Toward Operational Space-Based Space Surveillanceridl.cfd.rit.edu/products/publications/Lincoln...

• SHARMA, STOKES, VON BRAUN, ZOLLINGER, AND WISEMANToward Operational Space-Based Space Surveillance

VOLUME 13, NUMBER 2, 2002 LINCOLN LABORATORY JOURNAL 309

Toward Operational Space-BasedSpace SurveillanceJayant Sharma, Grant H. Stokes, Curt von Braun,George Zollinger, and Andrew J. Wiseman

On 1 October 2000, the Space-Based Visible (SBV) sensor on the MidcourseSpace Experiment (MSX) satellite was transitioned to operational status as aSpace Surveillance Network (SSN) sensor for the U.S. Air Force. This transitioncontinues a long Lincoln Laboratory history of technology insertion into thenation’s space control capability, which started in 1957 with the advent ofsatellite tracking at the Millstone Hill radar. The transition is an importantmilestone for the Advanced Concept Technology Demonstration (ACTD)program, which allowed the use of a “residual” experimental/demonstrationasset in an operational role directly supporting the warfighter. The informationdeveloped during the ACTD was also critical to the definition of and advocacyfor a follow-on operational constellation of space-based space-surveillancesatellites, which is now planned for funding starting in the 2003 fiscal year.

T - (SBV) sensor waslaunched on the Midcourse Space Experiment(MSX) satellite in April 1996. The MSX pro-

gram was sponsored by the Ballistic Missile DefenseOrganization (BMDO) primarily to gather phenom-enology data for missile-defense applications [1]. Ini-tial operations of the MSX satellite demonstrated theefficacy of using the SBV sensor for space surveil-lance. Following the completion of the BMDO mea-surements, the MSX satellite was incorporated intothe Space-Based Space Surveillance Operations(SBSSO) Advanced Concept Technology Demon-stration (ACTD) sponsored by the Office of the Sec-retary of Defense, the BMDO, and U.S. Air ForceSpace Command. The goal of the ACTD was todemonstrate operational space-based space surveil-lance, and to leave behind an effective system forSpace Command to operate.

The first operational space-based space-surveil-lance observations were supplied to Space Commandin April 1998, during the first year of the ACTD.During the following two years, the SBV sensor’s op-erations became progressively more capable, and its

observations substantially improved the quality of thedeep-space object catalog maintained by Space Com-mand. In addition, the SBV sensor validated the ca-pability of space-based space-surveillance sensors toprovide assured access to militarily important objects,by demonstrating a greater than 90% response totasking for targets of highest priority (Category 1 ob-jects in Space Command’s parlance).

This article provides a summary overview of theMSX/SBV satellite and the ACTD program, and pre-sents the operational lessons learned during the sec-ond half of the three-year-long ACTD. A detailed de-scription of the SBV sensor and the results of the firsthalf of the ACTD appear elsewhere [2]. The produc-tivity and effectiveness of the SBV are also describedby Space Command in the accompanying sidebar en-titled “Space Command Becomes New Owner ofSpace-Based System,” which contains the Air Forcestatement describing the transition of the MSX/SBVfrom ACTD status to the operating command. Anoverview of the intent of the ACTD program in gen-eral is provided in the subsequent sidebar entitled“What Is an ACTD?”


310 LINCOLN LABORATORY JOURNAL VOLUME 13, NUMBER 2, 2002

While the SBV sensor was used extensively andproductively during the missile tracking and phe-nomenology experiments [2–4], the primary aim ofthe SBV sensor was to validate the feasibility and effi-cacy of space-based space surveillance. Following thedepletion of the cryogen in the MSX’s SPIRIT 3long-wave infrared sensor, ten months after launch,the space-surveillance activities became the primaryfocus of the satellite operations. The space-surveil-lance activities were intended to fulfill the followingthree broad goals:

(1) Demonstrate the specific hardware and soft-ware included on the SBV, including the focal-planecameras [5], the signal processor [6] and the signalprocessor algorithms [7].

(2) Develop an effective concept of operations forspace-based space surveillance, including (a) under-standing the trade-offs between search-based andtasking-based operations [8], (b) achieving sufficientproductivity from the MSX/SBV to significantly im-

prove the space-object catalog maintained by SpaceCommand, and (c) demonstrating the power of high-accuracy angle/angle measurements to contribute sig-nificantly to initial orbit determination and mainte-nance of the space-object catalog.

(3) Convince the operational components of SpaceCommand that space-based space surveillance repre-sents an effective way to achieve their objectives, andwork with Space Command to integrate a space-based capability into the surveillance operations atCheyenne Mountain.

Over the past five years of on-orbit operations theSBV program clearly and unambiguously achieved allthree of these goals.

The MSX Satellite

The MSX satellite is an observatory-class spacecraftdeveloped by the BMDO primarily to collect phe-nomenology data in support of missile-defense ef-forts. The satellite hosts three primary imaging/spec-

S P A C E C O M M A N D B E C O M E S N E W O W N E RO F S P A C E - B A S E D S Y S T E M

The following press release was is-sued at Peterson Air Force Base,Colorado (AFPN, 25 October2000) [1]. Air Force Space Com-mand here became the newowner of the Midcourse SpaceExperiment satellite and its asso-ciated ground support infrastruc-ture, recently. The system pro-vides deep space surveillance andhas been operating since itslaunch in April 1996 under theBallistic Missile Defense Organi-zation.

The MSX space-based systemimproves AFSPC’s mission ofcollecting data related to deep

space orbits of military and com-mercial satellites without thelimitations inherent in groundsystems. These ground systemlimitations include location sen-sitivity, dependence on weatherand time-of-day requirements.

For the last three years,AFSPC worked with the BallisticMissile Defense Organization,Johns Hopkins University, Mary-land, and the Massachusetts In-stitute of Technology to extendthe MSX satellite’s life and ensureits viability as a space-based sys-tem for continued deep spacesurveillance.

“The Space-Based Space Sur-veillance Operation has helped toincrease our revisit rates on mili-tarily significant objects by 50%and has helped us to reduce ourlist of lost satellites by 80%,” saidMaster Sgt. Steve Ferner, AFSPCSpace Control Mission Team. “Ithas also enabled us to developsearch techniques that will be thestandard used in operations formany generations to come.”

Reference1. This press release came from web site

http://www.af.mil/news/Oct2000/n20001025_001612.shtml.



W H A T I S A N A C T D ?

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Demonstrations (ACTD)exploit mature and maturingtechnologies to solve importantmilitary problems. A decliningbudget, significant changes inthreats, and an accelerated paceof technology development havechallenged our nation’s ability torespond adequately to rapidlyevolving military needs. In addi-tion, the global proliferation ofmilitary technologies, resultingin relatively easy access to thesetechnologies by potential adver-saries, has further increased theneed to rapidly transition new ca-pabilities from the developer tothe user.

In early 1994 the Departmentof Defense (DOD) initiated anew program designed to helpexpedite the transition of matur-ing technologies from the devel-opers to the users. The ACTDprogram’s goal was to help theDOD acquisition process adapt

to today’s economic and threatenvironments. ACTDs empha-size technology assessment andintegration rather than technol-ogy development. The goal is toprovide a prototype capability tothe warfighter and to support theevaluation of that capability. Thewarfighters evaluate the capabili-ties in real military exercises andat a scale sufficient to fully assessmilitary utility.

ACTDs are designed to allowusers to gain an understanding ofproposed new capabilities forwhich there is no user experiencebase. Specifically, they providethe warfighter an opportunity to(1) develop and refine a conceptof operations to fully exploit thecapability under evaluation; (2)evolve operational requirementsas the user gains experience andunderstanding of the capability;and (3) operate militarily usefulquantities of prototype systemsin realistic military demonstra-

tions, and on that basis, make anassessment of the military utilityof the proposed capability.

There are three potential out-comes at the conclusion of theACTD operational demonstra-tion. The user sponsor may rec-ommend acquiring the technol-ogy and fielding the residualcapability that remains at thecompletion of the demonstrationphase of the ACTD to provide aninterim and limited operationalcapability. If the capability or sys-tem does not demonstrate mili-tary utility, the project is termi-nated or returned to thetechnology base. A third possibil-ity is that the user’s need is fullysatisfied by fielding the residualcapability that remains at theconclusion of the ACTD, andthere is no need to acquire addi-tional units.

Reference1. This material was taken from web site

http://www.acq.osd.mil/at/intro.htm.

troscopic sensors, which cover the wavelength rangefrom long-wave infrared to the ultraviolet band [1].The SBV provides broadband visible coverage of thespectral region from 300 nm to 900 nm. Figure 1shows the MSX satellite during its final integrationand test at Vandenberg Air Force Base, California,from which the satellite was launched. The core of theMSX contains the long-wave infrared sensor, calledthe SPIRIT 3. The SPIRIT 3 focal plane is cooled be-low 10 K by solid hydrogen cryogen, which is storedin the dewar visible at the center of the spacecraft.

The apertures of all the MSX sensors are located atthe top of the satellite, along with the sunshade forthe SPIRIT 3 sensor.

Figure 2 shows the entire suite of MSX instrumenttelescopes during integration of the satellite with thebooster. The first half of the booster faring, or nosecone, is shown already installed. The MSX satellite isseventeen feet long and weighs nearly six thousandpounds. As shown in Figure 1, the SBV sensor is com-posed of two elements: the seventy-three-pound tele-scope, which is co-boresighted with all the other



MSX sensors, and the one-hundred-pound electron-ics assembly toward the rear of the satellite.

The MSX satellite and sensors were integrated byand are operated by the Johns Hopkins UniversityApplied Physics Laboratory (APL). During space-sur-veillance operations of the satellite, the command in-formation for the SBV sensor and the MSX bus sys-tems is developed at Lincoln Laboratory in a facilitycalled the SPOCC (SBV Processing Operations Con-trol Center) [9] and forwarded to APL for upload tothe spacecraft. Data resulting from on-orbit opera-tions are returned to the SPOCC for calibration andprocessing into observations, which are provided tothe 1st Space Control Squadron (1SPCS), formerlyknown as the 1st Command and Control Squadron,(1CACS), located in Cheyenne Mountain, Colorado.Figure 3 shows the flow of tasking, commands, anddata among the various organizations involved in theoperations. Since the MSX satellite was designed for

missile-defense measurements, rather than space-sur-veillance operations, a number of constraints andcomplex system features must be accommodated inorder to conduct effective space-surveillance opera-tions. More detailed discussions of these constraintsare provided elsewhere [8–10].

Operational Impact of Space-BasedSpace Surveillance

After completing a successful demonstration of itsability to perform space-based space surveillance, theSBV sensor began contributing sensor operations inApril 1998, routinely responding to 1CACS tasking

FIGURE 1. The Midcourse Space Experiment (MSX) satelliteundergoing final integration and test at the Vandenberg AirForce Base launch processing facilities. The Space-BasedVisible (SBV) sensor is composed of a telescope assembly,shown at the top of the satellite, and the electronics assem-bly, shown near the bottom of the satellite.

FIGURE 2. The MSX satellite undergoing booster integra-tion on the launch pad. All of the sensors on the MSX arealigned along a common boresight. The SBV telescope isvisible to the left of the picture, covered with gold-coloredmultilayer insulation. An openable cover, used to maintaincleanliness of the optics during launch and early opera-tions, protects the SBV telescope. The SPIRIT 3 long-waveinfrared sensor sunshade is visible at the center of the pic-ture, and the ultraviolet/visible telescopes are located at theright of the picture. The first half of the booster faring (nosecone), which protects the satellite during launch, is shownbeing installed.

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and operational commands for eight hours per day.SPOCC, which receives daily tasking requests fromSpace Command, operates the spacecraft eight hoursper day, seven days per week. Descriptions of space-surveillance operations and data processing are pro-vided in previous publications [9, 11]. Table 1 sum-marizes sensor characteristics relevant to routinespace surveillance.

Two unique properties of the SBV sensor are beingexploited for space surveillance [12–14]. First, theSBV sensor is on an orbiting platform and has accessto the entire geosynchronous belt. Second, the widefield of view of the sensor allows efficient search op-erations and simultaneous multiple detections of resi-dent space objects (RSO). Surveillance data are col-lected in a sidereal track mode, in which the starsappear as point sources and the RSOs appear asstreaks. Routine surveillance data are then processedthrough the onboard signal processor to extract thestar and streak information, as illustrated in Figure 4.

The SBV sensor demonstrates the unique capabili-ties of a space-based space-surveillance sensor. Theunique nature of this sensor, however, has not pre-cluded improvements from being made to the entireSBV system. Modifications have been made to bothground and spacecraft systems and components. Im-provements undertaken early in the ACTD focusedon increasing the efficiency of scheduling observa-tions by collecting data on multiple RSOs simulta-neously and reducing the maneuver time required bythe MSX spacecraft. These efforts are described in de-tail in previous papers [2, 9]. The primary impact ofthe operational improvements has been to increase

the quantity and quality of SBV observations. The in-crease in productivity has not only allowed improvedsurveillance of deep-space objects but has also aidedin the collection of data on high-value RSOs.

The SBV sensor also provides a valuable contribu-tion in the identification of uncorrelated targets andin the detection of high-priority tasked RSOs. TheSBV sensor can make these contributions in regions

Table 1. SBV Sensor Characteristics

Spectral range 300–900 nm

Spatial resolution 12.1 arcsec/pixel

Field of view per CCD 1.4° × 1.4°

Aperture f number 15 cm, f/3

Number of frames per frameset 4–16 frames

Frame integration times 0.4, 0.625, 1, 1.6 sec

Frame sizes 420 × 420 pixels

FIGURE 3. The MSX/SBV ground network. The 1st Space Control Squadron (1SPCS) in Cheyenne Mountain, Colorado, theApplied Physics Laboratory (APL) at John Hopkins University, the 1st Space Operations Squadron (1SOPS) at Schriever AirForce Base in Colorado Springs, Colorado, and the SBV Processing and Operations Control Center (SPOCC) at Lincoln Labo-ratory make up the ground-based operations of the MSX satellite and the SBV sensor.

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1SPCS at Cheyenne Mountain

Applied Physics Laboratory (APL)

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that are not well covered by the current Space Surveil-lance Network (SSN), which is a worldwide distribu-tion of optical and radar sensors. Finally, the SBVsensor is also useful in generating observations thatresult in highly accurate position estimates of geosyn-chronous earth orbit (GEO) satellites.

Increasing Productivity

The impact of the modifications of both the ground-based and spacecraft systems has produced significantincreases in the productivity of the SBV sensor. Figure5 shows the average number of daily deep-spacetracks collected by SBV. In October 1997 the SBVsensor produced approximately 50 deep-space tracksper day; by October 2000 this number had increasedto nearly 400 tracks per day.

The productivity of the SBV sensor was increasedthrough a combination of efficient data collectionand efficient use of MSX spacecraft capabilities. Fig-ure 6 illustrates the increasingly effective time man-agement of the SBV system by showing the percent-age of time the SBV spends collecting data. Thispercentage is the fraction of time the charge-coupleddevice (CCD) sensors are collecting photons and pro-ducing framesets. The remaining fraction of the timeis spent maneuvering the satellite and processing thedata through the signal processor. A satellite maneu-ver is initiated immediately following the data collec-tion, and signal processing is performed during themaneuver. The effectiveness of the SBV sensor hasbeen increased by reducing the time the spacecraftspends maneuvering, and by reducing the effectiveprocessing time on board the spacecraft.

Efforts to increase the productivity of the SBV sen-sor focused initially on reducing the amount of timethe MSX spacecraft spent maneuvering. As a start, themaneuver durations were reduced from five minutesto three minutes for all maneuvers, independent ofmaneuver angle. With a shorter maneuver time, thefirst increase in productivity resulted from exploitingthe SBV sensor’s wide field of view to collect data onmultiple objects simultaneously. This wide observa-tional capability permitted the detection of more ob-jects with the same number of maneuvers. Productiv-ity was further enhanced by additional decreases inmaneuver duration and by an increase in MSX-space-craft data processing capability produced by a dualsignal processing upgrade.

FIGURE 5. The increasing average number of daily deep-space tracks collected by the SBV sensor have resultedfrom increasingly efficient use of spacecraft resources.Modifications of both ground and onboard software have in-creased the efficiency of maneuver scheduling and onboardsignal processing.

FIGURE 4. (a) The SBV sensor consists of a high-quality stray-light rejection telescope that contains four 420 × 420-pixelcharge-coupled devices (CCD) that generate (b) a raw frameset. (c) The onboard signal processor processes the focal-planeimages to yield star and streak reports that are used to construct (d) the signal-processed frameset image. The signal-pro-cessed data are used for routine space surveillance of resident space objects (RSO).

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The final boost in productivity resulted from theaddition of so-called pinch-point operations, whichenhance the efficient data collection of the MSXspacecraft by searching a small region of space withsmall quick maneuvers, and by placing the search re-gion in dense regions of the GEO belt. The effect ofthe quicker maneuver time is apparent in Figure 6,which shows the increased fraction of time spent col-lecting data. A more thorough description of pinch-point operations is provided in a later section.

Uncorrelated Target Discovery and Recovery

It is commonplace for the SBV sensor to detectstreaks that do not correlate with any known objectsin the catalog. This process occurs if a detection doesnot correspond to the predicted position of anyknown object to within a threshold of 125 millide-grees. Such a detection is called an uncorrelated tar-get, or UCT. In many cases, a UCT is actually a de-tection of an object currently in the catalog but whosepredicted position possesses significant errors. Theseerrors may exist because (1) the satellite has not beentracked for a long period of time and its cataloged ele-ment set is no longer accurate; (2) the satellite has re-cently maneuvered and has not been tracked since themaneuver occurred; or (3) the satellite was errone-ously associated with another satellite, commonly re-ferred to as a corrupted or mistagged element set. AUCT is also possibly a detection of a satellite that wasrecently placed into orbit but for which no elementset exists in the catalog. Finally, a UCT may be a de-tection of an object that has never before been seen byany sensor within the SSN, due to the object’s smallphysical size. Any UCTs that cannot be classified inany of these ways are likely to be false detections,caused by such factors as radiation events or anoma-lies in the signal processor of the SBV sensor.

The U.S. Space Command has developed a classi-fication scheme for objects that have been lost to theSSN, objects that need to be tracked for a variety ofreasons, or objects that are misassociated with anotherobject. This classification scheme, called the Atten-tion List, is also associated with the way in whichtasking priorities are given to sensors within the SSNon a daily basis, and it can be used to classify the suc-cessful association of UCTs. These categories are as

follows: (1) Lost, an RSO that has not been tracked byany SSN sensor within the last thirty days; (2) Atten-tion List, an RSO that has not been tracked within atleast five days, but which is not officially a Lost ob-ject; (3) Maneuvered, an RSO that has recently ma-neuvered and can no longer be associated with itscataloged position; (4) New Launch, an RSO that wasrecently placed into orbit but for which no elementset has been established; (5) Corrupted, an element setin the catalog that is seemingly misassociated withanother object; or (6) Uncataloged, an RSO that hasnever been previously cataloged.

Objects on the Attention List have a well-definedproblem, and action can be taken to track these ob-jects and remove them from the list, given the ad-equacy and availability of SSN sensors. In many cases,however, such as those associated with lost objects,maneuvered objects, new launches, corrupted ele-ment sets, and uncataloged objects, an object cannotbe classified until after it has first been discovered orrecovered. Once this discovery or recovery step is ac-complished, SSN sensors can then be tasked to gathermore information on that object, thus improving thequality of its element set. The SBV sensor has made asignificant contribution to the SSN by detecting and

FIGURE 6. The fraction of time used for collecting data is ameasure of how efficiently the MSX/SBV system is used.The data-collection time is the time required for collecting aframeset on a CCD. As the spacecraft maneuver time andsignal processing time have been reduced, the amount oftime devoted to data collection has increased from 5% tonearly 30% of the eight hours per day that is used for space-surveillance operations. The high efficiency of pinch-pointoperations is a result of performing short maneuvers.

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properly classifying numerous UCTs. With its widefield of view, the SBV sensor is able to recover objectswhose predicted positions deviate considerably fromtheir actual positions.

Specialized UCT processing software, which wasdeveloped at Lincoln Laboratory, can successfully as-sociate a detection with a previously cataloged object[15]. The SBV sensor has been highly successful inproperly cataloging UCTs, to a large degree specifi-cally because of its wide field of view. This capabilityallows for the successful detection of an RSO, even ifits actual position is far from its predicted location. Inthe case of the Ground-based Electro Optical DeepSpace Surveillance (GEODSS) system, which is thecollection of nine telescopes comprising most of theground optical sensors within the SSN, the effectivefields of view of these sensors are a factor of threetimes smaller than that of the SBV sensor, and, in thecase of the deep-space radars, the fields of view are anorder of magnitude smaller than that of the SBV sen-sor. Since the inception of the ACTD in October1997, the SBV sensor has exploited its wide field ofview by successfully assigning 160 detected UCTs toobjects in the catalog. During the ACTD, the SBVand Space Command teams reduced the Lost list as-sociated with the GEO belt from 63 objects, whichwere detectable to the SBV sensor, to merely 13.

Figure 7 shows the distribution in longitude of the

objects discovered or recovered by the SBV sensor,clearly indicating that most of the activity, particu-larly with respect to maneuvering satellites, occursover Europe. This region of the world has an unusu-ally high number of geosynchronous satellites that arefrequently maneuvered, or station-kept, in order tomaintain their restricted positions along the belt. It isalso important to note the significant support givenby the SBV sensor to new launches over central Asia.

High-Priority Object Tasking

For many years now, U.S. Space Command has useda system of prioritization to establish its daily taskingof the sensors within the SSN. Objects of highest pri-ority are referred to as Category 1 objects, and theseare the objects on which Space Command needs datathat day, with a high probability of success. These so-called Cat 1 objects may be satellites that are operatedby U.S. adversaries; they may be satellites that haverecently maneuvered and will be lost if they are nottracked immediately; or they may be satellites nearingreentry into the atmosphere, and updates are requiredat a high rate in order to establish a good predictionon their reentry point. The categories decrease in pri-ority to Category 2, 3, 4, and 5, with suffixes associ-ated with each, to indicate how many observationsand over what duration tracking data are requested.In theory, any given sensor within the SSN is sup-

FIGURE 7. Geographic distribution of high-priority uncorrelated target (UCT) objects discov-ered or recovered by the SBV sensor. This histogram clearly shows that the SBV sensor hashad a significant impact on supporting maintenance of the catalog over Europe and Asia.

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posed to respond to Category 1 objects with a veryhigh degree of success, Category 2 to a lesser degree,and so on down to Category 5.

Unfortunately, this goal is not reached in practice,with the one exception of the SBV sensor. Figure 8shows that the SBV sensor responds routinely to Cat-egory 1 tasking at or above the 90% level. So there is ahigh probability that U.S. Space Command will re-ceive tracking data on these objects when it tasks theSBV sensor. In contrast, the GEODSS system aver-ages around 30% in response to tasking of Category 1objects. The success rate of the GEODSS system isdue principally to its restriction to nighttime viewingand weather outages. As a consequence, Air ForceSpace Command has come to depend on the SBVsensor in the last three years to gather data on thesehigh-interest objects.

In addition to the 90% success rate of the SBV sen-sor in response to Category 1 objects, the sensor hascomplete global coverage, unlike any ground-basedsensor. Figure 9 shows where the SBV sensor hasgathered observations in response to Category 1 re-quests. The red triangles show the locations of currentdeep-space satellite-tracking radars. This figure illus-trates the contributions of the SBV sensor for regionsoutside of ground-based radar coverage.

Catalog Maintenance

SBV observations have proven valuable not only be-cause of their quantity and global coverage, but be-cause the quality of these observations has contrib-uted to the maintenance of an accurate catalog ofRSOs. This section illustrates the utility of SBV sen-sor observations to perform orbit determination ofgeosynchronous objects [16]. Three different require-ments for GEO orbit determination are explored.The first requirement is the capability to perform ini-tial orbit determination to catalog UCTs. The secondrequirement is the ability to perform long-term orbitmaintenance. The third and final requirement is theability to calculate very accurate orbits.

SBV sensor observations on a selected GEO satel-lite were used for this analysis. The accuracy of theSBV-derived orbit solution was assessed by compar-ing it to a more accurate reference orbit. The God-dard Space Flight Center routinely generates a preciseorbit for their Tracking and Data Relay System(TDRS) satellites in geosynchronous orbit. The pub-lished TDRS orbits are accurate to 50 to 60 m (1 σ)in position [17, 18]. The TDRS-4 satellite wastracked frequently by the SBV sensor and the Mill-stone Hill radar during a sixty-day maneuver-free pe-

FIGURE 8. The reliability of the SBV sensor in response to tasking of high-priority (Category 1)objects. This diagram shows a comparison of the number of high-priority satellites that weretasked to the SBV sensor on any given day, and the success to which data were gathered on thatobject. In comparison with the ground-based optical sensors in the Ground-based Electro Opti-cal Deep Space (GEODSS) system, the SBV sensor performs approximately three times morereliably in response to tracking these critical objects.

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riod. Both optical and radar tracking data were usedto compute orbits for TDRS-4, and the accuracy ofthese orbits was determined by comparing the orbitswith the reference orbits computed from GoddardSpace Flight Center element sets.

The capability to perform initial orbit determina-tion with SBV observations is necessary for the iden-tification of UCTs. The initial orbit-determinationprocess generates an element set for a set of UCT ob-servations, which can be compared to other elementsets, and can be used to direct sensors to observe theUCT. Figure 10 shows an example of the initial orbit-

determination process. In this example, about 5% ofthe orbit arc was sampled by the discovery observa-tions. The initial solution was refined by using least-squares estimation with all six observations, and Fig-ure 10 shows the resulting orbit error. The largeramplitude of the oscillations is a result of the eccen-tricity of the orbit not being well determined becauseof the small sampling of the orbit, although the errorsare small near the observations where the orbit is wellconstrained. The errors of the initial orbit-determina-tion solution are small enough to allow the object tobe recovered by the large field of view (1.4° × 1.4°) of

FIGURE 9. MSX/SBV coverage of high-priority objects. The red triangles show the locations of current ground-based deep-space satellite-tracking radars. The SBV sensor primarily provides data on high-priority deep-spaceobjects that are outside ground-based radar coverage. Three additional ground-based sites would be required—most likely situated in foreign countries—to achieve the same coverage offered by the MSX/SBV.

FIGURE 10. The quality of geosynchronous earth orbit (GEO) determinationduring the UCT discovery process increases as additional observations (indi-cated by red squares) are collected. Orbit error is reduced by an order of mag-nitude as the number of tracks increases from three to six. The right-side y-axisshows the orbit position error in millidegrees (mdeg).

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the SBV sensor within a few days of the discovery ob-servations. Once an element set is generated, furtherobservations are collected and the orbit is improved.The errors in the orbit rapidly decrease with an in-creasing number of tracks.

The growth of the geosynchronous satellite popu-lation combined with the small altitude band theyoccupy has increased concerns about the possibility ofcollisions between these objects. Highly accurate geo-synchronous orbits are required to address these con-cerns [19]. SBV sensor data has proven valuable ingenerating these high-accuracy orbits. Let us examinehow the fusion of SBV optical and radar data effi-ciently produces high-accuracy geosynchronous or-bits. The first two orbit solutions in Figure 11 repre-sent sparse SBV sensor data and sparse Millstone Hillradar data, respectively. The sparse SBV-data case rep-resents the amount of tracking necessary for routineobject tracking and catalog maintenance. The bars onthe left side of Figure 11 show the position errors inthe three orbit components (radial, along track, andcross track). The sparse SBV-sensor solution hassmallest errors in the cross-track direction because ofthe high angular accuracy of these observations. Theaccurate SBV-sensor measurement of the cross-trackcomponent permits the inclination of the orbit to bewell determined. The sparse Millstone Hill radar(MHR) data solution has the smallest errors in the ra-dial and along-track directions because of accuraterange and range-rate observations. The direct mea-

surement of range and range rate results in an accu-rate estimate of the semimajor axis and eccentricity ofthe orbit. The complementary nature of these obser-vations can be exploited by combining sparse opticaland radar observations to generate an orbit solutionthat is considerably better than what is achievablewith data from only a single source.

Finally, if sufficient SBV-only data (dense tracking)are available, it is also possible to generate an accurateorbit solution. The SBV sensor has proven very effec-tive in surveillance of deep-space geosynchronous ob-jects. The SBV sensor’s capability to collect accuratedata on low earth orbit (LEO) objects has also beenexplored; it is described in the sidebar entitled “Low-Earth-Orbit Observations: Solving Today’s Opera-tional Issues While Preparing for Tomorrow’s.”

Improving the Capability of the SBV Sensor

During the second half of ACTD operations, addi-tional improvements were made to both ground andspacecraft systems to enhance the capabilities of theSBV sensor. Space surveillance with the SBV sensorinvolves scheduling RSO observations, uplinkingSBV commands, collecting data, performing signalprocessing, maneuvering the MSX spacecraft, and fi-nally downloading and reducing data from SBV sen-sor observations. Because of funding constraints andspacecraft access limitations, SBV data collection islimited to a maximum of eight hours per day. Opera-tional improvements to increase the productivity of

FIGURE 11. The accuracy of GEO orbit determination. For routine space surveillance, orbit quality ofGEO objects is a function of type and quantity of data. The sparse SBV solution has smallest errorsin the cross-track direction, and the sparse Millstone Hill radar (MHR) solution has smallest errors inthe radial and along-track directions.The complementary nature of SBV optical data and MHR radardata permits an effective fusion of the two data types, resulting in accurate orbit estimates. An SBVtrack consists of two observations, and an MHR track consists of five observations.

SparseMHR

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L O W - E A R T H - O R B I T O B S E R V A T I O N S :S O L V I N G T O D A Y ’ S O P E R A T I O N A L I S S U E S

W H I L E P R E P A R I N G F O R T O M O R R O W ’ S

the Space-BasedVisible (SBV) sensor took greatpains to develop a sensor thatcould gather observations cover-ing objects in all orbit classes. Toaddress low earth orbit (LEO)targets, which must be detectedin the presence of the brightearth limb, the telescope designwas optimized for high off-axisrejection. This capability entaileda unique telescope design [1] andrequired very clean optics. Theultimate off-axis performance ofthe SBV sensor exceeded thelofty goals set for off-axis rejec-tion. However, because the op-erational shortfalls in the space-object catalog often involveddeep-space objects (any objectwith an orbital period exceeding225 minutes), little data were ac-quired on LEO objects over thecourse of the Advanced ConceptTechnology Demonstration. TheSBV sensor, however, made con-siderable contributions to thedeep-space catalog.

Anticipated needs in spacesurveillance over the next few de-cades include a requirement forshort timeline access to the orbitsof space objects. Achieving quickaccess to these orbiting objects(in a time period much shorterthan the object’s orbital period)by using ground-based sensors is

difficult because the sensors mustbe proliferated widely across theearth’s surface. This approach iscostly and requires access to nu-merous foreign sites.

Space-based sensors are muchbetter solutions to the require-ments for short timeline access toorbiting objects, including LEOobjects. In fact, the timeline re-quirements have been the mostpersuasive justification for a fol-low-on constellation of space-based space-surveillance sensors.

Given the long-term interestin understanding how to makespace-based space-surveillancesensors most effective for all or-bital regimes, Lincoln Laboratoryand Air Force Space Commandbegan an effort to gather observa-tion data on LEO space objects.The objectives were (1) to vali-date that the SBV sensor couldeffectively access and detect LEOtargets, (2) to verify that SBV tar-get tracks could be used to gener-ate accurate orbits for LEOs, and(3) to collect optical signaturedata covering a wide range ofLEO target types and geometrieswith respect to the sun.

Achieving the first objectivewas critical to demonstrating thatspace-based space-surveillancesensors could provide the shorttimeline access that is required by

Space Command across the en-tire target set. The objective wassuccessfully achieved as a by-product of achieving the secondand third objectives.

Conducting a series of obser-vations of known LEO targetsand processing the data to yieldupdated orbits achieved the sec-ond objective, namely, demon-strating the generation of accu-rate orbits with the SBV sensordata. Figure A shows an exampleof the result. In this case, a seriesof two SBV tracks were taken onspace object 23463, which is aRussian navigation satellite calledTsikada 1. The resulting orbitwas compared to a reference orbitgenerated by using ground-basedradar observations; Figure Ashows the resulting errors. Thequality of the SBV-only orbit er-ror for this object is on the orderof a few kilometers, which trans-lates to a timing error of less thana second and is representative ofthe accuracy of the LEO catalog.

Achieving the third objective,namely, generating an extensiveset of signature measurements ofLEO objects, has proved to be vi-tal to the requirements analysisand design for the follow-on op-erational constellation of space-based space-surveillance satel-lites. The space-object catalog



the SBV sensor are thus limited to increased effi-ciency in both scheduling RSO observations and indata processing onboard the spacecraft.

The remaining sections of this article discuss sub-sequent operational improvements that have beenimplemented during the second half of the ACTDperiod. The first of these improvements focuses on in-creasing the efficiency of data processing on board thespacecraft by exploiting redundant processing capa-bility onboard the spacecraft. The second improve-ment further increases the effectiveness of scheduling

RSO observations by concentrating data collectionon dense regions of the GEO belt, and by performingthese observations with a minimum number of ma-neuvers. These modifications represent a new modeof operations with emphasis on search operations ver-sus tasked operations.

Dual Signal Processor Operations

The SBV sensor was designed with redundant com-ponents to allow for reliable operations in space [5–7]. The primary components of the SBV sensor hard-

maintained by Space Commandcontains the radar cross sectionof each cataloged object. No op-tical signature data, however, areincluded in the catalog. Attemptsto convert from a radar cross sec-tion, usually measured at UHFwavelengths, to optical signaturesmeasured with wavelengths a fac-tor of 107 shorter, have met withlittle success. However, as de-scribed above, the current opera-tional shortfalls were most ex-treme in deep space, and as aresult LEO observations were not

generally tasked to the SBV. Dur-ing the surveillance of deep-spaceobjects, LEO objects are also cap-tured in the SBV sensor’s widefield of view. As a result of theseserendipitous detections, a cata-log of over 5000 measurementson over 2500 LEO objects at awide variety of sun/object/SBVgeometries was collected. Thisdatabase has been critical to therequirements analysis and designof the planned follow-on systemcalled SBSS (Space-Based Sur-veillance System). The capability

of proposed sensor constellationarchitectures can be realisticallyestimated by using the databaseof measured signatures. As ex-pected, short timeline access toobjects in all orbital regimes hasbecome a major requirement forthe operational constellation,and has been the leading prioritydesign objective.

Reference1. D. Wang, C. Wong, and R. Gardner,

“Space-Based Visible All-ReflectiveStray Light Telescope,” SPIE 1479,Surveillance Technologies, 1991, pp.57–70.

FIGURE A. SBV-only observations are capable of generating an accurate orbit of low earth orbit(LEO) satellites. The position error of an SBV-only orbit solution for the Russian navigation satelliteTsikada 1 is on the order of a few kilometers, which translates to a timing error of less than a second.

0 6 12 18 24Object 23463

Tsikada 1Russian navigation satellite

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ware consist of the camera, CCD sensor, analog elec-tronics, experiment controller, and signal processor.The experiment controller coordinates the operationof the CCD sensor and signal processor. The focalplane is configured such that a single failure will im-pact only two out of the four CCDs. Both the experi-ment controller and the signal processor have fully re-dundant channels. This redundancy has beenexploited in two ways in the SBV program. First, theredundant components were utilized in performingfinal testing of software upgrades to the onboard sys-tems. After thorough ground testing, software modi-fications for the SBV hardware were uploaded andfirst tested on the redundant component before beingimplemented in routine operations. This approachpermitted upgrades to the SBV system with minimalimpact on operations. The redundancy of the SBVsystem was also utilized to increase the processing ca-pability of the hardware. This section describes howthe redundant signal processor was used to increasethe capability to process data onboard the spacecraft.

In nominal operations with only one signal proces-sor, the CCD sensor is used to collect eight frames ofdata with an integration time of 1.6 sec per frame.The collected data are sent to the signal processor, inwhich star and satellite detections are extracted andstored as reports. The reports are initially storedwithin the memory of the signal processor and aremoved to the experiment controller before beingdownloaded to the ground. Data can be downloadeddirectly from the signal processor, although the pro-cess of downloading deletes the data from the signalprocessor. Once the data are moved to the experiment

controller, they can be downloaded multiple times.Figure 12 illustrates the timeline of processing datawith a single signal processor.

Figure 12 shows that approximately 200 secondsare needed to collect and process four successiveframesets of data. As stated earlier, the SBV hardwarewas designed and built with a redundant signal pro-cessor for reliability. The SBV hardware is capable ofoperating both signal processors. Figure 13 showshow the second signal processor is utilized.

Once one frameset has been collected by the CCDsensor and sent to the signal processor, the second sig-nal processor is immediately configured to receive thesecond frameset from the CCD sensor. The thirdframe is taken when the first signal processor is readyto receive the data. Finally, the second signal proces-sor is ready to receive the fourth frameset. The space-craft can begin maneuvering after the fourth framesetis taken, while the signal processor is processing thesedata. In dual signal processor mode approximatelyeighty seconds are needed to take four framesets ofdata, whereas approximately 160 seconds are neededto take the same amount of data with a single signalprocessor. In search operations all four CCDs areused sequentially, and dual signal processor search op-erations can cover twice as much area than single sig-nal processor search operations.

Dual signal processor operations are implementedby modifying onboard spacecraft software to togglethe two signal processors to receive the data from theCCD sensors. Once the memory in the signal proces-sor is filled up, the data are moved to the experimentcontroller after which they can be downloaded. The

FIGURE 12. Single signal processor. With one signal processor, the SBV sensor data must be processed through thesignal processor before the next frameset can be collected. The spacecraft can begin maneuvering while the lastframeset is being processed.

Signalprocessor

Time (sec)100500 150 200

Data collection

Signal processing

SBVsensor



onboard data flow is handled by a set of macros thatare loaded onboard the spacecraft to redirect the dataalternatively to both signal processors (a macro is a setof commands stored onboard the spacecraft and ad-dressable by a single command from the ground).

Geosynchronous Pinch Points

The increased efficiency of dual signal processing op-erations has made it possible to set up a search fencefor geosynchronous objects. This section motivatesthe location chosen for the search fence and the re-sulting sensor operating strategy. A geosynchronoussatellite is one in which its orbital period is synchro-

FIGURE 13. Dual signal processors. With two signal processors, a second frameset can be immediately collectedand sent to the second signal processor while the first signal processor is still processing. As with the single sig-nal processor case, the spacecraft can begin maneuvering while the last frameset is being processed.

nous with the rotation of the earth. If, in addition tohaving a period of one sidereal day, the satellite islaunched into the equatorial plane (an orbit with zeroinclination), the satellite seems to stay over a fixedpoint on the equator, as seen by an observer standingon the ground. This type of orbit is referred to as ageostationary orbit. If, however, the satellite’s orbitplane is inclined to the equator, the satellite appearsto an observer on the ground to follow a figure-eightpattern in the sky. This type of orbit is referred to as ageosynchronous orbit, but it is not geostationary. Fig-ure 14 illustrates the angles that define an orbit.

Geosynchronous and geostationary orbits are both

FIGURE 14. Satellite orbital elements. Six parameters are required to define the orbit of a satel-lite. The two shown in this diagram define how the orbit is inclined to the equator, namely, the in-clination i and the right ascension of the ascending node Ω, which defines the angular locationat which the orbit crosses the equator on its path into the northern hemisphere. The location ismeasured with respect to a fixed direction in the sky known as the vernal equinox γ.

Time (sec)100500

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used by the operators of communication, weather,and surveillance satellites to conduct their missions.In fact, an interesting pattern of behavior has devel-oped over the last forty years with regard to how satel-lite operators launch and maintain their geostationarysatellites and, ultimately, what happens to these satel-lites when they cease to be active. This behavior isdriven by the operator’s desire to exploit the naturaleffects of the sun, the moon, and the oblateness, orequatorial bulging, of the earth, in order to maximizethe satellite’s lifetime and to minimize its operational(fuel) costs. It is through a combination of these natu-ral effects and this behavior that creates what we referto as geosynchronous pinch points, or the systematicgrouping of satellites in specific regions of the geosyn-chronous belt [20]. By exploiting these pinch pointsthe SBV sensor is able to significantly increase its pro-ductivity in search mode. In order to appreciate thecauses of these pinch points and recognize how bestto exploit them, we must first understand the behav-ior of geosynchronous orbits.

Geosynchronous Orbit Characteristics

The natural effects of the sun, the moon, and the ob-lateness of the earth are referred to as lunisolar and J2geopotential perturbations. These forces produce,among other effects, a torque on the orbit plane of asatellite that results from the net out-of-plane forcecomponent that acts on the satellite. This torque pro-duces a correlated periodic variation in the inclina-tion and a precession of the orbit plane along theequator. The effect of precession is similar to the mo-tion that occurs when a spinning gyroscope is sus-pended on one end by a string; the gyroscope pre-cesses around the string in a direction dependentupon the spin direction of the gyroscope. This mo-tion is quantified by the rate of change of the rightascension of the ascending node, Ω, or the angle be-tween the location at which the orbit plane intersectsthe equator, on the satellite’s motion into the north-ern hemisphere, and an inertial reference pointknown as the vernal equinox γ . The effects are ob-served as a fifty-three-year periodic variation in theinclination and the ascending node [21, 22].

Geosynchronous-satellite operators, by thought-fully choosing the initial conditions of their satellite’s

orbit, establish a certain evolution of the orbit. Thereis a relationship between the inclination and the rightascension of the ascending node, due to the lunisolarand J2 effects. The goal of the majority of geosynchro-nous satellite operations is to maintain the relativeposition of the satellites over a point on the earth,thus producing a geostationary satellite. This goal re-quires that the inclination of the orbit remain nearzero, which can be accomplished only through rou-tine orbit maneuvers. However, performing a maneu-ver to change the inclination of an orbit is an exceed-ingly fuel-intensive operation, and, since the rate offuel consumption is the most important parameterdefining the lifetime of these satellites, careful plan-ning of these maneuvers is vitally important. As aconsequence, operators choose to launch their satel-lites in such a way as to minimize the need to performthese expensive inclination-altering maneuvers. Thiseffort is accomplished by launching the satellite withan initial inclination and ascending node that placesthe evolution along the curve shown in Figure 15.Since this philosophy is incorporated by the vast ma-jority of geostationary satellite operators, an interest-

FIGURE 15. Actual satellite data for geosynchronous satel-lites, showing the correlation of the inclination and rightascension of the ascending node due to lunisolar and J2

geopotential perturbations. The figure shows the initial con-ditions of inclination and ascending node used by mostgeosynchronous satellite operators. Data for satellites withinclinations less than 0.5° have been removed from the plot,since they are frequently maneuvered and because the rightascension of the ascending node becomes ill-defined if theinclination is near zero degrees (these low-inclination ob-jects always pass through the pinch-point region).

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ing pattern evolves in the orbits. It is this pattern thatforms the so-called pinch points.

Over thirty years have elapsed since satellites werefirst launched into geosynchronous orbit. Today thereare approximately 650 active and inactive geosyn-chronous satellites in orbit. Figure 15 shows actualsatellite positions, with respect to the inclination andthe right ascension of the ascending node, as they ex-ist today. It takes approximately twenty-seven yearsfor the inclination to increase to 15°, and nearlytwenty-seven years for the inclination to decreaseback to zero. Note that satellites with inclinations lessthan 0.5° have been removed from the plot, sincethey are frequently maneuvered to maintain their po-sition over a fixed point on the earth, and they gothrough the pinch point as well. Furthermore, theright ascension of the ascending node becomes ill-de-fined if the inclination is near zero, and this angle isdifficult to estimate accurately with data from theSSN.

The important feature to note is the structure ofthe geosynchronous population. During the activelife of these satellites, their inclinations are main-tained near zero and their corresponding ascendingnodes are maintained between –90° and 90°. When asatellite’s fuel is depleted, or when the satellite fails, itis no longer maneuvered. Over time the ascendingnode of the satellite rapidly evolves to 90° clockwise,and slowly evolves along the curve shown in Figure15. This cycle is completed in about fifty-three years.Because geosynchronous satellites have beenlaunched into earth’s orbit for only about thirty years,the satellites have evolved—at most—a little greaterthan halfway through the cycle. Satellites with incli-nations of 15° and near-zero ascending nodes, as seenin the right-most data in Figure 15, are some of thefirst geosynchronous satellites ever launched. Theseresults clearly show that, while active geosynchronoussatellites are maneuvered to remain their position onthe equator, the inclination and ascending node of in-active satellites evolve in predictable ways.

Figure 16 shows the distribution of ascendingnodes for the same set of satellites shown in Figure 15(satellites with inclinations less than 0.5° have onceagain been removed from the plot). This histogramshows the high concentration of orbits with ascend-

ing nodes between 20° and 90°. It is this clustering oforbits that forms the geosynchronous pinch points.These pinch points become even more clearly visibleif the population of the geosynchronous belt isviewed over a 24-hour period, as shown in Figure 17.In this figure, each satellite progresses from left toright along a sinusoidal trajectory. The contours indi-cate the number of distinct satellites passing througha 1.4° × 1.4° region of inertial space over a 24-hourperiod, which is the field of view of one charge-coupled device (CCD) on the SBV. The highest den-sity regions, or pinch points, are centered at 0° decli-nation and at approximately 65° and 245° in rightascension. Unlike with the data shown in the previousfigures, this data set contains all the geosynchronoussatellites, which explains the high concentration ofthose with zero declination (equatorial orbit).

Pinch points exist because, at these locations, ac-tive satellites on the belt are passing through these re-gions at the same time that older inactive satellites arecrossing the equatorial plane. Note the distinct high-density “tails” extending above and below the beltnear the two pinch points. This structure can easily beexplained by comparing Figure 17 with Figure 18,which depicts the sinusoidal orbit tracks of fifteen in-active geosynchronous satellites. In this figure, the si-nusoids have been systematically shifted in phase, sothat each orbit crosses the equator at a slightly differ-

FIGURE 16. Histogram of the right ascension of the ascend-ing node for geosynchronous satellites. The clustering ofascending nodes between 0° and 90° is the pattern referredto as the geosynchronous pinch points. Satellites with incli-nations less than 0.5° have been removed.

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ent location. This shift in phase occurs in reality, sinceeach inactive satellite is at a different location in itsfifty-three-year evolution.

Once these pinch points were understood, it was

the goal of the MSX/SBV team to determine an op-erational technique to exploit these regions, and thusincrease the SBV sensor’s productivity during itssearch periods.

Pinch-Point Implementation

Pinch-point operations require the pinch-pointsearch region to be continuously observed over thetwenty-four hours that geosynchronous objects re-quire to complete a single orbit. In practice SBV op-erations are limited to eight hours per day, and thecoverage of the search region has to be accomplishedby searching the region in small time increments overtwenty-four hours. By searching a region repeatedlywith a constant revisit interval, it is possible to createa search region that covers the entire geosynchronousbelt. Since the SBV sensor is orbiting the earth, a con-stant revisit interval implies that a given pinch-pointregion is searched once an orbit. The size of the searchregion results from a trade-off between the time re-quired to search the region and the amount of timethe region is continuously visible. Figure 19 showsthe resulting search pattern.

This figure shows a coverage box that is 30° wide

FIGURE 18. Orbit traces of fifteen geosynchronous satel-lites, with varying inclinations and locations of the ascend-ing node. This diagram clearly shows the cause of the tailsassociated with the pinch points, as seen in Figure 17. Be-cause of the shifted phase of the various sinusoids and thevarying inclination, regions of high concentration are foundboth north and south of the geosynchronous belt, in addi-tion to the main points on the belt.

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FIGURE 17. Satellite concentration in the geosynchronous belt over a 24-hour period. The conceptof pinch points becomes clear if the population of geosynchronous satellites is viewed with respectto the inertial coordinates of right ascension and declination over the course of one day. Thesepinch points, which are illustrated as lighter colored regions in the figure, are locations on the geo-synchronous belt that represent high concentrations of satellites at certain times of the day.

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in right ascension and 3° high in declination, andconsists of ten groups of framesets. Each group con-sists of a frameset from each of the four CCDs. Thespacecraft is maneuvered only between each group offramesets. The search region is covered by an array offorty framesets, and is covered from left to right.Since GEO objects move from right to left, objectsthat are leaving the search region are observed first,and objects that are just entering the region at the endof the search interval are observed last. This searchstrategy also results in a slight overlap betweenframesets in longitude, and the 30°-wide region inright ascension translates to a 25°-wide region in lon-

gitude. With dual signal processor operation approxi-mately thirty minutes are necessary to cover the pat-tern shown in Figure 19. Figure 20 illustrates howthis search region is covered every orbit revolution.

In practice, due to spacecraft operation con-straints, the pinch point is revisited twelve times perday, resulting in coverage of approximately 300° ofthe geosynchronous belt. The twelve pinch-point re-visits require approximately six hours of spacecraftoperations, which leaves approximately two hours fortasked operations. Figure 21 illustrates an exampletimeline, showing the distribution of pinch-point re-visits and the placement of two blocks of time de-

FIGURE 20. Four successive revisits of the pinch point illustrate the ability of pinch-point operations todetect objects with 0° inclination (green orbit) and with 15° inclination (blue orbit). As geosynchronoussatellites orbit the earth, satellites with inclinations between 0° and 15° pass through the pinch-point re-gion. The proper timing of twelve data collections at the pinch point allows the SBV sensor to cover upto 300° of the GEO belt.

T: Start time

T + 210 min

T + 70 min

T + 140 min

FIGURE 19. The pinch-point search region is covered by generating a pattern with the four-CCD SBV sensorarray. The complete pattern consists of forty framesets, which are grouped into ten four-frameset blocks. TheMSX spacecraft is maneuvered only between each of the four-frameset blocks. The resulting coverage is 30° inright ascension and 3° in declination.

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voted to tasked operations. The resulting operationtimeline results in six hours of search operations andtwo hours of tasked operations.

The scheduling of pinch-point data collection isalso limited by three time constraints that must be ac-commodated, as shown in Figure 22. The first con-straint is the time needed for contacts with the Ap-plied Physics Laboratory (APL). These are times usedby APL to upload commands and download data,and times when the MSX spacecraft is passing aboveAPL. The second constraint is times that the MSX

passes through the South Atlantic Anomaly (SAA).The SAA is a high-density region of high-energy elec-trons and protons. The high-density protons interactwith the CCD focal plane and prevent the detectionof RSOs [2]. The third constraint, as the pinch-pointregion is searched, are the times when lower satellitedensity regions are observed. The lower satellite den-sity region is a part of the geosynchronous belt lo-cated over the Pacific Ocean. It is desirable to positionthis 60° gap in pinch-point coverage over this PacificOcean region.

These three constraints are combined with the vis-ibility of the pinch-point region from MSX orbit, andthe timing of the pinch-point data collection is ad-justed by up to ten minutes to avoid conflicts. Figure23 illustrates an example of a complete data-collec-tion schedule.

The brief maneuvers required by pinch-point op-erations, combined with the faster speed of dual sig-nal processor operations, permit a large number offramesets to be taken. The number of commands re-quired to perform the pinch-point data collection ex-

FIGURE 22. Times for the pinch-point data collection are chosen to avoid conflicts with the Applied PhysicsLaboratory (APL) contacts with the MSX spacecraft, since these contacts are the primary means to upload com-mands to the spacecraft and download collected data. For maximum productivity, data collection is also avoidedwhen the spacecraft is in the South Atlantic Anomaly (SAA) and when the Pacific gap is passing through thepinch point.

0 hr 12 hr

APL contacts

24 hr

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24 hr

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Pacific gap

FIGURE 21. Twelve data pinch-point collection periods, dis-tributed over 19.4 hours. Two 70-minute blocks of time be-tween revisits of the pinch point are devoted each day totasking operations. These blocks are used for data collec-tion of high-priority objects and calibration objects.

0 hr 12 hr 24 hr

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Tasking



ceeded the command memory buffer aboard theMSX spacecraft. To reduce the number of these com-mands, a macro (consisting of commands to executedata collection from each set of four CCDs) was cre-ated and uploaded to the spacecraft. The macro per-mits four framesets to be taken with a single com-mand instead of multiple commands.

The spacecraft data handling was modified tomove the data to the experiment controller and retain

it until the experiment controller memory was reset.This modification increased the robustness of datadownloads. The data handling system was also modi-fied to permit all the data in the experiment control-ler to be downloaded at any available ground contact.This modification permitted nearly all the data to bedownloaded at least twice. These multiple downloadsalso required additional modifications in the groundprocessing system to appropriately handle the addi-tional data downloads.

As illustrated earlier, the pinch-point coverage hasresulted in increased productivity. Figure 24 illus-trates the efficient capability of pinch-point opera-tions by showing the distribution in both longitudeand inclination of all geosynchronous objects de-tected during pinch-point operations in a single day.Data from the tasked data takes are not included inthese results. Figure 24(a) illustrates the global cover-age with a planned gap only over the Pacific Ocean.Figure 24(b) shows that geosynchronous objects withboth low and high inclinations are being detected si-multaneously at the pinch points.

Metric Accuracy Improvement

In addition to increasing the productivity of the SBVsensor, we have also improved the quality of the met-ric observations. The metric accuracy of SBV sensorobservations is evaluated by a rigorous calibrationprocess. Observations of Global Positioning System(GPS) satellites are taken and compared to indepen-dently determined precise ephemerides for these sat-ellites. GPS satellites are used because they providerate and brightness characteristics that are similar togeosynchronous objects, which comprise the bulk ofSBV sensor observations. Analysis of factors that de-grade metric accuracy led to the identification of un-wanted spacecraft motion and contaminating radia-tion events as primary contributors to degradedmetric accuracy [23–25].

Two approaches were taken to reduce spacecraftmotion. First, SBV data were used to fine-tune thetime required to maneuver the spacecraft for a givenmaneuver angle. Unwanted spacecraft motion causesstars to streak on the focal plane. Analyzing the ma-neuver size and times that cause stars to streak made itpossible to determine the minimum maneuver time

FIGURE 23. Schedule showing twelve pinch-point data col-lection periods, subject to operational constraints. The datacollection can be scheduled to avoid APL contacts andminimize data collection in the SAA and the Pacific gap.

FIGURE 24. (a) The longitude distribution of detected GEOobjects, and (b) the inclination distribution of these objects.This example of data collected during a 24-hour period illus-trates the global coverage of pinch-point operations, and thesuccess of these operations in detecting high-inclinationGEO objects.

0 hr

Pinch-point search

Tasking

APL contactsSAA passesPacific gap

12 hr 24 hr

0 2 4 10 12 146 8

Inclination (deg)(b)

(a)

16

150

100

50

0

Dis

tinct

GEO

obj

ects



for a given maneuver angle. Second, it was deter-mined that the solar panels were free to track the sunduring routine data collection. Locking them in placeduring routine data collection minimized motion ofsolar panel, and limited the resulting motion on thespacecraft. The results of these modifications are vis-ible in the blue curve in Figure 25, which shows thedistribution of GPS residuals after the efforts to re-duce spacecraft motion were implemented. Compar-ing the blue curve to the green curve shows that alarger number of GPS residuals have smaller values.

Another factor that was determined to reduce theaccuracy of SBV sensor observations was the effect ofradiation events on the detected streak. A radiationevent is the interaction of a high-energy proton withthe focal plane. If a proton strikes a sensor pixel at theright time, the onboard signal processor has difficultydistinguishing the proton from the RSO detection.Metric observations are generated by fitting a line,weighted by intensity, through the RSO signaturedata. Since a typical radiation event has an intensitythat is greater than that for a RSO, the higher inten-sity tends to skew the line fit. Filtering out pixels withintensity above a threshold mitigates the effect of ra-diation events. The red curve in Figure 25 shows theresulting improvement in metric accuracy when theseradiation events are removed.

Summary

The SBV sensor has been successfully transitioned toan operational Contributing Sensor under Air ForceSpace Command sponsorship. Extensive operationalimprovements to ground systems and spacecraft sys-tems have increased the efficiency of data collection.In three years these improvements have increased theSBV sensor’s productivity from fifty deep-space tracksper day to nearly four hundred tracks per day. In ad-dition, the SBV sensor has demonstrated its effective-ness in discovering and recovering uncorrelated tar-gets (UCT) with its wide field of view and globalcoverage. The space-based sensor’s immunity to sur-face weather outages has also led to 90% acquisitionrate of high-priority objects. Accurate SBV-sensormetric observations can generate precise geosynchro-nous orbits for UCT discovery and recovery, catalogmaintenance, and other high-accuracy applications.

The Future of Space-Based Space Surveillance

The final measure of success of a demonstration sys-tem is the acceptance of the system into operationaluse and the procurement of a follow-on operationalsystem based on the results of the demonstration. TheSBV sensor, via the ACTD process, has met that finalmeasure of success. Starting in the 2002 fiscal year,

FIGURE 25. The metric accuracy of SBV sensor observations has been improved by reducing unwanted space-craft motion through more accurate modeling of spacecraft maneuvering and settling times, as shown in thegraph of the distribution of root mean square (RMS) Global Positioning System (GPS) residual data. Further im-provements have been made by removing the effect of radiation events on the detected spacecraft streak.

0 2

SBV goal

4 6 8

RMS GPS residuals (mdeg)

Distribution of GPS residualsStreak detection with spacecraft motion

Streak detection with radiation event

Radiation event

100

90

80

50

60

70

Per

cent

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resi

dual

s

Initial dataSpacecraft motion reductionRadiation event removal

68% < 1 mdeg



the U.S. Air Force has allocated funding for long-term operations of the SBV sensor, and constructionof an operational constellation of space-based spacesurveillance satellites. The system, now called theSpace-Based Space Surveillance (SBSS) system, illus-trated notionally in Figure 26, is expected to be com-posed of three to eight satellites, each carrying a wide-field visible sensor with heritage to the SBV sensor.

Developments in focal plane and processing tech-nology since the 1989 SBV sensor technology freezedate have been impressive, and will allow for signifi-cantly increased sensor capability. For instance, whenthe SBV sensor was designed, the CCD focal planeshad a limited number of pixels that had to be appor-tioned carefully between the objectives of maintain-ing good metric accuracy (each focal-plane pixel cov-ering a small angle) and having a wide-area searchcapability (each pixel covering a wide area). Today’sfocal-plane technology, as evidenced by LincolnLaboratory’s Solid State division’s five-million-pixelGEODSS upgrade CCD [26], allows enough pixelsto address both search capability and metric accuracywith no compromise. In addition, the SBV signalprocessor was constructed with special-purpose digi-tal signal processing chips, providing a processing ca-pability of about ten million fixed-point operations

per second. Today, many times that processing poweris available in general-purpose space-qualified proces-sors, which will be needed to process the informationfrom the larger focal planes. When combined into thenext-generation sensor, these technology improve-ments, along with gains in operational methods de-rived from the SBV sensor, offer an improvement inper-sensor productivity, beyond that demonstrated bythe SBV sensor, by a factor of more than ten.

While the underlying technology has evolved con-siderably, the lessons learned during the SBV sensor’stechnology demonstration and subsequent operationsendure as fundamental to the execution of space sur-veillance. The system engineers, designers, fabrica-tors, and operators of the SBV sensor have achievedexactly what they set out to do—bring space surveil-lance to space.

Acknowledgments

The work described here is a result of the efforts ofthe SBV team. Addition operations, development,and analysis were performed by the remaining mem-bers of the SBV team: Fred Morton, Jeff Cooper, BillBurnham, Elizabeth Evans, Pablo Hopman, andMarilyn Lewis. This work is sponsored by the De-partment of the Air Force.

FIGURE 26. The next-generation space-based space-surveillance system, cur-rently programmed into the U.S. Air Force planning process. This system isexpected to include three to eight satellites, each hosting a wide-field visible-band sensor based on the SBV design.



R E F E R E N C E S1. J.D. Mill, R.R. O’Neill, S. Price, G.J. Romick, O.M. Uy, E.M.

Gaposchkin, G.C. Light, W.W. Moore, T.L. Murdock, andA.T. Stair, Jr., “Midcourse Space Experiment: Introduction tothe Spacecraft, Instruments, and Scientific Objectives,” J.Spacecr. Rockets 31 (5), 1994, pp. 900–907.

2. G.H. Stokes, C. von Braun, R. Sridharan, D. Harrison, and J.Sharma, “The Space-Based Visible Program,” Linc. Lab. J. 11(2), 1999, pp. 205–238.

3. T.P. Opar, “Calibration and Characterization of the SBV Sen-sor,” Proc. 1993 Space Surveillance Workshop, Lincoln Labora-tory, Lexington, Mass., 30 Mar.–1 Apr. 1993, pp. 1–7 (LincolnLaboratory Project Report STK-206 II).

4. T.P. Opar, “SBV Data Collections in Support of Space Con-trol,” Proc. 1997 Space Control Conf., Lincoln Laboratory, Lex-ington, Mass., 25–27 Mar. 1997, pp. 11–23 (Lincoln Labora-tory Project Report STK-249 I).

5. B.E. Burke, R.W. Mountain, P.J. Daniels, and D.C. Harrison,“420 × 420 Charge-Coupled-Device Imager and Four-ChipFocal Plane,” Opt. Eng. 26 (9), 1987, pp. 890–896.

6. J.C. Anderson, G.S. Downs, and P.C. Trepagnier, “A SignalProcessor for Space-Based Visible Sensing,” SPIE 1479, 1991,pp. 78–92.

7. D.C. Harrison and J.C. Chow, “Space-Based Visible Sensor onMSX Satellite,” SPIE 2217, 1994, pp. 377–387.

8. R. Sridharan and G.H. Stokes, “Mission Planning and Auto-mation for Space Surveillance: A Case Study with the SBV”(Abstract), 2nd Int. Symp. on Reducing the Cost of SpacecraftGround Systems and Operations, Oxford University, Oxford,U.K., 21–23 July 1997.

9. W.F. Burnham, F.E. Morton, Jr., R. Sridharan, H.E.M. Viggh,A.J. Wiseman, and G.R. Zollinger, “Mission Planning forSpace-Based Surveillance with the Space-Based Visible Sen-sor,” J. Guid. Control Dyn. 23 (1), 2000, pp. 165–169.

10. H.E.M. Viggh, D. Blaufuss, F. Morton, A. Wiseman, and R.Sridharan, “SPOCC Mission Planning System Performance,”Proc. 1997 Space Control Conf., Lincoln Laboratory, Lexington,Mass., 25–27 Mar. 1997, pp. 25–35 (Lincoln LaboratoryProject Report STK-249 II).

11. J. Sharma, C. von Braun, and E.M. Gaposchkin, “Space-BasedVisible Data Reduction,” J. Guid. Control Dyn. 23 (1), 2000,pp. 170–174.

12. E.M. Gaposchkin, C. von Braun, and J. Sharma, “Space-BasedSpace Surveillance with the Space-Based Visible,” J. Guid.Control Dyn. 23 (1), 2000, pp. 148–152.

13. G.H. Stokes, “SBV Program Overview,” Proc. 1997 SpaceControl Conf., Lincoln Laboratory, Lexington, Mass., 25–27Mar. 1997, pp. 17–23 (Lincoln Laboratory Project ReportSTK-249 II).

14. J. Sharma, “Space-Based Visible Space Surveillance Perfor-mance,” J. Guid. Control Dyn. 23 (1), 2000, pp. 153–158.

15. G.R. Zollinger, J. Sharma, and M.J. Lewis, “SBV UncorrelatedTarget (UCT) Processing,” Proc. 1999 Space Control Conf.,Lincoln Laboratory, Lexington, Mass., 13–15 Apr. 1999, pp.175–188 (Lincoln Laboratory Project Report STK-254).

16. J. Sharma, “Orbit Determination Applications with theSpace-Based Visible Space Surveillance Sensor,” 10th AAS/AIAA Space Flight Mechanics Mtg., Clearwater, Fla., 23–26 Jan.2000, pp. 275–286.

17. D.H. Oza, D.T. Bolvin, J.M. Lorah, T. Lee, and C.E. Doll,“Accurate Orbit Determination Strategies for the Tracking and

Data Relay Satellites,” Flight Mechanics/Estimation TheorySymp, Greenbelt, Md., 16–18 May 1995, pp. 59–72.

18. B.J. Haines, S. Lichten, R. Muellerschoen, D. Spitzmesser, J.Srinivasan, S. Stephens, L. Young, and D. Sweeney, “A NovelUse of GPS for Determining the Orbit of a GeosynchronousSatellite: The TDRS/GPS Tracking Demonstration,” Proc. 7thInt. Technical Mtg. of Inst. of Navigation, Salt Lake City, Utah,20–23 Sept. 1994, pp. 191–102.

19. R.I. Abbot and J. Sharma, “Determination of Accurate Orbitsfor Close Encounters between Geosynchronous Satellites,”Proc. 1999 Space Control Conf., Lincoln Laboratory, Lexington,Mass., 13–15 Apr. 1999, pp. 71–83 (Lincoln LaboratoryProject Report STK-254).

20. K.S. Capelle and J. Sharma, “Geosynchronous Satellite OrbitPattern: Improvements to SBV Geosynchronous Belt Search,”Proc. 2000 Space Control Conf., Lincoln Laboratory, Lexington,Mass., 11–13 Apr. 2000, pp. 29–41 (Lincoln LaboratoryProject Report STK-255).

21. L.J. Friesen, A.A. Jackson IV, H.A. Zook, and D.J. Kessler,“Analysis of Orbital Perturbations Acting on Objects in OrbitsNear Geosynchronous Earth Orbit,” J. Geophys. Res. 97 (E3),1992, pp. 3845–3863.

22. E.M. Soop, Handbook of Geostationary Orbits (Kluwer,Dordrecht, The Netherlands, 1994).

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. is the assistant division leaderof the Aerospace division,where he specializes in analy-sis, design, and operations ofspace-surveillance systems,including the SBV and LIN-EAR programs. The SBVsystem provides the first space-based space-surveillance capa-bility to Air Force Space Com-mand in Colorado Springs,Colorado. The LINEARprogram utilizes space-surveil-lance technology developed forthe U.S. Air Force to searchfor near-earth asteroids. Beforecoming to Lincoln Laboratoryin 1989, he worked as a seniorscientist and operations man-ager at Geo-Centers Inc., acontracting company specializ-ing in fiber-optic sensors.Previously, he performednondestructive testing of laserfusion targets at Los AlamosNational Laboratory in NewMexico, and developed fiber-optic data-acquisition systemsand provided field support forunderground nuclear tests inNevada. He received a B.A.degree in physics from Colo-rado College, and M.A. andPh.D. degrees in physics fromPrinceton University.

is a staff member in the SpaceControl Systems group work-ing in the area of space-basedspace surveillance. He hasbeen overseeing Space-BasedVisible (SBV) developmentand operations for the pasttwo years, and he has beenextensively involved in SBVdata processing and utilization.His other research interestsinclude astrodynamics with anemphasis on orbit determina-tion. Before joining LincolnLaboratory in 1995, heworked at a small researchcompany outside of Bostonand at the Jet PropulsionLaboratory. He received B.S.and Ph.D. degrees in aerospaceengineering from the Univer-sity of Texas at Austin and anS.M. degree in aeronauticaland astronautical engineeringfrom MIT.

is an associate group leader inthe Space Control Systemsgroup, specializing in the areaof space-based surveillance. Heis currently the program man-ager of the SBV sensor on theMidcourse Space Experiment(MSX) satellite, and he is theMSX Surveillance principalinvestigator. He also serves asthe program manager for theSBIRS Surrogate Test Bed andthe SBIRS Midcourse Algo-rithm Development team atLincoln Laboratory, which issupported by the SBIRSProgram Office to performalgorithm and system riskreduction for SBIRS Low.Prior to his position at LincolnLaboratory, he was employedat the German Geodesy Re-search Institute in Munich andat the Earth Research Centerin Potsdam, working withGerman remote-sensing scien-tists to establish precisionorbits for the U.S. SpaceShuttle. He received a B.S.degree from Arizona StateUniversity, an M.S. degreefrom the University of Michi-gan, and a Ph.D. degree fromthe University of Texas atAustin, all in aerospaceengineering.



is a staff member in the SpaceControl Systems group, wherehe serves as a software lead forthe SBV program. In thisposition he develops algo-rithms for spacecraft opera-tion, analysis, and automation.His research interests includethe detection of fragmentationevents and close encountersbetween geosynchronoussatellites. He joined LincolnLaboratory in 1996 aftercompleting an M.S. degree incomputer engineering at CaseWestern Reserve University,where he completed a thesison decentralized networkrouters. He also has a B.A.degree in physics from KenyonCollege.

. is an associate staff member inthe Space Control Systemsgroup. He is a system engineerfor the SPOOC/SBV project,responsible for SBV sensorsupport and mission planning.He is currently developingcontrol software for a Beowulfcluster. He has a B.S. degree inelectrical engineering fromMichigan State University.

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