Before NASA launches a next-generation Earth-observing satellite dedicated to measuring how much moisture is bound up in the soil — a data point now largely missing in global change models — Goddard instrument designers will have to overcome one potentially tricky technological nuisance: radio interference from air-traffic control radars and even
The Soil Moisture Active Passive (SMAP) mission that NASA officially started earlier this year in response to the National Research Council’s first-ever Decadal Survey for Earth science will globally map soil moisture and the freeze/thaw state with unprecedented accuracy, resolution, and coverage when it launches in 2013.
The NASA-directed mission will carry a high-resolution synthetic aperture radar and a Goddard-developed radiometer that thermally images heat at long wavelengths.
Challenges to Overcome
However, building a radiometer and obtaining the type of global data scientists require are easier said than done.
To sense near-surface water, salinity, humidity, and other characteristics, instruments must penetrate vegetation and other barriers, which only can be accomplished using longer wavelengths either in the C- or L-band frequencies. Until recently, the L-band was off limits due to the prohibitively large antenna systems required to capture the long wavelengths. That left the C-band for these types of measurements — and in retrospect, not an ideal solution.
Overlap from active radio-service allocations has seriously affected remote-sensing products in the C-band. The radiometer on the Aqua spacecraft, for example, is plagued by interference, making it extremely difficult to use for soil-moisture remote sensing.
However, recent advances in antenna design now make L-band applications possible, which is why SMAP’s instruments will operate in the L-band (1.4 GHz) — a frequency that is devoted to “passive” remote-sensing applications. In other words, users at these low-frequency bands are not allowed to transmit data. They only may listen to the “static” from which they can derive the moisture data.
However, even that band might encounter interference from neighboring spectrum users, particularly from air-traffic control radars, said Jeff Piepmeier, the instrument scientist for Goddard’s radiometer.
“There is some anecdotal evidence that we will get interference,” he said. “What we want is quantitative data. What we want to see is whether there’s interference in cities, in rural areas, near airports.”
Quest for Quantitative Data
That’s why he is going on the offensive.
|Using Internal Research and Development funding, Piepmeier and his team are installing Goddard- and University of Michigan-developed receivers — hardware that will be used in the SMAP radiometer — on a Twin Otter aircraft to collect data over North America in October. Piepmeier’s team will then use the data to determine the level of interference and whether algorithms his team developed effectively mitigate the interference.
“With radiometers, all you do is listen,” Piepmeier said. “You listen to the static, which tells us about the Earth and the atmosphere. Radio signals are interference to us. In other words, one person’s noise is another person’s signal. What we don’t want is radar from airports or communications from an oil platform. What we really don’t want is to launch SMAP and find the data is unusable,” he said.
Goddard scientist Jeff Piepmeier will be building the radiometer for the Soil Moisture Active Passive mission, shown here in this artistís rendition. The mission is one of two that NASA began earlier this year.
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