My Question is, PLA ever tested Df-21d on any moving Target in sea?
That is a mystery that only the governments of China, the US, Japan, and South Korea know. There are no credible reasons why the Chinese boys would have complete faith in fanboy reports of a successful live test. The weapon determine the testing conditions. The DF-21 is supposed to be land based so that mean China would have a very limited testing area, unlike the US where we have access to the Atlantic and Pacific oceans. Today, no one can launch a ballistic missile without someone detecting it so the testing area for China would be under observations by US, Japan, and South Korea, from Earth all the way up to satellites.
Does that mean China cannot do discrete testings of individual components? All tests are 'rigged' tests so China can indeed perform such discrete testings. Take the over-the-horizon radar that supposedly is a crucial element. Instead of an aircraft carrier, oil tankers can be used. Both are surface ships that have many similar characteristics besides their common large sizes. China can also air drop the seeker head assembly to test its ability to distinguish a man-made object among sea clutter and this test can be done with no one the wiser. The list is considerable.
The problem with discrete component testings is that they do not reveal problems that may arise when components are interacting with each other or that certain environmental conditions may work against one's wishes. Take the OTH radar system again...
Over-the-horizon radar - Wikipedia, the free encyclopedia
The most common method of constructing an OTH radar is the use of ionospheric reflection. Given certain conditions in the atmosphere, radio signals broadcast up towards the ionosphere will be reflected back towards the ground. After reflection off the atmosphere, a small amount of the signal will reflect off the ground back towards the sky, and a small proportion of that back towards the broadcaster. Only one range of frequencies regularly exhibits this behaviour: the high frequency (HF) or shortwave part of the spectrum from 3 30 MHz. Given certain conditions in the atmosphere, radio signals in this frequency range will be reflected back towards the ground. The "correct" frequency to use depends on the current conditions of the atmosphere, so systems using ionospheric reflection typically employ real-time monitoring of the reception of backscattered signals to continuously adjust the frequency of the transmitted signal. Given the losses at each reflection, this "backscatter" signal is extremely small, which is one reason why OTH radars were not practical until the 1960s, when extremely low-noise amplifiers were first being designed.
We know about this property for decades. But there is a problem with relying upon the atmosphere...
Ionosphere - Wikipedia, the free encyclopedia
At night the F layer is the only layer of significant ionization present, while the ionization in the E and D layers is extremely low. During the day, the D and E layers become much more heavily ionized, as does the F layer, which develops an additional, weaker region of ionisation known as the F1 layer. The F2 layer persists by day and night and is the region mainly responsible for the refraction of radio waves.
Radar signals are radio waves. A long time ago, one of the Chinese boys here admonished me that radar signals are not radio signals. I just about busted a gut laughing. OTH radar signals are just as vulnerable to nature's fickleness as music or ham radio transmissions. The freq bands usually employed by OTH radars are the same. But I guess because the radar signals are generated by Chinese hands, the laws of physics does not apply...
...Anyway, this is just one of the many variables that test designers -- and I used to be one -- must be creative about especially if the country's safety is at stake.
Then at the end, after all the data from all the 'rigged' tests are analyzed and problems resolved, we must have a series of live and 'unrigged' tests under as many environmental conditions as possible. The enemy would know of the weaknesses of OTH radars so we must compensate somehow, after all, we cannot appeal to the enemy to attack us only when environmental conditions are favorable to us, correct?
There is no reason why the US government would keep a successful live DF-21 test a secret and China cannot launch such a test missile without our satellites knowing about it. That does not mean we should not take the DF-21 seriously. We must take its alleged claim of existence seriously. The MIG-25's history is instructive on how far we can go in inferences and speculations on a potential threat. What we inferred and speculated about the MIG-25, even though we were wrong in many aspects, were based upon our own capabilities. That mean the safe assumption is that if we can do it, the potential adversary could as well. That does not mean we should go into a panic but it mean if and once our worst fears are true, we are not surprised and that is halfway towards an effective counter.