High Frequency Doppler Research Articles

Evaluation of transesophageal Doppler detection of air embolism in dogs.

The authors assessed the use of a cylindrical sensor which transceives (transmits and receives) ultrasound in a 360 degree arc mounted on a standard esophageal stethoscope catheter to detect air embolism in dogs. They used electronic circuitry developed specifically to provide a continuous analog recording of high frequency Doppler energy as well as an audible signal. They found that the esophageal Doppler sensor was easy to position. In 25 of 30 dogs, the system distinguished Doppler sounds of venous air emboli that were present either in the superior vena cava, right atrium, right ventricle, or pulmonary artery from normal cardiovascular sounds. In the remaining five animals, Doppler heart and air emboli sounds were initially of poor quality, but improved with aspiration of air from the esophagus. In another five dogs, arterial air emboli produced by left ventricular injections also were detected. The esophageal sensor was sensitive to both venous and arterial air emboli ranging from 0.05--0.2 ml of air, and the authors consistently detected repeated intravenous injections of air throughout a 5-h time period. Optimal position of the sensor for detection of venous air emboli was at the level of the superior vena cava above its junction with the right atrium. Optimal position to detect arterial air emboli introduced via a left ventricular catheter was at the level of the aortic arch. Tissue analysis of the esophagus revealed no morphologic damage due to the sensor or transmitted energy.

Tracking the velocity and position of offshore vessels and floating objects with HF Doppler transponders

Experiments with a high-frequency (HF) Doppler radar and a frequency-shifting transponder show that the velocity and position of ships or floating objects can be tracked without the need for accurate differential-position measurements. Maximum-entropy spectral analysis permits tracking during rapid maneuvers using sampling times as short as 4 s.

A new technique for measuring tidal currents by using a two‐site HF Doppler radar system

We have measured surface currents by using a newly developed high‐frequency (HF) Doppler radar technique. The system was operated in lower Cook Inlet, Alaska, in July 1977. During one particular day, July 13, we analyzed the radar data collected every 30 min for 24 hours. By least squares fitting these data to two of the dominant tidal periods, 12.31 and 24.6 hours, we determined the spatial distribution of the tidal currents for each of these two periods. In addition, we determined the spatial distribution of the mean surface currents for this 24‐hour period. Our analysis of this data shows that near Kachemak Bay there is a distinct difference of the east‐west component of tidal flow in comparison with regions further from Kachemak. In addition, we observed longer period components for which we cannot determine the period. These components exhibited gyrelike spatial patterns. Our results demonstrate the use of HF radar techniques for resolving spatial and temporal currents which affect the nearshore environment.

Low‐Latitude Ionospheric High‐Frequency Doppler Dispersion Study

Studies of the Doppler frequency dispersion on a multiple high‐frequency phase stable propagation link were made at a low‐latitude site under a variety of geomagnetic disturbance conditions. Frequency stability of the system enabled us to measure ionospheric‐induced Doppler frequency changes as low as 0.01 Hz. The average Doppler frequency changes during development of the reflecting region at dawn can be characterized consistently by an exponential decrease of positive frequency versus time as the critical frequency of the F‐region increases. The power spectral density of the fluctuations in Doppler frequency was usually nearly white over the fluctuation period range 1 < τ < 20 minutes. The power spectral density magnitude correlates well with the planetary magnetic disturbance index Kp summed over the period of the observations. This result suggests that the observed effects are related to irregular movements of the F region. Occasional coherent‐appearing spectral peaks were observed that may be a manifestation of wave energy in the acoustic window propagated to ionospheric altitudes. No obvious relationship between the appearance of these coherent disturbances and the tropospheric weather pattern could be found.