The Altimeter Measurement Measurement Description

The first point to make about satellite altimeter data is that they are not simply the product of an instrument, but come from a measurement system. Several different observations need to be made to produce high-quality sea surface heights, and maintaining the quality of the final height observations requires high performance by all components of the system. As we proceed through this discussion of the various components, we will also briefly address the errors associated with each component, paying particular attention to whether a given component of the system might be prone to drift errors.

For the purposes of this discussion the system is separated into four basic components, illustrated schematically in Fig. 6.1. First there is the basic measurement of the distance from the satellite to the sea surface, which is called the satellite range and is so labeled on the figure. Second, to interpret this range as a height relative to some stable reference system, we also need to know the orbital height of the satellite in that reference frame, the zero point of which is labeled as the reference ellipsoid on Fig. 6.1. Third, one must realize that the satellite range is actually inferred from the travel time of a microwave radar pulse sent out from the altimeter, reflected from the sea surface, and then received again at the altimeter. To do the conversion from time delay to distance, we need to have an accurate value for the effective index of refraction along the path from the altimeter to the sea surface. Thus, the system requires measurements necessary to estimate the index of refraction of the atmosphere (troposphere and ionosphere). Fourth, since the altimeter's radar pulse actually samples an area of the sea surface that is relatively large (order of square kilometers), there is some ambiguity associated with how this average corresponds to mean sea level. Biases due to surface waves and the tidal phase must be considered in order to make a proper interpretation of the satellite range. These effects will be discussed together under the term surface effects. After discussion of these various components, this section will conclude with a brief summary of the errors in the overall system.

Satellite position and path

Satellite position and path

Altimeter Measurement

Figure 6.1 Schematic of the measurement geometry for satellite altimetry. The satellite's instantaneous position is shown by a large solid circle, and the smooth curve passing through this point represents the satellite's orbital path. The instantaneous shape of the sea surface is shown by a solid curve at the bottom of the figure and the reference ellipsoid is shown as a dashed line. The shaded ellipses represent the ionosphere and the troposphere. Note that these portions of the atmosphere are not to scale.

Figure 6.1 Schematic of the measurement geometry for satellite altimetry. The satellite's instantaneous position is shown by a large solid circle, and the smooth curve passing through this point represents the satellite's orbital path. The instantaneous shape of the sea surface is shown by a solid curve at the bottom of the figure and the reference ellipsoid is shown as a dashed line. The shaded ellipses represent the ionosphere and the troposphere. Note that these portions of the atmosphere are not to scale.

Understanding how altimeter range measurements are collected and converted to sea level measurements is conceptually straightforward, although the details are complex. The satellite transmits a radar pulse toward the ocean surface. After passing through the atmosphere, the pulse arrives at the atmosphere/ocean boundary, interacts with the ocean, and is then reflected back toward the satellite, again passing through the atmosphere. If we ignore atmospheric effects on the pulse propagation and the interactions between pulse and the ocean surface, the range h can be found by measuring the difference between the time of pulse transmission and the pulse arrival back at the satellite, multiplying by the speed of light, and dividing by 2:

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