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Terrestrial radio transmitters interfering with satellite

Posted: October 7, 2010

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Radio signals on Earth are proving troublesome for scientists attempting to use data from a European Space Agency satellite to measure soil moisture and ocean salinity.

Artist's concept of SMOS in orbit. Credit: ESA
ESA officials say errant signals from television, radio and security networks have ruined early data collections by the Soil Moisture and Ocean Salinity satellite, which launched in November 2009 and began science operations in May.

Orbiting 500 miles above Earth, SMOS uses an array of small antennas to measure faint radio signals containing clues on the properties of land and sea surfaces.

Arranged on three deployable arm-like structures, the satellite's 69 L-band antennas are stringed together in an interferometer array to maximize the sensor's sensitivity.

The MIRAS instrument collects microwave radiation coming from Earth's surface, a measurement known as the brightness temperature. Scientists studying the data can extract details of moisture levels in soil and the salt content of oceans.

The $439 million SMOS satellite is probing Earth's water cycle in a three-year mission.

The MIRAS instrument does its observations at frequencies between 1400 megahertz and 1427 megahertz, a band reserved for satellite research and radio astronomy by the International Telecommunications Union.

But the first global maps from SMOS show clusters of interfering signals scattered across southern Europe, the Middle East, India and China. The most offensive transmissions were coming from locations in or near cities like Madrid, Athens and Beijing.

According to ESA, the signals were either illegal transmissions within the SMOS frequency band or amplified interference from other bands leaking into the protected range.

SMOS data showing areas of interference identified by bright colors. Credit: CESBIO
"To ensure that the excellence of the mission would not be compromised, ESA embarked upon the tricky and lengthy process of having the illegal transmissions shut down and the excessive out-of-band emissions reduced," the space agency said in a statement.

Authorities are using the satellite's own data to trace the source of the signals. SMOS is designed to pinpoint moisture and salinity levels for its research mission, but officials are calling upon that precision to identify the locations of interference.

Officials are already seeing improvements as regulators retune over-powered transmitters and shut down those operating illegally.

Working with national spectrum management authorities, ESA first focused on improving the frequency interference in Europe. Spain, one of the early offenders, has reduced its unwanted transmissions and data there is already improved, according to ESA.

The SMOS mission is designed to shed light on Earth's water cycle. Soil moisture is linked to atmospheric humidity and the formation of precipitation, and scientists hope SMOS results will be considered by numerical weather prediction models to help make more accurate forecasts.

The salt content of the sea is a factor in ocean circulation, a network of conveyor belts that transport warm water to high latitudes and cool water to low latitudes, moderating the planet's climate.