Going the Distance: GOES-R and the Future of U.S. Geostationary Environmental Satellites
Nathaniel Feldman, Samuel Lim, Michael Madden, Jim O'Neal, and Kenneth Shere
The next generation of geostationary environmental satellites, GOES-R, represents a significant technological advancement in terms of the quality and quantity of meteorological and environmental data. Aerospace is supporting all aspects of this program, including acquisition, instruments, ground systems, communications, and architectural studies.
The GOES (Geostationary Operational Environmental Satellite) constellation provides continuous monitoring of meteorological conditions in the Western hemisphere. Operated by the National Oceanic and Atmospheric Administration (NOAA), the two active GOES spacecraft also monitor the space environment, receive and transmit search-and-rescue data, and relay ground-based environmental platform data.
A major upgrade to this system, known as GOES-R, is under development, with a first launch scheduled for late 2012. GOESR is a major step forward in the fields of weather, atmosphere, climate, and ocean monitoring. Its launch will mark the first technological advance in GOES instrumentation since 1994. The combined instrument downlink data rate will increase by a factor of 60, and the number of environmental product types will increase by a factor of 4. The amount of environmental data being rebroadcast to users throughout the hemisphere will increase by an order of magnitude. The total volume of information products will increase from roughly 43 to more than 150.
The Aerospace Corporation created conceptual designs that integrated the satellite bus, instruments, and communications payloads for GOES-R. Aerospace also performed reliability estimates for these segments. These designs and analyses provided a basis for the GOES-R reference architecture. (This reference architecture is not the government-recommended solution, but rather a basis for program planning and estimation only. GOES-R formulation contractors will be expected to define and develop their own architecture solution to meet government requirements.) NOAA and NASA also used Aerospace data in evaluating costs, technology maturity, and schedule for all aspects of GOES-R. Aerospace designs were further used as the basis for an industry-wide architecture study.
Architecture Studies
GOES-R system requirements include high reliability and long satellite service life. Aerospace studies suggested that GOESR could achieve these goals more easily by means of a distributed space-segment architecture. These goals could also be achieved using the current GOES architecture, in which the instruments, communications, and auxiliary services are consolidated on each of the two operational satellites. With a distributed space-segment architecture, these instruments, communications, and services would be deployed across four satellites—two in each orbital location. Placing the primary instruments (the Hyperspectral Environmental Suite and the Advanced Baseline Imager) on separate satellites would simplify the satellite design and increase satellite reliability. Separating components also simplifies infusion of new technology and allows for more efficient use of spare satellites: An instrument failure could provide an infusion opportunity; NOAA could replace only the affected satellite.
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These Aerospace Concept Design Center drawings illustrate the GOES-R distributed space-segment architecture. The first satellite carries the Advanced Baseline Imager, the Solar Instrument Suite, and the Geostationary Lightning Mapper. The second carries the Hyperspectral Environmental Suite and Space Environmental In Situ Suite. Each satellite has a dry mass of approximately 1500 kilograms. These engineering designs are used for instrument, interface, design, and cost analyses. |
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Other architectural options that Aerospace considered include keeping the instruments at geosynchronous orbit, but using a medium Earth orbit for communications and services; this would enable GOES-R to support other missions while helping NOAA transition toward its goal of a global observing system. Another option was to use a dedicated communications satellite at geosynchronous orbit to supply auxiliary services and to rebroadcast environmental data products to the user community. Another option was to use three satellites per orbital location—for example, one for each of the primary instruments and a third to carry communications and auxiliary services payloads.
This study provided the basis for a broad agency announcement issued by NOAA to evaluate alternative GOES-R architectures. Twelve contracts were issued as a result of this announcement. Aerospace assisted the GOES-R program office by cochairing the study, answering questions from vendors, and assessing their results. Reliability studies conducted by Aerospace were made available to the vendors and were widely used.
Aerospace is also assisting the GOESR program office prepare for the program-definition and risk-reduction phase. Results from the broad agency announcement, along with Aerospace technical documents and acquisition expertise, will be used in the program-definition and risk-reduction phase of GOESR acquisition.
Satellite Bus and Instruments
As part of the overall architecture study, Aerospace evaluated initial designs for the instruments and bus needed to support the GOES-R spacecraft lifetime of 15 years. The Aerospace Concept Design Center examined several alternative satellite designs to support 5 years of on-orbit storage followed by 10 years of operations. Aerospace created 13 design configurations for various consolidated, distributed, and medium-Earth-orbit constellations. These designs are continually updated and serve as NOAA's notional reference architecture; they're also used to create the program's independent cost estimate.
Improvements in hurricane coverage can be seen by comparing current GOES imager (left) and the simulated GOES-R Advanced Baseline Imager (right). The simulation represents improvements expected with the increased resolution of GOES-R imagery, which would enable improved feature detection (structure, moisture, temperature) and therefore contribute to more accurate forecasts. (University of Wisconsin) |
Aerospace performed trade studies and analyses of all the GOES-R instrument payloads. One primary instrument, the Advanced Baseline Imager, features 16 channels—two in the visible band and 14 in the near infrared and infrared. Spatial resolution is 0.5 kilometer in the visible band, 1 kilometer for the near infrared, and 2 kilometers for the infrared. In contrast, the current GOES imager has only five channels, with resolutions of 2 and 4 kilometers. The Advanced Baseline Imager can provide one full image of Earth, three images of the continental United States, and 30 mesoscale (1000 X 1000 kilometers) images every 15 minutes. Alternatively, it can provide a full Earth image every 5 minutes. The current GOES provides a full Earth image every 30 minutes; GOES-R will therefore provide six times more temporal coverage. Production has begun on this instrument, and Aerospace continues to support it with factory visits, technical reviews, and program reviews.
Aerospace likewise supported the design and analysis of the other primary instrument, the Hyperspectral Environmental Suite, which comprises a sounder and multichannel imager. Aerospace served on the NASA source-selection board for the instrument's formulation and continues to support program reviews and factory visits with NOAA. The Hyperspectral Environmental Suite is designed to provide high-resolution hemispheric soundings, mesoscale soundings of severe weather systems, and coastal waters imaging. The sounder will cover almost the entire hemisphere with 10-kilometer resolution in the infrared and a 1-hour refresh rate; the two current GOES sounders only cover the lower 48 states. For mesoscale soundings, the instrument will achieve 4-kilometer resolution in the infrared and have a single broadband visible channel for cloud detection. The exact number of sounding channels has not been established, but is expected to exceed 1500. In contrast, the current GOES sounder has 18 infrared channels. For monitoring coastal waters (mainland U.S. and Hawaiian), the Hyperspectral Environmental Suite will also provide at least 14 channels in the visible and near-infrared range, with 300-meter resolution or better and a 3-hour refresh rate.
GOES-R increased temporal coverage can be illustrated by comparing the 5-minute coverage of current GOES (left) with the 5-minute coverage from the simulated GOES-R (right). GOES-R can image the entire hemisphere in one-sixth the time it takes for the current GOES system. Such an increase in instrument performance places a greater burden on communication and data-processing systems. Aerospace is directly involved in improving communications throughput. (NOAA/ORA) |
Other Aerospace instrument support includes the Solar Instrument Suite, the Space Environmental In Situ Suite, and the Geostationary Lightning Mapper. The Solar Instrument Suite will have several instruments for monitoring solar activity—including an X-ray instrument for tracking and measuring solar flares. The Space Environmental In Situ Suite is designed to provide continuous measurement of Earth's ambient magnetic field along with the proton, electron, and alpha-particle fluxes at geostationary orbit. The Geostationary Lightning Mapper, sensitive enough to detect 70–90 percent of all lightning strikes, will help predict severe storms by continuously tracking the intensity, frequency, and location of lightning discharges; it will provide rapid information that could be correlated with radar returns, cloud images, and other meteorological data. Aerospace will support all these instruments through design, trades, satellite integration, and operations.
Communications
GOES satellite communications involve both direct transmission to and from a processing station on the ground as well as wide-area broadcasting. Each satellite sends raw data directly to a ground station, where the data are processed and archived. The processed data are compressed and transmitted back to each satellite. A transponder in the satellite downconverts the signal and broadcasts it over the visible Earth. Users equipped with suitable receivers can extract the information they need.
GOES-R communications will be far more challenging than earlier GOES satellite communications because the total raw data rate will probably be between 130 and 200 megabits per second. Even after lossless onboard compression, the onboard instruments can generate a combined raw data rate of up to 100 megabits per second on the downlink, and the GOES rebroadcast data rate after more extensive ground compression may be as much as 24 megabits per second. Previous GOES satellites were only required to transmit instrument data at 2.7 megabits per second to the Command and Data Acquisition Stations and broadcast data at 2.1 megabits per second. Requirements for data quality have also increased: The instrument data must achieve a bit-error rate of around 10-9 or better, far more stringent than the 10-6 for the current GOES downlink. The broadcast data must achieve a user receiver bit-error rate of around 10-6 to 10-9, as opposed to only 10-5 for the current user receivers.
The L-band spectrum currently used for instrument data downlink to the Command and Data Acquisition Stations (a few megahertz) is too narrow to meet the required 100 megabits per second, even with compressed data. As a result, instrument data will be transmitted in select portions of the X band. Even with migration to X band, bandwidth constraints still exist. Out-of-band emissions must be stringently controlled to prevent interference with other satellites using adjacent bands (e.g., the Deep Space Network downlink at 8400–8450 megahertz). The need for higher quality data in a constrained bandwidth further complicates matters, because it requires even greater power, which in turn increases the potential for out-of-band interference. This is true not only for the instrument data at X band, but also for broadcast data in L band.
NOAA has asked Aerospace to determine the feasibility and risk of integrating the core technologies into an end-to-end concept design that will meet system requirements. Accordingly, Aerospace has organized NOAA's communications engineering efforts along three principal lines: spectrum management and regulatory issues, analysis of specific core technologies, and development of an end-to-end communications testbed to verify feasibility.
Improvements in sounder coverage with the Hyperspectral Environment Suite can be seen by comparing the coverage of the continental United States provided by the current GOES (green box) and the hemispheric coverage of the GOES-R (blue cross-hatched). |
Spectrum management is a complex issue with far-reaching implications. Before the communications architecture can be developed, NOAA must find available bandwidth and reserve it through the National Telecommunications and Information Administration. This process takes years and requires planners to know which spacecraft contending for the same spectrum will be in orbit during the GOESR series time frame, which can run for 20 to 25 years from first launch, about 10 years from now. Spectrum has not yet been coordinated for the instrument data downlink and GOES rebroadcast uplink. Aerospace, with NOAA, is working to ensure that suitable X-band spectrum will be available and that interference levels in both L and X bands can be coordinated.
Aerospace has identified several core technologies that must be developed to support GOES-R requirements. These include bandwidth-efficient and power-efficient communications, forward error-correction coding and interleaving, L-band linearized amplifiers, filtering, synchronization, and commercial microelectronics suitable for use at the radiation levels of geostationary orbits. These technologies have been selected for investigation because they individually pose development risks and costs, and their interaction as part of a comprehensive system can have profound consequences with respect to data quality, data rate, control of out-of-band emissions, spacecraft mass and power levels, and compatibility with receiver system electronics. Thus, the main challenge is to integrate and test all these core technologies in an end-to-end system.
Improvements in smoke plume characterization can be seen in this comparison of the current GOES imager (left) and the simulated GOES-R imager (right). (University of Wisconsin) |
The communications testbed uses representative hardware and realistic channel conditions to help determine their cumulative effect on bit-error rate, out-of-band interference, and error containment. The testbed emulates end-to-end transmitter/receiver hardware and thus facilitates investigating potential alternatives for modulation and coding. It can migrate to PC-card implementation to serve as a pathfinder for the GOES rebroadcast receiver design. It can scale to instrument data rates to support all reasonable modulation candidates for the instrument data link. It can characterize bit-error-rate performance of commercially available codecs (coder/decoders). It can investigate bit-error-rate statistics and error patterns above and below a 10-9 bit-error rate for compressed/decompressed instrument data rates, which could not be achieved in a reasonable time using simulation.
NOAA has expressed a desire for stringent bit-error-rate requirements for its compressed data streams, but the science and research community has not yet specified how many erroneous pixels can be allowed for the Advanced Baseline Imager and for the Hyperspectral Environmental Suite data in a single scan, picture, frame, or data block. Aerospace is working with the science and research community to determine what bit-error pattern is preferable to provide appropriate data quality after decompression.
GOES-R reference architecture (view larger ). |
Ground System
The GOES-R ground system needs to substantially increase communications bandwidth, data processing, and archiving capabilities. To complicate matters, if the GOES-R space-segment architecture is distributed (as proposed in the reference architecture), satellite operations will need to control twice as many satellites and manage shared orbital locations. Moreover, ground facilities will need to include a remote backup location, so its survival will not be threatened by the weather at primary sites.
In light of these challenges, satellite operations need substantial automation along with integrated logistics support. GOES already employs substantial automation in product generation, but GOES-R may need virtually complete automation for this task.
Working with the program office and other contractors, Aerospace developed a GOES-R concept of operations to provide guidance for the program definition and risk reduction phase. As envisioned in this operational concept, the NOAA Satellite Operations Facility in Suitland, Maryland, will house mission management, product generation, and the Satellite Operations Control Center. It will receive telemetry and rebroadcast products directly from the satellite, but will not transmit the processed data uplink or bus commands to the satellites because of FCC restrictions. Commands will be generated at the Satellite Operations Control Center and relayed to a Command and Data Acquisition Station at Wallops, Virginia, for upload to the satellites. Likewise, GOES rebroadcast data will be assembled at Suitland and sent to Wallops for satellite upload, along with additional service data.
GOES-R will maintain a 30-day archive of raw data records and a 3-day archive of reconstructed unprocessed instrument data at full space-time resolution with supplemental information to be used in subsequent processing appended (Level 0). GOES-R will generate nearly 16 terabytes per day of meteorological and environmental (Level 2+) products. All calibrated instrument data and selected products are kept in permanent storage as part of the Comprehensive Large Array Data Stewardship System, known as CLASS. About 1 terabyte per day of data will be sent to CLASS.
Conclusion
The GOES-R system will transition to operations around 2014, with the first launch planned for late in 2012. The GOES-R satellite series will operate for more than 16 years, providing regional environmental imagery and specialized meteorological, climatic, terrestrial, oceanographic, and solar-geophysical data. GOES-R will support a wide variety of end users such as National Weather Service, Federal Aviation Administration, Environmental Protection Agency, and Department of Homeland Security. GOES-R products will be useful to much of America's industry, including agribusiness, transportation, and construction.
Aerospace participation in research, source selection, and program office activities has been instrumental in identifying difficult issues facing the GOES-R system. Aerospace's continued support in the upcoming acquisition phase can help ensure that the final architecture will be both feasible and powerful enough to meet the diverse user requirements. Aerospace expertise and continued involvement should enable NOAA to provide an improved geostationary weather and environmental sensing capability that can serve up to 2030.
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