Search

SigNals Of Opportunity: P-band Investigation (SNOOPI) will be the first on-orbit demonstration of remote sensing using Signals of Opportunity (SoOp) in P-band (240–380 MHz). P-band is needed to penetrate through dense vegetation and into the root zone. The longer wavelength of P-band also increases the unwrapping interval for phase observations. These observations hold the potential for spaceborne remote sensing of root-zone soil moisture (RZSM) and snow water equivalent (SWE), two variables identified as priorities in the 2017–2027 Decadal Survey for Earth Science and Applications from Space. SNOOPI will provide in-space validation of both the P-band SoOp technique and a science instrument prototype. SNOOPI technology validation goals will be met by targeting observations within 9 km of the SMAP calibration/validation sites in the continental United States. A secondary priority is collection of continuous phase data over snow-covered regions. These goals are evaluated under constraints of a limited data budget and mission lifetime, with a launch readiness in early 2022. Updates on the development of measurement models and mission planning to support SNOOPI are provided. A ground-based station will be deployed to monitor the noncooperative sources, in order to reduce risk due to uncertainty in knowledge of the broadcast power, spectrum shape, and orbital position.

In vacuum devices in which electrons are accelerated through a large potential difference immediately before impact with the fluorescent screen, loss of contrast can occur due to electrons which are back-scattered in the collision process. This paper describes an investigation into the behaviour of electrons back-scattered from screens having differing thicknesses of backing material. Measurements have been carried out of the back-scattering coefficient and the energy distribution of back-scatter electrons from a number of differing screen samples. From these measurements and the light output/applied voltage response curves of the screens, the light output due to back-scattered electrons compared with the light output due to the primary electrons in a variety of conditions can be calculated. Previous attempts to reduce contrast loss by using carbon, which has a low back-scattering coefficient, have not been very successful. It is shown that with careful choice of backing thickness and operating conditions it is possible to reduce contrast loss due to back-scattering to less than that from different sources such as internal light reflection in the faceplate glass.

The travelling-wave phototube (TWP) is a microwave detector of amplitudes modulation at optical frequencies. This paper gives an account of the theory of the TWP in terms of modified travelling-wave tube (TWT) coupled mode theory. The characteristics of the TWP are discussed and a comparison made between it and the TWT. The concept of equivalent impedance used to evaluate the performance of the TWP is introduced and its dependence on various parameters shown. Finally, the noise output is calculated and figures given for the performance of a practical device.