A brief overview of the Advanced Land Observing Satellite (ALOS) and its potential for marine applications

Ake Rosenqvist(1), Daisuke Ichitsubo(2), Yuji Osawa(2) Akihiro Matsumoto(2), Norimasa Ito(2) and Takashi Hamazaki(3),

(1)Earth Observation Research & Applications Center, JAXA, Harumi 1-8-10-X23, Chuo-ku, Tokyo 104-6023, JAPAN Email: ake.rosenqvist@jaxa.jp (2)ALOS Project Team, Japan Aerospace Exploration Agency, 2-1-1 Sengen, Tsukuba-city, Ibaraki 305-8505, JAPAN Email: [email protected], [email protected], [email protected], [email protected] (3)GOSAT Project Team, Japan Aerospace Exploration Agency, 2-1-1 Sengen, Tsukuba-city, Ibaraki 305-8505, JAPAN Email: [email protected]

ABSTRACT

The Advanced Land Observing Satellite (ALOS) is scheduled for launch by the Japan Aerospace Exploration Agency (JAXA) in JFY 2004. ALOS will carry three remote sensing instruments: an L-band polarimetric Synthetic Aperture Radar (PALSAR), an along-track 2.5 metre resolution stereo mapper (PRISM) and a 10-metre multi-spectral scanner (AVNIR-2). The successor of the JERS-1 satellite (1992-1998), ALOS will not only provide enhanced sensor performance, but also novel technologies for high accuracy positioning and attitude determination. This paper, its technical part based largely on [1] and Fig. 1. ALOS in-orbit configuration [2], provides a summary of ALOS characteristics, and brief discussion on its potential for marine applications Table 1. ALOS characteristics.

1. ALOS SATELLITE OVERVIEW Item Characteristics Orbit Sun synchronous, Sub recurrent With a mass of about 4,000 kg, ALOS is the largest Altitude 691.65 km satellite built in Japan (Fig.1), planned for launch by an Recurrent period 46 days, sub-cycle: 2 days H-IIA rocket from Tanegashima Space Center in Inclination 98.16 degree southern Japan in the fiscal year (JFY) of 2004. It will Generated power ~ 7 kW (end of life) be placed in a sun-synchronous orbit at 691 km, with a Weight Approx. 4,000 kg descending local Equator pass time at about 10:30 Data recorder 96 G bytes, solid-state (22:30 in ascending mode). The orbital revisit period is Data link 240 Mbps (via DRTS) 46 days, with a potential 2-day revisit capability for the 120 Mbps (direct down link) side-looking instruments. ALOS will carry three remote sensing instruments: PALSAR, PRISM and AVNIR-2 [3]. 2. REMOTE SENSING INSTRUMENTS

To accommodate the large data amounts generated by the three instruments, ALOS is equipped with an on- 2.1. PALSAR board 96 Gbyte solid-state data recorder. Down-linking The Phased Array type L-band Synthetic Aperture of all global data will primarily be performed directly Radar (PALSAR) is an active microwave sensor to Hatoyama Earth Observation Center (EOC), north of developed jointly by JAXA and the Japan Resources Tokyo, via JAXA’s Data Relay Test Satellite (DRTS). Observation Systems Organization (JAROS). It is an The DRTS was launched into a geostationary orbit enhanced version of the Synthetic Aperture Radar on (E90°) in September 2002, and it operates with a data JERS-1 (HH-polarisation; 35°off-nadir angle), also operating at L-band. The antenna consists of 4 rate of 240 Mbps (Ka-band). Direct transmission from ALOS to local ground stations can be be performed at segments, with a total size of 3.1 by 8.9 metres when a reduced data rate of 120 Mbps (X-band). deployed. Table 2. PALSAR characteristics [4]

Polarization Off-nadir angle (swath, resolution) Center frequency 1270 MHz / 23.6 cm Chirp band width 28 MHz (single pol.) 14 MHz (dual, quad-pol.) Polarization modes Single, dual, quad-pol. Off-nadir angle Variable: 7.2 - 51.4 deg. Swath width 70 km (single/dual) 30 km (quad-pol) 350 km (ScanSAR 5-beam) Noise Equivalent < -24 dB (single pol) Sigma-0 < -27 dB (dual pol) < -30 dB (quad-pol) < -25 dB (ScanSAR 5-beam) S/A (Range) ~ 23 dB (single/dual) ~ 21 dB (quad-pol) ~ 25 dB (ScanSAR 5-beam) Radiometric < 1 dB relative (within scene) Fig. 2. PALSAR observation characteristics accuracy < 1.5 dB absolute (between orbits) Data rate 240 Mbps (ScanSAR 120 Mbps) 2.2 PRISM PALSAR is a fully polarimetric instrument, operating The Panchromatic Remote-sensing Instrument for with either single polarisation (HH or VV), dual Stereo Mapping (PRISM) is a panchromatic (520-770 polarisation (HH+HV or VV+VH), or full polarimetric nm) radiometer with 2.5-metre spatial resolution [5]. It mode. The look angle is variable between 7° and 51° has three independent optical systems for nadir, (8-60° incidence angle). PALSAR can also operate in forward and backward looking to achieve along-track coarse resolution ScanSAR mode, with single stereoscopy. Each telescope consists of three mirrors polarisation (HH or VV) and 250-350 km swath width. and a CCD array for push-broom scanning. The swath width is 70 km in nadir-only mode, and 35 km during PALSAR can be technically operated in as many as triplet mode operations (Fig. 3) 132 different modes. From an applications point of view however, such a large number of potential mode combinations rather becomes contra-productive, as the risk for programming conflicts between users becomes a real issue. In order to minimise such mode conflicts, seven modes of operation have been identified as the default modes to be used (Table 3). The default mode selection was made as a compromise taking scientific criteria, programmatic aspects and satellite operational constraints into consideration.

The data recording rate is 240 Mbps in single-, dual- and full-polarimetric modes, which thus requires down-linking via the DRTS. The ScanSAR however operates at 120 Mbps, which allows direct down-link of data to local ground stations (within the ALOS ground network).

Table 3. PALSAR default observation modes

Polarization Off-nadir angle (swath, resolution) HH 34.3 deg. (70 km, 10 m) HH 43.4 deg. (70 km, 10 m) HH+HV 34.3 deg. (70 km, 20 m) Fig. 3. PRISM observation characteristics HH+HV 43.4 deg. (70 km, 20 m) HH+HV+VH+VV 21.5 deg. (30 km, ~30 m) ScanSAR (HH) 5-beam mode (350 km, 100 m) Table 4. PRISM characteristics. Table 5. AVNIR-2 characteristics.

Characteristics Characteristics Spectral range 520-770 nm Blue 420-500 nm Number of optics 3 (forward, nadir, backward) Green 520-600 nm Base-to-height ratio 1.0 Red 610-690 nm Spatial resolution 10 m (nadir) Near-Infrared 760-890 nm Swath width 70 km (nadir only) Spatial resolution 10 m (nadir) 35 km (triplet mode) Swath width 70 km Pointing angle +/- 1.5 deg. (across-track) Pointing angle +/- 44 deg. (across-track) Data rate 240 Mbps Data rate 120 Mbps

The PRISM telescopes are installed on both side of an The primary objectives of AVNIR-2 are disaster optical bench with precise temperature control. The monitoring and land cover mapping and with its forward and backward telescopes are inclined + and – across-track viewing capabilities (+/- 44°), observation 24 degrees from nadir to realise a 1.0 base-to-height of disaster areas within 2 days’ repeat can be foreseen. ratio. The wide field of view (FOV) provides fully The side-looking capacity also allows simultaneous overlapped three-stereo (triplet) images (35 km width) observations with the PALSAR – a unique property without mechanical scanning or yaw steering of the which can be expected to contribute to microwave- satellite (Fig. 3). To achieve full ground coverage with optical data fusion applications. a 35 km swath, two 46-day cycles are required, during which the three telescopes are tilted (+/– 1.5°) in across-track direction. 3. ALOS OBSERVATION STRATEGY

The prime mission of PRISM is global topographic Retrieval of bio- and geophysical parameters from mapping at a scale corresponding to 1:25,000 and remote sensing data in an operational manner is a generation of fine resolution digital elevation models. strong driver of current scientific work, requiring not only the availability of appropriate sensors and 2.3 AVNIR-2 inversion algorithms, but also that the data that are to The successor to the VNIR and AVNIR instruments on be utilised are acquired in a planned and systematic JERS-1 and ADEOS, the Advanced Visible and Near manner. Regional extrapolation of locally developed Infrared Radiometer type-2 (AVNIR-2) on ALOS is retrieval algorithms is in this context imperative if the a multispectral radiometer with 10 metres ground applications are to be more than of mere academic resolution. interest, and spatially consistent data over large areas – ranging from national to continental scales - thus become a requirement.

JAXA has acknowledged the critical need for regionally consistent data by setting aside a significant share of the ALOS acquisition capacity for this purpose, to establish dedicated a Global Data Observation Strategy in support to climate change research and environmental conventions [6]. The strategy is designed to provide spatially and temporally consistent, multi-seasonal coverage of all land areas on a repetitive basis, with all three sensors, during the life- time of the ALOS satellite. It also comprises consistent, repetitive coverage of all major coastal areas, lakes and inland seas.

The observation strategy, based on a number of basic acquisition concepts [7], is described in detail in [8].

Fig. 4. AVNIR-2 observation characteristics 4. ALOS TECHNICAL DEVELOPMENT 4.3. High Stability attitude control system When observing the land surface from a high altitude Very high accuracy mapping capacity is a major orbit, attitude stability is critical. In order to minimize mission driver for ALOS, and the following accuracy geometric distortion in the imagery, the attitude requirements have been taken into account in the movement (angular velocity) of the ALOS platform is satellite design [9, 1, 2]: stabilized within 0.0002 degree per 5 seconds, ß Satellite positioning information within 2.5 m; corresponding to 2.5 m, or one pixel, distortion within ß Satellite attitude information within 0.0002 deg. a 35 km square scene. Disturbances from major ß Long term attitude stability within 0.0002 degrees vibration sources, such as the data relay per 5 seconds communication antenna pointing mechanics, the ß Absolute time information for each pixel within AVNIR-2 pointing mirror drive mechanics, the solar 370 ms accuracy. array paddle drive mechanics and the PALSAR ß Minimizing thermal distortion of sensor’s optical antenna structure, are carefully controlled with a feed axes and between the optical axis and attitude forward technique and on-board parameter tuning. sensors (Star Tracker and Inertial Reference Unit) during the entire orbital period (~100 minutes). 4.4. Thermal Distortion During an orbital revolution, variations in the solar input along the orbit causes thermal distortion of 4.1 Precise Position and Attitude determination various components of the instruments and satellite To accommodate high accuracy mapping without the structure, subsequently resulting in instrument use of ground control points (GCP’s), new systems for performance degradation. position and attitude determination have been developed. A dual frequency carrier phase tracking In order to minimize the effects of thermal distortion, type GPS receiver provides 1 metre position accuracy ALOS features an integrated optical bench concept, and a high accuracy star tracker (STT) provides 0.0002 with the PRISM optics, star trackers (STT), inertial degrees accuracy in attitude, corresponding to a 2.5 reference unit (IRU), and jitter sensors (ADS), are all metre nadir pointing uncertainty on ground). The STT integrated on one rigid optical bench (Fig. 5). The is equipped with three optics (STO), two are used bench is covered by a Multi Layer Insulator (MLI) simultaneously and one is redundant. In order to allowing temperature control within +/-3 degrees (K). achieve the best star position accuracy possible, the optics use a low-thermal distortion structure, The satellite primary structure and truss members are implementing tight temperature regulations. also insulated with MLI and thermal expansion is cancelled by a negative expansion, so called CFRP, truss mechanism. Table 6. Star Tracker specifications

Item Characteristics Number of 3 (2 in operation, 1 redundant) trackers FOV 8 * 8 deg Magnitude 4 ~ 6.5 mag No. of stars 10*2 sets (acquisition) 5*2 sets (track) Star Position 9 arc sec (random) Error (3 sigma) 0.74 arc sec (bias) Fig. 5. PRISM, STT, IRU and ADS on the optical bench Output Rate 1 Hz

5. L-BAND MARINE APPLICATIONS 4.2. Absolute Time Clock Rather than utilizing a traditional on-board crystal PALSAR utilisation is primarily focused on terrestrial oscillator as internal clock, which require periodical applications, in particular global monitoring of forest calibration, the internal clock on ALOS is completely and wetlands [6] and crustal deformation measurements, synchronized within the accuracy of 404 ns (3 sigma) as well as DEM generation, disaster monitoring and to the GPS absolute time, yielding 1 ms order absolute geological resources surveys. These are established L- time accuracy. band applications which have been amply demonstrated by JERS-1 SAR. PALSAR can however also be expected to contribute L-band SAR sensitivity to sea surface waves and wind to marine and ice applications, to complement Envisat patterns is illustrated in Fig. 7, which indicates varying and Radarsat C-band observations with less common L- surface roughness, and presumably winds from the band data. south-east, off the islands of Samui and Phangan in the Gulf of Thailand. The relatively shallow off-nadir angle, JERS-1 SAR has not been utilised widely for marine fixed at 35° for JERS-1, does not seemingly affect the and ice applications, primarily due its comparably poor backscatter response, which also allows detection of noise floor (~ -18 dB), which was caused by reduced ships and suspected oil slicks (Fig. 8). power supply to the SAR antenna (325 W) following initial technical problems with the antenna deployment. Despite this shortcoming however, JERS-1 can be used to demonstrate the potential value of L-band SAR for certain marine applications, thus indicating a role for ALOS PALSAR.

Fig. 6 shows internal waves in the Andaman Sea, off the west coast of Thailand, that are clearly visible in the JERS-1 L-band SAR data. Notable is the temporal stability of the wave patterns, which extend over two adjacent JERS-1 swaths, acquired 24 hours apart on Jan. 24 (centre) and Jan. 25, 1997 (far left).

Fig. 7. Surface wind patterns observed by JERS-1 SAR on Nov. 25, 1997.

Fig. 6. Extract from the JERS-1 SAR Global Rain Forest Mapping (GRFM) mosaic of South-East Asia, providing an L-band SAR view of internal waves in the Andaman Sea (Jan. 23-25, 1997), Fig. 8. Ships in the Singapore Strait (JERS-1 SAR, Nov. 14, 1997) The limited radiometric sensitivity has however 7. REFERENCES restricted JERS-1 SAR applications to ice, and the utility of L-band SAR for retrieval of ice physical 1. Ichitsubo. A., Hamazaki T., Osawa Y. and parameters has instead been demonstrated using Pi- Matsumoto A. Development Status for the Advanced SAR, a Japanese airborne SAR system [10, 11]. Earth Observing Satellite, Proc. ISPRS Commission VII WG6, Kyoto, October 2003. To support such applications, as well as that of 2. Matsumoto A., Hamazaki T., Osawa Y. and terrestrial ice sheets, fine resolution PALSAR Ichitsubo. A. Development Status of ALOS’s Sensors, acquisitions over Antarctica and its surrounding oceans Proc. ISPRS Commission VII WG6, Kyoto, October are planned several times per year. PALSAR is 2003. expected to contribute to ship routing and monitoring of 3. Hamazaki, T., Overview of the Advanced Land sea ice movements in the Sea of Okhotsk, where the Observing Satellite (ALOS): Its Mission Requirements, Japan Coast Guard plan frequent observations during Sensors, and Satellite System, ISPRS Joint Workshop the winter season in the low resolution ScanSAR mode. “Sensors and Mapping from Space 1999”, Hanover, Germany, 1999. 4. Hamazaki, T., PALSAR Performance, NASDA doc. 6. SATELLITE DEVELOPMENT STATUS NBF99019, National Space Development Agency of Japan, Oct. 1999. The integration and testing of the ALOS Proto-Flight 5. Osawa, Y., PRISM: a panchromatic three-line Model (PFM) is currently in progress at JAXA sensor for mapping onboard ALOS, pp. 173 – 180, Tsukuba Space Center (Fig. 9), following successful Proc. of SPIE, vol. 3498, EUROPT, Barcelona, Sept. system development tests using the Mechanical Test 1998. Model (MTM), the Thermal Test Model (TTM), and 6. Rosenqvist A., M. Shimada, T. Igarashi, M. the Engineering Model (EM). Watanabe, T. Tadono and H. Yamamoto, Support to Multi-national Environmental Conventions and Terrestrial Carbon Cycle Science by ALOS and ADEOS-II – the Kyoto & Carbon Initiative. International Geoscience and Remote Sensing Symposium (IGARSS’03), Toulouse, France. July 21- 25, 2003. 7. Rosenqvist, A., Milne T. and Zimmermann, R., 2003. Systematic Data Acquisitions—A Pre-requisite for Meaningful Biophysical Parameter Retrieval? IEEE Transactions on Geoscience and Remote Sensing, Communications, Vol. 41, No. 7, pp.1709- 1711. 8. Rosenqvist A., M. Shimada, M. Watanabe and T. Tadono, Systematic Data Observation Strategies for ALOS PALSAR, PRISM and AVNIR-2. JAXA EORC proc. 2nd ALOS PI Workshop, Awajishima, Japan, Jan. 19-23, 2004. 9. Iwata, T. et al, Precision Attitude and Orbit Control System for The Advanced Land Observing Satellite (ALOS), AIAA Guidance, Navigation & Control Fig. 9. ALOS PFM at JAXA Tsukuba Space Center Conference, Austin, August 2003. 10. Matsuoka T., S. Uratsuka, M. Satake, A. Nadai, T. Following integration of all subsystems on the PFM, Umehara, H. Maeno, H. Wakabayashi, F. Nishio and proto-flight tests are being conducted, including both Y. Fukamachi. Deriving sea-ice thickness and ice types electrical and environmental tests (vibration, acoustic, in the Sea of Okhotsk using dual-frequency airborne separation shock and thermal vacuum tests). SAR (Pi-SAR) data, Annals of Glaciology, vol.34, pp. Transportation of the satellite hardware to the launch 429-434 (2002). site in Tanegashima, in southern Japan, will take place 11. Wakabayashi H., T. Matsuoka, K. Nakamura and during the summer of 2004. The launch is currently F. Nishio, “Estimation of sea ice physical parameters scheduled for the Japanese Fiscal Year 2004 (April using polarimetric SAR : Results from Okhotsk and 2004 – March 2005), possibly in December 2004. Lake Saroma campaign”, Annals of Glaciology, vol.33, pp. 120-124 (2001).