Ultrawideband electromagnetic interference to aircraft radios

A very recent FCC Final Rule now permits marketing and operation of new products that incorporate Ultrawideband (UWB) technology into handheld devices. Wireless product developers are working to rapidly bring this versatile, powerful and expectedly inexpensive technology into numerous consumer wireless devices. Past studies addressing the potential for passenger-carried portable electronic devices (PEDs) to interfere with aircraft electronic systems suggest that UWB transmitters may pose a significant threat to aircraft communication and navigation radio receivers. NASA, United Airlines and Eagles Wings Incorporated have performed preliminary testing that clearly shows the potential for handheld UWB transmitters to cause cockpit failure indications for the air traffic control radio beacon system (ATCRBS), blanking of aircraft on the traffic alert and collision avoidance system (TCAS) displays, and cause erratic motion and failure of instrument landing system (ILS) localizer and glideslope pointers on the pilot horizontal situation and attitude director displays. This paper provides details of the preliminary testing and recommends further assessment of aircraft systems for susceptibility to UWB electromagnetic interference.


Background
Ultrawideband (VWB) technology is typically characterized by the radiation and detection of baseband pulse signals, having a time duration of less than 1 nanosecond. A periodic sequence of these pulses can be shown in the frequency domain to appear as narrow-band signals at frequency spacing that is the inverse of the pulse repetition interval. Highly broadband antennas are required to transfer enough frequency content through the transmission medium to preserve the required degree of pulse shape characteristics. The fmt patent for a UWBtype communication system was issued to Gerald Ross, in 1973 [l], however the technology was referred to as baseband at that time. Because UWB technology is inherently a pulse modulated radio transmission scheme, blending of digital communications and RADAR sensor applications is greatly simplified. Some safety-related UWB applications address situational awareness needs in automobiles, like backup-warning systems, intelligent cruise control and collision avoidance. Some security-related UWB applications include sensors that can see into (and even through) boxes, bags, crates and walls, allowing detection of unauthorized equipment or intruders. UWB ground penetrating RADARS have been demonstrated to provide extensive information about buried pipes, weapons and facilities for military, geological, archeological and architectural applications. UWB systems can be implemented with very inexpensive and compact electronic components. Perhaps these characteristics hold the greatest promise for driving a revolution in new applications for consumer products. Designers and developers of wireless technology are promoting UWB technology for addressing the needs of high data rates, interoperability and location awareness that will be required for emerging wireless applications. primarily on FCC Part 15.209 spurious radiated emission limits [3]. Additional limitations are specified depending upon the stated application: imaging systems, vehicular RADAR systems, indoor UWB systems, and handheld UWB systems. The technical requirements for handheld UWB systems, as addressed in FCC Final Rule Part 15.5 19, are of primary concern when considering UWB technology applications within PEDs, particularly as a threat to aircraft radios. Handheld UWB system emission limit levels are specifically provided as effective isotropic radiated power (EIRP) from 960 MHz to above 10.6 GHz. Below  The FCC Final Rule states that the adopted standards "may be overprotective and could unnecessarily constrain the development of technology", and reveals the intention to issue a further rulemaking to "explore more flexible technical standards and to address the operation of additional types of UWB operations and technology". These statements appear to indicate that a relaxation of UWB radiated emission limits is planned for the near future.   [4], [5].) The RTCA publications contain numerous charts, clearly showing that typical PEDs radiate spurious signal amplitudes that are thousands of time less, at most fiequencies, than the FCC 15.209 limits require. In fact, the DO-233 analysis concluded that PEDs meeting FCC 15.209 limits could exceed interference limits for aircraft VOR and ILS localizer radios by a factor of over 1000 times, even after their emissions are attenuated by traveling from the passenger cabin to aircraft radio receivers. However, as noted by the DO-233 authors, the probability of a typical device radiating at the FCC l i m i t , on a particular aircraft radio channel is extremely low. UWB transmitters, on the other hand, emit equal-amplitude, narrow band signals at frequency spacing that is the inverse of the pulse repetition interval. When using pulseposition modulation and different clock frequencies, UWB transmitters emit narrow-band signals simultaneously at any frequency, even in safetycritical aircraft bands. There is clearly a very big difference between typical consumer devices, that radiate spurious signals nearly always far below 15.209 limits, and UWB devices, that may be intentionally designed to radiate at or near 15.209 limits.

FCC U W B Limits for Outdoor Handheld Systems
The final FCC rule explicitly states that "the operation of UWB devices is not permitted onboard aircraft, ships, or satellites.. .". This statement indicates that the FCC has documented EM1 concerns for UWB operation on board these vehicles. The FCC rule provides no guidance on how UWB devices can be restricted fiom operating in these vehicles, who is responsible for enforcing the restrictions, and what the penalties are.

NASYBagles Wings/United Airlines Test Project
To determine the threat power at the connector of a particular aircraft radio receiver, due to the spurious radiated emissions from a PED, losses due to propagation, antenna loss and cable loss occurring between the PED and the aircraft radio connector must be known. These losses can be identified as "interference path loss" (IPL). Since the RTCA/DO-199 & DO-233 studies, significant additional work has been performed by Eagles Wings Incorporated (EWI), Delta Airlines and NASA to better understand and quantify IPL. Previous analyses note that there are significant deficiencies in available data to allow estimation of the probability that a particular passenger location will have an IPL below a particular threshold [6]. A need was identified to extend the available IPL database on typical commercial transport aircraft. Such measurements are labor-intensive, and require exclusive access to airplane interior locations, exterior antenna systems, and avionics bay connections.
EWI submitted a proposal to NASA to work with United Airlines in resolving several technical issues related to IPL measurement data, including aircraft-to-aircraft repeatability, the type of test antenna, and IPL measurements at all passenger cabin seat locations. The proposal was supplemented with an evaluation of IPL mitigation techniques (ie. door/window exit seam shielding, and conductive window films), and assessment of aircraft RF cable and antenna health using newtechnology instrumentation tools. NASA issued a Purchase Order to work with EWI and United Airlines, on these goals. United Airlines was able to provide a limited number of flight-ready airplanes at an aviation storage facility in Victorville, California, including fuel, engineering and mechanic support. Measurements were performed during three one-week visits to Victorville, California.
Although UWB testing was not a part of the NASAEWI statement of work, all parties were interested in performing a preliminary test. After the contractually-required testing was completed, the E W n i t e d AirliedNASA team worked together to see if an UWB transmitter could affect operational aircraft radio systems. United airlines provided engineering and mechanic support, as well as fueled and operational airplanes. EWI provided engineering support, and NASA provided engineering support, UWB sources and instrumentation.

UWB Laboratory Signal Sources
UWB laboratory signal sources from a device manufacturer. The UWB sources are shown in Figure 2. Each UWB source has an internal 9V battery, and has a jack for external power using a 9VDC source. There is also a jack for switching odoff UWB pulses by using an external TTL clock signal. If an external TTL clock signal is not available, the unit has an internal lOMHz TTL clock that can be used by placing the CLOCK EXTANT switch in the INT Dosition.
In November 200 1, NASA procured several When operated, the UWB sources emit extremely short duration electrical pulses fiom their output jack. The manufacturer provided data specifications for the electrical pulses as follows: Output Voltage = 6.7 f 0.3 V (peak to peak).

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Because the UWB source output pulses are of such short duration, they contain frequency components that span several GHz. Figure 3 shows a close-up spectrum analyzer display of the UWB source output in the fiequency domain, measured at NASA LaRC.

UWB Testing On 3/22L2002, B737-200
UWB EMI assessment with a fully operational aircraft. The UWB signal source was operated using its internal 9V battery and loMHz internal clock, and connected to an antenna tuned for the fiequency band of the aircraft radio system being evaluated. A list of test equipment is provided in Table 1. Spectrum Analyzer and antennas were verified to be within-calibration schedule limits. UWB EMI assessment was performed on the VHF Omni-Ranging (VOR), instrument landing system (ILS) Localizer, ILS Glideslope, traffic collision avoidance system (TCAS), air traffic control radio beacon system (ATC), and VHF-Comm. aircraft radio systems as described herein.

43.1
VHF Voice Communications A conversation was initiated and maintained between the aircraft VHF radio and handheld VHF radio. The handheld VHF radio was operated fiom a location about 30 ft away fiom the nose of the aircraft, at both 118.02 MHz and 1 19.90 MHz. The UWB signal source was internally clocked (IOMHz), battery powered, and connected to the ETS 3 12 1 C dipole antenna (60-14OMHz balun, with element length set to 54.0cm), which was placed 1 meter away fiom the aircraft VHF-1 upper antenna (vertical polarization). Using the test-setup in Figure 4, the UWB signal amplitude was measured to be -23 dBm at 119.9 MEl[z, and less than -80dBm at 118.0 MHz. See Figure 5. No discernable effect in audio quality was observed during the conversation.

ILS Localizer
The local ILS Localizer beacon could not be acquired by the aircraft at the test location. The UWB signal source was internally clocked (IOMHz), battery powered, and connected to the ETS 3 12 1 C dipole antenna (60-140MHz balun, with element length set to 64.6cm), which was placed 1 meter away from the aircraft VOwLocalizer tail antenna (horizontal polarization). Cockpit instruments did not display any ILS Localizer information. No UWB effects were observed.

VOR
The local VOR beacon was acquired by the aircraft at the test location. The UWB signal source was internally clocked (1 O W ) , battery powered, and connected to the ETS 3 121C dipole antenna (60-14OMHz balun, with element length set to 64.6cm), which was placed 1 meter away from the aircraft VORLocalizer tail antenna (horizontal polarization). Cockpit instruments displayed appropriate navigation information for local beacon. No UWB effects were observed.

ILS Glideslope
The local ILS Glideslope beacon was marginally acquired by the aircraft at the test location. The UWB signal source was internally clocked (1 O m ) , battery powered, and connected to the ETS 3 121C dipole antenna (140-400MHz balun, with element length set to 2 1.2cm), which was placed 1 meter away fiom the aircraft Glideslope nose antenna (horizontal polarization). Cockpit i n s b e n t s displayed appropriate navigation information for local beacon. No UWB effects were observed.

ATC andTCAS
The local ATC interrogator and TCAS transponders on aircraft in the local airspace were acquired by the test aircrafl. The UWB signal source was internally clocked (IOMHz), battery powered, and connected to the AH Systems dual ridge horn (DRH) antenna. Because interference was observed with the horn antenna placed 1 meter away fiom the aircraft TCAS upper antenna, the test procedure was expanded to include locations inside the passenger cabin, with the doors closed. The "ATC Fail" indicator I~D on the coclmit disdav Dane1 illuminated and aimlane targets disameared from the TCAS disdav when the UWB simal source was turned ON. Video was recorded of the EMI situation. This failure was observed with the UWB source transmitting fiom the following locations: Outside the aircraft, -1Sm from the aircraft upper TCAS antenna, port side.
At all first class window locations (windows #I to 6), port side. (DRH Pol.  A few seconds after this spectrum analyzer trace was recorded, the UWB source was turned I 13.E.4-5 ON, and the trace was recorded again to obtain the red triangles. It can readily be seen that the ambient TCAS signal amplitudes were over 50dB higher than the UWB signal amplitudes, yet the UWB signals reliably interfered with the much higher level TCAS signals. (Note that during the 13 seconds before the second trace was recorded, several more ambient TCAS signals were observed by the spectnun analyzer. These can be readily identified as they follow the envelope of the other ambient TCAS signals, and should be disregarded.) The green diamonds show the signal amplitudes directly out of the UWB source, as measured using the Figure 5

test setup.
This test conclusively demonstrated serious air traffic control system failures due to a batteryoperated UWB transmitter being operated on-board the aircraft. The output power directly from the source was measured to be -30dE3m (as shown in   Generator was set to output 1 microsecond pulses at the desired pulse repetition frequency, and allowed external modulation of the UWB clock pulse by connecting a modulating signal to its "Control-Input" jack. When selecting the " F M modulation mode, the Hp 8 1 16A essentially provided a dithered mode of pulse spacing to the UWB source clock input (by deviating the pulse repetition frequency of the output clock pulses). When selecting the "AM" modulation mode, the HP 8 1 16A essentially provided an On-Off-Keying mode of pulse control to the UWB source clock input (depending upon whether the audio voltage output exceeded the TTL "1" level at the time of pulse generation). A Microcassette player audio signal was connected to the HP 8 1 16A control-input jack, while playing back a 30-minUte segment of voice audio (recorded from the Weather Channel). The Spectrum Analyzer, pulse function generator, TIC Tester and antennas were verified to be within calibration schedule limits. UWB EM1 assessment was performed on the ILS Localizer, ILS Glideslope, traffic collision avoidance system (TCAS), air traffic control radio beacon system (ATC), GPS, SATCOM aircraft radio systems as described herein.   Figure 8. Measured output data from the UWB source, when using both FM (dithered) and AM (on-off keying) modulation techniques are plotted in Figure 9. The UWB Source Clock. The ON-OFF keying (AM) and Dithered (FM) spectra are actually very dynamic, whereas these plots are merely a snapshot in time. Data was measured using the spectrum analyzer peak detector, with a 300 kHz resolution bandwidth.

UWB Testing On 4/12/2002, B747-400
Radiated signals from the UWB transmitter caused uncommanded motion and blanking of the Course Deviation Indicator bar on the aircraft Horizontal Situation Dimlav. Failures occurred only when applying FM to the UWB source clock input (dithered UWB), but not with AM (on-off keying) or no modulation. Because of time limitations, no attempt was made to transmit from the UWB source inside the aircraft, or to determine the minimum UWB transmit level at which the

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interference would occur. Video was recorded of the cockpit display anomalies due to UWB EMI.

ATC and TCAS The local ATC interrogator and TCAS
This test conclusively demonstrated serious ILS Localizer navigation failures due to a UWB transmitter being operated near the aircraft. The output power from the source was measured to be -2O&m, as shown in Figure 10. Adding a dipole antenna gain of 3 dF3 and subtracting the cable loss of 4dB, the UWB source transmitted an equivalent isotropic radiated power (EIRP) of -2 ldE3m. There was no attempt to determine whether -2 1 dl3m was the lowest EIRP at witch the failure would occur. Additional ILS Localizer testing was performed on 5/8/2002, and is reported in Section 4.5. cockpit display panel. and airplane targets disappeared from the TCAS disDlav when the UWB signal source transmitted out the Dilot's escaDe hatch. about 1 meter away from the toD TCAS antenna. The failure was reproduced when using different UWB clock frequencies when transmitting out the pilot's escape hatch. However, w i t h the UWB source transmitting from inside the cockpit or out the passenger cabin window, no effect was observed to the aircraft ATC/TCAS systems. Video was recorded of the cockpit display anomalies due to UWB EM.

ILS Glideslope
An "ATC Fail" message was observed on the aircraft ILS glideslope antennas-and the VOR/ILS Ramp Test Set are shown in Figure 8. This test conclusively demonstrated serious air traffic control system failures due to a UWB transmitter being operated outside the aircraft. The output power from the source was measured to be -30dBm. Adding a DRH antenna gain of 10 dE3, the UWB source transmitted an equivalent isotropic radiated power (EIRP) of -20dE3m. Additional ATC/TCAS testing was performed on 5 Table 3. Again, an external power supply and HP 8 1 16A Pulse Function Generator were provided by NASA to externally clock the UWB signal source, and a portable VOR/ILS Ramp Test Set (TIC Tester) was provided by United Airlines to allow transmission of ILS reference signals. The Spectrum Analyzer, pulse function generator, TIC Tester and antennas were verified to be within calibration schedule limits. UWB EMI assessment was performed on the ILS Localizer, ILS Glideslope, traffic collision avoidance system (TCAS), and air traffic control radio beacon system (ATC) aircraft radio systems as described herein.

ILS Localizer
The VOR/ILS Ramp Test Set was operated from the cockpit of the airplane (UAL Nose #1879), and the ILS localizer radio receiver was captured w i t h the 108.15 MHz test set reference signal. The UWB signal source was externally powered and externally clocked with the HP 8 1 16A Pulse Function Generator at 9.97MH2, to place a UWB frequency component at 108.15 MHz, coinciding with the ILS localizer test set channel. The UWB source was connected to two 50 ft lengths of RG2 14 coaxial cable connected inline, allowing the ETS 3 121C dipole antenna (60-14OMHz balun, with element length set to 64.6cm) to be placed anywhere within the airplane passenger cabin. A picture of the aircraft ILS Localizer antenna is shown in Figure 10.   o f 4 d B ( 1 0 0 f t , R G 2 1 4 @ 1 1 0~)   This test conclusively demonstrated serious ILS Localizer navigation failures due to a UWB transmitter being operated inside the aircraft, transmitting at power levels near those that the FCC has approved for marketing and operation of handheld UWB devices. A video was recorded of the cockpit display anomalies due to UWB EMI.

ILS Glideslope
The VORmS Ramp Test Set was operated from the cockpit of the airplane (UAL Nose #1879), and the ILS glideslope radio receiver was captured with the 334.55 MHz test set reference signal. The UWB signal source was externally powered and externally clocked with the HP 8 1 16A Pulse Function Generator at 9.98MHz, to place a UWB frequency component at 334.55 MHz, coinciding with the ILS glideslope test set channel. The VOWILS Ramp Test Set output attenuation was adjusted such that -17.2 dE3m was delivered to its antenna at 334.55 MHz. The UWB source was connected to a 50 fi length of RG214 coaxial cable, allowing the Schwarzbeck biconical antenna (very efficient in Glideslope frequency band) to be placed near the ILS glideslope nose antenna (horizontal polarization). The output power fiom the UWB source was measured to be -21 dBm, using the spectrum analyzer (as previously described). Adding a biconical antenna gain of 3 dB and subtracting the cable loss of 2.4 dl3 (50ft, RG 214 a 3 3 5 MHz), the UWB source transmitted an equivalent isotropic radiated power (EIRP) of -20.4 dBm.

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Radiated simals fiom the UWB transmitter caused erratic motion and retraction of the GS bar and GS Dointer and extension of the GS Fail flag on the HSI and ADI, resr>ectivelv. There was no difference when applying FM (dithered UWB), AM (on-off keying) or no modulation to the UWB source clock input. These failures were only observed when the UWB source was transmitting from outside the aircraft (in fiont of the nose), but not when transmitting fiom within the passenger cabin. A video was recorded of the EM1 situation.

Findings Summary
have collaboratively revealed that UWB device emissions can interfere with essential flight navigation radios. This work was performed as a voluntary supplement to general PED EM1 research on a non-interference basis. Table 4 provides an outline of the fmdings.
In summary, NASA, United Airlines and EWI

Conclusions
It has been conclusively demonstrated that a handheld, low-power UWB transmitter can interfere with aircraft TCAS, ATC, ILS localizer and ILS glideslope radios. Failure was demonstrated to occur on a B737 aircraft ILS localizer system with UWB EIRP levels as low as -4ldI3m. Measurements were pefiormed on two types of Boeing passenger jets. It is likely that EMI will occur at lower UWB EIRP levels on smaller regional airplanes, because of better electromagnetic coupling to aircraft antennas from their passenger cabins. Testing was very limited, and not likely to reveal the full degree of aircraft system susceptibilities. Several important aircraft systems were not considered at all, including RADAR altimeters, microwave landing systems, and DME. If the FCC 15.5 19 limits are modified,

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or particular devices exceed the limits, it is more likely that UWB EMI to aircraft ATCITCAS and other systems will occur. Testing demonstrated that modulation of the UWB signal greatly influenced the susceptibility threshold of the ILS localizer radio. It is likely that the modulation technique used in this test was not worst-case. A more detailed test, with careful attention to modulation parameters would likely induce failures at lower UWB EIRP levels. The VOR and VHF communication systems were not tested for increased susceptibility to modulated UWB, nor were they tested for susceptibility when receiving communication signals close to their receiver sensitivity limits. It is possible that these systems may have susceptibilities that are as yet undiscovered.
The focus of this testing was directed only towards handheld UWB systems. Other legitimate UWB applications, such as imaging systems, ground penetrating RADARS, surveillance systems, vehicular RADARS, and various communications and measurement systems may also pose a threat to air traflic control and aircraft navigation and communication systems. More detailed analysis and testing of UWB device impact upon flightessential aircraft navigation and communication systems, and air trafic control is strongly recommended, particularly before unlicensed devices are widely available.

Recommendations
including analysis and laboratory testing, fieldtesting on operational aircraft, and development of regulatory policies based upon authoritative technical merit. The radio signal structure for all aircraft navigation and communication systems should be studied to determine interfering signal modulation characteristics and levels that are required to impact aircrafl radio performance. Analysis of UWB device modulation approaches must be performed to identify the most threatening types, and quantify amplitudes required to threaten aircraft radio system performance. Closed-loop navigation system testing, incorporating actual flight hardware and trained pilots, should be performed to specifically describe symptoms and anomalies that may be caused by interfering UWB A three-element approach is recommended,

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signals, so that flight crews can readily determine whether they are experiencing an EMI situation. Aircraft field-testing is necessary for pre-screening to reveal likely problem areas, and for validation of analytical studies and laboratory test predictions.
The tests documented in this report fall into the prescreening category. Additional pre-screening is recommended before completing analytical studies on several aircraft systems, particularly VHF communications, VOR, and ILS Glideslope. Such prescreening should include minimizing desired signal amplitude available to the aircraft antenna, to optimize the potential for UWB signal interference. Prescreening should also include applying various modulations to the UWB source clock. Results of all analysis and test should be made publicly available for peer review and verification. Airlines, UWB device manufacturers, airborne radio manufacturers, universities and government (FAA, FCC, NASA, NTIA) should interact and cooperate to generate sound technical data and perform comprehensive analysis to develop regulatory policies with the safety and security of the public in mind.