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BAS Data Sheet No 6

Electronic Collision Avoidance

There have been many attempts through the years to develop an effective collision avoidance system that is practical for use in gliders.   In an ideal world, the system should supply timely, accurate and easily interpreted collision avoidance information to the glider pilot, warning him of all other flying machines that are likely to impinge on his flight path.   Bearing in mind that glider pilots spend a significant time turning in a thermal which may be shared with many other gliders in very close proximity, this is a demanding requirement.

There have recently been a number of developments that show promise in going some way to meet this requirement.   In this information sheet some of these systems will be discussed together with comments on their practicability, effectiveness and limitations.   The whole system of potential collision avoidance is subjective and emotive;  systems that some pilots might find useful will be considered worthless by others.

Visual Enhancement

Visual enhancement has been discussed in detail and trialled through the years.   The only real effective passive system has been found  to paint the glider black as it has been shown that contrast provides the most effective visual cue.   Unfortunately, for structural reasons related to skin temperature this is not possible in the majority of gliders, although it has been used for a number of light aircraft.   Enhancing visual acuity by using strobe lights has also been tried.   While a powerful strobe light can undoubtedly improve conspicuity, in a sunlit environment the power of the strobe has to be considerable if it is to be an effective visual cue.   A good practical yardstick is that if the strobe is painful to look at at close range, then it is likely to have some effectiveness in bright sunlight.   Unfortunately, the minimum electrical power consumption to operate a strobe light of this magnitude is of the order of 5 to 7 amps  -  not a practical proposition in a glider.   Furthermore, it is difficult to streamline the strobe so that it does not degrade the glide performance while retaining all round visibility.   An ultra bright LED based system is being developed but whether it will be able to provide a pulse of white light of sufficient intensity to be effective in bright sunlight is doubtful with existing technology

Electronic Systems

There are a number of electronic anti collision systems in development of varying effectiveness.   The major systems are discussed below together with my assessment of their effectiveness.   I have tried to be pragmatic in my comments as in many cases, the designers/vendors will have you believe that their particular system should be introduced immediately!

Before discussing specific systems, an overview of the existing transponder technology, also called Secondary Surveillance Radar (SSR) may be helpful.   The present transponder environment uses what is called Mode 3A/C.   This is a development of second world war IFF (Identification Friend or Foe).   The equipment provides for a ground transmitter/antenna, normally co-located with a conventional radar antenna, in which a coded pulse is transmitted on a frequency of  1030 MHz (1.03GHz).   This pulse is received by the airborne equipment which after a specified delay, transmits a coded pulse in reply on a slightly different frequency, 1090 MHz.   This is received by the ground station and processed to enhance the return on the controllers radar screen.   Mode A provides a position fix with no height information, while Mode C encodes the aircraft's barometric height  into the coded reply.   The majority of airborne transponders were developed some 30 years ago and suffer from two problems, both of which demand a relatively high current requirement of 3 to 4 amps.   While this is easily supplied in a powered aircraft with a generator, it is more of a problem with gliders, ultra lights and balloons.   The two components that are power hungry are the Travelling Wave Tube (TWT) and the mode C height encoder.   The TWT is an electronic valve that generates a 200 watt pulse to reply to the ground interrogation.   Currently specialised transistors are now available to replace the TWT.   The mode C height encoder in the aircraft is a simple electronic pressure transducer, but in order to maintain height accuracy, the pressure transducer has to be kept at a constant temperature of 60 C in a small electric oven.   This oven requires significant current to maintain the required temperature.   Modern transducers are now available that can be temperature compensated without an oven, thus significantly reducing the overall power consumption.   The 200 watt transmit pulse is attenuated by the losses in the co-axial cable and the antenna itself such that the effective radiated power is of the  order of 70 watts.   A current generation Mode A/C transponder using power transistors in place of a TWT, and temperature compensated pressure transducer has a power requirement of about 200 m/a

Mode A/C uses analogue technology and a new system based on digital technology, Mode S, has been developed.   Using Mode S it is possible to encode a considerable amount of information on to the reply pulse.   Among other data, this can include an unique aircraft identification and the current GPS position.   A Mode S transponder is also backwards compatible  ie it will work with current Mode 3A/C ground installations.   A CAA information sheet on the introduction of Mode S is available at http://www.caa.co.uk/docs/7/DAP_SSM_Mode_S_SSR_Factsheet.pdf

LAST

LAST is an acronym for Light Aviation SSR Transponder.   The development is being driven by the CAA  and has been trialled with varying results over the last few years.   Early prototypes used Mode A/C analogue technology with an effective power of only 20 watts.   While this produced acceptable results against modern ground installations, many of the existing ground stations have 1970's waveguides and the low power signal gets lost!   The present ICAO requirement specifies a minimum airborne transponder power of 70 watts and if the CAA continue to pursue the 20 watt power requirement, then the UK will eventually have to file for 'differences' from the ICAO specification or pressure EASA  to change the ICAO requirement.   As cost effective approved 70 watt Mode S aircraft transponders with an acceptable power requirements already exist on the commercial market, the LAST trial may well be overtaken by technical advancement.    That said, 20 watts or so works well with airborne Traffic Collision Avoidance warning Systems (TCAS) and Mode S ground installations as the receivers/waveguides use modern technology.  

Transponder Proximity Receivers

Many Traffic Proximity Alert System (TPAS) receivers are available on the commercial market, some little bigger than a thick credit card.   Surecheck TPAS, Proxalert R5, and Trafficscope are versions that are currently advertised.   They work by receiving and decoding Mode A/C airborne transponder transmissions, displaying the relative position and height of the radiating aircraft.   While they will warn you of an adjacent transponder equipped aircraft, it is not reciprocal, ie the other aircraft does not know you are there.   In the UK environment, I do not think there are many conflicts between transponder equipped aircraft and gliders, although this could well change if the carriage of transponders became mandatory in uncontrolled airspace.   One problem with transponders in a TPAS environment is that they only transmit when interrogated by a ground station;  if they receive no interrogations, then they remain 'silent'.    This means that TPAS receivers will not necessarily receive any warning signals unless the transponder is being interrogated by a ground station.  To correct this, most Mode S transponders employ a technique known as an 'extended squitter' whereby the transponder transmits a pulse encoded with the aircraft's GPS position at regular intervals.   The GPS position encoded in the data stream may be used by the proximity receiver to refine the potential collision hazard.

FLARM

FLARM is a Swiss developed electronic anti-collision system that has been developed for the gliding community in the European Alps.   It provides a collision warning between two adjacent aircraft when each is carrying a FLARM unit.  Reports indicate that it is very effective in Alpine flying where a number of gliders use well known mountain routes with no significant other traffic and high head on closing speeds.  The situation is a little different when flying away from mountainous areas, where the collision hazard is predominantly between gliders and powered/military traffic.   Furthermore, as it relies on EVERY aircraft carrying a FLARM device, its universal implementation is impracticable without legislation, particularly as the unit is not currently FAA or JAA/EASA approved.

Mode S Transponders

I am not an advocate for the mandatory carriage of Mode S or any other transponder in gliders on cross country flights.   However, cost effective, small and light Mode S units, such as the Filser TRT600, and the Trig TT21 are now available on the commercial market with a power consumption of only 150 m/a when in Mode S and increasing to around 200 m/a when being interrogated in Mode A/C.

At the moment a transponder does not directly provide anti collision information between transponder equipped aircraft unless TCAS or TPAS is also fitted, but I do not think it will be long before a simple built in or add on module will be available to display this information.  Meanwhile, a simple TPAS unit will give an effective proximity warning of the close presence of a transponder equipped aircraft.

A further consideration when fitting a transponder is antenna placement.   High power microwave transmissions of 1 GHz are known to be a potential health hazard and accordingly the antenna should not be mounted close to the pilot's head.   Ideally, the antenna, which is a short whip some 7 cms long, should be located on the underside of the aircraft with as short a length of co-axial cable as possible.  The favoured location in a glider is in a corner of the undercarriage bay, taking care that it is not fouled by the movement of the wheel.   The undercarriage doors are usually thin GRP which is transparent to microwave radiation   

Automatic Dependent Surveillance Broadcast (ADS-B)

This is an electronic system where aircraft avionics broadcast the aircraft position, altitude, velocity and other parameters completely automatically.   The aircraft positional data is derived from an on board GPS and is transmitted every few seconds via a digital data link without any involvement from the pilot.   The positional data can be received by other aircraft or an ADS-B ground station and used to provide a cockpit display of other traffic to pilots, or to Air Traffic Control surveillance services.   The range of the system is up to 150 miles, dependent on the height of the aircraft.   Any equipment that is developed is almost certain to have FAA or JAA/EASA approval permitting its legal installation in a wide variety of aircraft.

Unlike conventional radar, ADS-B works at low altitudes and on the ground.   It is being developed in Australia where the vast distances and low traffic density make a conventional ground radar system an expensive proposition.

A good overview of the ADS-B system can be seen at Garmin ADSB Overview FAQ

Summary

In summary, the development of ultra bright LED technology may improve visual conspicuity, while on the electronic front systems like Flarm, ADS-B and Mode S require that every aircraft has to carry a unit.   Mode S has considerable potential, mainly because it is already carried by the majority of larger commercial aircraft, but the high power requirements make it unsuitable for use on gliders and microlights.    The greatest potential is shown by the development of autonomous datalink systems and while Flarm has many attractions as purely a glider system, the ADS-B system has the advantage of universality as it can be legally carried by all commercial aircraft, GA aircraft and gliders. 

Finally, one downside of installing an electronic collision avoidance system that should not be forgotten is that reliance on electronic systems can subconsciously reduce the need for a visual lookout.   Visual lookout remains the main method of preventing a midair collision.

Nov 2012