Becoming a Pixel – Evolutions in pixel size and spectral detail

A brief historical perspective of the NASA Landsat program, the more recent transition to commercial satellite imagery, and the corresponding sensor spatial and spectral characteristics.

Remote sensing capabilities have improved dramatically in the last 40 years. The following is a brief overview of the history of earth observing multispectral sensors and how improvements in instrument design and spectral resolution have increased the application areas and types of analysis that can be performed using satellite imagery. The discussion starts at the beginning of the NASA Landsat program in the 1970s and extends through present day with commercial sensors operated by companies such as GeoEye and DigitalGlobe.

NASA launched Landsat 1 in 1972, which carried both a camera system and the first MSS sensor, a multispectral instrument with four bands (green, red and two in the near-infrared) with approximately 60m spatial resolution (resampled from a larger 68m x 83m field of view). With this launch began a revolution in the field of remote sensing. Although limited by today’s standards, Landsat MSS opened many doors for image analysis of the Earth’s surface. Importantly, based on the longevity and fidelity of the Landsat program, as of this year there are now 40 years of image data available for analysis.

NASA went on to launch Landsat 2 and 3, in 1975 and 1978 respectively, with MSS sensors containing nearly identical spectral and spatial characteristics as the MSS on Landsat 1. The only exception was that the MSS on Landsat 3 included a fifth band for measuring the thermal-infrared. In terms of spatial scale, the 60m resolution of these MSS instruments equates to a pixel size approximately equivalent to half of a professional soccer field. Many important analysis techniques were initiated in these early years. For example, the normalized difference vegetation index, which provides a relative measure of live green vegetation and is still used widely today, originated during the early years of the NASA Landsat and NOAA AVHRR programs.

The Landsat program next progressed to Landsat 4 and 5, launched in 1982 and 1984 respectively, which carried both the MSS sensor and a TM sensor. The newer TM sensor included a total of seven bands, six bands with 30m resolution (blue, green, red, one near-infrared, and two in the mid-infrared) and one band with 120m resolution (thermal-infrared). Landsat 6 was launched in 1993 but unfortunately failed to reach orbit. The most recent Landsat satellite, Landsat 7, was launched in 1999. This satellite carries the ETM+ sensor, which includes eight bands, the same seven bands as the TM sensor, with the addition of a panchromatic band at 15m resolution and an improved spatial resolution for the thermal-infrared band at 60m. Leveraging the 15m scale of the panchromatic band, the ETM+ sensor can be used to represent a ground area equivalent to a tennis court or moderately sized house, which is significantly improved from Landsat 1. This increase in spatial resolution and the accompanying greater number of bands achieved throughout the life of the Landsat program has enabled scientists to address an ever increasing array of Earth remote sensing applications, from agriculture and forestry to coral reefs and urban development.

There are plans for a Landsat Data Continuity Mission, to be launched in 2013, as well as numerous other government satellite programs, both within the U.S. and internationally; however, development in recent years has also moved into the commercial realm. Beginning with the IKONOS satellite [GeoEye], which was launched in 1999, satellite imagery was no longer limited to just government programs but now available from a commercial provider. IKONOS contains five total bands, four multispectral bands at 3.2m resolution (blue, green, red, near-infrared) and one panchromatic band at 0.8m resolution. Additional options now available on the market include: GeoEye-1 [GeoEye] (launched 2008; four multispectral bands at 1.6m and one panchromatic at 0.4m); QuickBird [DigitalGlobe] (launched 2001; four multispectral bands at 2.4m and one panchromatic at 0.6m); WorldView-1 [DigitalGlobe] (launched 2007; one panchromatic band at 0.5m); and WorldView-2 [DigitalGlobe] (launched 2009; eight multispectral bands at 1.8m and one panchromatic at 0.5m). This remarkable progression in resolution is enabling another leap forward in what can be accomplished using satellite imaging, literally putting global imagery in the palm of your hand.

As a parting thought, consider this… with today’s commercial satellites, it is now possible to say that you are a pixel. And if you’re lying down somewhere, such as on the beach, you’re about 4 pixels. It’s amazing technology.


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