An Italian Ground Station for the NASA-led Swift High Energy Astrophysics Mission

Transcription

1 SpaceOps 2006 Conference AIAA An Italian Ground Station for the NASA-led Swift High Energy Astrophysics Mission Luca Salotti Italian Space Agency, Rome, Italy ACU ASI BAT BSC FOV GNEST GRB GTO HPA ISAC LAN LEO LEOP MOC MIDEX MOU PI S/C UVOT XRT Swift is a MIDEX class NASA-led mission devoted to Gamma Ray Burst Astrophysics by means of a multiple payload sensible to wavelengths ranging from optical to soft/hard X- rays. The principal asset of the mission is a rapid reaction to GRB events powered by on board positioning and slewing capabilities with unprecedented swiftness which allow to probe and measure these events since 100 s after their first appearance in the sky. Swift has been successfully launched on November 20th 2004 from the KSC (FL, USA) into a 600 km circular orbit with 21 inclination. Italy and the UK are NASA partners for this endeavour. In particular, the Italian contribution includes the provision of the mission Ground Station of Malindi (Kenya) as well as the link connecting the Ground Station to the Mission Operation Centre in the USA. Since the launch the station is serving every pass of Swift relaying TLCs and acquiring TLM from the S/C in S-band. The MOC, located at the Pennsylvania State University (State College, PA, USA) is connected to the station by means of the operation network of the Italian Space Agency (ASI-net) through the gateways of JSC (Houston, TX, USA) and Fucino (Italy). Within this scenario real time TLM and TLC for Swift are on line relayed to the MOC while mass memory TLM is delivered off line. In this respect the ASI S-band Ground Station in Malindi has been updated in order to comply with Swift characteristics but also with the aim of evolving toward a multiantenna/multisatellite facility. During the first 14 months of Swift operation the station has served about 5300 passes displaying an efficiency level in excess of 99,5 %. This paper accounts for the main aspects of this ASI/NASA cross support as well as for the technical details of the updating of the facility. This is the first opportunity that an ASI Malindi Ground Station has to support a NASA-led scientific mission and represents a reference model for planning future collaborations involving this site. Nomenclature = Antenna Control Unit = Italian Space Agency Agenzia Spaziale Italiana = Burst Alert Telescope = Broglio Space Center (Malindi) = Field of View = Ground Network for Swift = Gamma Ray Burst = Geostationary Transfer Orbit = High Power Amplifier = Italian Swift Analysis Center = Local Area Network = Low Earth Orbit = Launch and Early Phase Operations = Mission Operation Center = Medium Size Explorer mission = Memorandum of Understanding = Principal Investigator = Spacecraft = Ultra Violet Telescope = X-ray telescope 1 Copyright 2006 by Italian Space Agency. Published by the, Inc., with permission.

2 G I. The Swift mission AMMA-RAY bursts (GRBs) were discovered in the late 1960s in data from the Vela satellites (Klebesadel, Strong, & Olson 1973). Continuous progress has been made in their understanding over the past thirty years and particularly since Now we know that they are bright (~few photons cm -2 s -1 flux in the kev band) flashes of gamma rays that occur approximately once per day and that have durations ranging from milliseconds to tens of minutes. The discovery by the ASI mission BeppoSAX (Costa et al. 1997) and ground-based observers (van Paradijs et al. 1997; Frail et al. 1997) of X-ray through radio afterglows allowed redshifts to be measured and host galaxies to be found proving a cosmological origin. GRBs are probably related to black hole formation at the endpoint of stellar evolution and definitely to bright beacons from the high redshift universe. The recent afterglow discoveries have illustrated that multi-wavelength Figure 1 - The Swift satellite (Courtesy of NASA/GSFC) studies are the key to further understanding of GRBs. To this goal a NASA-led mission was devised in the late 1990s with the key feature of capturing the afterglow of a GRB in a matter of minutes from the initial appearance of the GRB in the sky. Due to this capability the mission has been named Swift. Swift is a medium-sized explorer (MIDEX) mission successfully launched by NASA on November 20 th The hardware has been developed by an international team from the USA, the United Kingdom, and Italy, with additional scientific involvement in France, Japan, Germany, Denmark, Spain and South Africa. The primary scientific objective is to determine the origin of GRBs and to pioneer their usage as probes of the early universe. Swift's Burst Alert Telescope (BAT), is searching the sky for new GRBs and, upon discovery, triggers an autonomous spacecraft slew to bring the burst into the X-Ray Telescope (XRT) and Ultraviolet/Optical Telescope (UVOT) fields-of-view (FOVs). Such autonomy allows Swift to perform X-ray and UV/optical observations of >100 bursts per year within seconds of a burst detection. A complete description of the Swift mission and its scientific goals is contained in Gehrels et al The injection orbit of Swift is a LEO with altitude 600 km and an inclination of almost 20,5 over the equator. The contribution of Italy to the Swift mission is coordinated by ASI and is composed of an on board mirror for capturing X-rays, a data analysis and dispatching center (ISAC) and the mission Ground Station located in Malindi (Kenya). This Ground Station is connected to the general structure of the Swift ground segment (GNEST) through the ASI-net facility, an ASI owned data relaying network. This is the first time that an Italian Ground Station is used to support a NASA-led scientific mission. Figure 2 gives a general overview of the GNEST; the areas under Italian responsibility have been highlighted. 2

3 S ystem Architectu re Observation Requests SW IFT S-Band CM D/TLM 2.25 M bps dow nlink (R T & PB TLM ) 2 kbps uplink (Normal Commanding) M alindi G round Station Data Analysis Tools Science Teams, Co mmunity Kenya Quick-look Data & Products GSFC Swift Science Center H EASA RC ISAC U K DC M AF/M AR& DAS CM D/TLM 1 kbps dow nlink (A lerts & H/K) 125 bps uplink (ToO Requests) Quick-look L0 Data Fucino, Italy AS INet Fucino Gateway Swift Data Center Prod. FITS data GSFC Command, H/K, Sc ience 384 kbps Leased Line P SU M ission O perations C enter (M OC) JSC AS INet US Gateway S/W Updates Observatory Data BAT UVOT/XRT Spacecraft Flight GSF C Software PSU M aintenance Spectrum Astro Orbit Data SN Scheduling & Status GSFC Flight Dynam ics Facility 2-Line Elements NO RAD Observation Results NC C GSFC GSFC Front-E nd GCN TDRS Alerts, TOO Commanding, Contingency H/K, Tracking White Sands Complex (WSC) Commands Burst Alerts Alerts, H/K Satellites Optical Telescopes R adio Telescopes Tracking Data e.g. Chandra e.g. HET e.g. VLA November 13, 2001 Revision J Figure 2 General layout of the Swift Ground Segment with indication of the Italian contributions (Courtesy of NASA/GSFC) 3

4 II. The ASI Malindi site Since 1962 Italy has established an equatorial space base on the coast of the Indian Ocean near Malindi (Kenya) at a latitude of -3. Known as San Marco Base this site has been recently dedicated to its founder, the late Professor Luigi Broglio who passed away in 2001, becoming the Broglio Space Center (BSC). Since 2004 the BSC is managed by the Italian national space agency ASI. The base was active since the 1960s till 1988 for launching satellites and rockets from a complex of platforms three miles off shore the coast amid the Ngwana Bay. The facilities on the ground include two Ground Stations and a Remote Sensing Center. Current activities of orbital tracking at the BSC range from the support to LEO mission to the support to the flight of launchers (Ariane 5, Delta 2 and 4). In addition the Malindi stations are frequently inserted in the networks for the control of the early phases of the free flight of TLC satellites injected into GTO orbits (LEOP support). The Remote Sensing Center collects and dispatches earth images in different wavelength bands in the context of cooperation with NASA and ESA satellite programs. Also, geophysical measurements are accumulated by means using ozono-probes launched into the atmosphere. The BSC is linked with Italy via a redundant satellite data link which allow data/voice communications with the national Space Centre of Fucino (Avezzano, Italy). These links are part of the ASI operative data network ASI-net. Figure 3 - The Broglio Space Center in Malindi (base camp and S. Rita platforms). III. The Swift Ground Station After the signature of the MOU with NASA, ASI selected one Ground Station in Malindi (MLD-2) as the Swift Ground Station. This Ground Station was built in 1995 for supporting the ASI mission BeppoSAX which was in orbit in the period MLD-2 is equipped with a 10 m antenna and provides capability of tracking and control in S band. However, due to the peculiar characteristic of BeppoSAX, the station was not compliant with the current CCSDS standard for TLM/TLC; on the other hand, some crucial subsystems had arrived at the end of their operative life. Thus, the opportunity to serve Swift prompted a considerable effort of upgrading and reconditioning of the entire Ground Station. through a contract issued by ASI to the Italian firm Telespazio S.p.A. These activities, including the test phase, lasted for more than one year ( ). Main initiatives were: 1) Construction of a new control building for hosting equipment and operators; 2) Reconditioning of the antenna mechanical structure; 3) Substitution of all obsolete RF equipments; 4) Substitution of HPAs with more reliable SSPAs; 5) Install of new Base Band and Station Computer compliant with CCSDS TLM/TLC standards; 6) Install of a new Monitor & Control subsystem; 7) Install of a new station scheduling subsystem; 4

5 8) Install of a new station LAN; 9) Coding and install of a new Link Acquisition Procedure completely automatic; 10) Install of a new ACU; 11) Upgrade to a TCP/IP data relay network; 12) Substitution of the Frequency/Timing subsystem; At the end of this phase the Ground Station was not only fully compliant with the CCSDS standard for TLM/TLC and TCP/IP data link protocol, but also able to plan and execute automatic activities in a multiantenna/multisatellite environment. In addition, control from remote was introduced. One of the essential points that drove the update was the implementation of a new Link Acquisition Procedure. Actually, this feature, which on one side interface with the Monitor & Control subsystem and, on the other side, with the scheduling subsystem, is able to execute acquisition, tracking and data relay of satellite passes eliminating any manual operator activity. This procedure is based on a set of parameter easily configurable which, for its first implementation, has been adapted to Swift. Figure 5 - MLD-2 antenna Figure 4 - The Swift Ground Station (MLD-2) IV. The data network Since 2000 ASI has established a dedicated data transmission network whose name is ASI-net. The network has a star shaped architecture with pole and control center in Italy at the Space Center of Fucino and gateways at Malindi, Matera, Turin (Italy) and at three NASA facilities (JSC, KSC and MSC). Two legs of ASI-net were used in order to connect the MLD- 2 Ground Station with the JSC gateway in the US. From there NASA stretched a connection to the Swift MOC at Penn State University (State College PA). Also in this case a preparatory effort was implemented mainly to upgrade the bit rates to the Swift requirements and to increase the overall reliability of the link to the mission critical level (> 99,95 %). Furthermore, every branch of the link and every intervening equipment were provided with a redundant back up. A the end of the implementation phase a nominal bit rate of 512 kbps was dedicated to Swift together with a back up line of 128 kbps. This link carries TLM (real time and post pass transmission), TLC, voice and planning data over a TPC/IP layer. Control of the infrastructure is provided at the pole with a presidium around the clock. Also at the JSC a continuous monitoring is provided. In order to maximize the security aspects no Internet gateway has been inserted. 5

6 KSC Systems JSC Systems Swift PSU-MOC Systems KSC Communication Node JSC Communication Node Swift PSU-MOC Communication Node ASI Communication Gateway Other Services ASI Central Node (Fucino, Italy) Malindi, Kenya Ground Station Figure 6 - ASI-net connection of MLD-2 with the PSU MOC V. Operations At the end of a smooth launch campaign Swift lifted off on board a Delta 2 (7320) on November 20 th 2004 at 17:16:00 GMT from pad SLC -17A of Cape Canaveral. Injection into orbit was nominal. At 19:30:15 GMT MLD-2 acquired the spacecraft during its first pass over Malindi without the need of any search procedure. Real time TLM was smoothly relayed to the MOC and TLC activity started. At the end of the pass mass memory TLM was transmitted. This first successful pass represents the model of the current routine activity of MLD-2 which performs from 7 to 11 Swift passes every day. Since the injection into orbit the station has served 5359 Swift passes with an overall efficiency figure in excess of 99,5% (statistics of February 6 th 2006). Acquisitions are generally performed with the automatic mode where the activity of the operators is not necessary. These satisfying performances have allowed ASI to reduce the number of operators dedicated to the MLD-2 station with frequent un manned shifts. No major out of services have happened during this support; minor anomalies were due to human errors, to data link faults or to the hang-up of station equipment. The Swift mission is funded by NASA till the beginning of 2006 yet, given the well behavior of the spacecraft in orbit as well as the great interest displayed by the scientific community into the GRB data gathered by Swift, it is expected that an extension will be granted in the following months. VI. Conclusion The insertion of an Italian ground station into a NASA-led mission has represented a success. Moreover this program has given to ASI the opportunity to completely refurbish a Ground Station in Malindi and to upgrade the ASI-net network. The added value is that the new Swift station has acquired automatic capabilities, remote control, CCSDS compliance and a scheduling system which incorporates multimission/multiantenna concepts. These elements follow a general guideline of the ASI policy about the reduction of the costs of satellite operations and the interoperability with partner agencies. In order to completely attain this last goal the next steps include the upgrade of the station to the CCSDS SLE (Satellite Link Extension) level of compatibility. This feature, which was not needed to support the Swift mission, is however essential to connect the station with the state of the art remote control centers. 6

7 Figure 7 - Malindi Ground Stations tracking Swift 7

8 VII. Appendix Table 1 displays the complete parameter set of the Swift Ground Station: Frequency Frequency Range MHz (TX) MHz (RX) Polarization RHCP LHCP combination (RX) RHCP LHCP selection (TX) System G/T 21.3 db/ K at 5 elevation angle System EIRP 59 dbw Electrical characteristics TX Gain log FMHz/2025 dbi RX Gain log FMHz/2200 dbi Axial ratio within 1 db beam-width < 0.5 db Isolation between TX and RX paths 90 db Side-lobe envelope 2.5 < θ < θ 180 G log (θ) dbi G -10 dbi Pointing error (difference between commanded antenna position and actual antenna beam axis position): 50 mdeg (RMS) Tracking error (difference between signal source position and actual antenna beam axis position): 100 mdeg (at 3σ) for winds up to 70 km/h Mechanical characteristics Antenna diameter 10 m Steering Ranges AZ: ± 360 continuous EL: -1 to +91 continuous Tracking rate AZ: 20 /sec EL: 5 /sec Tracking acceleration AZ: 10 / sec2 EL: 5 / sec2 Tilt range ± 2.5 (East / West) Environmental conditions: wind speed full performance survival wind speed temperature 70 Km/h gusting 100 Km/h 200 Km/h (stowed) -40 C to +50 C Low Noise Amplifier Bandwidth to MHz Gain at central frequency at 25 C 50 db Gain slope 0.01 db/mhz Gain stability db/day; db/week 3rd order intercept point > + 20 dbm Noise temperature 55 K VSWR (input & output) 1.5:1 maximum Down-Converter Input frequency to MHz Frequency step size 1 KHz Output frequency and bandwidth 70 MHz ± 20 MHz Gain adjustment 30 db in 0.2 db step Frequency stability (using internal reference) ± 1 x 10-8 per month Noise figure 14 db Output impedance 50 Ohm Output VSWR 1.25:1 8

9 SSB phase noise Hz KHz KHz KHz Spurious output (including L.O.) -60 dbc Overall RX chain characteristics IF output level - 60 dbm ± 2 db (for a received IPFD of -150 dbw/m2) Gain stability ± 0.5 db over 24 hours ± 1 db over 6 months Gain ripple across band ± 0.3 db Gain slope ± 0.2 db/mhz Group delay in the D/C IF band: linear parabolic ripple ± 0.1 ns/mhz ± 0.05 ns/mhz2 1 ns p-p Power Amplifier Output frequency to MHz Output power (at saturation) 50 W Gain slope 0.05 db/mhz Gain stability ± 0.25 db/day Overall AM/PM conversion 5 /db Residual AM: below 10 KHz above 10 KHz -60 dbc -60 (1 + log FKHz) Noise and spurious per 4 KHz at rated gain -80 dbc Input VSWR 1.25:1 Up-Converter Input frequency 70 MHz + 20 MHz Input impedance 50 Ohm Input VSWR 1.25:1 Output frequency to MHz Frequency step size 1 KHz SSB phase noise Hz khz -90 khz khz Frequency stability (using internal reference) ± 5 x 10-8 per month U/C spurious output (including L.O. leak) -65 dbc Gain adjustment 30 db in 0.2 db step Carrier Sweep Wave form Symmetrical triangular Initial frequency setting Nominal up-link freq. ±1 KHz Frequency offset KHz Number of sweeping cycles Selectable Amplitude Selectable from 1 to 1000 KHz Sweep rate Selectable from 0,1 KHz/s to 175 KHz/s Overall TX chain characteristics Transmit level diagram 0 dbm at IF shall correspond to a maximum transmitted carrier EIRP Gain stability better than db over 24 hours Group delay (in any 10 MHz band): linear ± 0.05 ns/mhz 9

10 parabolic ripple ± 0.05 ns/mhz2 1 ns p-p Calibration Loop Input / Output frequency translation fixed to MHz Frequency stability (using internal reference) ± 1 x 10-8 per month Amplitude variation db maximum Output level setting adjustable in 0.2 db steps and in a charge of 20 db at least Spurious output - 60 dbc Phase delay variation at 100 KHz modulation 2 ns Group delay: linear parabolic ripple ns/mhz ns/mhz2 2 ns p-p SSB phase noise from 100 Hz to 300 KHz: above 300 KHz: < log f dbc/hz, f in Hz < -106 dbc/hz Telemetry Processing Input frequency 66 to 74 MHz Input Level range (AGC) -25 to -90 dbm RX acquisition range + 10 to KHz Loop BW 30 to 3000 Hz Input impedance 50 Ω VSWR 1.5 AGC time constant 1 ms, 10 ms, 100 ms Acquisition time 0.5 second (typical) Acquisition threshold C/No = 25 dbhz for 30 Hz loop BW Modulation PM/BPSK, FM/BPSK, PM/PCM, BPSK, QPSK Sub-carrier frequency up to 1.2 MHz PCM code NRZ-L/M/S, BP- L/M/S Data rate up to 5 Mbps Coding Viterbi / Reed-Solomon / Scrambling Time-tagging accuracy + 50 µsec + 10 µsec (with external 1-pps) Telecommand Processing Output Frequency 66 to 74 MHz Frequency stability (using internal reference) ± 5 x 10-8 per month Output level 0 to -40 dbm Output level setting accuracy ± 1 dbm Output impedance 50 Ω VSWR 1.2 Spurious outputs -60 dbc Modulation BPSK, QPSK, BPSK/PM; FSK/FM Carrier PM modulation index 0 to 2.5 rad Sub-carrier frequency up to 500 Kbps PCM code RZ, NRZ-L/M/S, BP- L/M/S Data rate up to 10 Kbps Ranging Measurement technique ESA Tone Standard ESA Code Standard Major tone frequency 100 KHz 300 KHz Minor tones 1 to 6 N/A Code length N/A 0 to 18 Integration time 0.25 to 2.5 sec PLL BW 0.1 to 8 Hz Measurement standard deviation 1 db vs theory 10

11 Phase measurement resolution Distance measurement resolution 1 nsec Frequency and Time reference Reference frequency 5 MHz Stability (24h) 1x10-12 IRIG-B output 5 MHz, 1 KHz Synchronization to UTC better than 100 ns (using GPS) Metereological Condition Indoor Outdoor Temperature 21 ± 3 C -20 C +50 C Relative Humidity max 70% non condensing 100% Rain N/A up to 10 cm/hr Winds N/A gusting to 100 Km/h Table 1 - Parameter set of the Swift Ground Station Acknowledgments L. Salotti thanks Niels Gehrels and Guido Chincarini, respectively US and Italian PIs of the Swift mission, for the continuous support provided during the implemention and operations of the Swift Ground Station. A special thanks also to Frank Marshall of NASA/GSFC, Lead of the Ground Network for Swift (GNEST) and to Patrizia Caraveo of the INAF. References Periodicals 1 Klebesadel, R. W., Strong, I. B., & Olson, R. A., 1973, ApJ, 182, L85. 2 Costa, E. et al., 1997, Nature, 387, van Paradijs, J., et al., 1997, Nature, 386, Frail, D. A. et al., 1997, Nature, 389, Gehrels, N.; Chincarini, G.; Giommi, P.; Mason, K. O.; Nousek, J. A.; Wells, A. A.; White, N. E. et al ApJ 611,

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