Algorithm Development for Localization using Time Difference of arrival and its Hardware implementation

Project Title

Algorithm Development for Localization using Time Difference of arrival and its Hardware implementation

Project Area of Specialization Electrical/Electronic EngineeringProject Summary

Transmitter localization is a both interesting and challenging task. Besides applying triangulation in combination with some type of direction finding receivers (using e.g. directional antennas, pseudo Doppler or the interferometer principle), the Time-Difference-of-Arrival (TDOA) method is widely used. In TDOA three or more (non-directional) receivers at different locations capture the unknown transmitted signal. Then it evaluates the differences in arrival time of the signals to calculate the transmitter’s position (multilateration).

Assume a signal is emitted by an (unknown) transmitter and is received by several receivers at different locations. Usually the signal arrives at different times at the different receivers due to the varying distances between transmitter and receivers. This difference in arrival time is called “TDOA”. A TDOA value can be measured between a pair of receivers. It should be emphasized, that we work on the time difference of arrival, since any absolute arrival times in relation to transmission times are in general not available (as opposed to other localization techniques like time-of-arrival, TOA).

Project Objectives

The objective of this project is to localize (pinpoint) any radio frequency transmitter within the city with the help of three receivers.

Project Implementation Method Establishing the SSH connections between the receivers and the master

In order to be able to start the reception of all receivers at least roughly at the same time (recommended is within 100 ms), it is required to configure SSH for key-based authentication instead of the default password-based login. For that purpose the user at the master (client) generates a pair of keys once. Then the public key is copied to the Rasperry PIs (server).

Raspberry PI based Receivers:

• 3 Raspberry PIs with SD cards (at least 8 GByte)
• 3 RTL SDR sticks (v3)
• SD card image for the Raspberry PI and librtl

Master PC:

• Linux PC or Windows PC with a virtual linux machine running 
• internet connection to receivers with SSH
• Matlab/Octave processing scripts 

A prerequisite for TDOA localization is the synchronization of the receivers. One possibility is to use a GPS receiver, that disciplines the Raspberry PI’s internal clock. However, the system utilizes a different method using a transmitter with known location as a reference, that is received well by all three receivers.Synchronization with a reference receiver works as follows: When the master triggers a measurement, each receiver first receives some samples of the reference receiver and then seamlessly switches to the unknown transmitter’s signal of interest. For synchronization, the master aligns the three receptions from the different receivers to each other on the time axis. Alignment is done in such a way, that the delay in the reference signal portion corresponds to the known distances of the reference transmitter to the receivers. Once the complete received signal is synchronized (or aligned) the TDOA of the unknown signals is simply calculated as the delay. Precisely spoken, this technique does not synchronize the receivers themselves, but rather the received signals.

System Operation

With the correlation function available for measuring the delay or TDOA and having synchronized the receptions, the overall signal processing for TDOA localization can be summarized:

1. Receive signals, transfer them to the master and consider them pairwise for further processing
2. Synchronize the received signals
   1. Calculate correlation of reference signal portion
   2. Discard invalid peaks of correlation function
   3. Use measured delay to synchronize
3. Measure unknown signals
   1. Calculate correlation of unknown signal portion
   2. Discard invalid peaks of correlation function
   3. Determine TDOA in samples and distance
4. Calculate hyperbola using geometry
5. Create a map with html and javascript (Google maps or Open Street Map) to display the results

Benefits of the Project

It can be used for localization of different transmitters of various signals e.g a DMR signal at 439 MHz, a mobile phone signal at 922 MHz, a FM signal at 96.9 MHz and an unknown signal at 391 MHz. It shows that localization is possible even with this simple setup.

Technical Details of Final Deliverable

• This project gives an introduction to time-difference-of-arrrival (TDOA) based localization of transmitters and presents a simple practical system using three RTL-SDRs to localize signals in a city.
• It will create a map with html and javascript (Google maps or Open Street Map) to display the results.

Final Deliverable of the Project HW/SW integrated systemCore Industry ManufacturingOther IndustriesCore Technology OthersOther TechnologiesSustainable Development Goals Industry, Innovation and InfrastructureRequired Resources

Item Name	Type	No. of Units	Per Unit Cost (in Rs)	Total (in Rs)
			Total in (Rs)	69943
raspberry pi 4	Equipment	4	10000	40000
memory card	Equipment	4	900	3600
raspberry pi chargers	Equipment	4	600	2400
HDMI cable	Equipment	3	150	450
RTL SDR V3	Equipment	3	7831	23493