Overview of Japan’s advanced land observing satellite-2 mission Shinichi Suzuki*a, Yuji Osawa a, Yasushi Hatooka a, Yukihiro Kankaku a, Tomohiro Watanabe a
a Japan Aerospace Exploration Agency, 2-1-1, Sengen, Tsukuba-city, Ibaraki 305-8505
ABSTRACT
The post-ALOS program has been defined in the basic plan for Japan’s space policy which was established by the Strategic Headquarters for Space Policy on June 2nd, 2009. It emphasizes the continuity of the ALOS mission: not only disaster monitoring but also land infrastructure management, earth environment and resource monitoring and so on. JAXA had completed the System Definition Review of the ALOS-2 satellite and ground system in February, 2009 and started phase B design of the new L-band SAR, satellite and ground system with the target launch in 2013. This paper introduces the mission and major specification of ALOS-2 satellite and L-band SAR.
Keywords: ALOS, ALOS-2, L-band SAR, satellite
1.INTRODUCTION
The Advanced Land Observing Satellite (ALOS) “Daichi” has been operated for more than three years since January 2006 to accomplish the missions including cartography, regional observation, disaster monitoring, and resource survey. Many users are expecting the ALOS follow-on program. JAXA had studied the conceptual design on the constellation of SAR satellites and optical satellites to satisfy the user requirement for disaster monitoring as the “Disaster Monitoring Satellite System”. Attaching importance to the continuity of ALOS data utilization, the post-ALOS program has been defined to consist of ALOS-2 (SAR satellite) and ALOS-3 (optical satellite) in accordance with the basic plan for Japan’s space policy. This paper introduces the mission and system overview of ALOS-2.
2.ALOS-2 MISSION
ALOS-2 succeeds to the L-band SAR observation of ALOS PALSAR and expands data utilization by enhancing its performance. Table 1 shows the major advantages of ALOS-2 to be compared with ALOS PALSAR.
Table 1. Comparison between ALOS PALSAR and ALOS-2
ALOS
PALSAR ALOS-2 Frequent observation -Revisit time: 46 days-Revisit time: 14 days
-Daytime observation is limited
by sharing with optical
observation
-No conflict
-Incidence angle : 8-60degrees -Right looking -Incidence angle: 8-70 degrees -Right / left looking
Spatial resolution-Strip map: 10m
-ScanSAR: 100m -Strip map: 3m/6m/10m - ScanSAR: 100m
-Spotlight: 1m x 3m
The major objectives of ALOS-2 are comprehensive land monitoring (land infrastructure management, resource management and resource exploration) and disaster monitoring by using L-band SAR. All image examples below (Figure 1 - 7) are using ALOS PALSAR data.
Invited Paper
Sensors, Systems, and Next-Generation Satellites XIII, edited by Roland Meynart,
Steven P. Neeck, Haruhisa Shimoda, Proc. of SPIE Vol. 7474, 74740Q · ? 2009
SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.831340
2.1 Land infrastructure management
1) Domestic observation
?Common usage of SAR data over Japan for such as regular monitoring of volcanoes and crustal movement
?Maintaining archived data for comparison
?More frequent observation than PALSAR in day time and night time with right/left looking capability of ALOS-2
Figure 1. Example of crustal deformation monitoring Iwo Island (November 11, 2006 and December 27, 2006)
2) Sea ice monitoring over Okhotsk 3)
?Near-real-time product delivery for Japan Coast Guard (JCG) in winter time
?More frequent observation (once every two days) than PALSAR (twice a week)
Figure 2. Example of sea ice monitoring
2.2 Resource management
1) Forest and wetland observation
?Detection of long-term and seasonal change of forest, wetland and desert in the world
?Global forest and wetland observation for the Kyoto and Carbon Initiative (K&C) project
Color composite of R= HH polarization, G=HV polarization, B=HH/HV polarization image.
Greenish color shows forested and purple color shows deforested or non-forest area.
Figure 3. Example of forest monitoring (PALSAR 50m Orthorectified Mosaic Image)
2) Deforestation monitoring
?Forest deforestation monitoring of tropical forests and boreal forest
?Frequent observation and rapid data deliver to operational users (IBAMA etc)
Figure 4. Deforestation in the West Rondonia for 11 years
3) Polar region monitoring
?Entire polar region observation by ScanSAR mode and regional area observation by strip map mode
Yellow square areas show high change regions(located at the root of Antarctica peninsula)
Figure 5. Example of polar region monitoring
4) Agricultural monitoring
? Investigation of paddy rice cultivation
? Seasonal observation over Japan with high resolution SAR data
2.3 Resource exploration
1) Resource exploration and development
? Basin mapping and oil slick monitoring
Figure 6. Example of oil slick monitoring
2.4 Disaster monitoring
1) Earthquake, Flood, Landslide, Volcano, Tsunami
? Regular data collection over Japan to be compared with the post-disaster data ? Rapid response to disaster to assess the damage
? Detects major road, bridge and buildings by high resolution SAR data (1-3m)
? JAXA corresponds to oversea disasters under the framework of the Sentinel Asia and the International
Disaster Charter.
Figure 7. Example of Emergency observation for Earthquake in China (May 12, 2008)
2.5 Potential use
Cartography of remote area (e.g. 1:50,000 map over South East Asia) is one of potential uses by using high resolution SAR image.
Analysis by GSI form ALOS raw data (c) JAXA, METI Geographical Survey Institute http://cais.gsi.go.jp/Research/topics/topic080604/in dex.html
3.ALOS-2 SATELLITE AND GROUND SYSTEM
3.1 ALOS-2 satellite
ALOS-2 equips the L-band SAR antenna just under its body and two solar array paddles with both sides (Figure 8).The L-band SAR succeeds to the active phased array antenna (APAA) technology developed for ALOS PALSAR. The APAA of ALOS-2 SAR allows Spotlight mode with electric beam steering in the azimuth direction adding to conventional Stripmap and ScanSAR modes. In order to cover wide area, ALOS-2 SAR has wider incidence angle from 8 to 70 degrees with electric beam steering in the range direction and also right/left looking capability by the satellite body tilting. ALOS-2 satellite is controlled to fly inside a tube of 500m radius along the reference orbit in order to have repeat pass SAR interferometry with high coherence.
Figure 8. ALOS-2 outline view
Table 2 shows the system specifications of ALOS-2. ALOS-2 will be put into the sun-synchronous orbit with local sun time of 12:00 in order to increase the frequency of observation in cooperation with foreign SAR satellites, optical satellites and ALOS-3. The revisit time of 14 days is the optimization between comprehensive cover and quick response.
Table 2. ALOS-2 system specifications
Orbit Sun-synchronous orbit Altitude: 628km Inclination: 97.9deg
Local sun time : 12:00 +/- 15min Revisit time: 14days
Number of cycle: 15-3/14
Orbit control: ≦+/-500m
Life time 5years (target:7 years)
Launch H-IIA launch
vehicle
Size 9.9m(x) x 16.5m(y) x 3.4m(z)
Mass ≦2000kg
Power generation More than 5200W (EOL)
Downlink X-band: 800Mbps(16QAM)
400/200Mbps(QPSK) Ka-band: 278Mbps(QPSK)
L-band SAR antenna X-band data link antenna
Data relay antenna
3.2 ALOS-2 SAR characteristics
The L-band SAR onboard ALOS-2 has enhanced performance than ALOS PALSAR. It has three imaging modes: Spotlight, Stripmap, and ScanSAR mode, and five observation modes. Fig. 9 shows the imaging modes of ALOS-2.
70°
8°
350 km
25 km x 25km
70°
50 km
observation area ：approx.1160km (right or left)
ScanSAR
swath:5scans 350km
Spotlight
swath:25km x 25km
Stripmap swath:50 or 70km
Az
Rg
8°
Figure 9. ALOS-2 imaging mode
The feature of each observation mode is as follows:
1) Spotlight mode
?
The azimuth spatial resolution is 1m for detailed observation of the disaster area. This mode uses electronic beam steering in the azimuth direction to increase the illumination time.
2) Ultra-Fine mode
?
Ultra-fine mode provides 3m resolution with 50km swath with all incidence angles. It is the basic mode to observe disaster in Japan and used to collect the base data for interferometry (InSAR). 3) High-sensitive mode
?
High-sensitive mode is designed for flood monitoring. It has better noise equivalent sigma zero (NESZ) to detect weak back scattering from flooded area. The spatial resolution is lower than Ultra-Fine mode but higher than that of PALSAR.
4) Fine mode
?
Fine mode is a conventional mode succeeding to PALSAR. The spatial resolution and swath are almost equal to those of PALSAR.
5) ScanSAR mode
?
ScanSAR mode is a conventional mode succeeding to PALSAR. The spatial resolution and swath are almost equal to those of PALSAR but ALOS-2 has dual polarization.
Table 3 and Table 4 summarizes the specification and system parameters of ALOS-2 SAR respectively.
Table 3. ALOS-2 SAR specification
Observation mode Spotlight
Stripmap
ScanSAR Ultra-Fine High-Sensitive Fine
Incidence angle8 to 70deg
Band width84MHz84MHz42MHz28MHz14MHz
Ground resolution 3m x 1m
(Rg x Az)
3m6m10m100m
Swath25km50km50km70km350km
Polarization Single Single/Dual Single/Dual/
Full/Compact
Single/Dual/
Full/Compact
Single/Dual
NESZ-24dB-24dB-28dB-26dB-26dB
S/A Rg25dB25dB23dB25dB25dB Az20dB25dB20dB23dB20dB Table 4. ALOS-2 SAR system parameters
Radar carrier frequency1236.5/1257.5/1278.5MHz
Band L-band
Wave length 22.9cm
PRF 1500 to 3000Hz
Range bandwidth14/28/42/84MHz
Polarization Single/Dual/Full/Compact
Look direction Right and Left
Antenna width 3m
Antenna length10m
Incidence angle8 to 70degrees
Range resolution3m/6m/10m/100m
Azimuth resolution1m/3m/6m/10m/100m
3.2.1 SAR antenna
ALOS-2 SAR is comprised of an active phased array antenna (APAA) and an electrical unit (ELU). The APAA enables electronic beam steering in both range and azimuth direction. The SAR antenna is 3m width and 10m length, and is composed of five identical electrical panels (Figure 10). Total 1080 radiation elements are driven by 180 Transmit-Receive-Modules (TRMs), which enable to steer the beam and to form the beam pattern for each imaging mode of Stripmap, Spotlight and ScanSAR. The satellite body is tilted in the roll direction of +/-30 degrees during the observation and returns to the nadir direction afterwards.
Figure 10. ALOS-2 SAR antenna
ALOS-2 adopts the dual receive antenna system (Figure 11). A full apertures or 3 out of 5 panels are used for transmission, while two separate partitions of antenna are used for reception. The pulse repetition frequency (PRF) can be reduced to half to allow wide swath coverage with high resolution.
Figure 11. Dual receive antenna system
3.2.2 Transmit-Receive-Modules (TRM)
The polarization can be selected as either Single (HH/VV/HV/VH), Dual (HH+HV/VV+VH), Full (HH+HV+VV+VH), or Compact polarimetry. The full polarimetry increases the PRF by alternative pulse of H and V, and limiting PRF leads to the narrow swath. ALOS-2 has the compact polarimetry as an experimental mode to reduce PRF by transmitting circular polarization (LHCP or RHCP) or 45 degrees linear polarization and receiving H and V (Figure 12).
Figure 12. Compact polarimetry (Experimental mode)
An output power of 34W is generated at the TRM output port with low loss and high power solid state power amplifier (SSPA) using Gallium Nitride (GaN) High Electron Mobility Transistor (HEMT), and the total output power is 5100W at the antenna output port with full aperture.
Another technique is the chirp modulation in order to decrease the influence of range ambiguity at high incidence angle. ALOS-2 uses up and down chirp with phase modulation (zero or pi using maximal length sequences), while PALSAR has down linear chirp only (Figure 13).
Figure 13. Chirp modulation
The influence of range ambiguity will be about 10dB less than no chirp modulation. Figure 14 shows the simulation images. The simulation area was selected as a distributed target and the data is from Pi-SAR which is an airborne L-band SAR. Image (c) is composite of (a) and (b), where the main signal level is equal to the ambiguity. By applying the chirp modulation, the ambiguity has disappeared as image (d).
Figure 14. Simulation images with/without chirp modulation
3.3 ALOS-2 Ground System
ALOS-2 will use the data relay satellite communication for TT&C and mission data transmission in the same way as ALOS. The mission data will also be acquired via X-band downlink with up to 800Mbps to JAXA Katsuura station and other ground stations. The “Satellite Control and Mission Operation System” will be developed together with the ALOS-2 satellite. The “Information System” will have data processing, archiving, cataloging and user service functions for the ALOS series. Figure 15 shows the overview of ALOS-2 Ground System.
Data Analysis System
(Cal/Val, Algorithm development, Science)
USB
（TLM,RNG ）
Antenna prediction
RNG USB
（CMD,RNG ）
USB （TLM,RNG ）X （Mission ）
USB
（CMD,RNG ）USB （TLM,RNG ）
X （Mission ）
USB
（CMD,RNG ）
Ka （CMD,RNG ）
Ka （TLM,RNG ）
Ka （Mission ）
CMD
CMD
CMD
CMD TLM
Katsuura
JAXA GN
GPS data
Observation
Request
USER
Observation
Request
Product
Observation Request
Product
Trajectory information
Level 0/1data Mission data
High latitude
ground station
Ground station of other organization Trajectory information
X （Mission ）
Satellite Control & Mission Operation System
Antenna prediction
Antenna
prediction
Antenna prediction
Flight Dynamics System
Data relay satellite
Calibration
coefficient ,Algorithm Level 0/1data
Other Space Agency
Information System
(Tasking, Collection, Processing, Exploitation and Dissemination)
EOC (Back up)
X
（Mission ）
JAXA SN (EOC/Tsukuba)
：ALOS-2 system ：Tracking network
Antenna prediction
Trajectory information
Figure 15. ALOS-2 Ground System
4. CONCLUSION
The ALOS-2 mission and system overview was introduced. JAXA is going to launch ALOS-2 in 2013 to continue the
ALOS data utilization.
REFERENCES
1. S. Suzuki, et al., ”The Post-ALOS program”, 27th ISTS, 2009
2. Y. Kankaku, et al., ” The overview of the L-band SAR onboard ALOS-2”, 27th ISTS, 2009