Vertical Navigation Radar
Matjaz Vidmar, S53MV
The two previous sections, "Modules" and "Assembly" discussed hardware details for the technician building and/or installing the proposed vertical navigation radar. On the other hand, it is very important for the final user to understand in detail the operation of a relatively new instrument like the proposed vertical navigation radar. This understanding is required both to make full use of a new instrument as well as establishing confidence with a new instrument and detecting possible malfunctions that could affect flight safety.
Even before assembling the whole instrument in the aluminum case it makes sense to connect together the main modules and perform some tests in the laboratory. An early version of the proposed instrument is shown under test on the following image:
During this test all alignments are optimized. In addition, in the RF section, the VCO has to be tuned for best linearity by cutting the central finger of the interdigital filter. The noise figure and gain of the LNA can be optimized by adding a few pieces of tinned copper foil.
The calibration of the radar is performed by an artificial radio path including cables to generate delays and attenuators to emulate propagation loss. The calibration hardware is shown on the following drawing:
The 0feet emulation includes the same two 1.5m-long (5feet) RG-142 antenna cables as installed on the aircraft, a total of 42dB (6dB+30dB+6dB) attenuation and an additional 1.2m RG-188 cable for the antenna & radio-path group delay. The result of the 0feet test is shown on the following image:
The 0feet test corresponds pretty well to what the instrument sees while the aircraft is on ground. Please note that the strong reflection at 0feet also raises the noise level at the top of the scale above 1000feet. This effect is normal and it should be checked by the pilot before flight!
The 25feet emulation includes all of the components of the 0feet emulation and in addition a 6dB attenuator and a 10m long section of RG-223 cable. 10m in polyethylene equal to 15m in free space or 7.5m (25feet) radar range in two directions. The result of the 25feet test is shown on the following image:
The threshold of the voice synthesizer is set by the pushbutton on the back panel. Depressing this pushbutton a vertical bar appears at the threshold intensity level as shown on the following image:
By keeping the pushbutton depressed, the bar starts moving. Releasing and depressing once again the pushbutton changes the direction of the bar as indicated by the small markers on the bar. Releasing the pushbutton also saves the selected threshold level in nonvolatile memory of the main processor.
If the threshold is adjusted too high, the voice synthesizer will fail to announce targets with weak reflections. On the other hand, if the threshold is adjusted too low, the voice synthesizer will also announce bad reflections with corrupted data and will repeat the message "zero", triggered by the antenna crosstalk when the maximum range is exceeded. The correct adjustment of the threshold depends on the particular antenna installation and requires a test flight!
The most common failure mode of all avionics are damaged antennas and/or broken or disconnected cables. At least the noise and the antenna crosstalk at 0feet should always be present. Therefore DO NOT USE THE INSTRUMENT if you see an "empty" display as on the following image:
A disconnected cable may even generate "ghost" reflections. Therefore DO NOT USE THE INSTRUMENT if you see a strange display as on the following image:
A normal display contains the reflection from ground, the antenna crosstalk and noise. The following image shows the display during normal operation at the maximum range of the described instrument at 5000feet:
While flying over a town at 900feet, many other reflections are visible above 900feet: the 900feet peak has a long "tail" extending to the top of the scale. The antenna crosstalk peak at the bottom at 0feet remains unaffected, indicating a correct operation of the instrument on the following image:
A vertical navigation radar is very useful while flying close to a mountain ridge. This potentially dangerous location is frequently used while gliding. In this case, many reflections from different altitudes result in a rather broad reflection on the display. Of course the maximum at 370feet shown on the following image is announced by the voice synthesizer as ground-proximity warning, rounded to "350":
A broad reflection peak always means very rough terrain below the aircraft. This is also the main reason why a vertical navigation radar needs a graphical display. Flying at 370feet may be completely safe over the sea or over flat terrain. The same altitude above a mountain ridge may quckly evolve into a crash!
On the other hand, the diffusion angle from the sea surface is much narrower, resulting in a narrow line at 1000feet on the following image:
While flying low over the sea surface, a double-pass reflection can be observed. The following image shows the crosstalk at 0feet, the main reflection from the sea surface at 500feet, a very narrow double-pass reflection radar-sea-aircraft-sea-radar at 1000feet and a small and rather broad peak at 3000feet from the coast.
Lakes have an even smoother surface than the sea, therefore they provide even stronger and narrower reflections than the sea. Besides the strong and narrow reflection peak at 500feet above the lake surface, reflections from trees at 4000feet surrounding the small lake are also visible on the following image:
Of course, the most important application of the described radar is assistance during all phases of the landing. The correct altitude can already be checked during the long final, as shown on the following image:
The voice synthesizer starts talking below 600feet. A typical image of a final on a large grass airfield is shown on the following image:
Below 100feet it is definitely necessary to move the sight on the runway, away from the instruments and concentrate on the landing flare. Of course, the voice synthesizer will guide the pilot during the final and landing flare like in the following example:
Final & flare from voice synthesizer
The proposed vertical navigation radar certainly adds a new dimension to VFR. Safety is improved through reducing the workload of the pilot and providing additional warnings at the same time. Of course, the user needs to gain familiarity with the new instrument, know its limitations, understand the results and be aware of potential failure modes.
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