Laurea Specialistica in Ingegneria Spaziale

2° anno

Corso ‘Stazioni di Terra’

Appendice9

I piccoli satelliti

Giorgio Perrotta

Anno Accademico 2007

Index

  • Definitions
  • Advantages and drawbacks of small satellites
  • Industry’s and Institutions’ response
  • Technologies
  • Technology vs. costs
  • Technology trends
  • Technology vs. system uses
  • Small satellite Near-term Missions
  • Small satellite Longer-term Missions
  • Small satellite Complementary Missions
  • Small satellite Future Missions
  • Concluding remarks

Definitions

Minisatellites: have typically a (wet) launch mass comprised in the 100-500 kg range and an installed DC power in the 200-800 W range. They offer good pointing performance and payload carrying capabilities and can be used for both near Earth and Geostationary Missions. The features make them a sort of ‘small conventional’ satellites and, as such, they suffer (mainly cost-wise) from many drawbacks of the larger spacecraft, while do not fully benefit from the advantages of the ‘small size’. Minisatellites are often used for unchallenging missions where a conventional ‘larger satellite’ would cost too much or have unnecessary features.

Examples of Minisatellites: Clementine scientific satellite

Microsatellites: usually they have a launch mass between 25 and 100 Kg (the upper mass range applies to the ‘wet mass’ if the microsatellite is equipped with a propulsion system). The prime DC power is in the 50-250 W range depending on technology. Microsatellites tend to be designed around the COTS concept and implement simplified project management and AIT : this allows achieving important savings in the procurement costs. These satellites can be used alone or in distributed systems, for remote sensing, telecom and science Missions.

Examples of microsatellite: UOSATs, Orbcomm

Definitions (2)

Nanosatellites: these have a dry launch mass in the 5 to 25 kg range and , in general, do not carry a propulsion system though they may be quite sophisticated. The installed DC power is in the 10 to 100 W range; they can be provided with full attitude control in three axes and offer sophisticated data communication capabilities. Nanosatellites, better deployed for lifetimes less than 3 years in LEO, can be used to inexpensively implement scientific, remote sensing, communication, technology research and educational missions, being characterized by a high affordability.

Examples of Nanosatellites: Tubsats, Itamsat, Opal

Picosatellites: objects having a dry mass between 0.1 and 5 kg (though most of them have a mass less than 2 kg) and an installed DC power in the 0.2 to 10 W (though many stay below 2 W). These spacecraft do not carry a propulsion system but their design can range from the extreme simplicity to a quite high complexity. Their low cost also allows to implement innovative distributed systems for a variety of uses, such as : educational, space research, technology test-beds, comms and sensing .

Examples of Picosatellites: Munin, Stensat, various Cubesat;

Advantages and Drawbacks of small satellites

Advantages

-lower production cost, implicit in the size reduction , and further enhanced by simplified project management procedures;

-cost reductions enhanced also by the COTS approach , since an inherent lower cost is compatible with the acceptance of greater risks;

-accessibility to more modern /compact / less power-hungry electronics associated to the COTS approach;

-easier implementation of spatially distributed systems ( e.g. : constellations, storms, formations) due to the lower unit cost;

-the availability of a ‘family’ of object characterized by different mass-DC power ranges, allows to optimize the costs vs. mission reqirements;

-better adherence to requirements of low-budget users;

Drawbacks

-mass and DC power limits are incompatible with ambitious missions;

-small satellites require new thinking and management procedures;

-still poorly understood and accepted by Industry and Institutions;

Industry’s and Institutions’ response

Small satellites have been differently ‘received’ in the USA and in Europe.

The American push for technology innovation, risk acceptance, free enterprise, the availability of new defense tools, has favourably pushed (we mean: with Government financial support) the development of these emerging technologies (mini, micro, nano and pico sats) mainly through the SME (Small Medium Enterprises) as opposed to the traditional Large Aerospace Companies;

In Europe the lack of real interest towards free enterprise and the large political power exerted by the big Aerospace Companies (they earn Governments’ financial support mainly to conserve jobs) has negatively affected the development of the ‘small sats’ technology. Moreover when the large Aerospace Companies felt that small sats could have plaid a good role in the European space market, they failed to see this as an opportunity but rather as a threat and acted accordingly. The ESA, notwithstanding the success of few smallsats in the upper mass range, have not yet succeeded in convincing the ESA members of the importance of supporting the full development of satellites below 100 kg (which is what the USA are currently pursuing).

Technologies (1)

COTS

  • The Commercial Off The Shelf (COTS) approach goes hand-in-hand with the micro and nanosats and can be also exploited in some minisatellite missions. The USA are putting a large effort in assessing the use of COTS for space use, given the decline of military expenses going to the Hi-Rel devices and the push for a dual-use approach to space systems definition;
  • However, efficiently selecting and adapting COTS to space implies a good understanding of the environment and devices’ physics and of the means to counteract potential deficiencies or weaknesses. Thus, terrestrial devices cannot simply be ‘exported’ for space use: rather, they must be ‘protected’ from the most dangerous space environment effects ( e.g.: radiations, temperature ranges, void..) to perform satisfactorily. These ‘protections’ can be simple and cheap (e.g. an heavy metal screening box) or complex and costly (e.g. current limiters, external anti-latchup circuits).
  • We estimate that the relative costs of a terrestrial, space-capable COTS, and Hi-Rel unit or devices are in the 1 :10:100 range, meaning that a ‘spatialized’ COTS unit is ten times less expensive than a full space-rated unit.

Technologies (2)

  • Besides COTS, the small satellites are inherently open to other new technologies, specially when forced by the dimensions of the spacecraft itself
  • In general the platform subsystems are based on devices whose mass and DC power drain is commensurate to the spacecraft size. The mass and DC power range of the smaller spacecraft ( nano and picosats) pushes the search for very innovative techniques even at the expense of ultimate performance, while for micro satellites one looks at a good compromise between performance/risk and mass/power.

Item / Picosat / Nanosat / Microsat / Minisat
Structure / xxxx / xxx / xx / x
Thermal contr / x / Xx / xx / xxx
Propulsion / - / x / xxx / xxx
Attit. Measur. / xxx / xxx / xxx / xx
Attit. Contr. / xxx / xxx / xxx / xx
On board D.H. / x / xxx / xxx / xx
Communic. / x / xx / xx / x

Technology vs. costs

  • Small satellites can be designed and produced at a significantly lower cost than the bigger satellites. The reasons for this statement are the following:
  • recurring costs are proportional to size. In fact the latter affects directly the materials, electrical parts number, the number of units or equipments, the manhours required for assembly and testing;
  • the documentation costs (a significant item in conventional large satellites) is less because of the reduced project complexity (number of units / subsystems to be traced);
  • the project management costs are also reduced because of the smaller working team and a much reduced (in an optimised programme) subcontractors’ number;
  • integration and testing costs are less by implementing clever strategies ( e.g. dedicated areas and experience-cumulating personnel) and simplified testing approaches (e.g. full-test one out of M and fast-check the remaining M)
  • The cost savings are much more than inversely proportional to the spacecraft’ mass size, specially for the two ‘lower’ categories (nano and picosats) and for multiple spacecraft systems.

Technology vs. System uses

  • Picosats, nanosats and microsats are ideal to implement spatially distributed systems based on a multiplicity of orbiting spacecraft.
  • Such distributed systems are:

-storms: fleets of space objects not necessarily maintaining any mutual position relationships among them. Storms can be injected in various orbits and, being not equipped with propulsion systems, evolve freely in space;

-constellations: groups of satellites coarsely maintaining certain relative positions among them. The spacecraft can be equipped or not with a propulsion system, depending on the orbit altitude and mission type. The constellations may include a small (say less than 6) or large (say greater than 20) number of satellites and are prevalently injected in LEO or MEO (e.g.: the GPS constellation) ;

-formations: constellations keeping very tight mutual geometrical relationships among them, for the purpose of implementing special Missions ( for scientific, remote sensing or telecommunication purposes). The spacecraft can be injected in LEO, MEO or GEO. This emerging technique is still at the study stage and waiting for highly performant nano or microsats;

Small Satellite Near-term Missions

  • Mini, micro and nanosatellites can be deployed to execute:

-dedicated missions ( one payload > one satellite : e.g: a store and forward communication system; a small multicolour camera for Earth’s observation)

-coordinated missions (multiple payloads > one satellite: e.g. panchromatic camera and multicolour cameras , or panchro camera with store and forward comms and direct-to-ground data relay)

-spatially-distributed missions (one or multiple payload > multiple satellites: e.g. a probe for near-Earth sounding of different physical phenomena and multiple satellites injected in orbits having different altitudes or inclinations) ;

-complementary missions (one smallsat works in support of a larger one: e.g: a mother-daughter assembly for interpherometry or 3_D Earth’s altimetry );

-substitution missions (one or multiple smallsats replacing a larger but older operational spacecraft: e.g. a GEO communication satellite is replaced by a small constellation of co-located minisatellites; a remote sensing spacecraft is replaced by one lighter, more modern and more performant mini or microsat);

  • The mission performance is determined by the spacecraft class / type.

Small Satellite Longer-term Missions

Considering that modern spacecraft can be designed with a payload mass and DC power fraction between 0.5 and 0.6, it turns out that the four small satellite classes can offer the following envelopes for payloads’ accommodation

class / Low mass / 50% fract / High mass / 50% fract / Low DC power / 50% fract / High DC pow / 50% fract
Mini / 50 / 25 / 500 / 250 / 200 / 100 / 800 / 400
Micro / 25 / 13 / 100 / 50 / 80 / 40 / 250 / 125
Nano / 5 / 2.5 / 25 / 12.5 / 10 / 5 / 80 / 40
Pico / 0.1 / 0.05 / 5 / 2.5 / 0.2 / 0.1 / 10 / 5

As can be seen the various spacecraft types can offer to potential Users very interesting payload accommodation capabilities. The above ‘gross’ performance are accompanied by attractive stabilization characteristics: three axis stabilization and highly performant attitude control is feasible with the latter types, while the smaller picosats can implement a 2 axis control with passive means.

Small Satellite Complementary Missions

  • Complementary missions are a reasonable and convenient use for small satellites, where they can demonstrate a good cost-effectiveness. For ‘complementarity’ we mean that the parent satellite leaves behind certain secondary missions or experiments the implementation of which would be very costly if done by that satellite (due to increased spacecraft resources’ demand). Implementing these secondary experiments with a smaller ‘companion’ spacecraft will lead to an overall lower system cost. There have been already examples of ‘complementary missions’ and others will certainly follow.
  • Substitution missions are less intuitive. In fact to replace a conventional satellite with an equivalent less massive and costly one, we must imagine that the technology has matured to the point of credibly and reliably doing the ‘switchover’, getting similar or better performance. For the time being such credibility is still lacking behind, therefore the ‘substitution missions’ are something for the future.

Small Satellite Future Missions

The most likely future market for micro, nano and picosats stays with spatially distributed systems, by some considered capable of replacing (in the medium –long term) the single spacecraft used for Earth monitoring and to support wideband communications.

Without doubt, exploiting the potential of small satellites in terms of cost-savings will be enhanced by the realization of wide-scale spatially distributed systems for small-delay information gathering purposes. Typical examples of Services optimally implemented through spatially distributed systems are:

-disaster relief, terrain observations, resources inventory /management;

-search and rescue, crisis management;

-medical and health information , emergency communications;

-deferred-time e-mail delivery to remote areas not reached by wired or wireless terrestrial networks;

-near-Earth real-time space-weather monitoring;

-world-wide weather monitoring from space;

Concluding remarks

  • Innovative technologies and emerging needs (cost reduction) are going to drastically change the way Space will be exploited in the near and far-term. Accordingly, the design of Space systems will migrate towards distributed systems constituted by multiple light-satellites;
  • The use of COTS will progressively and massively enter in the realization of space assets, but requires to be properly understood and managed;
  • The pervasive use of silicon in many fabrication technologies, besides the electronic ones, will be a key factor in the miniaturization and mass production of future micro, nano and picosatellites.
  • The exploitation of spatially distributed systems will increasingly become possible thanks to the availability of inexpensive micro,nano and picosatellites, all featuring very interesting characteristics;

The expanding and maturing micro,nano and picosatellite industry will favour the small medium enterprises more than the larger ones.