Multiple-Apogee Highly Elliptical Orbits for Continuous Meteorological Imaging of Polar Regions Challenging the Classical 12-H Molniya Orbit Concept
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Multiple-Apogee Highly Elliptical Orbits for Continuous Meteorological Imaging of Polar Regions Challenging the Classical 12-h Molniya Orbit Concept BY ALEXANDER P. TRISHCHENKO, LOUIS GARAND, LARISA D. TRICHTCHENKO, AND LIDIA V. NIKITINA DREAM: CONTINUOUS GLOBAL IMAGING. 1974. Since that time, the imaging technology has Satellite observations have transformed our percep- improved substantially. The Advanced Baseline tion of the Earth–atmosphere system since the first Imager (ABI) developed for the third generation of images of Earth from space were acquired in 1960. geostationary satellites, such as the U.S. Geostation- Nevertheless, a satellite meteorologist’s dream to ary Operational Environmental Satellite (GOES-R) observe weather at any point and time over the globe and the Japanese Himawari 8 and 9 (Himawari 8 remains elusive. This capability would represent launched in October 2014) can provide uninter- a breakthrough for short- and long-term weather rupted scans of the full Earth disk every 5 min. forecasting and narrowing uncertainties in the Sectors of 1,000 × 1,000 km can be acquired with a knowledge of Earth’s climate through better sam- temporal resolution of 30 s. These refresh rates can pling of the diurnal cycle. Continuous observation be effectively considered as “continuous imaging,” would also be beneficial for improving the quality of but a significant part of the globe still does not ben- satellite-derived essential climate variables (ECV). efit from similar capabilities. To a large extent, continuous imaging is achieved over the tropics and midlatitudes from geostationary CHALLENGE: POLAR REGIONS. Polar regions (GEO) satellites that rotate around the Earth at the are frequently called “the weather kitchens of the altitude of about 36,000 km in the equatorial plane Earth's climate.” Indeed, they exert great influence with the same angular rate as the planet. The satel- on global weather and show increased sensitivity lite can observe continuously with elevation angle to climate change. Orbital mechanics do not al- higher than 20° above the horizon a large region low GEO-type imaging over the poles. The polar that extends from the equator up to 62° latitude. regions are presently monitored from low Earth Meteorological imaging from GEO orbit started in 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 na- ture of LEO observations, the imaging refresh rate AFFILIATIONS: TRISHCHENKO—Canada Centre for Remote Sensing, is restricted by the orbital period and the width of Natural Resources Canada, Ottawa, Ontario, Canada; GARAND— Atmospheric Science and Technology, Environment Canada, Quebec, image swath. The frequency of observations is also Canada; TRICHTCHENKO AND NIKITINA—Canadian Space Weather a function of latitude. For example, achieving the Forecast Centre, Natural Resources Canada, Ottawa, Ontario, image refresh rate of 15(10) min at 60° latitude circle Canada would require 23(34) polar LEO meteorological CORRESPONDING AUTHOR: Alexander P. Trishchenko, Canada satellites with orbital and imaging characteristics Centre for Remote Sensing, Natural Resources Canada, 560 similar to the Joint Polar Satellite System (JPSS). Rochester Str., Ottawa, ON K1A 0E4, Canada These numbers are prohibitively large to achieve E-mail: [email protected] the goal of GEO-like imaging at high latitudes from DOI:10.1175/BAMS-D-14-00251.1 LEO systems; therefore an alternative solution is required. PB | JANUARY 2016 AMERICAN METEOROLOGICAL SOCIETY JANUARY 2016 | 19 Unauthenticated | Downloaded 09/29/21 09:14 PM UTC MOLNIYA ORBIT: CAPABILITIES AND DRAW- protons. The Molniya orbit crosses the proton radia- BACKS. Quasigeostationary coverage of polar tion belts at the region of maximum concentration of regions can be achieved from satellites in a highly energetic protons with energies up to several hundred elliptical orbit (HEO). The eccentricity of the HEO MeV. As an alternative, the 16-h three apogee (TAP) orbits is high by definition, and the satellite, in ac- HEO orbit was proposed, providing similar polar cordance with Kepler’s second law, spends most of the coverage as the Molniya HEO system while minimiz- time in the vicinity of apogee (i.e., the farthest point ing the proton ionizing hazards. The TAP orbit has a from the Earth’s surface). If the orbit altitude is high ground track with three apogee points repeatable over enough, and the orbit is oriented in such a way that two days. The constellation of two satellites in TAP the apogee is located over one of the two polar regions, orbit still repeats the ground track in 24 h. then a combination of only two HEO satellites can The Molniya HEO orbit was suggested for meteo- maintain a continuous view of the entire polar zone. rological imaging in 1990, but has never been used When satellite A leaves the optimal viewing zone and operationally for that purpose. A new wave of interest heads toward the perigee (i.e., the closest point to the in HEO satellite systems started a few years ago, when Earth’s surface), satellite B rises over the region to the scale of climate change happening in the Arctic maintain the complete circumpolar region in sight. polar region and the economic opportunities it may Interestingly, there are periods of several hours per open became evident. Recognizing the importance day of coincident (i.e., stereo-like) imaging from the and potential of the HEO system for polar observa- two satellites over most of the circumpolar area. Such tions, the World Meteorological Organization (WMO) a system could provide meteorological imaging and created the International Geostationary Laboratory communication capacity similar to GEO, but focused for Highly Elliptical Orbit (IGEOLAB HEO) Focus on the polar region. Two pairs of HEO satellites (i.e., Group to coordinate international efforts in this field, a total of four satellites) would be required to insure and included the HEO concept in the most recent continuous coverage of both poles. WMO implementation plan for the evolution of the The first HEO satellite system with period of Global Observing System. rotation equal to 12 h and called Molniya was imple- mented for communication purposes in 1965. It is established that a two-satellite Molniya HEO constel- lation 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 pe- riod—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 re- quired to maintain the orbit. Therefore, it is highly recommended that the orbit inclination of the HEO FIG. 1. Ground track for (a) 12-h Molniya, (b) 16-h system be set at the critical value. TAP, and (c) 14-h and (d) 15-h two-satellite HEO A significant drawback associated with the 12-h constellations. Yellow and magenta colors denote Molniya orbit is the risk linked to hazardous levels ground tracks for each satellite in a two-satellite of ionizing radiation due to passing the Van Allen constellation flying in the same orbital plane and belts. The highest danger originates from high-energy separated by 180° phase angle. 20 | JANUARY 2016 AMERICAN METEOROLOGICAL SOCIETY JANUARY 2016 | 21 Unauthenticated | Downloaded 09/29/21 09:14 PM UTC Until now, most HEO studies designed to provide for the family of HEO orbits suitable for continuous services over polar areas were structured along the observations of the Earth’s polar regions. Analysis 12-h Molniya, 16-h TAP, and 24-h Tundra orbit con- of ionizing radiation revealed the presence of a cepts that benefit from a ground track repeatable over local minimum in the region of orbital periods a 24-h period. A characteristic feature of these orbits between 14 h and 15 h. The total ionizing dose (TID) is the existence of a few (1, 2, or 3) standing apogee modeled via the Space Environment Information Sys- points as shown in Figs. 1a and 1b. tem (SPENVIS: www.spenvis.oma.be) at the reference A question comes to mind: Are the HEO orbital level of 50 krad over 15-yr period can be achieved with configurations previously developed for communica- 3.2 mm of aluminum shielding for the 14-h orbit and tion and surveillance applications also optimal for 3.3 mm for the 15-h orbit. This thickness of aluminum meteorological imaging of the polar regions? shielding is nearly 50% smaller than for the 12-h Mol- niya orbit. The total doses for protons are estimated to NOVEL SOLUTION: MULTIPLE-APOGEE HEO be 2–4 times smaller for the 14-h and 15-h orbits than SYSTEMS. We argue here that an HEO system with for the 12-h orbit. A reduced proton dose for MAP a small number of apogees is suboptimal for meteo- HEO systems is an important additional argument, as rological and climate applications. Indeed, with more proton radiation represents one of the most significant apogees, more evenly distributed imaging conditions risks to the HEO space mission. are obtained. This aspect is evident from Figs. 1c and Another important feature of the MAP configura- 1d, which shows the ground tracks for the 14-h and tions with orbital periods in the 14-h to 15-h range is 15-h HEO systems.