Satellite Laser Ranging in China
UCS Technical Working Paper
January 8, 2007
In September 2006, reports began to circulate that China had aimed ground-based lasers at U.S. satellites several times in the recent past. The reports contained few details about the incidents, leading to confusion about what actually occurred. These incidents were believed by some to be tests of Chinese laser anti-satellite (ASAT) weapons.
In a new analysis, we offer an alternate possibility of what may have happened, and suggest measurements that could distinguish between these uses.
A September 28, 2006 Defense News story reported that China had recently "fired high-power lasers at U.S. spy satellites flying over its territory in what experts see as a test of Chinese ability to blind the spacecraft."[1] ("Blinding" usually refers to causing permanent damage to an imaging satellite's detector.)
Shortly after this report appeared, Command Sergeant Major David Lady of the Joint Functional Combat Command for Integrated Missile Defense, was reported by Janes Defense Weekly to have stated that these incidents were discovered when operators noticed a "sudden decline in effectiveness as the satellites passed over China."[2] He was also quoted as saying that sensors at the Reagan Test Site on Kwajalein atoll detected laser light being reflected from U.S. satellites, saying they "sensed the projection of beams against the spacecraft and could identify the streams of photons." He reportedly stated that this was evidence of a Chinese laser countermeasure system.
However, Donald Kerr, Director of the Pentagon's National Reconnaissance Office, acknowledged that something had happened but that "it did not materially damage the U.S. satellite's ability to collect information."[3]
In addition, Gen. James Cartwright, who is in charge of U.S. military operations in space, said that the United States has not seen clear indications that China has intentionally disrupted American satellite capabilities.[4]
The Defense News report indicates that the targeted satellites were U.S. military imaging satellites. Laser illumination of these satellites is a concern because relatively low-power laser light could interfere with or potentially damage their sensitive detectors.
But were these "illuminations" anti-satellite attacks? The Defense News article above, for example, appears to equate illumination of the satellite with a laser attack, or tests or demonstrations of Chinese capability to carry out such attacks.
However, the statements quoted above from Kerr and Cartwright, both authoritative sources, cast doubt on the severity of whatever occurred and the intent behind it. These statements imply that the United States has indications China aimed lasers at a U.S. satellite or satellites, but that the illumination did not significantly affect the satellite, nor was it clear that was the intention.
The publicly available information is not sufficient to allow us to determine what actually happened in these incidents. One possibility is that China is developing laser weapons intended to disrupt satellites and has tested some version of them against U.S. satellites.[5]
However, there are other reasons to shine lasers at satellites, and we have analyzed a different possibility—laser ranging to determine satellite orbits—which appears to be compatible with the statements of Kerr and Cartwright but is not evidence of laser ASAT weapons. Moreover, based on our analysis, we suggest a way to distinguish whether the lasers used were appropriate for ranging or for a laser ASAT weapon.
China is known to have a network of satellite laser ranging (SLR) stations, which is part of an international network of such stations. SLR can be used to determine the orbits of satellites in low earth orbit to very high accuracy by reflecting short laser pulses off them and measuring the round-trip travel time. Although SLR is usually performed on a specific set of satellites, there is currently no international agreement that forbids any country from using SLR on any other country's satellites. If China used these SLR stations to track U.S. satellites as they passed over China, it could have led to effects consistent with the Kerr and Cartwright statements.
Our analysis shows that, although the lasers used for SLR are low power, an imaging satellite might notice the use of such lasers for tracking, and under some conditions an SLR laser could briefly obscure the images collected by the satellite in the immediate vicinity of the laser station (this effect is known as "dazzling"). There is also a very small probability (1 in 1,000) that under certain conditions, tracking an imaging satellite with one of the current Chinese SLR systems could damage the satellite's detector, but this possibility could be eliminated altogether by not using laser tracking when the satellite is nearly directly overhead. Future Chinese SLR systems are likely to be similar to the newer U.S. SLR systems, which use low power lasers that pose no danger of damaging a satellite sensor.
Information already possessed by the United States can help discriminate whether this incident looks more like an attack on the satellite's sensor or if it is more characteristic of satellite tracking. The Janes Defense Weekly story stated that a U.S. sensor at Kwajalein had detected laser light being reflected from a U.S. satellite. If the reflected laser light came from a Chinese SLR station, it would carry a signature that would be indicative of SLR, since the laser signal would consist of very short pulses at a low repetition rate—roughly 10 pulses per second. A laser system designed to disrupt a satellite's detector (either by dazzling or blinding) would be continuous rather than pulsed. Indeed, our analysis shows that a laser that produced short pulses at a low repetition rate would not be effective as an anti-satellite weapon, since the laser would almost always be off when the satellite was looking at it. Moreover, since the SLR laser is at a single frequency it could at most affect one of the satellite's multiple detectors.
We note that Kwajalein atoll is 5,500 kilometers from China. For a satellite at an altitude of 1,000 kilometers or less to be above the horizon and therefore visible from both China and Kwajalein it would have to be midway between them over the Pacific Ocean, some 2,500 kilometers from China. At that point the satellite would be very near the horizon as seen from China and would be in a position that the laser could not damage the satellite's detector.
As noted above, the use of SLR would be to allow China to accurately determine the orbit of satellites it was interested in tracking, especially low-altitude satellites that maneuver frequently or satellites whose orbits are not made public. If China was using its SLR capability in this way, it could simply be a means of space surveillance, to determine the orbits of satellites that pass over the region. However, while the SLR lasers would not in themselves represent a meaningful threat to the satellites, the high-quality tracking they could provide could be useful information for various methods of attacking satellites or interfering with their operation.
Notes
1. Vago, Muradian, "China Attempted To Blind U.S. Satellites With Laser," Defense News, 28 September 2006, http://www.defensenews.com/story.php?F=2125489, accessed 24 January 2007.
2. "US claims that China has used lasers to attack satellites," Janes Defense Weekly, 16 October 2006, http://www.janes.com/aerospace/military/news/jdw/jdw061016_1_n.shtml, accessed 24 January 2007.
3. Andrea Shalal-Esa, "China jamming test sparks U.S. satellite concerns," Reuters, 5 October 2006.
4. Elaine M. Grossman, "Top Commander: Chinese Interference with U.S. Satellites Uncertain," Inside the Pentagon, Vol. 22 No. 41, 12 October 2006, http://www.worldpoliticswatch.com/article.aspx?id=267, accessed 24 January 2007.
5. For an analysis of laser systems intended to disrupt imaging satellites, see The Physics of Space Security, Section 11.
