Position Estimate Using Radio Signals from Terrestrial Sources

The satellite navigation is supposed to be used in applications that need coordinates generally. However we can meet a plenty of satellite signal reception problems in a real environment, often called as difficult conditions. The difficult conditions hinder reliable positioning with required accuracy, often in applications that are important for saving or securing the safety of human lives (work of rescue teams, protection of people in large warehouses, safety of lone forest workers, etc.). The main reason for this is the weakness of the received satellite signals. In addition to that, the weak signals are also highly vulnerable by interference, spoofing or jamming, even with the low-power and often generated by low-cost devices. In spite of this, radio systems complementing and making the backup of the satellite positioning are searched. There are terrestrial radio systems using high-power signals with properties which are suitable for the positioning purposes. The most important property in conjunction with the sufficiently high power is a very sharp and possibly unambiguous correlation function. Besides the signals of systems used primarily for the navigation (such as eLORAN, e.g.) signals of some systems primarily dedicated to communication have the acceptable properties mentioned above. They are usually called Signals of Opportunity. As an example let us mention signals of the DVB-T, LTE, Wi-Fi, etc. In the field of indoor navigation signal strength fingerprinting is frequently used. However, for much larger open areas this approach is not the best one, because it requires a kind of a site survey to be done. In case of outdoor applications, the use of different principles has to be considered. The methods based on signal power and angles of arrival measurements have been found unsuitable because they may be misleading even in a lightly obstructed area. Range estimation using packet round trip delay (similar concept to the DME ranging used in aviation) is not possible because most of the systems do not have a stable response delay. However, some systems with synchronous transmitters (or base stations) can be used by the way of hyperbolic navigation concept. Luckily many of the systems with high transmitted power, such as TV broadcast or cellular data systems downlink have the precise transmission timing incorporated. This contribution describes utilizing the OFDM Signal of Opportunity for the positioning purposes. The proposed method is based on the peak search in an estimated radio communication channel model. The time differences of arrivals of signals from individual transmitter pairs can be measured in this way. At least two independent measurements must be used for the final two-dimensional position estimate by the iterative Levenberg-Marquardt algorithm. With three independent measurements the 3D positioning is available. Because the measured differences do not have generally to share the common timescale (only the measurement pairs have to), combining of more independent systems together is possible by nature of our approach. The validity of the above mentioned theoretical concept is demonstrated and verified by means of an experiment on real captured DVB-T (OFDM modulated) signals. Moreover, an analysis of transmitter geometry influence on the positioning accuracy is also shown and the graphical output for the real scenario is provided. Considering these results, future goals and improvement possibilities are formulated.