From Minitrack to NAVSTAR: The early development of the global positioning system, 1955–1975
The paper outlines the early days of satellite navigation, focusing on the work of the Naval Research Laboratory (NRL) on systems that contributed to the later Global Positioning System (GPS). NRL's innovations included the Minitrack system for space surveillance, the development of high-precision timing systems for use in space, the Timation navigation satellite program, and the first two Navigation Technology Satellites (NTS-1 and NTS-2). NAVSTAR-GPS emerged as a joint Department of Defense (DoD) program in 1973, by which time much of the GPS concept had been developed and a number of GPS technologies had been tested.
Wind Speed Retrieval Method for Shipborne GNSS-R
This study presents a sea surface wind speed retrieval method for the maritime environment, using forward-scattering signals from the global navigation satellite system (GNSS), which benefits from reduced costs compared with active radar detection. Most retrieval algorithms for GNSS-reflectometry (GNSS-R)-based wind measurement technique have predominantly focused on spaceborne and airborne platforms, whereas research related to shipborne platforms is relatively lacking. However, owing to the low altitude of shipborne receiving equipment, the Glistening Zone is concentrated in a small chip range. Therefore, based on the different slopes’ change in the front and rear stages of the time-delay waveforms, a threshold wind speed retrieval algorithm was established to improve the retrieval accuracy of shipborne GNSS-R. The experimental results using an actual ship show that the algorithm produces smaller root-mean-square error and mean error, and a maximum deviation of less than 2 m/s.
Similarities in protozoan and flatworm habituation behavior.
ONE problem in the study of habituation in protozoa is that similar results, given the same experimental manipulations, may not be obtained in metazoa. I shall compare habituation behaviour in the protozoan Spirostomum with that in the micrometazoan Rhabdocoel flatworm Stenostomum; it remains to be shown whether there is a correspondence between protozoa and metazoa both in behaviour and in their underlying chemical processes. Both these organisms contract in response to mechanical and touch stimuli: when subjected to repeated stimulation, they habituate1. Furthermore, the more intense the stimulus, the longer it takes for habituation to occur1.