# Positioning Using IRIDIUM Satellite Signals of Opportunity in Weak Signal Environment
## 5 Experimental Results
### Positioning Based on Iridium Satellite SOPs in Weak Signal Environment  
In this section, we realize the receiver positioning based on real collected IRIDIUM NEXT. The IRIDIUM NEXT signal Doppler-shift estimation results are given, then the positioning results based on IRIDIUM NEXT SOPs in weak signal environment is carried out. The entire collected IRIDIUM NEXT data (30 min) are divided into 5000 continuous data blocks (the time length of every data block is 360 ms), and the Doppler-shift will be estimated if the data block contains the Iridium burst. The Doppler-shift estimation results obtained by the traditional method are shown in Figure 8a,b. The Doppler-shift curves of the ring alert signal (downlink-only 7 channel)  
Figure 6: FFT results of the quadratic square IRIDIUM ring alert signal. Figure 7: Correlation results between local carrier and real signal. is shown in the left plot and the Doppler-shift curves of the primary message signal (downlink-only 11 channel) is shown in the right plot. The x-axis is the data block number and the y-axis is the Doppler-shift (the unit is Hz). Every circle in the plot denotes the Doppler estimation result from the corresponding data block. Generally, the Doppler-shift curve of the satellite is a kid of S-shape curve and the different S-shape curves belong to a different satellite, the Doppler value close to zero means the satellite is located near the top of the receiver, and the Doppler-shift value close to dozens of kHz means the elevation of the satellite is low. We can see that only two IRIDIUM NEXT satellite signal Doppler-shift estimation results can be obtained by the traditional method, which means in this case only two satellites can be seen by the receiver. In this case, when the two IRIDIUM NEXT satellites are near the top of the receiver, the Doppler-shift can be estimated by the traditional method since the signal is strong enough. However, the number of the Doppler-shift estimation results is very small, which means only several strong signals (average power is about 4 dB higher than for noise) can be used by the traditional method. Most signals are weak due to the occlusion environment and fail to estimate Doppler-shift by the traditional method. In this case, when the static receiver position is calculated by multi-epoch positioning method, the geometric dilution of precision of the instantaneous Doppler positioning algorithm is very negative since the satellites are in the same orbit and few Doppler-shift measurements are obtained. As a result, receiver positioning cannot be achieved since the algorithm does not converge. The QSA-IDE algorithm can estimate the IRIDIUM NEXT signal Doppler-shift in weak signal environment, as shown in Figure 9a,b. The meaning of the x-axis and y-axis in Figures 8 and 9 are the same. The Doppler-shift curves of the ring alert signal is shown in Figure 9a and the Doppler-shift curves of the primary message signal is shown in Figure 9b. We can see that four IRIDIUM NEXT satellites can be seen in this case, and both ring alert signal Doppler-shift curves and primary message signal Doppler-shift curves are more optimistic than for the traditional method. The Doppler curves in Figure 9a,b have exactly the same profile since the signals corresponding to every pair of curves belong to the same satellites. The new QSA-IDE method not only can see more Iridium satellites but also significantly increases the visible times of every satellite. The low elevation satellite signal Doppler-shift is also estimated successfully. The first satellite just begins to be seen at low elevation. These four satellites are located at the same orbit since the Doppler-shift curves of all these four satellites have the same shape. Although only one satellite can be viewed simultaneously under the occlusion environment, the Doppler-shift measurements of these satellites are enough for static receiver positioning by the multi-epoch positioning method.