SVOM: Challenge of implementing a worldwide coverage VHF Network in the equatorial region

Transcription

1 SpaceOps Conferences 28 May - 1 June 2018, 2018, Marseille, France 2018 SpaceOps Conference / SVOM: Challenge of implementing a worldwide coverage VHF Network in the equatorial region S. LACOUR 1 Centre National d Etudes Spatiales (CNES), Toulouse, 31400, France SVOM is an ambitious Chinese-French space mission dedicated to the detection and study of Gamma-ray bursts and their use for astrophysics and cosmology. The SVOM mission consists of a satellite carrying four instruments to detect, localize and observe Gamma-ray bursts, a VHF network to keep a permanent contact with the satellite and a ground segment including a wide angle camera and two follow-up telescopes. CNES is prime contractor for the alert system and responsible of the design and deployment of a minimum of 45 VHF stations across the entire intertropical band. CNES relies on a vast network of scientific partners to shorten deployment times as much as possible. This will be achieved through simplified hosting agreements with a large number of institutions, laboratories, space agencies or states. Alongside these activities, studies have been undertaken to develop reliable and robust VHF stations to minimize manpower during deployment, operation and maintenance due to the large number of stations to be deployed, often in remote locations and under severe environmental conditions. The goal is to have a fully operational VHF network before the launch of SVOM satellite, at the end of Nomenclature ANOC = Alert Network Operation Center C-GFT = Chinese Ground Follow-up Telescope CEA = Commissariat à l énergie atomique et aux énergies alternatives CNES = Centre National d Etudes Spatiales CNSA = China National Space Administration COTS = Commercial off-the-shelf CEA = Commissariat à l énergie atomique et aux énergies alternatives DORIS = Doppler Orbitography and Radiopositionning Integrated by Satellite F-GFT = French Ground Follow-up Telescope FPOC = French Payload Operation Center FSC = French Space Center GCN = Gamma-ray burst Coordinates Network GFT = Ground Follow-up Telescope GRB = Gamma-ray burst GRM = Gamma-Ray burst Monitor GWAC = Ground-based Wide Angle Camera ITU = International Telecommunication Union LHCP = Left Hand Circular Polarisation MTTF = Mean Time To Failure MXT = Microchannel X-ray Telescope NTP = Network Time Protocole PoE = Power Over Ethernet REGINA = REseau Gnss pour l Igs et la Navigation RHCP = Right Hand Circular Polarisation SVOM = Space-based multiband astronomical Variable Objects Monitor TDRSS = Tracking and Data Relay Satellite System VHF = Very High Frequency VT = Visible Telescope 1 Ground Station technician, CNES, DNO/SA/IS sebastien.lacour@cnes.fr 1 Copyright 2018 by CNES. Published by the, Inc., with permission.

2 I. Introduction SVOM (Space-based multiband astronomical Variable Objects Monitor) is a joint mission of the China National Space Administration (CNSA) and the Centre National d Etudes Spatiales (CNES) dedicated to the detection and study of gamma-ray bursts and their use for astrophysics and cosmology. Gamma-ray bursts could be separated between the short- and long-duration bursts at about 2 seconds. Short bursts would come from the coalescence of two massive and compact objects such as two neutron stars, or a neutron star and a black hole. In the case of long bursts, those that last for more than 2 seconds are produced at the end of a hypernova, a type of supernova. Gamma-ray bursts are considered as the brightest and the most energetic events in the Universe since the Big Bang and are isotopically distributed in the sky. The afterglow emission of a gamma-ray burst is the phase which follows the prompt emission. The remanent emission is not as brief as the prompt emission but gradually decreases on a timescale of hours, days or months. This makes it possible to carry out observing programs with ground or space telescopes, provided that there is a sufficiently precise position of the burst, in particular in the X-rays and in visible light. The information provided by the afterglow emission is crucial for a better understanding of the explosive phenomenon and the environment of gamma-ray burst progenitors. To be successful, such GRB mission requires a very high level of synergy between space-based and ground instruments since we need to explore phenomena which cover extremely broad dynamic ranges of times (from milliseconds for the prompt emission to weeks and months for the afterglow and/or the associated supernova), of wavelengths (from optical to GeV for the prompt emission, and from radio to X-rays for the afterglow and the supernova), and of luminosities (from magnitude ~ 6 for the brightest prompt optical flares down to magnitude > 28 for the faintest host galaxies). As a consequence, SVOM will be a highly versatile astronomy satellite design to study short bursts, long bursts and afterglows, with built-in multi wave-length capabilities (visible, X and Gamma), autonomous repointing and dedicated ground follow-up instruments. SVOM will have a broad science return thanks to its unique instrumental combination of 3 wide-field instruments: ECLAIRs, GRM, GWAC, and 3 Figure 1 - SVOM satellite (artist view) narrow-field instruments: MXT, VT, GFTs. (see Fig. 12 in Appendix)[1]. SVOM is expected to detect more than 200 Gamma-ray bursts over 3 years. CNES is prime contractor for the French payload development (ECLAIRs, MXT), for the alert system (VHF ground network) and for the French elements of the ground segment (payload monitoring center, scientific expertise centers). In the article, I will mainly present the alert network, its overall architecture and the simulations that helped us to define the theoretical optimal alert network needed to reach the mission specifications. We will also describe about the institutional, geographic and environmental constraints that led us to rethink this network and the number of stations to be deployed. Next, I will present the VHF stations in full details to explain how we designed them to achieve our objective of minimal maintenance and maximum availability. Finally, we will also report on the deployment progress one year after the start of activity and discuss about the future of this network once the SVOM project will come to its end. 2

3 II. VHF network alert A. Alert network architecture Figure 2 - Alert network architecture Technically, the alert system is organized as follows: A worldwide network of VHF stations ensures the reception of the messages transmitted by the SVOM satellite. The stations use the Internet to communicate with other entities and are all synchronized via the NTP service; The ANOC (Alert Network Operation Center) is responsible for the configuration, the monitoring and remote maintenance of the entire network of VHF stations. This center is hosted and operated by CNES. The FSC (French Science Center), hosted and operated by CEA at Saclay (France) receives and analyses in real time alert messages before dissemination them to the Chinese Science Center and to ground observatories. The French Payload Operation Center (FPOC) is in charge of operations for the instruments ECLAIRs and MXT. It is one of the French components of the SVOM ground segment. This center is hosted and operated by CNES Toulouse. As soon as a GRB is detected and localized by ECLAIRs telescope, SVOM alert network must make the fastest possible transmission of alert messages including the celestial coordinates of the GRB and some payload telemetries to the French Scientific Center, through the downlink of the satellite and the VHF station network. The French Scientific Center performs a first processing and control of received data then transmits it to its Chinese counterparts, at the Chinese Scientific Center. Finally, both centers will transmit pointing data to their respective tracking telescopes, the French Ground Follow-up Telescope (F-GFT) for France and the Chinese Ground Follow-up Telescope (C-GFT) and the Ground Wide Angle Camera (GWAC) for China. The FSC also transmits to the international community the information of gamma-ray burst detection, through the Gamma ray burst Coordinates Network (GCN). B. The main system constraints that led to the VHF alert network definition. Possible observation of GRB events requires to make available measurement of the GRB celestial coordinates made by SVOM satellite to scientific community in less than: 30 seconds in 65% of the cases; 20 minutes in 95% of the cases. 3

4 This requirement is essential to start observations from the ground as quickly as possible with SVOM project s automated telescopes and then with telescopes of the international community. Figure 3 - Alert message distribution strategy The availability of the system needed to meet the 30 second delay requirement is mainly dependent of the possibility to maintain a reliable and permanent contact between the ground and the satellite. SVOM will be a low earth orbit satellite, orbiting at 650 km with a slight inclination of 30 and as a result, the satellite s trajectory will oscillate between latitudes of +30 and -30. In order to guarantee a permanent and reliable link with SVOM satellite, several solutions were studied (use of the TDRSS network, deployment of a network of S-band simplified tracking stations) but for cost of development, simplicity, payload mass and reliability, use of MHz VHF band has been selected. This band is allocated to space exploration, satellite meteorology, mobile satellite communications and space research, in downlink and for the three ITU regions. Meanwhile, the choice of this technology has some drawbacks: Deployment of a large number of stations to cover the entire trace of the satellite Limited bandwidth SVOM won t carry a powerful payload and because of its agility, the link budget won t give so much margins especially in environments where industrial noise is important (airports, densely populated areas, ). During periods of solar activity, the phenomenon of ionospheric scintillation can cause significant signal losses (up to 20dB) especially in VHF and randomly over time with a scintillation of a few mhz to a few Hz. The robustness of the transmission coding will not, in some cases, overcome the phenomenon so scintillation must be integrated in the link budget. C. Host site requirements 1. Latitude: This requirement is closely related to the SVOM orbit with an inclination of 30. A host site will be more effective if its circle of visibility will remain at the maximum of this latitude. The Fig. 4 locates the optimal latitudes around the equator and between latitudes 20-26, for circles of visibility calculated at 10 of elevation angle. 4

5 Figure 4 - The perfect network (10 visibility circles) Simulations led to a very theoretical network of 36 stations distributed around the globe, ideally located on the equator (latitude = 0, longitude = * k) and close to the tropics (latitude = +/- 22, longitude = *k), k varying from 0 to 11, as shown in Fig. 4. Although the interest of a site, from the point of view of the coverage of the orbit, decreases beyond the tropics, until falling to zero around a latitude of 40, the potential sites will be studied until a latitude of 35/37 because some could be the only solution to perfect the coverage of the orbit, in particular in the ocean areas where there are not always islands at the wished latitudes. 2. Radioelectric activity Man-made noise has significant disruptive effects on the reception of VHF signals. This additional noise is generated by human activities like power generators, electrical and electronic equipment or vehicules [Ref. 2]. The numerous reports on this phenomenon attest to the fact that this subject is a big concern to everyone working in VHF or UHF bands. It appears that it is very difficult to precisely quantify this noise because it obviously depends on the location of the measurement, the time of day and the date on which it occurred. Also, the power and nature of man-made noise seems to change considerably with technological changes. A measurement campaign carried out in 2003 by the British company Mass [Ref. 3] at different typical locations shows substantial differences (from 10 to 25dB) between the measured average values and those of the ITU recommendation P.372-7, a recommendation based on measures done in the 1970s. Earlier in the project, it was foreseen to exclude potential sites located in heavily populated areas, but this solution would strongly penalize the search for potential host sites as a lot of our partners are in major urban centers. To overcome this problem, it was decided to increase the number of stations to reinforce the overall redundancy of the system. 3. Visibility The studies have shown that the minimum elevation angle to be taken into account for the design of the network couldn t be less than 10 but efforts will therefore be made, especially in areas with low man-made noise, to favor sites with no or little mask above 5. On the other hand, if host site is located in a very noisy area, reception should not be possible under good conditions up to an angle of elevation of at least 20, and in this case, potential masks may not be a problem. 4. Telecommunications The solution implemented to transmit alerts in real time uses a permanent and low speed connection over the Internet. Nowadays, we believe that this requirement is no longer a strong constraint as most of the places in the world are connected at least with a low speed Internet. 5. Accessibility A quick glance at the Fig. 4 shows that more than 2/3 of the area under SVOM track are oceanic areas. It is therefore reasonable to think that more than half of the stations of the VHF network (probably more than 20 stations) will be located on islands. The choice of these islands will be made primarily on the basis of an obvious criterion of 5

6 accessibility, because it seems unrealistic to install a station on an inaccessible island because it would greatly complicate its installation and its troubleshooting if necessary D. A race against time The development of VHF stations started mid-2017 and one of the most important challenges will be to have the most complete network possible before the launch of SVOM satellite in 2021 in order to perform the first system tests before the launch. To achieve this, CNES relies on its worldwide network of scientific partners that has been established with the deployments of the DORIS (orbit determination and ground positioning Fig. 5) and REGINA (GNSS figure Fig. 6) networks. The plentiful scientific partners of the SVOM project could also be asked to host a station of the alert network. Figure 5 - DORIS network Figure 6 - REGINA network Furthermore, CNES has decided to use simplified agreements to speed up administration procedures and to encourage as many organizations as possible to participate in the SVOM project. Specifically: All costs are covered by CNES Exclusion of liability foreseen for hosting sites Hosting sites can easily terminate agreement at any time Under condition, scientists of hosting site could request data to the SVOM Project team. E. Determination of the optimal network Taking into account known DORIS and REGINA host sites and our possible partnerships related to the SVOM project, we have drawn up a list of 86 potential host sites. Several scenarios were simulated to determine the optimal number of stations needed to meet the mission specifications of 65% of alert messages received within 30s. 6

7 Simulations have determined that a 35 stations network (Fig. 7) has performances near the specification (Fig. 8), especially in periods with no scintillation. In fact, we are moving towards the deployment of at least 45 stations in order to add redundancy to the system in the event of a station failure, trying to strengthen our network on both sides of large marine area. Figure 7 Optimal network with 35 VHF stations (10 visibilité circle) Figure 8 - Time to User cumulative distribution function (35 VHF station) III. VHF station overview The various constraints of the mission seen above require the deployment of a large number of stations across the entire intertropical band in location that are sometimes difficult to access and under various and sometimes harsh environmental conditions. From the beginning, CNES has been striving to develop a simple, powerful and robust VHF station that can be installed, commissioned and maintained as easily as possible and without any particular technical expertise. The specifications of our station were: Operating temperature (outdoor equipment): from -20 C to + 60 C Maximum thermal gradient: 10 C per hour Relative humidity: from 0 to 99% Pressure: from 900 to 1100 mbar Wind: Force 10 (Beaufort scale) Resistance to solar and UV radiations Resistance to saline mist (close to the ocean with permanent winds) Resistance to high rainfall in a very short time Interfaces with the host sites as simple as possible: low speed internet connection (<64kbps) and low energy consumption using PoE High MTTF to take into account the period of installation, validation of the network and the scientific mission Easy installation and start-up in less than 48 hours Zero hardware maintenance Performance checks and software maintenance operations remotely performed from the Network Manager at CNES Toulouse s facilities. Station should not contain electronic components with export restrictions 7

8 To fullfil these specifications, SVOM VHF station consists of a monobloc system with high performance VHF antennas, a Software-defined radio (SDR) module powered by PoE fixed on a strong adjustable support. This solution presents the following advantages: Reliability over time in any environment Easy installation and replacement in case of failure Low wind resistance in all directions Compactness for storage and transport. The chosen solution is to use COTS equipment as much as possible to minimize study and production time. However, careful consideration was given to the RF interfaces, the design and optimization of the demodulation algorithm as well as hardware and software optimizations in order to significantly increase the MTTF of the station. For economic reasons, omnidirectional antennas have been selected rather than directional antennas with pointing system. Selected antennas are small +4 dbic quadrifilar helix antenna which allow to capture signals with a correct gain up to low elevation of about 5. Two antennas are used to ensure good reception of the signal in both LHCP and RHCP polarities. The SDR module is confined inside a weatherproof rugged case and is based on a AD9361 components for the transceiver part and Zynq device for the digital part. These components have been widely studied in terms of reliability and there is a large amount of data on the subject. The AD9361 transceiver receives the RF signal and deals with the frequency downconversion, signal digitization and initial filtering. Although the transceiver incorporates a baseband filter, an extra filter was added upstream to reject disturbances due to the industrial noise close to the channel. The digital part of the station is based on a Zynq-type Xilinx component whose firmware and embedded software are composed of three main parts: Figure 9 SVOM VHF station The FPGA interfaces the transceiver to extract parameters of the input signal to demodulate then with a sliding FFT and finally to perform frame search and alignment. The CPU ARM1 is responsible for parameterizing the RF transceiver and retrieving useful data in CCSDS frames Communications with the outside world are provided by the CPU ARM0 Figure 10 Functional diagram of the SDR module The system is powered by a passive PoE supply to make the installation easier because a single cable can be used to carry the communication and power with a high degree of robustness against overvoltage and polarity reversal. The low overall consumption of the station (< 10W) allows to use a passive dissipation system which increases the reliability of the system. All data exchanges are done using Internet through several VPN links allowing a complete control of the level of security of the information. VPN links are established at the initiative of the SVOM VHF station to easily interconnect 8

9 will all the network configurations found on host sites and also to exclude any possibility of external connection to the station. The sending of useful demodulated packets (alert messages and onboard telemetry messages) is done using an HTTPS client and the REST protocol. The control, configuration and status of the station uses an HTTPS server and the collect of useful log files is based on the SFTP protocol. All services use the state of the art and best practices of computer security. Figure 11 Network policy IV. Current status The use of COTS, standardized protocols as well as the excellent knowledge of the selected technologies made possible to develop the first prototype of the VHF station in about 6 months. In April 2018, at the CNES facilities, RF compatibility tests between the board and the station were carried out with the Chinese integration team. These conclusive tests have demonstrated that the station's behavior is in line with the expected specifications and interfaces and will soon lead to a mass production of the stations. On the other hand, we are running a bit late in making contact with most of the selected host sites but the enthusiastic first feedback makes us confident about meeting the deadline and to have a fully operational VHF network before the launch in The development of simple-to-install stations with a zero maintenance design and minimal interfaces to the host sites as well as simplified administrative procedures seem to be important assets for a quick deployment of a global network of scientific stations. V. Future of the VHF network after SVOM mission VHF band was heavily used in the past but today most activity consists of meteorological satellites transmitting data and low resolution images. For this reason, it would be unlikely that the SVOM network could be reused for such future missions, except maybe for another GRB mission. However, through the SVOM project, CNES asserts its expertise in deployment of scientific station networks. The deployment of SVOM VHF network will also strengthen our cooperation with agencies and scientific research laboratories in many countries around the world. Our wish would be to share our lessons learned, strategies and procedures to make future deployments of scientific station networks easier for other space agencies or laboratories. 9

10 Appendix Figure 12 SVOM onboard and ground equipments References 1 B. Cordier, J. Wei, J.-L. Atteia, S. Basa, A. Claret, F. Daigne, J. Deng, Y. Dong, O. Godet, A. Goldwurm, D. Götz, X.Han, A. Klotz, C. Lachaud, J. Osborne, Y. Qiu, S. Schanne, B. Wu, J. Wang, C Wu, L. Xin, B. Zhang, S.-N. Zhang, The SVOM gammaray burst mission, Swift: 10 Years of Discovery (2014) pp A. Graham, N. C. Kirkman,P. M. Paul, Mobile Radio Network Design in the VHF and UHF Bands: A Practical Approach 3 Man-Made Noise Measurement Programme (AY4119) Final Report, MC/CC0251/REP012/2 Ed. 2 sept