## 3 Results  
### Transceiver Concept  
A transceiver, in the LRNTS sense, is a system capable both of transmitting and receiving navigation messages. Basically, it would be implemented as a compound device, consisting of:  
* a pseudolite generating GNSS navigation messages, which are transmitted via a transmitting antenna;
* a GNSS receiver, which receives the signals sent by other transceivers in the network. A conceptual diagram of such a transceiver node is given in Figure 3. Some functions are shared between the transmitter and receiver: AFS, frequency synthesizer, and CPU. The transceivers transmit and receive the signals at the same frequencies simultaneously. Therefore, interference between transmitted and received signals is likely to occur. This issue, called a near-far problem, is already recognized in terrestrial navigation applications, where augmentation of GNSS signals is needed, for example, in aircraft landing. Weak GNSS signals receive interference from strong pseudolite signals, which can completely block reception of the navigation messages by the GNSS receivers at the aircraft. Interfering signals can be processed in the transmitting and receiving phase. A latter approach is implemented, for example, in Successive Interference Calculation (SIC). This method reduces the effects of received signals with power above the ambient noise [53]. Antenna diagram shaping could help in reducing the near-far effect, which is summarized in [54]. A mitigation scheme combining two methods, Frequency Domain Excitation and Direct Reconstruction Cancellation, is used for solving for both continuous-wave and pseudolite-type interferences [55]. Narrowband interference can effectively be mitigated by temporal filtering methods, especially when the time domain and space domain methods are integrated. Such an antenna array Space Time Processing (STP) approach is elaborated in [56]. Interference between the transmitted and received signals in an LRNTS transceiver that operates in full duplex mode can be solved by using time-division multiplexing (TDM) or frequency-division multiplexing (FDM). In TDM, the transceiver alternates between transmitting and receiving at different times, effectively separating the transmitted and received signals in time. In FDM, the transceiver uses different frequencies for transmission and reception, avoiding the overlap of the signals. Furthermore, advanced filtering techniques can be used for minimizing interference between transmitted and received signals, for example, the usage of band-pass filters or notch filters, which eventually remove the transmitted signal from the received one. Additionally, using directional antennas can also reduce the amount of interference by limiting the amount of transmitted signal that reaches the receiver. The decision of where exactly to land the transceiver depends on the local terrain configuration. As stated before, the approximate locations for the transceivers were determined by examination of the 10 m DEM. The landing spot, which should be horizontal and with no significant obstacles nearby, can be selected using various landing techniques, for example, a multi-model hierarchical scene matching model [57] or an indirect scheme based on a two-point boundary value problem [58]. A method for obstacle detection during landing is elaborated in [59]. The sensitivity of the transceiver node's location does not influence the stability of the LRNTS mathematical model, which is statistically proven in latter subchapters. It means that the transceiver can be landed anywhere within a few tens of meters around the preliminary location if the spot is horizontal and without significant obstacles. The mathematical model of positioning using LRNTS, introduced in latter subchapters, assumes that both transmitting and receiving antennas of the transceiver refer to the same phase center, which is, generally, not the case.