Payload-Mass Trends for Earth-Observation and Space-ExplorationSatellites

M. Rast, G. Schwehm & E. AttemaESA Directorate for Scientific Programmes, ESTEC, Noordwijk, The Netherlands

IntroductionSpace Exploration and Earth Observation fromspace using satellites are both stillcomparatively young scientific disciplines.Remote sensing from space was born in theearly seventies, with the advent of the USLandsat satellite series. It has evolved rapidlyduring the past 25 years and today provides awealth of information for environmental

their technological sophistication, but also interms of their mass and size, has hadimplications both for the launch requirementsand ground-segment structures. However, thistrend towards ever larger space infrastructureshas slowly reversed in recent years, due to bothpolitical and financial constraints.

The differences in mission requirementsToday the requirements for Earth Observationmissions are more stringent, and thereforemore resource-hungry, than for SpaceExploration because our knowledge of theEarth is more advanced. The remote-sensingobservations made from orbit can be directlyvalidated in the terrestrial environment that isbeing investigated. Earth Observation missionsare only justifiable, therefore, if they have clearadvantages over alternative ground-basedmeasurements. Such advantages can includetime series of data, large-scale synopticviewing, and global access and coverage.

Earth Observation missions are generally moredemanding in terms of accuracy, stability,global coverage, revisit frequency, spatial andspectral resolution, as the targets have to bemeasured with high precision in order to satisfythe requirements of geo-biophysical retrievalprocedures and the related models. Sea-surface height, wind speed and surfacetemperature are three examples of suchgeophysical variables that need to be mappedfrequently and with high accuracy with the aidof precision space-borne satellite sensors (Fig.1). It is this combination of measurementprecision and high repetition rate that drivesEarth Observation sensor sizes and masses.

The major advantages of remotely sensed datalie in the synoptic context that theseobservations provide and in their timelycoverage of targets that could only be achievedwith enormous effort if one still had to rely onlyon ground-based and airborne measurements.Many of the more critical applications requirelong-term observation and thus a long satellite

This article reviews the factors and trends that have dictated the sizesof Earth Observation and Space Exploration satellites over the past 15 years and draws some conclusions regarding their expectedevolution in the future.

satellite payload-mass trends

research and other applications that are crucialto the future of mankind. Over the same timespan, space scientists have started toinvestigate both the near-Earth environmentand our planetary system with satellites andspace probes, starting with comparativelysimple ‘particles and fields’ missions andprogressing to complex, largely autonomousplanetary orbiters. The operation of space-borne observatories has helped astronomers toexpand their science into wavelength domainsthat are not accessible from ground-basedtelescopes due to the observational limitationsimposed by the Earth’s atmosphere.

As the goals have become more sophisticatedand the demand for ever more exacting datahas soared, there has been a perceivedtendency towards using larger platforms, likeEnvisat, to carry the wide range of instrumentsproposed for Earth Observation missions,compared with the smaller spacecraft beingused for Space Exploration. Given the currentpopularity of the ‘smaller, faster, cheaper’approach to space missions in general, now isperhaps a timely moment to examine whetherthe perception is indeed correct and whether itis a trend that will continue.

The fact that satellite payloads have evolveddramatically over the years not only in terms of

lifetime and high sensor stability. Such acombination of performance requirementsoften challenges the physical and technologicallimits, even today.

Earth Observation missions can be and arebeing commercially exploited and are makingan important contribution to the accurateforecasting and monitoring of economically andpolitically important parameters. The risk thatthese missions might fail therefore has to beminimised, which inevitably leads to amaximum-redundancy approach beingpursued onboard the satellite, adding to sensorcomplexity.

Space Science missions are generally moreexploratory in nature (Fig. 2), do not have tosatisfy operational requirements, andconsequently have less-stringent resolutionand stability requirements than EarthObservation missions. Moreover, the tasksassigned to Space Exploration satellites areusually highly focussed, whilst EarthObservation satellites have traditionally beendesigned to serve a broader range ofdisciplines and consequently of users.

These are prime factors in explaining the factthat Earth Observation payloads and missionshave tended historically to be larger than theirSpace Science counterparts.

Factors influencing mission scope andpayload complexityMono-disciplinary versus multi-disciplinarymissionsESA’s currently operating and planned EarthObservation missions are designed to serve awide range of scientific objectives andoperational user communities. The ocean/ice-oriented ERS-1 and -2 missions are alsoserving atmosphere and land applications. Thescientific astronomical observatory missions,on the other hand, are designed for veryfocussed missions and operate in very specificwavelength ranges, e.g. infrared, X-ray, or thesubmillimetre. The Solar System explorationmissions also have to address a wider range ofscientific objectives in order to gain the support

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Figure 1. A selection ofoperational remote-sensingproducts(a) Sea-surface height fromERS-2 Radar AltimeterObservations during the1997 El Niño event(b) Sea-surface temperaturein the Mediterraneanderived from ATSR(c) Gridded wind fields inthe Indian Ocean derivedfrom ERS-1 WindScatterometer data,indicating both directionand speed (a)

(b)

(c)

Figure 2. The Horseheadnebula region imaged byISO. The three brightreddish dots, visible in theinsets, are recently-bornstars

smaller spacecraft. In the case of operationalmeteorological satellites, the evolution fromNOAA’s Tiros series to ESA’s Metop satelliteseries provides a clear example of this trend.

Political and financial constraintsPolitical and financial conditions are also highlydetermining factors because the missiondesigners always try to fully exploit availablelaunch capacity, e.g. Ariane-4, Ariane-5 orSpace Station, as well as the available financialenvelope. This aspect has been clearlydemonstrated by the development of Envisatand Metop, which started life as a single largeso-called ‘Polar Platform’, with the presentEnvisat payload, plus a Wind Scatterometerand the complete NOAA TIROS payload (thelatter two now form the bulk of the Metopmission payload).

of the wider science community, given the smallnumber of flight opportunities that ESA canprovide within its severely constrained ScienceProgramme.

Operational versus research objectivesOperational remote-sensing missions requirehigh reliability, long lifetimes and a high level ofredundancy. Space Science missions canaccept a greater degree of risk because theydo not have to satisfy operational requirements,such as guaranteeing the provision ofcontinuous data inputs for numerical weatherforecasting.

Sensor technology evolution versus missionrequirements It is important when entering into discussionsabout the differences between operational andresearch missions to view missions andinstruments (sensors) separately.

Taking the evolution of space-borne SyntheticAperture Radars (SARs) in Europe as anexample, it is clear that owing to increasingobservational requirements, the mass,spacecraft size and resource demands havegone up significantly between the ERS-1/2type SAR (378 kg incl. the Wind Scatterometer)and the Advanced SAR on Envisat (830 kg). Onthe other hand, advances in technology,particularly in the electronics and detectorareas, have led to a reduction in the sizes andmasses of the individual instruments. Theevolution of interferometers and spectrometersis a good example in this respect. Overall,therefore, there is a balancing effect betweenthe demand for increased performance on theone hand, and the technical solutions beingdevised to satisfy the need for lighter andsmaller instruments on the other.

As far as the ‘active’ instruments like lidars andradars are concerned, the power requirementsdictate the size of the solar panels and thus theoverall mass budget for the mission. Togetherwith the complex electronic hardware needed,this often leads to higher mass budgetscompared with the scientific missions, whichtend to rely on passive instrumentation.

Small versus large satellitesBoth scientific and operationally orientedmissions can, of course, be implemented usinga number of smaller rather than one largespacecraft. This reduces the risk, but at thesame time results in a loss of synergisticcapability for the various instruments. The latteris one of the main reasons why EarthObservation satellites have tended to increasein size as more exacting requirements havebeen imposed, rather than opting for several

satellite payload-mass trends

A further financial constraint is the fact thatmany Earth Observation and Space Sciencemissions require specific orbits, which oftenprevent shared launches and hence theoptimum exploitation of the multiple-launchcapabilities of Ariane-4 or Ariane-5.

The influence of changing political and financialconstraints on the sizes of Earth Observationand Space Science missions is certainlyreflected in Figures 3 and 4.

Payload mass evolutionIn order to make a meaningful comparison of

Figure 5. Launch-mass/payload-mass ratios for past, current and future EarthObservation and Space Science missions

Earth-observation and scientific spacecraft andpayload masses, one has to break down thepayloads in each case into the sensor orinstrument element and the supportingmechanical and electrical equipment. In thecase of Space Science missions, for example,support equipment for probes, includingshielding and landing (e.g. parachute)equipment, should not, strictly speaking, becounted as ‘true’ payload.

In Figures 3 and 4, the launch masses ofvarious satellites (including platform, fuel andancillary equipment) are compared with the so-called ‘dry payload masses’, i.e. excludinginterfaces, fuel and harnesses. If one then looksat the launch-mass/dry-payload-mass ratiosdepicted in Figure 5, the ‘efficiency’ of thelarger satellites no longer looks so unfavourablecompared to the alternative of undertaking asequence of several smaller missions. Thishaving been said, both the Earth Observationremote-sensing graph and the one for SpaceExploration still indicate that the generaltendency, at least for the near future, is towardsreduced spacecraft sizes for both types ofmissions.

Is smaller also cheaper?By spreading the cost of a sequence of smallermissions over time, the immediate financialburden is lower compared with a large missionlike Envisat. This does not mean, however, thatthe total mission costs associated with meetingthe requirements of comprehensive EarthObservation or Space Exploration missions arenecessarily reduced when the route of ‘moreand smaller satellites’ is followed. To realise theoriginal mission objectives for a large payloadmission with small satellites, several missionshave to be flown simultaneously and thereforeat the end of the day the overall resourcedemands do not decrease.

The degree of risk associated with a suite ofsmaller satellites is less than that associatedwith a single large satellite with a morecomprehensive payload (i.e. not all eggs are inone basket!), but it is more difficult to fulfilsynoptic and coverage/synergy requirementswith a number of smaller satellites. The suite ofsmaller satellites also requires a flexiblelauncher family for multiple launches satisfyingspecific mission requirements, such ascontemporaneous observations and missionsynergy.

Thus, generally speaking, the ‘financial relief’ isonly of a temporary nature, as ultimately similarfinancial demands are spread out over a longerperiod of time, which in itself involves additionalcosts.

Past and future trendsUntil the early nineties, the primary limitation onspacecraft size was the available launchercapability and the readiness of the necessaryenabling technology, with missions tending toexpand to fill the available launch mass. Eventhough ESA had always had to comply withpredefined financial targets, only the Horizon2000 Programme brought hard upper limits forthe scientific Cornerstone or Medium-Sized

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Figure 4. Payload-mass histories for Science and Earth-Observation satellites

Figure 3. Launch-mass histories for Science and Earth-Observation satellites

However, where the overall mission goal iscomprehensive Earth or space ‘system’observation, such as for climate monitoring, theend result will be very similar, or even higherresource demands.

It is important to remember in this context thatthe limiting factor in recent years has been thelaunch capability, and not so much theavailability of financial resources. History alsoshows that the instruments (or payloads) inboth the Earth Observation and Space Sciencedisciplines can be expected to get smaller astechnology advances. However, this will playonly a subordinate role, because the growth inrequirements will drive sensor mass and sizeand balance out the ‘technological savings’.

The historical evolution of instruments andmissions in both Earth Observation and SpaceExploration has demonstrated that theuser/science community has responded topolitical pressure and financial constraint byfirst increasing and later decreasing the sizes ofits spacecraft in its efforts to make savings. Thenet effect has been a cyclic developmentscenario whereby the payload masses forcurrently planned launches tend to be of thesame order of magnitude as those 15 to 20years ago.

Given the much more sophisticated nature oftoday’s mission objectives, the imposition offurther reductions in mission size for eitherdiscipline – Earth Observation or SpaceExploration - would probably drivecapacities/capabilities below the threshold atwhich meaningful missions can be conducted,despite the greater capabilities of today’s – andtomorrow’s technology. Even if the overridinggoal is lower financial spending, a reasonablesized framework of Space Exploration andEarth Observation missions needs to bemaintained if Europe is to protect its scientificand cultural standing in the World. r

satellite payload-mass trends

missions. This setting of a priori definedfinancial limits has the unfortunateconsequence of ruling out some demandingbut extremely interesting long-term scientificmissions, such as those to the outer planets.

NASA has even gone a step further with itsExplorer or Discovery series, wheredeliberately low budget targets are set and thescientific return has to be optimised withinthese targets. However, NASA has theadvantage of benefiting from technologydeveloped as part of the former StrategicDefense Initiative, which has provided a wealthof innovative technology in terms of instrumentminiaturisation. These missions are veryfocussed in terms of scientific objectives,usually carrying just a few payload elements.Yet they are still designed with the launchercapabilities in mind, either in terms of cost orlaunchable mass for the required trajectory.The US has an advantage here too in that ithas a wide variety of launchers availablecovering a broad range of launch categories.Nevertheless, when one analyses the massfigures for a variety of NASA missions, thesame trend is found, i.e. the payload/dryspacecraft mass ratios for their ‘faster, better,cheaper’ missions are still in a comparablerange of 12 to 20%.

The recent imposition of the concept of‘affordability’ implies that not only EarthObservation, but also Space Explorationmissions must become smaller at the expenseof mission objectives and performancerequirements. Nevertheless, to realise theoriginal mission objectives for a large-payloadmission with small satellites, several missionshave to be flown simultaneously and thereforethe overall resource demands do notdecrease. Higher performance requires moresophisticated facilities if real scientific progressis to be made, which means increasedcomplexity and therefore risk.

At the end of the day, for both the SpaceScience and Earth Observation domains, theacquisition of increased knowledge tends todemand more resources rather than less.

ConclusionsLooking to the future of space exploration, withmankind pushing further and further into deepspace and possibly visiting other planets, thedemand for knowledge and the resultingrequirements will become even more exacting.The size of the individual missions could bereduced by splitting up the payloadcomplements to allow smaller, dedicated andmore focussed spacecraft to be flown.