International Civil Aviation Organization
WORKING PAPER / ACP WGM/WP10
AERONAUTICAL COMMUNICATIONS PANEL
THIRTEENTH MEETING OF WORKING GROUP M
Montreal, Canada18-21 November October 2008
Agenda Item 1(c)Use of the band 112 – 117.975 MHz by VDL Mode 4
Results of tests and frequency assignment planning criteria
(Presented by the Larry Johnsson)
SUMMARYThis paper presents the results of testing of VDL Mode 4 with the view to establish frequency assignment planning criteria for the use of the band 112 – 117.975 MHz by VDL Mode 4 while protecting ILS, VOR and GBAS systems from harmful interference. Frequency assignment planning criteria are proposed. Comments from the Spectrum Sub Group of the Navigation Systems Panel are in Appendix 4
ACTION
Action by the meeting is in paragraph 7.
1.Introduction
1.1From 29 September to 3 October 2008 additional testing of the use of the band 112 – 117.975 MHz by VDL Mode 4 was undertaken at KungsängenAirportin Norrkoping, Sweden. These tests were in addition to tests that had been undertaken between 1999-2001 in the United States and Germany. Also testing with VHF data broadcast systems (GBAS, GRAS) took place.
1.2The tests were in accordance with the test plans developed by ACP (2003-2007) and Eurocontrol (2004). These test plans were reviewed and agreed by the GNSSP/SSG.(2003) The test plans were amended with regard to the frequency range, as explained in paragraph 1.4 below. Also the comments from the GNSSP/SSG (April 2002) which were reviewed by the GNSSP prior to its agreement of the test-plan in 2003 were taken into consideration. VDL Mode 4 equipment used was production equipment; for earlier measurements (2001) prototype equipment was used.
1.3Testing was done with ILS-localizer, VOR and VDB-GBAS systems.
1.4At the World Radiocommunication Conference 2007 (WRC-07) of the International Telecommunication Union (ITU) the use of the band 108-117.975 MHz for air-ground communication systems (such as VDL-Mode 4) was restricted to the sub-band 112-117.975 MHz. Details on the allocation of the frequency band 108-117.975 to the Aeronautical Mobile (Route) Service (AM(R)S are in Appendix 1. As a result, compatibility of VDL Mode 4 with the ILS-Localizer only needs to be assessed with the ILS-Localizer operating at 111.950 MHz and the VDL Mode 4 operating at 112 MHz and higher frequencies.
1.5This paper was reviewed by the Spectrum Sub-Group (SSG)of the Navigation Systems Panel (NSP) in October 2008 with the view to provide comments on the proposed frequency assignment planning criteria necessary to protect ILS, VOR and GBAS from harmful interference. The comments from the SSG are in Appendix 4. The SSG could not agree at that meeting to the proposed frequency assignment planning criteria proposed in this paper and a revised version will be submitted to the next meeting of the SSG (March 2009). Comments from the SSG are expected to be submitted to the next meeting of WG M for review.
2.General comments
2.1VDL Mode 4 channel loading.
2.1.1For the development of realistic VDL Mode 4 channel loading (VDL Mode 4 interferer, ILS, VOR or GBAS victim) many different channel loading scenarios have been developed. For the testing of interference that can be generated by VDL Mode 4 a channel loading of 22% (i.e transmissions from one radio) was used. This is an unrealistic high level that can in practice not be achieved, but was used to simulate a high level of potential interference into the radio navigation aids (ILS, VOR, GBAS) operating in the band 108 – 117.975. Under worst case operating conditions transmissions from one aircraft radio will result in a maximum channel loading of 2.7% in practice.
2.1.2For the testing of interference from ILS, VOR and GBAS into VDL Mode 4 the channel loading of the VDL Mode 4 was set at 100%. The level of the interfering signals was increased until a message error rate of 2% was measured. The message error rate was measured over a period of 1 minute for each test.
2.2Equipment used in the testing
2.2.1The following equipment was used in the testing:
ILS VOR/Localizer receiver Honeywell RNA34BF s/n 01152
Rohde & Schwartz signal generator SMT 02 s/n 845376/017
VDL Mode 4 C.N.S. Systems AB VDL 4000-10-10 s/n 8.40-4000-100001
Telerad VDB transmitter EM 9009 s/n 491
Telerad VDB receiver RE 9009 s/n 74
HP Signal generator, VDL Mode 4 modulator E4431B s/n US38220123
Rohde & Schwartz spectrum analyzer FSL s/n 100341
VDL Mode 4 generator E4431B
2.3Polarization
2.3.1The polarization of ILS and VOR is horizontal and for VDL Mode 4 vertical. For VDB/GBAS the polarization is horizontal. Elliptical polarization is permitted. This would allow in practice for an isolation of the ILS/VOR signal to be received by the VDL Mode 4 antenna of about 10-20 dB. In some cases, for the purpose of calculating separation distances, examples are given in this paper that take into account a polarization discrimination of 10 dB. Polarization discrimination has not been applied when calculating separation distances with GBAS signals.
2.4ILS/VOR signal generator noise
2.4.1Due to the relative high noise level of the ILS/VOR signal generator, the results of the measurements showed worse performance for VDL Mode 4 than would be expected. This introduced a measurement uncertainty in the order of 5 dB (the measured ACR should be 5 dB better).This issue is further discussed in Appendix 3.
3.Testing of ILS-localizer
3.1Interference from the ILS Localizer into VDL Mode 4.
3.1.1Measurements were undertaken with the following test set-up
3.1.2Although VDL Mode 4 can only operate in the band 112-117.975 MHz, potential interference that can be caused into VDL Mode 4 has been tested with the ILS-localizer (signal generator) operating on 112.000 MHz as well as frequencies above and below,with VDL Mode 4 operating on 112 MHz. Measurements were taken with the ILS Localizer signal being tuned to the adjacent 25 kHz channels as shown in Table 1. The level of the (unwanted) ILS signal was increased until a message error rate of 2% was measured. The message error rate was measured over a period of 1 minute for each test.
3.1.3Measurement results are presented in Table 1 and Figure 1
Channel / -40 / -20 / -10 / -5 / -4 / -3 / -2 / -1 / 0 / 1 / 2 / 3 / 4 / 5 / 10 / 20 / 4-ACR / 69 / 69 / 65 / 63 / 61 / 59 / 58 / 49 / -8 / 49 / 57 / 59 / 61 / 63 / 67 / 69 / 70
Table 1 Adjacent channel rejection measurement results for VDL Mode 4 victim; ILS Localizer interferer
Figure 1 – ACR – VDL Mode 4 victim; ILS Localizer interferer
3.1.4Table 2 and Figure 2provide examples of the minimum separation distance (in km)between an ILS transmitter and a VDL Mode 4 receiver for given EIRP values for the ILS-Localizer (these values are typical for the front course sector of the ILS-Localizer). The ACR values used are as in Table 1.Also separation distances are given if a polarization discrimination of 10 dB is applied. Calculation of separation distances is further explained in Appendix 2.
Separation distance between ILS transmitter and VDL Mode 4 receiver (all distances in km)Channel / EIRP ILS 1kW / polarization / EIRP ILS 500W / polarization
(60 dBm) / correction (10dB) / (57 dBm) / correction (10 dB)
-40 / 0.933 / 0.295 / 0.661 / 0.209
-20 / 0.933 / 0.295 / 0.661 / 0.209
-10 / 1.479 / 0.468 / 1.047 / 0.331
-5 / 1.862 / 0.589 / 1.318 / 0.417
-4 / 2.344 / 0.741 / 1.660 / 0.525
-3 / 2.951 / 0.933 / 2.089 / 0.661
-2 / 3.311 / 1.047 / 2.344 / 0.741
-1 / 9.333 / 2.951 / 6.607 / 2.089
0
1 / 9.333 / 2.951 / 6.607 / 2.089
2 / 3.715 / 1.175 / 2.630 / 0.832
3 / 2.951 / 0.933 / 2.089 / 0.661
4 / 2.344 / 0.741 / 1.660 / 0.525
5 / 1.862 / 0.589 / 1.318 / 0.417
10 / 1.175 / 0.372 / 0.832 / 0.263
20 / 0.933 / 0.295 / 0.661 / 0.209
40 / 0.832 / 0.263 / 0.589 / 0.186
Table 2 - Separation distances between ILS-Localizer transmitters and VDL Mode 4 receivers as per Table 1
Figure 2 – Separation distances as per Table 2
- Include a polarization discrimination of 10 dB
3.1.4.2These separation distances apply to the course sector of the ILS Localizer (+/- 10⁰ relative to the (extended) runway centre line. Outside this area, the EIRP is about 10 dB lower.
3.1.5Conclusions
3.1.5.1With these results it was concluded that VDL Mode 4 can be used without frequency assignment planning constraints vis-a-vis the ILS if separated by at least 50 kHz (e.g. ILS-Localizer operating on 111.950 MHz and VDL Mode 4 on 112.000 MHz). This frequency separation is already provided by ICAO provisions and the ITU Radio Regulations. With regard to the separation distances it should be noted that these can be further reduced by correcting the measurements because of the noise level of the ILS signal generator (see paragraph 2.6 and Appendix 3). Also, since normally the desired field strength of the (wanted) VDL Mode 4 signal in the vicinity of airports is higher than the minimum assumed in the calculations, the actual minimum separation distance can be less than indicated. This would be an aspect to be considered when implementing the use of VDL Mode 4 on or around airports.
3.2Interference from VDL Mode 4 into the ILS-Localizer.
3.2.1Test set-up
3.2.2As the ILS Localizer can only operate in the frequency band below 112 MHz and VDL Mode 4 only above 112 MHz, the ILS Localizer frequency used during the tests was 111.950 MHz and the VDL Mode 4 was operating at 112.000 MHz and higher.
3.2.3The input level of the ILS-Localizer signal was set at -86 dB; measurements were taken with a maximum error of 4.5 µA in the ILS Localizer. Measurements were taken with a channel loading of the VDL Mode 4 transmitter at 22% and 2.7 %.
3.2.4The measurement results are contained in Table 3. (f(0)=111.950 MHz; second adjacent channel is 112.000 MHz)
Adj. 25 kHz channel # / 2 / 3 / 4 / 5 / 10 / 20 / 40ACR with loading 22% / 54.3 / 63.3 / 70.3 / 79.3 / 79.3 / 79.3 / 79.3
ACR with loading 2.7% / 74.3 / 78.3 / 79.3 / 79.3 / 79.3 / 79.3 / 79.3
Table 3 - Measurement results for ILS-Localizer victim; VDL Mode 4 operating on 112 MHz and higher.
Note: At an ACR of 79.3 the VDL Mode 4 signal level of0 dBm was input to the ILS receiver.
Figure 3 – ACR as per Table 3 (first adjacent channel not shown)
3.2.5Separation distances
3.2.5.1Separation distances (km) between ILS receivers and VDL Mode 4 transmitters were calculated as explained in Appendix 2 with the results as below in Table 4:
Adjacent channel # / 2 / 3 / 4 / 5 / 10 / 20 / 40ACR with loading 22% / 0.716 / 0.254 / 0.114 / 0.04 / 0.04 / 0.04 / 0.04
ACR with loading 2.7% / 0.072 / 0.045 / 0.04 / 0.04 / 0.04 / 0.04 / 0.04
Table 4Separation distances (km) between ILS Localizer (victim) and VDL Mode 4
Figure 4 – Separation distances as per Table 4
3.2.6Conclusions
3.2.6.1The separation distances in paragraph 3.2.5 were calculated with the minimum ILS signal level (-86 dBm) at the input of the Localizer receiver. Under worst case operating conditions a channel loading of more than 2.7 % is not expected. It had been concluded that transmissions of VDL from an aircraft will not cause harmful interference to another aircraft equipped with ILS.
3.2.7On-board compatibility of VDL Mode 4 transmissions and ILS receivers.
3.2.7.1The VDL Mode 4 transmits with an EIRP of 39 dBm. Assuming and isolation between the VOR and the VDL Mode 4 antenna of about 45 dB and 10 dB polarization losses (both antennas at the same side of the fuselage), the energy from the VDL Mode 4 transmissions at the ILS antenna would be in the order of -14 dBm. To protect the minimum ILS signal of -86 dB, a D/U of - 72 dB would be required. Higher antenna isolations (in the order of 60-70 dB can be expected when the ILS and VDL Mode 4 antennas are located on different sides of the fuselage. These criteria are met with a (worst case) channel loading of 2.7%. When implementing VDL Mode 4 on board an aircraft, necessary steps to avoid on-board interference from VDL Mode 4 into the ILS-Localizer needs to be secured Higher antenna isolation (e.g. NAV and VDL Mode 4 antennas at different sides of the fuselage would meet the protection with a VDL Mode 4 channel loading of 22 %.Channel loading figures in the order of 10-20 % can be expected from VDL mode 4 ground stations.
3.2.8Appearance of flag
3.2.8.1With the maximum VDL Mode 4 signal fed into the Localizer receiver, the flag was raised on the second adjacent channel with a D/U ratio of -79 dB; at this point the error in the ILS indicator was 20 µA. For this measurement the VDL Mode 4 channel loading was set at 22%. For this test the frequency of the localizer was 111.950 MHz and the frequency of the VDL Mode 4 was 112 MHz. No flag was raised when the interference was less than 4.5 µA with a (worst case) channel loading of 2.7%.
3.2.9Increased level of wanted ILS-Localizer signal
3.2.9.1The ILS-Localizer signal was increased in 6 steps of 5 dB from the minimum signal level of -86 dBm and, at the same time, the level of the (unwanted) VDL Mode 4 signal was also raised with the same value. No changes in the adjacent channel D/U ratios were observed. For this measurement the ILS-Localizer frequency was 111.950 MHz and the frequency for the VDL Mode 4 was 112 MHz.
3.2.10Audible interference
3.2.10.1During the tests, the effect of interference from VDL Mode 4 on the (Morse code) identification signal was observed during each measurement; the interference was that low that the proper identification of the Morse code by the flight crew was not impeded.
4.Testing of VOR
4.1Interference from VOR into VDL Mode 4
4.1.1Measurements were undertaken with the following test set-up.
4.1.2Measurements were taken with VDL Mode 4 operating at 112 MHz, 115 MHz and 117.950 MHz. The channel loading of the VDL Mode 4 was set at 100%. Measurements were taken with the VOR signal generator being tuned to the adjacent 25 kHz channels as shown in Table 5. A maximum error in the bearing of the VOR of 0.3 degrees was measured.
4.1.3Measurement results of the adjacent channel rejection are presented in Table 5 and Figure 5
Adjacent Channel Rejection (dB)Channel # / 112 MHz / 115 MHz / 117.950 MHz
-40 / 66 / 69 / 70
-20 / 66 / 68 / 67
-10 / 63 / 64 / 63
-5 / 60 / 60 / 59
-4 / 58 / 58 / 58
-3 / 57 / 56 / 56
-2 / 55 / 55 / 54
-1 / 38 / 38 / 38
0 / -8 / -9 / -8
1 / 34 / 34 / 34
2 / 55 / 56 / 55
3 / 57 / 57 / 57
4 / 59 / 59 / 58
5 / 60 / 60 / 59
10 / 65 / 64 / 63
20 / 68 / 67 / 66
40 / 68 / 69 / 66
Table 5- ACR for VDL Mode 4 (victim) and VOR (interferer)
Figure 5 – ACR as per Table 5
4.1.4Separation distances.
4.1.4.1Table 6 and Figures 6A-6C provide examples of the minimum separation distance between a VOR transmitter and a VDL mode 4 receiver for 50 W, 100 W and 1000 W EIRP values for the VOR transmitter. The ACR values used are as in Table 5.These separation distances can be further reduced if a polarization discrimination of 10 dB is applied. Calculation of separation distances is further explained in Appendix 2.
AdjacentChannel # / 112 MHz / 115 MHz / 117.950 MHz
ACR / VOR EIRP (W) / ACR / VOR EIRP (W) / ACR / VOR EIRP (W)
100 / 50 / 1000 / 100 / 50 / 1000 / 100 / 50 / 1000
dB / km / km / km / dB / km / km / km / dB / km / km / km
-40 / 66 / 0.4 / 0.3 / 1.3 / 69 / 0.3 / 0.2 / 0.9 / 70 / 0.3 / 0.2 / 0.8
-20 / 66 / 0.4 / 0.3 / 1.3 / 68 / 0.3 / 0.2 / 1.0 / 67 / 0.4 / 0.3 / 1.2
-10 / 63 / 0.6 / 0.4 / 1.9 / 64 / 0.5 / 0.4 / 1.7 / 63 / 0.6 / 0.4 / 1.9
-5 / 60 / 0.8 / 0.6 / 2.6 / 60 / 0.8 / 0.6 / 2.6 / 59 / 0.9 / 0.7 / 3.0
-4 / 58 / 1.0 / 0.7 / 3.3 / 58 / 1.0 / 0.7 / 3.3 / 58 / 1.0 / 0.7 / 3.3
-3 / 57 / 1.2 / 0.8 / 3.7 / 56 / 1.3 / 0.9 / 4.2 / 56 / 1.3 / 0.9 / 4.2
-2 / 55 / 1.5 / 1.0 / 4.7 / 55 / 1.5 / 1.0 / 4.7 / 54 / 1.7 / 1.2 / 5.2
-1 / 38 / 10.5 / 7.4 / 33.1 / 38 / 10.5 / 7.4 / 33.1 / 38 / 10.5 / 7.4 / 33.1
0 / 88 / - / - / - / 89 / - / - / - / 88 / - / - / -
1 / 34 / 16.6 / 11.7 / 52.5 / 34 / 16.6 / 11.7 / 52.5 / 34 / 16.6 / 11.7 / 52.5
2 / 55 / 1.5 / 1.0 / 4.7 / 56 / 1.3 / 0.9 / 4.2 / 55 / 1.5 / 1.0 / 4.7
3 / 57 / 1.2 / 0.8 / 3.7 / 57 / 1.2 / 0.8 / 3.7 / 57 / 1.2 / 0.8 / 3.7
4 / 59 / 0.9 / 0.7 / 3.0 / 59 / 0.9 / 0.7 / 3.0 / 58 / 1.0 / 0.7 / 3.3
5 / 60 / 0.8 / 0.6 / 2.6 / 60 / 0.8 / 0.6 / 2.6 / 59 / 0.9 / 0.7 / 3.0
10 / 65 / 0.5 / 0.3 / 1.5 / 64 / 0.5 / 0.4 / 1.7 / 63 / 0.6 / 0.4 / 1.9
20 / 68 / 0.3 / 0.2 / 1.0 / 67 / 0.4 / 0.3 / 1.2 / 66 / 0.4 / 0.3 / 1.3
40 / 68 / 0.3 / 0.2 / 1.0 / 69 / 0.3 / 0.2 / 0.9 / 66 / 0.4 / 0.3 / 1.3
Table 6 - Examples of separation distances between VDL Mode 4 receivers and VOR transmitters
Figure 6A – Separation distances (VDL Mode 4 victim) 112 MHz
(first adjacent channel not shown)
Figure 6B – Separation distances (VDL Mode 4 victim) 115 MHz
(first adjacent channel not shown)
Figure 6C – Separation distances (VDL Mode 4 victim) 117.950 MHz
(first adjacent channel not shown)
4.1.5Conclusion.
4.1.5.1It was concluded that with the separation distances required to protect VDL Mode 4 from interference that can be caused by VOR transmissions, no co-frequency use of VDL Mode 4 and VOR is recommended within the radio line of sight of the VOR ground station.
4.1.5.2When VDL Mode 4 is operating on the first adjacent (25 kHz) channel of VOR frequency it is recommended that VDL Mode 4 should not operate within a range of 10 to 40 km from the VOR station, depending on the EIRP of the VOR station.
4.1.5.3When VDL Mode 4 is operating on the second adjacent (25 kHz) channel of the VOR frequency, no frequency planning constraints are necessary to protect VDL Mode 4 from interference from VOR transmissions. However, when implementing VDL Mode 4, attention should be given to the possibility of interference within a radius of 1-5 km around a VOR station, depending on the EIRP of the VOR.
4.2Interference from VDL Mode 4 into VOR
4.2.1Test set-up
4.2.1.1The test set-up for the measuring interference from VDL Mode 4 into VOR was similar to that for testing interference into the ILS Localizer.
4.2.2Frequencies
4.2.2.1The frequencies on which the VOR testing took place were 112 [113] MHz, 115 MHz and 117.950 MHz.The frequency 113 MHz was used to measure the effect on frequencies lower than the nominal VOR frequency, as the VDL Mode 4 system cannot be tuned to frequencies below 112 MHz.
4.2.3During the tests, the input level of the VOR signal was set at -79 dBm; measurements were taken with a maximum error of 0.3 degree in the VOR indicator.
4.2.4The channel loading of the VDL Mode 4 was set at 22%.
4.2.5Measurement results are presented in Table 7.
Channel # / 112 MHz / 115 MHz / 117.950 MHzACR (dB) / ACR (dB) / ACR (dB)
-40 / 66.3 / 64.3 / 65.3
-20 / 67.3 / 65.3 / 66.3
-10 / 69.3 / 68.3 / 68.3
-5 / 71.3 / 72.3 / 70.3
-4 / 69.3 / 68.3 / 67.3
-3 / 67.3 / 68.3 / 66.3
-2 / 65.3 / 65.3 / 64.3
-1 / 12.3 / 10.3 / 9.3
0 / -14.7 / -17.7 / -19.7
1 / 14.3 / 12.3 / 12.3
2 / 65.3 / 65.3 / 66.3
3 / 67.3 / 67.3 / 67.3
4 / 69.3 / 69.3 / 68.3
5 / 72.3 / 72.3 / 70.3
10 / 69.3 / 69.3 / 70.3
20 / 69.3 / 71.3 / 71.3
40 / 71.3 / 70.3 / 72.3
Table 7 - Measured ACR for VOR (victim) receiver interfered with VDL Mode 4 signal
Figure 7 – Measurement results as per Table 7
4.2.6From the measurement results it can be concluded that the co-frequency D/U ration between VOR and VDL Mode 4 can be set at 20 dB. This implies that VDL Mode 4 can only operate on the same frequency as the VOR beyond the radio horizon of an aircraft at maximum range and height of the VOR designated operational coverage.
4.2.7Separation distances
Separation distances were calculated as shown in Table 8 and Figure 8 (see Appendix 2 for calculation methodology).
112 MHz / 115 MHz / 117.950 MHzChannel # / km / km / km
-40 / 0.080 / 0.101 / 0.090
-20 / 0.072 / 0.090 / 0.080
-10 / 0.057 / 0.064 / 0.064
-5 / 0.045 / 0.040 / 0.051
-4 / 0.057 / 0.064 / 0.072
-3 / 0.072 / 0.064 / 0.080
-2 / 0.090 / 0.090 / 0.101
-1 / 40.272 / 50.699 / 56.885
0 / 901.571 / 1273.503 / 1603.245
1 / 31.989 / 40.272 / 40.272
2 / 0.090 / 0.090 / 0.080
3 / 0.072 / 0.072 / 0.072
4 / 0.057 / 0.057 / 0.064
5 / 0.040 / 0.040 / 0.051
10 / 0.057 / 0.057 / 0.051
20 / 0.057 / 0.045 / 0.045
40 / 0.045 / 0.051 / 0.040
Table 8 Separation distances between VOR receivers (victim) and VDL Mode 4 transmitters (interferer)
Figure 8 – Separation distances as per Table 8 (first adjacent channel not shown)
4.2.7These separation distances are calculated with the minimum VOR signal level (-79 dBm) at the input of the VOR receiver with a channel loading of 22%. Under these conditions it can be concluded that transmissions from a VDL Mode 4 equipped aircraft will not interfere with the reception of VOR on another aircraft when operating on the second adjacent (25 kHz) channel and more. For VDL Mode 4 operating on the first adjacent (25 kHz) channel to the VOR frequency, it is recommended that theVDL Mode 4 shall not be used within a buffer of 60 NM around the VOR DOC.
4.2.8On-board compatibility of VDL Mode 4 transmissions and VOR receivers.
4.2.8.1The VDL Mode 4 transmits with an EIRP of 39 dBm. Assuming an isolation between the VOR and the VDL Mode 4 antenna of about 45 dB and 10 dB polarization losses (both antennas at the same side of the fuselage), the energy from the VDL Mode 4 transmissions at the VOR antenna would be in the order of -14 dBm. To protect the minimum VOR signal of -79 dBm, a D/U of -65 dB would be required. This is about the value shown by the measurements.