TDOA is not a mobile hunting mode, but can be used to compliment a mobile interference hunt. TDOA uses the Time Difference of Arrival technology introduced with the Anritsu Vision software, which can now be used with MIH to help you locate a general area of where an interfering signal may be found. This gives you a good idea of where to start Creating a Drive Route before using the mobile modes with a spectrum analyzer in the vehicle. This is easily accomplished in the office before you leave, or while out in the field and connected to a remote spectrum monitor system.
Remote Monitoring Probes positioned in a triangular pattern perform TDOA to identify a stationary signal’s longitude and Latitude coordinates. Use either Google Maps™, or a bit map to identify and geo-locate the interferer’s signal source.
To use TDOA to locate interference sources, the following considerations need to be made:
• Three probes (Anritsu MS2710xA series remote spectrum monitors) are required to do the TDOA triangulation of the RF source. Best results will be achieved if the interferer source is contained inside a virtual triangle made by connecting the remote spectrum monitor locations. TDOA works for sources outside of this triangle too and it may be used to locate a source that is many kilometers outside the triangle, but it is more accurate for sources that are close by.
• An Internet connection for the computer is required for MIH to connect to the remote probes. If this is not already built into the computer, a USB mobile broadband adapter or Wi-Fi adapter to a local hotspot from your cellular provider can be used.
• The source must be modulated. TDOA looks for features in the RF spectrum as measured at three locations. Those features are time-aligned, and the difference in the time for the signals to reach each receiver is used to calculate the location. If the signal of interest does not have features that can be aligned in time, then TDOA will produce meaningless results. Typically, that means the signal must be modulated.
• A clean IQ produces much more accurate results. The better you can set up the spectrum monitor to capture IQ data, the more accurate the position estimate will be. This means more time may be needed to set up each remote monitor and to adjust the frequency, span, reference level, and preamp settings to get the best possible IQ capture. Strong signals that are close by will be relatively better, but weaker signals, and especially distant signals, take some care to get meaningful results. This implies that you should either have a dedicated MIH operator in the vehicle, or be parked so you can devote your attention to setting up the measurement.
• Distance matters because of the uncertainty in any measurement made. You are looking for the intersection of three lines and for where those lines intersect at nearly right angles, then any uncertainty in the line positions produces a similar uncertainty in the intersection. However, if the lines approach each other at very shallow angles, then the lines may be within the distance of uncertainty for several kilometers. Distant sources outside of the triangle of the remote monitors will almost always produce lines that have very small incident angles, and that can greatly multiply the uncertainty in position.