Iran is claiming "its troops" shot down a U.S. reconnaissance drone as it "tried to cross the border a few days ago."
This report, by the way, originated with an "Iranian lawmaker" who passed the information to a Chinese news agency. Details of the alleged shootdown are virtually non-existent. In other words, take this one with an enormous grain of salt.
That's not to say that we don't keep an eye on Iran. Predator drones routinely fly along the Iran-Iraq border and over the Persian Gulf, monitoring infiltration routes for insurgent, and Iranian military activity. SIGINT assets keep tabs on communications and emitter activity, and overhead assets (read: satellites) cover key targets in Iran on a routine basis.
As for the alleged shootdown, it's within the realm of possibilities, but just barely. As we've noted before, tracking and successfully engaging a reconnaissance drone is more difficult that you might believe. For starters, Predators and other intel collection UAV typically have a very small radar cross-section, making them hard to detect with radar. Their slow cruising speed (typically less than 100 knots) makes detection and tracking even harder. On some radars, the Doppler gate has to be adjusted to look for extremely slow-flying targets, but that also introduces more clutter into the system, complicating the job for operators.
Iran does have a proficient SIGINT system, and tracking control and downlink signals from the drone could be helpful in an intercept. But assuming the Iranians actually have some idea of where the drone is (and Americans are creatures of habit), you still have to get the tracking information to the right air defense crews at the right time. And, despite recent C2 upgrades, Iran's air defense network is still beset by situational confusion and coordination problems. If Iranian SIGINT crews are able to pinpoint our drones, there's no assurance the information will reach the right AAA or SAM crew in time for a successful intercept.
If the Iranians actually managed to shoot down a Predator, there was probably an element of luck involved, i.e., a border observation post saw the drone, opened fire, and managed to hit it. A more likely scenario is that the UAV suffered some sort of malfunction and crashed in Iranian territory, allowing Tehran to claim that it was shot down.
Nothing I've seen suggests that the Iranians are any more proficient at tracking and engaging our UAVs than the Iraqis were during the "No-Fly Zone" era of the 1990s. Saddam's efforts to track and engage our drones were often good for a laugh, and they underscored the difficulty of the task. On more than one occasion, we watched (and listened) as Iraqi pilots tried in vain to locate and shoot down a Predator. They sometimes passed within a few thousand feet of the UAV, but were unable to detect it on radar, or visually.
But Iran is adept at exaggerating its military capabilities, and if I had to guess, I'd say this "shootdown" falls into that category.