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Satellites Peer Into India's Shrinking Aquifers

Gravity maps show disastrous draining in India's breadbasket

3 min read
Satellites Peer Into India's Shrinking Aquifers

12 August 2009—The Indian state of Punjab is considered to be the country’s breadbasket, producing about 1 percent of the world’s rice and 2 percent of its wheat, according to the Indian government. But Punjab might not retain its rank for long. According to data published this week in the journal Nature, the northwestern states of Haryana, Punjab, and Rajasthan lost about 109 cubic kilometers of groundwater between 2002 and 2008, or about three times the capacity of Lake Mead, the largest reservoir in the United States.

Scientists at NASA’s Goddard Space Flight Center and the University of California, Irvine, discovered the groundwater loss in new maps of the Earth’s gravity made using the twin satellites of the Gravity Recovery and Climate Experiment (GRACE) system. Unlike most Earth-observing satellites, GRACE doesn’t infer properties of the Earth’s surface from emitted or reflected radiation. Instead, the two satellites orbit in tandem, maintaining a distance between them of 220 kilometers. As the twin satellites fly, minor perturbations in the Earth’s gravity fields may cause the lead satellite to accelerate slightly before pulling the trailing satellite along and resuming the prescribed 220-km distance. A microwave ranging system on board GRACE tracks the minute changes in distance, and Global Positioning System receivers can pinpoint the location of the satellites over Earth with high precision. By constantly measuring the distance between the two satellites, the system can generate sufficient data to produce maps of the Earth’s gravity anomalies each month. Any changes in the mass of the land beneath the satellites over time are reflected in subsequent flights over the region.

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Top Tech 2023: A Special Report

These two dozen technical projects should make significant advances in the coming year

2 min read
Top Tech 2023: A Special Report
Edmon DeHaro

Each January, the editors of IEEE Spectrum offer up some predictions about technical developments we expect to be in the news over the coming year. You’ll find a couple dozen of those described in the following special report. Of course, the number of things we could have written about is far higher, so we had to be selective in picking which projects to feature. And we’re not ashamed to admit, gee-whiz appeal often shaped our choices.

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