Radiocommunications Agency  EMC Awareness

Radio Susceptibility

TETRA basestations and Fylingdales lock drivers out of their cars

GPS navigation vulnerable to TV antenna boosters

HMS Sheffield disaster

Problems with the U.S. radio system puts police officers at risk

TETRA basestations and Fylingdales lock drivers out of their cars

Description

Ken Yard of the Radiocommunication Agency described recent problems with the introduction of the TETRA services to the UK. Interference from this source to car alarms and immobilisers had caused over 12,000 call-outs on roadside recovery services in the last year alone.

He said that the problem was partly caused by TETRA base stations but the main cause was poor quality receivers (in the car system) with insufficient rejection of out-of-band transmitters. He hoped that this situation could be avoided with the new 868MHz band for car keyfobs.

In a similar way, high power radar pulses from the Fylingdales military base trigger the immobilising devices of many makes of cars and motorcycles - BMW, Mercedes and Jeep among them. Many have had to be towed out of range of the base before they can be restarted.

The RAF admits it is a problem but says it is down to the car manufacturers to change their frequencies. However, Jeep claims this is not possible because of government restrictions.

North York Moors National Park car park attendant Bill Peirson said that Jeep Cherokees, Mercedes cars and vans, and BMWs seemed to be worst affected by the radar. "As soon as the alarms go off, I go over to the owners and explain it's probably the Fylingdales radar that's caused it”.

"Motorbikes are the worst. There was a bike alarm screaming all afternoon recently and the rider didn't have any breakdown cover. I asked a friend in the village with a trailer to tow him away, and as soon as they were out of Fylingdales' range, it stopped."

Commentary

TETRA is a relatively new Europe-wide cellphone-like system operating in the 410-415 and 420-425MHz bands, whereas car alarm, immobiliser and central-locking systems controlled by ‘vehicle radio keyfobs’ are permitted to operate license-free at 433.72-434.12 throughout Europe. (Some older radio keyfobs in the UK operate around 418MHz.) .) The Fylingdales radar also transmits in the region of 400MHz.

Assuming that these transmitters do not emit excessive out-of-band noise (whether harmonics or spurious), the problem lies with the ‘adjacent channel’ performance of keyfob receivers.

Because keyfob systems are subject to intense price pressure, the radio circuits they use are generally low quality. Their transmitting frequency is not tightly controlled (and varies with temperature); and ‘super-regenerative’ receivers are used instead of the more selective superheterodyne type. Super-regenerative receivers tend to have very poor adjacent channel rejection.

The UK Government set up the Radio Activated Key Entry (RAKE) committee, which has recommended that radio keys for vehicle access should only be treated as a convenience feature and should never be used as the sole means of activation in safety critical situation.

References and links

“Compromise on 868 MHz”, page 14 of Low Power Radio Association News May 1999, describing a meeting on March 23rd 1999. The Low Power Radio Association is at: http://www.lpra.org.

A number of useful documents on this type of issue, including the 1997 RAKE guidelines, are available from http://www.radio.gov.uk/topics/low-power/lp-index.htm.

“Low power radio devices – a guide for the purchaser and systems designer”, a report for the Radiocommunications Agency by York EMC Services Ltd, University of York, http://www.yorkemc.co.uk/Research/index.htm

“EMC susceptibility of low power radio services”, M H Capstick, D A Pearce & L M McCormack, a report for the Radiocommunications Agency by York EMC Services Ltd, University of York, http://www.yorkemc.co.uk/Research/index.htm

Fylingdales: “Son of star wars leaves drivers stranded” by Paul Brown and Nigel Burnham, Wednesday December 18 2002, The Guardian.
To see this story with its related links, go to http://www.guardian.co.uk.

Note: Radars can interfere with radiocommunications, and vice-versa, too. See: “The Report of an Investigation into the Characteristics, Operation and Protection Requirements of Civil Aeronautical and Civil Maritime Radar Systems”, a report by Alenia Marconi Systems Limited, October 2002 for the Radiocommunications Agency and available from http://www.radio.gov.uk/topics/research/topics.htm#sharing

Links to Mitigation Techniques

	Installation	Design & Development	Resources
Adjacent channel rejection
Transmitter frequency control

GPS navigation vulnerable to TV antenna boosters

Description

At least three noise sources were preventing the use of GPS navigation aids in Moss Landing harbour (Monterey, California, U.S.A.) and at times up to 3km outside the harbour. The problem was well-known and reported at least once to the U.S. Coastguard and Federal Communications Agency (FCC), but it persisted until local engineers and scientists began to hunt them down.

The team roamed the waterfront with a spectrum analyser and receiver. They identified two culprits – all of them readily available, commercial-grade television antenna boosters fitted to yachts moored in the harbour.

The FCC found that these two preamplifiers came from the same factory, which sold units to at least four well-known U.S. brand names of consumer electronics equipment. The bad units apparently began with a design change in late 2000. In the autumn of 2001 the FCC succeeded in locating a third GPS jammer in Moss Landing harbour, which was also a VHF/UHF television antenna with preamplifier.

A study for the RA has looked at some implications of the vulnerability of GPS. It comments

“…protection of the GPS receivers using L1 has not been agreed in the UK. Therefore GPS receivers within the UK operate on a non-interference basis and can claim no protection from other users of the radio spectrum. Responses [to questionnaires] showed very little awareness of the underlying vulnerability of GPS/GNSS to interference or its potential loss of positioning data in situations of low satellite visibility. An example of GPS’s sensitivity to interference is that if the same rf power as a car remote door opener operating continuously and centred on GPS then reception would not be possible within a radius of less than ~0.5km.

…The UK is growing more reliant on GPS for fundamental activities. The fundamental weakness and vulnerabilities of GPS signal reception should be more widely publicised, especially for those services where significant inconvenience or critical impact could occur, although more GPS satellites may be launched.”

Commentary

The Global Positioning System (GPS) uses a network of numerous low-cost satellites in low earth orbit. Their on-board transmitters are not very powerful and their signal at the earth’s surface is less than the background noise of most receivers. Software algorithms are used to detect their signals despite this noise, but even so the signals are easily masked by co-channel interference.

The VHF/UHF antenna boosters/preamplifiers concerned had a flaw which caused them to self-oscillate at a frequency above 1GHz which was very dependant on temperature and supply voltage, and at times would coincide with the GPS ‘L1’ frequency. Sufficient power was radiated from the attached TV antennas and mains power cables to prevent GPS reception for some kilometres.

Such devices should be designed/manufactured to prevent self-oscillation.

References and links

“System Challenge – The Hunt for RFI – Unjamming a Coast Harbor” by James R Clynch, Andrew A Parker, Richard W Adler and Wilbut R Vincent of the Naval Postgraduate School, and Paul McGill and George Badger of the Monterey Bay Aquarium Research Institute, published in GPS World, January 2003 edition, pages 16 - 22, http:/www.gpsworld.com.

“US Coast Guard warning on GPS interference from antenna boosters, 10 August 2002”: http://www.ccg-gcc.gc.ca/mcts-sctm/GPSinterference_e.doc

Other examples of GPS interference can be found in the “Banana Skins compendium”, via a link from http:/www.compliance-club.com or at: http://www.compliance-club.com/archive1/Bananaskins.htm, especially (at the time of writing) numbers: 98, 119, 134, 207, 222 - 224, 227 - 232 and 236 - 238.

The ‘Volpe report’ on the vulnerability of GPS used in the transport infrastructure can be downloaded via: http://www.navcen.uscg.gov/gps/geninfo/pressrelease.htm, or direct from http://www.navcen.uscg.gov/archive/2001/Oct/FinalReport-v4.6.pdf.

“The Impact of GPS Signal Loss on UK Civil Infrastructure”, by S J Harding of QinetiQ, October 2001 (QINTEIQ/FST/CRMV/CR011937), available from http://www.radio.gov.uk/topics/research/topics.htm under the heading ‘Convergence and emerging technology’.

Links to Mitigation Techniques

	Installation	Design & Development	Resources
Circuit design – RF stability
Filters

HMS Sheffield disaster

Description

A well-known EMC-related catastrophe concerns the ‘Type 22’ frigate H.M.S. Sheffield, causing the loss of the ship and 20 lives, during the Falklands war.

The Sheffield’s search radar was switched off when the satellite communication system was used, because of interference from the radar. Without its search radar the Sheffield’s anti-missile defences could not be used, and this allowed an Exocet missile to hit the ship on 4th May 1982.

Commentary

Wartime is often a severe test of EMC performance, and lives are often lost when it is inadequate. Clearly, a satellite communications system on a warship should be able to be used without exposing the ship to danger from hostile activities.

The situation is complicated by the fact that on a warship both the radar and satellite antennas are located near to each other on the ship’s main mast, as high as possible to give the best performance. So the coupling between the two antennas will be quite high.

Added to this, either the radar system was transmitting harmonic or spurious signals that coincided with (or came too close to) the satellite communication frequency, or the satellite receiver did not have enough co-channel or adjacent channel rejection This story illustrates that although radio EMC issues are crucial for any military system, getting all the system parameters right is a very complicated matter and operational compromises often have to be made – perhaps with drastic consequences.

References and links

“EMC and Functional Safety, Impact of IEC 61000-1-2”, Dick Groot Boerle, Teamleader EMC Laboratory for Thales Nederland B.V., IEEE 2002 International EMC Symposium, Minneapolis, August 2002, http://www.ewh.ieee.org/soc/emcs.

The HMS Sheffield has its own website at http://www.bigwig.net/sheffield.

Other well-known military EMC-related disasters include the aircraft carrier U.S.S. Forrestal, see: “The tragic fire: July 29, 1967” at http://forrestal.org/fidfacts. The Zuni rocket that was accidentally fired from an F-4 Phantom and started the fire is believed to have been triggered by a combination of the powerful fields at deck level from the ship’s radar and an incorrectly fitted shielded cable connector.

Links to Mitigation Techniques

	Installation	Design & Development	Resources
Co-channel interference
Adjacent channel interference
Unintentional RF demodulation
Shielding enclosures
Shielding cables
Filtering

Problems with the US radio system puts police officers at risk

Description

In the USA, many of the emergency services (fire, police, ambulance, etc.) use a cellphone-like radio system which operates at 800MHz. The base-stations for these systems are often quite widely spread, to reduce the cost to the public purse.

These systems are known to suffer from ‘adjacent-channel’ interference, which seems to be on the increase due to crowding of the spectrum. The interference has resulted in documented cases where officers or other have been put at risk.

The main problem appears to be intermodulation in the RF front-ends of the handsets, caused by out-of-band signals from other licensed transmitters.

Commentary

In this case the sparseness of the basestations means that in some areas the signals are very weak and the handsets must struggle to pick up the signal from the background noise. In such situations a receiver’s adjacent-channel rejection becomes an important parameter.

Most radio receivers achieved their narrow channel bandwidths in the intermediate frequency processing stages. The bandwidths of the earlier (RF) stages are much wider, making them prone to interference from powerful signals at nearby frequencies. A solution is to improve the filtering of the RF stages.

References and links

“Interference to Public Safety 800MHz Radio Systems, Interim Report to the FCC, Dec 24, 2001”, http://www.apco911.org/afc/project_39/interim_report.pdf

Links to Mitigation Techniques

	Installation	Design & Development	Resources
Intermodulation
Adjacent channel rejection and notch filtering