## 1 Introduction  
GNSS (Global Navigation Satellite System) receivers are currently the most widely-used space-based PNT (positioning, navigation and timing) sensors. From infrastructure, such as the time-keeping systems of wireless telecommunication devices, to mobile devices or military weapons, they are now indispensable. However, the vulnerability of the satellite navigation systems has been an important issue for more than a decade, as GPS sensors must process signals with an extremely low power level. The seriousness of this vulnerability can be found in a statement issued by the Interagency GPS Executive Board, which reads \"GPS users must ensure that adequate independent backup systems or procedures can be used when needed\" [1]. For the past few years, there have in fact been attacks on the GPS L1-band close to the boarder of Korea, with one article stating that \"aircraft had to rely on alternative navigation aids\" during the attacks [2]. The ground-based radio navigation system introduced in this paper is an alternative/backup navigation system, which can overcome the vulnerability against intentional interference attacks through its use of a pseudolite network. Before the launching of GPS (Global Positioning System) satellites, pseudolites were used to test transmitters at the desert test ranges [3]. For the last twenty years, many researchers have extended the applications of these systems to various areas, including indoor positioning, the Mars navigation system and regional positioning systems [4; 5; 6; 7; 8; 9]. The ground-based radio navigation system has been studied as a regional positioning system of which the main applications are air vehicles. This type of system belongs to the synchronized pseudolite navigation system category, and these systems have a better system survivability than those in the asynchronized system category [4; 10]. In a CDMA (code division multiple access)-based system, users share the same frequency channel. This means that users can fail to acquire weak signals if there are strong signals from closely-located transmitters. This is known as the near-far issue. To avoid the near-far problem in the pseudolite sensors, previous researchers suggested a scheme that involved frequency offsetting and pulsing [11; 12]. We applied these two methods to avoid the near-far issue in our study. By applying the pulsing, we can allocate time slots to each signal, which means that there are no more collisions between signals. Earlier suggestions, such as RTCM SC-104 and RTCA (radio technical commission for aeronautics) 2000, recommended a randomized sequence, because the pulsed signal may have an aliasing effect on the navigation sensor. Because an additional frequency term caused by a periodic pulse could lead to a possible false lock in the tracking loop, they recommended accumulation of the received signal samples with a randomized pulse position. If the accumulation-and-dump time is long enough to use all of the sequence chips, received signal samples will appear to be continuous in the accumulation-and-dump filter. However, this method requires a long dwell time (e.g., at least 10 ms [11]), and the time is typically too long for some applications with high dynamics. In this study, we designed pseudolite transmitters with fixed pulse positions, for which the duty ratio is defined as 10%. We also suggested another algorithm that detects possible false locks in the navigation sensor. First, we introduce a mathematical model of the accumulation-and-dump filter output for a periodic pulsed signal, after which we explain why a false lock can occur. Secondly, two algorithms are suggested to detect a false lock in the frequency domain. After detecting a false lock with the multi-correlator characteristics, the algorithm performs a Doppler correction in a signal processing module. The implemented firmware was verified in the flight test with an unmanned helicopter.