Earth observation satellites only view a small portion of the world at any one time. Typically the ones used by Digital Earth Australia (DEA) only observe a small strip, north-south across the landscape, and over several days the whole landscape is finally observed in full. For example the Landsat satellites take 16 days to create a full coverage, and then only if the surface of the earth is not obscured by clouds. Hence there can be difficulties in capturing imagery over persistently cloudy locations such as Tasmania and northern Australia during the monsoonal months, and areas may not be observed at all for extended periods. Where the satellite data is being analysed as a time series, these long periods can cause important changes to be missed entirely, or parts of a cyclic environment to be misinterpreted (such as vegetation phenology and cropping cycles).
An additional limitation of the Landsat series is the availability of data due to the ageing of each satellite. Landsat 5 was operational for over 25 years, but for much of the later years, data were only acquired when sunlight directly illuminated its solar panels. This limited its operation across Australia, with coverage being seasonally dependent, and contracting north to a minimum in winter. In its last years the winter coverage usually only covered the northern coasts of Australia. Landsat 5 ceased regular operations over Australia in 2011, leaving just Landsat 7 until Landsat 8 was launched in 2013. Landsat 7 began service in 1999 as a replacement for Landsat 5. Initially Landsat 5 was switched off, but when Landsat 7 suffered a serious problem in 2003 due to the failure of its scan-line corrector (termed SLC-Off) Landsat 5 resumed service. The SLC-Off failure of Landsat 7 results in striping across every image from mid 2003 onwards, apparent in subsequent derived products. Landsat 8 has operated well since launch in 2013, with improved sensitivity, noise characteristics and data correction in comparison to the earlier sensors.
The result of the availability of the satellites is that the most consistent data availability occurs when two satellites are in operation (most of the 2003 to present period). The least data availability is in 2011 – 2012 when only Landsat 7 was available with data containing the SLC-Off striping issue. The overall data availability for the Landsat satellites is shown in Table 1.