Why Your ELB May Not Save You

The driving force behind the establishment of the system was maritime activity and this area has also seen most effort to date in the definition of standards. The maritime equivalent to the ELB is the EPIRB (Emergency Position Indicating Radio Beacon), which features a waterproof design.

The COSPAS/SARSAT satellites use a simple technique for locating an EPIRB/ELB. This involves the use of interferometric antenna techniques to establish the beacon position along a parallel to the satellite’s track, while a Doppler technique is used to pinpoint the position along the track. As the satellite passes overhead (or alongside), the Doppler shift of the transmitted 121. 5 MHz carrier abruptly jumps from positive to negative (from approaching target to receding target) and this transient, given the known position of the satellite, indicates the position of the beacon to within several kilometres accuracy. Aircraft with homing receivers can then pinpoint the beacon to initiate SAR operations.

Marine EPIRBs in Australia must comply with ECR 241, a standard which defines the physical design requirements and radio signal characteristics of the beacon. In order for the beacon to be tracked by COSPAS/SARSAT, it requires fairly tight constraints on the AM modulation and stability of the VHF carrier signal. If these are not met, the satellite may detect the beacon but cannot fix its position which largely nullifies the benefit of its usage. The specific requirement is for at least 30% of the transmitted power to be contained in the carrier and for the carrier to be within +/- 30 Hz of the nominal 121. 5 MHz beacon frequency.

Older generation airborne ELBs in most instances cannot meet this requirement, in no small part due to the design optimisations discussed above. ELBs of this class often operate with carriers overmodulated to maximise the power in the audible sidebands, this results in carrier phase jitter which interferes with Doppler tracking. In addition, many older designs are so simple in terms of circuit design, that the modulator circuit can load the oscillator and cause spurious frequency deviations in the transmitted carrier wave. As the satellite relies on detecting changes in carrier frequency produced by the Doppler effect, a weak carrier which is drifting or jumping about in frequency becomes a difficult proposition indeed and position location becomes impossible.

At this time no Australian manufacturers produce satellite compatible ELBs, although Standard Communications- Ph (02) 816 4755- and Kinetic Technology- Ph (03) 583 9566- produce ECR 241 compliant marine EPIRBs. The author was unaware of the scale of the problem until a chance meeting with the Chairman of the CAA, Dick Smith, who raised the subject and suggested this article following our Reece editorial in the April A A.

The CAA are in a uniquely difficult position because there is no current international standard for COSPAS/SARSAT compatible aviation beacons. The ICAO body tasked with defining the new international standard will not define the new standard until later this year, the Gulf War having caused the postponement of the standards meeting scheduled for earlier this year. This situation is further complicated by the direction taken by COSPAS/SARSAT, who plan to phase out the 121. 5 MHz frequency by the end of the decade, replacing it with the 406 MHz digital scheme currently in limited maritime use. The 406 MHz scheme involves the use of intelligent EPIRB/ELB devices which transmit burst mode digital signals which contain encoded identification unique to the platform carrying the beacon. Needless to say, the 406 MHz scheme will demand much increased transmitter power, relatively complex electronics and hence much larger or more potent batteries. The inevitable result will be a far more expensive and possibly larger beacon.

The CAA is therefore in the unenviable position of either having to go it alone in the interim and produce a unique Australian standard, or to do nothing until the new! CAO standard is defined and then adopt the international standard. Since the ICAO standard may very well be a 406 MHz beacon, the adoption of a uniquely Australian 121. 5 MHz standard could very well be a shortlived exercise, imposing a substantial cost burden upon operators. Whatever the outcome of the !CAO meeting, the ultimate life of the 121. 5 MHz standard is limited and any Australian standard will have to account for this.

Clearly the situation is complex and there are unlikely to be any simplistic answers. In the short term however, 121. 5 MHz is the only economical choice, particularly for general aviation aircraft.

The immediate problem faced by users of obsolete ELBs is that of establishing whether the device is or isn’t compatible with the satellite tracking system. There is no simple check, as the device must be tested by a suitably trained technician with a VHF band spectrum analyser which itself must be proven to be properly calibrated. Only then, by viewing the transmission spectrum from the ELB, can its suitability be established.

Dick Smith has proposed the carriage of marine EPIRBs, which are cheap, in the $250 range, and can be easily tested with dummy loads without producing false alarms. However, as portable devices they lack the impact switch and mountings for installation as permanent internal systems in aircraft. Therefore the marine EPIRB will not provide the crash resistance of the hardened and impact activated aviation ELB, and if the occupants of the crashed aircraft are severely injured or incapacitated, cannot be turned on. On the other hand where the occupants survive the crash in a condition where they can activate the EPIRB, it will provide a fix for the satellite and for SAR aircraft. In summary it is apparent that a large proportion ofAustralia’s ELBs are block obsolescent due to evolving technology and will need to be replaced with COSPAS/SARSAT compliant equipment. However, until the new ICAO standard is defined and adopted in Australia, the simplest measure is to carry, in addition to the internal ELB, an ECR 241 compliant marine EPIRB in the aircraft with the knowledge that it does suffer the limitations of a non-aviation rated device, and not to entertain any illusions otherwise.