Fine Spatial Resolution Optical Sensors

Keywords

spacecraft, orbits, camera, CCD.

Introduction

For a long time spaceborne remote sensing, especially sensors with fine spatial resolution, remained in the military domain (Aplin et al. 1997). In 1986, a breakthrough was realized with the launch of first French satellite Satellite Pour l' Observation de la Terre (SPOT) carrying sensors with spatial resolution as fine as 10 m. Later on to re-enforce the presumed United States (US) leadership in civil satellites, the US Land Remote Sensing Policy Act of 1992 allowed the commercialization of satellites carrying up to 1 m resolution sensors, and subsequently IKONOS, the first commercial satellite (0.81 m resolution), was successfully launched in September 1999. Because more than a dozen commercial satellites have now been launched within 0.5–10 m resolution range, a better distinction between these sensors is required: fine spatial resolution (FSR) for 1–10 m and very fine spatial resolution (VFSR) for 1 m and below.

This chapter first addresses generalities on the technology of FSR/VFSR optical sensors with their associated space platforms. It thus focuses on specific FSR/VFSR spaceborne sensors in orbit in 2009 with their available image data types, products and accuracy.

Space Platforms and Orbits

This section mostly focuses on Earth observation (EO) spacecraft, artificial satellites orbiting the Earth, carrying FSR/VFSR optical sensors. EO satellites obey the celestial mechanical laws as defined by Newton and Kepler for an unperturbed trajectory (Keplerian orbit) and by Gauss and Lagrange for a perturbed trajectory (osculatory orbit) (Escobal 1965, Centre National d' Études Spatiales 1980). A number of perturbations (Earth surface irregularities, atmospheric drag, etc.) slowly change the Keplerian orbit based on the two-body attraction of Newton's law into an osculatory orbit (Centre National d' Études Spatiales 1980). Information on orbits is often needed and orbital models are used depending of their utility and required accuracy (Bakker 2000):

• 1
  to calculate the satellite location on its osculatory orbit to compute Earth coordinates of scanned pixels, requiring high accuracy (metres) over a small time frame (seconds);
• 2
  to predict when the satellite will pass over a specific area, requiring low accuracy (km) but over a long time frame (days).

Many orbital models have been developed since 1960 using the same mechanical laws with Gaussian/Lagrange equations but the differences between the orbital models are mainly in the number and types of perturbations and the techniques to integrate them. As defined and adapted by the North American Aerospace Defense Command, Simplified General Perturbations (SGP), SGP4 and most-accurate SGP8 are the orbital models to be used for low and near Earth satellites (orbital period less than 225 min and altitude less than 6,000 km), while SDP4 and most-accurate SDP8 for highly-elliptic orbits are for deep-space satellites (orbital period greater than 225 min and altitude over 6,000 km). Other orbital models also exist to fulfil different requirements or for specific satellites.

An important factor for an artificial satellite is its final orbit because each orbit has associated advantages and disadvantages. The orbit altitude determines the sensor resolution, for example, DigitalGlobe decided to reduce the QuickBird-2 orbit altitude to achieve 0.61 m resolution instead of the 1 m resolution originally envisaged, without changing sensor characteristics. The orbit inclination determines the percentage of Earth that can be imaged (mainly in the highest latitudes), while other orbit variables determine the repeat cycle and the forelap depending on latitude. Consequently, orbit selection and sensor characteristics are closely related. Most, if not all, of the commercial EO spacecrafts have near-Earth, retrograde, quasi-circular, quasi-polar, geosynchronous and sunsynchronous orbits.

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