Copernicus's radar constellation will soon get a new reinforcement – the Sentinel-1D satellite has completed key checks in French Guiana and is entering the final phase of preparations for connection to the Ariane 6 rocket. The plan is for the spacecraft to take off from Kourou on Tuesday, November 4, 2025 to strengthen continuous imaging of the Earth's surface in all weather conditions and to compensate for the limitations brought by older equipment in orbit. The moment it connects to the Sentinel-1C satellite, launched at the end of 2024, Copernicus will once again have a double "radar sentry" for faster, more reliable, and denser coverage of the planet.

Double constellation as a standard for quick decisions

In a system like Copernicus, two operational satellites in the same mission are not a luxury but a necessity. The revisit interval over the same location directly determines how quickly a change can be detected – whether it's the outbreak of a flood, the appearance of a new landslide, a glacier shift, the spread of an oil slick, or overfishing. The 1C + 1D constellation will allow for shortening the waiting time for the next image and for denser time series, which means earlier warning and better-informed services that make decisions on the ground. This also restores the logic for which Sentinel-1 was originally designed: an operational service that doesn't stop even when clouds are thick and the night is long.

What Sentinel-1D brings compared to its predecessors

The heart of the spacecraft is the C-band Synthetic Aperture Radar (C-SAR), an instrument that sends and receives radar pulses and from them calculates a high-resolution image. The advantages of radar are well known: independence from daylight, resistance to clouds and precipitation, and precise measurement of changes on the ground and sea. Compared to the first generation (1A/1B), the new block (1C/1D) has been fundamentally modernized: more efficient energy consumption management, more robust computing and communication subsystems, and a GNSS receiver compatible with multiple systems for precise navigation and orbit determination – including the European Galileo, in addition to GPS and other constellations. This multi-constellation support provides a more stable, faster, and more accurate "heartbeat" to the satellite, which ultimately spills over into a more consistent geolocational footprint on every image.

Another important novelty is the introduction of a receiving system for AIS (Automatic Identification System) on newer platforms. AIS is a global standard by which ships automatically broadcast their identity and position. When the radar image and AIS signals are viewed together, a more complete story about traffic at sea is assembled: where vessels with an enabled transponder are, where "dark spots" without AIS appear, how movement matches weather conditions, waves, and currents. In practice, this means better maritime safety, more effective fishing surveillance, and faster reaction in search and rescue.

Why continuity is crucial: what changes with 1D

Sentinel-1A has already been in orbit for 11 years and is functioning beyond its designed lifespan. Thanks to Sentinel-1C, the constellation was again working in dual mode with 1A in 2025, but the arrival of 1D will strengthen operations for the next Copernicus cycle. In practice, this means: more orbits per day with useful acquisitions, more flexible switching between imaging modes (e.g., wide swath for rapid flood mapping or a narrower swath for urban deformations), and relieving the task planning burden between the two spacecraft. The effect is most visible in the shorter return time to critical locations and in the more stable availability of products through distribution systems.

Preparations in Kourou: from arrival to functional tests

After Sentinel-1D was delivered to French Guiana, the teams in Kourou completed the standard protocol: checking the pressure system and its integrity, then an "electrical check-out" for all main subsystems – power, thermal control, communications, computer, attitude control, and of course the radar line. The AIS antenna assemblies, which are transported separately for safety, were integrated onto the spacecraft and then checked with a final connectivity test to confirm the correct electrical connection and telemetry. These steps, although routine, create a "golden record" of data that is used as a reference for every subsequent system power-on – during fueling, connection to the rocket's upper stage, and the final vertical rollout to the launch pad.

Ariane 6: launch logistics and time window

The launch of Sentinel-1D is scheduled for November 4, 2025 on an Ariane 6 rocket in configuration 62 (two solid rocket boosters). The spacecraft targets an altitude of around 700 km in a sun-synchronous orbit. Ariane 6 enters the phase of regular operational missions in 2025, and precisely this kind of "service" delivery of a Copernicus radar satellite illustrates its key role in European autonomy of access to space. For the ground team, this means a well-rehearsed rhythm: final assembly of the "stack" (satellite + adapter + upper stage), fairing closure, rollout to the launch pad, and a series of final checks until T-0.

How the radar "sees" the world: a brief guide for readers

Unlike optical satellites that photograph with sunlight, radar measures its own reflections. The C-SAR emits waves with a frequency of about 5.4 GHz and reconstructs light-dark patterns from the return signal that depend on the roughness, moisture, geometry, and movement of the surface. Because of this, flooded soil, wet fields, and calm sea "sign" differently from dry soil, forests, or a wavy sea. In interferometry (InSAR), phase information from two images is compared and sub-centimeter movement of the ground is calculated – an excellent tool for landslides, mines, earthquakes, and urban stability. In maritime applications, wind maps are extracted from radar noise and the pattern of short capillary waves, and the geometry of dark spots reveals oil spills.

For whom the data is most valuable: from emergency services to agriculture

Emergency services and civil protection rely on Sentinel-1 for fast segmentation of flood plains, determining available roads, and assessing damage immediately after a storm. Maritime authorities use a combination of SAR and AIS to track fleet movements, monitor fishing zones, and spot suspicious patterns. Geological services monitor the stability of slopes, risky zones around active faults, or waste dumps. Urban planners and engineers detect slow subsidence of embankments, settlements, and roads. Agriculture builds indicators of crop stress from radar measurements of moisture and vegetation structure, which helps in insurance and irrigation planning. Climate scientists monitor changes in ice cover and glacier dynamics, while environmental organizations use radar to monitor wetlands, forests, and coastal ecosystems throughout the year.

Faster data return and better delivery to users

The Copernicus data delivery ecosystem has been thoroughly revamped in recent years. Sentinel-1C began preliminary operations in late March 2025 while the commissioning period was running in parallel, and its products were progressively made available to users. With the arrival of 1D, an even shorter time from acquisition to publication is expected thanks to optimized imaging plans, ground station capacities, and a flexible downlink schedule. For end users, this practically means a higher probability that they will download the image they need for the field on time.

Technical details: modes, resolutions, and orbital geometry

Sentinel-1 operates in several standard modes. Interferometric Wide Swath (IW) is the "workhorse" over land: a swath width of up to 250–300 km, ground-range pixels on the order of 5–10 m, ideal for deformations and flood maps. Extra Wide (EW) is preferred over coastlines and the open sea where it is most important to cover as much of the water area as possible. Stripmap (SM) is used for targeted areas in high resolution. The orbital setup with two satellites in a 180° phase provides better temporal density, and when interferometric coherence is needed, planners can "stitch in" special repeat passes over the same path – for detailed monitoring of a city, a dam, or a mine.

GNSS and precise orbit: why this is important to the user

The accuracy of scene geopositioning is just as important as the sharpness of the radar image itself. The new GNSS receivers with support for multiple constellations enable precise orbit determination (POD) with centimeter errors in post-processing. A better orbit means less bias in geocoding and more stable comparison of scenes over time – crucial for all series in which subtle shifts on the order of millimeters per week or month are sought.

AIS as a "second pair of eyes" over the sea

The radar image is excellent at showing everything that leaves a contrast on the sea – a ship, a wavy surface, an oil slick – but the radar doesn't "know" the name of the vessel. AIS complements this image with identity and course. At the level of operational centers, this means that a detected target on the SAR can be quickly paired with AIS messages – or marked as a "contact without AIS" if the transponder is silent. Such patterns are particularly interesting for monitoring protected marine areas, tracking corridors near energy facilities, and controlling illegal fishing.

What "better revisit" means in numbers

In ideal conditions, two satellites at opposite nodes of the orbit can halve the return time over the same location. This is not just a convenient metric for presentations, but an operational difference between "too late" and "on time". If it's a flood that is rising at a rate of tens of centimeters per hour, an additional image from the second satellite can determine whether a settlement will need evacuation within 12 hours or not. If it's about landslides, more frequent interferometric connections give an earlier signal that the slope has entered acceleration.

Industrial chain and reliability

Sentinel-1 is a product of a large European consortium that combines experience in radar electronics, power management assemblies, structures, and software. In the new block, the experiences from the commissioning and early operations of Sentinel-1C have been applied to testing methods, acquisition plans, and the optimization of the ground chain. This is one of the reasons why faster stabilization of Sentinel-1D is expected after entering orbit, with fewer risks and shorter technical pauses in the first weeks of the mission.

Open data and tools: where users work with the products

Copernicus remains true to the principle of open access. Sentinel-1 products are available through multiple interfaces and services for searching, processing, and downloading. More advanced users can automate protocols for downloading and processing, while field services will often use pre-configured maps with thematic layers. For those who want to understand the basics, a good start is to go through a brief overview of terms like the SAR instrument, GNSS, or AIS, and then try basic workflows in viewers and GIS tools.

Examples of use in the region: what it means for Croatia and the neighborhood

In our area, the radar constellation proves crucial in several scenarios. Along the Adriatic, these are winter and autumn cyclones that bring heavy rainfall, sudden flooding of torrents, and storms. On the karst and steep slopes of the Dinarides, the focus is on landslides and rockfalls, while in the flat parts of central and eastern Croatia it is important to map extensive flood zones along the Sava, Drava, and Kupa rivers. Maritime surveillance also includes seasonally increased yacht traffic and cargo and tanker traffic through the Adriatic, where the combination of SAR and AIS helps local port captains and port authorities. In agriculture, radar indicators of moisture and soil structure can improve damage assessments after hail or storms, which is important for both insurers and producers.

What is actually tested before fuel

The phase in which Sentinel-1D is "ready for finalization before fueling" means that all functional checks have been successfully completed. This includes "cold start" power-ons, redundancy tests (switching to backup power channels, RF lines, memory banks), checks of telecommands and telemetry, and simulations of typical operational scenarios in orbit. Only after this comes the fueling and oxidants for maneuvering thrusts, the closing of service panels, and the "instrument close-out" – access to the instrument is minimized and everything is left to electronic monitoring.

Operational lifespan and plan B

The designed life of Sentinel-1 is seven years, but operational experience shows that with good thermal control and careful resource allocation, the possibility of operation can be significantly extended. Despite this, the continuity of the mission never relies on a single satellite. Because of this, 1D is planned as an operational successor and partner, and the planning of future capacities is already being considered within the framework of the long-term Copernicus second-generation program. The useful effect for users: even when one satellite goes into planned service or faces an anomaly, the other continues to cover critical tasks.

From raw amplitude to user product

Raw SAR data goes through several levels of processing – from Level-0 and Level-1 products (e.g., GRD, SLC) to Level-2 and thematic maps. Each step includes a precise orbit, radiometric corrections, geocoding, and a whole series of filters that separate the useful signal from noise. Over the sea, for example, a wind field is reconstructed based on the radar's "granular" pattern; over land, terrain shift is calculated from scene comparisons. The new 1C/1D architecture with faster memories and a more reliable downlink helps a larger volume of data reach ground processors and users faster.

Viewed through the eyes of communities that depend on precision

Hydrology needs multi-day sequences to model inflows into retention areas and assess risk to settlements; transport wants to know which roads are passable after storms; energy controls wind farms and power lines on exposed terrains; tourism and maritime affairs rely on situational awareness of the route, waves, and wind. In each of these branches, a shorter revisit and greater delivery reliability brought by 1D reduce uncertainty in planning. When AIS is added to this, the picture at sea becomes more understandable and verifiable, which benefits everyone from port authorities to research institutions.

Calendar until liftoff

In the remaining weeks until November 4, 2025, the sequence that the teams have already polished on many missions follows: finalization of the spacecraft (closing panels, protection, and cables), fueling, integration on the upper stage of Ariane 6, connecting with the fairing, rollout to the launch pad, and the "wet dress rehearsal" and final countdown. On Day D, the time window and the state of the high layers of the atmosphere will determine the moment of liftoff, and the satellite will need a series of maneuvers to arrive at its target point and turn on the instrument in accordance with the commission plan.

Why it's important to follow even the "boring" checks

Such preparation periods rarely make headlines, but it is precisely in them that the conditions for years of reliable operation in orbit are created. Every screw and every voltage curve will get its signature in the logs; these values are later used when the software in orbit looks for an anomaly or when an engineer on the ground compares behavior before and after some automatic protection. Sentinel-1D has reached the point where only "cosmetics" remain before fueling and meeting Ariane 6 – and user communities across Europe are already counting on the data that will arrive in the first weeks after liftoff.

How users can prepare

Anyone who wants to take advantage of the full potential of 1D's arrival can already prepare workflows for faster data ingestion, update cloud masking in hybrid analyses (SAR + optical sensors), and make sure that geodetic systems and projections are consistently set in all tools. It's also good to check automated alarms for extreme events to trigger processing and dissemination to teams on the ground without manual intervention. In maritime centers, it's worth pre-defining connection rules for SAR detections and AIS, for example, thresholds for marking a "suspicious contact" if a target is seen on the radar without an active transponder.