As China began looking for traces of missing Malaysia Airlines flight MH370 within its borders, Chinese experts were divided over whether such a big jet could enter its airspace undetected. The move follows a request from Malaysia that it be given access to classified Chinese radar data.
Huang Huikang , China's ambassador to Malaysia, said China had started a search and rescue operation on its territory, checking the possible northern flight path of the Boeing 777-200 following its disappearance from radar screens on March 8 en route from Kuala Lumpur to Beijing. Of the 239 passengers and crew, 154 were Chinese.
Malaysian authorities appealed to China and other countries on Monday for radar data after satellite data showed the jet had either flown north towards Kazakhstan or along a southern corridor towards the southern Indian Ocean from its last known location over the northern Malacca Strait.
Foreign ministry spokesman Hong Lei did not say directly on Monday whether China would share its radar data, saying only that Beijing was willing to "proactively co-operate with Kuala Lumpur if it was beneficial to the search".
Watch: China is searching for Malaysian jet inside its territory
Chinese radar experts were divided over whether the plane could have flown undetected into Chinese airspace, but all agreed the radar data was so sensitive that a decision to share it could only be made "at the highest level".
Beijing-based military expert Li Jie said China had started a search of its territory because it did not want to miss any opportunity to locate the lost flight.
"There is still a slim chance that such a big plane could evade radar detection, so China should spare no effort to find it," he said.
Professor Li Jun , a data analysis researcher at the National Laboratory of Radar Signal Processing in Xian , Shaanxi , said "the possibility could not be ruled out" that the missing Boeing 777-200 had evaded radar while entering Chinese airspace.
A plane could dodge radar in two ways, Li said. Stealth military jets, for instance, use sophisticated coatings and special body design to minimise detection by radar. But a skilled pilot could also fly a civilian aircraft into a country without alerting its air defences.
"If [the pilot] flew low and made clever use of uneven terrain, he could have cheated radar and entered China, or any country, without being shot down or detected," he said. "The chance is remote but it cannot be ruled out."
To carry out such a mission, the flight crew would have to know the location of every radar station along the flight path, including military ones.
Although the whereabouts of military radar stations was supposed to be a tightly guarded national secret, "with the help of satellite images from Google Maps and some professional knowledge of the external features of radio antennas, you could mark out most radar stations on the route and chart a flight path through a 'shadowed corridor'," Li said.
Chinese radar and radio communications researchers were baffled by the incident and were following it closely, Li said.
The government may also have compiled raw data from civilian and military radar stations for detailed analysis.
Professor Su Hongtao , a radar tracking researcher at the national radar lab, agreed with Li that some pilots and aircraft could dodge detection through special manoeuvres, but doubted it were possible in a plane the size of a Boeing 777.
During the cold war, a young German pilot flew a small plane to Moscow, evading almost the entire air defence system of the Soviet Union, he noted.
"That certainly couldn't be done with a Boeing 777, which is much larger and faster, making it an easy target for any country's radar system," he said.
Singling out a suspicious object as large as a 777 on China's radar data should be easy, Su said.
Another source, a Beijing-based aviation expert who did not want to be named, said there was a greater chance the plane had headed south, but to concentrate more search resources there, the northern route had to be ruled out. "That's probably why China is carrying out the search at home," he said.
Professor Xu Rongqing , a radar imaging expert at the Harbin Institute of Technology, said the data collected by military radar could be mined for useful information about a country's air defence system, and that was probably why Malaysia was reluctant to share its with China immediately after the incident.
"Now that has become our headache. To share or not to share, that is the question," he said.
Xu said other countries' radar experts could glean a great deal of intelligence about Malaysia's air defences from the government's reports on the incident, and the same could happen to China.
"Chinese radar data contains lots of technical information that we definitely don't want to share with other countries," Xu said. "If it is to be shared, the decision must be made at the highest level."
Additional reporting by Adrian Wan in Kuala Lumpur