Iridium NEXT (Hosting Payloads on a Communications Constellation)
In 2007, Iridium Satellite LLC announced its plans to develop its Iridium NEXT constellation and start deployment in the timeframe 2015-2017. With the announcement came the offer of hosted payloads for government and scientific organizations. Iridium NEXT, in continuity to the current Iridium system of 66 satellites, will provide 24/7 real-time visibility over the entire Earth’s surface and its atmosphere. ICI (Iridium Communications Inc.) is the only MSS (Mobile Satellite Service) company offering global voice and data coverage. ICI owns and operates the constellation and sells equipment and access to its services. Satellites communicate with neighboring satellites via Ka-band ISLs (Inter-Satellite Links). Each satellite can have four ISLs: two to neighbors fore and aft in the same orbital plane, and two to satellites in neighboring planes to either side.
Hosted payloads on Iridium NEXT will provide an unmatched opportunity to meet Earth observation and government mission requirements in the near term at a fraction of the cost of designing, building, launching and maintaining dedicated platforms in space.
The Iridium NEXT system is expected to maintain Iridium’s current unique architecture that provides truly global coverage, with expanded capacity, higher data speeds, new services, flexible payload architecture capable of supporting future product enhancements, cost effectiveness in maintaining and operating the network, and a design to host secondary payloads.
Major parameters of the mission are given in Table 1. Each Iridium NEXT satellite has an allocation of 50 kg in mass, 30 cm x 40 cm x 70 cm in volume, 50 W of average power, and 100 kbit/s average data rate for each hosted payload.
Table 1: Hosted payload specifications of the Iridium NEXT Constellation
Background: Iridium is a MSS (Mobile Satellite Services) provider - the only network provider offering 100% worldwide coverage. The network is a very unique, resilient LEO (Low-Earth Orbiting) satellite constellation of 66 satellites plus in-orbit spares. The original Iridium constellation of 66 satellites plus 6 spares was launched between May 5, 1997 and May 17, 1998.
A comprehensive plan to replenish the Iridium constellation, known as Iridium NEXT will launch 66 new satellites to replace the current constellation, with launches expected to begin in 2015. Also planned are 6 in-orbit spare satellites and 9 ground spares. Iridium NEXT features increased subscriber capacity, higher data speeds, and capacity for hosting payloads.
Data handling for hosted payloads: Although the satellites in the Iridium system are primarily designed to support the Iridium communications mission, they have been adapted to accommodate hosted payload missions. Mission data and sensor telemetry and command data for these missions can be transported in near real-time utilizing the K-band network of crosslinks between satellites, feeder links to the ground, and teleports connecting the satellites through earth stations to a MPLS (Multiprotocol Label Switching) cloud called the Teleport Network.
Iridium operations manage deployment and operation of the Iridium system. Iridium NEXT would retain the capability to turn off the hosted payload, in an extreme emergency situation, to preserve the health of the Iridium satellite. Iridium enables a hosted payload command and data path to an MPLS cloud. A customer designated sensor operations facility would manage the hosted payloads in-orbit on the Iridium NEXT satellite using the command and data path provided by Iridium operations. These functions include:
- Sensor operations tables
- Updating software or firmware
- Data stream management (pull or push from the MPLS cloud)
- Anomaly resolution.
This Hosted Payload Operations Center will provide the data processing capability for the sensor data. It will receive the data stream from the MPLS cloud and processes the data for end users. End users can provide feedback to sensor operations and data processing.
Benefits: The hosted payloads offer a customer the following value proposition:
• Unprecedented geospatial and temporal coverage : 66 interconnected satellites with coverage over the entire globe
• Low latency : Real-time relay of data to and from payloads in space
• User control : Data collection and hosted payload access seamlessly through Iridium infrastructure or private gateways
• Cost effective : Access to space at a fraction of the cost of a dedicated mission
• Exclusive : No other opportunity like this is likely to become available in the coming decades.
• Consistent with 2010 U.S. President’s National Space Policy : Commercial capabilities, cost effective access.
Iridium Next SensorPod: A SensorPOD is a virtual container (enclosure) that is a designated subset of the total Iridium NEXT hosted payload volume and is applicable for small payloads and payload suites that only require a small portion of the available volume (Figure 1).
SensorPod “containers” can be arranged in varying configurations (i.e., stacked like blocks) to support many different customer experiments. SensorPod geometries can also be scaled on a case-by-case basis to accommodate specific customer payload needs. SensorPods are designed to be located and oriented in the hosted payload volume to provide both nadir and/or RAM FOV (Field-of-View) options. An example of a notional configuration which includes a combination of a “primary” nadir-viewing SP and multiple “secondary” SensorPods is shown in Figure 2.
Figure 1: SensorPod of Iridium Next (image credit: Iridium Satellite)
Figure 2: Schematic layout of an Iridium Next SensorPod (image credit: Iridium Satellite)
Customers may fill each SensorPod volume with one or several payloads as long as they remain within the overall sensor volume, mass, power and communication allocations. Customer payloads are provided with mechanical and thermal interface routing plates and conditioned electrical power and communications via a hub through external harnesses.
In June 2010, TAS (Tales Alenia Space) of France was awarded a contract from ICI (Iridium Communications Inc.) for the design and construction of 81 satellites — 66 operational satellites, six in-orbit spares, and an additional nine ground spares. In turn, TAS has selected OSC (Orbital Sciences Corporation) as a subcontractor for the integration of Iridium NEXT satellites and the hosted payloads in a facility located in Gilbert, AZ. 9) 10) 11) 12) 13) 14)
Iridium has also signed the largest single commercial launch deal ever with Space-X (Space Exploration Technologies Corp.) to be the primary launch services provider for Iridium NEXT.
In addition, Iridium entered into two comprehensive, long-term agreements with The Boeing Company for maintenance, operations, and support of Iridium’s satellite network. Under the first agreement, Boeing will continue operating Iridium’s current satellite constellation and will provide support for Iridium’s satellite control system. The second agreement is a new support services contract under which Boeing will become the exclusive operations and maintenance provider for Iridium NEXT. The combination of these agreements allows Iridium to benefit from having a single operator during the transition from the current constellation to Iridium NEXT.
Table 2: Specification of Iridium NEXT spacecraft (Ref. 11)
In Feb 2011, Iridium Communications announced that Orbital Sciences signed an agreement with Iridium that reserves hosted payload capacity on Iridium's next-generation satellite constellation, Iridium NEXT.
Orbital, as the satellite integrator and test sub-contractor for Iridium NEXT, will also be responsible for the integration of hosted payload platforms with the Iridium NEXT satellites. Orbital's role as the satellite integrator is critical to ensuring that multiple hosted payloads, including Orbital's capacity, can be accommodated simultaneously on the Iridium NEXT constellation. 15)
Launch: Iridium has contracted with SpaceX to launch the constellation on Falcon-9 v1.1 boosters in the timeframe 2015-2017. Ten satellites are on each launch and seven missions are planned.
Orbit: Circular polar orbit, altitude = 780 km, inclination = 86.4°, period = 101 minutes (the spacecraft are positioned in 6 orbital planes).
Figure 3: Artist's rendition of the Iridium NEXT spacecraft (image credit: OSC)
The 66-satellite main constellation (+6 in-orbit spares), configured in 6 orbital planes with 11 evenly spaced slots per plane, provides continuous global coverage as demonstrated by the RF footprints in Figure 4. This is achieved though cross-linked satellites operating as a fully meshed network that is supported by multiple in-orbit spares to provide real-time data downlink to the Iridium operated ground station network. The constellation has a design lifetime greater than 10 years in a polar orbit at 780 km with an inclination of 86.4°.
Figure 4: Illustration of RF overlapping footprints of the Iridium NEXT satellite constellation (image credit: Iridium)
Figure 5: Orbital coverage of the Iridium NEXT constellation of 66 spacecraft (image credit: Iridium Satellite)
Hosted Payload Missions:
In 2008, GEO (Group on Earth Observations) ' an international intergovernmental initiative with the goal of furthering the creation of a comprehensive, coordinated, and sustained Earth observing system or systems ' concluded that four missions stand out as prime candidates for flying on the Iridium NEXT platforms which would also be of benefit for climate observation. These are altimetry, broadband radiometry (Earth’s radiation budget), multispectral imaging (ocean and land) and GPS radio occultation. 16) 17) 18) 19)
There are several additional missions which could provide additional climate/weather observations of interest to various groups. The consensus was that a constellation approach to sensing, using the real-time communications backbone of Iridium, will enable unprecedented geospatial and temporal sampling, with a move from R&D-driven space programs to operational monitoring of the effects of global climate change. 20)
Figure 6: Timeline of the Iridium NEXT hosted payloads (image credit: Iridium Satellite, Ref. 19)
• Earth observation, atmosphere, and climate 21)
- GPSRO (GPS Radio Occultation). One or two instruments can be hosted in each plane (with GPS, GLONASS and Galileo tracking capability) 22)
- Ocean color
- Forest fire
- Earth radiation budget
- Ozone profile monitoring
- Solar irradiance
• Space weather and space situation awareness
• AMPERE for monitoring of the magnetosphere
• Low light imaging and cloud observations
• SensorPOD – small payloads 1-5 kg class in a 3U Cube volume, hosted on NEXT providing significantly more capabilities and longer mission life at low cost
• Aircraft monitoring –ADS-B receiver for next generation ATC/ATM
• AIS for maritime monitoring
On March 27-30, 2011, a GEOScan Planning Workshop was held in Annapolis, MD, USA. The dual theme concept of GEOScan involves System Science (SS) sensors on all 66 Iridium NEXT satellites as well as Hosted Sensor (HS) suites that can accommodate unique payloads in a standard, 14 cm x 20 cm x 20 cm [5.6 U] SensorPOD.
Each Iridium NEXT satellite has a total hosted payload allocation of 50 kg in mass, 30 cm x 40 cm x 70 cm in volume, and 50 W of average power. GEOScan is designed to fit into a hosted payload module, which has been allocated 5 kg in mass, 14 cm x 20 cm x 20 cm in volume, and 5 W of average power. In addition to these resources, the Iridium satellite design provides for an unimpeded 75° half-angle nadir FOV (Field of View), nadir pointing control to within 0.35° (pointing knowledge within 0.05°), spacecraft altitude control within 10 m, and spacecraft position control within 15 km (position knowledge within 2.2 km).
Table 3: Iridium NEXT hosted payload specifications and resource allocation for GEOScan
Figure 7: Allocation of the hosted payloads on the Iridium NEXT spacecraft (image credit: Iridium)
GEOScan (GEOscience Facility from Space) - a sensor suite of hosted payloads
GEOScan is a grassroots effort, envisioned as a National Science Foundation (NSF) globally networked orbiting observation facility utilizing the Iridium NEXT satellites, that will create a revolutionary new capability of massively dense, global geoscience observations. GEOScan capitalizes on the once-in-a-generation opportunity presented by Iridium with a facility that will benefit both society and a broad cross section of the scientific community through dramatic advancements in Earth and Space science. 23)
GEOScan, proposed as a globally networked orbiting facility utilizing Iridium NEXT’s 66-satellite constellation, will provide revolutionary, massively dense global geoscience observations and targets questions scientists have not been able to answer, and will not answer, until simultaneous global measurements are made. GEOScan dramatically lowers the logistical and cost barriers for transmitting “big data” from 66 satellites by using Iridium’s communications platform and COTS (Commercial-Off-The-Shelf )components. 24) 25) 26)
Background: A consortium led by researchers at JHU/APL ( Johns Hopkins University / Applied Physics Laboratory) is proposing a geoscience program that would give scientists the first continuous real-time look at the Earth's surface and atmosphere through a global network of sensors. GEOScan includes a "Hosted Sensor" program that provides an opportunity for university researchers and students, as well as small businesses, to address novel science topics and test new instruments and technologies in space at a low-cost. 27)
More than 100 volunteer scientists and engineers are working to implement the GEOScan program through a proposal to NSF. This committee, led by JHU/APL, is made up of scientific experts from a variety of geoscience disciplines including space, atmosphere, oceans and Earth science.
Four primary factors make this an unprecedented opportunity for geoscience discovery, while holding the potential to affect a paradigm shift in the way we conduct science from space: 28)
1) Truly global coverage provided by the constellation.
2) Massively dense-space-based measurements enable revolutionary new techniques such as tomographic imaging.
3) Because Iridium Inc. is a telecommunications company, the logistical and cost barrier of transmitting massive amounts of data from 66+ satellites is REMOVED.
4) Because the project plans to build nearly 70 2.5U GEOScan pods, advantage can be taken of the cost savings of scale for science from space instead of the highly costly "one of a kind" methods of the past.
Table 4: Key GEOScan advantages (Ref. 28)
Geoscience is at the dawn of a new era. Scientists are realizing that future discovery and understanding in geoscience research will rely on viewing the Earth as a complete and interactive system, rather than as a series of isolated observations of natural phenomena. There is growing scientific opinion that many of the open geoscience questions cannot be answered without global and continuous coverage of key measurements.
GEOScan science objectives: GEOScan will enable the next generation of discovery with the first globally networked orbital observation facility. GEOScan will:
• Measure Earth's outgoing radiation budget on a global scale with the temporal and spatial resolution necessary for studying the causal relationship between the Earth's infrared radiation and fast-evolving phenomena like clouds, dust storms and volcanic eruptions, as well as their effect on long term climate trends.
• Measure global mass flux variations at temporal and spatial resolutions that relate the flow of water mass in the oceans and atmosphere to secular trends in the global water cycle, cryosphere and climate.
• Image the Earth's radiation belt and plasma environment with an unprecedented temporal and spatial resolution that provides the details of the governing physical processes for large-scale global reconfigurations that drive space weather events and resultant societal effects.
Iridium’s Hosted Payload Program facilitates the effort, but it could be executed using any small-sat constellation. Each GEOScan sensor suite consists of 6 instruments:
1) a Radiometer, to measure Earth’s total outgoing radiation
2) a GPS Compact Total Electron Content Sensor, to image Earth’s plasma environment and gravity field;
3) a MicroCam Multispectral Imager, to provide the first uniform, instantaneous image of Earth and measure global cloud cover, vegetation, land use, and bright aurora
4) a Radiation Belt Mapping System (dosimeter), to measure energetic electron and proton distributions
5) a Compact Earth Observing Spectrometer, to measure aerosol-atmospheric composition and vegetation
6) MEMS Accelerometers, to deduce non-conservative forces aiding gravity and neutral drag studies.
The GEOScan system sensor suite is comprised of 6 instruments packaged to take advantage of the Iridium NEXT hosted payload allocation. This suite of instruments is designed to be batch manufactured to meet the cost and schedule constraints of the Iridium NEXT launch schedule and reduce costs through volume procurement, manufacture, integration, and test. The conceptual packaging of the suite of sensors is shown in Figure 8.
Figure 8: Illustration of the GEOScan sensor suite (image credit: GEOScan consortium)
Legend to Figure 8: GEOScan’s payload design uses a modular configuration for efficient assembly and testing. It also includes additional mass, power, data, and volume allocation for sensors proposed by scientific and government stakeholders.
CTECS (Compact Total Electron Content Sensors):
CTECS are GPS instruments that utilize a COTS receiver, modified firmware, a custom-designed antenna, and front-end filtering electronics. In a 24 hr period, a single GPS occultation sensor can provide several hundred occultations or total electron content (TEC) measurements distributed around the globe. Even with this number of occultations, latitude and longitude sectors still remain that are undersampled at any given instant because of the geometry of the GPS constellation.
GEOScan’s 66 CTECS will provide an unprecedented continuous global snapshot of Earth’s ionosphere and plasmasphere. The data will allow us for the first time to see the temporal and spatial evolution of the ionosphere/plasmasphere from 80-20,000 km with a 5 minute temporal resolution and 10 km height resolution with a measurement error < 3 TECU (Total Electron Content Unit) globally.
Furthermore, the gravity field will be derived using the satellites’ trajectories determined from the onboard CTECS GPS receivers, as well as from ancillary data from the MEMS accelerometers and Iridium star cameras. In short, the positions and velocities determined from the CTECS receiver can be differentiated to reveal the accelerations caused by the various dynamic (mass transport) processes that occur at the surface and in the atmosphere. By accurately tracking the orbit of each Iridium NEXT satellite and removing non-gravitational influences, the project can infer changes in Earth’s gravity field and learn about the processes that create these changes (e.g., large-scale water mass movement). Global diurnal water motion maps at 1000 km resolution, accurate to 15 mm of equivalent water height can be created on sub-weekly time scales with a time-integrated monthly resolution that matches the GRACE (Gravity Recovery and Climate Experiment) satellites.
Instrument: The CTECS sensor is a GPS receiver that has been designed to specifically make TEC (Total Electron Content), electron density altitude profiles, and ionospheric scintillation measurements. The CTECS sensor tracks the GPS L1 and L2 signals as they are occulted by the Earth. CTECS consists of a custom designed antenna, low-noise-amplifier, and the NovAtel OEMV-2 GPS receiver.
Table 5: CTECS specifications for GEOScan
The CTECS data downlink budget is customizable to the limitations of the spacecraft telemetry and is based on conservative estimates (e.g. scintillation occurs only at night but the calculation uses the mode at all times).
Note: The PSSC2 (Pico Satellite Solar Cell Testbed 2) nanosatellite served as a ow cost risk reduction for the upcoming SMC SENSE (Space Environmental Monitoring Nanosat Experiment) because it contains the Aerospace Corporation's CTECS (Compact Total Electron Content Sensor ) that characterizes the ionosphere by measurement of the occultation of GPS signals - a precursor of an instrument with the same function on SENSE. The PSSC2 nanosatellite of The Aerospace Corporation was deployed from the space shuttle flight STS-135 on July 20, 2011 (reentry on Dec. 7, 2011). After early orbit check-out, CTECS began normal operations mid August 2011. 29)
GEOScan will measure Earth’s outgoing radiation simultaneously and globally with a constellation of heritage-driven, two-channel radiometers carried by the Iridium NEXT commercial constellation of satellites. This constellation, each with a 127º field-of-view radiometer, will provide a global view of the Earth's TOR (Total Out-going Radiation) every 2 hours with better than 0.15% accuracy. The shortwave channel (0.2-5 µm) and total channel (0.2-200 µm) along with the longwave (determined form differencing the two channels) is calibrated to a precision of 0.09 Wm-2 with an accuracy of 0.3 Wm-2 using a NIST (National Institute of Standards and Technology) traceable calibration standard.
The GEOScan radiometer design draws direct heritage from the NISTAR instrument (TRL8) aboard DSCOVR (Deep Space Climate Observatory - aka Triana). NISTAR (National Institute of Standards and Technology Advanced Radiometer) is a high sensitivity, cavity-based radiometer, designed to measure the solar reflected and long-wave thermal emission from the full disk of the Earth as viewed from L1 (Lagrange point located on the Sun-Earth line 1.6 x 106 km from the Earth). The materials and construction techniques of the cavity detectors for GEOScan are based on those of the NISTAR detectors (TRL8).
Table 6: Radiometer design specifications for the GEOScan mission
Figure 9: Illustration of the radiometer (image credit: JHU/APL, Ref. 26)
CEOS (Compact Earth Observing Spectrometer):
CEOS can provide spectrally resolved information on the outgoing shortwave radiation from the Earth’s surface and atmosphere, which is critical fo calibrating climate models. CEOS can also determine the concentration of various aerosols and their effect on radiative transfer. As such, it will play an important role in achieving GEOScan’s goal of collecting high-quality data regarding the ERB (Earth’s Radiation Budget).
Aerosols represent an area of uncertainty within the climate modeling community. The IPCC (Intergovernmental Panel on Climate Change), in their 2009 report declared aerosols to be “the dominant uncertainty in radiative forcing.” Aerosols have direct effects on ERB, affecting atmospheric absorption, transmittance, and scattering of incoming and outgoing radiation. They also have indirect effects on ERB, as they act as condensation nuclei and affect cloud formation.
CEOS consists of an exceptionally small crossed Czerny-Turner spectrometer, a linear CCD array, and read-out electronics. Its modular design allows for easy substitution of optical elements, electronics, and optical sensors to rapidly and confidently customize the optical performance to meet a wide range of science goals. This design provides spectral measurements from 200 to 2000 nm with approximately 1 nm spectral resolution from 200 - 1000 nm and 3 nm from 1000 - 2000 nm. The foreoptics design provides a 1º FOV, which allows 14 km resolution.
CEOS has flight heritage on the O/OREOS CubeSat mission (launch Nov. 20, 2010) of NASA/ARC as part of the SEVO (Space Environment Viability of Organics) experiment, where it has been operating successfully on orbit so far. CEOS also shares heritage with the spectrometers on LCROSS and LADEE, two lunar science missions.
Figure 10: Photo of the CEOS spectrometer on the O/OREOS CubeSat (image credit: NASA/ARC)
MMI (Multispectral MicroCam Imager):
MMI is designed to provide multispectral images on both regional and global scales. The MMI provides multispectral imagery in the same footprint within a time of 30 s, each with a spatial resolution of ~450 m at nadir. The spacing of the satellites in the constellation (11 satellites per orbital plane), and the fact that one MMI is placed on every satellite, will allow complete multispectral global imagery to be acquired every 2 hours.
GEOScan’s MMI is a visible to near-infrared WFOV (Wide-Field of View) imager that uses a STAR-1000 CMOS imaging 1024 x 1024 array detector. MMI will use custom-designed strip filters oriented in the across-track direction. This will allow the imager to be used in a pushbroom mode. The attitude of the Iridium NEXT constellation will be carefully controlled because each of the satellites has cross-linked communication receivers and transmitters.
MMI uses refractive optics and will have a FOV of 33º x 33º to provide global coverage over a 2 h time interval. Each FOV footprint on the surface of the Earth is 465 km x 465 km. Images will be acquired every 29 s to provide continuous imaging in the along-track direction of the satellite. The trailing satellite in a given orbital plane lags the leading satellite by ~9 minutes. In this time interval, the relative drift in longitude between a spot on the Earth and Iridium is 250 km.
MMI requires a single 5 V power supply drawing 0.11 amp (0.55 W). This is lower power than typical cameras of this type. The MMI receives commands and transmits telemetry across an LVDS serial interface, which is commonly used between instruments and spacecraft. An Actel FPGA drives the detector control signals based on the command inputs, accepts detector image data and converts it to serial form to transmit. The FPGA is also capable of performing automatic exposure control, FAST compression or other simple algorithms. The camera produces a 1024 x 1024 pixel image with 10 bit pixels at one frame per second.
Figure 11: MicroCam with C-mount commercial optics (image credit: JHU/APL)
Dosimeter-based RBMS (Radiation Belt Mapping System):
GEOScan’s dosimeter payload will image radiation belt dynamics, including relativistic electron micro-bursts, global loss to the atmosphere, and variations in geomagnetic cutoffs of solar energetic particles. Each GEOScan payload will contain one pair of Teledyne micro dosimeters; one is an electron dosimeter with a 100 keV electronic threshold for registering a count, and the other is a proton dosimeter with a 3 MeV electronic threshold. A common thickness of shielding covers both dosimeters. Five unique lid-shielding choices will enable aggregating over multiple vehicles to obtain a dose-depth curve and electron and proton spectra. Shielding will be chosen to provide electron energy resolution from 0.3 to 5 MeV and proton resolution from 10 to 50 MeV. The dosimeter has more than sufficient dynamic range to measure the dose rate due to galactic cosmic rays at the geomagnetic equator for the most intense solar particle events and deep within the inner radiation belt.
Table 7: Teledyne UDOS001-K chip Dosimeter parameters
MASS (MEMS Accelerometer for Space Science):
MASS is a micromechanical, silicon-based accelerometer with unprecedented sensitivity compared to current accelerometers of similar size and power that can be used for Earth science applications. Current spaceborne accelerometers on GRACE and GOCE (Gravity field and steady-state Ocean Circulation Explorer) require more power (tens of watts) and mass (+50 kg) than can be accommodated within the GEOScan payload allocation. The GEOScan approach utilizes a constellation of low-noise MEMS accelerometers, which would assist in aggregately measuring the variations in Earth’s gravitational field as well as satellite drag for neutral density studies. Current commercial MEMS devices have demonstrated sensitivities in the 10 ng/√Hz range, with potential for 1 ng/√Hz — clearly suitable to compensate for the non-gravitational forces of 10-7 to 10-8 m/s2. The performance of this class of MEMS accelerometers has shown a consistent white noise floor on the order of 10-11 g2/Hz, making the device ideal for gravimetric measurements.
The GEOScan facility allows and encourages community users to propose for customized data acquisition from the facility system sensors. This is in addition to community user open access to standard system sensor data products through the NSF-JHU/APL Data Centers. Community users can customize data collection by acquisition date range and locations, measurement time and frequency, and parameters specific to the system sensors, such as integration time and compression for the optical imager, or a higher repeat rate measurement of certain bands by the spectrometer. GEOScan provides the ability for community users to tailor system sensor data to their research goals using measurements from anywhere on Earth taken at any time.
The data storage, processing, and downlinking process for GEOScan allows for continuous data collection from all instruments with less than 4 hours delay on data retrieval. A 2 GB SSR (Solid State Recorder) will provide ample data storage for a day’s worth of raw, compressed, and engineering data. Studies are being performed to support data storage options which cover one day up to a week. Currently, there is a 64.7% margin on data retrieval, and 20% margin on all SSR sizing calculations. All of this analysis requires that the PI hosted payload produce no more than 20 kbit/s of raw data that can be compressed into a 2 kbit/s data stream.
Furthermore, the project notes that in order to create a cyber infrastructure for the geosciences that enables transformative research and new understanding of the entire system, an architecture must be created that relies on synergy between measurement, data system, and user. To facilitate this approach, a broad effort requiring community input, scalable data management and web-based tools must be implemented. APL has been actively involved in meeting the future challenges of the geosciences by developing flexible, scalable, platform independent data management architectures for ingesting, processing, and serving out higher level data products to the community for the various NSF and internal programs that are actively managed, including: AMPERE, SuperDARN, SuperMAG, and GAIA.
A goal of existing programs is to create tools that reduce cost and save user/provider time through common interfaces and flexible applications, which have been instrumental in producing synergistic discovery.
Constellation system science:
GEOScan employs a full constellation approach to answer outstanding system science questions about the Earth and remote sensing of space environment state variables. Equipping the full constellation provides homogeneity of observation, thus simplifying analyses and reducing error in inherently global calculations. This suitably dense, homogeneous network enables the use of modern reconstruction techniques to image state variables and persistent measurement of global change across a wide range of temporal and spatial scales.
GEOScan climate science-measuring Earth’s energy balance:
GEOScan addresses Earth’s current state of energy balance and climate change via a homogeneous constellation of satellites observing the Earth 24/7 with hourly temporal resolution and spatial resolution ranging from 500 km for the broadband radiometer to 450 m for the imager. This revolutionary coverage will enable discoveries concerning many open science questions critical to our ecosystems and our habitability — notably how highly spatially and temporally variable phenomena aggregate to contribute to global change, and how global long-term changes affect smaller scales and surface processes where human beings live and work.
GEOScan’s most central climate instruments are extremely well calibrated radiometers, which will measure, for the first time, the ERI (Earth Radiation Imbalance). ERI is the difference between incoming radiation from the Sun and the TOR. TOR is the sum of reflected solar radiation and emitted longwave radiation. How ERI and TOR change regionally and globally, and on timescales from hourly to annually, is critical for understanding climate change.
According to climate models, current climate change, including the dramatic melting of Arctic sea ice and Greenland glaciers, results from an ~0.1–0.2% imbalance between incoming solar energy and TOR. Currently, space instruments measure incoming solar radiation to >0.03%. However, TOR has never been simultaneously, globally sampled, and is accurate to no better than 1% — not good enough to resolve the imbalance predicted by climate models. GEOScan’s global coverage of highly calibrated radiometers (0.3 Wm–2) will measure TOR at the necessary 0.1% accuracy level.
GEOScan’s time-variable gravity measurements focus on the large-scale, high-frequency spectrum of the gravity field that GRACE and other dedicated gravity field missions inherently cannot observe. These other missions cannot observe the high-frequency variations solely because of the limited sampling possible resulting from single satellite ground track (or satellite pair in the case of GRACE) measurements. The error caused by under sampling (independent of measurement accuracy) dominates any gravity solution at the daily to weekly timescales for a small number of satellites. This highlights the fact that with only one satellite pair, higher spatial resolution compromises temporal resolution and vice versa; the only way to improve both is to dramatically increase the number of satellites involved. GEOScan, with its 66 satellite constellation approach, addresses this short-coming in traditional gravity science investigations. The GPS measurements will track the orbits of the satellites to an accuracy of 2-3 cm, which, along with the data from the MEMS accelerometers, makes it possible to recover large-scale (>1000 km) gravity variations, which result mainly from the large scale movement of water mass.
Transformational space weather nowcasting and forecasting with the GEOScan constellation:
Significant progress has been made in the study of the Earth’s geospace environment over the last few decades. We have a firmly established understanding of the system dynamics on a climatological basis along with a basic understanding of the universal physics of smallscale processes of waves, instabilities, magnetic reconnection, and ENAs (Energetic Neutral Atoms). Yet accurate nowcast, much less forecast, of the details of individual space weather events remains elusive. We lack an understanding of the fundamental global properties of our system, such as determining what is the total energy input into the thermosphere, whether Hall or Pederson currents are primarily responsible for auroral current closure, and which mechanisms dominate radiation belt losses and their longitudinal extent.
Nowcasting and forecasting the global electron density field for space weather applications are difficult research operational challenges that have not yet been met. Operational requirements for electron density [Air Force, IORD-II (Integrated Operational Requirements Document-II)] include profiles of electron density from ~80 – 1500 km altitude, with ~ 5 km vertical resolution, and ~ 50-100 km horizontal resolution with errors in electron density < 10%. That is a global 3D electron density field from 80-1500 km altitude with 5 km vertical resolution and 50-100 km horizontal resolution. First principle models cannot achieve such resolutions and accuracy. Data assimilative models can achieve all the above requirements, but only with sufficient amounts of data.
In order to meet such stringent requirements over the entire globe, all the time, it is clearly necessary to have continuous global data coverage. This data coverage must be sufficient to sample the entire ionospheric profile with the required vertical resolution, and have the necessary horizontal resolutions. No existing data set, nor even combination of existing data sets, can meet this requirement. Thus, while in principle we have the theoretical understanding and numerical tools in place to provide required global nowcasts and forecasts of electron density, we do not have the necessary observational data. In addition to ionospheric nowcasting, there is a need for imaging the plasmasphere on a global, temporally updating scale. Plasmaspheric imaging is important since plasmaspheric densities impact the physics of the radiation belts. Plasmaspheric imaging to 20,000 km combined with radiation belt mapping of energetic electrons and protons will allow us to understand which loss processes dominate at different temporal and spatial scales. However, up until now there are almost no available direct measurements of plasmaspheric density.
GEOScan provides a transformative capability by providing the global data coverage necessary for the aforementioned investigations. GEOScan radio occultations will sample the ionosphere from 80 km altitude to the IRIDIUM satellite altitude of 780 km, Topside TEC observations provide information from the satellite altitude to the altitude of the GPS constellation (~ 20,000 km). Each GEOScan satellite will continuously monitor 10-15 topside TEC measurements to different GPS satellites, providing an almost overwhelming amount of topside and plasmaspheric data that can be used in tomographic imaging algorithms to obtain accurate, time evolving images of plasmaspheric density. For the radio occultations, each GPS receiver sees ~ 3 occultations at a time. Each occultation lasts ~ 1 minute. Over a 5 minute period a total of 66 x 3 x 5 = 990 occultations / 5 minute period are observed. While this data alone only provides ~ 600 km horizontal resolution, when combined with the copious amounts of ground GPS TEC data available (currently > 4000 sites and growing), we can easily achieve the required horizontal resolutions necessary to meet Air Force IORD-II requirements. A beautiful aspect of this constellation design is global continuous data that can be streamed to ground systems in near real-time. This allows, for the first time, global high-resolution, high accuracy nowcasts of electron density to be provided continuously in time. This is accomplished using global tomographic or data assimilation imaging methods such as IDA4D (Ionospheric Data Assimilation Four Dimensional. When combined with first principle models, where the global density field is used to reinitialize the model, it becomes possible to provide accurate forecasts that are only limited by the accuracy of the forward model.
GEOScan gratity imaging:
The time-variable gravity products created from GEOScan seek to provide new insights into the large-scale (>1000 km), short-term (< 1month) mass transport processes governing the global water cycle. Any process that involves the transport of water, such as the melting of glaciers in the cryosphere, changes in continental hydrology (e.g., groundwater), or other processes in the oceans and atmosphere, creates a change in Earth’s gravity field. By precisely measuring the variations of Earth’s gravity over time, the project can exploit this link and understand more about the behavior of these processes.
How the time-variable gravity field can be measured by GEOScan’s sensor suite is relatively straightforward, and is driven by the fact that changes in Earth’s gravity field, however small, will alter the trajectory of an orbiting satellite. Using the CTECS GPS receiver, the absolute position of each Iridium NEXT satellite will be precisely determined, down to the cm level. These positions can then be differentiated to create a time series of satellite accelerations that represent both gravitational and non-gravitational forces. Those accelerations caused by non-gravitational forces, such as atmospheric drag and solar radiation pressure will be accounted for by the information provided by the onboard MEMS accelerometers, leaving as a final product only those accelerations due to Earth’s gravity.
The GRACE mission was the first to highlight the value of time-variable gravity data; however, despite its tremendous success, GRACE suffers from the measurement sampling limitations related to having only a single satellite pair. Since gravity observations are essentially point measurements, the spatial and temporal coverage of a single satellite will never permit the observation of high-frequency events, and this is why the temporal resolution for GRACE is approximately one month. While GEOScan will not be able to match the spatial resolution of GRACE, the time variable data collected from the full constellation of Iridium NEXT satellites will allow the monitoring of large-scale processes at the Earth’s surface at time scales as short as one day. Global gravity data at this temporal resolution has never been collected before, and should be especially valuable to the ocean and atmosphere communities. Figure 12 demonstrates the potential quality of the GEOScan gravity products (bottom panel) from a single day’s worth of measurements, compared to the full high-resolution signal (top panel) over the same timeframe, as derived from a recent coupled Earth system model. As can be seen, a number of terrestrial and oceanic/atmospheric mass transport processes are clearly observed, with the spatial resolution corresponding to approximately 1000 km.
Figure 12: Illustration of the daily resolution expected from the GEOScan gravity products (bottom panel), as derived from a high-resolution coupled Earth system model (top panel). Units are in equivalent water height (image credit: GEOScan consortium).
Figure 13: Iridium NEXT mission timeline (image credit: JHU/APL, Ref. 26)
GEOScan’s development timeline is mated to Iridium NEXT’s development and launch schedule.
Hosted payloads of other customers on Iridium NEXT
ADS-B (Automatic Dependent Surveillance - Broadcast):
Satellite operator Iridium, through its new joint venture Aireon LLC, will be putting ADS-B (Automatic Dependent Surveillance-Broadcast) receivers on its next-generation satellite constellation as hosted payloads, aimed at bringing global, real-time aircraft surveillance for ANSP (Air Navigation Service Providers).
In June 2012, NAV CANADA and Iridium signed a contract for a joint venture to be run under the company Aireon LLC (McLean, VA, USA) with support from the U.S. FAA (Federal Aviation Administration) and suppliers Harris Corporation and ITT Exelis. The objective of Aireon is to take advantage of Iridium's hosted payload services using ADS-B receivers and to deliver a surveillance capability to ANSPs (Air Navigation Service Providers) around the world and their commercial airline customers. 30) 31) 32) 33)
ADS-B is a next generation commercial surveillance technology that supports radar-like separation standards. The system brings significant safety and efficiency benefits, offering properly-equipped and certified aircraft more flexible, fuel-saving routes through airspace previously managed using only procedural air traffic control. Aircraft with ADS-B automatically transmit accurate position reports with integrity every second to ATC (Air Traffic Control). As a result, ADS-B will reduce separation minima for equipped aircraft and allow more aircraft to follow the most efficient flight trajectory.
NAV CANADA corporation owns and operates Canada’s civil ANS (Air Navigation Service), providing for the safe and efficient movement of aircraft in Canadian domestic airspace and international airspace assigned to Canadian control. Through its coast-to-coast operations, NAV CANADA provides air traffic control, flight information, weather briefings, aeronautical information, airport advisory services, and electronic aids to navigation. NAV CANADA is the second largest air navigation service in the world by traffic volume and provides air traffic management for 1,200 flights per day, the busiest oceanic airspace in the world. -NAV CANADA will be Aireon's first customer to deploy the new satelliteborne surveillance capability in its North Atlantic airspace operations most of which is without surveillance at the moment.
In November 2012, Iridium Communications Inc. announced that it has finalized an agreement with NAV CANADA regarding Aireon LLC, a joint venture that will allow air traffic management agencies around the globe to continuously track aircraft anywhere in the world. For the first time ever, ANSPs (Air Navigation Service Providers) around the world will be able to track aircraft from pole-to-pole, including oceanic airspace and remote regions. The new capability will provide significant benefits to the aviation industry, including substantial fuel savings, a reduction in greenhouse gas emissions and enhanced safety and efficiency for passengers. 34) 35)
The ADS-B receiver payloads, to be mounted on each Iridium NEXT satellite, will operate independently and perform the air traffic surveillance function separately from the main mission of the spacecraft. The power for the ADS-B payloads will come from the main satellite bus and will be designed to work with the other subsystems, such as thermal management or communications systems. By sharing Iridium's spaceborne capability and ground infrastructure, these commercially hosted payloads illustrate how to avoid the cost of building and launching separate satellites, thereby reducing the expense and time required to put mission capabilities into space for government and private organizations via a public-private partnership model, says a representative. 36) 37)
The spaceborne ADS-B surveillance solution is set up as a joint venture between Iridium and NAV CANADA with support from the U.S. Federal Aviation Administration (FAA) and several other partners:
• Iridium will host the ADS-B receivers on its next-generation Iridium NEXT constellation.
• NAV CANADA is Aireon’s first customer and an investor in Aireon. The venture will be operated under a PPP (Public Private Partnership) between industry and the world’s major ANSPs.
• Harris Corporation is supplying 81 ADS-B payloads for the venture.
• ITT Exelis is providing systems engineering support.
Figure 14: The spaceborne ADS-B concept of operations (image credit: Aireon)
The Iridium architecture is unique in that all of the satellites are cross-linked, communicating with their neighboring satellites, allowing signals to be relayed from any point on the globe to a central ground location in Tempe, AZ (USA) in near real-time, with back-up locations in Alaska and Norway. The real time nature of relaying ADS-B surveillance data through the Iridium network is critical to achieving radar-like surveillance and reduced oceanic separation minima down to 15 NM (Nautical Miles) for aircraft equipped with appropriate communication and navigation avionics – enabling the full potential of benefits from such operations. No other existing or planned LEO-constellation has an equivalent capability. The Iridium architecture with built in redundancy and backup would provide a seamless experience to the Air Traffic Controllers when utilizing the Aireon surveillance capability. 38)
The Iridium NEXT LEO constellation is the world’s largest with 66 operational satellites, plus six on orbit and nine ground spares, providing a level of redundancy and system availability that is unprecedented. The Iridium satellite design has significant built-in redundancy and high reliability. The planned ADS-B payload receivers will have even higher reliability requirements and the design will include the ability to make on-orbit software updates to adapt to future changes in ADS-B formats, if required. No other LEO constellation has a comparable global coverage, system availability, redundancy or flexibility.
The Aireon global space-based ADS-B surveillance system is being developed under a joint venture between Aireon and NAV CANADA and will be operational in 2017. This transformational new global surveillance system will offer far reaching capabilities and benefits to the global aviation community. The global aviation community would benefit by early interaction and participation in the development and deployment of this capability.
Background: In the timeframe 2010, ADS-B is a land-based system and deployed primarily in high air traffic areas such as North America, Australia and Europe. Vital airways over oceans, mountains, remote areas and polar regions remain largely uncovered.
In the ground-based ADS-B system concept, each aircraft broadcasts its own GPS position along with other information like heading, ground track, ground speed, altitude. Receivers on the ground then receive this information and send it to air traffic control displays. The ADS-B information can be used to augment existing primary and secondary (transponder-based) radar or used in lieu of those radar technologies. Aircraft that broadcast this information are considered to be equipped with ADS-B Out. ADS-B is all about communications between aircraft, and also between aircraft and ground. Both are vital in ensuring safe flights and efficiency in terms of fuel use, time and emissions. ADS-B is an integral part of the planned efficiency drive towards 2020. 39) 40)
Taking advantage of the latest technology, ADS-B is designed to be retrofit on aircraft flying today. In its final form, ADS-B is designed to ease ATC (Air Traffic Control) as the number of approaches grows, enhancing safety and increasing airport capacity. In the air, the information provided by ADS-B enhances the pilots' traffic awareness, allowing more optimal flight levels leading to fuel savings. ADS-B is designed in two parts:
• ADS-B OUT provides a means of automated aircraft parameter transmission be tween the aircraft and the ATC.
• ADS-B IN provides automated aircraft parameter transmission between aircraft themselves.
Ground-based systems primarily use radar to provide aircraft surveillance. As part of the global ATM modernization, ANSPs (Air Navigation Service Providers), such as the U.S. Federal Aviation Administration (FAA) and NAV CANADA, are implementing new ADS-B systems. On-board ADS-B transmitters broadcast GPS position and other useful data; yet, ADS-B networks are limited by ground-based ADS-B towers, which collect this data for the ANSPs. The ground-based ATM infrastructure cannot monitor flights over oceans or remote regions of the globe where placing an ADS-B tower is not feasible, adds the representative.
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27) “APL Proposes First Global Orbital Observation Program,” Space Daily, Nov. 29, 2011, URL: http://www.spacedaily.com/.../APL_Proposes_First_Global_Orbital_Observation_Program/font>
28) Lars Dyrud, “GEOScan: A Geoscience Facility from Space,” NASA, Oct. 31, 2012, URL: http://science.gsfc.nasa.gov/670/seminar/2011_abstracts/Lars_Dyrud_abstract.html
29) “Final Nanosatellite Launched from Space Shuttle Atlantis,” Space Daily, July 25, 2011, URL: http://www.spacedaily.com/.../Final_Nanosatellite_Launched_from_Space_Shuttle_Atlantis 30) “Iridium to Revolutionize Global Air Traffic Surveillance With the Launch of Aireon(SM),” Iridium, June 19,2012, URL: http://investor.iridium.com/releasedetail.cfm?ReleaseID=684218 31) Marc Boucher, “NAV CANADA and Iridium Announce new Joint Mega Hosted Payload Venture: Aireon,” Space Ref, June 19, 2012, URL: http://spaceref.ca/commercial-space/nav-canada-part-of-largest-implementation-of-hosted-satellite-payloads.html 32) Jose Del Rosario, “A Hosted Payload Boost,” NSR, June 20, 2012, URL: http://www.nsr.com/news-resources/the-bottom-line/a-hosted-payload-boost/ 34) “Iridium Completes Formal Agreement for Global Air Traffic Joint Venture With NAV CANADA,” Iridium, Nov. 19, 2012, URL: http://investor.iridium.com/releasedetail.cfm?ReleaseID=722252 35) “Public Private Partnerships,” Aviation and Climate Change Seminar, ICAO Headquarters, Montreal, Canada, October 23-24, 2012, URL: http://www.icao.int/Meetings/acli/Documents/NEXA_24October-am.pdf 36) Courtney Howard, “Aireon selects Harris to provide ADS-B receiver payloads for Iridium NEXT satellites,” avionics intelligence, Aug. 16, 2012, URL: http://www.avionics-intelligence.com/articles/2012/08/aireon-harris.html 37) “Global Aviation Surveillance System,” Aireon brochure, July 5, 2012, URL: http://www.iridium.com/About/IridiumNEXT.aspx?section=Documentation 38) Space-based ADS-B surveillance and the impact on the air traffic management,” Twelfth Air Navigation Conference, Montreal, Canada, Nov. 19-30, 2012, URL: http://www.icao.int/Meetings/anconf12/WorkingPapers/ANConfWP141.1.1.ENonly.pdf 39) “Automatic Dependent Surveillance-Broadcast (ADS-B),” FAA, URL: http://www.faa.gov/nextgen/implementation/programs/adsb/ 40) Christine Vigier, “Automatic Dependent Surveillance Broadcast (ADS-B) Surveillance development for Air Traffic Management,” FAST Magazine, No. 47, January 2011, URL: http://www.airbus.com/fileadmin/media_gallery/files/brochures_publications/FAST_magazine/FAST47_5-adsb.pdf 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.
29) “Final Nanosatellite Launched from Space Shuttle Atlantis,” Space Daily, July 25, 2011, URL: http://www.spacedaily.com/.../Final_Nanosatellite_Launched_from_Space_Shuttle_Atlantis
30) “Iridium to Revolutionize Global Air Traffic Surveillance With the Launch of Aireon(SM),” Iridium, June 19,2012, URL: http://investor.iridium.com/releasedetail.cfm?ReleaseID=684218
31) Marc Boucher, “NAV CANADA and Iridium Announce new Joint Mega Hosted Payload Venture: Aireon,” Space Ref, June 19, 2012, URL: http://spaceref.ca/commercial-space/nav-canada-part-of-largest-implementation-of-hosted-satellite-payloads.html
32) Jose Del Rosario, “A Hosted Payload Boost,” NSR, June 20, 2012, URL: http://www.nsr.com/news-resources/the-bottom-line/a-hosted-payload-boost/
34) “Iridium Completes Formal Agreement for Global Air Traffic Joint Venture With NAV CANADA,” Iridium, Nov. 19, 2012, URL: http://investor.iridium.com/releasedetail.cfm?ReleaseID=722252
35) “Public Private Partnerships,” Aviation and Climate Change Seminar, ICAO Headquarters, Montreal, Canada, October 23-24, 2012, URL: http://www.icao.int/Meetings/acli/Documents/NEXA_24October-am.pdf
36) Courtney Howard, “Aireon selects Harris to provide ADS-B receiver payloads for Iridium NEXT satellites,” avionics intelligence, Aug. 16, 2012, URL: http://www.avionics-intelligence.com/articles/2012/08/aireon-harris.html
37) “Global Aviation Surveillance System,” Aireon brochure, July 5, 2012, URL: http://www.iridium.com/About/IridiumNEXT.aspx?section=Documentation
38) Space-based ADS-B surveillance and the impact on the air traffic management,” Twelfth Air Navigation Conference, Montreal, Canada, Nov. 19-30, 2012, URL: http://www.icao.int/Meetings/anconf12/WorkingPapers/ANConfWP141.1.1.ENonly.pdf
39) “Automatic Dependent Surveillance-Broadcast (ADS-B),” FAA, URL: http://www.faa.gov/nextgen/implementation/programs/adsb/
40) Christine Vigier, “Automatic Dependent Surveillance Broadcast (ADS-B) Surveillance development for Air Traffic Management,” FAST Magazine, No. 47, January 2011, URL: http://www.airbus.com/fileadmin/media_gallery/files/brochures_publications/FAST_magazine/FAST47_5-adsb.pdf
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.