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Using the Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM) data covering the region of the Three Gorges Reservoir, Changjiang, China, we have computed the water volume, length, and total and inundated areas of the reservoir, with the assumption that the water surface within the reservoir is flat. When the reservoir's surface water level is 175 m above the mean sea level, the computed values may be comparable to the official data published by the Chinese government.

Ambiguity estimation is critical for the positioning of a moving target truly. Traditional real-value Radon transform (RRT) has been used to estimate the slope of target's trajectory such that the ambiguity number can be derived. However, the unknown azimuth velocity of the target makes it difficult to determine the ambiguity number because the quadratic range cell migration (QRCM) caused by the platform velocity reduces the sensitivity of the RRT in the estimation. Also, the RRT does not work well when the signal-to-noise ratio (SNR) is low. Here, a method that uses the second-order Keystone transform (SOKT) to eliminate the QRCM and the modified fractional Radon transform (MFrRT) to estimate the ambiguity number was proposed. The method was simple and applicable in the low-SNR situation. Implementation considerations were presented. Finally, the effectiveness of the method has been shown using simulated and acquired synthetic aperture radar datasets.

To improve further the accuracy in the approximation of the range history for a synthetic aperture radar (SAR) onboard a mediumearth-orbit (MEO) satellite a fourth power term has been added to the advanced hyperbolic range equation (AHRE), and the new one is called a modified AHRE (MAHRE). Then, a two-dimensional spectrum using the MAHRE was derived, and the accuracy of the spectrum analysed. Promising results were obtained. Introduction:The integration time of one synthetic aperture of a medium-earth-orbit (MEO) synthetic aperture radar (SAR) can be long or a fine spatial resolution can be achieved. However, the long integration time means that the approximation in range history with a straight flight path within one synthetic aperture could become invalid. Thus, the use of a typical hyperbolic range equation can be questionable. An accurate but potentially complicated equation is needed for the processing of the MEO SAR data. Eldhuset [1] approximated the relative earth and satellite motion using a fourth-order Taylor expansion of the equation in azimuth time. Although Eldhuset achieved accurate range approximation, one drawback was the complicated algebraic operations involved in the development of imaging algorithms for data processing with the approximation. Then, Huang et al. [2] studied an advanced hyperbolic range equation (AHRE) consisting of terms up to the third order of the range equation in [1] and a linear term. They simplified the range equation of [1]. Using the AHRE, Huang et al. derived the 2D spectrum and then developed an advanced nonlinear chirp scaling (ANLCS) algorithm [3]. Closely examining the AHRE, one notices that the AHRE only compensates for the range history up to the cubic term. Under some circumstances, the compensation might not be enough or the level of accuracy might be inadequate. To increase the accuracy level, we modify the AHRE with an addition of a fourthorder term or modified AHRE (MAHRE). This addition is not simply a roll-back to the original range equation in [1]. Instead, the addition is directly applied to the AHRE. Thus, the MAHRE is still concise in expression.

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