©2011 Microcosm Inc. W23/1

23.1 LEO Communications Constellations

Historical ApproachesStuart Eves, Surrey Satellite Technology

As will be appreciated from Chaps. 8 to 10, satellitesin LEO have relatively limited coverage footprints on thesurface of the globe by comparison with their cousinshigher altitude orbits. Bearing in mind this footprint lim-itation, a system designer wishing to achieve a reason-able level of communications performance isautomatically driven towards a constellation involvingmultiple satellites. Traditionally, both satellites and theirlaunch vehicles have been expensive, and this raisesobvious questions about the financial wisdom of con-structing multi-satellite communications constellationsin LEO—would smaller numbers of satellites in higherorbits not represent a more logical investment? And yetthe most prolific satellite series in history, the RussianStrela-1 system, is a communications constellationwhich over its lifetime saw the launch of some 350 or sorelatively short-lived satellites. And in the 1990s, Irid-ium, Globalstar and Orbcomm all invested large sums inthe creation of LEO communications systems.

The explanation behind the apparent contradictionrelates to the user communities that these satellite sys-tems were endeavoring to serve, and the locations ofthose users on the surface of the Earth. These user com-munities were either mobile, with small, low-powerhand-held receivers, or, (in the case of the Strela-1 sys-tem), espionage agents who presumably had no desire toadvertise their presence by erecting a satellite dish on theroof! In most cases, such terminals will not be “coopera-tive,” (in the sense that the user will not necessarily beable to ensure a clear line of sight to the satellite, or usea highly directional antenna to track the satellite as itmoves across the sky). In order to establish a satisfactorylink budget to such an uncooperative terminal, it is nec-essary to ensure that the Effective Isotropic RadiatedPower (EIRP) from the satellites is sufficient to over-come these limitations. Specifically, the system designermust make certain that the free space path loss, (which isdictated by the range between the transmitter andreceiver), does not render the system infeasible.

The early Russian Strela-1 satellites were simple,mass produced devices. Approximately spherical, andlacking attitude control, they were equipped with rela-tively low gain, low frequency antennas, and werelaunched in batches of 8 into a 1,500 km altitude, highinclination orbit. Lacking a propulsion system, they weredeployed at intervals of a few seconds from the Cosmoslaunch vehicle, thereby gaining slightly different initial

orbital parameters which would cause them to driftaround their orbit plane relative to one another over time.More than one plane of these satellites was supported,but the lack of a station keeping system meant that theywere, for statistical reasons, unable to guarantee uninter-rupted coverage. The system was, instead, used to sup-port a store-and-forward communication system forRussian agents worldwide.

The Strela-1 constellation was eventually supersededby a more sophisticated system called Strela-2, (latermarketed commercially under the name Gonets in theWest). This constellation was composed of larger grav-ity-gradient stabilised satellites which could performreal-time communication, if both user and receiver werewithin the coverage footprint of the satellite, but couldalso relay data in a store and forward fashion if this werenot the case. Since they were gravity stabilized, the sat-ellites could exploit higher-gain, directional antennas,operating at higher frequencies than the Strela-1 system,and hence offering higher data rates. Like its predeces-sor, the Strela-2 system operated in high inclinationorbits, also approaching an altitude of 1,500 km. Thechoice of orbital altitude may have been dictated in partby the desire to keep the satellites below the worst effectsof the Van Allen radiation belts, although, (since all Rus-sian satellites during this era were pressurized designs),their electronics would have received a degree of shield-ing from the pressure vessel in which they were housed.

However, the Van Allen radiation belts certainly rep-resent a constraint on the orbital options open to the LEOcommunications system designer if a reasonable designlifetime is to be achieved. It is tempting to treat orbitalaltitude as a completely free parameter along with theother orbital parameters such as inclination and rightascension, but in practice, the radiation doses that a sat-ellite receives from protons trapped in the Earth’s elec-tromagnetic field at altitudes above 1,500 km will haveimplications for the relative amount of shielding requiredby the satellites, or the effective lifetime of the hardware,or both.

Due to the availability of lower latitude launch sites,access to GEO was easier for Western nations than it wasfor Russia. As a result, there was a greater focus on highaltitude communications, and significant investment inLEO communications constellations did not take placeuntil the 1990s. The increasing popularity of mobilecommunications led a number of providers to envisageglobal, satellite-based systems that would serviceregions where cellular towers were unavailable.

Several concepts were proposed to meet this commu-nications requirement, and three reached the stage ofactually launching satellites, Iridium, Globalstar andOrbcomm. These networks took different approaches to

23 Space Logistics and Manufacturing

Table 23web-0, Fig. 23web-0, Eq. 23web-0

W23/2 Space Logistics and Manufacturing 23.1

©2011 Microcosm Inc.

their constellation design, and it is instructive to comparethe different approaches that were adopted to both thespace and ground segments.

Iridium selected a system design that placed signifi-cant complexity in the space segment. The satellites wereequipped with inter-satellite links which allowed mes-sages to be passed between the satellites in real time. Thesatellites in the operational Iridium constellation wereoriginally configured in seven planes of 11 operationalsatellites, (and one spare per plane), giving a total of 77operational vehicles, (and hence the origin of the namefor the constellation, since iridium is the 77th element inthe periodic table). These 77 satellites were originallyillustrated in truly polar orbits at an altitude of 765 km.A subsequent revision to the Iridium constellationinvolved a move to a higher orbital altitude, 780 km,allowing each individual satellite to provide coverage ofa larger region of the Earth’s surface. This changeallowed Iridium to reduce the number of orbital planes to6, and the number of operational satellites to 66, (and atthis point the 11 spare satellites were “counted” in orderto preserve the rationale for name of the constellation;possibly because the 66th element in the periodic table,dysprosium, sounds more like a laxative than a satelliteconstellation!). The change to the number of planes wasalso accompanied by a change in the orbital inclinationto 86.4 deg. This was done because the orbit coveragepattern of the Iridium constellation requires precisemaintenance of the orbital altitude, and it was belatedlyrealised that truly polar orbits would result in the Iridiumsatellites repeatedly risking collisions as the vehicles indifferent planes passed directly over the poles.

The Iridium constellation design required carefulphasing between planes in order to minimize the numberof satellites required. As described in Sec. 10.6.2, thesystem relied on overlapping footprints between adjacentplanes (a design sometimes described in the literature asthe “streets of coverage” approach). The satellite foot-prints in adjacent planes intersect as illustrated inFig. 10-28 (Sec. 10.6.2) to ensure that there are no gapsin the overall coverage pattern. Clearly this pattern canbe maintained by satellites moving in the same directionin adjacent planes, but at the “seam” in the constellationbetween the ascending and descending passes, the satel-lites are moving in opposite directions, and the coveragefootprints move past one another. As a consequence, theplane separation between planes 1 and 6 of the Iridiumconstellation is approximately 25 deg, whereas the sepa-rations between the remaining 5 planes is on the order of31 deg.

There is a significant contrast between the Iridiumconstellation coverage and that provided by the the Orb-comm and Globalstar systems. The latter two constella-tions utilize lower inclination orbits, ensuring that thesatellites spend more of their time over the populatedregions of the globe (where the potential paying custom-ers are!). Due to the increased proportion of the time forwhich the individual satellites can provide an effectiveservice, both Globalstar and Orbcomm required fewer

satellites in their system design. In the case of Globalstar,an orbit altitude of 1,410 km and an inclination of 52 degpermitted the use of 48 satellites, consisting of 8 orbitplanes with six satellites in each. This constellation pro-vides continuous coverage of latitudes 70N to 70S,which corresponds to 80% of the Earth’s surface, butpretty close to 100% of the Earth’s population. In thecase of Orbcomm, the principal component of the con-stellation was 3 planes of 8 satellites at an inclination of45 deg and an altitude of 780 km. This was to be aug-mented by further planes of satellites passing over theequator and over the poles, for a total of 36 satellites.Again the coverage of the constellation was not global,and in some areas, gaps in the constellation pattern atlower inclinations resulted in less that 100% systemavailability. It should be noted that neither the Orbcommor Globalstar systems featured inter-satellite links, withthe result that their investment in terrestrial gatewayfacilities had to be proportionately greater than that ofIridium in order to deliver real-time connectivity into thePublic Switched Telephone Network (PSTN). Again thedistribution of the Earth’s continents represents a limit-ing factor, since there are inaccessible regions, (andespecially over the oceans!) where it is not practicable tosite a ground station, and so the satellites are required tooperate in a store-and-forward communications modeduring the times when they are out of sight of land.

It could be argued that Iridium’s choice of constella-tion configuration was inappropriate, particularly interms of the inclination that was chosen. In defence of theIridium constellation, it is frequently suggested that therewas a very good reason for the choice of orbit: that theUS government was paying for a proportion of the Irid-ium system in order to provide a means of communicat-ing with strategic submarine assets operating in Northernwaters. Nevertheless, it is hard to escape the conclusionthat Iridium could have saved themselves a significantproportion of their investment if only some of their orbitplanes had been near-polar.

What appears to have been overlooked, to somedegree, in the rush to deploy these LEO communicationssystems, is the geographic distribution of the potentialuser base. The Earth’s continents are significantly biasedto the Northern hemisphere, and so, consequently, is theEarth’s population. A graph illustrating this is providedat Fig. 23web-1.

Fig. 23web-1. The Distribution of the World’s PopulationAgainst Latitude.

Lati

tud

e (d

eg)

Coverage Requirement per Unit Latitude (%/10 deg band)

0

90

60

30

–30

–60

–90

0 5 10 20150 5 10 2015

SME-0291-01-C©2011 Microcosm

Table 23web-0, Fig. 23web-1, Eq. 23web-0

23.1 LEO Communications Constellations W23/3

©2011 Microcosm Inc.

One constellation proposal which clearly took thisfactor into account, (and which also exploited the factthat satellite orbits can be elliptical as well as circular),was the Ellipso concept.

Ellipso consisted of two different satellite components:

• The Borealis elliptical orbit element, whichinvolved 10 satellites in two retrograde orbitplanes at the critical inclination of 116.6 deg andwith argument of perigee values close to 270 deg,(i.e. in the southern hemisphere), The apogee alti-tude of 7,605 km and perigee altitude of 633 kmcorrespond to an orbit period of three hours, andthe apogee position over the northern hemisphereoptimizes its coverage of this region

• The Concordia component, which comprisedseven circular orbit satellites operating in the equa-torial plane at an altitude of 8,050 km. This equa-torial component provides coverage over a bandfrom 50 deg north to 50 deg south

The combined coverage pattern of the Borealis andConcordia components of the constellation is illustratedin Fig. 23web-2.

It can be seen that the coverage provided by the twocomponents overlap over the mid-latitude bands in thenorthern hemisphere, which is where the peak of the pop-ulation distribution occurs.

The fact that this coverage can be provided with just17 satellites is clearly attractive, but a word of caution isappropriate here. Both components of the Ellipso con-stellation operate in orbits that would experience a sig-nificant radiation dose, (principally from energeticprotons at these altitudes). While this is not necessarilyfatal to the concept, to achieve a comparable system life-time, appropriately hardened electronic components

would need to be selected to withstand this level of radi-ation. A satellite designer would also need to think care-fully about the choice of structural materials to enhancethe physical protection that is provided to the internal cir-cuitry. Clearly the net result of such shielding would beto make the satellites heavier, and, equally clearly, thiswould obviously incur additional launch costs.

It should also be noted that the retrograde orbitselected for the Borealis component is a more challeng-ing proposition for the launch system as a result of theneed to overcome some component of the Earth’s rota-tion, (the degree to which this is necessary will be dic-tated by the latitude of the launch site). The reason for itsselection is that it can be shown to meet the condition forsun-synchronizm outlined in Sec. 9.5.3. Though suchorbits are typically selected for imaging missions, thisregression of the line of nodes would potentially simplifythe satellite platform design, permitting fixed solar pan-els to be used, and also providing a permanent “coldface” for the satellite radiators.

Fig. 23web-2. The Coverage Provided by the Ellipso Constel-lation.

Borealis

Orbit Overlap

Concordia

Borealis

Orbit Overlap

Concordia

sBorealareereBoreaaaaaaaeeeeeBo sBBoo sssssBorealiBB aa iiaaBorealisrre ll ssB r isB r iBBoorrerreeaaalliiss

pOrbit Overrbit OverrlaaaplaOrbitOrbitOrbit ppppppppaaaaO e aO e aaaaaaallllllllverver apOvererOvOveverlap

aConcordConcordCoConcocordrdiaiaaaiaiia

SME-0292-01-C©2011 Microcosm

Table 23web-0, Fig. 23web-2, Eq. 23web-0