On 1 April 1960 US Army Signal Corps pilot Captain William M. Templeton lifted off in a Sikorsky H-19 Chickasaw helicopter from Camp Evans, part of the Fort Monmouth complex in New Jersey, for the short journey to Monmouth County Airport. On board he was carrying precious cargo. At the airfield, Captains Lloyd J. Petty and Robert C. Jones were preparing their L-23 Seminole aircraft to speed the package on the next leg of its journey. After takeoff they headed south, towards Bolling air force base in the southern suburbs of Washington, DC. On arrival their cargo was put in a vehicle and whisked away to the headquarters of the National Aeronautics and Space Administration, and within hours was being presented to President Eisenhower himself.
What cargo could be so precious as to warrant such treatment?
Inside the package were the first images taken by NASA’s TIROS-1 (Television Infrared Observation Satellite), which had been placed in a low-earth orbit after launching from Cape Canaveral aboard a Thor Able rocket at 0640 EST that morning. TIROS-1 carried two TV cameras: one with a wide-angle lens and one with greater magnification. Within 20 minutes of launch the satellite was transmitting its first TV pictures to the ground tracking station at Camp Evans, where Signal Corps personnel developed the first prints. Although primitive by today’s standards, they clearly showed the planet Earth, with the swirls of clouds associated with weather systems.
This was no April Fool’s joke – the era of remote Earth imaging from space for scientific purposes had arrived. And it had taken a very short time: only two years had elapsed since the US had launched Explorer 1, its first satellite. (At this point it should be noted that the military’s Corona photographic satellites had been operational from mid-1959. However, instead of transmitting their ‘take’ to the ground by radio, they employed an intriguing means of delivering imagery by dropping film canisters in a re-entry ‘bucket’. Once in the atmosphere the ‘bucket’ deployed a chute that was intended to be snagged by an aircraft for retrieval).
Although TIROS-1 suffered a system failure in June, its initial successes were such that it was obvious that scientists had a very valuable tool with which to further their knowledge of the planet and its weather. Over 40 launches have been undertaken (not all successful) in the TIROS series since 1960, and the latest Advanced TIROS-N satellites are the basis of NOAA’s current POES system (of which more later).
Today there are many satellites from many nations orbiting Earth, able to image the planet’s surface, monitor its weather, oceans and atmosphere, and record its ever-changing nature. While all of them have other primary missions, by the nature of their sensors many can also be used to monitor and record volcanic activity. For the sake of this article, however, we shall only look at the US-led satellite and sensor programmes that are commonly referred to in connection with volcanoes, and for which imagery is easily accessible to the scientific community and general public alike.
Types of orbit
Before examining the individual satellites and sensors it is worth taking a moment to look at how satellites work with regard to their orbits. According to their mission requirements they can be placed in orbits at various altitudes from the earth, but the two most useful for our purposes are geosynchronous orbit (GSO) and low-earth orbit (LEO, up to 2,000 kilometres). There are also medium-earth orbit (MEO) satellites, such as the GPS, GLONASS, BeiDou and Galileo satnav constellations that orbit twice a day at a little over 20,000 kilometres altitude.
A geosynchronous orbit is one in which the satellite remains in the same location relative to a point on the earth’s surface. The simplest form is a circular geostationary orbit (GEO) in which the satellite is directly above the equator at an altitude of 35,786 kilometres, and moving in the same direction (zero inclination) and at the same angular speed as the earth’s rotation. To place a satellite in orbit where it remains constantly above a point that is NOT on the equator requires an elliptical orbit with varying altitudes, although the major axis of the ellipse must always be 42,164 kilometres.
Geosynchronous orbits have obvious applications for persistent staring surveillance of a particular section of the earth’s surface. However, they operate at vast distances from the earth’s surface, so resolution of imagery is poor, and there is some lag during the transmission of signals.
Getting down and dirty
To get imagery of a much higher resolution then a LEO satellite is the obvious choice. In terms of obtaining the highest quality of image it would be best to have the lowest orbit possible, and some short-duration military reconnaissance satellites have had the ability to make very low passes before climbing back up to a more economic operating altitude. In practical terms that means greater than 600 kilometres, with around 700 kilometres being the norm. Below 600 kilometres the satellite encounters too much drag from the gases in the upper reaches of the earth’s atmosphere to be cost-effective in a long-duration mission.
In a 700-kilometre LEO a satellite circumnavigates the earth in around 100 minutes, allowing it to perform around 14 orbits every day. This can be exploited in a polar orbit to provide coverage of the whole of the earth’s surface every 24 hours. In this orbit the satellite travels in a north/south direction, overflying both poles at the ‘top’ and the ‘bottom’ of its orbit (90° inclination). Meanwhile, underneath it the world just keeps on revolving around the polar axis, so that each time the satellite passes up over a certain latitude the earth beneath it has been rotating for 100 minutes. Over the course of 24 hours the satellite can build up imagery of the whole globe in 14 swathes.
This can be further exploited in a refinement called a sun-synchronous orbit (SSO). In this the orbit, with an inclination of around 95°, is timed to always pass over a certain point on the globe at exactly the same solar (local) time every day. This means that the sun’s lighting and shadow patterns will, in essence, be the same for every pass.
Yet one more refinement is the frozen orbit. As we know, the earth is not a perfect sphere but an oblate spheroid with a bulge around the middle. Gravitational forces are greater at the poles than they are at the equator. As the earth precesses from equinox to equinox so gravitational forces on the satellite subtly alter through the seasons. Furthermore, the sun and moon also exert gravitational forces on a satellite. By offsetting the inclination (to about 98°) and introducing a tiny eccentric (elliptical) dimension the satellite effectively cancels out many of these ‘perturbations’, albeit at the expense of small altitude changes during each revolution. The overall effect is a much more stable orbit, in turn requiring less manoeuvring by the satellite to maintain its station. That means less fuel burn and a longer endurance. Most of the earth-monitoring LEO satellites are in this kind of orbit.
(Phew! Hope the brain isn’t hurting too much after all that orbital mechanics. I should point out that I am a complete ‘newb’ when it comes to this so I am expecting lots of corrections. Fire away please!)
There is an excellent description of the basics of orbital mechanics here, and an insight into ‘space junk’ and the need to occasionally reposition satellites to avoid conflicts:
and a very cool (it really is!) 3D satellite tracker here:
In Part 2 we shall look at some of the satellite programmes that regularly contribute to volcanological research.
ASTER photo answer: Four – from top they are Shiveluch, Bezymianny, Tolbachik and Kizimen
The image is stunning in high-resolution – you can see the lava lake and glowing lava flow at Tolbachik. Take a look at http://eoimages.gsfc.nasa.gov/images/imagerecords/80000/80226/tolbachik_ast_2013011_lrg.jpg but, beware, it is a large download (8MB) and the image unzips to a whopping 200MB