China's Space Program News Thread

[gelgoog]

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I would be very surprised (and disappointed) if China left out the ability to transmit on the new radio telescope.

It isn't necessary to study pulsars which is the design goal of FAST I think.

 

[Nutrient]

I would be very surprised (and disappointed) if China left out the ability to transmit on the new radio telescope.

The extra weight of the transmitter might have caused Arecibo's instrument platform to crash, destroying the radio dish. The central instrument platform, suspended over the 300 meter dish, weighed 900 tons; I don't know how much of that was the transmitter.

 

[Quickie]

It isn't necessary to study pulsars which is the design goal of FAST I think.

The extra weight of the transmitter might have caused Arecibo's instrument platform to crash, destroying the radio dish. The central instrument platform, suspended over the 300 meter dish, weighed 900 tons; I don't know how much of that was the transmitter.

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.

 

[silentlurker]

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.

Do you understand how radar astronomy works? When you're scanning objects millions of km away your transmitter becomes a point 1/r^2 intensity source regardless if the dish is 100m wide or 1000m wide.

 

[Quickie]

Do you understand how radar astronomy works? When you're scanning objects millions of km away your transmitter becomes a point 1/r^2 intensity source regardless if the dish is 100m wide or 1000m wide.

So? Are you a radio astronomer? What does the 1/r^2 intensity depend on in the end?

 

[silentlurker]

What does the 1/r^2 intensity depend on in the end?

I don't know what you mean by this, 1/r^2 is a proportionality law, it doesn't "depend" on anything

 

[siegecrossbow]

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Despite this we have a good idea of when the spacecraft is due to make its own Mars orbit insertion attempt.


By picking up signals sent between Tianwen-1 and Chinese ground stations amateur radio operators are able to determine information about the spacecraft's speed and position and calculate the arrival time at Mars.

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, an amateur radio operator who tracks spacecraft together with fellow enthusiasts, calculates that Mars orbit insertion around 7 a.m. EST (1200 GMT, 8 p.m. Beijing time), meaning Tianwen-1’s braking burn will begin up to half an hour before this. As will the Hope orbiter, Tianwen-1 will need a long burn of its main engines to slow the spacecraft enough to allow it to be captured by Mars's gravity. The main engines were tested through smaller maneuvers on the way to Mars.

 

[Quickie]

I don't know what you mean by this, 1/r^2 is a proportionality law, it doesn't "depend" on anything

That shows you don't know what you're talking about.

The signal intensity from a certain distance from a "point source" (as you have suggested for the radar dish) does depend on the signal output power (and therefore the signal source intensity depending on how the signal energy is distributed) originating from that signal source.

 

[silentlurker]

The signal intensity from a certain distance from a "point source" (as you have suggested for the radar dish) does depend on the signal output power

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.

You said "maximim transmitting signal power", so I assumed power was constant. Obviously if you increase the power intensity also increases at every range.

(and therefore the signal source intensity depending on how the signal energy is distributed) originating from that signal source.

The transmitting radio signal will just spread thinner (less energy) the bigger the dish

You seem to have it in your head that somehow the area of a radar dish affects the strength of the signal return. This is not true, as long as the target isn't sitting right in front of the dish.

At one million km away, it doesn't matter if you have 1 m^2 of dish radiating at 10W/m^2 or 10 m^2 of dish radiating at 1 W/m^2. As long as the output power is the same your signal return W/m^2 will be the same.

 

[Quickie]

You seem to have it in your head that somehow the area of a radar dish affects the strength of the signal return. This is not true, as long as the target isn't sitting right in front of the dish.

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.

At one million km away, it doesn't matter if you have 1 m^2 of dish radiating at 10W/m^2 or 10 m^2 of dish radiating at 1 W/m^2. As long as the output power is the same your signal return W/m^2 will be the same.

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.

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.