Tuesday, January 2, 2018

Regulating Radiated Emissions

Equipment installed on aircraft must be designed and tested to be compatible with other equipment and with the airborne environment.  Notably, radio frequency (RF) radiated emissions from one system may disrupt other aircraft systems such as radio receivers or present hazards to human health.

There are two edges to emissions: the source of the emissions and the receiver of the emissions.  The source of emissions is managed by compliance to limits that change based on frequency (a spectral mask).  The receiving system "susceptibility" to radiated emissions is tested by blasting the equipment with a barrage of signals representing the worst case (resulting from surrounding equipment radiating at their spectral limits).

RTCA DO-160 chapter 21 provides a basis for compliance to radiated emissions.  All equipment, whether intentional transmitter or unintentional, have components that produce radiated emissions.  A typical example is a clock used to run a processor.  The most stringent spectral mask has "notches" that limit emissions in aviation bands.

DO-160G Radiated Emissions Spectral Mask


RTCA DO-160 does not regulate intentional transmissions.  These are radio frequency signals that are produced as part of an aircraft communication system that utilize dedicated, approved spectrum. Underlying this is the concept that there is no conflict or overlap in authorized or licensed radio transmitters - that they are mutually compatible with other aircraft systems.  Regardless of the slice of authorized spectrum, all radio transmitters still must comply with the spectral mask elsewhere.

A notable issue relates to Iridium. Iridium utilizes time-domain multiple access (TDMA), where a common frequency is used half the time to transmit and half the time to receive.  The Iridium receiver can become overwhelmed by an Inmarsat transmitter, due to strong out-of-band emissions.  On the other side, Iridium out-of-band emissions can blank an installed GLONASS receiver.  RTCA MOPS apply guidance on all of the above systems in order to arbitrate a compatible solution.  I generally assume Iridium and Inmarsat operate mutually-exclusive if on the same airplane, and no GLONASS with Iridium; but there are possible options using filters.

Another example is managing intermodulation products, where emissions are created due to multi-channel transmissions.  For example, a limit is applied to each aero-Inmarsat terminal such that only higher-order intermodulation products intrude upon the GPS L1, 1575 MHz, carrier with sufficient attenuation.

Conducted emissions are also a factor, just not discussed here.

Limits are applied to radiated emissions by the FCC.

The FCC limits proximity less than 20 cm to portable (handheld) devices using Specific Absorption Rate (SAR) limits.

The FCC limits proximity, Maximum Permissible Exposure (MPE), to radio transmitters using guidance from Special Bulletin OET 56, Biological Effects and Potential Hazards of Radiofrequency Electromagnetic Fields.  

Satcom transmissions are above 13,750 MHz. The limit for controlled exposure (6 minutes) is 5 mW/cm2.  The limit for uncontrolled exposure (30 minutes) is 1 mW/cm2.

Fortunately, Satcom transmissions are non-ionizing. From OET65:
Ionization is a process by which electrons are stripped from atoms and molecules. This process can produce molecular changes that can lead to damage in biological tissue, including effects on DNA, the genetic material. This process requires interaction with photons containing high energy levels, such as those of X-rays and gamma rays. A single quantum event (absorption of an X-ray or gamma-ray photon) can cause ionization and subsequent biological damage due to the high energy content of the photon, which would be in excess of 10 eV (considered to be the minimum photon energy capable of causing ionization). Therefore, X-rays and gamma rays are examples of ionizing radiation.  
Ionizing radiation is also associated with the generation of nuclear energy, where it is often simply referred to as "radiation." The photon energies of RF electromagnetic waves are not great enough to cause the ionization of atoms and molecules and RF energy is, therefore, characterized as non-ionizing radiation, along with visible light, infrared radiation and other forms of electromagnetic radiation with relatively low frequencies. It is important that the terms "ionizing" and "non-ionizing" not be confused when discussing biological effects of electromagnetic radiation or energy, since the mechanisms of interaction with the human body are quite different.

The GEE/QEST tri-axis (GSAA) antenna offers one example MPE analysis.


At the highest transmit frequency of 14.5 GHz, the wavelength is 0.0207 m.

The near field radius is Rnf = 4.75 m

Emissions within the near-field are assumed up to the predicted limit anywhere within the near-field.

Main-beam emissions may exceed both controlled and uncontrolled emission limits throughout the near-field.  Humans should not intrude within the near field.

The Far Field radius is then Rff = 11.41 m

Power Density decreases linearly between the Near Field radius and the Far Field radius.

Power Density decreases as an inverse square of the distance when beyond the Far Field radius.

The controlled limits are met in-between the near-field radius and the far-field radius, at 7.22 meters (23.7 feet).

The uncontrolled limits are met beyond the minimum far-field radius, at 13.1 meters (43.1 feet).

No sidelobe emission exceeds the uncontrolled limit, and therefore no additional restriction is applied.

The main beam is elliptical in shape, inversely proportional to the aperture extent.  Ground operations are aligned such that elevation beamwidth reaches towards the ground and azimuth slews it around the horizon.  As transmissions are directed towards a satellite that will be at least five degrees above the horizon, the main beam emissions are necessarily attenuated pointing downwards.

In general, humans should assume the possibility of illumination by the main beam only if "at or above" the top of the aircraft fuselage.  It should be safe to operate without restriction while walking on the ground.

ARINC 791 anticipates the need for suspending transmissions on the ground for maintenance, such as during deice, when personnel are physically within the region above the antenna ground plane.  Operation on the ground is not always allowed due to licensing issues.  Procedures are required to ensure that transmissions are inhibited where the assumption is that the main beam is pointed directly at any persons working nearby.

For Ku/Ka band satcom equipment, the software design assurance level E provides little confidence that transmissions are inhibited, when pondering human safety.  The issue is worsened by the absence of any outward indication of radio frequency transmissions.  ARINC 791 provides for hardware interlock (removal of power) by a switch connected to the KANDU.

Inherent in normal maintenance actions is a means to inhibit reporting equipment faults as a result of disabling transmissions.

Stay tuned!

Peter Lemme

peter @ satcom.guru
Follow me on twitter: @Satcom_Guru
Copyright 2018 satcom.guru All Rights Reserved

Peter Lemme has been a leader in avionics engineering for 35 years. He offers independent consulting services largely focused on avionics and L, Ku, and Ka band satellite communications to aircraft. Peter chairs the SAE-ITC AEEC Ku/Ka-band satcom subcommittee, developing ARINC 791 and 792 characteristics and contributes to the Network Infrastructure and Interfaces (NIS) subcommittee developing Project Paper 848, standard for Media Independent Secure Offboard Network.

Peter was Boeing avionics supervisor for 767 and 747-400 data link recording, data link reporting, and satellite communications. He was an FAA designated engineering representative (DER) for ACARS, satellite communications, DFDAU, DFDR, ACMS and printers. Peter was lead engineer for Thrust Management System (757, 767, 747-400), also supervisor for satellite communications for 777, and was manager of terminal-area projects (GLS, MLS, enhanced vision).

An instrument-rated private pilot, single engine land and sea, Peter has enjoyed perspectives from both operating and designing airplanes. Hundreds of hours of flight test analysis and thousands of hours in simulators have given him an appreciation for the many aspects that drive aviation; whether tandem complexity, policy, human, or technical; and the difficulties and challenges to achieving success.

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