Thomas M. Roberts

Array aperture design using irregular polyomino subarrays

This paper presents data and a procedure for the use of irregular subarrays in planar wideband arrays. Using element-level phase shifters and time delay at subarray ports, the procedure substitutes polyomino subarrays in place of conventional rectangular subarrays to eliminate quantization lobes. The paper focuses on the following details of the design: the choice of subarray size for specified bandwidth, the selection of subarray type, and the tradeoff between bandwidth and peak sidelobe level. These items of the design procedure are detailed for arrays with 4- and 8-element subarrays.

Asymptotics and energy estimates for electromagnetic pulses in dispersive media: addendum

We review two references: one that appeared almost simultaneously with our paper [J. Opt. Soc. Am. A13, 1204 (1996)] and another that appeared later. We mention an earlier relevant paper, and we also correct two typographical errors in our paper.

Parameter variation with array size for broadband arrays of irregular polyomino subarrays

This paper has investigated the array parameters for arrays of irregular polyomino subarrays used to introduce time delay for large planar phased arrays, and compared these parameters with those of arrays using rectangular subarrays. It is shown that the gain reduction is only about 0.1 dB compared to rectangular subarrays and about 0.4 dB compared to an array with time delays at each element. Finally, it is shown that the array of irregular subarrays has no evident quantization lobes (quantization lobes do not decrease with array size), but has a number of low residual sidelobes that are shown to continually decrease with array size.

Performance envelope and reliability assessment of the NGST HEC cryocooler

The Northrup Grumman Space Technologies High Efficiency Cryocooler (NGST HEC) was designed to support a 10 Watt cooling load at 95 Kelvin while rejecting heat to an effective sink interface temperature of 300 Kelvin. This design is an example of the pulse tube with inertance tube variant of the Stirling thermodynamic cycle whose compressor section uses dual opposed pistons to minimize vibration imparted to any cooling load through the cold end. The Air Force Research Laboratory has characterized the extended performance envelope of this refrigeration system, including its off nominal design point performance and efficiency, its response to transient loading and rejection temperatures, and its cool down performance from ambient. In order to assess this systems long term ability to support extended continuous duty space missions, this cryocooler has been running continuously for over two years, as part of a five year study on whether significant degradation in performance can be measured over that time. Finally, comparison of this cryocooler to other similar space qualifiable refrigeration systems has been made.

Performance Degradation of Cryocoolers for Space Applications

The Cryogenic Cooling Technology Group at the US Air Force Research Laboratory supports development of prototype cryocoolers to meet Department of Defense requirements for space surveillance and target detection. Some coolers have shown deterioration in performance during the detailed laboratory characterization and endurance evaluation. Some of them have shown anomalies due to material choice, contamination, manufacturing defects, or some yet unknown factors. The degradation is either manifested gradually, such as due to leaks, or abruptly, such as vibration‐noise. The general causes and concerns for space cryocooler performance degradation are presented in the beginning followed by case studies of some of the developmental coolers, given in order to communicate lessons learned to the cryocooler community.