DREAM: CONTINUOUS GLOBAL IMAGING.

Satellite observations have transformed our perception of the Earth–atmosphere system since the first images of Earth from space were acquired in 1960. Nevertheless, a satellite meteorologist’s dream to observe weather at any point and time over the globe remains elusive. This capability would represent a breakthrough for short- and long-term weather forecasting and narrowing uncertainties in the knowledge of Earth’s climate through better sampling of the diurnal cycle. Continuous observation would also be beneficial for improving the quality of satellite-derived essential climate variables (ECV). To a large extent, continuous imaging is achieved over the tropics and midlatitudes from geostationary (GEO) satellites that rotate around the Earth at the altitude of about 36,000 km in the equatorial plane with the same angular rate as the planet. The satellite can observe continuously with elevation angle higher than 20° above the horizon a large region that extends from the equator up to 62° latitude. Meteorological imaging from GEO orbit started in 1974. Since that time, the imaging technology has improved substantially. The Advanced Baseline Imager (ABI) developed for the third generation of geostationary satellites, such as the U.S. Geostationary Operational Environmental Satellite (GOES-R) and the Japanese Himawari 8 and 9 (Himawari 8 launched in October 2014) can provide uninterrupted scans of the full Earth disk every 5 min. Sectors of 1,000 × 1,000 km can be acquired with a temporal resolution of 30 s. These refresh rates can be effectively considered as “continuous imaging,” but a significant part of the globe still does not benefit from similar capabilities.

CHALLENGE: POLAR REGIONS.

Polar regions are frequently called “the weather kitchens of the Earth's climate.” Indeed, they exert great influence on global weather and show increased sensitivity to climate change. Orbital mechanics do not allow GEO-type imaging over the poles. The polar regions are presently monitored from low Earth orbiting (LEO) satellites circling around the Earth at altitudes between 600 km and 900 km with a period of rotation close to 100 min. Due to the nature of LEO observations, the imaging refresh rate is restricted by the orbital period and the width of image swath. The frequency of observations is also a function of latitude. For example, achieving the image refresh rate of 15(10) min at 60° latitude circle would require 23(34) polar LEO meteorological satellites with orbital and imaging characteristics similar to the Joint Polar Satellite System (JPSS). These numbers are prohibitively large to achieve the goal of GEO-like imaging at high latitudes from LEO systems; therefore an alternative solution is required.

MOLNIYA ORBIT: CAPABILITIES AND DRAWBACKS.

Quasigeostationary coverage of polar regions can be achieved from satellites in a highly elliptical orbit (HEO). The eccentricity of the HEO orbits is high by definition, and the satellite, in accordance with Kepler’s second law, spends most of the time in the vicinity of apogee (i.e., the farthest point from the Earth’s surface). If the orbit altitude is high enough, and the orbit is oriented in such a way that the apogee is located over one of the two polar regions, then a combination of only two HEO satellites can maintain a continuous view of the entire polar zone. When satellite A leaves the optimal viewing zone and heads toward the perigee (i.e., the closest point to the Earth’s surface), satellite B rises over the region to maintain the complete circumpolar region in sight. Interestingly, there are periods of several hours per day of coincident (i.e., stereo-like) imaging from the two satellites over most of the circumpolar area. Such a system could provide meteorological imaging and communication capacity similar to GEO, but focused on the polar region. Two pairs of HEO satellites (i.e., a total of four satellites) would be required to insure continuous coverage of both poles.

The first HEO satellite system with period of rotation equal to 12 h and called Molniya was implemented for communication purposes in 1965. It is established that a two-satellite Molniya HEO constellation can achieve continuous coverage of the polar region 58°–90°N with a viewing zenith angle (VZA) less than 70°. Another HEO system—with a 24-h period—called Tundra is currently used by the satellite Sirius XM Radio service operating in North America. Both orbits, 12-h Molniya and 24-h Tundra, are launched with an orbit inclination equal to 63.4°. This value is called the critical inclination. It corresponds to zero rate of the apogee drift due to the second zonal harmonic of the Earth gravitational field, and insures a stable position of apogee over the polar zone. If the HEO orbit inclination differs from the critical value, then the apogee gradually drifts toward the equator. Orbital maneuvers are then required to maintain the intended orbit position. The farther orbit inclination is from the critical value, the more resources are required to maintain the orbit. Therefore, it is highly recommended that the orbit inclination of the HEO system be set at the critical value.

A significant drawback associated with the 12-h Molniya orbit is the risk linked to hazardous levels of ionizing radiation due to passing the Van Allen belts. The highest danger originates from high-energy protons. The Molniya orbit crosses the proton radiation belts at the region of maximum concentration of energetic protons with energies up to several hundred MeV. As an alternative, the 16-h three apogee (TAP) HEO orbit was proposed, providing similar polar coverage as the Molniya HEO system while minimizing the proton ionizing hazards. The TAP orbit has a ground track with three apogee points repeatable over two days. The constellation of two satellites in TAP orbit still repeats the ground track in 24 h.

The Molniya HEO orbit was suggested for meteorological imaging in 1990, but has never been used operationally for that purpose. A new wave of interest in HEO satellite systems started a few years ago, when the scale of climate change happening in the Arctic polar region and the economic opportunities it may open became evident. Recognizing the importance and potential of the HEO system for polar observations, the World Meteorological Organization (WMO) created the International Geostationary Laboratory for Highly Elliptical Orbit (IGEOLAB HEO) Focus Group to coordinate international efforts in this field, and included the HEO concept in the most recent WMO implementation plan for the evolution of the Global Observing System.

Until now, most HEO studies designed to provide services over polar areas were structured along the 12-h Molniya, 16-h TAP, and 24-h Tundra orbit concepts that benefit from a ground track repeatable over a 24-h period. A characteristic feature of these orbits is the existence of a few (1, 2, or 3) standing apogee points as shown in Figs. 1a and 1b.

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Ground track for (a) 12-h Molniya, (b) 16-h TAP, and (c) 14-h and (d) 15-h two-satellite HEO constellations. Yellow and magenta colors denote ground tracks for each satellite in a two-satellite constellation flying in the same orbital plane and separated by 180° phase angle.

Citation: Bulletin of the American Meteorological Society 97, 1; 10.1175/BAMS-D-14-00251.1

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A question comes to mind: Are the HEO orbital configurations previously developed for communication and surveillance applications also optimal for meteorological imaging of the polar regions?

NOVEL SOLUTION: MULTIPLE-APOGEE HEO SYSTEMS.

We argue here that an HEO system with a small number of apogees is suboptimal for meteorological and climate applications. Indeed, with more apogees, more evenly distributed imaging conditions are obtained. This aspect is evident from Figs. 1c and 1d, which shows the ground tracks for the 14-h and 15-h HEO systems. The 14-h two-satellite HEO system has 24 apogees (12 for each satellite). The ground track repeats over a 7-day period. The 15-h two-satellite HEO system has 16 apogees (8 for each satellite) repeated over a 5-day period. Due to a significantly larger number of apogee points in comparison to 12-h, 16-h, and 24-h HEO orbits, we refer to these configurations as multiple-apogee (MAP) HEO systems. This is a first and major consideration, but there are more, as presented below.

The selection of MAP orbits presented in Table 1 is based on our recent analysis of ionizing radiation for the family of HEO orbits suitable for continuous observations of the Earth’s polar regions. Analysis of ionizing radiation revealed the presence of a local minimum in the region of orbital periods between 14 h and 15 h. The total ionizing dose (TID) modeled via the Space Environment Information System (SPENVIS: www.spenvis.oma.be) at the reference level of 50 krad over 15-yr period can be achieved with 3.2 mm of aluminum shielding for the 14-h orbit and 3.3 mm for the 15-h orbit. This thickness of aluminum shielding is nearly 50% smaller than for the 12-h Molniya orbit. The total doses for protons are estimated to be 2–4 times smaller for the 14-h and 15-h orbits than for the 12-h orbit. A reduced proton dose for MAP HEO systems is an important additional argument, as proton radiation represents one of the most significant risks to the HEO space mission.

Table 1.

Some features of the MAP HEO orbits.

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Another important feature of the MAP configurations with orbital periods in the 14-h to 15-h range is that they can be at critical orbit inclination equal to 63.4°. It turns out that the orbit inclination for orbits with period greater than 15 h needs to be increased above the critical value to achieve the required coverage of high latitudes within a reasonable imaging altitude range. This feature also favors 14-h and 15-h orbits. Several other parameters of the MAP HEO orbits are presented in Table 1. The eccentricity ranges from 0.63 to 0.71, which insures sufficient dwelling time around the apogee point located over the polar region. The imaging time is 16 h per day over the polar zone, which is required to obtain uninterrupted coverage from a two-satellite system. The satellite altitude within the imaging period ranges from 26,600 km to 44,000 km, with midpoint around 39,900 km, which is comparable to the GEO height. The satellite latitude varies from 38.1° (15-h orbit) or 44.5° (14-h orbit) to 63.4° (i.e., equal to the orbit inclination), which is convenient for satellite tracking from ground receiving stations located in mid- and high-latitude zones. The perigee heights range from 2,131 km to 4,902 km [i.e., significantly higher than the 12-h Molniya orbit value (∼500 km)]. This is beneficial for reducing the atmospheric drag and collision avoidance with other satellites and space debris in the near-Earth region densely populated at altitudes below 2,000 km.

The spatial coverage achieved with 24 or 16 equally spaced apogees over the repeat cycle of 14-h and 15-h orbits is very uniform (Fig. 2). This is an important feature of the HEO system for climate applications, as it reduces biases in satellite-derived ECVs and other parameters caused by unequal distribution of observational angles. The coverage shown in Fig. 2a is obtained assuming a maximum VZA equal to 70° (i.e., minimum elevation angle of 20°). The 100% (continuous) coverage is achieved for a latitude range from the pole (90°) to slightly below 60°, while coverage of 50% is still achieved at 10° latitude, providing a valuable backup to all GEOs. As shown in Fig. 2b, the large number of apogees implies that the VZA ranges from zero to 70° for the entire region from the equator to 63.4° latitude. With a large number of apogees, any point in the circumpolar area can be continuously observed from multiple directions over the repeat period, thus dramatically increasing the diurnal cycle sampling. The distribution of viewing zenith angle versus relative azimuth and solar zenith angles is also quite diverse. The wide range of observational angles creates a unique capability for characterization of bidirectional reflectance properties of clouds and surface over polar latitudes. This capacity is also superior to that from GEO in the region of HEO-GEO overlap.

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Features of two-satellite 14-h HEO system for the Northern Hemisphere. (a) Map of temporal coverage (%) over the repeat cycle. Arcs correspond to five GEO satellites equally spaced along the equator. (b) Distribution of minimum viewing zenith angle.

Citation: Bulletin of the American Meteorological Society 97, 1; 10.1175/BAMS-D-14-00251.1

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Several other suitable MAP orbit configurations exist in addition to the 14-h and 15-h systems as shown in Table 1. They include, among others, the periods equal to 14.4 h and 14.77 h with 5 orbits over a 3-day repeat cycle and 13 orbits over an 8-day repeat cycle, correspondingly. In a two-satellite constellation for each hemisphere with satellite pairs launched in the same orbital plane and separated by 180° phase angle, the number of apogees increases two-fold.

VIEWING EARTH FROM HEO ORBIT: CHALLENGES AND NEW OPPORTUNITIES.

The HEO mission belongs to the GEO class of Earth Observing system due to a similar range of altitudes and image acquisition strategy (i.e., 2D scanning). The main challenge in viewing the Earth from HEO orbit is a variable field of view due to continuously changing altitude. Several assessment studies were completed to evaluate the suitability of new-generation GEO imagers, like the ABI planned for GOES-R and the flexible combined imager (FCI) planned for Meteosat Third Generation (MTG) satellites, to meet HEO imaging requirements. It was determined that the new-generation imagers are capable of acquiring imagery without gaps in HEO orbits while scanning as fast as from GEO orbit. The realistic sensitivity scenario analysis confirmed that the image geolocation accuracy will be similar to what is planned for GEO. The effective image resolution collected from HEO orbit varies with altitude; however, this is not perceived as a problem. The predetermined fixed grid map centered at the pole can be used for image remapping, similar to the GEO fixed grid. Normally, each satellite in a two-satellite HEO system should acquire imagery 16 h per day distributed symmetrically around the apogee point. This scenario creates a unique opportunity for stereo imaging from two satellites about 30% of the time.

Emerging technologies offer some new and unique applications, such as day-night imaging capacity currently available from the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi National Polar-orbiting Partnership (S-NPP) spacecraft. This is important because of long periods of nighttime conditions lasting for several months over the polar zones. Selection of bands on new-generation GEO imagers could be augmented with minor implications for the cost and technology to reflect the uniqueness of the polar environment, such as a need for better snow/ice, fog, and haze retrievals. The HEO orbit is well suited for observation of space weather (i.e., in situ magnetic field and ionizing radiation measurements). Studies have also shown the unique capability of the HEO orbit to improve the carbon cycle characterization over the Arctic and boreal zone using HEO infrared (IR) and visible-ultraviolet (VIS-UV) spectrometers, as reported in provided references.

SUMMARY.

A novel type of highly elliptical orbits termed as multiple-apogee (MAP) HEO systems with orbital periods between 14 h and 15 h is introduced. The MAP HEO two-satellite constellation for each hemisphere is designed to achieve continuous GEO-like imaging of the polar regions in an optimal way. These systems can be launched at critical orbit inclination and correspond to a local minimum of ionizing radiation with a relatively small proton component. These features simplify the process of orbit maintenance, reduce radiation shielding requirements, and favor a longer lifetime of the mission. In comparison to conventional HEO systems with a few apogees, like the classical 12-h Molniya concept, a MAP HEO constellation with multiple apogees achieves a more uniform geometrical sampling, which reduces view angle–dependent biases. These observational conditions are beneficial for high-latitude meteorological and climate applications.