Perlman has claimed that pCell technolâ ogy could eventually increase the capacity of today's cellular networks as much as a thousandfold. But experts say that ...
Satellites and Simulations Track Missing Methane Europe is spending €45 million to correctly gauge emissions of the greenhouse gas Methane emissions from oil and gas extraction, herding livestock, and other human activities in the United States are likely 25 to 75 percent higher than the U.S. Environmental Protection Agency currently recognizes, according to a meta-analysis of methane emissions research published recently in Science. While experts in remote sensing debate the merits of this and other recent challenges to the EPA’s numbers, definitive answers are already on order via a high-precision Earth observation satellite to be launched next year. The intensifying methane emissions debate has profound implications for cli‑ mate and energy policy. Natural gas consumption is rising, and methane’s global warming impact is more than 30 times as much as that of carbon dioxide, molecule for molecule, and second only to carbon dioxide’s in today’s net climate impact. Greenhouse gas inventories such as the EPA’s quantify emissions from the bottom up. They assume a leakage rate for each known source type and add up the sources. The EPA’s figures are being challenged by studies employing topdown methods, which use “inverse modeling” to transform point measurements of atmospheric methane into emissions estimates. Models of airflow are used to predict where measured methane is coming from and how fast it is flowing. Conclusions from the meta-analysis research, led by Stanford University energy resources expert Adam Brandt, track closely the results of the most recent and most comprehensive measurement-based estimate of nationwide methane emissions. The latter, published in November in the Proceedings of the National Academy of Sciences by a multi-institution Do you smell something? team of U.S.-based scientists, relies on more than A satellite might determine 12 000 air samples collected from towers across whether cattle or oil and gas wells the United States and flights by research planes are the bigger climate culprits.
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Perlman has claimed that pCell technol‑ ogy could eventually increase the capacity of today’s cellular networks as much as a thousandfold. But experts say that any per‑ formance gains will be limited by the pro‑ cessing capability at the data center and the speed and reliability of links to the ac‑ cess points. “You should be able to collect channel information, compute the signals, and send them back to the transmitters in less then 10 milliseconds,” USC’s Caire says. “According to our simulations, we see gains of about a factor of 10 for realistic scenarios. Much more than that, I’m skeptical.” At Columbia University in New York City, Perlman demonstrated pCell technology communicating with 4G LTE phones and other devices. Compatibility with LTE would allow users to roam seamlessly between the two networks without having to buy new handsets. Artemis’s engineers achieved this feat by simulating LTE base stations in soft‑ ware, using these virtual radios to inform the waveform calculations. However, MU‑MIMO experts argue that such compatibility will be much more difficult to maintain in realworld environments. Artemis is manufacturing pCell access points with small-cell provider PureWave Networks and is planning for large-scale tri‑ als in San Francisco. The company expects commercial rollouts by the end of 2014. Wireless experts say that Artemis has yet to prove it can overcome several difficult obsta‑ cles, including the large-scale coordination of transmissions from many access points and the integration of pCell clusters into existing cellular networks. “These are rigorous engi‑ neering challenges,” says Zhouyue “Jerry” Pi, a senior director of Samsung Research America in Dallas. “It’s not easy to make this kind of distributed MIMO work and create benefits.” Still, some experts concede that technolo‑ gies like pCell would make sense in congested hot spots such as airports, sports stadiums, and city centers—places where operators are already investing in dense clusters of small cells and where users don’t move around much. “But is it really going to revolutionize the data capacity of the whole world?” Pi asks. “I doubt that.” —a r iel bleicher
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Higher leakage rates would make natural gas more carbon intensive than coal
Not all experts are buying the latest figures, however. Harvard University environmental engi‑ neer Daniel Jacob says the latest inverse modeling study did not include enough data to reliably model emissions nationwide. And Jacob says they conflict with findings from top-down estimates that he and his colleagues assem‑ bled from two months of satellitebased measurements from 2004 before the premature failure of a methane detector on Europe’s Envisat satellite. (The satellite it‑ self operated from 2002 to 2012, five years longer than it was en‑ gineered for.) Their study affirms the EPA’s national estimate and attributes larger-than-expected emissions in the south-central U.S. primarily to livestock. Where Michalak and Jacob do concur is on what they call an ap‑ proaching revolution in top-down predictions due to satellite obser‑ vation. For methane, the revolu‑ tion kicks off with the €45 million (US $62 million) Tropospheric Ozone Monitoring Instrument (Tropomi), which the European Space Agency plans to launch next year on board the S entinel-5 Pre‑ cursor satellite. Like its predeces‑ sor on Envisat, Tropomi measures sunlight reflected off Earth and de‑ tects absorption of telltale wave‑ lengths to measure atmospheric levels of methane and other gases. But Tropomi is at least 10 times as sensitive as the Envisat instru‑ ment, according to Johan de Vries, Tropomi’s instrument scientist at Dutch Space, the satellite’s man‑ ufacturer. And its resolution is higher too, promising daily mea‑ sures for the entire globe at 7 by 7 kilometers. “It will mean that you can see sources of methane on the subcity level,” says de Vries. The data, predicts de Vries, will have political and commer‑ cial implications as top-down measurements shift the bal‑ ance of responsibility for meth‑ ane emissions from one state or economic sector to another. He anticipates another round of reassessment as topdown measurement of CO 2 improves. But that will take more work, because CO2’s ab‑ sorption signal is harder to iso‑ late than methane’s from the signals of other gases. “CO 2 measurements are at least five years behind,” says de Vries. —peter fa ir ley
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5 Earth-Imaging Start-ups Coming to a Sky Near You A tech boom in cheap, tiny, yet surprisingly able satellites Words like “provocative” and “disruptive” are rarely applied to the staid world of satellite imaging. But that’s exactly the kind of talk the industry is generating, with the launches of more than two dozen Earth-imaging sat‑ ellites in the last few months and more planned by year’s end. Those spacecraft, far smaller and cheaper than tradi‑ tional satellites, are being lofted not by the usual aerospace players but by venture-capital-funded start-ups. In many cases they’ll orbit in large constellations capable of revisiting sites several times a day. Though their per-pixel resolution won’t compete with that of satellites from market leader Digital Globe, they’ll provide fresher data at a lower price. The press has made much of start-up Skybox Imaging’s projected ability to count all the cars in every Walmart parking lot in the United States on Black Friday. Other uses
Clockwise from top left: PlanetiQ; UrtheCast; NASA; Skybox Imaging; Dauria Aerospace
in 2007 and 2008. Anna Michalak, one of the study’s authors and an expert in remote sensing and inverse modeling who also hap‑ pens to be at Stanford, says the team’s national methane estimate is accurate to within 10 percent and shows that EPA’s inventories have some holes. “The total from our atmospheric measurements does not equal the sum of the parts counted in the inventories. We need to look at whether there are line items that are underestimated by the inventories, or missing entirely,” says Michalak. A regional estimate by the authors of the PNAS report for the heavily drilled south-central U.S. states, meanwhile, points to oil and gas operations as a likely underc ounted methane source. Accounting methods comparable to those used in the EPA’s inventories (which do not break down emissions by region) yield methane esti‑ mates of 3 million metric tons of carbon per year, only about a third of what Michalak and her colleagues found. The report’s authors conclude that the re‑ gion’s oil and gas fields leak five to seven times as much methane as the EPA inventories capture. Natural Resources Defense Council staff scientist Vignesh Gowrishankar says those higher leakage rates—at least 7.5 percent of gas production instead of the 1.5 percent assumed by the EPA— would make electricity generated from natural gas more carbon in‑ tensive than coal-fired power. The EPA says it is paying attention. In January, EPA administrator Gina McCarthy told a U.S. Senate p anel that the EPA was working with other agencies to “assess emis‑ sions data” for methane and “ad‑ dress data gaps.”
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