Temporal: The forgotten resolution

Time, Copyright: scanrail / 123RF Stock Photo

Time, Copyright: scanrail / 123RF Stock Photo

Temporal resolution shouldn’t be forgotten when considering satellite imagery; however it’s often neglected, with its partners of spatial and spectral resolution getting the limelight. The reason is the special relationship spatial and spectral has, where a higher spectral resolution has meant a lower spatial resolution and vice-versa, because of limited satellite disk space and transmission capabilities. Therefore, when considering imagery most people focus on their spatial or spectral needs and go with whatever best suits their needs, rarely giving temporal resolution a second thought, other than if immediate data acquisition is required.

Temporal resolution is the amount of time it takes a satellite to return to collect data for exactly the same location on Earth, also known as the revisit or recycle time, expressed as a function of time in hours or days. Global coverage satellites tend to have low earth polar, or near-polar, orbits travelling at around 27,000kph and taking around 100 minutes to circle the Earth. With each orbit the Earth rotates twenty-five degrees around its polar axis, and so on each successive orbit the ground track moves to the west, meaning it takes a couple of weeks to fully rotate, for example, Landsat has a 16 day absolute revisit time.

Only seeing the part of the Earth you want to image once every few weeks, isn’t very helpful if you want to see daily changes. Therefore, there are a number of techniques satellites use to improve the temporal resolution:

  • Swath Width– A swath is the area of ground the satellite sees with each orbit, the wider the swath the greater the ground coverage, but generally a wider swath means lower spatial resolution. A satellite with a wide swath will have significant overlaps between orbits that allows areas of the Earth to be imaged more frequently, reducing the revisit time. MODIS uses a wide swath and it images the globe every one to two days.
  • Constellations – If you have two identical satellites orbiting one hundred and eighty degrees apart you will reduce revisit times, and this approach is being used by ESA’s Sentinel missions. Sentinel-1A was launched in 2014, with its twin Sentinel-1B is due to be launched in 2016. When operating together they will provide a temporal resolution of six days. Obviously, adding more satellites to the constellations will continue to reduce the revisit time.
  • Pointing – High-resolution satellites in particular use this method, which allows the satellites to point their sensors at a particular point on earth, and so can map the same area from multiple orbits. However, pointing changes the angle the sensor looks at the Earth, and means the ground area it can observe can be distorted.
  • Geostationary Orbits – Although technically not the same, a geostationary satellite remains focussed on an area of the Earth at all times and so the temporal resolution is the number of times imagery is taken, for example, every fifteen minutes. The problem is that you can only map a restricted area.

Hopefully, this has given you a little oversight on temporal resolution, and whilst spectral and spatial resolution are important factors when considering what imagery you need; do spent a bit a time considering temporal needs too!

Rosetta: Extra-terrestrial Observation

Full-frame NAVCAM image taken on 9 August 2014 from a distance of about 99 km from comet 67P/Churyumov-Gerasimenko. Image: ESA/Rosetta/NAVCAM

Full-frame NAVCAM image taken on 9 August 2014 from a distance of about 99 km from comet 67P/Churyumov-Gerasimenko. Image: ESA/Rosetta/NAVCAM

Most people will have seen last week’s news about ESA’s Rosetta spacecraft arriving at comet 67P/Churyumov-Gerasimenko and the animated images of the ‘rubber-duck’ shaped object taken from Navigation Camera (NavCam), part of Rosetta’s Attitude and Orbital Control System. The arrival generated many headlines, from the 10 years it took to catch the comet, through the history making first rendezvous and comet orbit, to the final part of the mission and the intention to land on the comet. However there was little detail about the remote sensing aspect of the mission, which we feel is a missed opportunity as it’s using many of the techniques and methodologies employed in Earth observation (EO).

The orbiter part of Rosetta carries eleven different remote sensing experiments with a wide variety of sensors gathering data about the comet before the lander touches down. Amongst the instruments on-board are three separate spectrometers; a visible and infrared thermal imaging spectrometer (VIRTIS) focussing on temperature and geography; an ultraviolet imaging spectrometer (ALICE) looking at gases and the production of water and carbon dioxide/monoxide; and finally ROSINA has sensors for measuring the composition of the comet’s atmosphere and ionosphere.

The VIRTIS instrument has two channels; the VIRTIS-H channel is a high spectral resolution mapper operating from 2 to 5µm, whereas the VIRTIS-M is the mapper operates at a coarser spectral resolution and one of its main products will be a global spectral map of the comet’s nucleus. This instrument has already been used to undertake measurements of Earth. In November 2009, on Rosetta’s third Earth fly-by, VIRTIS measurements were compared to existing EO instruments from ENVISAT/AATSR, SCIAMACHY and MODIS. Overall, there was a strong correlation with the EO data, but differences were also seen – especially in the 1.4µm water absorption feature.

VIRTIS has a key role in supporting the selection of the November’ landing site, a task that has become more difficult now the comet has been imaged in detail and is seen to have a complex shape. In addition, recent VIRTIS measurements have shown the comet’s average surface temperature to be around minus seventy degrees centigrade, which means the comet is likely to be too warm to be ice covered and instead must have a dark, dusty crust.

Remote sensing is playing a huge part in the Rosetta mission and it should be celebrated that these instruments will gather data over the next eighteen months to help scientists determine the role comets play in the evolution of planets. It will be amazing if remote sensing techniques developed to explore, monitor and analyse our planet, will be the same techniques that help determine if the water on Earth originally came from comets.