Vilhelm Gregers-Hansen

An Improved Empirical Model for Radar Sea Clutter Reflectivity

A fundamental characteristic of radar sea clutter, important for radar performance evaluations, is its apparent reflectivity defined a σo (m2/m2). “Apparent” is used as a reminder that any measurement of sea clutter reflectivity includes the effects of propagation and shadowing close to the sea surface. Sea clutter reflectivity depends on many factors, including sea state, wind velocity, grazing angle, polarization, and radar frequency. An empirical sea clutter model proposed by Horst, et al. (1978), the so-called GIT model, has found widespread acceptance in the radar community. However, this model does not always agree with what is the most complete experimental database of sea clutter reflectivity available to the radar systems engineer. The 1991 edition of F. E. Nathansons book provides seven tables of measured sea clutter reflectivity data summarized from approximately 60 sources. The large deviation between the GIT model and this database, in particular at lower sea states, has prompted NRL to develop an improved model for sea clutter reflectivity based on these tables. The model is a function of radar frequency, polarization, sea state, and grazing angle. The functional form of this empirical model was chosen such that the average absolute deviation (in dB) between the model and the experimental data was minimized.

WARLOC: A high-power coherent 94 GHz radar

This paper describes the development of a high-power, coherent radar system at W-band and discusses potential applications of radars with this new capability. Previous radars in this frequency band were limited by available power-amplifier technology to about 500 W of average power; WARLOC radar represents an increase in power, by 20 times, over previous coherent radars at 94 GHz. This performance improvement is possible due to the development of a gyroklystron amplifier specifically for this and future radars in this frequency band. The gyroklystron amplifier tubes deliver 100 kW peak power and 10 kW of average power at a center frequency of approximately 94 GHz. Other novel features of this radar include the use of highly overmoded waveguides and rotary joints for the transmission of power from the final power amplifier (FPA) to the antenna, and a high-power quasi-optical duplexer. The system uses a relatively large 1.8 m diameter (580-wavelength) Cassegrain antenna, which required the development of an antenna with an rms surface accuracy of 0.0025 in, to obtain long-range detection and identification of small objects. Test data show an antenna gain of 62.5 dB, confirming that the needed surface accuracy was achieved. Two mobile shelters house the radar system, permitting relocation to various test sites. WARLOC is presently operational at the Naval Research Laboratorys Chesapeake Bay Detachment facility, Maryland. It is being employed in radar imaging of airborne and surface objects, and in the scientific study of propagation effects and atmospheric physics phenomena.

Code inverse filtering for complete sidelobe removal in binary phase coded pulse compression systems

Pulse compression is used in radar systems to improve range resolution while maintaining a high duty cycle. In addition to practical implementation constraints, the key issues for the selection of a pulse-compression waveform are mismatch loss, peak / integrated range sidelobes, and Doppler tolerance. While much progress has been made in the design of nonlinear frequency modulated (FM) chirp waveforms satisfying these requirements, the corresponding performance for binary phase-coded waveforms is often inadequate. In order to improve the range sidelobes achieved with phase-coded waveforms, specially designed mismatched pulse compression filters can be used. Several such approaches have been described in the literature since 1959. This paper review these techniques and highlight a particular approach using infinite impulse response (IIR) filters, which has received little attention in the past. Using this technique the performance for a number of binary phase codes of different length have been determined and their Doppler tolerance is investigated.

An empirical sea clutter model for low grazing angles

The most fundamental characteristic of sea clutter, as used in radar performance evaluation, is its apparent reflectivity defined as σ° (m2/m2). The word apparent is used here as a reminder that any measurement of sea clutter reflectivity inevitably includes the effects of propagation close to the sea surface. Sea clutter reflectivity depends on many factors including sea state, wind velocity, grazing angle, polarization, and radar frequency. A comprehensive tabulation of measurements from around 60 sources were included in the 1991 edition of Nathansons book [1] and this probably represents the most complete database of sea clutter reflectivity available. Also included in this book by Nathanson was a detailed description of an empirical sea clutter model proposed by Horst et. al. [2], the so-called Georgia Technical Institute (GTI) model. This model has found widespread acceptance in the radar community although its technical basis may be somewhat vague.

High-power millimeter-wave transmitter for the NRL WARLOC radar

High power millimeter wave instrumentation radars have a number of important applications ranging from defense missions to basic scientific studies At the Naval Research Laboratory, a new high power 94 GHz radar named WARLOC has been developed. This radar employs a high power gyro-klystron as the final power amplifier and was developed during 1996-2001. The WARLOC radar has been integrated as a transportable system, using the 100 kW peak, 10 kW average power gyro-klystron amplifier, a low-loss transmission line, a quasioptical duplexer, and a Cassegrain antenna. The transmitter operation and waveguide system is the subject of this paper.

Radar systems trade-offs vacuum electronics vs. solid state

During the first 50 years of radar, vacuum electronics devices (VEDs) were the major source of power for their final power amplifiers (FPA). While VEDs continue to be used in many current operational radars, solid-state transmitters are becoming a viable alternative in many applications due to their advantages in availability, maintainability, modularity, and, sometimes, performance. In most cases, a solid-state transmitter must be part of an integrated radar design process. Attempts to replace high-power vacuum tube transmitters in an existing radar, with footprint- and performance-equivalent solid-state versions, have proven difficult since VED based transmitters traditionally have been low duty cycle and solid-state transmitters need to be operated at a relatively high duty-cycle in order to be cost-competitive. This talk reviews system level pros and cons between vacuum tubes and solid-state devices for new radar systems. The important issues of cost, maintainability, and reliability, are application specific and thus difficult to address in general terms.

A stacked analog-to-digital converter providing 100 dB of dynamic range

The dynamic range requirements of a modern radar system far exceed the dynamic range attainable with available ADC technology. Traditional ways of overcoming these dynamic range limitations, including STC, AGC, and IF limiting, have serious drawbacks. The stacked ADC concept was developed to address the dynamic range limitations without the drawbacks of the traditional mitigation techniques. To demonstrate the concept, an experimental system based on the concept was built and tested. The resulting system demonstrated a dynamic range improvement over COTS ADCs of 30 dB, providing a total dynamic range of 100 dB.

The structure of turbulence in clouds measured by a high power 94 GHz radar

The Naval Research Laboratory (NRL) has recently developed a 3–10 kW average, 80 kW peak power 94 GHz radar with scanning capability, WARLOC (W Band Advanced Radar for Low Observable Control). This radar is powered by a gyroklystron developed by a team led by NRL. One application has been to image clouds. New capabilities of WARLOC include imaging with greatly improved sensitivity and detail as well as the ability to detect much lower cloud returns. At short scale lengths (∼10 m), the cloud reflectivity has a speckle pattern indicating that it is governed at least in part by stochastic processes. Here WARLOC is used to measure correlation functions and turbulence spectra in clouds. In the inertial range, the Kolmogorov prediction for the correlation function index (2/3) agrees well with the data, but the assumption of isotropy does not. Furthermore, for longer scale lengths, the fluctuations appear to be wave like in the vertical direction, but not in the horizontal direction.

WARLOC: a high-power millimeter-wave radar

A high-power, coherent, W-band (94 GHz) millimeter-wave radar has been developed at the Naval Research Laboratory. The radar employs a 100 kW peak power, 10 kW average power gyroklystron as the final power amplifier, an overmoded transmission line system, and a quasioptical duplexer, together with a 6 foot Cassegrain antenna, a four-channel receiver, and state-of-the-art signal processing. Developed as a research radar for the investigation of tactical Navy radar applications in the millimeter wave band, additional radar measurement studies include cloud physics, propagation, and forward and backscatter studies.

Transmitter noise compensation - a signal processing technique for improving clutter suppression

A new signal processing technique, referred to as transmitter noise compensation (TNC), has been developed at the Naval Research Laboratory (NRL) to improve the capability of existing radars to suppress returns from strong clutter. This technique compensates for intra-pulse transmitter noise, as well as power supply instabilities, by capturing and processing an accurate replica of each transmitted pulse. Subsequently, through pulse-to-pulse comparisons, the measured transmit errors are used to derive a digital filter which compensates for the transmitter noise in the digital signal processor (DSP). This paper describes the transmitter noise compensation technique, its theory of operation, and the results from an experimental effort to demonstrate its feasibility. This experiment has confirmed the validity of the TNC concept and demonstrated an improvement of 15 dB or higher.