Nº 1 2013 > Mobile monitoring
Mobile monitoring and direction-finding station
An ITU-compliant solution
D. Brückner, H. Kranich, U. Trautwein MEDAV GmbH, Uttenreuth, Germany
Spectrum monitoring can help government authorities ensure public security. With this is mind, the German authority for spectrum regulation and monitoring — Bundesnetzagentur — published a tender for the procurement of a mobile monitoring and direction-finding system for temporary and event-related use, for example during world sporting championships.
The tender specified a shelter containing equipment for direction finding in the very high frequency/ultra-high frequency (VHF/UHF) band from 20 to 3000 MHz and monitoring capabilities for the extended VHF/UHF frequency range (9 kHz to 6000 MHz). The operational requirements included rapid deployment and flexible remote-control access. The technical requirements specified extremely accurate direction finding, and reliable detection and direction finding of short-term emitters.
This article describes the solution delivered by MEDAV GmbH, namely ITU-compliant VHF/UHF monitoring and direction-finding equipment housed in a container that can be moved to different locations using a standard container truck or vessel. The system — "called the MDF 300‑5m mobile shelter solution"— can be assembled and disassembled rapidly without any tools.
Because the system is based on the concept of software defined intelligence architecture, it can be upgraded with additional functionalities simply by installing additional software. With a completely or partially retracted mast, the system can be operated in vehicles or vessels in motion. The system supports different operating modes, making it possible to save power, and it is designed to support connections to a central station with very low transfer rates.
Mechanical and operational aspects
The container is equipped with monitoring and direction-finding devices in the frequency range from 9 kHz to 6 GHz for monitoring, and 20 MHz to 3 GHz for direction finding. The system can be operated by remote control using techniques based on the Global System for Mobile Communications (GSM), the Universal Mobile Telecommunications System (UMTS), fixed-line wide area network (WAN) or INMARSAT BGAN. The antennas are easy to assemble and have excellent direction-finding accuracy and sensitivity because of their wide aperture and separation.
The container has cooling, heating and ventilation and is equipped with remote control for monitoring and shutdown, and has a burglar alarm and a video surveillance system. The antenna mast can be retracted in an emergency.
The monitoring and direction-finding antennas are mounted on top of a mast of a total height of 13 metres, giving a total antenna height of 15 metres. The combined antenna plus mast are designed to withstand a maximum wind speed of 140 km/hour and 3 cm of ice.
The system is normally powered by a 230 VAC 50 Hz power supply, but can be operated using an internal battery if the external power supply fails. Air conditioning and heating guarantee a temperature of 22°C inside the container. The system can be operated in four different modes:
General operation mode: All measurement equipment runs in remote or manual (local) mode. The monitor scans and searches for emitters of interest. Emitters can be demodulated (analogue and digital) and analysed. The direction finder calculates the line of bearing of emitters. The monitor and direction finder can run independently. Wideband and narrow-band recordings can be made for subsequent offline demodulation, analysis and documentation. All results are stored in a database. Line of bearing results come from two or more direction-finding sensors, which are used for localization. The calculated positions are drawn on a map.
Intercept mode: This mode is used to save energy. Only the monitor runs, while the direction finder and other functionalities are switched off. The monitor checks frequencies or frequency ranges for activities. If an active emitter is detected, the system powers up completely.
Direction finding mode: This mode is used to save energy. Only the direction finder is operating, while monitoring and other functionalities are switched off. The direction finder can be controlled remotely, using triggers for automated recording.
Standby mode: All measuring equipment is switched off. Only the remote control communication system is running. All personal computers are in stand-by mode.
In the event of technical problems, the system falls into failure mode. For example, failure mode will be activated automatically by failure of the power supply or by over-heating. Only the communication system keeps running for status requests and restart activities.
The measurement system
Software defined intelligence architecture (SDIA) technology ensures a state-of-the-art system with software upgrades throughout the lifetime of the monitors and direction finders. Processing is carried out on virtual devices running on standard information technology equipment, such as servers, workstations and notebooks using the newest versions of Linux or Windows operating systems. Only a small amount of specialized hardware — mainly antennas and tuners — is used, so the system can be updated easily.
The specialized hardware uses standard interfaces, and so can easily be replaced by newer devices.
In the multifunctional communication sensor’s front-end, the synchronized tuners work in parallel without any signal loss. Growth potential and flexibility in the design of software upgrades over the life-cycle allow for the system characteristics to be continuously adapted to current needs. Functions can be added, for example beam forming or special modes for special signals.
The DF-A0038 is a portable wideband direction-finding antenna suitable for fixed and mobile mast-mounted applications. This antenna covers a frequency range from 20 MHz to 3.0 GHz. An integrated monitoring antenna provides a sensitive omnidirectional signal on the same axis as the direction-finding antenna, without interference between them.
One of the most important steps of building an interferometer-based direction finder is the calibration of the direction-finding antenna. There should be no interfering transmitter in the surroundings of the test field. All instruments and cables have to be calibrated separately to achieve a precise and reproducible result. MEDAV calibrates all antennas in a remote test area in South Africa.
The direction-finding receiver box comprises five receivers. Each tuner is connected to the appropriate antenna element, so no commutation or other switches are required. This enables the direction finder to detect and process very short bursts (1 millisecond) which are used by frequency hopper radios.
A separate antenna matrix and an intercept receiver are used for monitoring purposes and provide listen-in functionality.