Test Data Indicate Possible Ka-band Weakness for UAV Links
PARIS — Ongoing tests using military Ka-band satellite links to a ground terminal simulating an unmanned aerial vehicle (UAV) raise questions about how much bandwidth can be offloaded from UAVs flying below even mild cloud cover on otherwise sunny days, Astrium Services has concluded.
“This whole nirvana you hear about massive data rates — well, show us what tests you’ve done,” said Dylan Browne, Astrium Services vice president for sales and marketing. “What we have found so far is a tough reality: Some customers will have to be kicked off the satellite link because of changed priorities in the military command at a time when bandwidth is not available.”
Astrium Services is using 10 megahertz of civil Ka-band frequency converted to the neighboring military Ka-band frequency aboard the Hylas-1 telecommunications satellite. Hylas-1, which was launched in November and declared ready for service in March, was designed to test this kind of payload flexibility.
Astrium’s preliminary conclusions are that the pitch and roll of UAVs combined with even intermittent clouds or dust concentrations could pose challenges for the use of Ka-band in conflict areas.
In a presentation here to the MilSpace 2011 conference organized by SMi Group of London, Browne said the company is well aware that Ku-band has become too popular over the Middle East, South Asia and North Africa to permit much growth in UAV data links using that frequency.
With Ku-band satellites now spaced 2 degrees apart in geostationary orbit, rather than the previous standard of 4 degrees, a 1-meter, 16-watt antenna on a UAV transmitting at 10 megabits per second or more is likely to interfere with other satellites in the region. It is one of several reasons for the migration to Ka-band.
The U.S. Department of Defense and other military organizations are planning the gradual rollout of Ka-band capabilities in the coming years, notably the Warfighter Information Network-Tactical (WIN-T) terminals using the Wideband Global Satcom system’s Ka-band links.
Startup satellite operator Yahsat of the United Arab Emirates has a sizable amount of Ka-band satellite capacity being readied for government use, and London-basedis building three large Ka-band satellites for near-global coverage for military and corporate customers.
What Astrium Services has found is that “all of these [satellite operators] are bending metal without having talked to the ISR customers,” said Browne, referring to the military acronym for intelligence, surveillance and reconnaissance.
For its tests using the Hylas-1 satellite, which is located at 33.5 degrees east longitude, Astrium — which owns Paradigm Secure Communications, the operator of Britain’s Skynet telecommunications satellites — received authorization from the British government, Browne said. The company has also filed initial reservations with global regulators for military Ka-band satellites at various geostationary orbit positions, he said.
The problem is how to use adaptive coding and other techniques to squeeze as much capacity from the UAV-to-satellite link as possible given the enormous increase in bandwidth requirements aboard these vehicles for streaming video. A Global Hawk UAV, Browne said, will consume as much as 294 megabits per second of bandwidth, compared with today’s average of less than 10 megabits.
Astrium has hosted British and NATO delegations to view the test site, where a 1.2-meter deployable antenna is stationed and used to simulate a UAV.
A cloud passing between the antenna and the Hylas-1, even on an otherwise sunny day, can reduce bandwidth by 50 percent or more. The same is true on days of high dust concentrations when the antenna is simulating maneuvers that result in giving the satellite a low elevation angle during transmission.
“In that case, if you are running three full-motion video streams, which one do you drop?” Browne asked. “We all love the upside to Ka-band — all that free extra spectrum — but the problem is, how do you profit from it? How do you prioritize your transmissions? And what data rate do you give to your soldiers on the ground? These issues have not been discussed, but they are big issues for us.”
Astrium Services plans to continue the testing in various weather conditions in the coming weeks. The company has negotiated a pre-emptible lease with Hylas-1’s owner, Avanti Communications of London, meaning the capacity will remain available until Avanti sells it to a long-term commercial customer.
In the coming weeks Astrium will conduct tests to simulate Ka-band links to mobile ground vehicles and aboard aircraft, Browne said.