Why doesn't rigging mess with radar ?

[transmitterdan]

Originally Posted by Dave_S

Thanks to all, it makes sense.



When I did a survival at sea course, the instructor said that radar was a complicated instrument to read and basically useless without training. The B&G salesman said that was the old ones, these new ones are intuitive and I'll be fine with a quick run down on it.



I assume the truth is in the middle ?



Any views on this.

Radar is like a lot of things on a boat. You have to use them when the situation is clear and you don't need it. By watching with your eyes and matching that with what you see on radar makes it 100 times easier when you need radar in anger.

Read the manual and try all the features. Learn why they are useful (or not) and when they might help you. But the reality is that if you are already competent with navigation then using radar is not a big step away. If radar confuses someone then I suspect they have a more serious training deficiency that is way more important.

Even a 15 year old color LCD radar is pretty easy to use. The new 4g and HD units are really good but anything built in this century will be plenty easy to use.

[a64pilot]

I don't think the 4G radar is chirp, I think it is a continuous transmitted constant power, but with frequencies that sweep up and down, the return signal based on its frequency will determine the time since that frequency was transmitted and based on time distance can be computed as speed of light is a constant.
However if it does work like that, how does it handle Doppler shift?
I've not seen a decent operating principle of the thing in print myself.

[Jim Cate]

Originally Posted by a64pilot

I don't think the 4G radar is chirp, I think it is a continuous transmitted constant power, but with frequencies that sweep up and down, the return signal based on its frequency will determine the time since that frequency was transmitted and based on time distance can be computed as speed of light is a constant.
However if it does work like that, how does it handle Doppler shift?
I've not seen a decent operating principle of the thing in print myself.

I suspect that when dealing with targets with relative motion in the order of a few m/sec that the doppler shift is vanishingly small and simply disregarded.

Jim

[transmitterdan]

Originally Posted by a64pilot

I don't think the 4G radar is chirp, I think it is a continuous transmitted constant power, but with frequencies that sweep up and down, the return signal based on its frequency will determine the time since that frequency was transmitted and based on time distance can be computed as speed of light is a constant.
However if it does work like that, how does it handle Doppler shift?
I've not seen a decent operating principle of the thing in print myself.

FMCW (aka 4G) is a chirp. But it does not use pulse compression at the receive end. Instead it multiplies the received signal by the transmitted signal. The output of the multiplier produces the sum and difference frequency between the transmitted and received signals. The sum is filtered out leaving only the difference. The received frequency will a little different than the transmit frequency owing to the round trip speed of light time to the target (the transmit frequency has increased by the time the reflected signal returns). The exact difference in frequency being a linear function of the out and back distance to the reflecting target. There is a "magic" box (circulator) in the antenna that can separate the transmitted from the received energy so the transmitter can be always on without interfering with the receiver. The chirp can either be a linear ramp up or down in frequency with the results being the same in either case.

A target moving away from the transmitter will be shown ever so slightly further away than it was when illuminated due to doppler shift but at boat speeds this error is insignificant. It's actually a interesting case study in the uncertainty principle (can't know both position and speed simultaneously).

Simrad make both FMCW and pulse compression radars.

[Wotname]

Originally Posted by transmitterdan

Radar is like a lot of things on a boat. You have to use them when the situation is clear and you don't need it. By watching with your eyes and matching that with what you see on radar makes it 100 times easier when you need radar in anger.

Read the manual and try all the features. Learn why they are useful (or not) and when they might help you. But the reality is that if you are already competent with navigation then using radar is not a big step away. If radar confuses someone then I suspect they have a more serious training deficiency that is way more important.

Even a 15 year old color LCD radar is pretty easy to use. The new 4g and HD units are really good but anything built in this century will be plenty easy to use.

Very very good advice IMO.

Although last century units are pretty easy to use also

I remember watching a fisherman/pawner back in the 80's using confidently picking up close in underwater reefs with his radar - all with careful control of range, gain, sea state clutter etc etc. I could never achieve this; I stuck with picking up stuff above the water...