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Landsat 8 and Landsat 7 comparison, First Look

by Santosh Singh

Landsat Data Continuity Mission or Landsat 8 has been long awaited since the failure of the Scan Line Corrector onboard the Landsat ETM+ satellite sensor, just 4 years after its launch in 1999. There was 22% data loss in each scene which severely affected efforts on Climate change and various other Environmental studies. NASA was forced use other satellites as filler for the time until Landsat satellite was launched. Actual LDCM was used for this purpose only. NASA had invited proposal from commercial imaging companies to provide data that met LDCM specifications. After an evaluation of proposals received from industry, NASA cancelled the Request for Proposals in September 2003. In August 2004, a memorandum from the White House Office of Science and Technology Policy (OSTP) directed Federal agencies to place Landsat-type sensors on the National Polar-orbiting Operational Environmental Satellite System platform. In December 23, 2005, the OSTP issued a memorandum directing NASA to implement the LDCM in the form of a free-flyer spacecraft carrying an instrument referred to as the Operational Land Imager (OLI). NASA and the USGS are now implementing the OSTP directive. In December 2009, a decision was made to add a thermal infrared sensor (TIRS) to the mission payload.


Now the satellite has already been launched and a pre-commissioning sample data has been provided by NASA. This data contains following bands:

Band #     Spectral RegionBand 

Band 1     Visible
Band 2     Visible 
Band 3     Visible 
Band 4     Near-Infrared 
Band 5     Near-Infrared
Band 6     SWIR 1
Band 7     SWIR 2 
Band 8     Panchromatic
Band 9     Cirrus 
Band 10     TIRS 1
Band 11     TIRS 2

 

Band 1 to Band 9 are acquired by Operational Land Imager(OLI) and Band 10/11 are acquired by Thermal Infrared Sensor(TIRS). OLI is similar to Landsat ETM+, has all bands at 30m resolution except for Panchromatic which is at 15m resolution. TIRS acquires both bands at 100m resolution. Below is comparative spectral profile of both sensors:

 

Above is how both sensors compare spectrally. Six of the bands are equivalent to those of the ETM+ sensor currently in use on Landsat 7. The two additional bands on OLI are an ultra-blue band centered at 443 nanometers for coastal and aerosol studies, as well as a cirrus band centered at 1,375 nanometers for cloud detection. OLI doesn’t collect data in the thermal infrared wavelengths. Both OLI and TIRS are push broom sensor capable of acquiring 12bit data with and excellent Signal to Noise ratio.

Sample Data:

We downloaded the sample data from USGS Website. If you want to download it here is the link.

First look at the panchromatic band was enough to observe the advanced technology used in the LDCM:

Panchromatic band of Landsat 8 satellite sensor. Excellent resolution and radiometric details easily distinguish this data from older Landsat ETM+ data shown below. © NASA/USGS

Below is the Landsat ETM+ image of the same area:

 

Grainy nature of Landsat ETM+ plus is immediately noticeable once compared with Landsat 8′s Panchromatic band. Image Credit: NASA

The discernible change in quality is the result of higher Signal to Noise ratio of the Landsat 8′s OLI sensor.  The pushbroom technology employed enable acquisition of data in 12 bits with high SNR.

Below we will see some zoomed in portions of above images. Although both panchromatic bands are at 15m resolution, the LDCM’s Pan band seems to be better in terms of spatial resolution, therefore efficiently utilizing the data space.

 

Landsat 8′s Panchromatic band(Above) and Landsat 7′s Panchromatic band(below) zoomed to same scale. Noise and degradation in spatial resolution can be seen in ETM+ image. Image Credit: NASA/USGS

As expected OLI’s narrower panchromatic band is  providing better spectral contrast over land surfaces. Also, the push-broom sensor’s higher signal-to-noise ratio makes it possible to narrow the spectral bands and move them away from atmospheric absorption features, thereby reducing the sensitivity of the changes in the atmosphere. The biggest change is in the near infrared (band 5 in OLI). That band is substantially narrower and will be less sensitive to atmospheric conditions than Landsats 5 and 7.


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