Solid State Advantage
In 2006, EWR released the revolutionary solid state E700 PDR system, creating a new category of portable Doppler weather radars. This ground breaking modular concept was designed for field use by military personnel – and was quickly adapted by the U.S. Marine Corps and the U.S. Air Force. The E700 PDR radar consists of three primary modules: a radome unit which contains all of the RF components, digital transceiver, and the antenna assembly; a radar processor enclosure; and a pneumatic mast. The radome unit was specifically designed to be mounted on the easily deployable pneumatic mast, which can raise to radar to a height of up to 50 feet. The modular architecture allows for easy field set-up and tear-down, and quick field replacement of components, which is critical to a highly dynamic field deployment. The E700 PDR can be unpacked, set-up, and operational within 30 minutes. A number of standard mounting options exist, which allow the user to mount the radar on top of a roof or building, or attach the mast to a mobile trailer or platform. The key enabling technology for this rugged, reliable, and highly portable radar system is the use of a solid state transmitter.
The Solid State Difference
Traditionally, most commercial weather radars employed magnetrons or klystron tubes to generate the high power RF energy. In a solid state radar transmitter, the Radio Frequency (RF) energy is generated and amplified by transistors instead of a vacuum-tube based device. Today’s Solid State amplifiers offer a number of advantages over their tube-based cousins, particularly the lower-cost magnetron. These benefits should be carefully considered when selecting a system, particularly in mobile and portable radar applications.
Magnetrons are generally the lowest cost transmitter solution, particularly at higher frequencies such as X-band. However, the transient oscillation nature of a magnetron results in random phase operation. Pulses from a magnetron are not coherent and require the radar to use “coherent-on-receive” processing, which limits the clutter cancellation ability of the radar and limits some advanced phase-based algorithms for multi-trip echo correction, for example. Fully-coherent radars, such as those using solid-state or klystron transmitters generally have better clutter filtering capability and allow for more varied waveform selection (such as pulse compression waveforms) and pulse repetition interval selection. Moreover, solid-state amplifiers generally can switch pulse widths, frequency, and pulse repetition intervals at a pulse-by-pulse basis.
Magnetron transmitters are generally not frequency agile. Thus, the transmit frequency cannot be easily changed like in a solid-state radar. This can be problematic for mobile radars that may need to avoid particular frequencies for local interference issues. Klystrons allow for some frequency-variability, but are generally narrowband due to the device physics, whereas solid-state amplifiers at X-band may have an operating bandwidth in the hundreds of MHz.
In many weather radar applications portability and ruggedness are key driving factors in the selection of a system. The elimination of tube amplifiers greatly enhances the reliability of the radar, particularly during rough transport, and allows the radar to be operated immediately upon a cold start. In addition, the lifespan of a solid-state transmitter is generally much longer than for a magnetron based transmitter, where the magnetron may need to be replaced after just a few years of operation.
The high voltages required in tube-based radars may also be a concern for many users, particularly those that may need to service their systems in the field.
Pulse Compressed Waveforms
Pulse compressed waveforms amplified by a solid-state transmitter can provide sensitivity comparable to much higher power tube-based transmitters. For example, a 1 kilowatt solid state transmitter producing a 50us pulse compressed waveform with a 1 MHz bandwidth has the same sensitivity and range resolution as a 1us pulse transmitted by a 50 kilowatt tube-amplifier. By using solid state amplifiers, peak power is traded for average power with no loss in system performance. In an EWR radar, the pulse compressed waveform’s inherent “blind-range” is mitigated by transmitting a short pulse at a slightly different frequency to “fill-in” the blind-range of the long pulse.
Solid State Value
Historically, the higher cost of the solid state amplifier has been a driving factor when specifying a radar system. A decade or more ago, the cost per watt of solid-state amplifiers made them prohibitive for use in many commercial applications, particularly at higher frequencies such as X-band. However, over the past several years the rise of Gallium Nitride (GaN) transistor development for the telecom and defense industries have driven the cost per watt down significantly. Even microwave ovens, once the most significant user of low-cost magnetrons, are beginning to incorporate solid-state amplifiers to leverage the transistors ability to vary frequency and amplitude. Commercial aviation and marine radars, which have similar performance and reliability needs as portable weather radars, are beginning to transition to all solid-state RF sections.
A New Generation of Radars
The new generation of EWR products includes the dual-polarization E800 radar. The E800 is designed as a cost effective, polarimetric radar system for users requiring a higher resolution radar while maintaining the advantages of a smaller footprint and low infrastructure costs. This unit is an excellent choice for mobile applications, gap filling in existing networks and fixed sites where installation of a larger system would be difficult. The E800 radar can operate in a switched or simultaneous polarimetric mode, which provides for measurements of differential reflectivity and phase in the simultaneous mode, while providing the capability to measure linear depolarization ratio in the switched mode. In many klystron-based dual-polarization radars, the system can only operate in the simultaneous polarization mode since the single transmitter output is split into two (for horizontal and vertical polarization). Switched operation would require the use of costly high-power RF waveguide switches and loads, which are not easily integrated into a portable or mobile radar application. In the E800, each polarization channel’s transmit path is controlled independently on a pulse-by-pulse basis, allowing the user to program a myriad of waveform and scan strategies to best optimize the radar’s performance against the mission at hand.