IEEE Geoscience and Remote Sensing Magazine - September 2021 - 36

The information in Figure 3 demonstrates the diversity
of the sensor types used for measuring the different
geophysical variables, which contributes to the overall
robustness of the EOSC but, more importantly, to how
complementary these observing systems are. We see that
ozone, trace gases, and, to a lesser extent, aerosols are well
retrieved from IR, VIS, and UV sensors (albeit with low
vertical resolution) but poorly inferred from MW sensors,
except for limb-sounding MW sensors, while temperature
and water vapor profiles are well retrieved from passive
MW and IR radiometers. Note that the best information on
hydrometeor profiles and ocean surface winds comes from
specially designed active sensors (cloud and precipitation
radars and radar scatterometers, respectively). Considering
Figure 3 as a whole, even though there is a predominance
of empty (white) cells indicating that no useful information
is contributed for those geophysical variables by that sensor
type, at least one and as many as six sensor types provide at
least high-relevance (yellow or better) capability to observe
each of the major geophysical variables.
The full capability-assessment array (i.e., the ability of
the entire EOSC or individual sensors/constellations to satisfy
the needs of a specific application) may be thought of as
3D-the geophysical variables in one dimension, the sensor
type in a second, and the attributes of the variables (Table 1)
in the third. Figure 3 depicts an average over attributes. In
fact, the attributes of different sensor systems are quite diverse.
As an example, Figure 4 presents the typical horizontal
coverage from a selection of sensors. These maps reflect the
different orbits and observing patterns of the sensors. In a
limited time window, compositing the coverage of the different
sensors that make up the EOSC results in data gaps as
well as areas of overlap within which there may be different
performance attributes, including resolution and accuracy.
OBSERVING SYSTEMS OF THE FUTURE
The WMO Integrated Global Observing System (WIGOS)
vision [6] is a comprehensive assessment of what the global
constellation of observing systems might look like in the
2040 time frame. It identifies several key factors in the evolution
of the EOSC leading up to the 2040 timeframe. These
include 1) the requirements from new and changing applications,
including the quickly developing coupled Earth
system models; 2) the advances in satellite remote sensing
capabilities; 3) a need for greater resiliency; 4) increases in
the number and diversity of satellite providers; 5) a larger
diversity in the types and quality of the observations; 6)
maintaining international planning, coordination, and
cooperation; and 7) the expectation that the observations
provided by WIGOS be made freely available in unrestricted
international exchange.
INSTRUMENTS AND PLATFORM IMPROVEMENTS
Evolving application and user requirements will drive improvements
in the EOSC. There are numerous potential improvements
in remote sensing technology: future passive
36
MW and IR sensors will have more spectral information and
higher horizontal resolution. Advances in technology have
the potential to support high temporal- and horizontalresolution
hyperspectral IR and MW sounders and imagers
to be deployed in GEO [17], [37], [38]. However, it is likely
that technological advances will be needed to demonstrate
a GEO MW instrument that simultaneously achieves an
acceptable signal-to-noise ratio and useful temporal and
horizontal resolution. Active MW instruments will make
use of multiple frequencies and polarizations, and signals
of opportunity will increasingly be exploited (e.g., in [39]).
In general, validity ranges, i.e., the range of values retrieved
with sufficient accuracy, will be extended. For example,
cross-polarization capabilities in scatterometers will allow
for the retrieval of higher wind speeds than the current signal
saturation, which occurs at approximately 30 m/s.
In addition to the traditional GEO and LEO orbits for
meteorological satellites, other orbits will be used. Lowinclination
(equatorial) orbits are already used for tropical
missions, such as COSMIC-2 [40], and highly elliptical orbits
have been the subject of several studies and proposals,
(e.g., in [41]).
Another trend in new missions is constellations of SmallSats
with masses of generally fewer than 1,000 kg. CubeSats,
a class of SmallSats constructed from standard 10-cm
cube components and with masses of fewer than 10 kg,
are becoming increasingly popular with rapidly evolving
capabilities engendered by the 10-cm standardization [42].
Multiple SmallSats and CubeSats are often deployed at low
(i.e., 400-500 km) altitudes, leading to higher temporal
and horizontal coverage as well as increased horizontal
resolution in measuring the atmospheric profiles of temperature
and humidity, as well as other atmospheric and
surface properties. Due to their low cost, SmallSats can also
be flown in specific formations, providing new capabilities.
For example, SmallSats flying in formation can track features
to estimate winds and accurately measure multiangle,
multispectral reflectance from surfaces that can be used to
determine a number of surface and cloud properties, such
as albedo and vegetation properties [43], [44]. The CYGNSS
mission [12] launched eight satellites in low-inclination
orbits on 15 December 2016. Each CYGNSS satellite has
a mass of 28.9 kg. Commercial satellite companies have
launched large numbers of SmallSats and CubeSats. In the
case of RO observations, these data are proving useful for
NWP [45].
The trend toward smaller platforms will continue in the
future due to advances in technology and the availability
of reusable designs and components, leading to less costly
and faster fabrication and the greater availability of inexpensive
launch options [46]. This will provide significant
capabilities for high temporal refresh for certain parameters.
However, the need for in-orbit, high-quality, and highresolution
observations of certain parameters will remain,
and this will likely require maintaining larger satellites in
the near future, at least for some parameters.
IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE SEPTEMBER 2021
IEEE Geoscience and Remote Sensing Magazine - September 2021

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