Executive Summary

In late 2022, NASA’s Earth Science Division (ESD) requested the Committee on Earth Science and Applications from Space (CESAS) of the National Academies of Science, Engineering, and Medicine conduct a short study that would assess the potential use of a proposed multi-user, robot-tended, uncrewed commercial space platform as a potential host for a large number of Earth remote sensing instruments. Specifically, NASA requested that the study:

• Consider a notional platform in a Sun-synchronous orbit (SSO, nominally 600–800 km) capable of hosting 20 or more instruments; and
• Consider the impacts on NASA’s ability to achieve Recommendation 3.2 from the 2017 decadal survey¹ regarding the recommended additional observations classified within the Designated Observables, Earth System Explorer, Incubation, and Earth Venture elements, as described in Table 3.3, Observing System Priorities.

In its request (see Appendix A, Statement of Task), NASA further specified that potential impacts on the existing Program of Record were not within the scope of the committee’s assessment; however, the committee could comment in general terms on considerations for the use of the notional platform versus existing plans to accomplish survey priorities. In addition, the committee was invited to comment on how the use of the notional platform would compare with the deployment of its instrument complement via multiple free-flyers in different orbits, the impacts of possible consolidation of a platform at a single nodal crossing time, and how well any single orbit could satisfy the survey priorities. In subsequent discussions with NASA, the committee was given additional guidance regarding the characteristics to be assumed for the notional platform; these are the “working assumptions” shown in Chapter 1.

Many of the factors that would determine whether the notional platform is a viable option as a host for instruments to address decadal survey priorities are beyond the scope of this study. With that caveat, the committee’s principal observations and conclusions are as follows:

• NASA, the National Oceanic and Atmospheric Administration (NOAA), the U.S. Geological Survey (USGS), and U.S. international partners conduct Earth science observations from space utilizing instruments on platforms that are placed in a variety of orbits according to measurement needs.
• The assumed single platform would be placed in a Sun-synchronous polar orbit at an altitude of 600–800 km. Constrained to operate at a particular altitude and inclination, and with a fixed ground-track repeat time and fixed local solar time for observations, the platform would be compatible only with a subset of the Earth observations recommended in ESAS 2017.² Furthermore, it would be optimal for an even smaller subset of these because of differing

___________________

¹ National Academies of Sciences, Engineering, and Medicine (NASEM), 2018, Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space, Washington, DC: The National Academies Press, https://doi.org/10.17226/24938.

² Ibid.

measurement requirements and compromises that are likely in accommodating a suite of 20 or more instruments (see Chapter 4 and Figure 4-1).³ For example:

• A number of key Earth observations are not suitable for any platform placed in SSO. These include precision altimetry measurements that would otherwise be limited by solar tide aliasing.
• A number of key measurements are not well suited for a platform at the assumed 600–800 km altitude of the notional platform. These include global measurements of vegetation height and structure and vertical profiles of tropospheric horizontal winds (from lidar), which are currently only possible from platforms operating at significantly lower altitudes.
• The near-polar inclination of the notional platform is less well suited to measurements over the tropics.⁴
• The inclination of the notional platform does not allow complete coverage of the polar sea ice.⁵
• A single platform will not address conflicting repeat cycle times or ground-track repeat times.⁶
• While, for the most part, this report does not address accommodation issues, key characteristics of the platform, including its pointing accuracy and stability, place limits on the number and types of instruments that can be hosted.⁷
• The notional platform is envisioned for launch in 2025–2027. Even if the platform were in place in this time frame, it is highly unlikely that the full capacity of the platform, which is assumed to be 20 or more instruments, could be utilized if populated with the class of instruments assumed by ESAS 2017. The suitability of CubeSat- and SmallSat-class instruments, which typically have shorter development times, was not evaluated.

___________________

³ The committee examined measurement needs in the three new program categories recommended by the 2017 decadal survey: Designated Observables (ongoing or scheduled in the next few years), Earth System Explorers (competitive opportunities for medium cost-capped missions), and Incubator (requires sensor development). The Designated measurements are already designed to fly in orbital configurations that are optimal for the prescribed measurement and are mostly inconsistent with the single notional orbit and platform. This reduced the sensor-to-platform match to the Explorer and Incubator categories.

⁴ For example, the Tropical Rainfall Measuring Mission (TRMM) was placed in non-Sun-synchronous orbit at an altitude of 400 km with an inclination of 35 degrees to the equator. Its successor, the Global Precipitation Measurement (GPM) Core Observatory satellite, flies at an altitude of 407 km in a non-Sun-synchronous orbit that covers the Earth from 65°S to 65°N. This orbit was chosen because of the specific scientific goals of the mission, ensuring sufficient overlap with other constellation spacecraft for cross-calibration and covering a large portion of Earth’s surface for data acquisition with minimal ground-track repeating. The chosen orbit allows an optimal instrument coverage with minimal time to achieve global coverage. Spacecraft & Satellites, 2023, “GPM Core—Mission & Spacecraft Overview,” https://spaceflight101.com/spacecraft/gpm-core-mission-spacecraft-overview.

⁵ ICESat-2 was placed in a non-Sun-synchronous orbit at an altitude of approximately 500 km with a 92° inclination for global coverage from 88°S to 88°N latitudes. The satellite has a 91-day repeat cycle.

⁶ This would include, for example, many of the required Earth observations on nadir-viewing instruments; therefore, the separation distance between adjacent ground tracks controls horizontal resolution. Examples include measurements of ice topography and surface mass change. These measurements require dense ground tracks (separations of a few hundred kilometers at the equator) for each repeat period, which will likely not be consistent with the timing requirements of other instruments on the notional multi-instrument platform (e.g., revisit times less than 1 week).

⁷ In addition to orbital constraints, the platform itself may limit the number and types of instruments that can be hosted. For example, interferometric synthetic aperture radar measurements require a large deployable antenna on a platform with accurate attitude control, accurate pointing, and a highly stable platform. A notional platform flying in a lower orbit could support ice elevation measurements, but only if the platform had the requisite pointing control. Measurements of Earth’s gravity field, which are currently made via highly accurate ranging measurements of the distance between two spacecraft flying in tandem, could not be implemented using the notional platform as one of spacecraft.

• As a host platform, the proposed platform shares some attributes with the International Space Station (ISS). Indeed, some of the concepts for the notional platform include what would effectively be an uncrewed, robot-tended ISS follow-on that would be placed in SSO, an orbit more advantageous for many Earth observations than the current ISS orbit.
  • As demonstrated on the ISS and in constellations of satellites orbiting in close formation, the notional platform holds promise as a platform that would support coincident, near-simultaneous measurements.
  • The proposed platform would also be uncrewed, which could simplify operations, provide a very low microgravity environment, and allow observations through a less contaminated space environment.
  • However, a key factor influencing ISS utilization for Earth observations has been a NASA policy that provides low-cost transportation options for chosen missions/instruments.
• Although not studied in detail, the committee notes the potential use of the notional platform as a host for smaller-size instruments that might be developed for use in, for example, NASA’s Earth Venture line.
  • The Earth Venture-Continuity strand is of particular interest given the need to find cost-effective follow-ons for measurements initiated with larger, more costly instruments or for measurements that need to be sustained (e.g., those generating climate data records). However, as noted above, the utility of the notional platform in this role is constrained by an orbit that may be mismatched to measurement needs.⁸

An informal review of measurement approaches suggests that perhaps 10 investigations could operate effectively on a single SSO platform with noon crossing time. Similar or smaller numbers of measurements are compatible with dawn-dusk or other daytime crossing times. However, when other factors such as orbit altitude are considered, the number of compatible measurements likely shrinks somewhat, particularly considering the diversity of passive and active sensors and requirements on spatial resolution, field of view, and revisit times. Further study would be needed to arrive at an optimal architecture; however, it does appear likely that several satellites of smaller size would enable a more compelling portfolio of science measurements than a single platform with 20 instruments at a given crossing time and altitude.

The committee notes that the greatest potential for the notional platform could be the ability to expedite instrument technology demonstration and incubator projects that may otherwise struggle to find opportunities for deployment in space given relative priorities compared to other parts of the NASA ESD portfolio, such as Earth System Explorers and Earth Venture. Instrument demonstration and incubator projects can often benefit from operation in space with orbits that are not necessarily optimized for science but offer opportunities to retire risk and advance sensor technology and retrieval algorithms.

Finally, the restricted scope of the NASA study request, as well as the committee’s limited time, resources, and expertise, did not allow for more than a top-level analysis of the practicality of the notional platform. In particular, the committee does not comment on a key attribute and critical enabling technology of the proposed platform: its capability for robotic operations that include instrument swaps.

___________________

⁸ In addition, the generation of climate data records—long-term, well-calibrated time series—can, in some cases, only be realized over multiple missions, each flying in a similar orbit. The notional platform would be obviously limited as a host for more than one of these types of measurements.