to where you are now, and it shows a silly speed. Well into 3 digits. And it takes about 5 seconds to settle down to the actual speed. (Which is proebably where I got the 5 seconds figure from).

I've read a few interesting documents about sat navs and the logs that they create. Apparently, they cannot tell you precisely how fast you were going at a certain point on the road, or at a particular time. The data is not accurate enough. Neither can they state that you were in a particular spot at a particular point in time. But given a few pieces of data at known intervals, the error in calculating a particular speed or a particular point reduces significantly. So they cannot say that at the point of collison you were traveling at 100 mph. But they can say with a high degree of certainty that you travelled the 100metres up to the point of collision at an average speed of between 95 and 105 mph.