ALOS-4 (DAICHI-4)

ALOS-4, formally designated Daichi-4 and cataloged under NORAD ID 60182, is a Japanese Earth-observation satellite operated by the Japan Aerospace Exploration Agency (JAXA). Launched on June 30, 2024 (July 1 local Japanese time), it carries an advanced L-band synthetic-aperture radar system designed to observe the Earth's land surface with high resolution and broad coverage. With a launch mass of 2,990 kg, it ranks among the heavier dedicated Earth-observation platforms in low orbit today, continuing a lineage of Japanese SAR satellites that stretches back more than three decades.

Mission and Purpose

ALOS-4's primary instrument is PALSAR-3, the third generation of the Phased Array type L-band Synthetic Aperture Radar developed in Japan. This sensor is the direct successor to PALSAR-2, which flew aboard ALOS-2, and it inherits the strengths of that heritage while extending the capabilities available to researchers and operational users.

L-band synthetic-aperture radar occupies a distinct and practical niche in Earth observation. Unlike optical sensors, which depend on sunlight and are blocked by clouds, an L-band SAR actively transmits microwave pulses and records the echoes that return from the surface below. Because L-band wavelengths—roughly 23 centimeters—can penetrate through cloud cover, rain, and even into the upper layers of vegetation canopies, the technology is particularly valuable for monitoring tropical forests, wetlands, agricultural fields, and terrain that would be invisible to cameras for months at a time due to persistent cloud cover. This makes the data useful for applications such as deforestation mapping, disaster response, subsidence monitoring, and geological surveys.

ALOS-4 is the fourth satellite JAXA has operated carrying an L-band SAR payload. Its predecessors were JERS-1, the original ALOS (also called Daichi), and ALOS-2 (Daichi-2), forming a continuous record of L-band SAR data collection that now spans decades. This long observational continuity is scientifically significant: it allows researchers to detect slow surface changes—millimeter-scale ground deformation, gradual land-cover transitions, or long-term subsidence—that only become apparent when consistent measurements are compared across many years. ALOS-4 is intended to extend that record further into the future.

While the specific operational objectives and mission status as currently recorded in public tracking catalogs are not fully detailed, the satellite's design and sensor payload clearly situate it within JAXA's broader commitment to land-surface monitoring on a global scale.

Orbit and Tracking

ALOS-4 occupies a sun-synchronous orbit (SSO), circling Earth at an inclination of 97.9° to the equatorial plane. Its apogee stands at 636 km and its perigee at 634 km, making the orbit very nearly circular with less than 2 km of eccentricity. At this altitude the satellite completes one full revolution of the Earth approximately every 97.3 minutes, yielding roughly 14 to 15 orbits per day.

A sun-synchronous orbit is a specific near-polar trajectory in which the satellite's orbital plane precesses at exactly the same rate as the Earth progresses around the Sun. The net result is that the satellite crosses any given latitude at approximately the same local solar time on every pass. For a radar instrument like PALSAR-3 this timing consistency is less critical for illumination—since the sensor provides its own illumination—but it remains operationally useful for scheduling data downlinks, coordinating ground station contacts, and enabling systematic repeat-pass interferometric measurements, in which two radar images acquired from nearly the same vantage point at different times are combined to detect subtle surface deformation.

The orbit's near-circular shape at roughly 635 km altitude keeps the satellite within what is generally considered the operational band of low Earth orbit, well below the inner Van Allen radiation belt. This altitude offers a balance between reasonable ground resolution, manageable orbital decay rates, and broad swath coverage per pass.

ALOS-4 is tracked continuously by the global network of radar and optical sensors that feeds the US Space Surveillance Network, and its position is cataloged under international designator 2024-123A. As of the time of this writing, the satellite remains in orbit with no scheduled or recorded reentry date.

Design and Operator

ALOS-4 was manufactured by Mitsubishi Electric Corporation, one of Japan's primary space systems integrators and a long-standing contributor to JAXA's satellite programs. Mitsubishi Electric has been involved in the construction of multiple Japanese Earth-observation and communications spacecraft, and its role in ALOS-4 continues that industrial relationship.

The satellite's total mass of 2,990 kg reflects the substantial hardware required to accommodate a large-aperture phased-array radar antenna, the electronics needed to generate, transmit, and receive high-power microwave pulses, onboard data storage, and the power generation systems—solar arrays of considerable area—needed to sustain active radar operation. SAR satellites are inherently power-intensive; the radar transmitter must fire thousands of pulses per second during imaging passes, placing significant demands on the electrical power subsystem.

JAXA, the operating agency, was established in 2003 through the merger of three predecessor Japanese space organizations. It serves as Japan's national space agency and conducts programs spanning launch vehicles, human spaceflight, planetary science, and—critically for ALOS-4—Earth observation. JAXA maintains its own ground infrastructure for commanding satellites and receiving data, and ALOS-4 data is made available to domestic and international users through established data distribution channels consistent with JAXA's policies for its Earth-observation missions.

The ALOS series has historically been notable for JAXA's relatively open data access policies, particularly for disaster response and scientific research, and ALOS-4 fits within that broader programmatic context.

Significance and Legacy

The launch of ALOS-4 in mid-2024 marks a meaningful step in the continuity of Japan's L-band SAR observational record. Each generation of the Daichi series has arrived with improved technical capabilities over its predecessor, and PALSAR-3 continues that progression. From JERS-1's foundational observations in the 1990s through ALOS and ALOS-2, Japan has assembled a global dataset of L-band radar imagery that is widely used in both research and applied contexts—by forestry agencies monitoring carbon stocks, by geological surveys tracking fault motion, and by disaster management authorities responding to earthquakes, floods, and volcanic events.

Japan's geography makes Earth observation with SAR technology particularly resonant as a national priority. The country sits at the junction of four tectonic plates, experiences frequent seismic activity, and is exposed to seasonal typhoon systems that bring intense rainfall and flooding. The ability to acquire reliable surface data under any weather conditions and at any time of day is not merely academically desirable—it has direct practical relevance for civil protection and infrastructure monitoring within Japan and across the broader Asia-Pacific region.

By extending the operational lifespan of Japan's L-band SAR capability, ALOS-4 ensures that the multi-decade dataset accumulated by its predecessors does not face a gap in coverage. Such continuity is difficult to recover once lost: a gap of several years in consistent radar observations can make long-baseline interferometric analyses unreliable and interrupts the seasonal and annual time series that support forest carbon accounting under international frameworks.

The satellite's placement in a sun-synchronous orbit at approximately 635 km also positions it to operate alongside or in coordination with other Earth-observation platforms, potentially enabling multi-sensor data fusion that combines the all-weather penetrating capability of L-band radar with the complementary strengths of optical, multispectral, or C-band radar sensors operated by other agencies.

How to Spot It

ALOS-4 is a large spacecraft—nearly 3,000 kg—with broad solar arrays necessary to power its radar system. In principle, objects of this size in a ~635 km sun-synchronous orbit can be visible to the naked eye under favorable conditions: shortly after local sunset or before local sunrise, when the ground is in darkness but the satellite's solar panels are still illuminated by the Sun. At inclination 97.9°, ALOS-4 traces a steeply inclined ground track that carries it across nearly all latitudes, meaning observers across much of the globe may have opportunities to see it pass overhead.

Because it is a radar satellite rather than an optical one, ALOS-4 carries no reflective optics or mirror surfaces specifically designed for ground illumination, and its brightness will depend on the orientation of its solar arrays relative to the observer and the Sun at the moment of any given pass. Visibility is therefore variable and is best determined using current orbital elements from a real-time tracking tool—such as those provided on this site using the NORAD catalog entry 60182—which can compute precise rise, transit, and set times, maximum elevation, and predicted brightness for any observer location.