Power of Arrival (PoA) is the default location algorithm for locating interference signals and it places a bullseye icon on the map to indicate a location estimation. Time Difference of Arrival (TDOA) is a method for locating an RF emitter based on the differential time of arrival of the transmitted signal to 3 remote probes. TDOA may not always produce a single unique estimation, so the PoA estimate can help in choosing the most likely location for the RF source. Angle of Arrival (AoA) provides bearing information for the position of a transmitted signal.
Power of Arrival
Power of Arrival (PoA) is a Geo-location algorithm that uses spectrum monitor receivers to capture and compare instantaneous power levels detected at three physically separated locations. By detecting relative power levels, an approximate location of a signal of interest can be found. PoA works with both modulated and non-modulated (CW) signal types. PoA techniques can also be used with burst or pulsed signals. Measurement signals for each receiver are time synchronized in order to maximize accuracy.
Angle of Arrival
Angle of Arrival (AoA) requires Options 400 and 401. One advantage of AoA vs TDOA/PoA is that only one location is needed into order to perform geo-location. Both TDOA and PoA require three positions in a ‘triangle’ format. However, AoA in turn only provides bearing information, ie the signal of interest lies somewhere along a trajectory line from the position of the AoA monitor to the interference signal’s position.
Directional antennas are used by the AoA algorithm to computer bearing lines. Power is automatically measured at each antenna port, where calculations are made based on received signal strength.
Figure: Angle of Arrival shows a setup example for AoA. Either the MS27103A (12 or 24 RF Inputs) or the MS27102A (outdoor monitor with the option of 4 or 6 RF Inputs) can be used for the AoA application.
Angle of Arrival
Time Difference of Arrival
Time Difference of Arrival (TDOA) is a very powerful technique to locate interference sources and other modulated broadcasters. However, it is not universally useful, and it does take considerable care and experience to use it effectively. Many sources of interference are CW or just noisy electronics. There is nothing about these type of signals that can be time-aligned and those emitters will not be locatable with TDOA.
To use TDOA to locate interference sources, the following considerations need to be made:
• The source must be modulated. TDOA looks for features in the RF spectrum as measured at 3 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 diagram produces much more accurate results. The better you can setup 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 target receiver, adjusting 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.
• Distance matters. 3 target receivers are used to do the TDOA triangulation of the RF source. Best results will be achieved if the source is contained inside a triangle made by connecting the target receiver locations. TDOA works for sources outside this triangle, and 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.
The reason distance matters is due to the uncertainty in any measurement made. We are looking for the intersection of three lines. 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 the triangle of the target receivers will almost always produce lines that have very small incident angles, and that can greatly multiply the uncertainty in position.
TDOA requires additional set ups and IQ Capture. This chapter provides exercises for the following setup and monitoring activities:
