Abstract
The development of small-satellite technologies allows the low Earth orbit intersatellite link (LEO-LEO) occultation method to observe the Earth’s atmosphere with global coverage and acceptable costs using electromagnetic signals, in which the L/X/K/M band and short-wave infrared band signals have been well demonstrated to be suitable. We hence need to investigate the impacts of orbital and constellation parameters on the number and spatiotemporal distribution of LEO-LEO occultation events for best-possible LEO-LEO occultation mission design and optimization at the targeted mission size. In this study, firstly, an occultation events location simulation model accounting for the right ascension of the ascending node (RAAN) precession was set up and the concept of a time-dependent global coverage fraction of occultation events was defined. Secondly, numerical experiments were designed to investigate the orbital parameters’ impacts and to assess the performance of LEO-LEO occultation constellations, in which the Earth is divided into 5° × 5° latitude and longitude cells. Finally, the number, timeliness, and global coverage fraction of occultation events for two-orbit and multi-orbit LEO-LEO constellations were calculated and analyzed. The results show that: ① the orbit inclination and RAAN are the main impacting parameters followed by orbital height, while the RAAN precession is a relevant modulation factor; ② co-planar counter-rotating receiving and transmitting satellite orbits are confirmed to be ideal for a two-satellite LEO-LEO constellation; ③ polar and near-polar orbit constellations most readily achieve global coverage of occultation events; near-equator orbit constellations with supplementary receiving and transmitting satellite orbit planes also readily form the occultation event geometry, though the occultation events are mainly distributed over low and low-to-middle latitude zones; and ④ a well-designed larger LEO-LEO occultation constellation, composed of 36–72 satellites, can meet the basic requirements of global numerical weather prediction for occultation numbers and timeliness, yielding 23,000–38,000 occultation events per day and achieving 100% global coverage in 12–18 h.
Highlights
Occultation is a well-proven remote sensing method that requires at least one electromagnetic signal transmitting satellite (Tx) and one receiving satellite (Rx) to observe the Earth’s atmospheric thermodynamic state variables [1,2,3,4,5,6], chemical composition [7,8,9,10,11], dynamics [12], and ionospheric electron density profiles by using particular frequency signals [13]
Based on a pair of Rx and Tx satellites, the impacts of the orbital parameters on the number and spatial–temporal distribution of LEO-LEO occultation events were analyzed using the variable separation approach, in which the analyzed target parameter was set as a discrete variable while other parameters were fixed as certain values
From the results and discussion we can draw the following conclusions: 1 the inclination and right ascension of the ascending node (RAAN) are the main orbital parameters impacting the occultation event number and coverage, followed by the orbital height; in particular, the RAAN precession is an important orbital perturbation that cannot be neglected in the LEO-LEO occultation constellation design; 2 co-planar and counter-rotating Rx and Tx orbits were confirmed to provide an ideal two-satellite LEO-LEO constellation; 3 polar orbit and near-polar orbit
Summary
Occultation is a well-proven remote sensing method that requires at least one electromagnetic signal transmitting satellite (Tx) and one receiving satellite (Rx) to observe the Earth’s atmospheric thermodynamic state variables (e.g., pressure, temperature, and humidity) [1,2,3,4,5,6], chemical composition (e.g., the greenhouse gases H2 O, CO2 , CH4 , N2 O, O3 , and CO) [7,8,9,10,11], dynamics (e.g., line-of-sight wind speed) [12], and ionospheric electron density profiles by using particular frequency signals [13]. As an advanced technique, low Earth orbit intersatellite link (LEO-LEO) microwave occultation (LMO), using various X/K band and M band frequencies around the 22 GHz and 183 GHz water absorption lines, can exploit both the refraction and absorption of the signals to solve the temperature–humidity ambiguity problem; LMO enables us to retrieve pressure, temperature, and humidity profiles without background information [5,15]. Rx and Tx orbital parameters on the number and spatial–temporal distribution of LEO-LEO occultation events and the exploration capability of LEO-LEO occultation constellations need to be deeply investigated and assessed as they are significant to LEO-LEO mission design and optimization. The impacts of the Rx and Tx orbital parameters on the LEO-LEO occultation event numbers and distribution were investigated and assessed using a two-satellite constellation with one LEO Rx and one LEO Tx based on simulation datasets by the variable separation approach.
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