Term
| What is electromagnetism? |
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Definition
| Magnetism developed by an electrical current passing through a wire coiled around a metal bar that induces a magnetic field. |
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Term
| If electromagnetic waves were visible and moving, describe what pattern they would have? |
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Definition
| Like a series of sine waves. |
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Term
| What is the effect of wavelength on attenuation? |
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Definition
| Attenuation has a greater affect on a shorter wavelength and weakens the energy of the beam. |
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Term
| How are frequency and wavelength related? |
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Definition
| Inversely, because wave speed is constant. |
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Term
| What does the attenuation of radar waves depend on? |
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Definition
| The wavelength of the emitted waves and the size and composition of the particles encountered. |
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Term
| Explain why absorbed energy that is later re-radiated back to the antenna is not recognized: |
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Definition
| Energy is absorbed and reradiated at a different wavelength. This energy that is different and unrecognizable to the radar the energy that was originally transmitted. |
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Term
| Name two ways to measure pulse length: |
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Definition
| In units of time it takes the radar to send one pulse (expressed in microseconds) or in units of distance (horizontal) from the front edge to the back edge of the pulse (expressed in meters) |
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Term
| What does the amount of energy transmitted by the radar depend on? |
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Definition
| The duration or length of the pulse. |
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Term
| Define pulse repetition frequency (PRF): |
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Definition
| The rate at which the pulses are transmitted in a unit of time. |
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Term
| What is the effect of pulse length and PRF on radar range? |
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Definition
| Pulse length affects the minimum range of the set, because the transmitter must be shut off by the time the reflected signal returns. Pulse repetition frequency affects the maximum range. The signal must have time to return to the set before the next pulse is transmitted. |
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Term
| Measured from the leading edge to the back edge of the pulse: |
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Definition
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Term
| The rate at which pulses are transmitted: |
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Definition
| Pulse repetition frequency (PRF). |
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Term
| The period of waiting for a returned pulse: |
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Definition
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Term
| Is limited by how often pulses are broadcast: |
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Definition
| Maximum unambiguous range. |
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Term
| Occurs when energy is received from an old pulse after transmission of the next pulse: |
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Definition
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Term
| What happens if an antenna moves at too great an angle between pulses? |
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Definition
| The number of pulses returned per target will be too low, and some regions may not be probed for targets. |
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Term
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Definition
| Range folding occurs when energy is received from an old pulse after transmission of the next pulse. |
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Term
| Where can the maximum power density of the radar beam be found? |
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Definition
| Along the centerline of the beam. |
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Term
| What occurs when a target occupies only a small portion of the beam, thus hiding or altering the true characteristics of the target during display? |
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Definition
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Term
| Explain the term “below beam effects” |
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Definition
| Missed targets below the beam. |
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Term
| What is beam blockage the result of? |
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Definition
| The radar beam striking large obstructions near the antenna site. |
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Term
| How far should two targets be separated, in order to be displayed as separate targets? |
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Definition
| By more than one beam width. |
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Term
| What is the ability of the radar to distinguish between two targets at the same direction but at different ranges called? |
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Definition
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Term
| How does the cone of silence affect the viewing of meteorological targets that move into it? |
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Definition
| Meteorological targets appear to weaken and disappear as they move into the cone of silence. |
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Term
| Normally, what does the radar beam do with respect to height as it moves away from the antenna? |
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Definition
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Term
| List the two types of anomalous propagation (AP) and briefly describe the atmospheric conditions that cause each to occur. |
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Definition
- Superrefraction—which occurs when warm, dry air overlies cool, moist air, as in an inversion. The result isthat the radar beam bends below its normal path.
- Subrefraction—occurs when water vapor contentincreases and temperature decreases with height. The result is that the radar beam travels above its normal path.
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Term
| When the radar beam is being refracted below its normal path, what is this process called? |
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Definition
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Term
| What is the straightening of the radar beam upward called? |
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Definition
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Term
| What effect does superrefraction have on the displayed echo heights on the radar? |
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Definition
| Displayed echo heights are overestimated. |
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Term
| What effect does subrefraction have on the displayed echo height on the radar? |
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Definition
| Displayed echo heights are underestimated. |
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Term
| List five assumptions that the basic radar equation makes? |
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Definition
- The particles are small, homogeneous spheres whose diameters are much smaller than the radar’s wavelength (Rayleigh scattering)
- The particles are spread uniformly throughout the contributing region (sample volume)
- Precipitation throughout the sample volume is the same (all rain or all snow—no mixed precipitation)
- The main lobe of the antenna beam pattern is adequately described by mathematical notation.
- Attenuation and multiple scattering are negligible.
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Term
| The reflectivity measurement received for each sample volume is simply a measurement of what? |
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Definition
| Power, expressed in watts. |
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Term
| The strength of a thunderstorm return increases from 49dBZ to 52dBZ. By how much has the strength increased? |
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Definition
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Term
| How does the use of the decibel system, to compare returned radar energy, allow increases or decreases to be expressed? |
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Definition
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Term
| What is the best resolution for base reflectivity product and how are the sample bins averaged? |
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Definition
| The .54nm reflectivity product. Average of four successive .13nm bins. |
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Term
| Describe how the 1.1nm base reflectivity product is displayed: |
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Definition
| Displays the highest of every two .54nm bins. |
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Term
| Describe how the 2.2nm base reflectivity data is displayed: |
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Definition
| Displays the highest of four .54nm bins. |
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Term
| What are the maximum degrees of phase shift in electromagnetic energy that can be related to a correct velocity? |
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Definition
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Term
| At how many degrees of phase shift do velocities become ambiguous? |
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Definition
| Greater than or equal to 180°. |
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Term
| If radar energy strikes a stationary target, what kind of phase shift is experienced? |
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Definition
| The electromagnetic energy experiences no phase shift. |
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Term
| If a target moving toward the radar causes the energy to be backscattered, will it be at a higher or lower frequency? |
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Definition
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Term
| What is the Doppler shift? |
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Definition
| The change in position of the backscattered wave as the radar interprets it to be moving is called the Doppler shift. |
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Term
| What is meant by the term radar coherency? |
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Definition
| It is when a radar produces a pulse at the same frequency as the preceding ones and can remember the specific frequency to determine the frequency shift, and therefore, the radial velocity. |
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Term
| The ability of the radar to compare the frequency of each new pulse with that of the preceding pulse is known as what? |
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Definition
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Term
| By convention, how are velocities toward the radar expressed? |
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Definition
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Term
| By convention, how are velocities away from the radar expressed? |
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Definition
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Term
The image below represents a single dipole exhibiting a frequency shift; use it to complete the following question.
[image]
If the antenna is at point B and facing directly toward E, how is the radar detecting the target velocity?
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Definition
| It shows the target as stationary because the motion is perpendicular to the beam; therefore, the target’s velocity is depicted as zero. |
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Term
The image below represents a single dipole exhibiting a frequency shift; use it to complete the following question.
[image]
If the antenna is at point A and facing directly toward D, how is the radar detecting the target velocity? |
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Definition
| The motion is directly away from the beam. Therefore, the full component or full velocity is detected. |
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Term
The image below represents a single dipole exhibiting a frequency shift; use it to complete the following question.
[image]
If the antenna is at point F and facing directly toward C, how is the radar detecting the target velocity?
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Definition
| The motion is neither parallel nor perpendicular to beam, therefore only part of the target’s velocity is detected. |
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Term
The image below represents a single dipole exhibiting a frequency shift; use it to complete the following question.
[image]
If the antenna is at point D and facing directly toward A, how is the radar detecting the target velocity?
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Definition
| The motion is directly toward the beam. Therefore, the full component or full velocity is detected. |
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Term
If the environmental wind is from 240° at 30kt and the antenna is pointing toward 180°, what is the velocity the radar detects?
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Definition
Using the equation Vr = Va (cos θ)
Since θ = 60° (the difference between 240° and 180°)
and Va = 30 knots
Then:
Vr = 30kt (cos 60°)
Vr = 30kt (0.5)
Vr= 15kt
Since the motion is toward the radar it is expressed as a negative, therefore the answer is –15kt. |
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Term
| Define unambiguous velocity: |
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Definition
| The limit to the speeds the WSR–88D can measure without error. |
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Term
| In radar terms, what are the speeds that exceed the maximum unambiguous velocity called? |
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Definition
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Term
| What limits velocity detection? |
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Definition
| The wavelength of the radar. |
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Term
| How does increasing the PRF affect the chances of aliasing? |
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Definition
| Increasing the PRF increases the Nyquist co-interval. |
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Term
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Definition
| It is the tradeoff where an increase in PRF increases the maximum unambiguous velocity but decreases the maximum unambiguous range. |
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Term
| What is the Doppler dilemma a compromise between? |
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Definition
| The maximum range and the maximum velocity. |
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Term
| A PRF of 4,000 pulses per second results in what kind of velocity detection and range? |
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Definition
| Since 4,000 pulses per second is a high PRF, velocity detection will be high, but the range will be very short. |
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Term
| What is the square of the standard deviation of the distribution known as? |
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Definition
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Term
The sample volume represents the smallest amount of the atmosphere that can be instantaneously sampled by the radar. Why is this? |
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Definition
| The energy returned from particles separated by less than the sample volume’s dimensions will be averaged together as a single return. |
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Term
Use the diagram below to answer the following question.
[image]
Which of the diagrams best shows a precipitation echo with large particles and high internal shear? |
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Definition
| Figure C, since large particles reflect more power back to the radar there will be more area under the curve. |
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Term
| What is most likely occurring when you observe low base reflectivity values with high spectrum width values? |
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Definition
| Early stages of convective development. |
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Term
| What is most likely the most likely cause of uniform spectrum width values with isolated areas of higher values? |
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Definition
| Stratiform precipitation with embedded convection. |
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