Platforms and orbits
Platforms are ‘vehicles’ on which sensors are mounted to emit and record EM radiation. Two common platforms in remote sensing are aircrafts (both manned and unmanned) and satellites.
Aircrafts can be operated at altitudes up to 20 km. They are mostly used to acquire aerial photographs. The advantage of using aircrafts is that high spatial resolution images (down to 5×5 cm pixels) can be acquired for a targeted region at a particular time. The disadvantage is that images normally cover small areas in extent and are expensive to acquire. The use of an aircraft as a platform is normally preceded by flight planning, where pre-defined routes for the aircraft movement, aircraft altitude, dimensions of photographs, etc. are defined. More details on aerial survey missions can be found here.
In recent years, the use of Unmanned Aerial Vehicles (UAVs) has become popular. We refer to the UAV section for a more detailed discussion of the options that this technology provides.
When one wants to monitor larger areas (even with global coverage) and/or obtain synoptic views of the same target/area, satellites are normally the platform of choice. Many of the commercial remote sensing images are acquired by sensors on board of satellites. A rocket is used to launch one or more satellites into space where the satellites are placed in a predefined orbit to image the earth for some period of time. Various parameters characterize a satellite’s orbit. These include: orbital altitude, inclination angle, period, repeat cycle and type. These orbital parameters to a large extent determine the monitoring capabilities of the satellite. We discuss them below.
Orbital altitude refers to the height (vertical distance) of the satellite above the earth’s surface. Most commercial satellites orbit at an altitude of 500–1000 km. Orbital altitude influences, to a very large extent, the spatial coverage and resolution of the resulting image. In most cases, the higher the orbital altitude of a satellite, the larger the spatial coverage (the area that it covers on the earth) and the lower the spatial resolution of the resulting image. Satellites operating from a relatively low orbital altitude are mostly preferred for agricultural applications, especially in smallholder systems, as they provide more spatial detail (they have higher spatial resolution) and this improves chances of identifying the typical small fields. Images from DigitalGlobe satellites, which were mostly used in the STARS project, were taken at orbital altitudes of between 450 and 770 km (see Table 3.1).
Orbital inclination angle is defined as the angle (in degrees) between the orbital plane and the equatorial plane. Together with the field of view of the sensor, the inclination angle determines the earth’s latitudinal extent that a satellite can image. The inclination angle of a satellite is set in accordance with what it was launched to monitor. For example, satellites with very low inclination angles (e.g., 300) may be intended to monitor areas around the tropics only, i.e., areas between latitudes 300 south and 300 north. Well-known commercial sensors such as Landsat and DigitalGlobe’s ranges of satellites have orbital inclinations of larger than 900. This means that they observe all areas between latitudes 900 south and 900 north.
Orbital period is the time (in minutes) that it takes a satellite to complete one full orbit. This has implications for the life span of the satellite (i.e., determines number of orbits), but it also determines image quality, temporal resolution and spatial resolution of the resulting images. The orbital period of most commercial sensors ranges between 90 and 100 minutes.
Repeat cycle is defined as the number of days between two successive, identical orbits from a set of identical satellites. In other words, it refers to the frequency at which EO sensors acquire images of the same portion/part of the earth’s surface. Repeat cycle is typically measured in days. Satellites that have high repeat cycles (i.e., high acquisition frequency) produce, for the same period, more images than those with a low repeat cycle. For agricultural purposes, satellites with high repeat cycles are preferred, as chances of getting more images during the cropping season are improved. There is always a trade-off between repeat cycle and spatial resolution. Satellites with high repeat cycles mostly produce low spatial resolution images, and vice versa. This problem is gradually being solved by new satellite technology that strives to achieve both. Examples are RapidEye and SPOT6/7.
Orbital type Based on the orbital properties above, three common orbital types can be distinguished: polar, sun-synchronous and geostationary. These are described below.
Polar orbits have an inclination angle between 80 and 1000. By virtue of this angle, these satellites observe virtually the whole globe. DigitalGlobe’s range of satellites falls within this category, and so do the Landsat satellites and many other commercial satellites. Satellites with this orbital type normally have an orbital altitude between 600 and 1000 km.
Sun-synchronous orbits appear to be in the same position from the perspective of the sun always. Satellites in sun-synchronous orbits have high inclination angle and cross the equator at the same local time in every orbit. Most satellites in this orbit cross the equator at around 10.30 am local time. This is probably an optimal timing for combination of good sun illumination and cloud cover. The satellite has the capability to record night time images (which results in thermal bands) on the return leg of its orbit. Landsat, SPOT, IRS and DigitalGlobe’s range of satellites are all in sun-synchronous orbits.
Geostationary: satellites that have an inclination angle of zero (i.e., they are placed above the equator) and orbital altitude of an approximate (and very high) 36,000 km are said to be in geostationary orbit. Satellites in this category have an orbital period of one sidereal day, which is equal to the rotational period of the earth around its own axis. The altitude of these satellites leads to low spatial resolution but high spatial coverage for the resulting images. Most satellites in this orbital type category are used for meteorological and telecommunication purposes. Images from such satellites are not directly useful for smallholder agricultural applications, though obviously weather data is important to farmers.