PARIS — A 13-year-old group of nations providing Earth observation satellite imagery free of charge after natural disasters anywhere in the world is being forced to evolve with the imminent retirement of its most-utilized spacecraft and the privatization of many of its satellite sources.

The International Charter on Space and Major Disasters, formed in 1999 by the French space agency, CNES, and the 20-nation European Space Agency (ESA), has grown to 14 member nations and agencies — Russia is about to become the 15th — plus 41 nations that have created Authorized Users of the charter and can activate it on their own.

More than 90 other nations have access to the charter through the United Nations or through regional disaster-response networks such as the 32-nation Sentinel Asia network.

Between 2000, when it first entered into operation, and 2012 the charter has been activated 369 times, according to Philippe Bally, the charter’s coordinator at ESA.

Half of the emergencies have related to floods, many from tsunamis.

The charter makes use of about 20 satellites with optical and radar sensors operated by governments and companies in the United States, Canada, France, Germany, Brazil, Argentina, China, Japan, South Korea, Taiwan and India.

In a press briefing at CNES here March 28, Bally said 110 nations have taken advantage of what he called the charter’s “virtual constellation” of satellites in the past decade.

Notable for its absence in the constellation is Italy’s four-satellite Cosmo-Skymed radar observation system. Italy is not a member of the charter.

Once the charter is activated by an Authorized User, whether on the user’s own behalf or on behalf of another nation, satellite operators with spacecraft about to overfly the affected region respond by programming their spacecraft to image the disaster area.

Bally said this usually takes around three hours. Between 24 and 48 hours later, imagery is taken and transformed, within six to eight hours after acquisition, into maps and other disaster-mitigation products made available to emergency crews at the scene.

For the last decade, the charter’s most-used satellite has been the French Spot 5 medium-resolution optical spacecraft, which is owned mainly by the French government but used commercially by Astrium Geo-Information Services.

Now well past its planned retirement date, Spot 5 will be removed from service and placed into a graveyard orbit in 2015, CNES has said.

Catherine Proy, the charter’s project chief at CNES, said the French agency has been involved in 80 percent of the charter’s 369 activations since 2012. On average, she said, France’s Spot satellites — Spot 5 and the now-retired Spot 4 spacecraft — have been producing 100 images for charter activations per year. These include images from Spot’s archive as well as those taken in direct response to a charter activation.

The charter’s other satellite sources include privately owned satellites from DigitalGlobe of Longmont, Colo. But DigitalGlobe, which makes a living selling imagery and related products, typically does not provide images directly to the charter.

Instead, the charter receives DigitalGlobe imagery from the U.S. National Geospatial-Intelligence Agency (NGA), DigitalGlobe’s biggest customer.

The charter’s two Authorized Users in the United States are the U.S. Geological Survey and the U.S. National Oceanic and Atmospheric Administration.

The NGA’s status has made it easy to field U.S. high-resolution optical imagery for charter purposes without having to negotiate with the private sector. Up to now, the U.S. satellites have been the only ones in the charter’s constellation that are privately owned.

That is about to change. Spot 5 will be succeeded by two smaller satellites, Spot 6 and Spot 7, both wholly owned by Astrium Geo-Information Services. Unlike the NGA in the United States, no French government agency has agreed to make bulk purchases of Spot 6 or Spot 7 imagery.

Proy said CNES is negotiating with Astrium on future use of Spot 6 and Spot 7. Astrium and DigitalGlobe, as well as other private-sector satellite operators, regularly make images available free of charge to respond to a major natural disaster. But the images are not necessarily delivered in the quantity or speed needed for disaster response.

A second issue facing the charter is that an increasing number of the satellites in its virtual constellation are high-resolution spacecraft, including the two French Pleiades satellites, which are owned by the French government but used by Astrium.

Higher-resolution satellites have the advantage of highlighting details, such as individual structures damaged in a city, which might escape notice in a lower-resolution image. But higher resolution also means a smaller swath width, which could mean less frequent coverage of disaster-stricken areas.

Bally said that ideally, the charter members would like to make use of commercial satellites such as Spot 6 and Spot 7, or the satellites that are part of the U.K.-based Disaster Monitoring Constellation, and then overlay these images with higher-resolution imagery.

Bally said the arrival in 2014 of the Sentinel-2 satellites, to be part of the environment-monitoring network operated by the European Commission, will help fill the void left by Spot 5’s retirement.

Peter B. de Selding was the Paris Bureau Chief for SpaceNews.