How can we install a radar on radio telescopes like FAST or GMRT? Recently I came across an article mentioning that ever since the failure of Arecibo telescope, we only have one asteroid radar imaging telescope which is Goldstone observatory.
The article also mentions the need of Asian (or any other country) radio observatory to upgrade their telescopes to telescope + radar system to take the place of Arecibo.
This made me wonder how can we upgrade such system instead of starting all over again (building a new telescope that could handle radar imaging as well).
The article is quite informative. It is a summary of &quot;a paper published in the journal Scientia Sinica Information&quot; which appears to be Discussion on the requirements and feasibility of constructing China's near-Earth asteroids radar system. While it is written in Chinese the tables and figures alone are very informative.
They are going for a &quot;planetary scale&quot; or very long range system; out to 0.1 AU or 15 million km for an asteroid is ambitious, and what's really interesting it sounds like their transmitted radar beam will com from &quot;four or five 35-metre (115-foot) diameter radio dishes&quot; rather than one big one.
I've pasted some bits into google translate so far and it looks like they will try to have the option to use the multiple transmitting dishes coherently

(3) X 和 Ka 频段上行天线组阵技术. 在国家高技术发展计划支持下, 我国在上行天线组阵技术研 究方面也已经取得了突破, 成功实现了对地球同步静止轨道通信卫星在 C 频段 (发射频率 6 GHz) 的 3 个天线上行组阵技术验证, 达到了 80% 的合成效率. 后续需要针对频率更高的 X 和 Ka 频段上行天 线组阵, 重点开展上行链路相位延迟变化准确估计技术、时延和相位对齐的精确控制技术和大回路系 统标校技术等研究.

X and Ka band uplink antenna array technology. With the support of the national high-tech development plan, China has also made breakthroughs in the research of uplink antenna array technology, and successfully achieved the C-band communication satellites in geosynchronous geostationary orbit. (Transmit frequency 6 GHz) 3 antenna uplink array technology verification, achieved 80% synthesis efficiency. The follow-up need for higher frequency X and Ka band uplink antenna array, focus on accurate estimation of uplink phase delay changes Research on precise control technology of technology, time delay and phase alignment and large loop system calibration technology.


Rather than build one giant dish for transmitting like Goldstone Solar System Radar which is

...a large radar system used for investigating objects in the Solar system. Located in the desert near Barstow, California, it comprises a 500-kW X-band (8500 MHz) transmitter and a low-noise receiver on the 70-m DSS 14 antenna at the Goldstone Deep Space Communications Complex. It has been used to investigate Mercury, Venus, Mars, the asteroids, and moons of Jupiter and Saturn. The most comparable facility was the radar at Arecibo Observatory, until that facility collapsed. GSSR now stands alone.

...they will use several 35 m transmitting dishes and an even larger number and more widely separated array of receiving dishes.
Table 7, &quot;China asteroid detection multi-bases radar system layout station composition&quot; is a list of transmitting sites. Obviously not all can be coherent and they will be used in a variety of ways.

It's really hard to add a new multi hundred kilowatt transmitter to an existing very large dish like a 70 m DSN or FAST. You can read more about that in answer(s) to What is a Beam Waveguide dish and why does the Deep Space Network use them?. The big 70 m DSN dishes use the focus up between the primary and secondary; it is a real challenge to add more hardware there. The image below shows a DSN 70 m dish, for scale, the red lines in the dish itself are a safe walking path and going up each arm of the secondary reflector are stairways for people, not ants.
I think that the Chinese project is quite ambitious but it's a next step in technology, rather than a makeshift retrofit that would interrupt availability of currently very busy large dishes, and it seems that adding a transmitter to FAST is not an option. From this answer to How will the closure of the Arecibo dish impact deep space communications? (found here):

With the loss of Arecibo, Goldstone's DSS-14 now becomes the world's largest and most powerful radar dish.  (China's 500 meter FAST dish is larger, but has no transmitter and is purely passive.)  Sky and Telescope reports that &quot;Arecibo offered 18 times the sensitivity of other existing facilities, such as NASA's Goldstone receiver.&quot;  It also states

Arecibo is also irreplaceable for scientists. Even though it’s technically the second-largest radio dish in the world (China’s Five-hundred-meter Aperture Spherical Telescope, or FAST, recently broke the record Arecibo held for decades), the observatory has unique capabilities, among them its radar. “FAST cannot do radar, it’s specifically incapable of doing active observation,” Springman explains. Because of that, FAST can’t take Arecibo’s place in planetary defense by characterizing asteroids and their orbits.


See also answers to

What will succeed the Arecibo Observatory?
Arecibo: Advantages of Giant Dish?
What are monostatic radar observations, and how will Deep Space Network's DSS-13 be used to observe asteroid 1999 WK4's flyby of Earth?


From Yahoo News China urged by scientists to create biggest radar system to save lives threatened by asteroid strike (similar/same article reprinted in several places, including intellasia.net, toysmatrix.com):

The proposed system would use four or five 35-metre (115-foot) diameter radio dishes in Kashgar, Xinjiang, to send powerful beams into space. Returned signals would be picked up by large antennas across the Chinese mainland in a number of cities, including Jiamusi, Beijing, Tianjin, Shanghai and Kunming.
The radar detection range would be expected to exceed 0.1 astronomical unit – or a tenth of the mean distance between the Earth and the sun.
The project “will fulfil the responsibility of our nation to the mankind,” said the team led by Li Haitao with the People’s Liberation Army’s Beijing Institute of Tracking and Telecommunications Technology in a paper published in the journal Scientia Sinica Informationis on Tuesday.
Continuing to rely solely on information provided by the Americans for a threat that could cause a global catastrophe would have a negative impact on China’s rising status in the international community, the researchers said.
The US had two planetary radars but one of them – the Arecibo Observatory in Puerto Rico – collapsed in December because of ageing, hurricane damage and a shortage of funding for repair and maintenance. This leaves the Goldstone Solar System Radar in a Californian desert as the last line of defence, and things could go wrong.
“A single failure of the system would leave us without an essential tool to ensure our protection. Such a failure occurred as recently as 2019-20, when the Goldstone transmitter was inoperative because of klystron (high power microwave vacuum tube) failures,” said professor Jean-Luc Margot, a planetary radar expert with the University of California, Los Angeles.
“I am glad to hear that China is considering building a radar system to study near-Earth asteroids,” he added.


From this answer to Why is the reflector on this millimeter-wave antenna spinning?:

Below are photos of one of the 70 meter Deep Space Network telescopes for talking to deep space spacecraft. This one is in the Goldstone complex. From the relative sizes in the image, the secondary mirror is close to 8 meters in diameter. Considering the size and mass of the secondary (those are stairs for humans on each leg, and the red lines in the dish are &quot;safe paths for walking&quot;) other, more advanced techniques to scan electronically have been developed, but the concept is the same.

above: Photo credit JPMajor, creative commons CC BY-NC-SA 2.0. click for larger.

above: From commons.wikimedia.org click for larger.