SMAP Part 2
Table 5: List of SMAP soil moisture Core Validation Site candidates. Sites marked with asterisk (*) have been qualified for core validation status for at least one of the SMAP spatial scales (36 km, 9 km, and 3 km)
Figure 41: Location of all SMAP soil moisture core validation site candidates (image credit: NASA/JPL,Caltech and Partners)
The performance of the SMAP soil moisture products will be assessed over the core validation sites. Root mean square difference (RMSD), unbiased RMSD (ubRMSD), bias and correlation are computed for each site. The mission success is evaluated based on the average of the metrics. Figure 42 shows an example of the comparisons for assessing the SMAP soil moisture products over one of the core validation sites (Little Washita, OK, USA in this case). These reports are used to track the performance of the SMAP products and updated weekly with latency of the in situ data ranging from 1 day to 1 month, and upon product updates.
Figure 42: Example of rehearsal results over the Little River core validation site obtained with simulated SMAP L2SMP product using SMOS brightness temperature. The report over the site shows the time-series of the up-scaled in situ and SMAP retrieval (top), scatter plot and metrics of the comparison (bottom middle), and maps of the entire site and the pixel with stations used highlighted with red (bottom left and right, respectively), image credit: NASA/JPL,Caltech and Partners (Ref. 61)
• August 2015: The following references deal with the validation of the SMAP data at various core validation sites, of algorithm development, of radiometer calibration techniques, of initial RFI results, etc. - reported at IGARSS 2015. 62) 63) 64) 65) 66) 67) 68) 69) 70) 71) 72) 73) 74) 75) 76) 77) 78)
• July 28, 2015: The Canadian government has announced funding projects at five Canadian universities to analyze and compile measurements collected by NASA’s Soil and Moisture Active Passive (SMAP) satellite. 79)
- The SMAP mission will provide measurements of soil moisture and determine whether the ground is frozen or thawed in the Canadian boreal environment and other cold areas of the world. These measurements will help to produce global maps of soil moisture, helping scientists to better understand how changes in weather and climate affect the cycling of Earth’s water and carbon. This data could help improve weather forecasting including more accurate flood and drought predictions. With new insights into changing weather and water conditions, Canadian farmers will be able to better understand crop yields and get early warnings of soil conditions that could lead to crop-damaging pests.
• July 10, 2015: Mission managers at NASA/JPL, Pasadena, California, are assessing an anomaly with the radar instrument on NASA's SMAP (Soil Moisture Active Passive) satellite observatory. The radar is one of two science instruments on SMAP used to map global soil moisture and detect whether soils are frozen or thawed. 80)
- On July 7, at about 2:16 P.M. PDT, SMAP’s radar halted its transmissions. All other components of the spacecraft continued to operate normally, including the radiometer instrument that is collecting science data. - An anomaly team has been convened at JPL and is reviewing observatory and instrument telemetry and science data. Telemetry indicates no other issues with the spacecraft.
• May 19, 2015: NASA begins SMAP science mission operations. — During SMAP's first three months in orbit, referred to as SMAP's “commissioning phase”, the observatory was first exposed to the space environment, its solar array and reflector boom assembly containing SMAP's 6 m reflector antenna were deployed, and the antenna and instruments were spun up to their full speed, enabling global measurements every two to three days. The commissioning phase also was used to ensure that SMAP science data reliably flow from its instruments to science data processing facilities at NASA/JPL in Pasadena, California, and at NASA/GSFC (Goddard Space Flight Center) in Greenbelt, Maryland. 81)
- "Fourteen years after the concept for a NASA mission to map global soil moisture was first proposed, SMAP now has formally transitioned to routine science operations," said Kent Kellogg, the SMAP project manager at JPL. "SMAP's science team can now begin the important task of calibrating the observatory's science data products to ensure SMAP is meeting its requirements for measurement accuracy."
Figure 43: High-resolution global soil moisture map from SMAP's combined radar and radiometer instruments, acquired between May 4 and May 11, 2015, during SMAP's commissioning phase. The map has a resolution of 9 km (image credit: NASA/JPL, Caltech, GSFC)
- Together, SMAP's two instruments, which share a common antenna, produce the highest-resolution, most accurate soil moisture maps ever obtained from space. The spacecraft’s radar transmits microwave pulses to the ground and measures the strength of the signals that bounce back from Earth, whereas its radiometer measures microwaves that are naturally emitted from Earth’s surface.
- "SMAP data will eventually reveal how soil moisture conditions are changing over time in response to climate and how this impacts regional water availability,” said Dara Entekhabi, SMAP science team leader at MIT (Massachusetts Institute of Technology) in Cambridge, MA. “SMAP data will be combined with data from other missions like NASA's GPM (Global Precipitation Measurement), SAC-D/Aquarius and GRACE (Gravity Recovery and Climate Experiment) to reveal deeper insights into how the water cycle is evolving at global and regional scales."
- The first global view of SMAP's flagship product (Figure 43), a combined active-passive soil moisture map with a spatial resolution of 9 km, shows dry conditions in the Southwestern United States and in Australia's interior. Moist soil conditions are evident in the U.S. Midwest and in eastern regions of the United States, Europe and Asia. The far northern regions depicted in these SMAP maps do not indicate soil moisture measurements because the ground there was frozen.
- Over the next year, SMAP data will be calibrated and validated by comparing it against ground measurements of soil moisture and freeze/thaw state around the world at sites representing a broad spectrum of soil types, topography, vegetation and ground cover. SMAP data also will be compared with soil moisture data from existing aircraft-mounted instruments and other satellites.
- Preliminary calibrated data will be available in August 2015 at designated public-access data archives, including the National Snow and Ice Data Center in Boulder, Colorado, and Alaska Satellite Facility in Fairbanks. Preliminary soil moisture and freeze/thaw products will be available in November, with validated measurements scheduled to be available for use by the general science community in the summer of 2016 (Ref. 81).
• May 7, 2015: SMAP ground truth campaign. SMAPEx-4 (Soil Moisture Active Passive Experiments-4) field campaign in Australia. Around 40 scientists are studying the Australian soil from the ground and air — the first major soil moisture field campaign conducted since SMAP launched Jan. 31, 2015. The three-week study, conducted from May 2 to May 22, is designed to validate soil moisture measurements from SMAP. 82)
- The location of the Yanco region in Australia is a remote region, about 600 km west of Sydney. The aircraft, also carrying a radar and radiometer, provides microwave backscatter and brightness temperature observations at high resolution to help verify SMAP’s products. The aircraft flies for about six hours during a SMAP overpass and mimics SMAP’s readings in terms of wavelength, viewing angle and resolution ratio.
- On foot, scientists are measuring soil moisture directly. They use probes that stick into the ground and measure the amount of water in the top inches of the soil. These data are used to evaluate the calculated soil moisture measurements from aircraft and SMAP. The Yanco region has a diverse climate, soil, vegetation and land cover, which allows for more rigorous testing of the SMAP algorithm over a variety of surface types and conditions.
- Instrumentation and sampling strategy: 83)
The main instruments used to collect coincident SMAP observations are the Polarimetric L-band Multi-beam Radiometer (PLMR) and the Polarimetric L-band Imaging Synthetic aperture radar (PLIS). As shown in Figure 44(b), PLMR and PLIS will be configured in pushbroom mode on a scientific aircraft, achieving a swath width of 6 km at altitude of 1 km. PLMR is a dual-polarized (Vertical and Horizontal) radiometer operating at a center frequency of 1.413 GHz. Six beams of PLMR have viewing angles of ±7°, ±21.5° and ±38.5°, resulting in 1 km resolution and 90° across track field of view. PLIS is a quad-polarized (HH, HV, VH, and VV) radar operating at frequency of 1.245-1.275 GHz. Its two 2x2 patch array antennas incline at an angle of 30° from the horizontal on both sides of the aircraft, thus the antenna gain is within 2.5 dB of the maximum gain between 15° and 45°, resulting in ~7 m resolution and a ~2 km gap in the middle of 6 km swath.
In SMAPEx-4, eight replicated flights over the SMAP flight area are planned in coincidence with SMAP coverage, plus one flight for the Aquarius flight area on the day when both SMAP and Aquarius overpasses are coincident. Flight lines are designed with a spacing of 5 km so that the outer beams of PLMR will be overlapped with adjacent flight lines to ensure a full coverage of PLMR over the entire flight area for SMAP. Meanwhile all six focus farms will also be observed by PLIS (Figure 45). Being limited by maximum flight duration, over half of the flight area of Aquarius will be observed by PLMR to represent pixel soil moisture variability. Additionally, a RFI detector and multi-spectral sensors in visible, near infrared, short wave infrared, and thermal infrared bands will be used on the aircraft to provide supplementary information on RFI, vegetation water content, and land surface temperature.
Figure 44: (a) Airborne L-band radiometer (PLMR), L-band radar (PLIS), and multispectral sensors; (b) sensor configuration; and (c) airborne sampling strategy (image credit: SMAPEx-4 partners)
Figure 45: SMAPEx-4 flight lines and coverage of PLIS over focus farms (image credit: SMAPEx-4 partners)
• May 6, 2015: The maps (Figure 46) of global soil moisture were created using data from the radiometer instrument on NASA's SMAP (Soil Moisture Active Passive) observatory. Each image is a composite of three days of SMAP radiometer data, centered on April 15, 18 and 22, 2015. The images show the volumetric water content in the top 5 cm of soil. Wetter areas are blue and drier areas are yellow. White areas indicate snow, ice or frozen ground. Evident in the image sequence are regions of increased soil moisture and flooding caused by precipitation during this period in Southeast Australia, Bangladesh and Argentina. 84)
- These as yet uncalibrated soil moisture images will be evaluated and improved during the calibration and validation phase of SMAP's science mission, which begins on May 10, 2015.
Figure 46: The images from April show the volumetric water content in the top 5 cm of soil. Wetter areas are blue and drier areas are yellow. White areas indicate snow, ice or frozen ground (image credit: NASA/JPL)
• April 21, 2015: With its antenna now spinning at full speed, the SMAP observatory has successfully re-tested its science instruments and generated its first global maps, a key step to beginning routine science operations next month. 85)
Figure 47: SMAP radar image acquired from data from March 31 to April 3, 2015. Weaker radar signals (blues) reflect low soil moisture or lack of vegetation, such as in deserts. Strong radar signals (reds) are seen in forests. SMAP's radar also takes data over the ocean and sea ice (image credit: NASA/JPL,Caltech, GSFC)
- The radiometer data from the instrument test have been processed to map microwave emissions from Earth's surface, expressed as brightness temperatures in Kelvin and at a horizontal spatial resolution of about 40 km. The Amazon and Congo rainforests produced strong emissions, depicted in red shades, due to their large volumes of biomass. Brightness temperatures in the Sahara Desert reach about 300 Kelvin due to its low moisture content. The impact of soil moisture is evident over a large region south of the Great Lakes, where an increase in soil moisture due to precipitation in March resulted in relatively cool brightness temperatures of about 200 K. Similar impacts of rain on soil moistures and brightness temperatures are seen in Namibia and Botswana, Africa, where there was significant rainfall in late March.
- With its spin-up activities complete, the observatory's radar and radiometer instruments were powered on from March 31 to April 3 in a test designed to verify the pointing accuracy of the antenna and the overall performance of the radar and radiometer instruments. The radar data acquired from the test have been processed to generate instrument data products with a spatial resolution of about 30 km.
- SMAP's radiometer detects differences in microwave pulses transmitted to the ground by the instrument that are caused by water in soil. It measures Earth's natural microwave emissions at the frequency of 1.4 GHz. Around the globe, the most striking difference in these natural emissions is between water and land surfaces. A desert emits microwaves at about three times the rate a lake does. Because the difference is so large, even a small amount of moisture in soil causes a change that a radiometer can measure accurately.
- The SMAP mission is required to produce high-resolution maps of global soil moisture and detect whether soils are frozen or thawed. SMAP's radar has two data acquisition functions: one for SAR (Synthetic Aperture Radar) processing to produce radar measurements at a spatial resolution of 1 to 3 km, and another for low-resolution processing to produce radar measurements at a spatial resolution of 30 km. The SAR function will be used over land surfaces and coastal oceans during routine science operations, while low-resolution processing will be exercised over land as well as over global ocean areas. - Since the SAR function was only turned on for limited durations during the March 31 - April 3 test, mission scientists did not obtain enough SAR data to produce global high-resolution maps. Beginning April 13, SMAP will start conducting regular SAR observations that will enable high-resolution global mapping of land surfaces about every two to three days.
- Scientists will combine measurements from SMAP's radar and radiometer sensors to capitalize on the strengths of each and work around their weaknesses. The radar alone can produce a soil moisture measurement with a spatial resolution of about 3 km, but the measurement itself is less accurate than the one made by the radiometer. The radiometer alone achieves a highly accurate observation of soil moisture but with a much poorer spatial resolution of about 40 km. By combining these separate measurements through advanced data processing, SMAP will provide the user community with a combined soil moisture measurement that has high accuracy and a resolution of 9 km. The advanced processing required to combine these active and passive measurements is now being functionally checked out, and is the last step in SMAP's postlaunch checkout process. SMAP will offer the individual radar and radiometer data, among other data products.
• March 27, 2015: SMAP completes procedure to spin up its antenna to final science measurement rate. The 6 m reflector antenna is now ready to wrangle up high-resolution global soil moisture data, following the successful completion of a two-part procedure to spin it up to full speed. On March 26, mission controllers at NASA/JPL commanded SMAP's spun instrument assembly to increase its rotation speed from the initial rate of 5 rpm achieved on March 23 to its final science measurement rate of 14.6 rpm. 86)
- Throughout the gradual process, which took approximately 80 minutes, onboard guidance, navigation and control software managed the spin-up acceleration level, allowing the spacecraft to maintain its Earth-pointing (nadir) attitude. Initial data indicate the antenna spin-up procedure went as planned. Mission controllers will now analyze the spin-up process and the stability of the observatory at its final spin rate.
• March 24, 2015: Mission controllers at NASA/JPL have commanded the 6 m antenna on SMAP to begin spinning for the first time. The partial spin-up is a key step in commissioning the satellite in preparation for science operations. - Last week, mission controllers sent commands to release the locking mechanism that prevented the observatory's spun instrument assembly — the part that spins — from rotating during launch and deployment of the reflector. The spun instrument assembly includes the spin control electronics, radiometer instrument and reflector antenna. - Yesterday, in the first step of a two-step procedure, the spun instrument assembly was spun up to its initial rate of five revolutions per minute (rpm), a process that took about a minute. Initial data indicate the partial antenna spin-up procedure went as planned. 87)
Figure 48: NASA's SMAP spacecraft antenna starts spinning (image credit: NASA/JPL)
- Because of the large size (mass) of the spun instrument assembly and its relatively rapid angular acceleration during spin-up, SMAP's spacecraft bus rotated in the opposite direction during this process to balance the angular momentum. It reached a peak rate of up to 11 degrees per second. Once the spun instrument assembly spin rate stabilized at five rpm, the spacecraft’s reaction wheels quickly restored the spacecraft bus to a non-rotating, stable attitude. Onboard flight software then turned the observatory back to its science-gathering orientation, with the spin axis pointing straight down to the ground and SMAP's solar array pointed toward the sun.
- The observatory will remain in its current configuration with the spun instrument assembly rotating at five rpm for about three days to allow ground controllers to assess the observatory's performance at this spin rate before proceeding to the next step. On March 26, after ground analysis of this first antenna spin-up step is completed, mission controllers plan to increase the antenna's spin speed to its final rate of approximately 15 rpm.
• March 13, 2015: One ecosystem where scientists would most like to understand the effects of changing freeze/thaw cycles is boreal forests, the great ring of green covering the land nearest the North Pole. The forests of Alaska, Canada, Scandinavia and Siberia cover almost 15 percent of Earth's land surface. The Arctic is warming more quickly than lower latitudes, and the way these forests respond to this rapid change could provide valuable clues about our planet's warmer future. 88)
- But we know very little about how the boreal forests are changing. Millions of square miles have no roads or even villages. "What we have now are very sparse, seasonal measurements from the ground, according to John Kimball of the University of Montana, Missoula, and a member of the science team for NASA's SMAP mission. We do have long-term, global satellite data sets that are sensitive to freeze-thaw, but they tend to be very coarse. That means each measurement averages the status of a large area. Like a mosaic made of large tiles, these data cannot show much detail.
- That's about to change. By the end of April, SMAP will begin monitoring the frozen or thawed state of the landscape north of 45º north latitude (about the latitude of Minneapolis) every two days. The primary mission of SMAP is to measure the amount of moisture in the top few cm of soil globally, but it also detects whether that moisture is frozen or in liquid form. SMAP's radar measurements, with "tiles" only 1 km by 3 km across, will reveal far more detail than scientists now have about the freeze/thaw status of the land surface.
- Why is greater detail needed? In the Arctic, the timing of the spring thaw can vary considerably within a small area. Because the returning sun is low on the horizon, the shadowed north side of a hill may remain icy many days after plants have started growing again on the sunlit south side. Those early spring weeks are critical in the short Arctic growing season. According to John Kimball, once the vegetation thaws, boom! Photosynthesis takes off. The highest rates of photosynthesis can be obtained within a few weeks after the thaw, and a later thaw can mean much lower vegetation growth for the season. We need observations at what I call the landscape level to more precisely monitor those patterns and changes.
- During photosynthesis, plants absorb carbon dioxide from the air. The carbon stays in their wood, roots and leaves, and when they die, most of it remains in the soil. That makes undisturbed forests what scientists call carbon sinks — places that remove carbon from the atmosphere. Longer unfrozen seasons in the Arctic give forests more time to grow and spread, increasing the extent of the carbon sink.
- On the other hand, climate warming has increased the occurrence of droughts and wildfires in the Arctic. A burning forest spews enormous amounts of carbon into the atmosphere; in scientific terms, it is a carbon source. Thus, global climate change is causing the northern forests both to absorb and to release more carbon.
• March 9, 2015: NASA's SMAP observatory has successfully completed a two-day test of its science instruments. The observatory's radar and radiometer instruments were successfully operated for the first time with SMAP's antenna in a non-spinning mode on Feb. 27 and 28. The test was a key step in preparation for the planned spin-up of SMAP's antenna to approximately 15 rpm in late March. The spin-up will be performed in a two-step process after additional tests and maneuvers adjust the observatory to its final science orbit over the next couple of weeks. 89)
- Based on the data received, mission controllers concluded the radar and radiometer performed as expected. The controllers are based at NASA's Jet Propulsion Laboratory, Pasadena, California; and NASA's Goddard Space Flight Center, Greenbelt, Maryland.
- The first test image illustrates the significance of SMAP's spinning instrument design in producing more comprehensive maps. For this initial test with SMAP's antenna not yet spinning, the observatory's measurement swath width — the strips observed on Earth in the image — was limited to 40 km. When fully spun up and operating, SMAP's antenna will measure a 1,000 km swath of the ground as it flies above Earth at an altitude of 685 km. This will allow SMAP to map the entire globe with high-resolution radar data every two to three days, filling in all of the land surface detail that is not available in this first image.
- The radar data illustrated in the upper panel of Figure 49 show a clear contrast between land and ocean surfaces. The Amazon and Congo forests in South America and Africa, respectively, produced strong radar echoes due to their large biomass and water content. Areas with no vegetation and low soil moisture, such as the Sahara Desert, yielded weaker radar echoes. As expected, the dry snow zone in central Greenland, the largest zone of the Greenland ice sheet where snow does not melt year-round, produced weaker radar echoes. Surrounding areas in Greenland's percolation zone, where some meltwater penetrates down into glaciers and refreezes, had strong radar echoes due to ice lenses and glands within the ice sheet. Ice lenses form when moisture that is diffused within soil or rock accumulates in a localized zone. Ice glands are columns of ice in the granular snow at the top of glaciers.
- The test shows that SMAP's radiometer is performing well. The radiometer's brightness temperature data are illustrated in the lower panel of Figure 49. Brightness temperature is a measurement of how much natural microwave radiant energy is traveling up from Earth's surface to the satellite. The contrast between land and ocean surface is clear, as it is in the radar image. The Sahara Desert has high brightness temperatures because it is so hot and has low soil moisture content. The India subcontinent is currently in its dry season and therefore also has high brightness temperatures. Some regions, such as the northeast corner of Australia, show low brightness temperatures, likely due to the high moisture content of the soil after heavy rainfall from Cyclone Marcia in late February.
Figure 49: SMAP first light - radar radiometer data, Feb. 27-28, 2025 (image credit: NASA/JPL-Caltech)
• Feb. 26, 2015: The JPL control team is reporting the successful deployment of the boom and antenna in two separate steps — comparing this important event prosaically with a cowboy at a rodeo raising his arm (boom) to unfurl a huge golden lasso (antenna) that will soon spin up to rope the best soil moisture maps ever obtained from space. - The deployment of the boom and the mesh reflector (antenna) onboard of SMAP represents in effect a key milestone on its mission to provide global measurements of soil moisture. 90)
- Following its picture-perfect launch and insertion into orbit, mission controllers performed a series of health checks of the observatory's subsystems. They also ran successful initial health checks of SMAP's radiometer and radar science instruments, powering them on for 30 hours in receive-only mode and processing the data. Then on Feb. 18, mission controllers successfully commanded SMAP's 5 m two-hinged boom to unfold and extend.
- On Feb. 24, the team commanded SMAP's reflector antenna at the end of the boom to deploy. SMAP's reflector boom assembly is an advanced, low-mass rotating deployable mesh reflector antenna system that supports the collection of SMAP's radar and radiometric measurements in space. It is the first-ever spinning and precision mass-balanced deployable mesh reflector antenna, and is the largest spinning mesh reflector ever deployed in space.
- Astro Aerospace experts have preliminarily determined that the deployed natural frequency of the reflector boom assembly in orbit is nearly identical to prelaunch model predictions. This provides confidence in the health of the deployed reflector and in its performance once spun up.
- For launch, the flexible mesh antenna, which is edged with a ring of lightweight graphite supports called a perimeter truss, had been tightly folded and stowed into a volume of just 30 cm x 120 cm. Upon deployment, the truss slowly opened, like a camp chair, to its full diameter of almost 6 m. Despite its size, the reflector has a mass of only 25 kg. With its supporting boom and launch restraints, the entire reflector and boom assembly has a mass of just58 kg.
- Kent Kellogg, the SMAP project manager at JPL said: "Deploying large, low-mass structures in space is never easy and is one of the larger engineering challenges NASA missions can confront in development. This week's result culminates more than six years of challenging reflector and boom assembly development, system engineering and an extensive test campaign. With this key milestone in our rear-view mirror, the team now looks forward to completing the remainder of our commissioning activities and beginning routine science operations for this important mission with broad applications for science and society."
- Later this week, SMAP's science instruments will be checked out with the deployed reflector antenna in a non-spinning configuration. This will mark the observatory's first operation with the reflector and boom assembly functioning as an antenna to view Earth. It will also mark the first time SMAP's radar high-power amplifier will transmit a signal.
- In about a month, after additional tests and maneuvers to adjust the observatory to its final 685 km, near-polar operational science orbit, SMAP's "lasso" antenna will do a sort of Texas two-step, spinning up in a two-stage process to 14.6 revolutions per minute. By rotating, the antenna will be able to measure a 1,000 km swath of Earth below, allowing SMAP to map the globe every two to three days.
• All four ELaNa CubeSats were ejected from the second stage per the mission timeline (1 hr and 45 minutes after liftoff), and are flying free. Prior to the deployment, the second stage of the Delta-2 rocket performed an 8 second retrograde maneuver to lower the orbit of the vehicle for the release of the CubeSats. Three P-PODs were installed on the second stage, filled with two 3U CubeSats and two 1.5U satellites.
• About 57 minutes after liftoff, SMAP separated from the rocket's second stage into an initial 661 x 685 km orbit. After a series of activation procedures, the spacecraft established communications with ground controllers and deployed its solar array. Initial telemetry shows the spacecraft is in excellent health (Ref. 23).
Sensor complement: (SMAP)
The instrument name is the same as the mission name. As with most microwave instruments, the antenna is the dominant instrument subsystem that both determines the ultimate measurement performance and governs spacecraft accommodation.
The key instrument requirements were determined by the SMAP science working group to be:
1) dual-polarization (linear H and V) L-band passive radiometer measurements at spatial resolution of 3 km
2) linear HH, VV and HV L-band radar (SAR) measurements
3) a wide swath to ensure global three-day refresh time for these measurements (1000 km swath at the selected orbit altitude of 685 km).
4) co-located L-band active radar measurements and passive radiometer measurements at a constant incidence angle near 40º
The radiometer and radar resolution requirements at L-band dictate that a relatively large antenna aperture must be employed. A shared antenna/feed approach is utilized for accomplishing the required simultaneous radiometer and radar requirements. The overall SMAP architecture is shown in Figure 53, which includes the rotating RBA (Reflector Boom Assembly), feed assembly, radiometer electronics subsystem, and radar electronics subsystem. 91) 92) 93) 94) 95)
A deployable conically-scanning antenna structure of 6 m diameter is selected for the instrument design. In 2009, Astro Aerospace (Carpinteria, CA, a business unit of Northrop Grumman) received a contract from JPL for the design and development of RBA. The antenna consists of an AstroMeshTM reflector and a single feed horn shared by an L-band radar and an L-band radiometer. Whereas the radiometer resolution is defined in the standard manner as the real-aperture antenna footprint, the higher resolution radar measurements are obtained by utilizing SAR (Synthetic Aperture Radar) processing (Figure 64).
Two design modifications were introduced in 2011 (Ref. 91):
• The addition of active thermal control to the instrument spun side to provide a more stable, settable thermal environment for the radiometer electronics
• A “sequential transmit” strategy for the two radar polarization channels which allows a single high-power amplifier to be used.
Because the rotating RBA is shared by the radiometer and radar, the RF signals from the Earth must be separated by diplexers into the active and passive bands, respectively. These diplexers are located on the spun side and are shown as part of the radiometer subsystem. All of the radiometer electronics are located on the spun side of the interface to minimize front-end losses, with slip rings providing a telemetry, signal, and power interface to the spacecraft. The more massive and more thermally dissipative radar electronics are on the fixed side, with the transmit/receive pulses routed to the spun side via a two channel RF rotary joint.
A major milestone in the development of the RBA CDR (Critical Design Review) was completed on December 13-14, 2011 for the SMAP payload. The spun portion of the RBA has a nominal mass of 49 kg; the entire RBA, including launch restraint equipment, has a mass of 65 kg. The launch restraint equipment attaches the compact, stowed reflector to the side of the spacecraft for launch. 96)
The radiometer measures microwave emissions for H and V polarization brightness temperatures, and provides 3rd and 4th stokes parameters that are used for RFI mitigation. The radiometer digital electronics includes digital spectral filtering for RFI mitigation. The key to high radiometric measurement accuracy is achieving repeatable, characterizable, and monotonic responses over temperature and time. Therefore, the thermal design employs passive design features for short-term stability, such as the use of a titanium thermal isolator and radome for the feed. In addition, active thermal control is used for additional seasonal stability, as well as for temperature set-point adjustment, which enables the operational temperature to be changed on-orbit to avoid gain non-linearities. This set-point feature was added to the design as a result of lessons learned from other microwave radiometer missions that exhibited this undesirable behavior. Radiometer performance requirements are summarized in Table 6. 97)
A frequency diplexer allows the radar and radiometer to share a common instrument antenna. The diplexer simultaneously provides high RF isolation between the radar and radiometer frequencies and pre-select filtering for RFI (Radio Frequency Interference) rejection in a compact and low-RF-loss package. It also accommodates high-power handling capability for the radar.
The overall SMAP instrument architecture is shown in Figure 50. Because the rotating reflector is shared by the radiometer and radar, the RF signals from the Earth must be separated by frequency diplexers into the active and passive bands. These diplexers are located on the spun side of the observatory as shown in Figure 50. Note that all of the radiometer electronics are located on the spun side of the interface to minimize front-end losses, with slip rings providing a telemetry, signal, and power interface to the spacecraft. The more massive and more thermally dissipative radar electronics are on the fixed side, with the transmit/receive pulses routed to the spun side via a two-channel RF rotary joint.
Figure 50: Simplified instrument functional diagram (image credit: NASA)
Table 6: SMAP radar and radiometer requirements
Table 7: SMAP instrument key antenna requirements
The instrument antenna design is an offset-fed reflector arrangement (0.7 f/D) with a 6 m projected aperture and vertical and horizontal linear polarizations selectable through an OMT (Orthomode Transducer). Zenith deck mounting offered a number of system design and performance advantages including (1) lowest overall spun mass/inertia; (2) lowest RF transmission line losses to the radiometer and radar; and (3) lowest overall system noise temperature for radiometer measurements because most feed spillover ‘sees’ cold space. The zenith mounting location and conical scanning requirement, however, posed a significant constraint on the flight system design to avoid intrusions into the instrument antenna FOV (Field of View), the solar array design was a particular challenge (Figure 51). The zenith location also partially but significantly blocks visibility to the GPS (Global Positioning System)—this contributed to a decision in Formulation to forego on-board GPS capability for orbit position and ephemeris and to instead use Doppler tracking and ground-based time synchronization. The most significant design drivers on the antenna came from the radiometer: beam efficiency, reflector surface emissivity, and temperature knowledge to achieve the antenna noise temperature requirements. Radar drivers on the antenna performance were gain stability, sidelobes, and cross polarization levels. Key instrument antenna performance requirements are summarized in Table 7.
Figure 51: Final solar array simplified to reduce panels and deployments; allowed slight penetration into instrument antenna FOVs (image credit: NASA)
Legend to Figure 51: As power requirements matured in the design, the solar array was simplified to a three-panel configuration (with only two deployments) that protruded slightly into the instrument antenna FOV. The resulting RF pattern and performance perturbations from the intrusion can be removed in ground science data processing; however, high-fidelity RF analysis (confirmed by scale model testing) showed that perturbations are very minor and may be negligible. The stowed configuration was also significantly driven by launch vehicle fairing packaging and resulted in having no exposed cells when stowed. This iterative design example is representative of the close interaction between science, instrument, and spacecraft design teams to arrive at the least complex design to satisfy mission requirements.
Mechanically, the spin subsystem and spun platform assembly provide the spin function; launch lock and release; mechanical support for the radiometer, feed and RBA (Reflector Boom Assembly); and spun-side electrical control functions (RBA deployment motors control, thermal control, telemetry, etc.). The heart of the spin subsystem is a Boeing-provided BAPTA (Bearing and Power Transfer Assembly) that includes the spin motor, bearings, 65 slip rings for power and digital telemetry transfer across the spinning interface, and the RF rotary joint used by the radar. The BAPTA is enclosed within the cylindrical instrument core structure that also provides the mounting platform for all spun-side assemblies. ICE (Integrated Control Electronics) control the RBA deployment motors, spun-side thermal control, and radiometer command and telemetry. The Radiometer Front-End Electronics and passive RF components such as the diplexer are mounted to the feed horn. The Radiometer Electronics and Spin Control Electronics are mounted to separate structures to optimize spun mass properties. A CCA (Cone-Clutch Assembly) is the structural interface between the spun platform assembly and the spacecraft, and also locks the spun platform and offloads the BAPTA bearings during launch. Design features are incorporated to attenuate pyroshock levels for the spun electronics and bearings.
Table 8: Instrument mass and power breakdown (reflects CDR estimates with growth contingency applied)
Collectively, the instrument spinning elements form the SIA (Spun Instrument Assembly). The SIA is largely balanced, by design, by adjusting the antenna optical prescription, by appropriately offsetting the feed and spun electronics assemblies from the spin axis using varying strut lengths, and by including small pre-launch adjustable ballast/balance masses at key locations (mostly on the RBA) in the design. An aggressive mass properties management program insures that as-built spun-side mass properties are tracked; the prelaunch adjustable ballast/balance masses allow correction for as-built mass characteristics to ensure proper on-orbit balance is achieved. Extensive analysis and modeling of the system dynamics and control behavior demonstrated that robust and stable balance is achieved by the fixed instrument balance design approach coupled with the spacecraft pointing control authority. Practical SIA on-orbit adjustable balance mechanisms could only provide a small fraction of total available pointing margin and were therefore not included in the design. The spun momentum of the SIA is compensated by the spacecraft’s reaction wheels and ACS (Attitude Control System ). This arrangement simplified the instrument design as well as overall control and fault protection design for the observatory; however, it places a constraint on the maximum spun momentum of the SIA of 364 Nms (at CDR the estimated spun momentum was 326 Nms). Another instrument configuration benefit is that most of the spun mass is concentrated near the rotation axis, which greatly reduces the sensitivity of spun momentum to mass growth.
Instrument RBA (Reflector Boom Assembly):
The 6 m shared reflector posed a number of unique challenges for SMAP:
• Packaging and deployment to accommodate fairing volume and spacecraft constraints
• Spinning and pointing control of such a structure
• Use of a mesh deployable for radiometric measurements
• Accurate pre-launch RF pattern characterization, accounting for spacecraft interaction effects.
The NGAS (Northrop Grumman Aerospace System's) deployable AstromeshTM (AM) reflector was selected and its mass properties are ideally suited for SMAP’s application. These reflector designs have been used in geosynchronous communications satellite applications including MBSAT (Mobile Broadcasting Satellite) of Thuraya, Inmarsat-4, and Alphasat. SMAP uses a derivative of Astro’s heritage antenna design, AM-Lite (Figure 52), which accommodates smaller aperture sizes and fits within a smaller stowed volume.
Figure 52: The 6.5 m AstroMeshTM Lite engineering model reflector (image credit: NGAS - Astro Aerospace)
This is the first known application of this kind of deployable reflector design within a high-performance microwave radar and radiometer antenna system, operating in LEO and also in a spinning configuration. Each of these ‘firsts’ poses unique challenges for SMAP that have been addressed within the mission and system design, and in the verification and validation (V&V) approach. The overall configuration and basic components of the stowed and deployed reflector are shown in Figure 53.
The reflector uses a perimeter truss consisting of composite tubes to support front and rear webs of fiber-reinforced tape. The reflector surface is a 20 OPI, gold-plated molybdenum wire mesh held in place with a net that attaches to a front web to provide the proper spacing to form the mesh’s parabolic shape. The perimeter truss is attached to a prime batten, which in turn attaches to the two-segment boom. The boom is made of two graphite/epoxy tubes connected by hinges that allow the furled reflector to be stowed for launch to fit within the launch vehicle fairing. Pyro-initiated releases are required to open launch restraints before the boom can be deployed.
The RBA deployment is a significant driver to the mission and observatory design. The boom and reflector are deployed separately (boom first, followed nominally by the reflector two days later). The LEO orbit drove the thermal designs to ensure there were no “hot spots” that could either overheat or introduce large thermal gradients, which could add deployment risk. An operational constraint is imposed to idle spacecraft guidance and attitude control during deployment to eliminate ACS reaction loads on the RBA. This, in turn, places requirements and constraints on the deployment duration to be completed within 40 minutes (a spacecraft power constraint). Heritage reflectors have been deployed over long periods (hours) to prevent motor overheating, so new motor and thermal designs were required for SMAP to accommodate the short deployment time and thermal loading.
Figure 53: SMAP’s reflector-boom assembly stowed and deployed configurations (image credit: NASA)
The <0.5 K radiometer antenna temperature calibration requirement places challenges on the mesh and web RF emissivity and mesh temperature knowledge (mesh emissivity < 0.0035, mesh emissivity knowledge to 0.001, mesh temperature known to 60ºC). Early measurements of mesh materials were completed to confirm it was acceptable for soil-moisture applications (Figure 54). The mesh density (OPI) was assessed to meet the L-band emissivity requirements. 20 OPI was selected as the best trade between mass properties and RF performance (lower mass and easier to stow and manage than 40 OPI density, and although the 10 OPI mesh has lower mass it was found to be too lossy for L-band radiometer measurements). The mesh temperature knowledge is another key parameter required for antenna pattern correction. The mesh temperature cannot be measured directly on-obit, so it will be determined by ground test-verified models to provide on-orbit temperature estimates.
Heritage reflectors have been flown in geosynchronous orbits. The LEO environmental effects on the mesh had to be considered during the mesh qualification, including effects of the extensive thermal cycling on-orbit, effects of atomic oxygen, solar ultraviolet (UV) radiation, ESD (Electrostatic Discharge) charging, solid particles, and PIM (Passive Intermodulation) concerns. The mesh qualification program determined that “cold welding” from thermal cycling or gold flaking due to CTE mismatch were not credible failure modes. The mesh is not susceptible to oxidizing from the atomic oxygen. The RF performance would be negligibly impacted if there were a small hole or tear in the mesh from a solid particle micrometeroids. PIM was determined not to cause an interference concern for science or telecom functions. In communication applications of the mesh reflector, the webs are typically painted with a conductive paint for ESD considerations. However, for SMAP, this type of paint contributed too much radiometric loss and had a significant impact in the overall antenna error budget, so an effort was made to eliminate the use of the paint. However, it was determined that the un-painted webs would degrade unacceptably in the vacuum UV environment, so a paint was ultimately selected that was both low loss and tolerant to atomic oxygen and UV. This is an example of how critical RF performance had to be carefully balanced with other design considerations such as operating in LEO.
Figure 54: Mesh emissivity test setup and sample (image credit: NASA)
Another challenge related to the RBA has been the extensive modeling and component level testing for a piecewise verification of the antenna RF performance. Radar and radiometer performance are highly dependent on antenna pattern characteristics such as gain, half-power beamwidth, sidelobe and cross-polarization levels, and electrical pointing. SMAP’s large antenna size and gravity effects make it impractical and high risk to conduct an end-to-end antenna test on the ground in a flight-like configuration and environment (to “test as you fly”). The antenna pattern RF performance is being verified by a combination of test and analysis. Scattering from the boom, solar panels, and other parts of the observatory structure affect the antenna pattern. Moreover, the radar and radiometer have stability requirements for pattern characteristics. GRASPTM (Geodetic Reference Antenna in Space) is used to model the antenna in the presence of the observatory to determine the effects that could change the antenna pattern, such the FOV changes as the antenna rotates, reflector and boom thermal distortions, and dynamic distortions from spacecraft. As an independent verification, a high-fidelity 1/10th scale model of the observatory was tested to confirm the GRASPTM model (Figure 55). The flight feedhorn pattern characteristics will be measured and applied to the GRASPTM RF model to form the final-prelaunch antenna performance. The performance will be verified and uncertainties will be refined during post-launch commissioning and during the science calibration and validation campaign.
Figure 55: 1/10th scale model undergoing antenna range measurements (image credit: NASA)
RFI (Radio Frequency Interference) mitigation:
The L-band spectrum region (Figure 56) is heavily used and SMAP’s measurements are made over land areas, where most potential interferers are located. This contrasts with the similar L-band Aquarius mission, where the primary target is the relatively RFI-quiet ocean. It’s also worth noting here that RFI is evolving and generally increasing over time and that trend is expected continue into the future; SMAP must therefore be prepared to operate not only in the RFI environment as it exists today, but also in the RFI environment that may exist later in this decade. To meet radiometric accuracy requirements, SMAP has adopted aggressive measures to identify and mitigate the effects of RFI. Because SMAP is a global mapping mission with continuous, near-real-time generation of data products, any RFI mitigation techniques must lend themselves to reliable automation in ground processing software. The nature of the RFI threat differs somewhat for the radiometer and radar channels, which are treated in turn (Ref. 97).
Figure 56: SMAP radar and radiometer operate within congested spectrum allocations that require RFI mitigation strategies to be applied (image credit: NASA)
Radiometer: The RFI power in the radiometer’s 24 MHz bandwidth will positively bias the observed brightness temperature, resulting in an erroneous dry bias in soil moisture estimates if uncorrected. While large RFI impacts (> ~40–50 K) can be detected and discarded, causing data loss, smaller RFI contributions are more difficult to detect and more likely to impact science product accuracy. Errors resulting from RFI corruption of even ~0.5 K are significant, therefore the radiometer includes a digital receiver to enable RFI detection and mitigation.
An extensive effort has been conducted to characterize the RFI environment expected for the radiometer. Two primary sources of RFI information have been utilized: a set of airborne observations in the United States and observations from the SMOS (Soil Moisture and Ocean Salinity) L-band radiometer mission of ESA (European Space Agency). The results show a variety of sources are present, including pulsed and narrowband, with some limited evidence of “broadband” continuous sources. SMOS data do not provide detailed RFI source information, but do provide a global characterization of observed RFI power levels. In particular, SMOS data show increased RFI levels in global regions outside the Americas.
Available characterization data shows RFI sources are either pulsed or narrow-band [i.e., continuous wave (CW)–like] in nature. Pulsed sources can be detected in the time-domain, if the radiometer detector is sampled at a sufficiently high temporal resolution. The radiometer has a fundamental sampling frequency at the radar PRF of 3.2 kHz, which allows ground-based sub-millisecond RFI detection and mitigation using simple time-domain pulse thresholding strategies. To detect and mitigate CW sources, the radiometer’s 24 MHz bandwidth is digitally filtered into 16 x 1.5 MHz subbands (Figure 57). Detected powers in these subbands will be telemetered to the ground at ~ 1 ms temporal resolution. The resulting ~ 1.5 MHz x 1 ms spectrogram dataset can be utilized in a variety of RFI detection methods, including channelized pulse detection, cross-frequency algorithms, or “peak-picking” methods.
Figure 57: Radiometer band divided into 16 x 1.5 MHz spectral subbands for RFI detection (image credit: NASA)
In addition to time/frequency discrimination, the radiometer digital subsystem will also compute the first through fourth moments of observed fields both in the 24 MHz “full-band” (i.e., 3.2 kHz) and 1.5 MHz “subband” (16 channel x 1 ms) datasets. The availability of these moments will enable computation of the full-band and subband kurtosis for RFI detection, which has also been shown effective against several source types.
Radiometer electronics design: The SMAP radiometer electronics design is largely adopted from the Aquarius design with one major difference: the SMAP radiometer uses a superheterodyne digital receiver to enable advanced RFI mitigation and full Stokes polarimetry. The system design includes a highly linear microwave receiver with internal calibration sources and a digital signal processor for RFI detection. The frontend comprises an RF cable-based feed network, with frequency diplexers and coupled noise source, and a radiometer frontend (RFE) electronics package. Internal calibration is provided by reference switches and a common noise source inside the RFE. The RF backend (RBE) downconverts the two 1413 MHz channels (for vertical and horizontal polarizations) to an intermediate frequency (IF) of 120 MHz. The IF signals are sampled two analogtodigital converters (ADCs) in the radiometer digital electronics (RDE). The RBE local oscillator and RDE digital clocks are syncrhonized to ensure coherency between the sampled IF signals. The RDE performs additional filtering, subband channelization, crosscorrelation for measuring third and fourth Stokes parameters, and detection and integration of the first four raw moments of the signals. These data are packetized and sent to the ground for calibration and further processing. 98)
A block diagram of the radiometer electronics is shown in Figure58 with photographs of hardware accompanying each block. Starting on the right, external noise source and directional couplers, diplexers, RFE and RBE compose the the radiofrequency (RF) electronics subsystem. Testing shows the noise figure is approximately 1.5 dB. The RF signal enters the system and the radiometer band is selected by the diplexers. The RFE amplifies the signal and applies calibration sources. The RBE shifts the frequency of the L-band signal an IF suitable for sampling by the ADCs in the RDE. Special consideration was given to specifying system linearity because of the of the system (~80 dB) and presumed presence of RFI. Measurements indicate the system operates at most 25 dB below 1 dB compression, resulting in <0.2 K of nonlinearity error before correction. Testing was performed to characterize the nonlinearity and residual error after correction is shown in Figure 59.
The RDE performs digital signal
processing to the vertical and horizontal polarization channels output
by the RF subsystem. The RDE will be the first spaceflight processing
to produce radiometer data in all four Stokes parameters and first four
raw moments integrated across the fullband channel and 16 subband
channels. This new capability enables aggressive RFI mitigation to be
applied to the data. In the DSP algorithm, a
Figure 58: SMAP radiometer block diagram showing signal paths and frequency plan. Photographs of the hardware accompany each block (image credit: NASA/GSFC)
Figure 59: Estimated error after nonlinearity correction is applied as function of input antenna temperature for horizontal polarization. The average error is less than 40 mK. Vertical polarization is similar (image credit: NASA/GSFC)
A repeatability test was done in thermal vacuum testing in part because of these new features. The radiometer hardware was thermal cycled while observing a stable coldFET at its input. The changes in calibrated output between test plateaus is shown in Figure 60. The project team found the performance to be comparable to the Aquarius radiometer pre-launch test results.
Figure 60: Calibration repeatability of the radiometer electronics during several thermal cycles of testing (image credit: NASA/GSFC)
The proliferation of RFI in the terrestrial environment is a major driver to the SMAP radiometer spaceflight electronics design. The implemented design contains features specifically developed to mitigate RFI and to meet or exceed science mission requirements. The design implementation employed both hardware and software applications that work in concert to enable the detection and removal of RFI-saturated data. The hardware architecture includes a digital channelized radiometer/polarimeter with a superheterodyne frontend. To process the science data and remove RFI-saturated data, a complex ground algorithm is employed. The system outputs data packets containing time, frequency, polarization, and statistical diversity (8 packets with 360 samples each) that are processed by the ground algorithm to detect and remove RFI. RFI. Figure 61 shows the Radiometer electronics installed to the SMAP Observatory. After a successful ground integration and test campaign, including extensive thermal and electrical testing, the electronics are awaiting launch no earlier than January 2015 and subsequent on-orbit checkout.
Figure 61: Radiometer electrical components installed to spun-side assembly of the Observatory (image credit: NASA/GSFC)