Understanding GNSS Interference Localization Using TDOA

Localizing sources of GNSS interference is important for effective countermeasures. It enables timely identification and mitigation of interference sources, ensuring the stability and accuracy of navigation systems. Since most GNSS interference signals are wideband, the Time Difference of Arrival (TDOA) method is particularly effective for their localization. Wideband signals are well-suited to correlation analysis, which is essential for accurately determining the time of signal arrival and ultimately pinpointing the interference source. This precision makes TDOA a powerful tool in maintaining the integrity of GNSS-dependent systems.

Time Difference of Arrival (TDOA) is a method used to locate an interference source by measuring the difference in the arrival times of a signal at multiple points (probes). When interference originates from a source, the signal reaches each reception point (antenna) at slightly different times due to varying distances from the source. By knowing the time differences, it’s possible to calculate the source’s position. This method requires precise synchronization of all reception points to measure arrival times accurately and determine the coordinates of the interference source.

• Geometric Arrangement of Probes and Interference Source
  The spatial configuration of probes relative to the interference source significantly impacts localization accuracy. When the source is positioned within or near the triangle formed by the probes, the TDOA system achieves high precision. However, if the source is located outside this triangle, especially along the lines extending through two probes, accuracy diminishes due to the effect known as Geometric Dilution of Precision (GDOP).

The image illustrates the concept of Geometric Dilution of Precision (GDOP) in a TDOA-based interference localization system. The three red triangles represent the positions of the receivers, and the color gradient shows the accuracy of position estimation across different areas. In the central area within the triangle, where accuracy is highest. Outside this zone, accuracy degrades significantly. This visualization highlights the importance of placing probes strategically and ensuring that the interference source lies within the coverage area to achieve optimal localization accuracy.

• Time Synchronization Between Probes
  Accurate measurement of signal arrival time differences requires precise time synchronization among all probes. Even minor discrepancies can lead to significant errors in determining the interference source’s coordinates. Utilizing high-stability clocks synchronized with GNSS helps minimize these errors.
• Signal Bandwidth
  Wideband signals allow for more precise determination of arrival times, thereby enhancing TDOA accuracy. Conversely, narrowband signals may complicate arrival time estimation and reduce localization precision.
• Signal Strength and Signal-to-Noise Ratio (SNR)
  Higher signal strength and SNR facilitate accurate arrival time determination. Low SNR can make signal detection challenging and decrease measurement accuracy.
• Multipath Effects and Reflections
  Signal reflections from buildings, the ground, and other objects can cause multipath propagation, leading to errors in arrival time measurements. These effects are particularly pronounced in urban environments and can significantly degrade localization accuracy.
• Number of Probes
  Deploying a greater number of probes can improve localization accuracy, as additional data points help compensate for errors and reduce uncertainty.

Predicting exact accuracy under varying conditions is challenging due to the influence of multiple factors. However, if the interference source is within the coverage zone, an accuracy range of 100–200 meters can generally be expected.

GPSPATRON offers a fully automated system for detecting and localizing GNSS interference using Time Difference of Arrival (TDOA) technology. This system operates without user intervention, providing real-time and historical geolocation results accessible via an intuitive map interface.

Required Hardware and Options

To implement TDOA with GPSPATRON, the following components are essential:

• At least three GP-Probe TGE2-CH3 4G RFSA: A high-performance, three-channel GNSS probe equipped with an integrated RF signal analyzer. This device is designed to detect, classify, and localize GNSS spoofing and jamming.
• TDOA Software Option: An additional feature for the GP-Probe TGE2 that enables the transmission of raw IQ data to the GP-Cloud for interference localization using the TDOA method.

• GP-Cloud Platform: A web application that processes data from GP-Probes, performing real-time analysis to detect and classify GNSS interference, and localize sources using TDOA algorithms.

How It Works

1. Detection: The GP-Probe TGE2 continuously monitors GNSS signals on three channels and sends the data to GP-Cloud. When interference such as jamming or spoofing is detected, GP-Cloud sends commands to all nearby GP-Probes to send raw IQ samples from the built-in RFSA to the server.
2. Data Collection: With the TDOA option enabled, the probe captures raw IQ data, precisely synchronized with the Pulse Per Second (PPS) signal from its internal GNSS receiver. This synchronization ensures accurate time-stamping of the captured data.
3. Data Transmission: The probe transmits the synchronized raw IQ data to the GP-Cloud platform via its built-in 4G modem or Ethernet connection.
4. Analysis: GP-Cloud processes the received data using TDOA algorithms to calculate the time differences in signal arrival across multiple probes. By analyzing these time differences, the system triangulates the probable location of the interference source for each timestamp.
5. Visualization: The results are presented as heatmaps within the GP-Cloud interface, visually indicating areas with the highest likelihood of interference origin. This allows users to monitor and respond to GNSS interference events effectively.

As we cannot predict the exact accuracy of interference localization for your specific case, you can refer to our engineering parameters that influence localization accuracy. These key specifications provide insights into the factors impacting TDOA accuracy, helping you better understand the potential performance in your setup.

1. RF Signal Analyzer ADC IQ Rate
   • Specification: 12-bit, 60 MSPS
   • Impact: A higher sampling rate increases the bandwidth, enhancing the system’s resolution for correlation processing, which is critical for TDOA accuracy. By capturing signals in finer detail, it allows for more precise time difference measurements across probes. This parameter is fundamental, as it directly impacts the system’s performance. When the data is sent to the cloud, it is compressed to 4-bit, reducing bandwidth usage while retaining essential signal information for accurate interference localization.
2. Noise Figure
   • Specification: 6 dB, Max
   • Impact: A low noise figure enables more accurate time difference measurements for weak signals by reducing the impact of internal noise.
3. Local Oscillator (TCXO) Specifications
   • CF: 40 MHz
   • Frequency Stability: ±50 ppb over -40ºC to +85ºC
   • Phase Noise: -95 dBc/Hz @ 10 Hz, -120 dBc/Hz @ 100 Hz, -140 dBc/Hz @ 1 kHz, -150 dBc/Hz @ 100 kHz (10 MHz carrier)
   • RMS Jitter: 400 fs at 40 MHz carrier
   • Impact: The TCXO provides stable reference frequency with low phase noise, which is crucial for maintaining precise time synchronization across probes under spoofing and jamming.
4. Raw IQ Data Size 
   • Specification: 64k samples each second (4 bits @ 60 MSamples), synchronized with PPS
   • Impact: The larger the size of the data, the more accurately the time difference can be determined by correlation analysis.
5. Automatic Operation
   • Specification: Automatic initiation of raw IQ data transmission to the cloud upon detection of interference or anomaly on a probe
   • Impact: Automatic triggering ensures that raw data is sent to the central processing system from multiple nearby probes without user intervention, enabling faster response times and seamless operation. This minimizes delays in interference detection and localization, enhancing system efficiency.
6. PPS Accuracy
   • Specification: ≤ 20 ns in clear sky conditions
   • Impact: High PPS phase accuracy is crucial for synchronizing data from multiple probes. The ≤ 20 ns accuracy ensures minimal timing errors, directly improving TDOA localization precision.
7. Holdover Mode
   • Specification: Maintains PPS accuracy during jamming or spoofing
   • Impact: The holdover mode stabilizes the Pulse Per Second (PPS) signal when a probe is under jamming or spoofing, allowing the TDOA algorithm to continue functioning accurately. This is essential for maintaining localization accuracy under adverse conditions.

• Simpler Antenna Requirements
  TDOA does not require sophisticated antenna system. Standard commercial GNSS antennas with built-in preamplifiers are sufficient, making deployment easier and more cost-effective. In some cases, even existing GNSS antennas already installed at the user’s site can be repurposed for TDOA, further reducing installation requirements.
• No Calibration Required
  Unlike Angle of Arrival (AoA) systems, TDOA does not require complex calibration after installation. This simplifies the setup process and minimizes maintenance, as no additional adjustments are needed once the antennas and receivers are in place.
• Flexible Placement
  TDOA has less restrictive siting requirements compared to AoA, allowing greater flexibility in selecting deployment locations. TDOA systems are easy to deploy quickly, especially in urban environments where additional receivers can be positioned to counteract the shadowing effects of tall buildings.
• Effective with Wideband, Low SNR, and Short-Duration Signals
  TDOA performs well with wideband, complex signals and can accurately localize low signal-to-noise ratio (SNR) and short-duration signals. Wideband signals are particularly suited to TDOA, as the method’s performance improves with increased bandwidth. TDOA also benefits from correlation processing, which can enhance weak signals, making it possible to locate sources even under low SNR conditions.
• Noise and Interference Suppression
  The correlation processing in TDOA allows for effective suppression of uncorrelated noise and interference, improving its ability to locate signals even with low signal-to-interference ratios (SINR). Signals unique to one or two receivers are automatically disregarded, isolating the interference source more precisely.
• Multipath Mitigation
  TDOA can be less affected by multipath interference due to wavefront distortion from local obstacles. With sufficient signal bandwidth and advanced processing, TDOA systems can minimize multipath effects, making them suitable for dense urban environments where reflections are common.
• Flexible Geometry for Probe Placement
  TDOA accuracy is based on Geometric Dilution of Precision (GDOP), which depends on the spatial arrangement of receivers rather than the baseline distance between them. This allows flexible placement of probes around the interference source, making TDOA effective in diverse layouts and environments.