# A Review of Wireless Positioning Techniques and Technologies: From Smart Sensors to 6G
## 3 Wireless Technologies
### Global Navigation Satellite System (GNSS)
#### 3.1.2 Indoor GNSS Satellites  
The indoor GNSS satellites aim to utilize the GNSS signals indoors, by assuming that they are always present with lower power signals, compared to outdoors; therefore, very sensitive receivers can be deployed for capturing and retransmitting them [70]. The indoor GNSS satellites include (i) Pseudolites, (ii) Repeaters, (iii) Repealites, and (iv) Grin-Locs, which are detailed below. _Pseudolites_ are pseudo-satellites, and thus, small in size transceivers, acting as noise code generators for generating local, ground-based GNSS signals [71], as shown in Figure 13a.They broadcast their own signal structure, which includes the GPS frequencies L1 (1575.42 MHz) and L2 (1227.6 MHz). Similar to GPS, they suffer from multipath, near-far problems, as well as time synchronization issues. For instance, in [72], the positioning system utilizes at least four visible satellites without timing and relies on the navigation signal simulator and pseudolites. For diminishing the near-far effect on the receiver, pseudolite antennas that matched the GPS satellite number were deployed. The drawback of this system is the assumption that the information stored in the pseudolites is identical to the actual orbiting satellite; therefore, a range of errors arise. _Repeaters_ are used to re-broadcast the signals of the outdoor constellation indoors. A repeater is an RF switching device, which receives the outdoor signals and switches them among the multiple re-radiation antennas, one at a time. Its operation includes boosting the reception by using a reception antenna, a signal amplifier, and internal rebroadcast antennas. It consists of two antennas, the active and passive, which are connected with a direct current (DC) power, supplied directly via a coaxial cable. The active is deployed outdoors and contains a low noise amplifier, whereas the passive antenna is deployed indoors and re-radiates the signals. In [73], two scenarios were evaluated, the direct and repeater setup. The direct setup includes a direct connection of the GNSS receiver to an outdoor antenna, whereas the repeater setup includes the GNSS repeater attached to an outdoor antenna, which relays the signals to the GNSS receiver. It was concluded that the TTFF for the repeater setup scenario had a lower TTFF median time, compared to the direct connection scenario. On the other hand, the repeater scenario resulted in the introduction of additional noise and, thus, a lower carrier-to-noise ratio. The technical issues raised from using TDoA measurements for the indoor repeater-based positioning system are also discussed in [74]. _Repealites_ are terrestrial transmitters acting as a satellite and broadcast the signal of the outdoor constellation indoors. In essence, a repealite-based system is a combination of  
Figure 13: Indoor GNSS Satellites: (**a**) Pseudolites reproduced with permission from [PERSON], _Preliminary Testing of Pseudolite to Improve GPS Precision_, published by ResearchGate, 2004, and (**b**) Grin-Locs-based positioning system, reproduced with permission from [PERSON], _A GNSS-based Inverted Radar for Carrier Phase Absolute Indoor Positioning Purposes_, published by IEEE, 2016.  
pseudolites and repeaters. They use a single GNSS signal transmitted by the indoor antenna, and the different signals received from the different repealites are distinguished by adding a specific delay to each one. For instance, the system in [75] consists of three repealites, which are evaluated for the carrier phase ambiguities. The carrier phase ambiguities were estimated using the code measurements with a binary phase shift keying modulation applied to the carrier or by gathering information from the environment (i.e., code, carrier phase, and relative signal strength), aiming to eliminate the problems associated with the code measurements. It was concluded that the information from the environment approach provided better accuracy [75].