ALOS-4

ALOS-4 (Daichi-4, Advanced Land Observing Satellite-4) is Japan's fourth-generation L-band synthetic aperture radar mission, developed and operated by the Japan Aerospace Exploration Agency (JAXA) [^eoportal-alos4]. The satellite was launched on 1 July 2024 aboard the H3 launch vehicle (third flight, H3 F3) from Tanegashima Space Center and placed into a sun-synchronous orbit at 628 km altitude with a 14-day repeat cycle and 12:00 local time at descending node [^jaxa-alos4-launch][^eoportal-alos4].

Built by Mitsubishi Electric Corporation (MELCO) on the JAXA Advanced Satellite Bus-3, ALOS-4 carries PALSAR-3 (Phased Array type L-band Synthetic Aperture Radar 3) as its primary payload [^eoportal-alos4]. PALSAR-3 succeeds PALSAR-2 aboard the still-operational ALOS-2, which continues data acquisition in extended mode alongside ALOS-4. The principal advance is a dramatic expansion in imaging capacity: PALSAR-3 ScanSAR mode achieves a 700 km swath at 25 m resolution using four simultaneous scan beams, compared with PALSAR-2's 350 km swath at 100 m in wide-area mode [^eoportal-alos4].

PALSAR-3 supports three imaging modes: Spotlight (3 m x 1 m resolution, 35 km scene), Stripmap (3, 6, or 10 m resolution, 100 to 200 km swath), and ScanSAR (25 m, 700 km swath). All modes operate at L-band with selectable centre frequencies of 1236.5, 1257.5, or 1278.5 MHz and dual polarisation (HH or HH+HV) [^eoportal-alos4]. L-band wavelength (~23.8 cm) provides deep canopy penetration, making ALOS-4 effective for detecting sub-canopy forest disturbance that shorter-wavelength radars miss.

A secondary payload, SPAISE3 (Space-based AIS Experiment 3), receives Automatic Identification System signals from ocean vessels using eight antennas with digital beam-forming, improving detection in high-traffic shipping lanes over the predecessor SPAISE2 on ALOS-2 [^eoportal-alos4]. An optical inter-satellite data relay (LUCAS) was demonstrated on 23 January 2025, achieving 1.8 Gbps downlink at 1.5 micrometre wavelength via a geostationary relay satellite [^eoportal-alos4]. ALOS-4 entered nominal operational status by late 2025, with commercial distribution handled by Tenchijin, Inc. from February 2025 [^eoportal-alos4].