EMC Awareness
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Radiocommunications Agency EMC Awareness |
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Aviation/Aerospace
Poor bonding causes interference with navigation and communications
Video Walkman interferes with critical instruments
Why cellphones are banned during airflights
Examples of satellite interference
| Poor bonding causes interference with navigation and communications |  |
Description
A pilot complained about navigation and communication equipment on his plane becoming inoperable when flying through rain. Studies have shown that when an aircraft flies through rain, static electricity on the aircraft skin can exceed 100,000 volts and arcing between poorly bonded aircraft surfaces can be a source of broadband RF noise.
Author
Masone recalls a flight test that involved flying EMI specialists into a storm.
Immediately upon entering the storm, Masone heard a high-pitched squealing sound
from the pilot’s headset. Its intensity was such that the pilot had to
remove his headset. The navigation display then went black. All navigation and
communication equipment was inoperable in a whiteout condition with freezing
rain and snow! Upon exiting the storm, all navigation and communication functions
returned to normal operation. Back at the hangar, high voltage testing led to
the discovery of a poor bond between two surfaces on the horizontal stabilizer.
Commentary
‘Tribocharging’ is the name given to the build-up of electrical charge on a surface or object due to friction with another material. Basically, electrons become detached from one material and attached to the other. Walking with plastic or rubber soled shoes across a synthetic carpet can cause a person to become ‘tribocharged’ up to as much as 20kV.
Tribocharging also occurs when one of the materials is a liquid or a gas. Water is generally conductive, but pure water or ice are not, so hail or snow in a storm can cause aircraft surfaces to charge up. Some parts will charge more than others, so if they are metal and they are not properly bonded together, arcing can occur between parts of the aircraft.
Arcing creates powerful emissions across the entire radio spectrum, all at once, which would be picked up by the various antennas on the aircraft and would be bound to cover any frequencies used by navigational and communications equipment, causing interference and even complete loss of signal.
References and links
“The case for combining EMC and environmental testing”, W H Parker, W Tustin and T Masone, ITEM 2002, pages 54-60, http://www.interferencetechnology.com.
Links to Mitigation Techniques
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| Electrical bonding |  |
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| Arc suppression | |
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| Video Walkman interferes with critical instruments | |
Description
Following take-off from ‘BDL’ airport in a DC9 in July 1999, whilst
climbing to cruising altitude, the captain’s 
radar
altimeter gave an error message (‘flagged’) and the traffic and
ground proximity alert systems subsequently showed ‘FAIL’. These
three critical instrument malfunctions were independently verified by two other
flight crew.
The problem continued throughout the climb to 35,000 feet when it became possible for a flight attendant to check the passenger cabin to see if any passenger portable electronic devices were in use. She discovered that a Video Walkman was in use in seat XX. After it was switched off, the problems cleared up.
Commentary
Radiated emissions from digital processing products (such as computers, portable video/DVD players, etc.) occur at discrete frequencies – the harmonics of its internal ‘clock’ oscillators. If these emitted frequencies coincide with the frequencies used by various navigational instruments, such as radar altimeter, they could cause interference. The emissions could come directly from the product’s printed circuit board or the devices mounted on it, but are mostly emitted by leads attached to the product (such as headphone leads) acting as ‘unintentional antennas’. Closer to the product, the emitted fields are more intense.
Much of the electronic equipment for the avionics instruments is located under the floor of the passenger compartment, which may not provide any shielding. Certain seats in the passenger cabin of an aircraft will be closer to certain avionic equipment, and/or may be closer to antennas located outside the aircraft. So portable electronic devices (PEDs) may be more liable to cause interference when they are used by people sitting in those seats.
One would usually expect equipment that meets emissions standards such as required for CE, FCC or VCCI marking not to cause interference, but not all equipment complies with these standards, and sometimes a ‘bad batch’ could get manufactured which had higher emissions than expected.
However, radiated emissions test standards measure using antennas in the ‘far field’ and avionics equipment aircraft’s external antenna could be in the ‘near field’ – or at least significantly closer than is assumed by the test standards. For example, CISPR 22 (EN 55022) measures emissions at 10 metres distance and does not guarantee freedom for interference for domestic radio receivers that have antennas closer than 10 metres to the product tested.
References and links
NASA Aviation Safety Reporting System (ASRS) report dated 1st May 2002, download from: http://asrs.arc.nasa.gov/report_sets/ped.pdf. This example is No. 443329 in the ASRS report.
There are many other examples of suspected PED interference problems in the ASRS report, and there are also numerous examples and useful links in the next reference.
“Electromagnetic Interference with Aircraft Systems: why worry?” article ref: RVS-J-97-03 by Peter B. Ladkin and colleagues, University of Bielefeld – Faculty of Technology, Networks and Distributed Systems, Research group of Prof. Peter B. Ladkin, Ph.D., at: http://www.rvs.uni-bielefeld.de/publications/Incidents/DOCS/Research/Rvs/Article/EMI.html
“Avionics and Portable Electronics: Trouble in the Air” by Albert Helfrick, presented at the Aircraft Electronics Association Annual Convention, Nashville, TN, 1996. Avionics News Magazine, September 1996. Available from http://bluecoat.eurocontrol.fr, Public Reports.
Links to Mitigation Techniques
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| Circuit design for emissions control | |
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| Filtering with CM cable-mounted chokes | |
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| Filtering | |
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| Shielding of area and volume | |
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| Shielding of cables | |
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| Why cellphones are banned during airflights | |
Description
In October 2002, a set of avionic equipment was tested under controlled conditions in a test chamber for susceptibility to cellphone interference. General aviation avionic equipment, representative of earlier analogue and digital technologies, was used. The equipment, comprising a VHF communication transceiver, a VOR/ILS navigation receiver and associated indicators, together with a gyro-stabilised remote reading compass system, was assembled to create an integrated system.
The tests covered the cellphone transmission frequencies of 412MHz (Tetra), 940MHz (GSM900) and 1719MHz (GSM1800), including simultaneous exposure to 940 and 1719MHz. The applied interference field strengths were up to 50 volts/metre for a single frequency, and 35 volts/metre for dual frequencies.
The following anomalies were seen at interference levels above 30 volts/metre, a level that can be produced by a cellphone operating at maximum power and located 30cms from the victim equipment or its wiring harness.
Most anomalies were observed at 1719MHz.
The results of the tests endorse current policy that restricts the use of cellphones in aircraft.
The CAA will remind operators about the specific risk from cellphone usage on the flight deck, and recommend that confirmation be obtained from passengers at check-in that cellphones in their luggage have been switched off.
Commentary
Cellphones are an example of a Passenger Electronic Device (PED) that pose particular problems for aircraft because of the relatively high powers of their RF emissions compared to other PEDs (such as a video walkman or laptop computer, discussed elsewhere).
Much of the electronic equipment for the avionics instruments is located under the floor of the passenger compartment, which may not provide any shielding. Certain seats in the passenger cabin of an aircraft will be closer to certain avionic equipment, and/or may be closer to antennas located outside the aircraft. So cellphones may be more liable to cause interference when they are used by people sitting in those seats.
Whereas the weak emissions from most PEDs are most likely to cause interference when an emitted frequency coincides with a frequency already being used by radiocommunications or navigation systems, the more powerful emissions from a cellphone are capable of causing interference in any electronic circuits, if the phone is close enough, through demodulation in non-linear devices such as transistors, diodes, surge protectors, ICs, etc.
Many aircraft flying today use avionics navigation and communication systems designed well before cellphones became commonplace, and as a result they may lack the necessary immunity.
References and links
Effects of Interference from Cellular Telephones on Aircraft Avionic Equipment, CAA Paper 2003/3, http://www.caa.co.uk/docs/33/CAPAP2003_03.PDF. The CAA is the UK’s Civil Aviation Authority.
For the above report and many others on aircraft interference issues, including those relating to cellphones, go to the CAA homepage at http://www.caa.co.uk and click on ‘Search’, then type ‘interference’ into the search box (also try searching by ‘cellphone’ or ‘PED’).
“Phones in flight can be fatal”, IEE Review, June 2003, page 16, from sales@iee.org.uk, http://www.iee.org/shop or http://www.iee.org.uk/Library.
Items numbered 447036, 460415, 467979, 487546, 495128, 504194 and 524699 in the NASA Aviation Safety Reporting System (ASRS) report dated 1st May 2002. Download the report from: http://asrs.arc.nasa.gov/report_sets/ped.pdf.
“Mobile may have caused Swiss air crash” by Fiona Fleck, Daily Telegraph 11 Jan 2001, page 16.
Other examples of cellphone interference with aircraft can be found in the “Banana Skins compendium”, via a link from www.compliance-club.com or at: http://www.compliance-club.com/archive1/Bananaskins.htm, especially (at the time of writing) numbers: 112, 113 and 187.
Links to Mitigation techniques
Clearly, ensuring that all cellphones carried onto an aircraft by a passenger, or in their hold baggage, are switched off is a good mitigation technique. But guaranteeing that nobody forgets or uses their cellphone during take-off, flight or landing requires the fitting of ‘cellphone detectors’ that sound an alarm if they detect the characteristic transmissions from cellphones in stand-by mode or in use. At least one company makes such detectors. It would clearly be better to check hold baggage in this way before it is loaded into the hold of the aircraft.
Designing avionics systems that are less susceptible to interference, without adding too much weight in additional shielding and filtering, requires the full range of EMC-by-design techniques.
| Installation | Design & Development | Resources | |
| Circuit design for reducing demodulation | |
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| PCB layout for EMC | |
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| Shielding of areas and volumes | |
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| Shielding of cables | |
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| Filtering with CM cable-mounted chokes | |
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| Filtering | |
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| Examples of satellite interference | |
Description
The wake shield experiment was launched February 1994, but the small satellite used could not be deployed due to EMI with its attitude control system. This was caused by inductive coupling (crosstalk) between the unshielded attitude control sensor cable and the power bus of the spacecraft. This was an unpleasant lesson learned at the cost of a failed experiment.
The Gamma Ray Observatory satellite experiment launched in 1991 experienced a transponder lockup that prevented the spacecraft from receiving control commands. EMI from a ground source (plus design problems) was the cause.
The NOAA-11 weather satellite was launched in 1988. In September 1991 a series of phantom commands were observed and determined to be caused by EMI due to a noisy VHF (Very High Frequency) environment.
The NOAA-12 weather satellite was launched in 1991. In September 1991 it experienced phantom commands when it flew over Europe, due to the heavy commercial VHF environment over Europe.
Commentary
Great care and attention to detail goes into the design and construction of satellites, because of the enormous cost of putting them into space. But there is also great pressure to keep weight down to reduce the launch costs, or to get more value out of the satellite given the payload that can be managed by the launch vehicles. This pressure can result in cables screens, enclosure shields and filters not being employed where necessary.
Also, it is clear that the electromagnetic environment in space can be affected by terrestrial communications and other electronic activities, and it can be difficult to know what environment to design for.
The NASA report (see below) states: “The seemingly continuous stream of technological advances in electronics has important future implications for EMC in regard to spacecraft and spacecraft payloads. The effects of electromagnetic interactions in electrical/electronic systems are of ever-growing concern because of the increasing susceptibility of system components to EMI, use of automated electronic systems, and pollution of the electromagnetic environment with emissions.”
References and links
The above incidents are items 2.2.2, 2.2.4, 2.2.5, and 2.2.6 in NASA Reference Publication 1374: “Electronic systems failures and anomalies attributed to electromagnetic interference” published in July 1995. Download it from: http://trs.nis.nasa.gov/archive/00000296.
Links to Mitigation Techniques
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| Routing cables | |
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| Cable shielding | |
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| Software protocols to reject interference | |
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| Saturn launch vehicle, range safety interference | |
Description
During on-pad checkout at the Kennedy Space Centre prior to one of the early development test flights of the Saturn launch vehicle, the range safety receivers detected an extraneous signal. Because these receivers processed commands for engine cut-off, arm and destruct, a thorough investigation was conducted.
The spurious signals were caused by the multitude of telemetry transmitters located on board to collect test data, however, none of these were operating near the range safety receiver frequency.
Further investigation determined that the various RF signals were ‘mixing’ and producing intermodulation products in a non-linear circuit created by metalwork that was not properly bonded, namely the hinged cable tray covers and chain handrails on the gantry.
Commentary
Non-linear circuits (e.g. all semiconductors and ICs) will demodulate RF signals, as has been mentioned elsewhere in these examples, and this is a common cause of lack of immunity to RF fields.
But non-linear circuits will also generate harmonics of the RF signal they are exposed to, and whenever more than one frequency is present at the same time they will produce intermodulation frequencies at the sum and difference of the RF signals.
Intermodulation in non-linear circuits is used in most high-quality radio receivers and spectrum analysers, where the non-linear circuit is known as a ‘mixer’, but it can happen in any non-linearity including oxidised or corroded metal connections and dissimilar metal junctions, that are not well-bonded at RF.
This can be a big problem for the mating connectors in the cables carrying high-power RF signals from a transmitter to its antenna – even very tiny non-linearities at the metal-to-metal connections between the mated halves can create sufficient levels of harmonic or spurious signals to contravene their licensing requirements.
References and links
The above incident is item 2.1.1 in NASA Reference Publication 1374: “Electronic systems failures and anomalies attributed to electromagnetic interference” published in July 1995. Download it from: http://trs.nis.nasa.gov/archive/00000296.
Links to Mitigation Techniques
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| Preventing intermodulation | |
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| RF bonding of metalwork | |
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