424. Modern spacecraft – Antique specifications
Spacecraft now and of the future are being controlled by EMC requirements of the past. Little has been done by the launch vehicle/spacecraft manufacturers to abandon MIL‑STD‑461C which was released in 1986 because most of the electronics equipment being used aboard current launch vehicles is approved by similarity and heritage to MIL‑STD‑461C and its predecessors. Twenty years later these electronic equipment items are still not tested to today’s MIL‑STD‑461E requirements because there is a risk that the items will fail to meet the requirements and thus the cost will increase if it becomes necessary to redesign the equipment. That cost is insignificant compared with the cost of losing an entire mission!
In the 20 years that have elapsed since MIL‑STD‑461C was released, the EMC environment has undergone major changes. High speed digital devices have been created that have fundamental clock and bus frequencies that span the entire LV/SC frequency range from the UHF Band Flight Termination Systems through S-Band telemetry and C-Band tracking transponders. Personnel involved in ground operations can carry and use hand held transceivers and cellular telephones close by sensitive electronics equipment. There are now many more orbiting receivers and emitters, plus range assets have increased dramatically since 2001. It’s way past time to bring requirements up-to-date!
It is important to note that daily KSC (Kennedy Space Center) monitoring has detected levels from off site emitters that are theoretically beyond the horizon and at times detected levels higher than the theoretical free space maximum. This is possibly due to multipath and atmospheric ducting effects.
The vehicle may fly closer to an emitter during launch and thus be exposed to higher field levels than it is exposed to on the launch pad. There are also downrange emitters that can cause strong fields at the vehicle. In this case the trajectory of the vehicle must be considered. Data bases that are developed by the Joint Spectrum Center are used to determine these levels. The Launch Services Program has recently funded Aerospace to predict ascent field levels for each mission based on the flight trajectory.
In addition, once the spacecraft separates from the vehicle the on-orbit fields must be considered if it will be in a near earth orbit. It is common for tracking radars to use spacecraft as targets of opportunity and field levels from both US and other emitters can be as high as 100’s of volts/meter. Additionally there are other extremely high level emitters (over the horizon back scatter RADAR, etc.) that produce levels in the 1000’s of V/m that SC trajectories may inadvertently cross. Table 3 shows the worst case ascent and on-orbit field levels being specified in the proposed MIL‑STD‑1541B. Some of the emitters reflected in this table such as Cband tracking radars are mitigated, however some can not be, especially foreign emitters.
(Extracts from: “Modern Spacecraft – Antique Specifications,” Ron Brewer, Launch Service Program, Analex Corporation, IEEE International Symposium on EMC, Portland, OR, USA, August 14-18 2006, ISBN: 1-4244-0294-8/06.)
425. Equipotential design of systems
Using the original concept, the system failed for EFT testing at 1kV in a capacitive coupling clamp. The reason was that the distributed control units and the central screen were connected using screened cables, but the screen was terminated at both ends by a pigtail connection. By changing the screen termination into a low impedance connection, mounted directly at the chassis entrance of the modules, the EFT test passed up to 5kV.
The failure of temperature sensors has been found many times in practice, always with similar reasons of failing: no good equipotential reference over the complete system. Typical for a set of sensors and transducers is the fact that they are very distributed over larger systems. Because in some cases the termination at both ends of a screened cable causes problems, the screen is not terminated at all, or only at one side. Which does not really offer a good protection at common mode level of interference, and certainly no more at higher frequencies of the ambient noise.
Most of the problems are occurring because subparts of a larger system are not well interconnected. In this case, the problem can be solved by ‘insulating’ the sensor itself (ex. by using optocoupled systems, or differential mode signal transmission), and by connecting the screen of the cable in a good way to the chassis as the incoming point of the central control unit. For safety reasons, special care must be taken for PE requirements, ending sometimes in an extra (parallel to the cable screen) PE wire connection.
(Extracts from: “Equipotential Design of Systems: Examples from Practicing,” J Catrysse, W Debaets, N Dediene, EMC Europe 2000, 4th European Symposium on EMC, Technologisch Instituut vzw, Brugge, 11-15 Sept 2000, ISBN: 90‑76019-14-2.)
426. Failures at electricity distribution substation
This study into disconnector-related EMI was initiated following series of failures experienced at Brenner substation – an Eskom 275/88kV open-air substation situated in Gauteng, South Africa. In particular it was noticed that Bandwidth Management Equipment (BME) installed in a cabinet inside the substation’s telecommunications room would fail for a period of approximately 10 seconds each time disconnectors were operated in an adjacent high voltage yard [2].
The BME is a crucial part of the microwave communications link between the site and the National Control master station, and it takes 20 – 30 seconds to re-establish this link if the BME fails. Another cause for concern was that the BME occasionally failed during line faults [2].
(Extracted from: “Testing Hypotheses Concerning the Flow of Common Mode Current in a Substation,” CD Walliser, JM Van Coller, PH Pretorius and AC Britten, EMC Europe 2000, 4th European Symposium on EMC, Technologisch Instituut vzw, Brugge, 11‑15 Sept 2000, ISBN: 90-76019-14-2.)
427. Patriot missile system interference
The Wall Street Journal reports that military investigators are exploring the possibility the electromagnetic interference may have been the cause of two friendly fire incidents during the Iraq war involving Patriot missiles that resulted in downing of two allied fighters and the deaths of three airmen.
According to the Journal report, investigators have ruled out either manual error by the operators of the Patriot missile batteries, or mistakes by the missiles themselves, and are now focusing on whether the extremely close positioning of multiple missile batteries on the ground resulted in elevated levels of EMI that interfered with the systems’ high-powered radars.
Military officials admit that the Patriot missile batteries were moved around the battlefield during the war to protect U.S. and British ground troops, and at times were clustered in close proximity to one another. And, although all military systems are tested for EMI, the Journal quotes one source who said: “If you look at the intensity of the radiation in that battlefield area, I don’t believe anyone would say that particular environment had been duplicated before. It was very, very intense.”
(Extracts from “Patriot Missile Systems may be EMI Susceptible,” NewsBreaks, Conformity, September 2003, page 48. Also see Banana Skin No. 299.)
428. Pilots pick up baby monitor transmissions
CNN reports that pilots approaching Luton airport in Great Britain recently picked up more than the monotone of the air traffic controller over their radios. Authorities reportedly worked 12 hours to track down the sound of a squealing infant that was picked up on the normal communications frequencies. They ultimately traced the noise to a baby monitor in a home located near the airport. Broadcasting babies aren’t new. As we’ve previously reported (See Conformity, October 1997), or own Federal Aviation Administration receives numerous reports of similar incidents here in the United States as wireless communications devices proliferate.
(Extracts from: “Pilots pick up Baby Monitor Transmissions,” NewsBreaks, Conformity, August 2003, page 88. Also see Banana Skins No. 225 and 299.)
The regular “Banana Skins” column was published in the EMC Journal, starting in January 1998. Alan E. Hutley, a prominent member of the electronics community, distinguished publisher of the EMC Journal, founder of the EMCIA EMC Industry Association and the EMCUK Exhibition & Conference, has graciously given his permission for In Compliance to republish this reader-favorite column. The Banana Skin columns were compiled by Keith Armstrong, of Cherry Clough Consultants Ltd, from items he found in various publications, and anecdotes and links sent in by the many fans of the column. All of the EMC Journal columns are available at: https://www.emcstandards.co.uk/emi-stories, indexed both by application and type of EM disturbance, and new ones have recently begun being added. Keith has also given his permission for these stories to be shared through In Compliance as a service to the worldwide EMC community. We are proud to carry on the tradition of sharing Banana Skins for the purpose of promoting education for EMI/EMC engineers.
