NISAR (NASA-ISRO Synthetic Aperture Radar) Mission
NISAR is a joint NASA-ISRO (Indian Space Research Organization) Earth-observing mission with the goal to make global measurements of the causes and consequences of land surface changes. Potential areas of research include ecosystem disturbances, ice sheet collapse and natural hazards. The NISAR mission is optimized to measure subtle changes of the Earth's surface associated with motions of the crust and ice surfaces. NISAR will improve our understanding of key impacts of climate change and advance our knowledge of natural hazards.
NISAR will be the first satellite mission to use two different radar frequencies (L-band and S-band) to measure changes in our planet's surface less than a centimeter across. This allows the mission to observe a wide range of changes, from the flow rates of glaciers and ice sheets to the dynamics of earthquakes and volcanoes.
At the IAC (International Astronautical Congress) in Toronto, Canada ( Sept. 29-Oct. 3, 2014), NASA Administrator Charles Bolden and K. Radhakrishnan, chairman of ISRO, signed two documents to launch a NASA-ISRO satellite mission to observe Earth and establish a pathway for future joint missions to explore Mars. 1) 2) 3) 4) 5)
Under the terms of the new agreement, NASA will provide the mission's L-band SAR (Synthetic Aperture Radar), a high-rate communication subsystem for science data, GPS receivers, a solid state recorder, and a payload data subsystem. ISRO will provide the spacecraft bus, an S-band SAR, and the launch vehicle and associated launch services. A launch of NISAR is targeted in 2021.
NASA had been studying concepts for a SAR mission in response to the National Academy of Science's decadal survey of the agency's Earth science program in 2007. The agency developed a partnership with ISRO that led to this joint mission. The partnership with India has been key to enabling many of the mission's science objectives.
The first civilian SAR satellite in history, called SEASAT, was launched by NASA in 1978. SEASAT's L-band (24 cm wavelength) SAR operated for three months before the failure of the spacecraft's power system. SEASAT led to a series of NASA space shuttle-based radar missions and inspired the development of spaceborne SAR systems worldwide. Launching another free-flying scientific SAR in the US has proven elusive, despite strong demand from the science and applications community (Ref. 7). 6)
The NISAR mission concept is the current incarnation of NASA's answer to the NRC (National Research Council) Decadal Survey response of 2007 for previously unavailable data and insight in three earth science domains: Deformation, Ecosystem Structure, and Dynamics of Ice (DESDynI).
The DESDynI mission concept has undergone several revisions over the years from quite drastic measures, such as the removal of a second spacecraft platform, and collaboration with an international partner, to minor such as adjustments of specific target areas.
Through discussions between NASA and ISRO on the possibility of a joint radar mission, it became clear that the goals originally identified for DESDynI-R were of great interest to the ISRO science community. In January 2012, ISRO identified targeted science and applications that were complementary to the primary mission objectives, agricultural monitoring and characterization, landslide studies, Himalayan glacier studies, soil moisture, coastal processes, coastal winds and monitoring hazards. For many of these objectives, the addition of an S-band
Since January 2012 when the initial L- and S-band SAR mission concept was put forward as a partnership, JPL and ISRO teams have been attempting to refine the science plan and its implications for the mission. In September 2013, ISRO received initial approval from the Government of India for jointly developing with NASA the L- and S-band SAR mission. A Technical Assistance Agreement (TAA) between ISRO and Caltech/JPL was enacted on September 30, 2013. NASA Administrator Charles Bolden and K. Radhakrishnan, Chairman of ISRO, signed the NISAR Implementing Arrangement (IA) on September 30, 2014.
The Indian Space Research Organization (ISRO) will partner with NASA's Jet Propulsion Laboratory (JPL) on this proposed mission, and ISRO is baselining to provide a large portion of the mission in launch, bus, infrastructure, and an S-band instrument. JPL would provide the L-band InSAR instrument, antenna, telecom, GPS and solid-state recorder.
These two InSAR instruments combined could produce data rates of upwards of 5 Gbit/s for intervals on the order of 60 seconds, and sustained rates for global mapping of deformation objectives on the order of 2 Gbit/s. These projected rates and duty test the bounds of today's infrastructure for moving this data from orbit to the ground.
Science objectives: NISAR's science objectives are based on priorities identified in the 2007 Decadal Survey and rearticulated in the 2010 report on NASA's Climate-Centric Architecture. NISAR will be the first NASA radar mission to systematically and globally study solid Earth, ice masses, and ecosystems. NISAR will measure ice mass and the land surface motions and changes, ecosystem disturbances, and biomass, elucidating underlying processes and improving fundamental scientific understanding. The measurements will improve forecasts and assessment of changing ecosystems, response of ice sheets, and natural hazards. NASA also supports use of the NISAR data for a broad range of applications that benefit society, including response to disasters around the world. In addition to the original NASA objectives, ISRO has identified a range of applications of particular relevance to India that the mission will address, including monitoring of agricultural biomass over India, monitoring and assessing disasters to which India responds, studying snow and glaciers in the Himalayas, and studying Indian coastal and near-shore oceans.
All NISAR science data (L- and S-band) will be freely available and open to the public, consistent with the long-standing NASA Earth Science open data policy. With its global acquisition strategy, cloud-penetrating capability, high spatial resolution, and 12-day repeat pattern, NISAR will provide a reliable, spatially dense time-series of radar data that will be a unique resource for exploring Earth change (Table 1-1).
Table 1: NISAR characteristics
The NISAR flight system design, development, integration, testing and operations are a joint venture, with equivalent-scale contributions from both JPL and ISRO. The suite of flight systems consists of the launch vehicle and free-flying observatory. The NISAR observatory is designed around the core payloads of Land S-band SAR (Synthetic Aperture Radar) instruments, designed to collect near-global radar data over land and ice to satisfy the Level 1 science goals. In addition to the two radar instruments, the NISAR payload includes a GPS (Global Positioning Ssystem) receiver for precision orbit determination and onboard timing references, a solid-state recorder, and a high-rate data downlink subsystem to enable transmission of the high-volume science data to the ground. 7) 8) 9)
Figure 1: NISAR spacecraft in deployed configuration, with annotation of key instrument elements. Spacecraft bus is provided by ISRO. There are 24 L-band transmit receive modules (12 per polarization) and 48 S-band modules (image credit: NASA/JPL, ISRO) 10)
Figure 1 shows the fully integrated and deployed observatory system. The 12 m RAR (Radar Antenna Reflector) is at top, supported by the RAB (Radar Antenna Boom). The boom is attached to the RIS (Radar Instrument Structure), which is itself attached to the ISRO I3K Spacecraft Bus. Extending on either side of the bus are two solar arrays each with three panels that together supply approximately 4 kW of power when illuminated (i.e. at all times when not in eclipse or off Sun-pointing). The radar payload integration (L-band and S-band integration) will occur at JPL, and the overall observatory integration will occur at ISAC (ISRO Satellite Center) in Bangalore, India. The main elements of the system are illustrated in Figures 1 and 2.
Figure 2: Spacecraft in stowed configuration (image credit: ISRO, NASA)
Spacecraft mass of ~ 2800 kg. Mission life = 3 years (5 years consumables). A modified I3K spacecraft bus of ISRO is used. The NASA project management is at JPL.
The NISAR spacecraft will accommodate two fully capable synthetic aperture radar instruments (24 cm wavelength L-SAR and 10 cm wavelength S-SAR), each designed as array-fed reflectors to work as SweepSAR scan-on-receive wide swath mapping systems. The mapping scenario calls for frequent sampling over broad areas to create a time series and allow for noise reduction through stacking methods. Thus, a high-rate instrument and data downlink systems are required. The average capacity of the envisioned data downlink is of the order of 26 Tbit/day, supporting the instruments which can produce at L-band from 72 Mbit/s in its lowest bandwidth mode to over 1500 Mbit/s in the most demanding high-bandwidth, multi-polarization mode. Table 2 summarizes the overall mission characteristics.
NASA contributions include the L-band SAR instrument, including the 12-m diameter deployable mesh reflector and 9-m deployable boom and the entire octagonal instrument structure. In addition, NASA is providing a high capacity solid-state recorder (with 9 Tbit capacity at end of life), GPS receiver, 3.5 Gbit/s Ka-band telecom system, and an engineering payload to coordinate command and data handling with the ISRO spacecraft control systems. ISRO is providing the spacecraft and launch vehicle, as well as the S-band SAR electronics to be mounted on the instrument structure. The coordination of technical interfaces among subsystems is a major focus area in the partnership.
Figure 3: Artist's concept of the deployed NASA–ISRO NISAR spacecraft (image credit: NASA/JPL-Caltech)
Table 2: Overview of NISAR mission characteristics
• May 26, 2018: According to a GAO (Government Accountability Office) assessment, the joint collaboration between NASA and ISRO to orbit an advanced SAR (Synthetic Aperture Radar) imaging satellite is moving forward toward a 2021 launch date. 11)
- "The NISAR project continues to track a risk that process differences between NASA and its development partner, the Indian Space Research Organisation (ISRO), could negatively affect cost and schedule, but a recent project assessment concluded that collaboration between the two organizations has been effective," the GAO report stated.
- "For example, in July 2017, the project signed an updated cooperative project plan that outlines how the two organizations should interface on topics such as requirements and technical information," the assessment added.
- The satellite "will study the solid Earth, ice masses, and ecosystems. It aims to address questions related to global environmental change, Earth's carbon cycle, and natural hazards, such as earthquakes and volcanoes," the review said."The project will include the first dual frequency synthetic aperture radar instrument, which will use advanced radar imaging to construct large-scale data sets of the Earth's movements."
• January 30, 2018: Northrop Grumman's Astro Aerospace has successfully completed a CDR (Critical Design Review) of the AstroMesh radar antenna reflector for the NISAR (NASA-ISRO SAR) satellite. Post CDR, the program will move into the "build" phase of the AstroMesh radar antenna reflector in preparation for the scheduled 2021 launch date. Northrop Grumman will use its proprietary AstroMesh deployable mesh reflector for NISAR's large aperture antenna, building an ultralight and extremely stiff reflector suited for high frequency communications and radar applications. 12)
• Aug. 16, 2016: Astro Aerospace, a Northrop Grumman Corporation company, has completed the PDR (Preliminary Design Review) of the AstroMesh® radar antenna reflector for the NISAR (NASA-ISRO Synthetic Aperture Radar) satellite. The antenna reflector, furnished by Astro Aerospace, is part of the NISAR L-band synthetic aperture radar managed by NASA's Jet Propulsion Laboratory.
• NISAR passed its Preliminary Design Review (PDR) on 21-23 June 2016. Key Decision Point-C (KDP-C), the review to confirm the mission for detailed design and development, was held on August 23, 2016 (Ref. 7). NISAR is currently in Phase C: Subsystem developments and instrument prototyping. 13)
• In April 2016, a contract was signed between Airbus DS and JPL with the objective to provide the latest generation flash memory SSR (Solid State Recorder) of Airbus DS for the NISAR mission. So far, the flash-based SSR of Airbus DS has exceeded 40 months of operation in orbit onboard SPOT-6 (launch on Sept. 9, 2012), the first commercial satellite to deploy this technology. The Airbus DS flash-based mass memory products have also been launched on SPOT-7 (launch on June 30, 2014) and on Sentinel-2 of ESA (launch June 23, 2015). This product will offer more than 10 Tbit storage capacity in a unit with a mass < 25 kg. 14) 15)
- Compared to the previous SDRAM-based generation, this flash-based solution offers 60% better performance, is also 2.5 times lighter, 5 times smaller and consumes 3.5 times less power. Widely used in mass-market electronics, flash technology has now proven that it meets the very strict quality standards required for space missions fulfilling all requirements in orbit.
• Status in the spring of 2015: Formulation (Phase B) preliminary design. 16)
Launch: A launch of NISAR is planned for December 2021 on an ISRO GSLV (Geosynchronous Satellite Launch Vehicle) Mark II launch vehicle of ISRO from SDSC (Satish Dhawan Space Center) in Sriharikota, India. 17)
Orbit: Sun-synchronous dawn-dusk orbit, altitude of 747 km, inclination = 98.4º, LTAN at 18 hours, repeat cycle of 12 days.
Figure 4: A snapshot of the Reference Science Orbit orbital elements at the first ascending equator crossing are given in the following table and are specified in an Earth-Centered True Equator and Equinox of Epoch coordinate frame. During every 12-day repeat cycle, NISAR will execute 173 orbits, which will provide global coverage of the Earth (image credit: NASA/JPL-Caltech)
During science operations, NISAR will fly within a diamond-shaped orbital corridor defined for each of the repeat cycle's 173 orbits and tied to the rotating Earth (Figure 4-3). This corridor is defined to enable accurate correlation of science observations from pass-to-pass and cycle-to-cycle, supporting assessment of changes in the science targets. The dimensions of the diamond were calculated as an upper bound on acceptable error produced by a non-zero baseline between passes/cycles between three primary factors of phase unwrapping error, geometric decorrelation and topographic leakage, but ultimately dominated by the former (phase unwrapping error, i.e., high fringe rate in regions of large topographic relief).
The center of the Diamond is defined by the 173-orbit reference trajectory (referred to as the Reference Science Orbit), which is fixed to the Earth's surface and is exactly repeated every 12 days. The Diamond can be thought of as a fixed altitude, longitude and latitude profile that spans the entire repeat cycle; a conceptual representation of this corridor is shown in Figure 5. To maintain the Diamond, the JPL Navigation team plans on executing maneuvers over the long ocean passes (Atlantic and Pacific) as much as possible not to impact science data collection (Ref. 7).
Figure 5: Actual versus reference trajectory for NISAR as maintained within the diamond (image credit: NASA/JPL-Caltech)
Commissioning Phase: The first 90 days after launch will be dedicated to Commissioning, or IOC (In-Orbit Checkout), the objective of which is to prepare the observatory for science operations. Commissioning is divided into sub-phases of Initial Checkout (ISRO engineering systems + JPL Engineering Payload checkout), Deployments, Spacecraft Checkout and Instrument Checkout. Philosophically, the sub-phases are designed as a step-by-step buildup in capability to full observatory operations, beginning with the physical deployment of all deployable parts (notably the boom and radar antenna, but not including the solar arrays which are deployed during Launch Phase), checking out the engineering systems, turning on the radars and testing them independently and then conducting joint tests with both radars operating.
Figure 6: Mission timeline and phases for NISAR. The mission timeline for NISAR will be divided into launch, a 90-day commissioning or in-orbit checkout period, followed by 3 years of nominal science operations, and 90 days of decommissioning (image credit: NASA/JPL)
Science Operations Phase: The Science Operations Phase begins at the end of Commissioning and extends for three years and contains all data collection required to achieve the Level 1 science objectives. During this phase, the science orbit will be maintained via regular maneuvers, scheduled to avoid or minimize conflicts with science observations. Extensive Calibration and Validation (CalVal) activities will take place throughout the first 5 months, with yearly updates of 1-month duration.
The observation plan for both L- and S-band instruments, along with engineering activities (e.g., maneuvers, parameter updates, etc.), will be generated pre-launch via frequent coordination between JPL and ISRO. This plan is called the "reference mission;" the science observations alone within that reference mission are called the "reference observation plan" (ROP). The schedule of science observations will be driven by a variety of inputs, including L- and S-band target maps, radar mode tables, and spacecraft and ground-station constraints and capabilities. This schedule will be determined by JPL's mission planning team, and the project will endeavor to fly the reference mission, which includes these science observations exactly as planned pre-launch (accommodating for small timing changes based on the actual orbit). Periodic updates are possible post-launch which will lead to a new reference mission.
Routine operations of NISAR are dominated by Orbit Maintenance Maneuvers, science observations and data-downlink. Additional activities will include continuous pointing of the Solar Array to maximize power, continuous zerodoppler steering of the spacecraft, and potential periodic yaw-turns to shift from left-looking vs right-looking attitudes to support phases of science observations.
Decommissioning Phase: Decommissioning phase begins after the 3-years of the primary science phase and after any extended operations phase (e.g. NASA Senior Review) have concluded. This phase extends for 90 days. NASA deorbit and debris requirements are not applicable for NISAR, however the project must comply with ISRO's guidelines to safely end the mission. ISRO adheres to the IADC Space Debris Mitigation Guidelines, IADC-02-01, Revision 1, September 2007.
Sensor complement (L-SAR, S-SAR)
Background: Conventional SAR systems suffer from limitations in swath width and resolution: generally one cannot have both wide swath coverage and high resolution. In designing SAR systems, one usually has some trade-off between the two, because the limited antenna dimensions constrain the trade space. ScanSAR techniques allow wider swaths employing a burst mode of data acquisition, with the radar beam pointed electronically to different subswaths in elevation. ScanSAR achieves wider swath at the expense of degraded azimuth resolution or fewer looks. It was first demonstrated on NASA/JPL's SIR-C mission in 1994 and was one of several key techniques that enabled the follow-on SRTM mission to successfully measure more than 80% of the Earth's topography. 18)
NASA and ISRO will share science and engineering data captured at their respective downlink stations, and each organization will maintain their own ground processing and product distribution system. The science teams and algorithm development teams at NASA and ISRO will work jointly to create a common set of product types and software. The project will deliver NISAR data to NASA and ISRO for archive and distribution. NASA and ISRO have agreed to a free and open data policy for these data (Ref. 7).
Table 3: Major instrument characteristics for NISAR
The L-SAR (L-band Synthetic Aperture Radar) instrument is the focus of the NASA-chartered science goals for NISAR. To meet these goals, it will be heavily utilized during the mission. Current mission scenarios have the instrument on and collecting data for 45-50% per orbit on average, with peaks as high as 70%.
The L-SAR is a side-looking, fully polarimetric, interferometric synthetic aperture radar operating at a wavelength of 24 cm. The L-SAR is capable of 242 km swaths, 7 m resolution along track, 2-8 m resolution cross-track (depending on mode) and can operate in various modes including quadpolarimetric modes, i.e. transmitting in both vertical and horizontal polarizations, and receiving in both the same polarizations transmitted, and cross-polarizations. A cross polarization mode, for example, receives the horizontally polarized component of the return signal when vertically polarized pulses were transmitted, and vice versa. From the NISAR science orbit, the instrument's pointing accuracy is such that the L-SAR data can be used to produce repeat-pass interferograms sensitive to large-scale land deformation rates as small as 4 mm/year.
To meet the requirements of all science disciplines, the L-SAR radar instrument is designed to deliver fast sampling, global access and coverage, at full resolution and with polarimetric diversity. The technological innovation that allows this performance is the scan-on-receive "SweepSAR" design, conceived and refined jointly with engineering colleagues at the DLR (German Aerospace Center) under the DESDynI study phase.
SweepSAR (Figure 7) requires the ability to receive the echoed signal on each element independently, such that localized echoes from the ground can be tracked as they propagate at the speed of light across the swath. As an echo moves from receive element to receive element, the signals from neighboring elements must be combined to form a continuous record of the echo. Given the width of the swath (~244 km), returns from two or more echoes must be processed simultaneously. This operation is best performed using digital combining techniques, so the received echo is digitized immediately upon reception, filtered, decimated, and then sent to a signal combiner.
On transmit, the entire radar feed aperture is illuminated, which creates a narrow strip of radiated energy on the 12 m reflector that illuminates the full 242 km swath on the ground. On receive, the echo illuminates the entire reflector, and that energy is focused down to a particular location on the radar feed aperture depending on the timing of the return. The narrowness of the receive beam on the ground (due to the wide reflector illumination) minimizes ambiguity noise so that individual pulse can be tracked separately as they sweep across the feed.
Figure 7: Sweep-SAR technique illustration of enabling "SweepSAR" concept, which allows full-resolution, multi-polarimetric observations across an extended swath (> 240 km). By transmitting energy across the full feed aperture, a wide swath is illuminated on the ground. Each patch element on the feed can receive independently, allowing localization in time, hence space, of the return echo scattered from the ground. Note: Transmit and Scanning Receive events overlap in time and space. Along-track offset shown is for clarity of presentation only (image credit: NASA/JPL)
The SweepSAR L-band and S-band radars are being designed to work independently or together. The L-band hardware will be built at JPL, and the S-band electronics portion at ISRO. The feed apertures at L- and S- band are built by JPL and ISRO, respectively, as well phase-matched to their respective electronics and cabling. In this sense, each radar is a self-contained instrument up to the radiated energy from the feed aperture. Thereafter, both will share the same reflector, with a nearly identical optical prescription (F/D=0.75). Because a distributed feed on a reflector-feed antenna has a single focus, much of the radiated and received energy is not at the focus. Since S-band wavelength is 2.5 times shorter than L-band, yet the feed is the same length to achieve identical swath coverage, the S-band system has greater deviations from the focus. Thus, the design has been iterated to derive the best offset, tilt and phasing of each radar to balance the performance across the two systems. This analysis has been done independently by the JPL and ISRO teams, then cross-compared to validate.
For the radars to operate together as a dual-frequency system, it is necessary to share oscillator and timing information to lock their pulse repetition frequency together, which will be done with simple interfaces. Another concern is the coupling between the feed apertures. In the current design, the two apertures will be mechanically and electrically separated, to keep the coupling manageable.
Filtering, decimation, calibration estimation and combining are done in a set of FPGAs or ASICs on each radar. This complication exists for both L-band and S-band and leads to a multiplicity of parallel processing efforts in the spaceborne electronics. The SweepSAR technique was demonstrated in an airborne configuration to show its efficacy. 19)
With SweepSAR, the entire incidence angle range is imaged at once as a single strip-map swath, at full resolution depending on the mode, and with full polarization capability if required for a given area of the interest. Azimuth resolution is determined by the 12-m reflector diameter and is of order 8 m.
Because the radar cannot receive echoes during transmit events, there are one or more gaps in the swath if the radar's pulse rate is fixed. NISAR has the ability to vary the pulse rate in order to move the gaps around over time. The data can then be processed to gapless imagery by interpolating across the gaps.
Over most of the world, the instruments will be operated independently. The requirements for range resolution, polarization and radar modes supported by the instrument are science target dependent. The instrument supports a fixed set of polarizations and bandwidth combinations of those listed in Table 4. The physical layout of the payload is depicted in Figure 8.
Table 4: Supported polarizations and bandwidth combinations
Figure 8: NISAR instrument physical layout (image credit: NASA, ISRO)
The Shuttle Imaging Radar-C was the first orbiting multi-frequency, multipolarization SAR around Earth and demonstrated the value of having multiple wavelengths. Possible benefits include:
• Use of S-band in polar regions can reduce the impact of the ionosphere, since the S-band signal will be 5 times less sensitive than L-band to ionospheric perturbations.
• Use of L-band and S-band jointly will allow a good estimate of the ionosphere using dual-band mitigation techniques (Rosen et al., 2010).
• Use of L-band and S-band jointly to extend the range of sensitivity for biomass estimation and surface deformation, and aid in estimating soil moisture.
• Use of L-band and S-band jointly to study differential surface roughness and volume scattering effects, improving classification of natural surfaces.
• Use of L-band and S-band jointly or separately to study decorrelation rates of natural surfaces, improving the utility of interferometry for change detection, and change classification.
These capabilities will provide researchers with a fundamentally new global (at L-band) and globally distributed (at S-band) data set for research. It is important to note that the system downlink is at present fully tasked, so opportunities for dual-band collection must be balanced against alterations to the nominal observation plan.
NISAR's SweepSAR scan-on-receive timing is illustrated in Figure 9 for a typical L-band mode of operation – dual-pol (HH/HV) 20 MHz bandwidth split spectrum mode. In this mode, the 20 MHz signal is positioned at one end of the allowable spectrum and a separate 5 MHz bandwidth signal is transmitted sequentially in time at the opposite end of the spectrum (Table 5). This is performed using linear frequency modulated waveforms with identical chirp rates, but with different pulse extents: the 20 MHz pulse extent is 20 ms, while the 5 MHz pulse extent is 5 µs, giving a total pulse extent of 25 µs. This split spectrum approach is implemented for the purpose of ionospheric correction of interferometric products. The 20 MHz band provides the main dual-pol observation, with repeated observations over time allowing interferometric comparison, while the 5 MHz band of the same co- and cross-pol measurements is used primarily for forming low-resolution maximally band-separated dual-pol interferograms. By unwrapping and scaling the phase of the 5 MHz band, it is possible to derive an estimate of the ionospheric contribution to the interferogram. 20) 21)
Figure 9: Time and spatial scales at issue in NISAR's SweepSAR implementation, and illustration of gaps in the swath for fixed PRI operations. NISAR's PRI is 606 μsec for the nominal mode of operation, which corresponds to a range distance of around 91 km, or 140 km – 180 km of ground range, depending on the incidence angle, while the full extent of the received echo is greater than 240 km ground range. The pulse duration for nominal modes is 25 µs, corresponding to a 3.7 km range extent over which the receivers are blanked to avoid interference by the transmit pulse, leading to blind range gaps of 5.6 km (far range) – 6.8 km (near range) on the ground. Other PRIs and pulse durations lead to different spacing and extent of gaps (after M. Villano, DLR).
Note that since NISAR has dual waveform generators, it is possible also to create HH and VV signals simultaneously with different center frequencies. This allows 20 split-band operation (HH/HV in one band and VV/VH in the other band) using a 20 µs pulse duration for each. This so-called "quasi-quad pol" mode of operation is being considered for optimizing some science retrievals.
Table 5: System and processing parameters for nominal dual-pol operation of NISAR
The PRI (Pulse Repetition Interval) for this mode of 606.06 µs corresponds to the 1650 Hz pulse repetition frequency needed to balance ambiguities against data rate. The entire incidence angle range is imaged at once as a single strip-map swath, at full resolution depending on the mode, and with full polarization capability if required for a given area of the interest. Because the received echo extends in time longer than the PRI, transmit events will occur during the receive window. The receivers are blanked during these transmit events creating blind ranges as illustrated in Figure 9.
For fixed PRI operations, these gaps are persistent strips of blanked ranges. The extent of the gaps is doubled in processing: Because range resolution is achieved by a matched filtering operation with a replica of the transmitted chirp, data near the edges of the gap are only partially resolved. The impact of gaps are therefore felt over a 13.4 km extent (mean over the swath), with full resolution at the edges, degrading to coarser and coarser resolution as the middle of the gap is approached from each edge.
For the purpose of tracking requirements, the gap is bookkept as containing no useful data. Hence in Figure 10, which shows the noise performance of the L-band radar, there are two gaps in the curves indicating this 13.4 km region. This creates three distinct subswaths, with no coverage in the gaps unless the PRF is altered in some way to move the gaps around . 22) 23) Ascending and descending coverage can mitigate this coverage loss to an extent. This paper discusses the implications of these gaps on achieving NISAR science objectives and on the overall usability of the data.
Figure 10: NESZ (top) and Ambiguities (bottom) for the dual-pol mode and constant PRI operation (image credit: NASA)
Impact of gaps:
NISAR has an observation plan that covers all Earth's land and ice covered surfaces at least once each 12-day cycle from the ascending and descending portions of the orbit. Except for the fine resolution observations of ice sheets in the polar extremities, there is no attempt to narrow the swath of any acquisition as a function of latitude, though above 60 degrees north latitude, every other observation is culled since there would be complete redundancy in these observations. The fact that the ground tracks converge with increasing latitude does not help to fill the gaps in by combining ascending-only or descending-only swaths, as it does to build up overlap at the swath edges, since gaps are at a fixed ground range and don't converge with latitude. When there is substantial swath overlap approaching 60 degrees north, these gaps may be filled in by neighboring swaths, but then above 60 degrees, the swath culling will open the gaps again. Thus for ascending-only or descending-only coverage, certain regions of Earth will simply not be mapped. The two gaps in the dual-pol swath are roughly 10% of the total area, so 10% of Earth would not be covered from ascending or descending directions in any given 12-day cycle. The gap could be moved around from cycle to cycle, to ensure every area is mapped.
Most areas however will be covered by using both ascending and descending observations. Given that ascending swaths will fill in at least 90% of the area missed by descending passes, and vice-versa, only about 1% (10% of 10%) will not be mapped in the combined ascending or descending passes, leaving diamond-shaped holes. In this case orbit convergence will help to fill in gaps in this 1% in the higher latitudes.
Science Performance: The NISAR mission is designed to meet a specific set of measurement requirements, which are summarized in Table 1 (Ref. 24), and which have been quite stable over the period of mission development. (NISAR is planned for launch in 2021 and is presently in its critical design phase.) The science measurements are characterized in terms of quantities that are derived from radar imagery, not in terms of the imagery itself: For solid earth and cryosphere objectives, the measurements are expressed in terms of vector displacements of the surface of Earth. For ecosystems, the measurements are in terms of biomass, disturbance, and areal classification of wetlands and active agriculture. For all these requirements, the algorithmic approaches mandate the use of data acquired over time and from ascending and descending orbits to synthesize the data products. As such having gaps in the swath of any given scene do not necessarily mean that requirements cannot be met. 24)
The NISAR project has developed an end-to-end science performance estimation tool that utilizes the mission observation plan, the performance of the radar instrument (for example as shown in Figure 10), and a detailed error model for propagating radar measurement errors to errors in the science measurements described above. In the model simulations to date, the gaps in the swath are assumed to exist. In all cases, the impact of the gap is felt in loss of samples in time and space, but the models predict that requirements can be met. Figure11 shows an example of a simulation output for biomass estimation, wherein a year of dual-pol observations are used to estimate biomass from the observed backscatter.
Figure 11: Example of output of the NISAR biomass performance model showing accumulated histogram of errors over the globe based on the planned observations and including transmit gaps. The requirement specifies that 80% of the regions where biomass is below 100 Mg/ha must be have an error better than 20 Mg/ha. The final paper will show similar results for other requirements (image credit: NASA)
In summary, the gaps that exist in the NISAR swaths have impacts on the continuity of imagery in space and time, but the requirements of the mission can be met formally. Scientists typically prefer to work with full-coverage imagery, however, but for NISAR may prefer higher quality imagery with gaps to continuous images that results from varying the PRI and processing across the gaps. Further work, to be presented in the final paper, is required to explore various mitigation strategies and their impacts on the science performance.
GDS (Ground Data System)
The GDS includes the tracking stations, data capture services, the communications network and end party services (Figure 12). The stations, services and communications are NASA multi-mission capabilities managed by GSFC (Goddard Space Flight Center). The GDS will send the raw science data to the SDS (Science Data System), which converts the downlinked raw data into Level 0a and Level 0b data that are the starting point for the science data processing Ref. 7).
Figure 12: NISAR ground stations (including the NASA Near-Earth Network stations in Alaska, Svalbard and Punta Arenas; ISRO stations in Antarctica, Shadnagar, Bangalore, Lucknow, Mauritius, Biak), control center and launch location (Sriharikota (SDSC), India) , image credit: NASA/JPL
Science Data System:
The SDS converts the Level 0b data into Level 1 to Level 2 science data products2 that the NISAR mission provides to the science community for research and applications. The SDS facility is designed to process data efficiently and distribute data products in a timely manner to the community as required to meet mission objectives. The SDS facility includes computer hardware dedicated to operational data production. The SDS facility is planned as a cloud-based hybrid SDS, with all elements cloud-enabled. This allows for some processing to be done at JPL and some to be distributed to the external cloud. The science and algorithm development teams will have access to cloud instances separate from the production instances to enhance algorithmic accuracy and performance.
The SDS is controlled through a cloud-based production management system at JPL ( Jet Propulsion Laboratory) in Pasadena, California. JPL is responsible for implementation of software to generate Level 1 radar instrument data products and Level 2 products. The science team is responsible for generating Level 3 geophysical data products for calibration and validation purposes. As funds permit, software for Level 3 products may be migrated to the production system to generate larger areas of Level 3 products.
To facilitate the software development process, the SDS will establish a mechanism for developmental instances of the SDS to be made available to the algorithm development and science teams. These developmental instances will be logically separate from the production system but will allow development and testing of the software that will be used to automatically generate the science data products once NISAR is in orbit.
NASA NEN (Near Earth Network): NISAR will downlink both to ISRO ground stations (see below) and to NASA (Near Earth Network ) stations. For the NASA stations, Ka-band antennas will be used at one or more complexes. The specific antenna complexes currently
JPL MOC (Mission Operation Center): JPL will perform mission operations from multiple buildings at JPL in Pasadena, California, all of which are considered to make up the MOC. The existing multi-mission EOMOC (Earth Orbiting Missions Operation Center) will provide operations teams with consoles, workstations, voice and video displays. Navigation and GPS operations will be conducted from other JPL locations.
JPL Science Data Processing Facility: JPL science data processing will be done using the JPL SDS (Science Data System). SDS software and storage will be hosted by cloud services, likely AWS (Amazon Web Services) in Oregon.
NASA DAACs (Distributed Active Archive Centers): NASA's EOS (Earth Observing System) operates DAACs around the United States and has been interoperating with foreign sites. For NISAR, the ASF (Alaska Satellite Facility) DAAC has been selected. The DAAC will utilize AWS cloud services for processing, storage and distribution.
ISTRAC (ISRO Telemetry, Tracking & Command Network): The ISRO ISTRAC facility in Bangalore will be used for spacecraft operations and to schedule and operate a set of S-band Telemetry, TTC (Tracking & Commanding) stations.
NRSC (National Remote Sensing Center): The ISRO NRSC (National Remote Sensing Center) operates an Earth science acquisition, processing and dissemination center in Hyderabad. For NISAR, this center operates two Ka-band stations as part of their IMGEOS (Integrated Multi-Mission Ground segment for Earth Observation Satellites), one near NRSC in Shadnagar, India, and another remote station in Antarctica. The station in Shadnagar is also referred to as the SAN (Shadnagar Acquisition Network).
SDSC (Satish Dhawan Space Center) SHAR (Sriharikota Range): SDSC SHAR, with two launch pads, is the main launch center of ISRO located at 100 km north of Chennai. SDSC SHAR has the necessary infrastructure for launching satellite into low earth orbit, polar orbit and geostationary transfer orbit. The launch complexes provide complete support for vehicle assembly, fueling, checkout and launch operations.
WANs (Wide Area Networks): WANs will be used for long-distance exchanges among NISAR facilities. All WANs will consist of circuits carrying TCP/IP-based traffic.
The NISAR observatory's telecommunications system provides for one uplink path and three downlink paths. The uplink path is from ISRO's command center at ISTRAC through the observatory's S-band antenna mounted on the ISRO spacecraft bus. The three downlink paths are as follows: tracking and engineering telemetry, from the same S-band antenna back down to ISRO's spacecraft operations center at ISTRAC; instrument data from both L- and S-band systems, through the shared spacecraft Ka-band antenna (provided by ISRO) to ISRO's NRSC (National Remote Sensing Center) facilities near Hyderabad via ISRO's Ka-band ground stations at Shadnagar (India) and Antarctica; and the same instrument data and engineering telemetry through the shared spacecraft Ka-band antenna to NASA NEN (Near-Earth Network) stations (Figure 13).
ISRO's 2.88 Gbit/s Ka-band system provides for science data downlink to Indian ground stations with an effective information rate of 2.0 Gbit/s. The Ka-band downlink to NASA ground stations will be at 4.0 Gbit/s with and information rate of 3.45 Gbit/s via a JPL provided transmitter. ISRO supplies the Ka-band electronics and a 0.7m HGA (High Gain Antenna) mounted on the spacecraft's nadir surface to be used by both ISRO and JPL Ka-band transmitters, through a JPL provided and controlled switch. The antenna gimbal and control of the gimbal will be provided by ISRO. There will be 15 to 20 downlink sessions per day, with average session duration of less than 10 minutes. Note that there are separate Ka-band telecom transmitters, but they share the same Ka-band antenna. This system is fully redundant and cross-strapped except for the antenna and Ka-band gimbal.
Figure 13: NISAR telecommunications links include Ka-band downlink to NASA and ISRO stations at 4 Gbit/s and 2.88 Gbit/s, respectively, and S-band uplink and downlink from and to ISRO ground stations (image credit: NASA, ISRO)
Ka-band Communications: ISRO's NRSC facility operates an Earth science downlink and processing center in Shadnagar, India, near Hyderabad. It is also referred to as the SAN (Shadnagar Acquisition Station). This facility is the primary center for ISRO Ka-band communications from the observatory during nominal science operations. NRSC plans to place a Ka-band reception antenna on this facility within the existing IMGEOS (Integrated Multi-Mission Ground segment for Earth Observation Satellites) facility at SAN. ISRO also plans to use another Ka-band ground station (Bharati in Antarctica) for science data downlinks. Primary playback of science data, however, will utilize NASA stations of the NEN (Near Earth Network) at the ASF (Alaska Satellite Facility) and Svalbard (Norway). These stations are shown in Table 6 and Figure 14.
Table 6: NISAR Ka-band ground stations
Figure 14: Locations of NISAR Ka-band ground stations (NASA stations in Alaska, Svalbard and Punta Arenas, and ISRO stations in Shadnagar and Antarctica are shown), image credit: NASA, ISRO
Mission Planning and Operations
Since nearly all objectives are best satisfied with regular repeated observations of any given science target, the NASA-ISRO joint science team will create an overall science observation strategy that establishes a nominal repetitive observing baseline prior to launch. It is anticipated that the Joint Science Team will alter the nominal observation plan during the course of the mission. Applications and other government users may also request plan changes. The project team will strive towards accommodating these within the project constraints. These post-launch updates to the Reference Observation Plan will be applied on a quarterly or semiannual frequency basis, with accommodation of urgent response requests in response to natural hazards and other emergencies (Figure 15).
The Joint Science Team will rely on Mission Operations and the Project Science Team to understand the implications of any changes to the observation plan. Changes will be specified through target/mode/attributes as is currently done. Mission Operations Team will then rerun the mission scenario simulation to examine resource (power, thermal, data downlink, cost) constraint violations. The Project Science Team will apply the updated Candidate Observation Plan through the science performance models to see if there are any impacts to L1/L2 science requirements. If resource violations or performance impacts are identified, iteration will be required.
Figure 15: Flowchart showing steps to be followed for long-term re-planning of Reference Observation Plan. This process will be followed periodically (roughly every 6 months) for updating the Reference Observation Plan during operations (image credit: NASA, ISRO)
JPL will develop the coordinated observation plan that takes into account spacecraft power, maneuvers, data throughput sizing and availability of downlink channels. That plan will be sent to ISRO for uplink to, and execution on, the observatory. JPL manages all L-band SAR instrument operations, with the ISRO uplink station serving as a pass-through for L-band instrument commands. ISRO manages all S-band SAR operations. All instrument operations are guided by the coordinated observation plan, with specific commands/sequences to implement the plan developed by the respective organizations. Navigation is led by JPL, with maneuver design provided from JPL to ISRO to implement the maneuvers. Maneuver implementation is fed back to JPL as input for the next maneuver planning process. In the same vein, JPL provides the telecom sequence for the NASA-provided Ka-band telecom subsystem used for all science data downlink, while ISRO feeds back to JPL the ISRO-provided Ka-band telecom subsystem downlink contacts. JPL is responsible for producing the required science data specified by NASA and delivering them to NASA DAAC(s). The ISRO NRSC (National Remote Sensing Center) will process and distribute the required science data specified by ISRO.
Mission operations will be a joint JPL-ISRO effort. Day-to-day observatory operations will be conducted at the ISTRAC center in Bangalore. ISTRAC monitors and controls the spacecraft, downlinking spacecraft telemetry to a local archive from where JPL can pull data as needed. All science data is downlinked via the JPL Ka-band telecom, initially processed, and archived first in the JPL Science Data System, and then in the ASF DAAC, from where ISRO can pull the data as-needed. In addition, a subset of L-band and S-band data, specified by ISRO/ SAC (Space Applications Center),Ahmedabad, India, will be downlinked directly to India (NRSC ground station) via the spacecraft Ka-band telecom.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).