## 4 Flight Experiment Results  
Real-time flight tests were conducted to evaluate an implemented user sensor using a ground-based radio navigation system, in this case the synchronized pseudolite navigation system shown in Figure 11. In this system, the transmission times of the pseudolites are synchronized to a specific pseudolite time known as the master pseudolite time. Detailed information about time synchronization can be found in earlier work [10]. Because the master pseudolite time is a local system time, we created a log file that included both a GPS NMEA (National Marine Electronics Association) time tag and the local system time for the purpose of analyzing the navigation accuracy performance. Assuming 10 Hz data output from a sensor, the ideal log time synchronization error from the GPS time is 0.05 s, which corresponds to one meter considering the 20 m/s dynamics of the test vehicle. We did not synchronize the local system time to the  
Figure 11: The ground-based radio navigation system using the synchronized pseudolites. GPS time, because the basic idea of this local navigation system is to assume the vulnerability of the GNSS signal. Log synchronization using the GPS NMEA was only done for a performance analysis during the developmental stage. We did not use an IMU (inertial measurement unit) sensor to compensate for the lever arm between the local navigation system antenna and the GPS antenna. We can expect that a horizontal bias of less than one meter would be added to the navigation results due to the lever arm. In the case of the vertical direction, it can be corrected mostly with body-frame information, because the UAV helicopter is supposed to keep its pitch constant. As described earlier [10], pseudolites installed on the ground use dual-frequency bands as one of the near-far problem resolutions. During the flight test, the master pseudolite and the slave pseudolites uses different bands. Possible near-far problems between the slave pseudolite signals are assumed to be mitigated by the pulsing scheme. To avoid cross-correlations in the satellite navigation system, pseudolites can use the frequency offset. For pseudolite sensors using both GPS and pseudolite transmitted signals, in-band offsets at satellite-signal spectral nulls are used with a single RF frontend. Because our system is an alternative navigation system, it uses out-of-band offsets to prevent hostile energy sources from interfering with the GPS signal. The unmanned helicopter shown in Figure 12 was used to carry the ground-based navigation sensor during the flight test. A radio navigation sensor using a pseudolite signal from the ground does not have good vertical performance characteristics, because it has a relatively large vertical DOP (dilution of precision). Therefore, the cruise height of an air vehicle should be kept as high as possible so as to prevent poor condition numbers during the positioning calculations. During the flight test, we planned the height of the unmanned helicopter to be several hundred meters, because the safety of the vehicle should be guaranteed against strong wind. While the planned height was high enough to obtain the line of sight from all transmitters during the cruise mission, the expected vertical DOP was not good. The pseudolites were installed in a coastal area; the horizontal distances between them were set to dozens of kilometers. The multipath signal reflected from the sea surface was one of the critical issues to resolve before the flight test could be conducted. We attempted to install ground stations as far as possible from the coast line to avoid possible direct reflections from the sea surface, and we used a spatial diversity combining technique with the time synchronization sensors installed at the same site with the transmitters, as described in previous research [10, 17].