where [N.sub.F] and [mathematical expression not reproducible] denote the navigation gain in the final phase and the look angle at the switching instant.
Considering the variation of look angle under the PN guidance in the initial phase, the look angle should satisfy the condition: [N.sub.I] < 2, [sin.sup.-1] [eta] < [[sigma].sub.M] < [cos.sup.-1] (2[eta]/[N.sub.I] - 2), or [N.sub.I] >2, [[sigma].sub.M] > max([sin.sup.-1] [eta], [cos.sup.-1] (2[eta]/2 - [N.sub.I]).
For initial guidance phases, the condition of [N.sub.I] < 1 and [mathematical expression not reproducible] can ensure that the look angle will increase from [mathematical expression not reproducible] to the limit value [mathematical expression not reproducible] during the time of ([t.sub.s] - [t.sub.0]), monotonously.
For each look angle measurement, the frequency of the VNA was changed from 1 to 8 GHz for a total of 100 discrete frequency points as in the case of the simulation.
We also notice the scattering from cavity that experiences the same behavior of the tree's body as the look angle varies during the measurement.
Based on the above assumptions, then, (1) can be rewritten as a summation over scatterer and provides a twodimensional (2D) data by considering different look angles as the following for a finite number of point scatterers (M) that form the tree-body:
It can be seen from Figure 4(d) that look angle of APN increased and stabilized at 21.5.
It can be seen from Figure 5(d) that look angle of APN increased reversely to -16[degrees].
In most cases, look angle of proposed law is small.
(ii) The coverage of radar cannot provide valid echoes with a zero-Doppler centroid when the look angle is less than the minimum of the range.
For the above issues, this paper introduces a general approach to compensate the Doppler centroid frequency for the spaceborne SAR which works with a look angle within an effective range.
where [phi] is the look angle of radar (shown in Figure 1); [R.sub.s](t) and [R.sub.t] are the magnitudes of [R.sub.s](t) and [R.sub.t](i), respectively.