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Tower Proximity Warning Transponder System This work has been published in the Teen Ink monthly print magazine.

   History of the Problem

From the time that an impending object is seen, to the time of possible impact, a small-aircraft pilot has only six seconds on average, to react and avoid the situation. The "see and avoid" system by which many pilots navigate around obstructions typically requires twelve or more seconds of active reaction time. This is why all radio and TV broadcast towers are known as Awidow makers' to the aviation society.

Currently, the Federal Aviation Administration has no laws requiring the use of collision warning systems or relative coordinate position systems in non-commercial aircraft. Although many aircraft owners have chosen to invest in some non-standard flight safety equipment, there is still a large percentage of pilots who fly by sight. One popular piece of equipment available for use on small planes is the receiver and processing equipment for the Global Positioning System (GPS). The GPS is a geometry-based satellite data interpretation System, using a maximum of eight out of a possible 24 communications satellites for radar information at any given moment. While the GPS is a very useful, albeit expensive, tool for aircraft accident prevention, many aircraft owners would rather use standard maps and instrumentation to avoid large obstructions.

While modern aircraft are very quick and efficient, they are also more dangerous in relation to shorter reaction-time availability. Were a large, multiple-engine, private jet to collide with a large communications tower, the entire surrounding community would be showered with debris and tower segments. Power and phone lines would be knocked down. Traffic would be stopped for miles. Perhaps dozens of lives would potentially be lost in a given accident.

Weather conditions play a very significant role in the loss of visual contact with tall dangerous structures. Often, weather conditions in the northeast change rapidly and unpredicted low ceilings may sweep over a region of widely varied terrain. About 70 percent of the accidents surveyed occurred in the low-light and low-visibility conditions most often present during the foggy autumn and winter seasons.

Quincy: A Concerned Local Community

The controversial seven-hundred foot communications tower in Quincy, Massachusetts has been the center of attention in broadcast and print media recently. This concern is a product of the public awareness of lenient FAA regulations and low visibility in thunderstorm, fog and heavy snowfall conditions. The main complaint filed with the Quincy Zoning Board is from the residents in the adjacent town of Milton relative to the hazards involved with medical helicopter flight paths less than half a mile away. There is an extremely high probability that a slight radar or satellite-based positioning system malfunction could someday cause an accident. For medical helicopters that do not even use a positioning system, the probability of a potential crash in harsh weather is significantly greater.

Necessity for the Transponder System

An updated version of the GPS could provide the information necessary to avoid a collision with large towers. However, aircraft with only the updated GPS (or an equivalent ground location determination system) would still need an alternate warning system for supplementary obstruction detection. For any low-flying aircraft, including the many helicopters and planes which have no collision warning system, a simple transponder receiver and processor could be used to coordinate with the tower distress transmitters. This is what our entry team has designed.

In order to address the concern for a low cost, easily integrated aircraft warning transmitter on a tower with communications antennas installed, one must first consider several factors. Standardization of signals, federal and state laws, communications band frequencies, and effectiveness versus cost are the main determinants in establishing a national standard for a large obstruction warning system. However, these factors are not yet relevant for a transmitter design in mere prototype format. Each of the following designs functions efficiently either in conjunction with the GPS or as a separate system.


The first of our four tower proximity warning transponder system (TPWTS) designs relies on a standard, slightly modified caution signal. The use of a simple omni-directional multi-element, vertically polarized array antenna with 500 milliwatts of input power would yield a threshold signal strong enough to reach aircraft receivers within a several-mile radius of the tower. The digital data sent by this antenna would include tower height, latitude and longitude; the frequency of data repetition should not exceed about 5 times per second to increase accuracy of reception. With these design characteristics, the specific signal field of the transmission for the Quincy site would be a broadcast "donut" of about 6 miles in radius; the exact signal strength is unpredictable due to environmental and receiver noise floor variation.

For the purpose of compatibility, an aircraft owner would need to purchase an inexpensive corresponding TPWTS. I receiver. The receiving antenna design consists of three forward-facing antennas mounted on the underside of the aircraft. The TPWTS. 1 is recommended for planes which use the GPS. Whereas the overall system cost (including possible maintenance) is minimal, the simplicity of the system would not provide accurate enough primary directional and relative position information for independent use.


The TPWTS.2 is slightly above the TPWTS.1 in complexity and reliability. While the TPWTS.1 utilizes an omni-directional, multi-element, vertically polarized array antenna, the TPWTS.2 uses a directional antenna with 3dB horizontal beam width of 45 degrees. This directional antenna would be enclosed in a weatherproof beacon housing at about 400 feet and would rotate at a frequency of one cycle per second, transmitting a data sentence every 125 milliseconds. This data burst would include tower height, latitude, longitude and cardinal direction of the signal. The processing for the direction data is synchronized with the rotating antenna, such that exact relative compass direction of the tower is known to the pilot. Every time the beacon completes a cycle, the aircraft processor receives graphable data, based on change in relative direction and aircraft velocity. Expenses for this system are greatest due to maintenance; a rotating antenna may easily break down and specialized repair services would be difficult to obtain.


Our third proposed transponder system utilizes the same basic transmission equipment as the TPWTS.2. However, the TPWTS.3 does not require any Moving parts. While the antenna is the same for both designs, the TPWTS.3 requires eight antennas and a specialized planar frame for mounting individual bays. Each of these antennas is separated by 45 degrees, and represent each of the cardinal directions, totalling 360 degrees. The transmitter sends out the constant data strings, along with the variable heading specific for each of the eight directions, in sequence for each antenna switch, without the need for multiple transmitters. Eight antennas were chosen because too many would be expensive and weighty, too few antennas would not provide accuracy of proximity.


The last of our four transponder system designs entails a highly complex, specifically tailored variation of the TPWTS.2. The transmission of data bursts involves four electronically-steerable antennas, capable of separately sweeping each 90 degree field of signal by means of phase shift technology. Depending on the distance between the three or four main beams of a communications tower, the arrangement of the antennas must be altered for the total of the electronically steerable fields to equal 360 degrees. The transmitter would quadrilaterally partition its TPWTS.2-like compass directions and switch transmission lines from antenna to antenna, in sequence. Similar technology is currently being applied in the field of broadcast engineering; however, the equipment involved is nonstandard. Although this system would work well for a prototype, it would not be employable as a national standard.


In conclusion, the TPWTS.2 is theoretically the most economic, efficient and widely applicable design. A prototype of the TPWTS.2 would, of course, be planned in much more detail than in this summary. Since the technology involved has not previously been applied, the test flight receiver prototype would need to be monitored with various equipment sensors and recording devices. The signal strength and rotation mechanism stability would be monitored from within the transmitter and on several ground locations. In addition, several weeks of recorded synchronization checks would assure that the TPWTS.2 was ready for real-life test situations. Perhaps months or years of documented success in the possible Quincy prototype site would prove that the TPWTS.2 could be used as a national precedent for tower proximity warning transponder systems.

Despite the eyesore of seven bright red lights polluting the dusk horizon, local community members would at least feel safer knowing that the chances of a medical helicopter collision have dropped considerably.u

This work has been published in the Teen Ink monthly print magazine. This piece has been published in Teen Ink’s monthly print magazine.

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