Good news for the detection of deforestation, but what are the implications of changes in sensor characteristics compared to previous satellites?
The series of Landsat satellites has provided a unique archive detailing changes in the land surface since 1972. The 8th satellite was launched successfully on February 11th 2013, and with on board checks and calibration complete, NASA handed it over to the United States Geological Survey (USGS) in May. Its first images have now been processed and are available to download free of charge via GLOVIS. I’m very excited because I’ve just downloaded and processed my first scene, over Sierra Leone and Liberia, including one of my study sites, the Gola Rainforest National Park:

Landsat 8 scene covering southern Sierra Leone and western Liberia, captured 2nd June 2013. This is a 6-5-4 composite, stretched to accentuate the difference between intact forest (dark green), degraded forest (light green) and non-forest (purple). Image courtesy of the U.S. Geological Survey.
Though there has been at least one Landsat satellite in orbit since Landsat 1 was launched in July 1972, there have been a few hiatuses (hiati?) in Landsat coverage, periods of time where few high quality images are available in the archive for much of the world. The most notable are in the early- to mid- 1990’s, caused by funding uncertainty, mis-management and the loss of Landsat 6 at launch; and November 2011 until May 2013.
This most recent gap has only been partial, with two satellites collecting some data: Landsat 7 was launched in 1999 but since May 31st 2003 it has had a fault with its Scan Line Corrector (SLC), causing the loss of some data within each scene. Landsat 7’s sensor (ETM+) is, like the TM sensor of Landsats 4 & 5, a whisk-broom sensor, which collects data via a mirror that scans side-to-side, parallel to the satellite’s track. The SLC is a set of two mirrors that corrects for the forward motion of the satellite to keep the scan lines parallel to each other. SLC-off data therefore has wedges of data missing:
The result of this is that 22 % of the image data is missing, in wedges that increase in size towards the edge of the image. Only the very center of the scene is unaffected:
Though Landsat 7 has continued to successfully collect data from this date onwards, the missing strips make using this data for land-use change difficult. Incredibly however another source of Landsat data is available, despite no new Landsat satellite being launched for almost 10 years after this date. This is because Landsat 5, a satellite launched on the 1st of March, 1984, continued collecting useful data until November 16th 2011, when a fault developed in its transmission system. While this has been partially fixed, and Landsat 5 recently set a Guiness World Record for the longest continuously operated satellite, at 28 years 10 months, it has been collecting little data since that point.
The commissioning of Landsat 8 is therefore very welcome. It will be widely used by a huge range of organisations, but my interest is particularly regarding its use for mapping forest cover changes to quantify deforestation and degradation. I intend to use Landsat imagery (clouds permitting) to update land-use change analyses I’ve already performed for sites in Sierra Leone, Cameroon, Gabon, Uganda, Peru and Colombia, and have begun downloading the necessary images, as you can see above.
Different sensor characteristics: OLI vs ETM+/TM
Before analysing these images though it is important to understand that the sensor on Landsat 8 differs in many ways from that found on earlier Landast satellites. That sensor characteristics should change with time as technology improves is only natural, and has happened before in the Landsat series. The primary Multi-Spectral Scanner (MSS) sensor of earlier Landsats was superseded by the Thematic Mapper (TM) sensor on Landsat 4 & 5, increasing the resolution to 30 m and increasing the number of spectral bands (though both also carried MSS sensors for data continuity reasons). Landsat 7 features a sensor called the Enhanced Thematic Mapper+ (ETM+), which was a relatively small update of the TM sensor – it added a panchromatic band at twice the resolution (15 m) to allow finer details to be distinguished, however the spectral bands and basic design remained the same.
The Operational Land Imager (OLI) on Landsat 8 is very different to ETM+ however, representing much more than just an evolution of the earlier sensor. Its whole design is different: it is a pushbroom rather than a whiskbroom sensor, resulting in fewer moving parts (no SLC to go wrong here!) and a higher signal-to-noise ratio. But more importantly from a user perspective is that the wavelengths have changed: 3 new bands have been added, and most of the other bands now detect a smaller range of wavelengths:

Spectral bands of Landsat 8 vs Landsat 7 sensors. Note the additional bands in Landsat 8, and that the duplicated bands are mostly narrower. Image provided by NASA, source http://landsat.usgs.gov/ldcm_vs_previous.php
In theory these narrower wavelengths should improve the ability of the sensor to discriminate different landcover types, so they are good news. But the lack of continuity is a problem: a classification algorithm applied to one sensor should not be blindly applied to another anyway, but even with good cross-calibration doing so would not be appropriate here. Even the widely used NDVI will differ between the old and new sensors, with the near infrared band on OLI being much narrower than the equivalent band on previous satellites (compare OLI band 5 with ETM+ band 4 on the graph above). I will be performing some comparisons between near-contemporaneous Landsat-7 and Landsat-8 scenes over the next few months to see how much effect this has.
While changing wavelengths is in some ways unhelpful, the additional bands are another story. I’m particularly excited about the new Band 1, which is designed to help detect hazy, high-altitude cirrus clouds, which can be hard to spot but can subtly change spectral characteristics.
The future
Optical satellite data will remain a very useful tool far into the future, so I’m hopeful the Landsat dataset will continue to be added to for many decades yet. However, the US Congress has not as yet committed funding for any more Landsat satellites, and there is no long-term commitment by the US to keep Landsat series operating indefinitely. With the Landsat archive having been made open and free in 2008, and with scenes collected worldwide, Landsat provides an incredibly service to the rest of the world, but there is no guarantee this will continue.
With the need for continuous, consistent satellite data in mind, the EU is funding ESA to launch a series of Landsat-like satellites as part of its Sentinel series, part of the Global Monitoring for Environment and Security (GMES, now known as Copernicus) initiative. Sentinel 2 is based around a wide-swath, 10 m resolution sensor, with a pair of satellites able to provide images of the whole land-surface every 5 days. The first satellite will be launched in 2014, with partner and replacement satellites budgeted into the 2030’s, using technology and expertise developed from the SPOT satellite series among others. While there have been many other Landsat-like satellites launched in the past, and many are operational now, Sentinel 2 is the first that, like Landsat, will offer ‘free and open access for all’. I hope there will be a Landsat 9, but if not then it is good to know that Landsat 8 is there to bridge the gap between now and the start of the Sentinel era.













