Introduction to Optical Satellite Imagery

From Football Field to Football: A History of Advances in Optical Satellite Imagery

Satellite imagery became publically available in 1972, and led to the founding of NPA Satellite Mapping (NPA) as a consultancy in the same year. Since then, the evolution in capabilities of both optical satellites and data processing has been staggering.

ERTS-1, (Landsat-1) orbited the Earth as the only dedicated Earth Resources Satellite for two and a half years. Upon the failure of its Return-Beam Vidicon (RBV) sensor within only 2 weeks of launch, the 4 spectral band Multi Spectral Scanner became the unexpected primary sensor, capturing over 100 images a day with pixels representing a ground sampling size similar to a football pitch. Mostly, re-imaging attempts were limited to a minimum of 16 day intervals. Additionally, obtaining early Landsat imagery of regions located outside the range of a ground station or a relay satellite was a huge challenge.

Image resolutions and the number of commercial satellites increased through the 1980s and 1990s. Continuing the mission to keep a detailed record of the Earth’s environment, Landsat-4 and -5 increased resolutions to 30 m and included shortwave infrared bands which greatly improved geological applications. Subsequently, SPOT satellites brought pointable sensors to the commercial world, a highly valued capability for any cloud affected area, and another leap in resolution, to 10 m, was taken.

With the breakup of the Soviet Union, NPA made the next leap, distributing 2 m imagery scanned from Russian KVR-1000 image films, all of which had been returned to Earth by parachute. It was older technology, but it worked! Such imagery was much in demand as it enabled the mapping of buildings, small roads and tracks, which greatly increased the applications of satellite imagery for clients wishing to gain insights into the operational challenges in their remote project areas. Nevertheless, more often than not, obtaining required coverage from archive remained a major challenge.

Landsat-5 image of Phalaborwa in South Africa, acquired in 1984
Spot-6 image of Phalaborwa, South Africa
Phalaborwa, South Africa, acquired by Landsat-5, 1984.
Improvements in technology are demonstrated by this image acquired by SPOT-6. Individual blocks are visible within instability of the northern pit edge at Phalaborwa.

The high resolution digital image revolution arrived shortly before the turn of the century. IKONOS brought sub-meter resolution digital imagery with pointing of its sensor sufficiently flexible to enable capture of same pass stereo, a key requirement for detailed and low noise elevation mapping. However, relatively few launches or notable resolution improvements occurred in the first decade of this century, and the new high resolution optical satellites were mainly those now in the Digital Globe constellation (Quickbird, WorldView-1, -2, GeoEye).

The last 10 years has seen the number of commercial optical imaging satellites rapidly increase. Today, NPA supply imagery captured from over 250 satellites. While most have better than 5 m resolution, several constellations comprise of sub-meter sensors. Around a dozen now provide imagery in the 70 - 30 cm range; the size of a football. Such constellations include the DigitalGlobe satellites, with the inclusion of 30 cm WorldView-3 & -4, Airbus’ Pléiades-1A,-1B, and the Korean Multi-Purpose Satellites (KOMPSAT-3,-3A). Most recently, the Chinese added GaoJing (SuperView-1) 01 to 04 to this list, which are the first of a planned 16 satellite constellation of 50 cm sensors, four higher resolution as well as synthetic aperture radar (SAR) and a fleet of video and hyper-spectral cameras.

The last 10 years has seen the number of commercial optical imaging satellites rapidly increase. Today, NPA supply imagery captured from over 250 satellites. While most have better than 5 m resolution, several constellations comprise of sub-meter sensors. Around a dozen now provide imagery in the 70 cm - 30 cm range; the size of a football. Such constellations include the DigitalGlobe satellites, with the inclusion of 30 cm WorldView-3 & -4, Airbus’ Pléiades-1A,-1B, and the Korean Multi-Purpose Satellites (KOMPSAT-3,-3A). Most recently, the Chinese added SuperView-1 01-04 to this list, which are the first of a planned 16 satellite constellation of 50 cm sensors, including four higher resolution satellites, synthetic aperture radar (SAR) and a fleet of video and hyper-spectral cameras.

Topping the numbers game by far is Planet, operating over 175 low cost ‘Dove’ nano-satellites with 3 – 5 m resolution along with 13 SkySat satellites at 80 cm resolution. Planet’s model is to offer high revisit imagery or information within a subscription based monitoring service.

This resolution journey is expected to continue with greater numbers of advanced satellites also reducing revisit times. Such expanded high resolution constellations are likely to be first instigated by the improved Chinese SuperView satellites, Digital Globe’s Scout & Legion constellations and Airbus’ next generation Pleiades satellites. Many of these should be operational by the 50th anniversary of Landsat-1 returning its first images, providing more data per square kilometer than is present in a full 175 x 185 km Landsat-1 frame.

Landsat-1 MSS image over eastern England, acquired 1975Landsat-1 MSS imagery acquired in 1975 over eastern England.
Landsat-1 MSS image over London, acquired 1975Central London from the Landsat-1 MSS imagery acquired in 1975.
WorldView-3 Image over Auckland, 30 cm resolutionWorldview-3 imagery over Auckland, New Zealand at 30 cm resolution.

With resolution improvement being the primary focus of the satellite development race, funding and data bandwidth limitations have often restricted the spectral capabilities of many high resolution satellites to just the common four visible and near infrared bands. However, this has enabled the parallel development for some sensors hosting multi-spectral capabilities as their strengths. These sensors offer great value to the task of mapping or monitoring the Earth’s environment and resources.

In 1999, NASA launched the Japanese ASTER sensor, which provided an enhanced spectral view of the Earth, collecting imagery across 14 spectral channels (visible to thermal) at 15 – 90 m spatial resolution. With Landsat-6 not achieving orbit, defects with the Landsat-7’s sensor and program funding doubts, the data taken for granted by many scientists and for multiple applications was in jeopardy. However, battling technical problems, Landsat-5 entered the record books as the longest-operating Earth-observing satellite mission (1984-2013) when NASA successfully launched Landsat-8 in 2013. This enabled continuity of the Landsat program, with much improved radiometric quality and 15 – 30 m resolution. In 2015 and 2017, ESA’s 10 – 20 m Sentinel-2A and -2B joined Landsat-8 with the contribution of vast amounts of wide swath multi-spectral imagery in bands ranging from the UV blue, through the visible and near infrared to shortwave infrared (SWIR). Digital Globe’s WorldView-2 gave eight visible and near infrared bands and WorldView-3 incorporated these with another eight SWIR channels. Government restrictions prevent the SWIR data being distributed at more than half their full resolution, at 3.7 m.

We have at our disposal a wide range of optical satellite image options, delivered by satellite operators across all continents of the world. NPA Satellite Mapping is proud to be an official distributor/reseller for the vast majority of them, and has been since the launch of the first commercially available resources, in 1972. Our in-depth knowledge of the industry and its continual developments enables us to connect our clients with the right imagery for the job in hand.

For more information on how optical imagery can benefit your project, please contact Richard Chiles.



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