Friday, 11 January 2013

How NASA's Flying Lab Stays Connected to Earth...??

Mars and Antarctica have a lot in common—they're both cold, inhospitable places with terrible broadband service. The crew of Operation IceBridge, NASA's airborne survey of glaciers and ice shelves in the Arctic and Antarctic works with networking constraints similar to those of the Curiosity rover, keeping in contact with its ground crew at worse-than-dialup speeds using the lowest-bandwidth method possible: Internet Relay Chat.

In February of 2010, after seven years of operation, the final laser sensor on NASA's Ice, Cloud, and Land Elevation Satellite (ICEsat) failed. With its replacement not slotted to be launched until 2015, NASA launched Operation Ice Bridge to conduct aerial surveys to fill in the gap. Flying 500 meters above the surface in a precisely-planned pattern over the Antarctic ice sheets, the OIB aircraft—operated by the National Suborbital Education and Research Center at the University of North Dakota—carries ice-penetrating radar, a gravimeter for measuring variations in the density of the ice below, and an Airborne Topographic Mapper—a laser altimeter that combines GPS data with laser measurements to build a precise record of the elevation of the ice sheets.

But because of the poor satellite coverage in the Antarctic, the refitted vintage Douglas DC-8 airliner can't use the Inmarsat BGAN service it normally uses for voice and data communication channels. "Like most high bandwidth satellite systems, the constellation is in geosynchronous orbit," David Van Gilst, NSERC's network engineer, told Ars in an e-mail interview. "So once you get past 72-73 degrees latitude the satellites are so low in the sky as to be problematic. Past 80 degrees latitude, they're below the horizon."

Bandwidth is a big enough problem for the land-based scientists in the Antarctic, who get about 10 hours a day of broadband (with some aggressive management of ground station dishes) from NASA's Tracking and Data Relay Satellite-F (TDRS-F) and the re-purposed GOES-3 weather satellite. But it's not practical for OIB's flying laboratory (operated by the National Suborbital Education and Research Center at the University of North Dakota) and other science on the move to try to lock in on TDRS or GOES-3; the only real option is to use the Iridium satellite network—the satcom equivalent of dial-up. Iridium's 66 low-orbit satellites zip around Earth in near-polar orbits (inclined at 86.4 degrees relative to the equator). Since they orbit at just 470 miles or so above the Earth—much closer than even the GPS satellite network, let alone geosynchronous communications satellites—they don't require directional antennas or the kind of broadcast power needed for most satellite communications. But the tradeoff is that the satellites quickly pass in and out of range, and connections have to be passed off from one to the other. While Iridium advertises data rates of up to 10 kilobits per second, the best that they usually can manage over a single connection is a quarter of that.

So Operation Ice Bridge squeezes all it can out of Iridium the old-fashioned way—by multiplexing over PPP Multilink. The DC-8 is equipped with an array of Iridium-based modems, each of which dials into a modem on a land line at NASA's Ames Research Center in San Jose. By aggregating the connections, the OIB flying lab gets about 9600 bits per second of bandwidth. "We've experimented with as many as 8 channels," said Van Gilst, "but with Iridium's lower level of reliability (modems will tend to hang up and have to redial periodically, particularly on a moving, banking aircraft) and the relatively high latency, PPP starts to see diminishing returns beyond 4 or 6 channels."

Over that BBS-worthy bandwidth, Operation Ice Bridge pushes and pulls three main types of data. One of them is IRC chat, which allowed for the crew on the DC-8 to coordinate with the ground crew in Punta Areas, Chile, and has also been used during the summer to coordinate with weather forecasters for thunderstorm-chasing over Kansas. During the latest Antarctic mission, IRC was also used to communicate with students in 49 school classrooms in the US and Chile. One IRC server is aboard the aircraft and another at the ground station, reducing the number of connections that need to be handled over the narrow IP network pipe. The DC-8 also sends back an ASCII-based telemetry stream over the Iridium connection, providing the aircraft's location as well as some meteorological sensor data. "This data is of interest to weather modeling groups," Van Gilst said, "as there is often not a lot of in-situ measurement data available. And simply knowing where the aircraft is via a Google Earth KML is extremely helpful, allowing ground crew to return to the airport to receive the aircraft when we're about to return." Another stream of data that comes over the multiplexed Iridium channels is a feed of weather and satellite data from the ground station. "The OIB missions generally do not need much in the way of real-time situational awareness data, as the flight plans are fixed at takeoff," said Van Gilst, "but the scientists have found it helpful to have access to model run data while we're returning to base in order to get a jump on planning the next day's flight." Other missions flown by NASA's Earth Science Division—such as the ARCTAS atmospheric monitoring mission in the Arctic in 2007—have changed their flight plans in mid-air based on updated satellite data; in the case of ARCTAS, the data would be used "to chase forest fires in Northern Canada, locating flare ups from GOES and MODIS satellite data," Van Glist said.
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