China's Space Program News Thread

[silentlurker]

Why not? The size of the dish has a direct relation to the amount of signal energy collected from the target and therefore the received signal strength

Yes, a bigger dish does collect more energy and allow you to detect further objects. But making a bigger dish output the same amount of power as a smaller dish does not change the signal response at a distance as you originally claimed.

That's not how it works. With a "1 m^2 of dish radiating at 10W/m^2", the signal intensity is at 10W/m^2 starting from the radar dish and it will keep on reducing the further away the target is. The same goes with the "10 m^2 of dish radiating at 1 W/m^2" starting with a signal intensity of 1 W/m^2 at the dish and reducing to a much smaller intensity by the time it arrives at the same target. So, the signal reflected back will be much reduced for the latter case.

Alright, lets look at your logic here. You're saying that only the initial signal intensity (W/m^2) matters, not the emission area (m^2)? Think carefully about how ridiculous that is. You're proposing that 100 flashlights in a grid are just as effective at spotting an object far away in the dark as 1 flashlight is. They have the same inital signal intensity after all!

 

[Quickie]

Yes, a bigger dish does collect more energy and allow you to detect further objects. But making a bigger dish output the same amount of power as a smaller dish does not change the signal response at a distance as you originally claimed.

Not sure what you're trying to say.
But signal intensity at the source does determine the signal intensity at the distance according to the inverse square law.

Check out the link

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""The intensity of radio waves over distance obeys the inverse-square law, which states that intensity is inversly proportional to the square of the distance from a source. Think of it this way: double the distance, and you get four times less power. ... Notice how steeply the intensity drops off at each point.""

Alright, lets look at your logic here. You're saying that only the initial signal intensity (W/m^2) matters, not the emission area (m^2)? Think carefully about how ridiculous that is. You're proposing that 100 flashlights in a grid are just as effective at spotting an object far away in the dark as 1 flashlight is. They have the same inital signal intensity after all!

What are you talking about? Having 100 flashlights together in a small place ("in a grid" as you say) will roughly have 100 times the intensity of 1 lightbulb.

Okay to help you understand. Imagine the radio telescope as a car headlight with the lightbulb at the focal point of the reflecting parabolic dish. Increasing the size of the parabolic disc of the car headlight while keeping the same lightbulb (analogous to keeping the same amount of radio signal power) will increase the area of the lighted spot and at the same time reducing the light intensity.

Look, you look like you're really not up to the mark to discuss this kind of subject, and I'm not here to educate you on the most basic of these things.

So we better move on from here, as this is all out of topic anyway.

 

[foxmulder]

The size of the FAST dish is probably way past the optimal size for an active radar dish. The transmitting radio signal will just spread thinner (less energy) the bigger the dish is for a certain maximum transmitting signal power source that can be possibly built.

The optimal size for an active radar dish for detecting asteroids is probably closer to the size of the Arecibo Observatory dish.

You are not correct in 2 rounds:

1) Laser do work the way you describe not radio signals.

2) FAST "dish size" -aperture can be adjusted by the software.

 

[Quickie]

You are not correct in 2 rounds:

1) Laser do work the way you describe not radio signals.

2) FAST "dish size" -aperture can be adjusted by the software.

I don't think so for point 1. See my previous post about the car headlight analogy.

As I've said before, that's all out of topic here. So better don't go there.

 

[silentlurker]

What are you talking about? Having 100 flashlights together in a small place ("in a grid" as you say) will roughly have 100 times the intensity of 1 lightbulb

Weird how you can acknowledge that having 100 flashlights in a grid has roughly 100 times the intensity of 1 lightbulb but not that 10 m^2 patches of 1W/m^2 radar emitters has roughly 10 times the intensity of 1 m^2 of the same emitter.

 

[gullible]

what time will Tianwen1 expect to be inserted today?

 

[siegecrossbow]

what time will Tianwen1 expect to be inserted today?

8:00PM Beijing time but we won’t know till 9 due to thruster burn (roughly half an hour), blockage behind Mars, and signal time lag due to distance.

 

[Quickie]

...but not that 10 m^2 patches of 1W/m^2 radar emitters has roughly 10 times the intensity of 1 m^2 of the same emitter.

Where did I say that? You're putting words in my mouth.

Anyway, the radar dish doesn't have many units of "1W/m^2 radar emitters" spread all over its surface. The only emitter source is located at the final focal point of the dish. The same emitter power spread over a larger dish is always going to lead to smaller signal intensity.
Of course, the total signal energy transmitted from the dish is going to be the same as the smaller dish with the same emitter power but with higher signal intensity radiating from the dish.

This is the most basic of how a radio telescope function.
This is the most that I can help you.

Time to move on to a more interesting topic like the Tianwen-1.

 

[Rachmaninov]

Sorry to budge in but Quickie you might want to revisit what you’ve mistakenly / misleadingly stated.

You first stated that there is “less energy” transmitted if a bigger dish is used:

The size of the FAST dish is probably way past the optimal size for an active radar dish. The transmitting radio signal will just spread thinner (less energy) the bigger the dish is for a certain maximum transmitting signal power source that can be possibly built.

Then you went on to say (and I paraphrase) that actually a larger dish would gather more reflected signal from the target, which is correct but goes against your statement above that it’s too large to be optimal:

Why not? The size of the dish has a direct relation to the amount of signal energy collected from the target and therefore the received signal strength.

You went on to basically regurgitate exactly what silentlurker said about the inverse square law whilst insisting you’re helping him understand:

With a "1 m^2 of dish radiating at 10W/m^2", the signal intensity is at 10W/m^2 starting from the radar dish and it will keep on reducing the further away the target is. The same goes with the "10 m^2 of dish radiating at 1 W/m^2" starting with a signal intensity of 1 W/m^2 at the dish and reducing to a much smaller intensity by the time it arrives at the same target. So, the signal reflected back will be much reduced for the latter case.

Remember the parabolic dish is not exactly perfect. It behaves somewhat like a point source of a radio signal in one aspect i.e. the signal intensity reduces as the distance gets further from the radar dish.

Then finally you admitted that the total energy transmitted is going to be the same, which is correct but contradicts your first statement quoted above:

Of course, the total signal energy transmitted from the dish is going to be the same as the smaller dish with the same emitter power but with higher signal intensity radiating from the dish.

Sometimes fudging things work, sometimes it doesn’t. Let’s just do the respectable thing, own it up like many other senior members here who openly admits their mistakes, and move on to more news about Tianwen or something.

 

[Deino]

Allegedly from 7:30 Beijing time on, a live stream will report on Tianwen-1's orbital insertion.

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