SENSOR CHARACTERISTICS

1-3 SENSOR CHARACTERISTICS

1-31 Geometrical characteristics

The geometrical characteristics are expressed in terms of :

1-311 Accuracies of location

    Geometric quality is expressed on the basis of the particular analyses that is applied on the images : apart the local distortion, some order of priority for specifications of the different accuracies are given :
  • first, the highest priority is put on the multispectral registration for spectral analyses or use of multispectral indices like the NDVI or new indices that could be generated using the SWIR or blue bands.
  • then comes the capability to correctly locate the High Resolution pixels acquired simultaneously, relatively to the VEGETATION pixels,
  • as the temporal evolution is one of the most important feature analyzed from the VEGETATION data, the multitemporal accuracy is particularly good,
  • finally, the absolute location accuracy allow adequate positioning of each time serie on other maps or geographical information.
  • Local distortion reflects the sampling accuracy within a small area and can be expressed as a quadratic mean of differences between the actual pixel position and a reference regular position. This characteristic is consistent with the registration and multitemporal accuracies and such that the deviation for each spectral band is not more than 0.3 pixel,

     

      Fig. 131-1 : Diagram of errors to be consiudered.

  • Each individual spectral relative location for one date is represented by B0, B2, B3, B4 (blue, red, near infrared and short wave infrared spectral bands). The " mean multispectral relative location " is defined as the center of the smallest circle including all spectral measurements relative locations. The multispectral registration error is measured as the diameter of that circle : xS.

  • At one particular date, the corresponding High Resolution " multispectral pixel " is located at H relatively to its computed location X. The collocation error is the distance between H and the mean multispectral relative location : d .

    • Fig. 131-2 : Collocation.

  • At different dates, the mean multispectral relative locations is located inside a smallest circle, the center of which is defined as the " mean multitemporal relative location ". The period that is considered for the definition of that circle is one year. The multitemporal registration error is measured as the diameter of that circle : xT.

  • Finally, the absolute location error is defined as the distance between the mean multitemporal relative location and the computed location : D.

    • Fig. 131-3 : Absolute location.

  • That particular scheme for definition of geometric errors puts a higher priority on the multitemporal registration than on absolute location of each multispectral pixel. However, the specification for the absolute location error of each multispectral measurement (one particular date) is inferred from the specifications on xT.

      Fig. 131-4 : Multitemporal registration.

    The following specifications for the errors assume non biased errors and are given as the values of two standard deviations S2() (corresponding approximately to a probability of 5% to have errors larger than the specified value in case of gaussian distribution) :

  • multispectral registration specification : S2(xS) is significantly less than 0.3 km, with an objective value of 0.1 km,
  • collocation specification : S2(d) is less than 0.3km,
  • multitemporal registration : S2(xT) is less than 0.5 km with an objective value of 0.3km,
  • absolute location specification : S2(D) is less than 1 km with an objective value of 0.5 km.
  • For the blue band, as it is used mostly for atmospheric corrections, at least for the first flight model of the instrument, the specifications for xS, d, xT, and D can be relaxed to be of the order of 1km.

      Fig. 131-5 : Geometric accuracy.

    Registrations

    • Multispectral : < 0.2 km
    • Multitemporal for one year : << 0.5 km

    Absolute location < 0.8 km

1-312 Spatial resolution variation

    Resolution comparison between linear array and scanner :

     

      Fig. 131-6 : Spatial resolution variation.

    See appendice 1 for spatial resolution and sampling.

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