Military Radio Communications Research in Australia

An overview of recent research by the Australian Defence Science and Technology Organisation in the field of military radio communications is presented. A philosophy for improving digital radio system performance over complex, variable channels is outlined. A key breakthrough, called PDF-directed adaptive radio, which can provide substantially greater throughput over HF channels whilst minimising bit-error rate and delay, is described. Simulation results for fast adaptive Schemes applied to both serial-tone and parallel-tone HF modems are presented and shown to significantly out-perform fixed rate modems and modems employing hybrid automatic-repeat-request schemes. A new detector scheme is discussed which has superior performance to conventional detectors for digital traffic in the presence of inter-symbol interference and impulsive noise

Neural networks and early fast Doppler for prediction in meteor-burst communications systems.

Thesis (Ph.D.)-University of Natal, Durban, 1994.In meteor-burst communications systems, the channel is bursty with a continuously fluctuating signal-to-noise ratio. Adaptive data rate systems attempt to use the channel more optimally by varying the bit rate. Current adaptive rate systems use a method of closed-loop decision-feedback to control the transmitted data rate. It is proposed that an open-loop adaptive data rate system without a decision feedback path may be possible using implicit channel information carried in the first few milliseconds of the link establishment probe signal. The system would have primary application in low-cost half-duplex telemetry systems. It is shown that the key elements in such a system would be channel predictors. The development of these predictors is the focus of this research. Two novel methods of predicting channel parameters are developed. The first utilises early fast Doppler information that precedes many long duration, large signal-to-noise-ratio overdense trails. The presence of early fast Doppler at the trail commencement is used as a toggle to operate at a higher data rate. Factors influencing the use of early fast Doppler for this purpose are also presented. The second method uses artificial neural networks. Data measured during trail formation is processed and presented to the neural networks for prediction of trail parameters. Several successful neural networks are presented which predict trail type, underdense or overdense, and peak trail amplitude from the first 50ms of the trail's lifetime. This method allows better estimation of the developing trail. This fact can be used to implement a multi-rate open-loop adaptive data rate system

The study of an adaptive bit rate modem for meteor scatter communications

Includes bibliographical references

LOCATION, ORBIT AND ENERGY OF A METEOROID IMPACTING THE MOON DURING THE LUNAR ECLIPSE OF JANUARY 21, 2019 & TESTING THE WEAK EQUIVALENCE PRINCIPLE WITH COSMOLOGICAL GAMMA RAY BURSTS

Location, orbit and energy of a meteoroid impacting the moon during the Lunar Eclipse of January 21, 2019 During the total lunar eclipse of January 21, 2019 at least two meteoroids impacted the moon producing visible flash lights on the near side. One of the impacts occurred on the darkest side of the visible lunar face and was witnessed by many astrophotographers. In this paper we present estimations of the location, impact parameters (velocity and incoming direction), orbit and energy of the meteoroid, as obtained from images and videos collected by amateur astronomers in Colombia, the Dominican Republic, Morocco, USA, Canary Islands, Cape Verde, Czech Republic, Austria, and Germany. Astrometric measurements on the images put the impact location at selenographic lat = -29.43 and lon = -67.89 while photometric measurements predict the flash brightness of Gf = 6.7. The novel Gravitational Ray Tracing (GRT) technique is used to estimate the orbital properties and radiant of the impactor. We find that that the meteoroid impacted the moon with a speed of 13.8 km/s (70% C.L.) and in a relatively shallow angle, (6 of visible light in a short time (0.3 seconds). The total impact energy was ~0.5 tons of TNT which correspond to a body with a mass ~20 kg and a diameter of ~30 cm. If our assumptions are correct, the crater left by the impact will have ~10 meters across and it could be detectable by prospecting lunar probes. These results arose from a timely collaboration between professional and amateur astronomers which highlight the importance of citizen science in contemporary astronomy. Testing the Weak Equivalence Principle with Cosmological Gamma Ray Bursts Gamma Ray Bursts (GRBs) with rapid variations at cosmological distances are used to place new limits on violations of the gravitational weak equivalence principle (WEP). These limits track intrinsic timing deviations between GRB photons of different energies as they cross the universe, in particular in the KeV to GeV energy range. Previous limits in this energy range have involved only the gravitational potential of local sources and utilized temporal variability on the order of 0.1 seconds. Here WEP violation limits are derived from sources with greater distance, faster variability, and larger intervening mass. Specifically, GRB sources with redshifts as high as 6.5 are considered, with variability as fast 0.2 milliseconds, and passing the gravitational potentials of inferred clusters of galaxies distributed randomly around the line of sight. WEP violation limits are derived from data from GRB 910711, GRB 920229, GRB 021206, GRB 051221, GRB 090429, and GRB 090510. The strongest constraint in the very early universe comes from GRB 090429 which limits gamma (500 keV) - gamma(250 keV) \u3c 1.2 x 10-13. The strongest overall constraint comes from GRB 090510 which yields a WEP violation limit of gamma(30 GeV) - gamma (1 GeV) \u3c 6.6 x 10-16. This strongest constraint is not only a new record for WEP violation limit for gamma-ray photons and in the early universe, but the strongest upper bound for Delta gamma that has ever been recorded between any two energy bands

Potential markets for a satellite-based mobile communications system

The objective of the study was to define the market needs for improved land mobile communications systems. Within the context of this objective, the following goals were set: (1) characterize the present mobile communications industry; (2) determine the market for an improved system for mobile communications; and (3) define the system requirements as seen from the potential customer's viewpoint. The scope of the study was defined by the following parameters: (1) markets were confined to U.S. and Canada; (2) range of operation generally exceeded 20 miles, but this was not restrictive; (3) the classes of potential users considered included all private sector users, and non-military public sector users; (4) the time span examined was 1975 to 1985; and (5) highly localized users were generally excluded - e.g., taxicabs, and local paging

Monitoring the Long Wavelength Transient Sky with the LWA1 Telescope

Radio transient astronomy has received a vastly increasing amount of interest within the last few decades. In this time, several new sources have been discovered and many more have been predicted. These sources are spread throughout the radio spectrum, and many emit strongly within the low frequency (10 - 100 MHz) regime. The first station of the Long Wavelength Array (LWA1) is a compact array of 260 dual polarization dipole antennas operating between 10 and 88 MHz. With good sensitivity, high time and frequency resolution, and an instantaneous field of view up to ~ 20,000 deg2, the LWA1 an ideal instrument for searching for transient phenom- ena. This dissertation presents transient work done with the LWA1, which includes a search for prompt emission from gamma ray bursts as well as a blind search for un- specific transients. These searches resulted in new limits on astronomical transients and the discovery of radio emission from large meteors (fireballs). This dissertation also presents a highly sensitive followup study on the fireball emission, which has yielded new insight into the origin of the emission, suggesting that it is emission of plasma waves within the plasma trail

Mechanisms of turbulent mixing in the Continental Shelf bottom boundary layer

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1999The bottom boundary layer is an important dynamical region of shallow water flows. In this thesis, the problem of turbulent mixing in the coastal bottom boundary layer is investigated with a unique set of field measurements of velocity and sound speed that span a significant fraction of the boundary layer obtained over a six-week long period in the late summer of 1996 on the New England shelf. The energetics of the turbulent fluctuations are investigated by testing simplified budgets for turbulent kinetic energy and scalar variance. The turbulent kinetic energy budget is locally balanced while the scalar variance budget is not, probably due to turbulent diffusion. The direct effects of stratification are consistently significant only in the outer part of the boundary layer, where the flux Richardson number is approximately equal to a critical value of 0.2. Turbulence closure is investigated in terms of non-dimensional profiles of velocity and sound speed. Close to the bottom, the results are consistent with Monin-Obukhov similarity theory, while in the outer part of the boundary layer other scales including the height of the boundary layer are important for setting the turbulent length scale.My doctoral work was supported by the Office of Naval Research under grants N000149S10373 and N0001496109S3