- Email: [email protected]
NTS—A nanosatellite space trial
ARTICLE IN PRESS Acta Astronautica 66 (2010) 1475–1480
Contents lists available at ScienceDirect
Acta Astronautica journal homepage: www.elsevier.com/locate/actaastro
NTS—A nanosatellite space trial Elliott Coleshill , Jeff Cain, Franz Newland, Ian D’Souza COM DEV Ltd., Cambridge, Ontario, Canada
a r t i c l e in fo
abstract
Article history: Received 29 January 2009 Received in revised form 2 November 2009 Accepted 8 November 2009 Available online 5 December 2009
The COM DEV Mission Development Group has recently launched a nanosatellite mission called NTS (Nanosatellite Tracking of Ships). NTS is a space trial, intended to provide proof-of-concept validation for a COM DEV AIS (Automatic Identification System) radio that has been specifically designed to receive ship AIS transmissions from low Earth orbit. The AIS system uses the very high frequency (VHF) band and provides detailed information about each equipped vessel. Not only does this system provide information such as the location of a vessel, but it also provides heading, engine status and other crucial information about the ship. Designed for terrestrial use, the AIS system traditionally has a range of only 100 km and uses a local self-organized time-division multiple access (SO-TDMA) scheme to ensure transmissions from all ships in any selforganized cell do not overlap. Receiving these signals from a space platform presents an opportunity to provide wide area monitoring of shipping activity. Detection of AIS signals from space will provide a new capability to track and monitor large maritime vessels even when there are large distances from traditional shore based detection systems. The NTS program was designed to be a low cost demonstration satellite and provide rapid risk mitigation to assist in the development of a future constellation of spacecraft that could provide operational AIS from space (AIS-S) reception and dissemination. The scope of the NTS program was kept to a minimum with focus on the design, development and demonstration of a new payload. The nanosatellite bus was developed using a combination of pre-existing designs by the University of Toronto Institute of Aerospace Studies/Space Flight Laboratory (UTIAS/SFL). The resulting bus design was a combination of their CanX-2 and Generic Nanosatellite Bus. The NTS spacecraft is able to provide the capability to detect AIS signals from low earth orbit with multiple, short AIS signal collection cycles over its planned mission lifetime. The paper presents an overview of the NTS spacecraft, mission concept and preliminary results obtained from the flight. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Nanosatellite AIS Automated information System NTS
1. Introduction The Automatic Identification System (AIS) [1] is a shipto-ship and ship-to-shore system that is used as an aid for collision avoidance and vessel traffic management. An AIS Corresponding author.
E-mail addresses: [email protected] (E. Coleshill), [email protected] (J. Cain), [email protected] (F. Newland), [email protected] (I. D’Souza). 0094-5765/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2009.11.007
signal is a short message broadcast by a ship at two different VHF frequencies and includes information about the ship, its course, speed, crew and cargo. AIS transmitters are mandated for specific classes of vessels and are being voluntarily added by others (including search and rescue boats and aircraft). Class A is a mandatory service for all ships of 300 gross tonnage and upwards engaged on international voyages, cargo ships of 500 gross tonnage and upwards not engaged on international voyages and passenger ships irrespective of size. Class B is a voluntary
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E. Coleshill et al. / Acta Astronautica 66 (2010) 1475–1480
AIS service for pleasure vessels. Typically, the AIS system has a range of 50–100 km, which limits any long-range ship position knowledge. In addition, AIS transmitters use a SO-TDMA scheme to allow all ships within a selforganizing cell to broadcast their information without overlapping other transmissions from ships within the same cell. In this paper, overlapping messages are often referred to as message collisions. The ability to collect AIS messages from space would provide global marine traffic awareness and provide input to a large number of applications of national interest, such as search and rescue, national security, environmental study and shipping economics. There is, however, one major problem that must first be solved for space-based AIS radios. Due to its large field of view, a spacecraft monitoring AIS signals will detect thousands of ships over multiple self-organized cells producing collisions between messages. These message collisions make it very difficult to extract the messages from individual ships. The problem with detecting and extracting AIS messages from space, where collisions between messages occur, has been studied for several years now [2]. Most solutions to date require long observation times over the area of interest; a low likelihood of message detection being addressed by using a long collection time to improve the overall detection statistics. COM DEV Ltd. has developed and tested, using simulations and aircraft trials [3], an AIS radio intended to address the collision problem inherent in reception of AIS signals from space. The purpose of this radio is to achieve a much higher level of ship detection per spacecraft pass. Following success of the terrestrial trials, COM DEV subsequently worked with UTIAS/SFL to design, develop and launch a satellite called NTS; using a responsive space platform [4] and carrying the COM DEV Ltd. AIS radio.
2. NTS Spacecraft The NTS spacecraft is a platform dedicated to collecting AIS signals from space. It uses a generic nanosatellite bus platform, provided by UTIAS/SFL and a custom built AIS from space (AIS-S) receiver and antenna developed by COM DEV Ltd. The overall goal of the NTS mission is to provide proof-of-concept data as verification that the AIS radio operates as intended and is capable of extracting collided AIS signals. Specifically, the objectives of the mission are:
to collect samples of AIS messages from low Earth orbit,
to measure the RF environment within the 160– 162 MHz range and determine noise levels and,
to provide a platform to qualify an engineering model and basic capability of a COM DEV Ltd. AIS radio in preparation for the operational unit to be used on future missions. Fig. 1 provides a view of the NTS satellite. NTS is a 6.5 kg satellite. The bus design is based on a combination of the
Fig. 1. View of the NTS Nanosatellite showing three of four UHF antennas, AIS payload antenna, S-Band patch antenna and multiple solar cells.
electronics from the pre-existing UTIAS/SFL CanX-2 [5] design and the structure for the Generic Nanosatellite Bus [6] (GNB). The payload and antenna were custom designed to suit the NTS mission. The primary structure of NTS forms a 20 20 20 cm cube. Each face has two strings of solar cells providing continuous power independent of the spacecraft attitude. Communications are relayed through four monopole antennas mounted at the corners of the assembly and an S-band patch on two sides for data download. The payload uses a single monopole antenna tuned to the 160–162 MHz frequency band. The launch of NTS occurred on April 28, 2008, on the Antrix (ISRO—Indian Space Research Organization) PSLVC9 flight as one of the tertiary nanosatellite payloads.
3. Mission Operations NTS is flying at an altitude of 620 km and has a payload antenna footprint of approximately 5000 km. Fig. 2 illustrates the footprint over the equator, the colors/ profile illustrating the different gain contours of the antenna. Primary communications with the NTS spacecraft are performed via the UTIAS/SFL ground station in Toronto, Canada. Due to memory constraints on-board the House Keeping Computer (HKC) the payload is only commanded to operate for a maximum of 90 s at a time. Currently a full 90 s of data takes 3–5 days to download over the S-Band link at UTIAS/SFL. In order to facilitate a faster download of data a secondary ground station is
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execution of all time tag commands in the HKC queue the data are downloaded over several ground passes. 3.3. Decoding and extraction The final task combines all the data together forming a complete set for each AOI recorded during the last sequence of time tag commands. Each AOI data file is demodulated to extract the AIS messages and their contents saved for plotting and analysis. 4. Initial Results
Fig. 2. An STK view of the 5000 km NTS footprint over the equator.
being constructed at the University of Aalborg in Denmark. Operations of the NTS satellite payload are performed using two basic modes, full recording and partial recording. When using the full recording mode the payload is powered on, configured and the data recorder is turned on. Recording continues until the payload memory has been completely filled, at which point the HKC will start requesting the data for transmission to the ground. The partial recording mode provides the ability to perform small bursts of recording over several different areas of the globe. In this mode data from a series of areas of interest (AOI) can be collected during a single orbit. The planning cycle for both modes is exactly the same and is detailed in the next sections.
3.1. Planning and script generation During the planning and script generation task the mission analysts determine the AOI for the next round of recording. If only a single area of interest is desired, a full record mode will nominally be used. If there are multiple AOI required within an orbit or two, multiple partial recordings will be performed. Recording time is dependent on the number of AOI required. A sequence of time tagged commands is then generated to configure the payload and turn on the data recorder as the satellite passes over the AOI.
3.2. Execution and download Once the sequence of commands has been created and verified on the flatsat models of the payload and HKC, the commands are uploaded to the satellite into the time tag queue and await execution over the AOI. After the
Since the launch of NTS, 40 different locations around the globe have been recorded. These locations were selected based on low, medium and high-density ship traffic regions with an overall goal of getting a global view of the ship traffic density and a capability assessment of the AIS-S payload. NTS has provided COM DEV with the means to analyse the AIS spectrum environment and the issues with message collisions. The very first payload operation was over a region where a low density of ships was expected. The AIS signals collected in this region were extremely clean with very few collisions. A sample of the signal environment and phase for this region is shown in Figs. 3a and b, respectively. The characteristic synchronization marker and message trailer can be seen in Fig. 3b, showing a single message that can be extracted trivially. Figs. 3c and d show the signal strength and phase for a region with a greater number of ships (a medium traffic region). The collisions between messages are clearly visible. In particular, Fig. 3d shows two overlapping messages. The first message’s synchronization marker can be seen, but the overlap between messages masks the message trailer and subsequent message’s synchronization marker. As of September 2008, approximately 60 min of ship traffic data have been collected. Within this timeframe, NTS has successfully captured AIS messages from almost 12 000 different targets. These include slightly over 11 600 class A ships, 160 base stations, approximately 50 class B ships and 2 SAR aircraft. It is interesting to note that the lower power class B transmissions were not targeted for collection by the receiver. Their collection is indicative of the receiver’s gain sensitivity and signal to noise ratio. Fig. 4 provides a global view of the data collected from launch until November 2008. The figure is a tiling of several snapshots during that time frame and does not represent a complete coverage of the globe. The image is only intended to show the extent of ship detections using only observations of 90 s at a time, tiled over a large portion of the globe. In addition, certain regions such as the Baltic have a high in-band interfering signal and appear blank on the map. Because the ships at any given location on this map have been observed for only 90 s (compared to an operational AIS-S system which could observe for 10–15 min), the COM DEV receiver can be seen to detect many more ships than would a space qualified commercial receiver. In fact, over 1000 unique MMSIs have been detected in 90 s. In some low density shipping
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Fig. 3. A number of AIS signals at baseband and a single demodulated NRZI AIS signal for a low ship density area (a and b respectively) and a medium ship density area (c and d respectively).
Fig. 4. All AIS messages collected to date from the NTS satellite.
areas, commercial receivers would be able to detect ships as well, however, the number of individual messages will be lower, preventing detailed tracking of ship movement.
The NTS payload provides the capability of comparing the COM DEV approach to AIS message reception to that of a commercial receiver. To accomplish this, the AIS
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Fig. 5. Google Earth views of NTS AIS receiver vs. Commercial AIS receiver. Left: COM DEV AIS-S receiver AIS message reception; Right: Commercial AIS receiver AIS message reception.
Fig. 6. STK views of the NTS footprint showing ships detected overlaid with theoretical antenna footprint and gain patterns for a number of different 90 s data collection cycles. The theoretical receive antenna gain contours are plotted at 30 s intervals, and are arbitrarily color coded orange for peak gain, red for 2 dB below peak, yellow for 7 dB down and green for 12 dB down.
signals are passed through a commercial receiver using multiple settings to determine the maximum number of ships that can be detected by standard Gaussian minimum shift keying (GMSK) demodulation techniques. With the first 50 s of data collection over the Atlantic, the COM DEV receiver detected 590 ships whereas the commercial receiver detected only 104. Fig. 5 provides an illustration of this data where the left image contains the NTS receiver results and the right image contains the commercial AIS receiver results for the same 50 s of data. If a longer time of observation was available, many more ships could be
detected by NTS. The commercial receiver would also detect more ships, and perhaps, depending on the ship traffic density, both might detect all the ships. Performance depends on the ability to detect as many messages as possible, however. In this case, NTS emerges with superior performance. Also, the ability to detect many more messages automatically implies a greater probability of detection. Fig. 6 provides an illustration of the distribution of ships in the field of view of the NTS AIS antenna for three different collection cycles. The ships collected can be seen
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to be distributed throughout the field of view, with no clear correlation between the theoretical receiver gain pattern (based on theoretical satellite pointing) and numbers of ships detected. No such correlation would necessarily be expected, in particular due to actual ship distribution being the primary driver of distribution, but also due to the receiver pattern not accounting for ship transmitter power or transmitter antenna gain, actual NTS pointing during pass collection and other sources of variability. What is more significant however is that ships are detected out to, and even beyond, the limb, demonstrating the wide area detection capability of the NTS satellite. Where Fig. 5 may suggest some gain sensitivity for the standard AIS receiver (this is apparent due to the non-circular detection footprint pattern for the standard receiver; the edges of the field of view, which correspond to the lower gain regions, do not capture any ships), Figs. 5 and 6 both suggest that such sensitivity is less significant for the COM DEV approach. 5. Future Work NTS has satellite to provided a step is to missions as
proven to be an excellent first on-orbit trial test the COM DEV AIS receiver and has vast amount of data for analysis. The next increase the functionality in subsequent follows:
Increase memory. The current NTS only has 15 MB of
storage on-board which limits the usage to 90 s ‘‘snapshots’’ of data collection. Increasing the on-board memory will provide the ability to gain statistics on detection during a complete fly over, rather than just a snapshot of a single spot. Increase downlink speed. The download speed of NTS is 32 kbps and requires data to be downloaded during multiple ground station passes. Increasing the download capabilities will allow for higher utilization and faster data refresh rates for a given region.
A more capable nanosatellite could be launched with the upgrades listed above. Ideally, however, a constellation of such AIS satellites would be launched, to provide regular global updates. With the large payload footprint, a constellation of six such spacecrafts could provide global updates approximately every 90–100 min, for example. Upgrading to a microsatellite-class would also be beneficial, allowing each spacecraft in the constellation to
support a dedicated Payload Data Downlink and up to a 100% duty cycle for the payload, due to the greater available power. 6. Conclusions In conclusion, this paper has presented an overview of the COM DEV NTS mission; including details on the satellite, operations and some of the results obtained. Since its launch in April 2008, the NTS mission has performed several recording cycles around the globe. The data from the COM DEV payload has been compared to the results from commercial receivers. In all cases the COM DEV receiver has successfully detected significantly more ships than the commercial receiver. The NTS nanosatellite mission has also provided further proof that AIS monitoring from space is not only feasible and can provide operationally useful data, but that with careful design, a performance surpassing standard AIS receiver detection can be attained even in the presence of signal collisions.
Acknowledgments The authors would like to acknowledge the dedication and effort of the many people on the COM DEV and UTIAS/ SFL teams who have made the NTS spacecraft and mission a success. References [1] /http://en.wikipedia.org/wiki/Automatic_Identification_SystemS. [2] T. Eriksen, G. Hoye, B. Narheim, B. Meland, Maritime Traffic Monitoring Using a Space-Based AIS Receiver. Proceedings of the 55th International Astronautical Congress, Vancouver, Canada, October 4–8, 2004. [3] I. D’Souza, J.S. Cain, W. Chen, F. Newland, Nanosatellite Tracking of Ships: A Satellite Demonstration of AIS Signal De-Collision. Proceedings of the NATO Military Sensing Symposium SET-130, Orlando, Florida, US, March 2008. [4] J.S. Cain, F. Newland, F. Pranajaya, R. Zee, Rapid Development of Proof-of-Concept Missions. Proceedings of the 6th Responsive Space Conference (AIAA/6th), 2008. [5] K. Sarda, S. Eagleson, C. Caillibot, C. Grant, D.D. Kekez, F. Pranajaya, R. Zee, Canadian Advance Nanosatellite Experiment 2: Scientific and Technological Innovation on a Three Kilogram Satellite, Acta Astronautica 59 (2006) 236–245. [6] S. Eagleson, K. Sarda, S. Mauthe, T. Tuli, R. Zee, Adaptable, MultiMission Design of CanX Nanosatellites. Proceedings of the 20th Annual AIAA/USU Conference on Small Satellites, Logan, Utah, August 2006.
Contents lists available at ScienceDirect
Acta Astronautica journal homepage: www.elsevier.com/locate/actaastro
NTS—A nanosatellite space trial Elliott Coleshill , Jeff Cain, Franz Newland, Ian D’Souza COM DEV Ltd., Cambridge, Ontario, Canada
a r t i c l e in fo
abstract
Article history: Received 29 January 2009 Received in revised form 2 November 2009 Accepted 8 November 2009 Available online 5 December 2009
The COM DEV Mission Development Group has recently launched a nanosatellite mission called NTS (Nanosatellite Tracking of Ships). NTS is a space trial, intended to provide proof-of-concept validation for a COM DEV AIS (Automatic Identification System) radio that has been specifically designed to receive ship AIS transmissions from low Earth orbit. The AIS system uses the very high frequency (VHF) band and provides detailed information about each equipped vessel. Not only does this system provide information such as the location of a vessel, but it also provides heading, engine status and other crucial information about the ship. Designed for terrestrial use, the AIS system traditionally has a range of only 100 km and uses a local self-organized time-division multiple access (SO-TDMA) scheme to ensure transmissions from all ships in any selforganized cell do not overlap. Receiving these signals from a space platform presents an opportunity to provide wide area monitoring of shipping activity. Detection of AIS signals from space will provide a new capability to track and monitor large maritime vessels even when there are large distances from traditional shore based detection systems. The NTS program was designed to be a low cost demonstration satellite and provide rapid risk mitigation to assist in the development of a future constellation of spacecraft that could provide operational AIS from space (AIS-S) reception and dissemination. The scope of the NTS program was kept to a minimum with focus on the design, development and demonstration of a new payload. The nanosatellite bus was developed using a combination of pre-existing designs by the University of Toronto Institute of Aerospace Studies/Space Flight Laboratory (UTIAS/SFL). The resulting bus design was a combination of their CanX-2 and Generic Nanosatellite Bus. The NTS spacecraft is able to provide the capability to detect AIS signals from low earth orbit with multiple, short AIS signal collection cycles over its planned mission lifetime. The paper presents an overview of the NTS spacecraft, mission concept and preliminary results obtained from the flight. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Nanosatellite AIS Automated information System NTS
1. Introduction The Automatic Identification System (AIS) [1] is a shipto-ship and ship-to-shore system that is used as an aid for collision avoidance and vessel traffic management. An AIS Corresponding author.
E-mail addresses: [email protected] (E. Coleshill), [email protected] (J. Cain), [email protected] (F. Newland), [email protected] (I. D’Souza). 0094-5765/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2009.11.007
signal is a short message broadcast by a ship at two different VHF frequencies and includes information about the ship, its course, speed, crew and cargo. AIS transmitters are mandated for specific classes of vessels and are being voluntarily added by others (including search and rescue boats and aircraft). Class A is a mandatory service for all ships of 300 gross tonnage and upwards engaged on international voyages, cargo ships of 500 gross tonnage and upwards not engaged on international voyages and passenger ships irrespective of size. Class B is a voluntary
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AIS service for pleasure vessels. Typically, the AIS system has a range of 50–100 km, which limits any long-range ship position knowledge. In addition, AIS transmitters use a SO-TDMA scheme to allow all ships within a selforganizing cell to broadcast their information without overlapping other transmissions from ships within the same cell. In this paper, overlapping messages are often referred to as message collisions. The ability to collect AIS messages from space would provide global marine traffic awareness and provide input to a large number of applications of national interest, such as search and rescue, national security, environmental study and shipping economics. There is, however, one major problem that must first be solved for space-based AIS radios. Due to its large field of view, a spacecraft monitoring AIS signals will detect thousands of ships over multiple self-organized cells producing collisions between messages. These message collisions make it very difficult to extract the messages from individual ships. The problem with detecting and extracting AIS messages from space, where collisions between messages occur, has been studied for several years now [2]. Most solutions to date require long observation times over the area of interest; a low likelihood of message detection being addressed by using a long collection time to improve the overall detection statistics. COM DEV Ltd. has developed and tested, using simulations and aircraft trials [3], an AIS radio intended to address the collision problem inherent in reception of AIS signals from space. The purpose of this radio is to achieve a much higher level of ship detection per spacecraft pass. Following success of the terrestrial trials, COM DEV subsequently worked with UTIAS/SFL to design, develop and launch a satellite called NTS; using a responsive space platform [4] and carrying the COM DEV Ltd. AIS radio.
2. NTS Spacecraft The NTS spacecraft is a platform dedicated to collecting AIS signals from space. It uses a generic nanosatellite bus platform, provided by UTIAS/SFL and a custom built AIS from space (AIS-S) receiver and antenna developed by COM DEV Ltd. The overall goal of the NTS mission is to provide proof-of-concept data as verification that the AIS radio operates as intended and is capable of extracting collided AIS signals. Specifically, the objectives of the mission are:
to collect samples of AIS messages from low Earth orbit,
to measure the RF environment within the 160– 162 MHz range and determine noise levels and,
to provide a platform to qualify an engineering model and basic capability of a COM DEV Ltd. AIS radio in preparation for the operational unit to be used on future missions. Fig. 1 provides a view of the NTS satellite. NTS is a 6.5 kg satellite. The bus design is based on a combination of the
Fig. 1. View of the NTS Nanosatellite showing three of four UHF antennas, AIS payload antenna, S-Band patch antenna and multiple solar cells.
electronics from the pre-existing UTIAS/SFL CanX-2 [5] design and the structure for the Generic Nanosatellite Bus [6] (GNB). The payload and antenna were custom designed to suit the NTS mission. The primary structure of NTS forms a 20 20 20 cm cube. Each face has two strings of solar cells providing continuous power independent of the spacecraft attitude. Communications are relayed through four monopole antennas mounted at the corners of the assembly and an S-band patch on two sides for data download. The payload uses a single monopole antenna tuned to the 160–162 MHz frequency band. The launch of NTS occurred on April 28, 2008, on the Antrix (ISRO—Indian Space Research Organization) PSLVC9 flight as one of the tertiary nanosatellite payloads.
3. Mission Operations NTS is flying at an altitude of 620 km and has a payload antenna footprint of approximately 5000 km. Fig. 2 illustrates the footprint over the equator, the colors/ profile illustrating the different gain contours of the antenna. Primary communications with the NTS spacecraft are performed via the UTIAS/SFL ground station in Toronto, Canada. Due to memory constraints on-board the House Keeping Computer (HKC) the payload is only commanded to operate for a maximum of 90 s at a time. Currently a full 90 s of data takes 3–5 days to download over the S-Band link at UTIAS/SFL. In order to facilitate a faster download of data a secondary ground station is
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execution of all time tag commands in the HKC queue the data are downloaded over several ground passes. 3.3. Decoding and extraction The final task combines all the data together forming a complete set for each AOI recorded during the last sequence of time tag commands. Each AOI data file is demodulated to extract the AIS messages and their contents saved for plotting and analysis. 4. Initial Results
Fig. 2. An STK view of the 5000 km NTS footprint over the equator.
being constructed at the University of Aalborg in Denmark. Operations of the NTS satellite payload are performed using two basic modes, full recording and partial recording. When using the full recording mode the payload is powered on, configured and the data recorder is turned on. Recording continues until the payload memory has been completely filled, at which point the HKC will start requesting the data for transmission to the ground. The partial recording mode provides the ability to perform small bursts of recording over several different areas of the globe. In this mode data from a series of areas of interest (AOI) can be collected during a single orbit. The planning cycle for both modes is exactly the same and is detailed in the next sections.
3.1. Planning and script generation During the planning and script generation task the mission analysts determine the AOI for the next round of recording. If only a single area of interest is desired, a full record mode will nominally be used. If there are multiple AOI required within an orbit or two, multiple partial recordings will be performed. Recording time is dependent on the number of AOI required. A sequence of time tagged commands is then generated to configure the payload and turn on the data recorder as the satellite passes over the AOI.
3.2. Execution and download Once the sequence of commands has been created and verified on the flatsat models of the payload and HKC, the commands are uploaded to the satellite into the time tag queue and await execution over the AOI. After the
Since the launch of NTS, 40 different locations around the globe have been recorded. These locations were selected based on low, medium and high-density ship traffic regions with an overall goal of getting a global view of the ship traffic density and a capability assessment of the AIS-S payload. NTS has provided COM DEV with the means to analyse the AIS spectrum environment and the issues with message collisions. The very first payload operation was over a region where a low density of ships was expected. The AIS signals collected in this region were extremely clean with very few collisions. A sample of the signal environment and phase for this region is shown in Figs. 3a and b, respectively. The characteristic synchronization marker and message trailer can be seen in Fig. 3b, showing a single message that can be extracted trivially. Figs. 3c and d show the signal strength and phase for a region with a greater number of ships (a medium traffic region). The collisions between messages are clearly visible. In particular, Fig. 3d shows two overlapping messages. The first message’s synchronization marker can be seen, but the overlap between messages masks the message trailer and subsequent message’s synchronization marker. As of September 2008, approximately 60 min of ship traffic data have been collected. Within this timeframe, NTS has successfully captured AIS messages from almost 12 000 different targets. These include slightly over 11 600 class A ships, 160 base stations, approximately 50 class B ships and 2 SAR aircraft. It is interesting to note that the lower power class B transmissions were not targeted for collection by the receiver. Their collection is indicative of the receiver’s gain sensitivity and signal to noise ratio. Fig. 4 provides a global view of the data collected from launch until November 2008. The figure is a tiling of several snapshots during that time frame and does not represent a complete coverage of the globe. The image is only intended to show the extent of ship detections using only observations of 90 s at a time, tiled over a large portion of the globe. In addition, certain regions such as the Baltic have a high in-band interfering signal and appear blank on the map. Because the ships at any given location on this map have been observed for only 90 s (compared to an operational AIS-S system which could observe for 10–15 min), the COM DEV receiver can be seen to detect many more ships than would a space qualified commercial receiver. In fact, over 1000 unique MMSIs have been detected in 90 s. In some low density shipping
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Fig. 3. A number of AIS signals at baseband and a single demodulated NRZI AIS signal for a low ship density area (a and b respectively) and a medium ship density area (c and d respectively).
Fig. 4. All AIS messages collected to date from the NTS satellite.
areas, commercial receivers would be able to detect ships as well, however, the number of individual messages will be lower, preventing detailed tracking of ship movement.
The NTS payload provides the capability of comparing the COM DEV approach to AIS message reception to that of a commercial receiver. To accomplish this, the AIS
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Fig. 5. Google Earth views of NTS AIS receiver vs. Commercial AIS receiver. Left: COM DEV AIS-S receiver AIS message reception; Right: Commercial AIS receiver AIS message reception.
Fig. 6. STK views of the NTS footprint showing ships detected overlaid with theoretical antenna footprint and gain patterns for a number of different 90 s data collection cycles. The theoretical receive antenna gain contours are plotted at 30 s intervals, and are arbitrarily color coded orange for peak gain, red for 2 dB below peak, yellow for 7 dB down and green for 12 dB down.
signals are passed through a commercial receiver using multiple settings to determine the maximum number of ships that can be detected by standard Gaussian minimum shift keying (GMSK) demodulation techniques. With the first 50 s of data collection over the Atlantic, the COM DEV receiver detected 590 ships whereas the commercial receiver detected only 104. Fig. 5 provides an illustration of this data where the left image contains the NTS receiver results and the right image contains the commercial AIS receiver results for the same 50 s of data. If a longer time of observation was available, many more ships could be
detected by NTS. The commercial receiver would also detect more ships, and perhaps, depending on the ship traffic density, both might detect all the ships. Performance depends on the ability to detect as many messages as possible, however. In this case, NTS emerges with superior performance. Also, the ability to detect many more messages automatically implies a greater probability of detection. Fig. 6 provides an illustration of the distribution of ships in the field of view of the NTS AIS antenna for three different collection cycles. The ships collected can be seen
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to be distributed throughout the field of view, with no clear correlation between the theoretical receiver gain pattern (based on theoretical satellite pointing) and numbers of ships detected. No such correlation would necessarily be expected, in particular due to actual ship distribution being the primary driver of distribution, but also due to the receiver pattern not accounting for ship transmitter power or transmitter antenna gain, actual NTS pointing during pass collection and other sources of variability. What is more significant however is that ships are detected out to, and even beyond, the limb, demonstrating the wide area detection capability of the NTS satellite. Where Fig. 5 may suggest some gain sensitivity for the standard AIS receiver (this is apparent due to the non-circular detection footprint pattern for the standard receiver; the edges of the field of view, which correspond to the lower gain regions, do not capture any ships), Figs. 5 and 6 both suggest that such sensitivity is less significant for the COM DEV approach. 5. Future Work NTS has satellite to provided a step is to missions as
proven to be an excellent first on-orbit trial test the COM DEV AIS receiver and has vast amount of data for analysis. The next increase the functionality in subsequent follows:
Increase memory. The current NTS only has 15 MB of
storage on-board which limits the usage to 90 s ‘‘snapshots’’ of data collection. Increasing the on-board memory will provide the ability to gain statistics on detection during a complete fly over, rather than just a snapshot of a single spot. Increase downlink speed. The download speed of NTS is 32 kbps and requires data to be downloaded during multiple ground station passes. Increasing the download capabilities will allow for higher utilization and faster data refresh rates for a given region.
A more capable nanosatellite could be launched with the upgrades listed above. Ideally, however, a constellation of such AIS satellites would be launched, to provide regular global updates. With the large payload footprint, a constellation of six such spacecrafts could provide global updates approximately every 90–100 min, for example. Upgrading to a microsatellite-class would also be beneficial, allowing each spacecraft in the constellation to
support a dedicated Payload Data Downlink and up to a 100% duty cycle for the payload, due to the greater available power. 6. Conclusions In conclusion, this paper has presented an overview of the COM DEV NTS mission; including details on the satellite, operations and some of the results obtained. Since its launch in April 2008, the NTS mission has performed several recording cycles around the globe. The data from the COM DEV payload has been compared to the results from commercial receivers. In all cases the COM DEV receiver has successfully detected significantly more ships than the commercial receiver. The NTS nanosatellite mission has also provided further proof that AIS monitoring from space is not only feasible and can provide operationally useful data, but that with careful design, a performance surpassing standard AIS receiver detection can be attained even in the presence of signal collisions.
Acknowledgments The authors would like to acknowledge the dedication and effort of the many people on the COM DEV and UTIAS/ SFL teams who have made the NTS spacecraft and mission a success. References [1] /http://en.wikipedia.org/wiki/Automatic_Identification_SystemS. [2] T. Eriksen, G. Hoye, B. Narheim, B. Meland, Maritime Traffic Monitoring Using a Space-Based AIS Receiver. Proceedings of the 55th International Astronautical Congress, Vancouver, Canada, October 4–8, 2004. [3] I. D’Souza, J.S. Cain, W. Chen, F. Newland, Nanosatellite Tracking of Ships: A Satellite Demonstration of AIS Signal De-Collision. Proceedings of the NATO Military Sensing Symposium SET-130, Orlando, Florida, US, March 2008. [4] J.S. Cain, F. Newland, F. Pranajaya, R. Zee, Rapid Development of Proof-of-Concept Missions. Proceedings of the 6th Responsive Space Conference (AIAA/6th), 2008. [5] K. Sarda, S. Eagleson, C. Caillibot, C. Grant, D.D. Kekez, F. Pranajaya, R. Zee, Canadian Advance Nanosatellite Experiment 2: Scientific and Technological Innovation on a Three Kilogram Satellite, Acta Astronautica 59 (2006) 236–245. [6] S. Eagleson, K. Sarda, S. Mauthe, T. Tuli, R. Zee, Adaptable, MultiMission Design of CanX Nanosatellites. Proceedings of the 20th Annual AIAA/USU Conference on Small Satellites, Logan, Utah, August 2006.