The SMAP orbit is a 685-km altitude, near-polar, sun-synchronous 6am/6pm, 8-day exact repeat, frozen orbit.
- Near-polar provides global land coverage up to high latitudes including all freeze/thaw regions of interest
- Sun-synchronous provides observations of the surface at close to the same local solar time each orbit throughout the mission, enhancing the ability of the analysis algorithms to detect changes and improving overall science accuracy
- Consistent 6:00 a.m. observation time is optimal for science, minimizes effect of Faraday rotation, and minimizes impact on spacecraft design
- Frozen orbit provides minimal altitude variation during an orbit, benefitting radar design and accuracy
- 685-km altitude is an exact 8-day repeat orbit, advantageous for radar change detection algorithms
- Orbit provides optimum coverage of global land area at three-day average intervals, and coverage of land region above 45N at two-day average intervals
SMAP’s radar, ceasing operation in early July 2015, accurately measured the echoes of very short radio frequency (RF) pulses that bounced (“backscatter”) off the Earth’s surface. The amount of backscatter that was returned to the radar changed with the amount of moisture in the soil – wetter soil caused more backscatter to reach the radar. The radar measurement is also quite sensitive to whether the soil is frozen or thawed – which in turn was used by SMAP to determine when different regions of the Earth under spring thaw or fall freeze. The radar operated at microwave frequencies (L-band), where the RF pulses were not much affected by weather or by a moderate amount of vegetation that might cover the soil. The radar used a special technique known as “synthetic aperture” to resolve the RF backscatter over much smaller surface areas that would otherwise be possible. This technique allowed SMAP to measure soil moisture with a 9 km (6 mile) area resolution and freeze-thaw state with a 3 km (2 mile) area resolution. After SMAP launched, scientists had gone through an extensive process to calibrate the radar measurement to actual soil moisture measurements at pre-selected sites around the world to assure the measurements were accurate under a wide variety of ground conditions.
- Frequency: 1.26 GHz
- Polarizations: VV, HH, HV (not fully polarimetric)
- Relative accuracy (3 km grid): 1 dB (HH and VV), 1.5 dB (HV)
- Data acquisition:
- High-resolution (SAR) data acquired over land
- Low-resolution data acquired globally
SMAP’s radiometer is a very sensitive receiver that accurately measures the naturally occurring radio frequency (RF) energy given off by the Earth’s surface. The radiometer operates like an infrared camera (or night vision goggles), where warmer objects appear proportionally “brighter” than colder objects and allow their temperature to be accurately measured without being in direct contact. The radiometer receives energy in a narrow microwave band. This frequency has been set aside by international agreement for applications which involve only a receiver (and no transmitting is allowed in that band). This allows SMAP’s radiometer (and, in other frequencies, ground-based radio telescopes) to operate without interference. This frequency band also allows the radiometer to be not much affected by weather or by a moderate amount of vegetation that may cover the soil. Within this frequency band (L-band), water appears relatively ‘cold’ (about 100K) and dry soil appears relatively ‘warm’ (about 300K) to the radiometer. With this great difference between wet and dry soils, the radiometer allows SMAP to produce very high soil moisture accuracy (4%) by simply measuring the microwave ‘temperature’ of the land surface. After SMAP launches, scientists have completed an extensive process to calibrate the radiometer measurement to actual soil moisture measurements at pre-selected sites around the world to assure the measurements are accurate under a wide variety of ground conditions.
- Frequency: 1.41 GHz
- Polarizations: H, V, 3rd & 4th Stokes
- Relative accuracy (36 km grid): 1.3 K
- Data collection:
- High-rate (16 sub-band) data acquired over land
Integration time for high-rate, full-band data with averaging is typically ~40 ms
- Low-rate data acquired globally
What it is and why it's needed: SMAP’s most prominent feature is its large spinning instrument antenna. The antenna is the ‘eye’ of the instrument. Its large size – 6m (20 ft) – and gold-plated wire mesh (or screen) surface focuses the radio frequency (RF) energy collected by SMAP’s radar and radiometer. The area measured is a ‘spot’ on the earth’s surface only 36 km (25 miles) diameter. This is the smallest area for which the instrument directly measures soil moisture. However, using a radar process called “aperture synthesis” that area was reduced to 1 km (or 0.5 mi). The antenna works like a flashlight where the feedhorn, located at the base of the antenna boom, illuminates the large reflector that in turn produces a narrow beam that illuminates the Earth. The beam is tilted at a 40° angle so that the ‘spot’ on the Earth is shifted about 500 km (310 miles) from directly under the observatory. When the antenna spins, the spot moves in a circle 1000 km (620 miles) in diameter around the observatory – this forms the very wide measurement swath of the instrument that is key to enabling SMAP to measure the entire Earth every 2-3 days (depending on latitude). The 14.6 rpm spin rate is set to insure that the radar and radiometer measurement timing produce continuous measurements around the circle.
What is unique about it: Large antennas like SMAP’s pose special challenges for space missions because they must: (1) fold up (“stow”) like an umbrella to fit within the limited space inside the launch vehicle fairing; (2) have low weight for launch; and (3) provide a precision surface to focus the RF energy to the desired spot on the Earth. SMAP’s antenna is a new, advanced design that is very low weight and has an ultra-compact stow volume (the 20 ft reflector is stows to 1 ft diameter inside the fairing). The reflector and boom weigh only 56 and 55 pounds, respectively. Antennas like SMAP’s have been used for communication satellites (typically with larger sizes). SMAP, however, is the first use of this kind of deployable antenna for a science measurement, and it is also the first use in a spinning application. The science measurement and spinning usage posed additional development challenges for SMAP in addition to the challenges inherent in developing a large deployable structure for space application.
How will we make sure it will work as designed? Confidently deploying large, lightweight structures in space is one of the larger engineering challenges that NASA missions can confront in development. For these structures, it was impossible to fully test them on the ground in exactly the same way and in exactly the same environments as they would be deployed and operated in space. Special “gravity compensation” equipment was developed for ground testing that often posed more difficulties than the space environment, because it would add more equipment and hardware that most go along with the deployment of the actual flight structure (and it never actually completely represents the microgravity of space). Gravity offloading limited how much testing could be done over temperature and in vacuum. Because of all these limitations, SMAP’s antenna deployment functions were verified by a combination of rigorous component-level tests over space-like environments, and by sophisticated analyses to show how the structure would behave under both the expected (“nominal”) and potentially unexpected conditions that might be encountered in space. The combination of test and analysis, combined with the large performance margins built into the design to overcome unexpected conditions, ensured that SMAP’s antenna would deploy successfully and perform as needed in its application, even though we were not able to conduct a complete deployment and spinning test of the full system on the ground.
- Conically-scanning deployable mesh reflector shared by radar and radiometer
- Diameter: 6 meters
- Rotation rate: 14.6 RPM
- Beam efficiency: ~90%
- Swath width: 1000 km
- Spatial Resolution:
- Radiometer (3 db IFOV): 38 km x 49 km
- SAR: 1-3 km (over outer 70% of swath)
The spacecraft bus for SMAP was designed to do the jobs necessary to operate the rotating instrument for its mapping mission in Earth orbit. Within its box-like enclosure, various electronic assemblies and equipment are housed. These include the power and computer systems, the electronics for SMAP’s radar, radio equipment for communicating with ground stations, a fuel tank and small rocket engines (called thrusters) for propulsion, and devices called “reaction wheels” used to control the spacecraft’s orientation. The solar array produces about 1,500 Watts of electric power (about the same amount of power as used by a typical hair dryer). This power operates the spacecraft’s systems and powers the electric motors and electronics of the spinning instrument section mounted on the spacecraft’s top surface.
Before it reached space and deploys the solar arrays and antennas, the Observatory was stowed in a much more compact form, so it could fit inside the protective fairing provided by the launch vehicle for its ride into orbit. At launch, the Observatory weighed a little over one ton (2,086 pounds), of which about 179 pounds was propellant stored in the spacecraft’s propellant tank. During its ride into space on a Delta II rocket, the Observatory must withstand large “g” forces (about 7 g’s at their peak) and violent shaking – levels similar those withstood by high-performance military aircraft!
Once it is in space, the spacecraft is able to determine how it is oriented using specialized sensors, some of which are able to sense the location of the Sun, and others that are able to “see” star patterns, which are compared to a sky catalog stored in its computer. Using these sensors and its computer system, the spacecraft “knows” how it is oriented to better than 1/10 of one degree. This is the angle swept by the hour hand of a clock in 10 seconds!
The spacecraft also has an “autopilot,” not unlike those used in airplanes, that uses its orientation knowledge and reaction wheels to steer itself, keeping the instrument’s spin axis pointed towards the Earth and its solar array pointed towards the Sun. During the mapping mission, the spacecraft will fire its small thrusters approximately once every two to three months to carefully control its orbit, so that the instrument views the Earth’s surface in a repeatable manner. This allows SMAP’s science team to compare soil moisture data obtained from the Earth’s various land areas accurately over multiple orbits.
As it flies over the Earth’s surface during mapping, the Instrument makes near-continuous measurements, generating large amounts of data that are then stored by the spacecraft’s computer memory. The data are transmitted to waiting NASA ground stations two or three times per each orbit. This process is in some ways similar to how a high-speed Internet connection operates. Once received on the ground, the SMAP instrument data are moved over actual internet links to SMAP’s science data processing system at JPL, where the data undergo a great deal of further calibration and processing to create the soil moisture and freeze/thaw measurements. Over its planned three-year prime mission phase from 2015-2018, SMAP returned about 135 Terabytes of information from orbit – for comparison, that’s enough data to fill the memories of 270 laptop computers with 500 Gigabytes of storage capacity each.