Today on Post Reports, we talk to national security reporter Shane Harris about exclusive reporting from The Washington Post on the vast aerial surveillance program behind the Chinese spy balloon.Read more: The U.S. intelligence community has linked the Chinese spy balloon shot down on Saturday to a vast surveillance program, and U.S. officials have begun to brief allies and partners who have been similarly targeted.Why balloons? The technology is old but effective, according to Shane’s sources. “The real advantage that the balloon has is that it actually moves very slowly,” Shane said. “That balloon could hover over a target at an altitude of about 60,000 to 80,000 feet, where it might be very hard to see. And it can stay there potentially for hours.”The United States hasn’t been great at detecting the balloons before now. In some cases, the balloons had been characterized as UFOs. Shane breaks down what this renewed concern about Chinese surveillance means for U.S.-China relations going forward — and why so many countries spy on each other.