VesselSat AIS constellation
VesselSat refers to a constellation of two microsatellites, a ground station, all related software, and the operations framework; it was built from summer 2010 through the end of 2011 by LuxSpace Sarl, of Betzdorf,Luxembourg, and leased to ORBCOMM (ORBCOMM is the exclusive licensee for the AIS data collected by VesselSat-1 and VesselSat-2). The objective of VesselSat-1 and -2 is to collect spaceborne AIS (Automatic Identification System) message data in support of maritime tracking services. 1) 2)
AIS (Automatic Identification System) is a technology embarked on all vessels ≥ 300 GRT, which is used as anti-collision system. Many coastal countries have established shore based receiving stations to monitor the vessel traffic. However, the reach of these stations is limited to more or less 100 nautical miles. Satellite AIS is a new emerging technology that provides a cost effective solution for monitoring vessel traffic and the individual positions of ships around the world. Such vessel monitoring information is of particular interest of ship owners and port authorities but raises also expectations to be useful for supporting maritime policy and the creation of maritime awareness information. Satellite AIS is considered as an add-on to the coastal stations that extends the vessel monitoring capability for safety and security aspects to a global scale for both the institutional and private sector.
Background: At the start of the project, LuxSpace had available the first generation AIS payload (Pathfinder-2) that had been built and was flying on Rubin-9.1 (launch on Sept. 23, 2009), and a first generation RFPA (Radio Frequency Power Amplifier). Furthermore, the satellite was based on the existing microsatellite platform that was developed by a current LuxSpace engineer in a previous job. This consisted of a completely thought-out concept for a small tumbling satellite, including various system level aspects, and two already produced complete satellite structures. These structures were used for VesselSat-1 and -2 and the concept of the platform was applied.
Figure 1: Illustration of the VesselSat microsatellite (image credit: LuxSpace)
LuxSpace is the owner and operator of the VesselSat spacecraft and acted as the systems integrator, but worked closely with the subcontractors in the design of the subsystems. LuxSpace in principle did not build the hardware itself. For the VesselSat project, the only subsystems that were built at LuxSpace were the magnetorquers.
The overall concept of the satellite is based on a simple, robust structure that fulfils several roles: structural strength, thermal contact and inertia, electrical grounding and radiation shielding. The satellite is built up around an internal structure of three vertical plates connected to a honeycomb core sandwich base plate via angle brackets. Outer panels are also sandwich plates that are mounted on the outside of the structure and are not load-carrying. Electronics is carried in standard size modules that are suspended between the plates on rails, and additional units are screwed to the outside of the two structural panels on the outside (underneath the outer sandwich panels).
The structure is heavy, but has several big advantages (Ref. 1):
• It is cheap, simple to build and robust
• The screwed interconnections give a little flexibility, leading to a complete lack of resonances in the structure
• Good thermal contact and the mass of the structure provide a well balanced internal
• Good electrical contact provides an excellent grounding scheme that is beneficial for low EMI (Electromagnetic Interference).
• The thick Aluminum panels and electronics boxes provide radiation shielding for the electronic components inside.
The satellite carries a single on-board computer (OBC), and double TM/TC chains and a double payload receiver. The failure of one of these double chains induces no loss in performance.
EPS (Electric Power Subsystem): Power is provided by 5 solar panels on side and top faces, connected to a PCU (Power Control Unit), which takes care of proper charging of the two strings of batteries. The battery voltage is provided to the subsystems on the three parallel power buses; each subsystem takes care of down-conversion from the bus voltage to its own required voltage(s).
Attitude: No controlled attitude – the spacecraft spins slowly.
• Attitude knowledge from sensors
• Limited control of attitude possible with magnetorquers.
The VesselSat platform is relying on two 3-axes sun sensors (each one consists of 3 photodiodes in a pyramidal configuration), one 3-axes magnetometer (HMC1053, from Honeywell), and two 2-axes gyroscopes (LPY510AL, from STMicro). The attitude determination of the satellite is made on ground, based on housekeeping telemetry (sensor value sampling, nominally at 1Hz). The project is able to ask for very long sampling durations (>3h), that are used to:
- calibrate the sensors (the focus is on magnetometer data)
- establish the accuracy of the on-ground attitude determination algorithm.
RF communications: Two telecommand receivers with integrated GPS receivers each cover one half hemisphere with one polarization and one with the opposite polarization. Together they thus provide spherical coverage independent of satellite attitude. The two GPS receivers are linked both to two GPS antennas on opposite side faces through a coupler designed by LuxSpace.
Two telemetry modulators are available, each with their own RFPA. One of the two is being selected for nominal use, the other is a cold backup. The telemetry modulators share the same UHF antennas as the telecommand receivers; they are connected through a failsafe antenna coupler specifically designed by LuxSpace, that provides the correct phasing between the four antennas and performs switching from transmit to receive mode, so that the receivers are properly isolated from the transmitters in all cases.
The telecommand receivers are based on the same architecture as the AIS receiver, though operating at a different frequency. The telecommand messages are formatted as AIS messages. This allows for using the AIS receivers as backup channel for telecommand, of course requiring a VHF uplink instead of a UHF uplink. This functionality was used operationally for VesselSat-2 while a dedicated ground station was not yet ready, and LuxSpace’s Redu station was used.
UHF up-/downlink – redundant transmitters and receivers:
• Uplink 9.2 kbit/s, downlink from 64 up to 512 kbit/s, various modulations, error correction, encryption selectable
• Insensitive to satellite attitude: omnidirectional antenna coverage.
As can be seen in the functional architecture, some subsystems are redundant. However, failure of almost any one of the redundant systems will lead to some degradation of performance.
The OBC and software was designed to be robust and capable to recover from all problems it might encounter. It includes latch-up detection and watchdogs to guard against SEU problems – ensuring a rapid reboot of the OBC to minimize downtime to a few seconds, and so minimizing the loss of data acquired by the AIS receivers. If communication is lost for any reasons short of a catastrophic failure in the satellite, it can remain unattended for extended periods (weeks) without deteriorating. If no contact is made for 24 hours, the satellite goes into security mode, listening for the ground station and sending a short signal every minute. If the power level in the battery drops to a low level, the satellite also goes into low power mode, shutting down subsystems to conserve power, and waits for the ground station to contact it.
Figure 3: VesselSat-1 on the launcher with the primary payload Megha-Tropiques (image credit: LuxSpace)
Launch of VesselSat-1: The microsatellite (29 kg) was launched as a secondary payload on the ISRO flight PSLV-C18 from SDSC (Satish Dhawan Space Center) on the east coast of India on Oct. 20, 2011. The primary payload on this mission was the Megha-Tropiques spacecraft, a cooperative project of ISRO and CNES (France). 3) 4)
Orbit of VesselSat-1: Near-circular low inclination orbit, altitude of ~ 865 km, inclination = 20º, period ~ 102 minutes (~14 rev./day). The advantage of the low inclination orbit of VesselSat-1 is that it has high repeatability in ground coverage and gives many contacts per day with a near-equatorial ground station. Two stations about 180° apart in longitude together allow for contact every orbit.
Secondary payloads on the VesselSat-1 flight were:
• SRMSat, a nanosatellite (10.9 kg) of SRM (Sri Ramaswamy Memorial) University, Chennai, India.
• Jugnu (the Hindu word for firefly), is a nanosatellite (3 kg) of the Indian Institute of Technology (IIT), Kanpur.
• VesselSat-1, a microsatellite (29 kg) of LuxSpace, Luxembourg (a company of OHB Technology AG). VesselSat-1 carries an AIS (Automatic Identification System) payload for the detection of ships in the ground segment. Orbcomm is the exclusive licensee for the AIS data collected by VesselSat-1.
Launch of VesselSat-2: The microsatellite (29 kg) was launched on January 9, 2012 as a secondary payload on the CZ-4B (Long March 4B) vehicle from the TSLC (Taiyuan Satellite Launch Center) in China. The primary payload on this flight was ZY-3 (Zi Yuan-3), a remote sensing spacecraft of MLR (Ministry of Land and Resources), China. The launch provider on this flight was CGWIC (China Great Wall Industry Corporation). 5) 6) 7)
Figure 4: Photo of VesselSat-2 on top of the LM-4B (image credit: CGWIC)
Orbit: Sun-synchronous orbit, altitude = 506 km, inclination = 97.5º.
Figure 5: Ground tracks of VesselSat-1 in low-inclination orbit and VesselSat-2 in SSO (image credit: Luxspace, Ref. 9)
Mission status of VesselSat-1 and VesselSat-2:
• In January 2014, both VesselSats are operating nominally, their performance is very stable. On January 9, 2014, VesselSat-2 was 2 years on orbit, while VesselSat-1 is in its 3rd year of operations (Ref. 8).
• In April 2013, both VesselSats are operating nominally, the performance is stable. 8)
The benchmark for operational performance is the number of vessels that are detected per day. The total fleet of AIS equipped vessels is on the order of 50,000. These are distributed very unevenly. Large concentrations are around Europe, along the Chinese and Japanese coast and the Gulf of Mexico and US coastlines. These are the regions where detection is most difficult, but in part this is alleviated by well-established coastal AIS receiving stations that cover a large part of the most congested areas. The major contribution of space-based AIS is over open oceans where ground based AIS reception is not available. 9)
VesselSat-1 captures around 23,000 vessels per day, while VesselSat-2 sees more than 30,000 per day. The difference is due to the fact that VesselSat-2 is in polar orbit, and sees the entire globe, while VesselSat-1 is in low-inclination orbit (20°), which limits the accessible region to around 48° North and South.
• So far (mid-2012), both satellites have performed above expectation. The backup telecommand functionality through the payload receivers was used successfully, and a software update was successfully implemented. The only failure seen thus far was the MMC memory cards on VesselSat-1, which was not able to withstand the radiation environment; a software workaround for this was implemented, restoring the satellite to full functionality within a few weeks (Ref. 1).
• VesselSat-2 was launched on January 9, 2012 from China into an SSO orbit, and was operated initially through the LuxSpace ground station at Redu, using the backup uplink through the VHF channel and the payload AIS receivers. First contact was again made by technicians on-site at the ground station on the first days, and then the checkout and activation was performed through remote contact to Redu from Luxembourg. The satellite was declared operational after 1 week (Ref. 9).
Figure 6: First snapshot of AIS data from Vesselsat-2, three days after launch (image credit: LuxSpace, Ref. 2)
• After the IOT (In-Orbit-Test) and Check-Out-Phase in Nov. 2011, VesselSat-1 was integrated into the existing OG2 (ORBCOMM Second Generation) network system. The OG2 consists of 18 satellites carrying AIS payloads.
• For the IOT phase, two LuxSpace staff were on-site at the ground station to perform the initial contacts and check-outs from there, to remove possible problems of a remote link to the ground station. The satellite signal was acquired on the first pass, and within a few passes, the satellite state was determined, and the IOT checks were started. The payload antennas were deployed on the 4th pass and the payload was activated. The satellite started returning all payload data from the 3rd day after launch and was declared fully operational after 2 weeks.
• Prior to launch, based on experience from Pathfinder-2 and other ISS missions, the project estimated that the satellites could capture up to 500,000 AIS messages per day (24 hours). In flight, VesselSat-1 actually collects about 900,000 messages per day, and VesselSat-2 up to 1,200,000. The higher number for VesselSat-2 is due to the fact that it flies at much lower altitude, having a link budget that is 4.8 dB better than that of VesselSat-1.
The contractual target was to detect 16000 unique vessels per satellite per day (24 hour period). In fact, VesselSat-1 averages about 22-24000, and VesselSat-2 over 30000.
Figures 7 and 8 show the performance since the launch of VesselSat-1 and of VesselSat-2, respectively. Two gaps are clearly seen in the VesselSat-1 operations. The first corresponds to a ground station problem, the second one to an on-board memory problem.
VesselSat-1 encountered an on-orbit failure of the MMC (Multi Media Card) memory after about 4 months on orbit. It was discovered because this is due to the write/erase circuitry of the card, which was known but not published. The memory card is used for storage of AIS data as well as housekeeping data. A workaround to this situation was devised where the RAM memory is used to emulate a virtual MMC card. Writing a software patch for this took about two weeks and it was thoroughly tested on the flat-sat on ground. It was uploaded to the satellite and activated, and the operation of the satellite was successfully restored.
This shows the robustness of the overall design allowing the project to recover from this failure almost completely — the more limited amount of memory available limits the amount of housekeeping that can be collected between downloads. The AIS messages were already being filtered beforehand (keeping only one per ship per several minutes) which already reduced the amount of payload data to be stored, so that in nominal operation all collected data can be stored between downloads. The MMC card will be replaced by another solution that will avoid this specific weakness. - As a last remark on this point, Pathfinder-2, in which the same methods for screening components were used, is still operational more than 2½ years after launch, providing validation for LuxSpace selection, screening and design criteria.
Figure 7: Overview of vessels detected per day by VesselSat-1 (image credit: LuxSpace)
Sensor complement: (AIS receivers)
Vesselsat-1 and -2 carry a double AIS VHF receiver. The two AIS receivers are fully independent. The receivers are connected to an antenna array, which is phased as two long dipoles in this configuration (Figure 2). On Vesselsat, an orthogonal dipole antenna is used that gives a reasonable directivity at low complexity.
• Each receives all four AIS channels (2 current channels, 2 future 'space' channels)
• Messages are demodulated, time tagged and stored
• Double messages (received by both receivers) can be filtered out
• Messages can be filtered to keep only one per unique ship per several minutes (period can be set)
• Payload memory sufficient for more than 24 hours of data – with circular buffer, so will overwrite old data when full.
The AIS receivers can be used as backup telecommand receivers. The architecture of the AIS and TC receivers is very similar, except the front-end. The commands are sent in a modified AIS message format, which can be decoded by either receiver and are passed on to the OBC. This will then differentiate between telecommand and actual AIS messages. This functionality was used on VesselSat-2 with LuxSpace’s Redu ground station which only has a VHF uplink capability.
Figure 8: Overview of vessels detected per day by VesselSat-2 (image credit: LuxSpace)
In order to operate both satellites in their different orbits, a ground segment was designed consisting of multiple ground stations with a central processing unit in the Orbcomm server center to link them together. The design driver was the required high level of automation in the ground segment enabling automated satellite operations under nominal conditions without operator interference. The ground segment software consists of three main modules (Ref. 9).
1) The GCA (Ground station Control Application) runs locally in the ground stations and automatically configures and controls the ground station equipment, handles the ground station telemetry acquisition and performs satellite tracking. In addition, the GCA executes the satellite operations, including the payload data downlink, according to the VesselSat satellite operations protocol.
2) The RFA (Remote Frontend Application) runs in the Orbcomm server center and synchronizes all ground stations in the network, exchanging the downlink status and the current state of the satellites. In addition, the RFA processes and stores the satellite telemetry received from the ground stations.
3) The ROI (Remote Operator Interface) enables the operators to control and monitor multiple satellites and ground stations from one interface. It connects to the different ground stations in the network via the RFA displaying the current status of ground stations and satellites and allowing the operators to directly send or queue (timed) telecommands to a satellite.
There are currently three active ground stations in the network, each handling up to 16 satellite passes per day. After the downtime of the first ground station due to weather damage in November 2011, the ground segment has been working continuously and smoothly.
Luxspace operates its own fully automatic ground station which was developed and built for Vesselsat:
• Quadruple Yagi antenna
• Fully automated operations as nominal condition
- Satellite is automatically contacted by ground station when in range
- Satellite state data, housekeeping data is automatically downloaded
- AIS data is automatically downloaded
- All data is automatically forwarded to data center
• Full manual operations is possible
• Nominal manual operations:
- Programming specific housekeeping acquisition
- Programming attitude control maneuvers.
Figure 9: Photo of the ground station (image credit: LuxSpace, Ref. 2)
1) J. Buursink, G. Ruy, K. Schwarzenbarth, B. van Schie, J.-B.Frappé, Ph. Ries, H. Moser, “VesselSat: Building two Microsatellites in one Year,” Proceedings of the 4S (Small Satellites Systems and Services) Symposium, Portoroz, Slovenia, June 4-8, 2012
4) “ORBCOMM Announces Launch of AIS-Enabled Satellite,” Space Daily, Oct. 14, 2011, URL: http://www.spacedaily.com/reports/ORBCOMM_Announces_Launch_of_AIS_Enabled_Satellite_999.html
5) “China makes first orbital Launch of 2012,” Spaceflight 101, Jan. 9, 2012, URL: http://www.spaceflight101.com/ziyuan-3-launch.html
6) “ORBCOMM Announces Successful Launch of VesselSat2,” Space Ref, January 10, 2012, URL: http://www.spaceref.com/news/viewpr.html?pid=35698
7) Liu Bo, Ling Fei, “Long March,Easy And Reliable Access To Space For Small Satellites,” Proceedings of the 64th International Astronautical Congress (IAC 2013), Beijing, China, Sept. 23-27, 2013, paper: IAC-13-B4.5.1
8) Information provided by Jeroen Buursink of LuxSpace Sàrl, Betzdorf, Luxembourg.
9) J. Buursink, Gh. Ruy, B.van Schie, J-B.Frappé, K. Schwarzenbarth, P. Ries, H. Moser, “First year on orbit of VesselSat-1 and -2,” Proceedings of the 9th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 8-12, 2013, paper: IAA-B9-0405P
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.