VOLUME 28 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY AUGUST 2011 Spatial and Temporal Sampling of Polar Regions from Two-Satellite System on Molniya Orbit ALEXANDER P. TRISHCHENKO* Network Strategy and Design, Meteorological Service of Canada, Environment Canada, Ottawa, Ontario, Canada LOUIS GARAND Data Assimilation and Satellite Meteorology Research, Science and Technology Branch, Environment Canada, Dorval, Quebec, Canada (Manuscript received 22 October 2010, in final form 28 January 2011) ABSTRACT There has been a significant increase of interest in the building of a comprehensive Arctic observing system in recent years to properly and timely track the environmental and climate processes in this vast region. In this regard, a satellite observing system on highly elliptical orbit (HEO) with 12-h period (Molniya type) is of particular interest, because it enables continuous coverage of the entire Arctic region (588–908N) from a constellation of two satellites. Canada is currently proposing to operate such a constellation by 2017. Extending the pioneering study of S. Q. Kidder and T. H. Vonder Haar, this paper presents in-depth analysis of spatiotemporal sampling properties of the imagery from this system. This paper also discusses challenges and advantages of this orbit for various applications that require high temporal resolution and angular sampling. 1. Introduction Increased attention to the Arctic weather is also paid because of significant concerns related to a changing cli- Product latency and refresh rate are among key crit- mate, as this region is broadly anticipated to be the most ical features of a satellite observing system. The ability affected by increasing temperature (Solomon et al. 2007). to acquire and deliver imagery of the earth over large The enhanced melting of sea ice allows significant eco- areas with high temporal resolution is getting increasingly nomic opportunities related to transportation and natural important for weather monitoring, allowing more reliable resources exploration. Deriving high-quality climate re- nowcasting and longer-term forecasts to ensure the safety cords from satellites also requires good temporal sam- of marine, ground, and air transportation. This is espe- pling, especially for rapidly evolving variables such as cially true for the harsh and rapidly changing Arctic en- cloud and radiation. vironment, where climate conditions often put at risk the The current paradigm of satellite meteorology relies survivability of humans in critical situations. The high on the combination of geostationary (GEO) and low temporal frequency of the imagery is very beneficial to earth orbiting (LEO) satellites. The LEO satellites be- track various dangerous environmental conditions that long in most cases to the category of polar orbiters. are common at high latitude such as polar lows, fog, air- Because of synchronous motion of the GEO satellites craft icing, sea ice movements, and volcanic ash transport. with the earth’s rotation, they are perceived as station- ary platforms located over a given nadir position on the equator. The GEO satellites permit continuous obser- * Current affiliation: Canada Centre for Remote Sensing, Ottawa, Canada. vation of the weather within their area of coverage. The constellation of several GEO satellites is currently op- erating around the earth at any time, which allows Corresponding author address: Alexander P. Trishchenko, Canada Centre for Remote Sensing, 588 Booth Street, Ottawa ON K1A0Y7, continuous coverage of tropical and midlatitude regions Canada. of the earth up to approximately 608. The GEO con- E-mail: [email protected] stellation has been operating as part of the international DOI: 10.1175/JTECH-D-10-05013.1 977 978 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 28 World Weather Watch Program since 1979. However, by a sufficient number of VIS and IR channels, and to good-quality observations between 608 and 908 latitude carry a suit of detectors for space weather observations zones in the Northern or Southern Hemispheres cannot (PCW 2010). The Russian Federation proposes the be obtained from GEO systems because of oblique view multifunctional space system ‘‘Arktika,’’ which will geometry. High-latitude observations are currently pro- consist of several spacecrafts, including those on HEO vided by LEO satellites. These satellites have a typical orbit. orbital period of about 100 min, and the swath is nor- The advantages of the Molniya orbit for the provision mally below 2500 km. Consequently, there is currently of communication services and some other purposes no source of continuous imagery for the southern or over high-latitude regions have been clearly demon- northern polar regions obtained with sufficiently high strated. Despite this significant success, that orbit has refresh rate. The spatial discontinuity and lack of ade- not been exploited so far for operational meteorological quate temporal sampling significantly affects many ap- applications. This is due to three major factors: com- plications, especially those related to atmospheric motion plexity of the observational geometry created by the vector retrievals; radiation budget; surface parameters; constantly changing satellite altitude and speed, de- estimation of bidirectional reflectance distribution func- manding pointing accuracy for imager and spacecraft, tion (BRDF); and studies linked to cloud life cycle, no- and risk associated with the harsh radiation environment. tably frontal passages. The first two factors, variable altitude and pointing, As demonstrated by Kidder and Vonder Haar (1990), combined with the need for precise image registration, it turns out that continuous coverage of the polar areas impose very strict requirements on the imager scanning can be achieved from satellites launched on a highly system and satellite attitude knowledge and control. The elliptical orbit (HEO). The HEO term covers a number ionizing radiation on Molniya HEO orbit, unlike LEO of potential orbits. A well-established HEO is the and GEO configuration, includes a significant portion of Molniya (‘‘lightning’’ in Russian) orbit used intensively trapped protons that affect spacecrafts twice per orbit by the Soviet Union and the Russian Federation for when passing through the Van Allen radiation belts. communication purposes since 1965. This orbit has Recent feasibility studies conducted for HEO missions a 63.48 inclination, and the orbital period is close to half have concluded that with currently available technology a sidereal day. The spacecraft has a 24-h repeatable it is possible to build a HEO system providing imagery for ground track with two apogees located 1808 apart. The meteorological applications (WMO 2010). In this paper, concept of using high inclination orbits to improve earth we report some new results obtained during preliminary observations over polar latitudes was recently endorsed phases of the PCW system development in Canada. One by the World Meteorological Organization (WMO) major conclusion derived from this effort is that a two- in its ‘‘Vision for the Global Observing System (GOS) satellite system on Molniya orbit is capable of providing in 2025’’ adopted by the 61st session of the WMO Ex- observations with viewing zenith angle (VZA) , 708 ecutive Council (EC-LXI) (WMO 2009). The WMO (recognized limit for quantitative retrievals) continuously document recommends operational implementation of (100% coverage) in the latitude zone between 588 and 908 visible (VIS) and infrared (IR) HEO imagers to monitor and nearly continuous observations in the latitude zone high-latitude phenomena related to winds, clouds, vol- between 458 and 588 with coverage greater than 72% canic ash plumes, sea ice, snow cover, vegetation prop- (;85% on average). The area of continuous coverage erties, and wild fires with sufficient temporal resolution goes down to 388 if VZA , 908. (WMO 2009). The paper is organized as follows: Section 2 provides To address the need in the HEO meteorological a summary of equations and features of Molniya HEO. observing system, the Focus Group for Highly Ellip- Section 3 presents results on the HEO capabilities re- tical Orbits was established as a part of the WMO In- lated to temporal sampling. Section 4 discusses the se- ternational Geostationary Laboratory (IGEOLAB) lection of apogee points and the choice of single orbital initiative. In the framework of IGEOLAB HEO focus plane versus two planes for the constellation. The HEO group, two countries, Canada and the Russian Federa- constellation provides unique capabilities in terms of tion, have announced their plans for HEO missions in viewing geometry for the study of cloud and surface bi- the time frame 2013–16 (WMO 2010). Canada proposes directional reflectance characteristics. This is presented in the Polar Communication and Weather (PCW) satellite section 5. Section 6 concludes the article. Material in system that will consist of a pair of HEO satellites. Each appendix A illustrates orbit layout and provides defini- PCW satellite will be equipped to provide communica- tion of orbital parameters. Appendix B discusses the tion services over the selected high-latitude region, to potential problem of the imaging instrument exposure to carry a multispectral meteorological imager characterized direct sun during observations from Molniya orbit. AUGUST 2011 T R I S H C H E N K O A N D G A R A N D 979 2. Basic properties of Molniya orbit of the oblateness of the earth, its gravitational field U(r) is not spherically