Emiliano Ortore

A small satellite constellation for continuous coverage of mid-low earth latitudes

When located on a geostationary orbit, a satellite keeps a steady position with respect to a generic point on the Earth’s surface and this characteristic allows for important advantages. A continuous longitudinal coverage of the Earth’s surface (higher latitudes excluded) is a result of using a three geo-satellite constellation. Nevertheless, there are also several drawbacks related to the geostationary orbit employment. The need to consider alternative satellite constellations has begun to arise from these disadvantages; these constellations, in spite of having very similar characteristics to the geostationary system, are able to overcome the complexity, the costs and the launching site problems connected with a geostationary satellite. For equatorial orbits, the Four-Leaf Clover System represents a profitable alternative compared to the traditional geostationary system. As far as high Earth latitudes are concerned, there are different operational constellations, such as Molniya and Tundra, capable of ensuring the continuous coverage of a region and generally taking orbits with a critical inclination into account (63.43 deg). The aim of this paper is to demonstrate that it is possible to create a satellite constellation capable of ensuring a continuous coverage of mid-low Earth latitudes. After a general study of the orbits employed to date, followed by a general graphical representation, a constellation of eight small satellites in multi-synchronous orbits makes the achievement of this paper’s aim possible. Several possibilities for application follow, both for telecommunications and remote sensing missions.

Satellite constellations in inclined multi-stationary orbits

After a general approach to the concept of multi-synchronous orbit (MSO) missions, which includes some past investigations on the use of orbits alternative to the Geostationary one, a selection of satellite constellations in MSOs, providing continuous and complete longitudinal coverage of the Earth, has been carried out. The concept of four-leaf clover system (FLCS), already introduced in the past for the equatorial–tropical belt, has been generalized, taking into account orbit inclinations up to the critical value (63.43°) and the consequent different operational scenarios. The temporal evolution of ground tracks and coverage of periodic orbits have been investigated in depth. A comparison between a Geostationary system and an inclined FLCS, in terms of costs and performance, pointed out the advantages of using these latter orbits.

Interlink and coverage analysis for small satellite constellations

In this article, the use of both the well-known Walker constellations and a proposed Randomic one are studied focusing on the data exchange between small satellites and their coverage of the Earth. The term Randomic is adopted to define constellations that are composed of small satellites placed in orbits occasioned by ‘catching a ride’ on launchers intended for other payloads. Thus, if on the one hand this ‘piggyback’ configuration has the merit of saving launch costs, on the other hand the small satellites will generally end up in orbits not ideally suited to their intended mission. In any case, in order to ensure an appropriate communication chain between the satellites, Bit Error Rate has to be kept below certain levels and this in turn requires keeping satellites within appropriate limits of distance and relative velocity. Following a statistical analysis over the past decade of launches, aimed at establishing the most exploitable orbits for ‘piggyback’ configurations, an investigation into the use of this type of constellation has been conducted. The comparison with an optimal Walker configuration has highlighted the better performance of the Walker and the significant reduction of mission expenditure of the Randomic constellation.

The "Ulingo" Mid-Low Latitudes Observation (MILO) Mission

This chapter aims at proposing a small mission devoted to observe the mid-low latitude regions. The selected orbit, called multi-sun-synchronous (MSS), represents an innovation with respect to the classical sun-synchronous orbit. The advantage offered by this kind of orbit, more suitable for observing tropical regions, will be evaluated and described. The satellite will be equipped with three optical sensors: a medium-high spatial resolution VIS-NIR multi-spectral sensor, allowing the surface monitoring and land-use and land-cover studies; a medium spatial-resolution 3-band thermal (MIR-TIR) sensor allowing the surface temperature (LST, SST) estimate and hot-spot (fires, volcanic eruption, etc.) detection; a panchromatic VIS-NIR camera for night-time observation able to reveal artificial and natural lights. Further, such an orbit allows the observation of the same region of the Earth at different local-times. In this way, the diurnal cycle of surface temperatures can be reconstructed with a 2.4-hour local-time interval. The amount of data acquired by an equatorial station has been estimated. Orbital perturbations effects have been taken into account in order to verify the feasibility of the mission; the Results have been reported in another chapter.