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Navigation hen ESAs second deep space antennabecame available in late 2005Cebreros in Spain, the Agency couldbegin using a powerful new navigationtechnique particularly important for inter-planetary craft: delta-DOR. Delta-DORcontributed to the successful orbit insertion ofVenus Express around the planet in April 2006,and it is expected to be a fundamental tool fornavigating all of ESAs current and futureinterplanetary missions. Routine navigation ofa spacecraftaround the Solar System relies on twotracking methods: ranging and two-wayDoppler. Precisely measuring the time it takes radio signals to travel to andfrom a spacecraft gives the distancefrom the ground station (two-wayrange), while measuring the signalsDoppler shift provides the crafts velocityalong that line-of-sight (range-rate).The other two position coordinates,against the sky background, areobtained only indirectly from the motionofthe ground station as the Earth rotates.This imposes a daily sinewave oscillationRoberto Maddè, Trevor Morley, Ricard Abelló,GuntherSessler & Javier de Vicente Directorate of Operations &Infrastructure,ESOC, Darmstadt, Germanyesa bulletin 128 - november 2006ANew Technique for ESAs Deep SpaceNavigationRosettas close flyby of Mars in February 2007 will be assisted by thelatest addition to ESAs tracking techniques. ESAs deep space antennaat Cebreros in Spain (inset) will play a critical role Madde 11/9/06 1:40 PM Page 68 Navigation hen ESAs second deep space antennabecame available in late 2005Cebreros in Spain, the Agency couldbegin using a powerful new navigationtechnique particularly important for inter-planetary craft: delta-DOR. Delta-DORcontributed to the successful orbit insertion ofVenus Express around the planet in April 2006,and it is expected to be a fundamental tool fornavigating all of ESAs current and futureinterplanetary missions. Routine navigation ofa spacecraftaround the Solar System relies on twotracking methods: ranging and two-wayDoppler. Precisely measuring the time it takes radio signals to travel to andfrom a spacecraft gives the distancefrom the ground station (two-wayrange), while measuring the signalsDoppler shift provides the crafts velocityalong that line-of-sight (range-rate).The other two position coordinates,against the sky background, areobtained only indirectly from the motionofthe ground station as the Earth rotates.This imposes a daily sinewave oscillationRoberto Maddè, Trevor Morley, Ricard Abelló,GuntherSessler & Javier de Vicente Directorate of Operations &Infrastructure,ESOC, Darmstadt, Germanyesa bulletin 128 - november 2006ANew Technique for ESAs Deep SpaceNavigationRosettas close flyby of Mars in February 2007 will be assisted by thelatest addition to ESAs tracking techniques. ESAs deep space antennaat Cebreros in Spain (inset) will play a critical role Madde 11/9/06 1:40 PM Page 68 on the range and range-rate data relatedto the position ofthe spacecraft. Theseposition components, though, can onlybe deduced to much lower accuracy.Also, when the spacecraft is close to thecelestial equator, the calculationsvery poorly determined. The craftsvelocity components in the plane-of-skyare not measured and can only be foundfrom how the position changes from dayto day. This means that tracking overseveral days is necessary and calls forvery high-fidelity modelling ofthespacecrafts motion.The tracking system at ESAs 35m-diameter deep space antennas (DSAs),at New Norcia in Western Australia andCebreros near Madrid provides veryaccurate measurements. Typically, therandom errors on range are about 1mand on the two-way range-rate less than0.1mm/s. Nevertheless, the limitationsdescribed above mean the accuracy ofresulting orbit determination may notbe good enough for navigation duringThe quasar is usually within 10º ofthespacecraft so that their signal pathsthrough Earths atmosphere are similar. In principle, the delay time ofthequasar is subtracted from that ofthespacecrafts to provide the delta-DORmeasurement (the Greek symbol deltais commonly used to denote difference).The delay is converted to distance bymultiplying by the speed oflight. A complication is that the quasar andspacecraft cannot be measuredsimultaneously. In practice, three scansare made: spacecraft-quasar-spacecraftor quasar-spacecraft-quasar, and theninterpolation between the first and thirdconverts them to the same time as thesecond measurement, from which thedelta-DOR data point is calculated.As two angles are required to define adirection, full exploitation ofdelta-DOR calls for measurements from twodifferent baseline orientations, the closerto 90º the better. The error in the delta-DOR measurement translates into anangular error that diminishes withcritical stages ofa mission. This isespecially the case on approaching aplanet before landing, performing aswingby or insertion into orbit.However, ESA can now augment theconventional tracking by measurementsknown as Delta Differential One-wayRange(delta-DOR).NASAs Deep Space Network (DSN)has provided delta-DOR data since 1980and has aided the navigation ofESAmissions since 1986. In 1992, the navigational accuracy ofUlysses on its approach to Jupiter wasimproved by the addition ofdelta-DORmeasurements. In the second halfof2003, 56 delta-DOR measurementsfrom the Goldstone (California, USA)-Madrid baseline and 49 from theGoldstone-Canberra (Australia) baselinewere processed at ESOC for MarsExpress. For the release ofBeagle-2 andinsertion into Mars orbit, this provideda 7-fold reduction in the navigationuncertainty compared with the standardlonger baselines. Maximising thebaseline is limited by the need for thespacecraft and quasar to be mutuallyvisible from both antennas for longDuring each scan, signals are sampledand recorded in the stations. Therecorded data are transferred to ESOC,where they are processed to extract thedelay.A spacecraft signal is normally asequence offrequency-spaced tones(either dedicated DOR tones producedby the transponder or harmonics ofthetelemetry subcarrier), each tone with itsfull power contained in a few Hertz ofbandwidth. In contrast, quasar signalslook like noise buried in the antennasoverall noise. For this reason, twodifferent algorithms (based on thesignals characteristics) are necessarywhen extracting the delay in the signalarrival times at the two stations. Also, the accuracy improves ifthetones are further apart in frequency. Soa wide bandwidth is important.With the Cebreros DSA-2 antennacoming into operation in September2005, ESA had the potential for makingdelta-DOR measurements for the firsttime. With DSA-1 at New Norcia inWestern Australia, the baseline is11650km. However, even with thisbasic infrastructure, the system had tobe upgraded for delta-DOR: modifyingthe receivers at each station, a newarchitecture for the communicationlinks from the stations to ESOC, thedevelopment ofa correlatorto extractthe delays from the raw data recorded ateach station, and a flight dynamicssystem able to use the measurements.The system upgrade was completed inless than 10months, driven by the needto have an operating and tested delta-DOR capability before the VenusExpress launch in November 2005. Theimproved system could then help tonavigate the craft between the planetsand into the critical orbit insertion.The Venus Express orbit had tocalculated to very high accuracy, so anThe ESA Delta-DOR ConceptThe delta-DOR technique for navigatinginterplanetary spacecraft is based on asimple but effective concept. It uses twowidely separated antennas to simul-taneously track a transmitting probe inorder to measure the time difference(delay time) between signals arriving atthe two stations. The technique ofmeasuring this delay is namedDifferential One-way Range (DOR).Theoretically, the delay depends onlyon the positions ofthe two antennasand the spacecraft. However, in reality,the delay is affected by several sources oferror: for example, the radio wavestravelling through the troposphere,ionosphere and solar plasma, and clockinstabilities at the ground station.Delta-DOR corrects these errors bytrackinga quasar in a direction close tothe spacecraft for calibration. The chosenquasars direction is already knownextremely accurately by astronomicalmeasurements, typically to better thanofa degree (a nanoradian).Operations &Infrastructureesa bulletin 128 - november 2006esa bulletin 128 - november 2006www.esa.intwww.esa.int ESA's New Norcia Deep Space Antenna, about 150km north of Perth,Western AustraliaMeasuring the time it takes a radio signal to travel from Earth to thespacecraft and back gives the distance (range, r). The Doppler shift inthe frequency gives the speed along that line (range-rate, vnavigating the spacecraft requires knowing the actual velocity (v)through space. Traditionally, the missing elements were provided bymeasuring the spacecrafts movement against the sky backgroundover several days Navigation ESAs second Deep Space Antenna, at Cebreros, SpainDelta-DOR tracking of a deep space probe and a nearbyquasar from DSA-1 (New Norcia) and DSA-2 (Cebreros).The quasars position is already known with greatprecision from astronomical catalogues, so the actualmeasurements will reveal the distortions added by theionosphere, for example, allowing them to be removedfrom the probes tracking Madde 11/9/06 1:40 PM Page 70 uncertainty ofonly 1nanosecond (abillionth ofa second) was imposed onthe delta-DOR time-delay measurements.This corresponds to an angular accuracyofroughly a millionth ofa degree …better than 4km on the probes positionat a distance of150millionkm.With only two stations available, ESAcan provide delta-DOR tracking withjust one baseline, and can track thespacecraft only in the portion ofspacevisible between New Norcia andCebreros.The ideal case for delta-DORpurposes would be to have another deepspace antenna at American longitudes,preferably in the southern hemisphere.This would provide a baseline almostperpendicular to the current one,completely resolving the angularposition ofthe spacecraft.With such a baseline, ESA could beindependent ofoutside help for delta-DOR tracking.Creation ofthe delta-DOR system wasdone step by step. Several elements ofthe existing infrastructure had to bemodified and some created exnovomeet the highly demanding require-ments on a very tight schedule.New Norcia and up to 1.4 Gbyte/hourfrom Cebreros, or up to 95% oftheavailable bandwidth.The correlator The data are finally collected andprocessed in a correlatorexplicitlydesigned for delta-DOR processing. Thechallenge in this case consisted ofcontaining the costs (thus building asoftwarecorrelator instead ofthe morecomplex and expensive hardwarecorrelator normally used) and the verytight schedule. Defining the softwarerequirements and identifying theinterfaces with all the other elementswas a demanding task, requiring theanalysis ofsimilar processors developedby NASAs Jet Propulsion Laboratoryand radioastronomy systems. TheDepartment ofAerospace andAstronautical Engineering oftheUniversity ofRome La Sapienzadeveloped this software correlator. Thehost machine is an off-the-shelfserverwith enough computational power toprocess the data to meet the 24-hourconstraint.Flight dynamics supportAn important role during all phases isplayed by the ESOC Flight Dynamicsteam, who support the planning,execution and evaluation ofdelta-DORobservations by: …identifying suitable quasars near tothe direction ofthe spacecraft, andproviding visibility information; …providing accurate orbit predictionstothe correlator, including derivedquantities like expected one-way rangeand range-rate values for both thespacecraft and quasar for bothstations;…processing the reduced DOR datawithin complex software to generatethe delta-DOR residual (the differencebetween the actual measurement andits value predicted from mathematicalmodels) and, together with theprocessing ofthe conventional data,to determine the spacecraft orbitalparameters. Receiver modificationsThe existing Intermediate FrequencyModem System (IFMS) receiver had tobe modified for simultaneous receptionofmultiple signals and to synchronisethe raw data, essential for achieving therequired accuracy.The IFMS is a multi-mission receiverdeveloped by British Aerospace underESA contract for a large variety ofroutine tracking purposes … telecommandtransmission, telemetry reception, datadecoding, ranging and Dopplermeasurements. In order to supportdelta-DOR measurements, the IFMSwas upgraded to receive up to eightchannels in different portions ofthedownlink spectrum with a relative time-tag synchronisation among the channelsofbetter than 1nsec. Remoteinstallation ofthe software (anothercharacteristic feature ofthis receiver)then allowed a fast upgrade ofthereceiving system in both antennas. Forredundancy, two ofthe three receivers ineach station were upgraded.Two External Storage Units (ESUs),each an off-the-shelfserver, were addedto each station to offload the storageburden from the receiver. They alsopermit fast formatting and long-termstorage ofthe data. The two ground stations are usuallyremotely operated from the GroundFacilities Control Centre in ESOC.Orbit predictions required to point theantennas to the object are delivered tothe stations on a routine basis. Theunique delta-DOR feature is theproduction by the Flight Dynamicsteam ofpredictions for quasars. Theoperations ofall ground elementssupporting delta-DOR (ground stations,correlator, Flight Dynamics, communi-cations, ESOC facilities) are scheduledaccording to Flight Dynamicsrequirements, which mesh with stationusage by other missions, and incoordination with a delta-DORobservations planning team. The datarecorded at the ground stations areretrieved offline via the correlatorworkstation during or just after theobservation itself.Based on the raw data, and on theprediction files provided by FlightDynamics, the correlator extracts thedelay between the signal arrival times atthe two stations required for the orbitdetermination soft-ware. These aredelivered to the FlightDynamics team tocalculate the space-crafts orbit.The Validation Campaignwith Mars ExpressTesting ofESAs delta-DOR system began inlate 2005 using Rosettaand Venus Express.Around the same time,DOR measurementswere made ofpairs ofquasars (one ofeachpair representing thespacecraft) so that thecorrelation ofthequasar signal could bevalidated. In Januaryand March 2006, testDOR data wereobtained from Mars Express.Ofall these tests, those with MarsExpress were the most important. Whilein orbit around Mars, its trajectory isdetermined using only Doppler data,with a resulting error in its positionrelative to the planet ofusually less than200m. Our knowledge ofthe positionofMars itselfhas about the sameaccuracy. Mars Express could thus beused to evaluate the real accuracy ofthedelta-DOR measurements.The Mars tests revealed a correlatorproblem that caused the delta-DORmeasurements to be wrong on the orderof5nsec. After this was corrected,processing ofthe six sets ofDOR datashowed that all but one ofthe delta-DOR measurements were accurate tobetter than 0.5nsec (the goal was1nsec). The other gave 0.7nsec; this wascaused by using a quasar 15º from thespacecraft (the standard is within 10º).The Operational Venus Express Campaign Following these encouraging results,and although the project was still in itsvalidation phase, it was decided to makedelta-DOR measurements ofVenusData transferOnce the data have been stored on theESUs, they are transferred to thecorrelator at ESOC for processing. Thequantity is substantial (up to 11Gbyte)… mainly from the quasar observations.Furthermore, they must reach ESOCwithin 12hours in order to be used fornavigation within 24hours oftheobservations. To cope with theserestrictions and to keep the costs down(so no dedicated data links), existingresources had to be used. Both stationsare connected to ESOC via a triangularnetwork, where each side has a 2Mbit/scapacity. For the delta-DOR datatransfer, the capacity is used on a best-effort basis, on both the direct line(single hop) and the indirect line (dualhop). The busy lines, especially for NewNorcia, required special data retrievaland stacking algorithms. (See also NewCommunication Solutions for ESAGround Stationsin A high throughput was achieved: anaverage ofup to 1.2Gbyte/hour fromOperations &Infrastructureesa bulletin 128 - november 2006esa bulletin 128 - november 2006www.esa.intwww.esa.int Two IFMS receivers were upgraded at each ground station The system updated for delta-DOR: the improved receiver, thestorage units at each station, the modified wide-area networkand the development and installation of a software correlator at Areas of mutual visibility (greater than 10º above the horizon) between DSA-1 (New Norcia), DSA-2 (Cebreros) and a hypotheticalstation in Paranal (Chile), highlighting the advantage of having a third antenna Madde 11/9/06 1:41 PM Page 72 uncertainty ofonly 1nanosecond (abillionth ofa second) was imposed onthe delta-DOR time-delay measurements.This corresponds to an angular accuracyofroughly a millionth ofa degree …better than 4km on the probes positionat a distance of150millionkm.With only two stations available, ESAcan provide delta-DOR tracking withjust one baseline, and can track thespacecraft only in the portion ofspacevisible between New Norcia andCebreros.The ideal case for delta-DORpurposes would be to have another deepspace antenna at American longitudes,preferably in the southern hemisphere.This would provide a baseline almostperpendicular to the current one,completely resolving the angularposition ofthe spacecraft.With such a baseline, ESA could beindependent ofoutside help for delta-DOR tracking.Creation ofthe delta-DOR system wasdone step by step. Several elements ofthe existing infrastructure had to bemodified and some created exnovomeet the highly demanding require-ments on a very tight schedule.New Norcia and up to 1.4 Gbyte/hourfrom Cebreros, or up to 95% oftheavailable bandwidth.The correlator The data are finally collected andprocessed in a correlatorexplicitlydesigned for delta-DOR processing. Thechallenge in this case consisted ofcontaining the costs (thus building asoftwarecorrelator instead ofthe morecomplex and expensive hardwarecorrelator normally used) and the verytight schedule. Defining the softwarerequirements and identifying theinterfaces with all the other elementswas a demanding task, requiring theanalysis ofsimilar processors developedby NASAs Jet Propulsion Laboratoryand radioastronomy systems. TheDepartment ofAerospace andAstronautical Engineering oftheUniversity ofRome La Sapienzadeveloped this software correlator. Thehost machine is an off-the-shelfserverwith enough computational power toprocess the data to meet the 24-hourconstraint.Flight dynamics supportAn important role during all phases isplayed by the ESOC Flight Dynamicsteam, who support the planning,execution and evaluation ofdelta-DORobservations by: …identifying suitable quasars near tothe direction ofthe spacecraft, andproviding visibility information; …providing accurate orbit predictionstothe correlator, including derivedquantities like expected one-way rangeand range-rate values for both thespacecraft and quasar for bothstations;…processing the reduced DOR datawithin complex software to generatethe delta-DOR residual (the differencebetween the actual measurement andits value predicted from mathematicalmodels) and, together with theprocessing ofthe conventional data,to determine the spacecraft orbitalparameters. Receiver modificationsThe existing Intermediate FrequencyModem System (IFMS) receiver had tobe modified for simultaneous receptionofmultiple signals and to synchronisethe raw data, essential for achieving therequired accuracy.The IFMS is a multi-mission receiverdeveloped by British Aerospace underESA contract for a large variety ofroutine tracking purposes … telecommandtransmission, telemetry reception, datadecoding, ranging and Dopplermeasurements. In order to supportdelta-DOR measurements, the IFMSwas upgraded to receive up to eightchannels in different portions ofthedownlink spectrum with a relative time-tag synchronisation among the channelsofbetter than 1nsec. Remoteinstallation ofthe software (anothercharacteristic feature ofthis receiver)then allowed a fast upgrade ofthereceiving system in both antennas. Forredundancy, two ofthe three receivers ineach station were upgraded.Two External Storage Units (ESUs),each an off-the-shelfserver, were addedto each station to offload the storageburden from the receiver. They alsopermit fast formatting and long-termstorage ofthe data. The two ground stations are usuallyremotely operated from the GroundFacilities Control Centre in ESOC.Orbit predictions required to point theantennas to the object are delivered tothe stations on a routine basis. Theunique delta-DOR feature is theproduction by the Flight Dynamicsteam ofpredictions for quasars. Theoperations ofall ground elementssupporting delta-DOR (ground stations,correlator, Flight Dynamics, communi-cations, ESOC facilities) are scheduledaccording to Flight Dynamicsrequirements, which mesh with stationusage by other missions, and incoordination with a delta-DORobservations planning team. The datarecorded at the ground stations areretrieved offline via the correlatorworkstation during or just after theobservation itself.Based on the raw data, and on theprediction files provided by FlightDynamics, the correlator extracts thedelay between the signal arrival times atthe two stations required for the orbitdetermination soft-ware. These aredelivered to the FlightDynamics team tocalculate the space-crafts orbit.The Validation Campaignwith Mars ExpressTesting ofESAs delta-DOR system began inlate 2005 using Rosettaand Venus Express.Around the same time,DOR measurementswere made ofpairs ofquasars (one ofeachpair representing thespacecraft) so that thecorrelation ofthequasar signal could bevalidated. In Januaryand March 2006, testDOR data wereobtained from Mars Express.Ofall these tests, those with MarsExpress were the most important. Whilein orbit around Mars, its trajectory isdetermined using only Doppler data,with a resulting error in its positionrelative to the planet ofusually less than200m. Our knowledge ofthe positionofMars itselfhas about the sameaccuracy. Mars Express could thus beused to evaluate the real accuracy ofthedelta-DOR measurements.The Mars tests revealed a correlatorproblem that caused the delta-DORmeasurements to be wrong on the orderof5nsec. After this was corrected,processing ofthe six sets ofDOR datashowed that all but one ofthe delta-DOR measurements were accurate tobetter than 0.5nsec (the goal was1nsec). The other gave 0.7nsec; this wascaused by using a quasar 15º from thespacecraft (the standard is within 10º).The Operational Venus Express Campaign Following these encouraging results,and although the project was still in itsvalidation phase, it was decided to makedelta-DOR measurements ofVenusData transferOnce the data have been stored on theESUs, they are transferred to thecorrelator at ESOC for processing. Thequantity is substantial (up to 11Gbyte)… mainly from the quasar observations.Furthermore, they must reach ESOCwithin 12hours in order to be used fornavigation within 24hours oftheobservations. To cope with theserestrictions and to keep the costs down(so no dedicated data links), existingresources had to be used. Both stationsare connected to ESOC via a triangularnetwork, where each side has a 2Mbit/scapacity. For the delta-DOR datatransfer, the capacity is used on a best-effort basis, on both the direct line(single hop) and the indirect line (dualhop). The busy lines, especially for NewNorcia, required special data retrievaland stacking algorithms. (See also NewCommunication Solutions for ESAGround Stationsin A high throughput was achieved: anaverage ofup to 1.2Gbyte/hour fromOperations &Infrastructureesa bulletin 128 - november 2006esa bulletin 128 - november 2006www.esa.intwww.esa.int Two IFMS receivers were upgraded at each ground station The system updated for delta-DOR: the improved receiver, thestorage units at each station, the modified wide-area networkand the development and installation of a software correlator at Areas of mutual visibility (greater than 10º above the horizon) between DSA-1 (New Norcia), DSA-2 (Cebreros) and a hypotheticalstation in Paranal (Chile), highlighting the advantage of having a third antenna Madde 11/9/06 1:41 PM Page 72 www.esa.int Express and use them operationally.Fifteen data points derived fromsessions on five occasions in March andearly April 2006 augmented a total of45NASA measurements obtained at thesame time, mainly from the Goldstone-Canberra baseline.Pre-launch analysis had shown that,under normal circumstances, thenavigation accuracy needed for insertioninto orbit around Venus could be achievedwith range and Doppler data only. Delta-DOR increased confidence, because itcould confirm the basic correctness ofthese conventional orbit solutions. Also, delta-DOR covered thecontingency case ofthe spacecraftswitching to its basic safe mode duringthe last few days before arrival at Venus.In that case, thrusters would fireautonomously and perturb the orbitwith a velocity increment ofunknownmagnitude and direction andimprecisely-known timing. Delta-DORwould reveal the orbit much faster thanconventional data.Analysis showed that the quality ofthese ESA measurements was onlyslightly inferior to those obtained usingNASAs 34m antennas. The mostaccurate were obtained with NASAs70m dishes. Although the delta-DORdata substantially reduced thenavigation uncertainties, the improve-ment was not as marked as that forMars Express. This was mainly due to acombination ofunfavourable geometryand problems achieving consistentmodelling ofsmall accelerations fromsolar radiation pressure and possibleoutgassing from the spacecraft. Despite this, the single mostimportant navigation parameter, theminimum altitude above Venus atarrival, was only 3km higher than thepredicted 386km. Even with all theinformation available after the event, itis not possible to distinguish entirelybetween small navigation errors and thesmall difference between the actual andexpected performance ofthe orbitOn 25 February 2007, Rosetta will swingby Mars at a planned altitude of250km. Errors in the swingby are fuel-expensive to correct afterwards, so it isplanned to make both NASA DSN andESA delta-DOR measurements, mostlyin January and February.In early 2007, Rosetta will appear inEarths southern sky, at the limit ofsimultaneous visibility from Goldstoneand Madrid for NASA. It is expectedthat very few, ifany, DOR measure-ments can be made from this baseline.This means that, in order to exploitdelta-DOR capabilities to obtaincomplete direction information (that is,use two baselines), one must be ofNASA stations and the other ofESAstations … a truly complementaryarrangement between two spaceagencies.Future interplanetary ESA missionswill also benefit from this technique. It isexpected that it will help BepiColomboto make significant fuel savings in itscorrection manoeuvres. In preparation,a SMART-1 tracking campaignvalidated the capability ofthe system torecord and process dedicated DORtones transmitted by the spacecraft.Finally, collaboration with NASA andJapan will be improved by thedevelopment ofdata translators toexchange data and results. This willgreatly extend the number ofbaselinesavailable for delta-DOR observations,benefiting everyone involved in thenavigation ofdeep space probes. Operations &Infrastructurewww.esa.int ESAs delta-DOR measurements will be important for combiningwith NASAs information during the Rosetta flyby of Mars inFebruary 2007 Madde 11/9/06 1:41 PM Page 74