Hybrid geolocation systems

Locating transmitters is an essential mission for all regulators and organizations that are assigned radio monitoring tasks. This pertains especially to unlicensed and illegal transmitters as well as stations interfering with other radio services. Traditionally, direction finders (DF) have been used for location tasks. New solutions, based on the time difference of arrival (TDOA) principle are an excellent complement to classical DF systems.

TDOA – the concept

Prerequisite for a TDOA-based location is a sufficient number of receivers in the vicinity of the transmitter. Since these receivers are at slightly different distances from the transmitter, the same signal reaches them at slightly different times. Based on these time differences, the location of the transmitter is calculated.
Given that electromagnetic waves propagate at approx. 300 000 km/s, the time accuracy of the system has to be in the nanosecond range.

TDOA – The Rohde & Schwarz way

Rohde & Schwarz has deliberately decided to implement TDOA as an optional add-on to the current generation of receivers and direction finders. This approach has numerous advantages. The most important ones are:

  • The excellent sensitivity of the system provides a high signal-to-noise ratio (SNR). This leads automatically to higher location accuracy. Furthermore, higher SNR compensates inaccuracies due to smaller signal bandwidths. Thus, narrowband signals are located much more precisely or even found at all. Thanks to the superior overall performance of the equipment, it is extremely well suited for operation in dense urban environments. There, TDOA is very advantageous but the signal scenario is very challenging for the devices.

  • TDOA, angle of arrival (AOA) and even a hybrid solution are possible, offering always the best option for each task.In the case of direction finders, the same device can even serve as a TDOA receiver or classical DF.

  • The equipment is available for sophisticated, ITU-compliant monitoring tasks when no location mission is running. While locating transmitters is an important task, experience shows that only a minor fraction of time is spent for it. The system is used to the greatest possible extent.

  • The time a signal takes to travel within the device from the antenna input to the signal processing stage is taken into account when issuing the timestamp. This unique Rohde & Schwarz development provides higher location accuracy. In addition, it makes it possible to freely combine all TDOA-capable equipment.

Additional benefits:

Existing devices can easily be extended. Thus, there is no need to find additional monitoring sites, a task that becomes increasingly challenging. In addition, existing infrastructure like power supply and communications for remote control is already available. The upgraded station is immediately ready for the next location mission.

Rohde & Schwarz offers the broadest portfolio of TDOA-capable devices and systems on the world market. A large range of indoor models as well as various outdoor systems are ready for operation, and they can all be flexibly combined. Thus, customers can choose the equipment according to their most important tasks.

Hybrid: TDOA and AOA – the best of both worlds

Both principles, TDOA and AOA have their respective advantages. TDOA works well with broadband emissions, typical for modern communications signals like DVB-T, DAB, LTE, or WiMAX™. The concept of TDOA even allows compensating effects of reflection and multipath propagation, making it especially powerful in dense urban environments. AOA has its strengths when it comes to narrowband, unmodulated or continuous-wave (CW) signals.

Consequently, the perfect system would integrate TDOA and AOA, where the operator decides case by case which one is better suited or even selects a combination of both. That is exactly what the new hybrid TDOA-AOA solutions from Rohde & Schwarz provide:
the best of both worlds.

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