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8 October 2026/5 min read

L-band arrives: what NISAR changes for alpine monitoring

Free, calibrated L-band radar from NISAR has been public since July 2026. For snowy, rough and fast-moving alpine slopes, it is the missing wavelength.

By Julian Walder

On 20 July 2026 NASA released fully calibrated L-band radar products from NISAR, the joint NASA and ISRO mission launched on 30 July 2025. The data are free, open to anyone with an Earthdata login, and cover nearly all of the planet's land and ice. For high-alpine hazard monitoring, it is the most significant new data source since Sentinel-1 began its C-band record in 2014.

Here is why, and what it changes in practice.

The facts

  • Launch: 30 July 2025. Pre-calibration "beta" products, more than 100,000 files acquired before February 2026, were released on 27 February 2026. Calibrated "provisional" products for the full observation plan followed on 20 July 2026; they have been validated at a limited set of sites and remain provisional pending wider validation.
  • Archive: observations from the first year of science operations are being added over the coming months, with the complete record expected by the end of 2026.
  • Revisit: a 12-day exact repeat orbit, imaging the same location twice every 12 days when ascending and descending passes are counted.
  • Resolution: 3 to 10 metres, depending on mode. Swaths are 240 kilometres wide.
  • Latency: 36 to 72 hours from acquisition to the availability of processed products.
  • Products: geocoded single-look complex images for rapid interferogram generation, geocoded unwrapped interferograms with coherence layers from nearest-neighbour 12-day pairs, and geocoded backscatter, all global; pixel-offset products over designated cryosphere regions.
  • Access and licence: through the Alaska Satellite Facility, NASA Earthdata and the asf_search Python package. All NISAR science data are freely available under NASA's open data policy, with no stated restriction on commercial use.

Why wavelength matters at altitude

A radar interferogram only works where the ground looks similar enough to the radar between two passes. Scientists call that coherence. C-band, the wavelength used by Sentinel-1, is sensitive to small changes in the surface: fresh snow, growing vegetation, or motion of more than a few tens of centimetres per year are enough to scramble the phase. In the high Alps these conditions are the norm, not the exception.

L-band uses a longer wavelength. It penetrates vegetation and dry snow better, it tolerates rougher surfaces, and it can follow faster motion before the phase becomes unreadable. ESA's analysis of the Blatten collapse put this to the test with archived L-band data from the ALOS-2 and SAOCOM missions: the Kleines Nesthorn was visibly moving from 2016 and accelerating to more than 150 centimetres per year by August 2024, a rate at which an annual C-band product struggles. ESA's own write-up notes that C-band radar has difficulty in high alpine terrain, in dense vegetation and wherever motion exceeds tens of centimetres per year.

The catch, until this year, was access. Those L-band archives are not open. High-resolution ALOS-2 products are sold commercially, and SAOCOM data over Europe are distributed under agreement. A regional screening service could not be built on them. NISAR removes that barrier.

Where Sentinel-1 still leads

None of this makes C-band obsolete. The Sentinel-1 constellation was renewed in 2026: Sentinel-1D launched in November 2025, its data opened on 17 April 2026, and by early July 2026 Sentinel-1C and 1D had re-established the nominal 6-day repeat between them, with Sentinel-1A phased out. The constellation offers something NISAR cannot: an archive back to 2014, which is what makes back-tests of past events possible, and a 6-day repeat per track that is twice as dense as NISAR's 12 days.

Sentinel-2 adds a third view: 10 metre optical imagery every five days, which tracks surface features on glaciers that move too fast for any radar phase method and provides snow and cloud masks.

Looking ahead, ESA's ROSE-L mission, planned for launch in 2028, is designed to add a second L-band source with 5 to 10 metre resolution and a revisit of three or six days depending on mode. A contract for the next generation of Sentinel-1 was awarded in June 2026. The direction of travel is clear: more wavelengths, denser revisit, all free.

What this means for cadence

Put the three sources together over a Swiss alpine region and the picture looks like this. Every six days a Sentinel-1 pass from at least one track; every twelve days an ascending and a descending NISAR pass; every five days a Sentinel-2 scene, cloud permitting. In a typical month a given slope is observed by radar from several geometries at two wavelengths, plus optical.

That supports a weekly screening cycle for change detection, with a monthly consolidation step in which the time series is re-estimated across all sensors and the slope ranking is refreshed. It does not support hourly monitoring, and it never will; satellite revisit is measured in days. The hand-off to ground radar, cameras and observers, which measure every few minutes, remains the only route to hours of warning.

What Watchberg does with it

Watchberg processes NISAR L-band, Sentinel-1 C-band and Sentinel-2 optical data in-house with open tools, builds per-slope motion histories, and flags the onset of acceleration. The L-band stream is new, and the first full year of NISAR over the Alps will only be complete at the end of 2026, so the claims about what it adds are projected and to be validated. The pilot programme planned for 2027 is designed to measure, slope by slope, how much coherence L-band recovers on snow, vegetation and fast-moving ground compared with C-band alone, and how much earlier an acceleration is detected when both are fused.

The physics gives good reason to expect a large difference. The data to prove it now exist, and anyone can download them.

Sources

  • NISAR
  • L-band
  • Sentinel-1
  • Data

Related

Join the 2027 pilot programme.

Watchberg is preparing pilots with hazard offices and operators in three Swiss cantons. If you carry responsibility for alpine slopes, let us show you what a weekly briefing would look like for your region.

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Pilot programme 2027. In discussion with hazard offices in three cantons.