The objective of this work is to investigate the Iridium L-band downlink burst signal in terms of the cross-ambiguity characterization for single burst and multicarrier multi-burst mode, to demonstrate its suitability and capability as Signal of OPportunity (SoOP) for navigation. The Iridium bursts are collected and examined by demodulating without any prior knowledge for frame verification. Then, the ambiguity function (AF) for the two modes is investigated and deduced to assess the waveform properties with resolution and sidelobes in both the Doppler and delay domains. It has been found that, the simplex burst with maximum length of 20.32ms presents 40μs delay resolution and 49Hz Doppler resolution, and its AF yields a peak to interference floor ratio of 26.64dB, which are the best capability for the single burst mode. The multichannel multi-burst mode produces a significant improvement for the time delay resolution even if the AF profile degradation should be accepted. Issues relating to the AF property improvement also are identified during this study.