Further Considerations

 

While the implementation listed above would work, there are many further aspects that need to be considered in creating a final system.

1.1     Multiple echoes

Reception of multiple echoes can already be noticed as a potential problem in the experiment that was conducted.  In the experiment, it was possible to use human judgment in order to determine which received pulse was the correct echo.  However, this will not be the case in the final system.  The correct echo is typically the last echo received.  Therefore, the microprocessor should incorporate instructions to choose the last echo seen within a predefined period of time. 

 

A possible algorithm could choose the amount of time to wait for an echo to be equal to the approximate time expected.  This would vary as the aircraft descended, and could start with a value of the time it takes a signal to travel 80m.  As the aircraft descended, the timeout value could be set to equal the actual time from the last signal plus an error factor.  

1.2     Time Between Pulses

The time between pulses is factor that needs to be considered.  Possibly the best solution is to simply have the microprocessor emit a pulse following reception of the previous echo.  In this manner, a pulse will never conflict with reception of an echo, and the time between pulses will increase as the aircraft descends, resulting in higher accuracy as the plane gets closer to the waterıs surface.

1.3     Speed of Sound

The speed of sound is not a constant, but rather a function of temperature and humidity.  As a result, it will change with differing weather conditions.  The altimeter requires a method of ascertaining the correct speed of sound in order to make accurate calculations.

Two methods have been discussed in order to calibrate the altimeter.

 

The first method involves using additional devices, such as a thermometer and hygrometer, and calculating the speed of sound based on the measured values.  These devices might already be available on the aircraft, in which case the altimeter would need to be interfaced with them, or they might need to be included within the altimeter design.

 

The second method would make use of the altimeter itself.  A secondary system could be used to reflect a calibrating pulse off another part of the aircraft, which is at a fixed known distance from the altimeter.  The time taken to receive the echo would then be used in conjunction with the measured distance to determine the speed of sound. This system would be self-calibrating and could be set to recalibrate itself every few seconds as the aircraft descended to allow for varying climatic conditions.

1.4     Environmental Conditions

The altimeter will likely be exposed to high winds and possible submersion in water.  As a result, it must be housed in a casing that is waterproof as well as strong.  The microphone should be protected from wind as much as possible, as wind would add extra noise to the signal received, possibly hampering accurate measurement.

1.5     Mounting Point

An important consideration in designing the altimeter is where it will be mounted on the aircraft.  Mounting it on a wing would probably be inadvisable, as it might interfere with flight dynamics. A possible location may be at the same location as where the struts of the pontoons of a floatplane are mounted.