3-hours elliptical orbit constellation

Background

The motivation for finding orbit types other than the geostationary one led Spacesys to revisit the elliptical orbits, considering that the World gross product and population is concentrated in a 20° to 70° north latitude belt. As a matter of fact, many other orbit types- including the ICOs, the LEOs, Molnya, Tundra and the 8-hours period,  have specificities that are either unfavourable or unfit for certain communication applications in the above latitude belt.

 Accordingly we looked at one or more of the following features:

-        sunsynchronicity: to exploit  the benefits stemming from being able to put satellites in a dawn-dusk orbit were they are nearly continuously illuminated  by the sun;

-        short orbital period  but not as short as that of  typical LEOs ;

-        non-negligeable orbit ellipticity: in order to exploit the advantages stemming from disposing of  two orbital arcs (close to perigee and apogee) with very different features, enhancing the operational flexibility;

-        apsidal line stability with the time: this implied to fix the orbit inclination to either 63.4° or 116.6°

A small family of orbits meeting all the above requirements were found by the Spacesys founder in 1993 - and duly patented. The first member of this 'orbits family'  has the following characteristics:

-        elliptical, with eccentricity of 0.347;

-        inclination of 116.6° (i.e. retrograde) ;

-        orbit period of 3 hours;

-        semimajor axis: 10560.27 km;

-        perigee /apogee altitudes: 516.9 km /7847.38 km;

A spacecraft injected in this orbit makes 8 revolutions in one solar day. The two variables available for mission optimization are the RAAN  and the perigee argument.  Values of the latter of 90° or 270° optimize the coverage  for the northern and, respectively, the southern Earth hemisphere.  Values of the RAAN close to 6.a.m. or 6 p.m. give  the orbit plane an orientation allowing a nearly continuous sun illumination of the satellites. Achieving the required orbit starting from a lower altitude circular orbit is quite inexpensive too.

Applications

The potential applications are those exploiting the key features of the orbit type which is suitable to build up small constellations – 3 to 6 spacrcraft- injected in one or two orbital planes;

Impact on Mission: 

-        lower altitude than GEO > less demanding link budgets

-        long uninterrupted contacts (w.r.t. LEOs)  with ground sites

-        excellent coverage  of northern ( southern) latitudes for regional communications

  thanks to the inclination and apogee height

-        excellent satellites visibility from high latitude Earth’ terminals

-        roundtrip delay much shorter than with geo satellites

Impact on spacecraft design:

-        reduced orbit disturbances w.r.t.  LEOs;

-        spacecraft almost always sunlit > better Sun energy capture

-        nearly invariant attitude and thermal environment; black sky as an heat-sink

-        moderate delta-V for orbit injection

-        moderate size and cost

Example of constellation

-     multiple satellites for daylight-nightime emergency, service  or secure comms (in private networks)  between   sites  in the northern latitude belt

-        six satellites distributed in two orbital planes at 180° in RAAN

-        uninterrupted coverage for latitudes greater than 20° north

-        some service discontinuity for latitudes lower than 20°

The concept is now being revisited by Spacesys also in view of other applications more tailored to present and near-term needs  

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