Modern Signals Require a Modern Approach to Spectrum Monitoring and Emitter Location
By I.C. Tillman and Raymond Shen, PhD, Agilent Technologies

Today’s modern off-the-air signals have higher carrier frequencies, wider bandwidths, lower power and complex modulation formats. Even though traditional monitoring stations are typically equipped with expensive directional and omni-directional antennas, direction finders, racks of RF receivers, spectrum analyzers, DF processors, RF switches and cables, computers and communications gear, they experience great difficulty in detecting these modern signals. In addition to dealing with the complexities of these signals, traditional monitoring stations are usually placed too great a distance away from the dense, urban areas of the city to perform adequate spectrum monitoring. Placing traditional monitoring stations in areas with adequate proximity to signals of interest would be cost prohibitive in most countries. Thus, traditional monitoring stations are difficult to site, deploy and fund for the case of detecting, classifying and locating modern communications emitters.

In an ideal world, monitoring stations would be portable, connected and synchronized to enable close proximity monitoring and either time difference of arrival or power-based geolocation of signals of interest. The stations would be affordable, rugged, low power and low maintenance. They would have broad frequency coverage and enough processing bandwidth and horsepower to handle any kind of traditional analog or modern digital signal. Spatially-distributed and time-synchronous networks of RF sensors are easier to site, simpler to deploy and more cost effective for use in dense urban environments of modern digital signals. [1]

Applications
For military frequency managers working tirelessly during critical tests to defend their spectrum from encroaching commercial transmissions, a network of RF sensors can help detect frequencies in-use versus plan to provide automated, real-time reports on changes that include frequency, bandwidth, amplitude, time and location. Such a network is depicted in Figure 1. The same could be said for large event spectrum management (such as for wireless microphone use at the Olympics). RF sensors can also be used to detect and locate intended or unintended interferers of vital communications frequencies. This would be useful for any kind of publicly or privately-used communications infrastructure (military, airports, railway). From this perspective, frequency regulators benefit greatly from using RF sensors. Other applications of the RF sensors include identifying and locating rogue emitters. Some venues, such as hospitals, prisons and other secure areas, require radio silence.

RF sensors distributed throughout an area of interest are well-suited to additional markets and applications, including:

• ITU-R Monitoring
• Frequency and range management
• Spectrum policy enforcement (hospitals, airports, private companies)
• Spectrum Security (including TEMPEST, building and facility monitoring)
• Cellular phone detection and location
• Signal propagation measurements
• Signal search and collection
• Emitter location and direction finding
• Spatially-distributed and time-synchronous RF sensor networks, as a measurement platform and as a modern approach to spectrum monitoring, are here to stay.

Technical Features
Agilent’s N6841A RF Sensor is an example of a product with these kinds of capabilities. It has been designed to be networked together, be tightly synchronized, and conduct a range of spectrum monitoring, emitter location, and signal analysis functions. The N6841A has a 20MHz to 6GHz frequency range, 20MHz processing bandwidth and excellent dynamic range. It has a weatherproof, sealed enclosure qualified to IP67 for water and dust intrusion. This allows for outdoor placement near the antenna, so that the RF feedline to the antenna can be minimized (typically less than 2m), thereby improving the overall RF performance of the sensor. Outdoor sensors are synchronized using GPS and achieve better than 20 nS timing accuracy across the network. Indoor sensors can use the Precision Timing Protocol defined in IEEE-1588 to achieve similar synchronization results without having each sensor connected to GPS.

There are a number of applications in which the sensor can be used; thus, there are a number of software packages that interface with the RF sensor hardware. Agilent’s Signal Surveyor software, shown in Figure 2, is a very robust spectrum monitoring, search, intercept, classification and collection software package. It can operate up to four sensors
simultaneously and has advanced signal thresholds and detection, advanced filtering and alarm tasks to facilitate automation of many otherwise time-consuming tasks. Signal Surveyor is ideally suited for RF survey, interference detection and mitigation and IQ. It is often used as a tipper for the N6854A Geolocation Server Software.

The N6854A Geolocation Server Software, shown in Figure 3, extends the capabilities of a wide area network of N6841A RF sensors with integrated real-time RF emitter geolocation and mapping. This software computes the estimated position of emitters based on several unique geolocation methods. It usually operates in conjunction with a signal search system that provides signal detection and characterization and initiates geolocation measurements once a signal of interest is detected. Results are logged to the database and may also be displayed real-time in the user interface. [2] The N6854A employs one of three algorithms to estimate the position of an emitter: Time-Difference-Of-Arrival, Received Signal Strength, and an adaptive Hybrid method which uses both time and power to determine the estimated position.

The 89601B Vector Signal Software will support live measurements from the RF sensor, facilitating remote signal analysis. IQ files made with the RF sensor are fully compatible with the VSA software in a post-processing mode. [3]

Additionally, the RF sensor has a fully documented API that gives programmers access to the full complement of measurement functions available in the RF sensors (both individually and in groups). With this powerful suite of software combined with a network of synchronized RF Sensors, you can make virtually any of the measurements available from the traditional monitoring station and at a fraction of the cost and hassle of installation.

For more information about Agilent’s RF Sensor and applications for over-the-air spectrum monitoring, emitter location and signal analysis, go to: www.agilent.com/find/rfsensor.

References
[1] Report ITU-R SM.2211 “Comparison of Time-Difference-of-Arrival and Angle-of-Arrival Methods of Signal Geolocation”, June 2011.
[2] Agilent N6854A Technical Data Sheet, Publication #5989-9207EN, Printed in USA, October 14, 2010.
[3] Agilent 89601B Vector Signal Analyzer Brochure, Publication #5990-6553EN, Printed in USA, February 1, 2011.

About the Authors
I.C. Tillman is a Business Development Manager with Agilent Technologies responsible for Emissions Analysis, RF Sensor & Geolocation Technologies. I.C. has over 25 years of experience in the test and measurement world.

Raymond Shen, Ph.D. is an R&D manager with Agilent Technologies. Raymond has over 13 years of RF and wireless expertise, frequently speaking on new signal location methods as well as emerging communication standards.

Agilent Technologies
www.agilent.com
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