Ionospheric effects on linear FM rectangular envelope signals of space-borne SAR are considered in this paper. Two kinds of effects due to the ionosphere are presented and the relative formulas are derived

including compressed pulse shift and second phase error. It can be seen that the compressed pulse shift is proportional to total electron content （TEC） and inverse proportional to the square of signals frequencies

and that the second phase error is proportional to TEC and the square of signals bandwidth while in inverse proportional to the square of signals frequencies at the same time. The ionospheric effects at P

L

C bands are also investigated by comparison based on the formulas derived above. The result shows that signals at P-band will be severely affected by the ionosphere. The effects will be obvious at L-band if the signals have a large bandwidth or TEC values are high. At C-band

ionospheric effects are neglectable.