From ERS-1 to NISAR: 35 Years of SAR at ASF

In August 1991, an antenna on the University of Alaska–Fairbanks campus tracked the European Remote-Sensing Satellite 1 (ERS-1) as it passed over interior Alaska. During that pass, the Alaska Satellite Facility (ASF) received its first satellite data downlink. ERS-1 had launched just a few weeks earlier. The European Space Agency satellite carried a C-band synthetic aperture radar (SAR) that could image Earth's surface through clouds and darkness, making it especially useful for observing Alaska and the polar regions.

At the time, ASF was known as the Alaska SAR Facility, a new ground station built to receive synthetic aperture radar data from satellites passing over Fairbanks.

August 2026 marks 35 years since that first downlink. In the years between ERS-1 and today's missions, the facility has supported a long list of satellites, expanded its ground station, and built an archive that now holds decades of Earth observations.

Some of the satellites that collected those observations have been gone for years. Their data are still being used. And now NISAR is adding to that record.

Why Fairbanks?

Fairbanks is an unusually good place for a satellite ground station. At about 65 degrees north latitude, it sits beneath the paths of many satellites traveling in polar and high-inclination orbits. As they circle Earth, those satellites pass within range of high-latitude ground stations several times a day. ASF can have nine or 10 contacts with the same polar-orbiting spacecraft in a 24-hour period.

Each pass lasts only a few minutes. An antenna picks up the spacecraft as it rises above the horizon, tracks it across the sky, and stays with it until it drops out of view. During that time, science data can be transmitted to the ground and, depending on the mission, other communication with the spacecraft can take place.

It's something that happens so routinely at ASF that it's easy to overlook how much those few minutes matter. A satellite collects observations as it orbits hundreds of kilometers above Earth. When it comes within range of Fairbanks, those observations have a path to the ground.

The location made particular sense for synthetic aperture radar, as it doesn't need sunlight to image the surface and clouds don't get in the way as they do with optical imagery. Those are useful qualities anywhere, but especially in Alaska, where winter brings long periods of darkness and cloud cover can linger.

That combination of geography and technology shaped much of ASF's early work.

The Early Missions

ERS-1 was quickly joined by other radar missions. Japan's JERS-1 launched in 1992, carrying an L-band radar. In 1994, SIR-C/X-SAR flew aboard Space Shuttle Endeavour twice, collecting radar observations at several frequencies.

ASF's role was growing at the same time. In 1994, it became a NASA Distributed Active Archive Center, adding long-term data preservation and distribution to the work already happening at the ground station.

ERS-2 and Canada's RADARSAT-1 followed in 1995. RADARSAT-1 became an important source of observations of the polar regions and was at the center of one of the decade's more ambitious radar mapping projects. In 1997, the RADARSAT Antarctic Mapping Mission used hundreds of satellite passes to create the first high-resolution radar map of the entire Antarctic continent. ASF's location enabled the facility to repeatedly receive data from RADARSAT-1 as the spacecraft passed over high northern latitudes.

As more missions were added, ASF was no longer simply a ground station receiving SAR data. It was also maintaining a growing archive and making those observations available to researchers.

By 2003, the original name no longer described everything the facility was doing. The Alaska SAR Facility became the Alaska Satellite Facility. The acronym, conveniently, didn't have to change.

Data That Outlasts the Mission

Japan's Advanced Land Observing Satellite, ALOS, launched in 2006, carrying PALSAR, an L-band synthetic aperture radar. L-band radar uses a longer wavelength than the C-band instruments of missions such as ERS and RADARSAT. That gives researchers different information about forests and vegetation, ice, and changes to Earth's surface. ASF served as the ALOS data node for the Americas, providing access to mission data.

ALOS stopped operating in 2011, but its observations remain part of ASF's archive and are used by researchers today. Older data often become more useful with time because they give scientists something to compare with newer observations. A radar image collected 15 or 20 years ago can show what a place looked like before a wildfire, a landslide, a changing glacier, or years of surface movement. That's a large part of the value of maintaining a long-term archive. The spacecraft may be gone, but the observations remain useful.

ASF's radar holdings also include data that were not collected from space. NASA's AIRSAR and UAVSAR collected radar observations from instrumented aircraft, allowing scientists to make detailed measurements over specific areas. UAVSAR has flown over earthquakes, volcanoes, glaciers, landslides, wildfire scars, and other places where researchers are interested in how the surface is changing. Those airborne observations became another part of ASF's growing radar record.

Sentinel-1 Changes the Scale

Sentinel-1A launched in 2014 as part of the European Union's Copernicus Earth observation program, and it changed the amount and frequency of SAR data available to researchers. Rather than collecting occasional observations over selected areas, Sentinel-1 was designed to acquire C-band radar data systematically and repeatedly across large parts of the world. Those data were also openly available. 

For researchers, that meant having enough observations to track change over time rather than relying on a few snapshots. A glacier could be tracked through multiple seasons. Scientists could compare the ground before and after an earthquake or follow subtle movement over months or years. Flooding could be mapped even when storms and clouds obscured the surface. Sea ice could be followed from one acquisition to the next.

Sentinel-1B joined Sentinel-1A in 2016 and increased the frequency of those observations. Today, Sentinel-1C and Sentinel-1D continue the mission.

At ASF, the Sentinel-1 archive grew quickly, both in the amount of incoming data and in the number of people using it.

Those observations now sit alongside radar data collected decades earlier. ERS-1 and ERS-2. JERS-1. RADARSAT-1. SIR-C/X-SAR. ALOS PALSAR. AIRSAR and UAVSAR. More than a decade of Sentinel-1. They came from different countries, different agencies, and different generations of radar instruments. Together, they give researchers a record of Earth's surface that reaches back more than three decades.

NISAR Joins the Record

In July 2026, NISAR became the newest SAR mission to join that history. The NASA-ISRO Synthetic Aperture Radar mission carries both L-band and S-band radar instruments and is designed to repeatedly observe Earth's land and ice surfaces. Scientists will use those observations to study earthquakes, volcanoes, landslides, and glaciers, as well as changes in forests, wetlands, agriculture, and other ecosystems.

ASF supports the distribution of the mission's NASA L-band data. On July 20, 2026, calibrated provisional NISAR L-band data became publicly available, with additional observations being added as they are processed.

For ASF, there's a natural connection between NISAR and that first ERS-1 downlink. ERS-1 was part of an early generation of satellite radar missions that showed what SAR could contribute to Earth science. NISAR arrives 35 years later with far more ambitious coverage and an established record of earlier radar observations.

ASF has changed along with the missions. What started with one antenna now includes multiple antennas tracking spacecraft over Fairbanks. The archive has grown from terabytes to petabytes. And while working with SAR once required specialized systems and expertise just to get the data, researchers today can search and access decades of observations online from almost anywhere.

ASF's work isn't limited to SAR anymore, either. The facility has supported other Earth-observing missions over the years, and its antennas are now part of NASA's Near Space Network.

Outside, the basic work of communicating with satellites—especially SAR platforms—still looks surprisingly familiar. An antenna turns as a spacecraft comes within range of Fairbanks. It follows the satellite across the sky for a few minutes while data make their way to the ground. Then the spacecraft disappears over the horizon. Another one will be along soon.

ASF has been making those connections for 35 years.