How does radar work?
Lots of people are familiar with the radar guns used by highway patrols and baseball pitching coaches. In these handheld devices, pulses of radio energy are generated and sent out from the radar gun. If something is in the field of view of the radar gun, it reflects back the pulses. Electronics in the radar gun measure the frequency of arrival of reflected pulses and can determine the speed of the target towards or away from the radar gun. The electronics ignore pulses whose frequency of arrival are not changing (in other words, the target is not moving).
SMAP’s radar works in much the same way. In SMAP’s studies, the pulses are not used to look for motion, though. Instead, SMAP measures the quality and quantity of the reflections coming back from Earth’s surface. These data can then be interpreted for the amount of water that is in the top 10 centimeters (4 inches) of soil over an area that measures 3 km x 3 km. The radar measurements can tell scientists if the soil is desert-dry, swamp-wet, or pasture, savannah, or forest.
Pulses of radio energy are generated in the radar’s transmitter electronics, pass up through the feedhorn and are bounced off the antenna to a small patch of land on Earth’s surface. The reflection comes back the same way, reflecting off the antenna into the feedhorn and back into receiver electronics for analysis and recording.
SMAP’s antenna spins while it transmits and receives radar signals (microwaves at a frequency of 1.2 GHz) from a small patch of Earth’s surface. The spinning antenna combined with SMAP’s orbital motion sweeps out a series of intersecting loops that create a swatch 1000 km (621 miles) wide. The detector measures the amount of microwave energy it is receiving and that value is recorded approximately 3000 times per second. These data are the sent down to Earth where maps of radar return strength are constructed for interpretation by scientists.
How does a radiometer work?
SMAP’s radiometer works in much the same way that a camera’s light meter works. Where a light meter has a sensor that tells the camera how much light is passing through the lens, SMAP’s radiometer measures and records the changing amount of radio frequency microwaves that are emitted by Earth’s surface.
As SMAP’s antenna spins, it is reflecting and focusing microwaves from a small patch (40 km x 40 km grid) of Earth’s surface into the radiometer’s detector. The detector measures the amount of microwave energy it is receiving and that value is recorded every approximately 3000 times per second. These data are then sent down to Earth where maps of microwave emissions are constructed for interpretation by scientists.
Why does SMAP’s antenna spin?
SMAP’s antenna spins in order to cover a wide swath of Earth’s surface as it passes over our planet during each orbit. If the antenna didn’t spin, SMAP would measure soil moisture over a very narrow strip of Earth. Radar measurements Saturn’s moon Titan by NASA’s Cassini spacecraft are called “noodles” because of the long, narrow maps they make.
By spinning the antenna at the correct speed (14.6 revolutions per minute), the antenna sweeps out overlapping loops 1000 km (621 miles) in diameter. The overlapping loops are combined to make wide ribbons which overlap each other and map all of Earth every two days near the North and South Poles and every 3 days near the equator.
How did SMAP’s antenna fit in the rocket?
SMAP’s antenna is a wire mesh that was designed to unfold in the micro-gravity environment of low Earth orbit. At launch it is folded into a small cylinder (about 1.8 meters (yards) long and 42 centimeters [16.5 inches] in diameter). A system of motors and springs works together to unfurl the antenna, draw it taught into the desired paraboloidal shape, and then snap the edges in place so it retains its shape. When it is unfurled, the antenna has a diameter of 6 meters (19.8 feet).
The antenna is not strong enough to support itself in Earth’s gravity so testing the mechanism required a special fixture to support the antenna as it unfurled for testing.
Why don’t radio waves go straight through SMAP’s antenna?
The answer to this question requires a look at how electromagnetic waves and conductors work first.
The electric field in electromagnetic waves can “induce” electrons in conductors (metals) to move back and forth, changing the speed of the electrons with the continuously changing shape of the wave. The shape of the wave indicates the strength (voltage) and direction of the electric field. The peak-to-peak (or valley-to-valley) distance in a wave is called the wavelength. The wavelength of light is less than one millionth of a meter (you’ve seen one thousandth of a meter on a ruler that shows millimeters). The wavelength of radio waves extends from millimeters to thousands of meters and longer.
The antenna on your car receives radio signals from a broadcast station which move electrons in the strip of metal. (Then downstream electronics convert the motions of the electrons to sound.) The reverse is done with electronics at the radio station, converting the sound to voltages that move electrons in the tall radio station antenna(s) to create a radio signal that radiates from the antenna(s). The receiving effect occurs across the spectrum whenever a conductor is illuminated with electromagnetic waves from visible light to radio. (Ultraviolet light, x-rays, and gamma rays carry so much energy that they knock loose electrons or pass right through the metal.)
A careful look at a mesh antenna like SMAP’s shows that it is made of metal. A close look would show your distorted reflection of each section of mesh and of course light from background objects coming through the gaps in the mesh. Light waves passing a conductor have less influence on a conductor than radio waves. So to reflect light, we need to make a continuous mirror. Radio waves have a larger area of influence so they can be reflected from mesh that is not too open (though longer wavelength radio waves could be reflected from it).
The electrons moving in the metal reflector generate an outgoing electromagnetic wave that matches the wavelength of the incoming electromagnetic wave. The generated wave can add to the incoming wave such that their sum is zero. Instead, the generated wave goes back out the way the incoming wave came in: this is reflection.
SMAP’s antenna was optimized for work with L band radio waves that its radar and radiometer use. Light goes through the antenna but these radio waves are reflected by the antenna.
What is the electromagnetic spectrum? Where are SMAP’s instruments on it?
The electromagnetic spectrum refers to the range of waves that our bodies and our instruments detect as gamma rays, x-rays, ultraviolet waves, light (in colors spanning the rainbow), infrared waves (heat), and radio waves. They are produced by a variety of processes in atoms and their nuclei, molecules, and in wires and cables.
“Electromagnetic” refers to the combined electric and magnetic waves that travel together. Depending on their frequency (how fast the paired waves oscillate), they are described as being in one of the categories mentioned above.
Sound waves, seismic waves, and ocean swells are all examples of waves that are not electromagnetic.
SMAP’s radar and radiometer operate in the radio section of the electromagnetic spectrum. Engineers and scientists have given names to various frequency ranges across the radio spectrum. SMAP’s instruments operate in the microwave range (like your microwave oven) in the L band. The radar operates at a frequency of 1.26 GHz (1.26 billion cycles per second) and the radiometer operates at 1.41 GHz.
How can I see SMAP when it is in orbit?
SMAP will be visible, like most Earth-orbiting spacecraft, during dusk or dawn from your location. It will look like a slowly moving star crossing the sky approximately from north to south or from south to north. Observing at the correct time will be essential since it will only be visible for a few minutes. Due to the requirements for its observations of our planet, the orbit it will be using may only allow it to be visible once or twice a day every fourth or every eighth day.
SMAP is likely to be difficult to see from the ground and may require binoculars or even a carefully pointed telescope. While its antenna is large, the antenna has so many gaps in the mesh it is made of that there isn’t much material to reflect sunlight to the ground. (It behaves differently with radio waves.) The body of the spacecraft is relatively small so it won’t be especially bright either.
Once SMAP is on orbit more detailed instructions will be provided so you can determine when to look for it and where in the sky to look.
What is soil?
Soil is much more than crushed rock. Think about the difference between beach sand and the soil in a garden. Soil has particles of rocks and minerals included with decomposing biological material and living things: plant roots, worms and a variety of bugs, bacteria, and sometimes larger creatures. Water or air fill pores between soil particles.
The amounts of the various components of soil affect what can grow there and how the soil might be used for human purposes, such as building structures.
What is soil moisture?
Soil moisture is a measure of the quantity of water in a sample of soil. Volumetric water content is measured based on the volume of the soil sample and the air and the water in the sample compared to the volume of water (alone) in the sample. Gravimetric water content is measured by weighing (“massing”) the weight of the pristine sample and then drying it and measuring the weight of the dried sample. Both measures of water content are unit-less ratios. Most of the time both values are less than 1.0, though the numbers themselves differ from each other due to the difference in measuring technique.
How is soil moisture measured on the ground?
Researcher collect soil samples in special containers that are sealed immediately after the sample is collected so no moisture evaporates from the sample. Back in the laboratory, the container+sample is weighed on a precision laboratory balance.
The open sample is next placed in a low temperature oven. The oven gently heats the sample for as long as three days. When the sample is completely dry throughout, it is weighed again.
The difference in weights is the amount of moisture in the soil, which was lost to evaporation in the oven.
Why does data release take so long?
SMAP data have to be processed and converted from “engineering” values to the scientific units used for analysis. The science team and project will be very careful to post data that are correct, and this collection and cross-checking takes time.
Some data, such as maps of soil moisture, can be processed and posted much more rapidly. These will be available on the SMAP website.
Can I measure soil moisture in my yard?
Yes! Measurements can be made anytime. Tracking seasonal changes or simply before/after a storm will be interesting as well. Local areas might have different types of soil that hold moisture differently and can be measured to demonstrate the differences.
The basic idea is to (1) record the date, time, and place you (2) collect a sample and then (3) measure its weight immediately (before its moisture starts to evaporate). Then (4) dry it over the course of a few days and (5) weigh it again. Finally, (6) record your results.
Protocols for making these measurements in your kitchen laboratory are still being developed. Having a high quality scale will be necessary. The sample can be collected with a small shovel that places the sample in a zip lock bag. Drying the sample can be done under heat lamps.
More specific instructions and details will be published as soon as they are available.
How do microwaves measure soil moisture?
Over many years, scientific research has been done to understand the relationship between the “appearance” of the Earth’s surface at 1.0-2.0 GHz microwave frequencies and the amount of moisture present in the surface soil being observed. This research has led to the development of mathematical formulas and algorithms that can accurately relate soil moisture content to measurements of microwave brightness and reflectivity – these are the types of measurements made by SMAP’s radar and radiometer instruments. Simply put, soil with a higher moisture content appears “brighter” when viewed at microwave frequencies, and is more “reflective” at those frequencies as well. To get accurate results with this type of microwave measurement technique and mathematical formulas, other properties of the soil are needed, such as soil type (e.g., sand, loam, clay), and surface temperature.
SMAP’s radar and radiometer observe the Earth’s surface from space at about 1.2 GHz, a frequency regime found to provide the best results with the remote sensing approach outlined above. The radar provides “active” illumination of the Earth’s surface and measures the strength of radar pulses reflecting off of the Earth back into space. The radiometer passively measures the intensity of natural microwave emissions from the surface. These two measurements are used together in a synergistic way to calculate soil moisture content with greater accuracy than either one could achieve on its own.
How do microwaves measure temperature?
Every material object in the universe whose temperature is above absolute zero (that includes literally everything) emits electromagnetic (EM) radiation. Electromagnetic radiation includes waves that our bodies and our instruments detect as gamma rays, x-rays, ultraviolet waves, light (in colors spanning the rainbow), infrared waves (heat), and radio waves.
The amounts of EM radiation of these different types coming from a warm object have different amounts depending on the temperature of the object. An ice cube glows invisibly in the infrared and radio frequencies while a light bulb with a tungsten filament puts out some light and even more heat. Knowing the temperature of an object, it is possible to predict how much of each type of EM radiation is coming from the object. The reverse works too: measure how much EM radiation is coming from an object and its temperature can be determined.
SMAP’s radiometer measures three frequencies (wavelengths) of microwaves that are particularly strong around temperatures of 0°C (32°F, the freezing point of water). By looking at the ratios of the amounts of energy in those wavelengths, the temperature of the surface can be measured.
Why measure soil moisture?
Measuring soil moisture gives scientists, government planners, and politicians an idea of the status of land not just locally but also regionally, nationally, and internationally. These data help
- Monitor Drought
- Predict Floods and Landslides
- Support Farm Production
- Weather Forecasting
Why is NASA measuring soil moisture?
SMAP is a satellite designed to do fundamental research about the moisture in soil on a worldwide basis and over years of measurement, encompassing all four seasons. But even data obtained for research reasons has value to practical users of such data when they are available (see the FAQ Why measure soil moisture?).
SMAP’s science team includes researchers and practical users. They will all be working to understand processes in the soil, its interaction with nearby and distant bodies of water, and Earth’s atmosphere. They look for previously unrecognized interactions and for new or better uses of these data as applied to their areas of expertise.
How can I see SMAP data?
SMAP data will be publicly available after being deposited in archives intended for this purpose. Complete data may take several months for posting. The data have to be processed and converted from “engineering” values to the scientific units used for analysis. The science team and project will be very careful to post data that are correct, and this collection and cross-checking takes time.
Some data, such as maps of soil moisture, can be processed and posted much more rapidly. These will be available on the SMAP website.
What’s a carbon cycle?
The carbon cycle is the complex set of interactions involving several different carbon-based gases (among them the so-called “greenhouse gases”) that take place between the atmosphere, land, and oceans of the Earth. There are many “sources” of carbon generation; some of the most significant of these are associated with human activity such as fossil fuel consumption for energy production and transportation. The Earth’s environment also includes several carbon “sinks,” including bodies of water that chemically absorb certain gases, and vegetation that consumes carbon dioxide from the atmosphere via photosynthesis. Other factors, such as the size and distribution of the polar ice sheets and glaciers, play a significant role in the carbon cycle as well. Scientific research has shown that the atmosphere’s properties for retaining heat, or reflecting some of the energy from the Earth’s surface back towards the surface, are dependent upon the type and amount of carbon-based gases present, as well as their distribution through the atmosphere.
Many different carbon-based gases are released into the atmosphere through plant respiration and decay, the interaction of the oceans and atmosphere, as well as human activity, including agriculture and the burning of fossil fuels as an energy source. The concentration and distribution of carbon in the Earth’s atmosphere plays a significant role in weather and climate behavior.
What’s a water cycle?
The water cycle refers to the movement of water between different parts of the Earth’s environment, including the atmosphere, natural reservoirs such as oceans and lakes, and the soil and rock formations making up the Earth’s land surfaces. In its different forms (water vapor, liquid water, and ice), water moves through the Earth’s environment cyclically, subject the complex interactions between land areas, oceans, and the atmosphere. This process is illustrated starts with liquid water from different sources – bodies of water, moist soil, and vegetation among others – is transported into the Earth’s atmosphere via evaporation and transpiration, where it is suspended in gaseous form and contributes to cloud formation. Areas within the atmosphere that become saturated with water vapor lead to precipitation, returning water to the Earth’s surface again in liquid form. Surface water, in turn, moves dynamically across, into, and beneath the land surface, re-accumulating in bodies of water, as well as underground reservoirs (e.g., groundwater).
What’s an energy cycle?
The energy cycle describes the interactions between energy sources within the Earth’s environment. These interactions are very complex, and even small changes in them can lead to significant changes in long-term climate behavior. A simple illustration of the major elements of the energy cycle is shown in Fig. xx below. Both the Sun and the Earth’s interior are sources of energy. That energy is distributed (and some of it is lost) within Earth’s environmental sphere in ways that contribute to both short-team weather and long-term climate behavior.
Why doesn’t SMAP use gravity gradient orientation instead of 3-axis stabilization?
While in orbit, the SMAP spacecraft is controlled by an onboard computer, not unlike the autopilot systems found in many aircraft. In order for the instruments to work properly, the pointing of the spacecraft’s spin axis needs to be controlled accurately (to within 0.5 deg) in all three dimensions (or axes) in a continuous manner. Gravity gradient orientation, which relies on tiny differences in the Earth’s gravitational pull acting on different parts of the spacecraft for stabilization, typically provides only about 10-30 deg of orientation accuracy, and so is not suitable for SMAP.
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