P. van Genderen

OFDM waveforms for frequency agility and opportunities for Doppler processing in radar

Frequency agility is an important feature when radar operates in jammed environments. With the development of the orthogonal frequency division multiplexing (OFDM) waveform in the communications, the opportunity for a new generation of digital agile radar rises. Old and expensive multi-channel analog agile front ends could be replaced by simpler and cost effective single channel transceivers where the coherent switches in frequency are done digitally. The novelty of the paper is to investigate an agile OFDM waveform so that Doppler can be solved despite the apparent frequency agility. The OFDM performance is assessed from the radar point of view, using the ambiguity function as a tool. Our approach considers a pulse burst, each pulse being implemented with OFDM chips. Three patterns for frequency agility from pulse-to-pulse are studied. Preliminary suggestions to accomplish coherent processing in an agile context are derived.

A multi frequency radar for detecting landmines: design aspects and electrical performance

A radar has been designed and built for application in the problem of anti-personnel landmine detection. The radar is based on the principle of a Stepped Frequency Continuous Wave (SFCW) transmission scheme. The novelty of the system is, that it transmits 8 frequencies at the same time through one antenna and that it repeats this procedure 16 times with shifted frequency offsets in order to collect a set of 128 frequency samples. The resolution in the synthesized range profile in air is just over 3cm, while the total frequency coverage is from 400MHz upto 4845 MHz, so ultra wide band. The architecture of the radar however is based on Commercial Off The Shelf narrow band technology. The acquisition time for collection of a full set of 128 frequencies covering the ultra wide band is less than 2 msec. This short time was achieved by simultaneously transmitting 8 frequencies and using Direct Digital Synthesis for signal generation. The concept of SFCW radar is prone to errors in phase. Therefore a calibration facility is built in. First results of the system while it is still in the setting-to-work phase are encouraging.

OFDM Signals as the Radar Waveform to Solve Doppler Ambiguity

The OFDM communication signal is proposed for spread spectrum radar signal generation. A radar signal processing technique is developed to solve the Doppler ambiguity in the pulsed Doppler radar by exploiting the multicarrier structure of the OFDM and the random phase modulation on the carriers in a Doppler compensation scheme. The novel processing technique is analyzed in detail, through both analytical derivations and Monte Carlo simulations, to demonstrate the feasibility of using the OFDM modulation with random phase coding for radar signals.

Multi-carrier radar waveform schemes for range and Doppler processing

A processing technique that utilizes the OFDM communication waveform as the radar waveform is considered. The double use of the waveform for communications and radar enables establishing radar networks communicating through the radar beam. The processing technique exploits the Doppler sensitivity of the OFDM to reduce the Doppler ambiguity. An enhancement to the waveform and the processing is proposed to completely eliminate the Doppler ambiguity.

Interleaved OFDM Radar Signals for Simultaneous Polarimetric Measurements

A novel pair of mutually-orthogonal radar waveforms is proposed for simultaneous polarimetric measurements (SPM). The proposed waveform pair exploits the orthogonality features between different subcarriers within a single orthogonal frequency division multiplexing (OFDM) chip, and is called interleaved OFDM (I-OFDM). With I-OFDM signals the isolation limitation defined by the bandwidth time (BT) product for frequently used waveform pairs like linear frequency modulated (LFM) and phase-code modulated (PCM) signals, can be overcome. The performance features of I-OFDM signals are theoretically analyzed; the application of I-OFDM signals and the corresponding signal processing scheme in SPM are presented. A significant increase of the polarimetric measurements efficiency by utilization of the proposed I-OFDM signals has been verified by experiments using an operational radar system.

Wideband OFDM pulse burst and its capabilities for the Doppler processing in radar

The use of multicarrier (MC) signals could support a new generation of radar that would provide simultaneously RF agility and Doppler processing. Easy to generate in digital signal processing (DSP), the well-known orthogonal frequency division multiplexing (OFDM), originally a communication waveform, is a first choice candidate. Its flexible structure makes both radar concepts of continuous and pulse burst waveforms, compatible. In this paper, the focus is on the pulse burst. The novelty of the paper is to investigate capabilities for Doppler processing of the OFDM pulse burst in both narrowband and wideband cases. Since the waveformpsilas bandwidth-time (BT) product is high, the analysis starts with the narrowband approximation (very low speeds) and rapidly moves on to the true wideband version (higher speeds) of the Doppler effect, in an elementary scenario with a single point target.

Ground penetrating impulse radar for detection of small and shallow-buried objects

The video impulse system ground penetrating radar system for detection of small and shallow buried objects has been developed. The hardware combines commercially available components with components (e.g. antennas) specially developed or modified for being used in the system. Antenna system has been designed so that it provides sufficient spatial resolution and power budget together with small coupling and ringing. The GPR has been tested in different environmental conditions and has proved its ability to detect small and shallow buried targets.

Wideband ambiguity function and optimized coded radar signals

A system efficiently combining radar and communication requires signals with a large time-bandwidth product to achieve high resolution for radar and high data rate. The narrowband approximation of this wideband signal leads to errors. The narrowband conditions in literature are a gross indication of when the narrowband treatment of a signal is invalid. In this paper a mathematical expression is derived quantifying the error committed when applying the narrowband approximation. This expression decomposes the difference between wideband and narrowband correlation in two components: one only depending on the physical problem parameters (a, tau) and the signal bandwidth, duration, and carrier frequency; the other depending only on the particular signal assumed. This paper proposes a method to evaluate quantitatively when the narrowband approximation is inadequate, and a minimization algorithm to select a signal coding that minimizes the error caused by treating a wideband signal correlation with the narrowband approximation.

System description of a stepped frequency CW radar for humanitarian demining

A multiple frequency stepped frequency continuous wave radar for detecting flush buried and surface laid antipersonnel landmines was designed and built. The system covers a frequency band from 400 MHz up to 4845 MHz. The range resolution is 3.4 cm and the maximum unambiguous range is 4.3 m. The antenna configuration is a bistatic one with two Archimedean spirals with opposite sense of rotation. The instrumented dynamic range is 40-50 dB per frequency channel. After synthesis of the range profile the dynamic range is significantly higher. The system parameters are aimed at imaging algorithms exploiting the phase evolution over the antenna footprint in particular. One example of the results achieved with and without synthetic aperture radar processing is presented.

A risk-based object-oriented approach to sensor management

Sensor systems play a critical role in providing situational awareness and threat assessment. Management of adaptive multifunction radars and sensor suites in todays combat systems is mostly based on an operator-defined set of priorities. Such a set can be very complex and requires a thorough understanding of the sensor capabilities and performance and of the operational needs. Changing such a set in strongly varying scenarios is prone to creation of sub-optimal system performance. In this paper a novel approach to assigning priorities to individual objects in the environment of a naval vessel is presented. This priority assignment is accomplished by means of dynamic evaluation of the risk imposed by each object with respect to the completion of the mission. More in particular a three-stage sensor management system is proposed that first uses the uncertainty related to each objects attributes, such as the state vector, classification and identification to determine the allocation of surveillance, track or recognition tasks. The sensor manager then selects the most appropriate sensor for the task and finally distributes the available time budget based on priorities in case of multiple tasks. The risk is estimated by a dynamic Bayesian network modeling relevant operational knowledge. The nodes in this DBN represent the different object states in time and the events leading to them. A computer simulation was developed to prove the viability of this concept.