1、European Radiocommunications Committee (ERC) within the European Conference of Postal and Telecommunications Administrations (CEPS) SHARING BETWEEN TERRESTRIAL FLIGHT TELEPHONE SYSTEM (TFTS) AND RADIO ASTRONOMY IN THE 1.6 GHz BAND Paris, May 1992 Reports are being issued from time to time by the Eur
2、opean Radiocommunications Committee (ERC) of CEPT to inform industry, operators, users and other interested parties of the work in hand, provisional conclusions and future activities in specific areas of radio frequency management. Such Reports give more details than is normally possible in a Recomm
3、endation and allow an opportunity for comment to be made on the work carried out so far. in most cases, it would be hoped that a formai CEPT Recommendation could be issued on the subject of the Report m due course, taking into consideration any comments received on the Report. Reports are formally a
4、pproved by, and issued in the name of, the Committee itself. In general the detailed preparation of Reports, and further work on the subject, will be done by Working Groups or Project Teams. Thus, any reference in the Reports to the ERC should be taken to include the whole framework of the ERC, incl
5、uding its Working Groups, Project Teams, etc. MICHAEL GODDARD European Radiocommunications Committee (ERC) STDaCEPT ERC REPORT III-ENGL I1942 S 2326414 0015059 271 Convrieht 1992 the EuTonan Conference of Postal and Telecommunications Administrations ICEPT) STDmCEPT ERC REPORT 1%-ENCL L9eiZ I 232641
6、4 00150b0 T53 111 ERC REPORT 11 Page 1 SHARING BETWEEN TERRESTRIAL FLIGHT TELEPHONE SYSTEM (TITS) AND RADIO ASTRONOMY IN THE 1.6 GHz BAND 1 INTRODUCTION During the preparation for the WARC-92, the ERC tried to find a suitable frequency band for terrestrial aeronautical public correspondence, since t
7、he band allocated at MOB-87 was only 2 x I MHZ and the new requirements were 2 X 5 h4Hz. The first band suggested for the uplink was 1700-1705 MHz, sharing was however difficult due to the small earth stations tracking low earth orbit (LEO) satellites in the METSAT service. The fiequency band 1670-1
8、675 MHz was then suggested, since it is difficult for the METSAT to use this band as a downlink, without causing interference to the radio astronomy service in the lower adjacent frequency band. Since the TPTS only includes a limited number of ground stations, the possibility for an allocation in th
9、is band should be investigated. This report includes the summary of the compatibility study between TFTS and Radio Astronomy Service. 2 TECHNICAL PARAMETERS FOR THE TERRESTRIAL FLIGHT TELEPHONE SYSTEM The technical parameters used in the calculations have been received from ETSI RES-5, which is work
10、ing with the standard for the TFTS-system The parameters are preliminary since the ETS has not yet been approved. The maximum power used in the calculation was 44 dBm and the attenuation of the spurious emission was 65 dB. Each ground station will be designated as either high medium or low power. Th
11、e high power ground stations, called en- route stations, are for en-route coverage. Medium ground stations are for use mainly during the climb and descent phases of fiight and low power ground stations, called airport ground stations, are only for use when aircraft are on the ground. The en route st
12、ations are spaced 380 + 20 km apart. 3 HARMFUL INTERFERENCE LEVEL FOR RADIO ASTRONOMY SERVICE In the frequency band 1660-1670 MHz, both spectral line and continuum observations are takirig place. The interference criteria that has been used is taken from CCIR Report 224. The requirements are that th
13、e PFD shall not exceed the levels for more than 10% of time, but since its considered probable that reflections from overflying aircraft will account for a large proportion of the lo%, the interference level has been calculated for 1%. 4 SHARING 4.1 Cases of interference The TFS-system can cause int
14、erference in three ways. 1) Since the radio astronomy is very sensitive to interference, even spurious at low levels can cause interference at large distance. 2) The TFTS-carriers can cause interference outside the RAS band due to the large bandwidth used for continuum measurements, which influence
15、the radio astronomy receiver selectivity. 3) Receiver intermodulation. STD-CEPT ERC REPORT LL-ENGL 1992 1 232b4L4 00150bl 92T SS ERC REPORT 11 Page 2 4.2 Calculations Two cases have been studied: - An airport 16 km from the radio astronomy site. For separation distances less than 100 km, the dominat
16、ion propagation mechanism is diffraction rather than tropospheric scatter. The propagation estimates are, taken from CCIR Recommendation 370-5. - A radio astronomy site close to the sea. The maximum distance from a TFTS ground station is assumed to be 200 km, the propagation model used was troposcat
17、ter and ducting. In both cases a negative margin is obtained, but sharing is possible if proper site shielding is achieved together with the methods below. For continuum observation it may be necessary to avoid the lower frequencies. Measures to achieve greater attenuation: a) Out-of-band emission R
18、ecognishg that most of the wide band emissions are caused by the power amplifier and that the current specification (65 dB below the carrier) can be met by the aircraft transmitters, then a better performance can be expected fiom the ground station transmitters which are not subject to rigorous avia
19、tion requirements. So it should be possible to improve the out-of- band emission by 10-20 dB. b) e.i.r.p. The calculations were made on the assumption that transmitters would operate at full power, it should be possible to arrange the system pian such that a 10 dB lower e.ir.p. sufficient near the c
20、ritical RAS sites. c) Filters It is unlikely that more than 6 dB attenuation will be available even if TITS transmission are restricted to above 1673 MHz. There wi inevitably be some loss in e.i.r.p. at the transmitted frequency, this will be made worse as more filter sections are added to increase
21、the attenuation. This approach also has an undesirable effect on frequency planning. Note that measurements on a PA may show that sideband power rolls off with increasing separation from the carrier, in which case some advantage could be gained by using the higher frequencies at critical sites even
22、if filtering is not used. d) Polar Diagram Control A radiated pattern which incorporates a notch in the direction of the RAS site could be produced by the TFTS antenna. A reduction of 10 to 20 dB should be possible. This would involve ihe use of an additional element, either parasitic or driven, ass
23、ociated with each transmitting antenna at the TFTS ground station. This approach could with advantage be kept as a fallback move should interference levels be found over a period of time to be greater than predicted. The loss of coverage would need to be taken into account. If two spaced TFTS ground
24、 stations were involved coverage could be completed from one or the other except at the protected site. For further information about the calculations, see Annex. 5 CONCLUSION The conclusion is that sharing is possible between RAS and TFS, if proper site shielding is achieved through careful choice
25、of TFS antenna site, together with other measures. Proper site shielding should be possible to achieve, since the number of stations is limited. For continuum observation it may be necessary to avoid the lower frequencies. It should be noted that there are radio astronomy sites a few meters fkom the
26、 sea, so there exist large coordination distances. ERC REPORT 11 Page 3 Spectral line observations Continuum observations ANNEX 1 HARMFUL INTERFERENCE LEVELS FOR RADIOASTRONOMY Assumed Input Power Spectral Bandwith Power flux-density Power (=) (dBW (dB(W/m2) Flux-density 0.02 -220 -194 -237 10 -205
27、-181 -25 1 *) ( dBW/(m2Hz) d (km) 16 30 The requirements are that the PFD at the sites shall not exceed the levels stated above for more than 10% of time. Fieldstrenght PFD Margin (dB(CLV/m) (dB W/(mzHz) (dB1 56 -202 -35 42 -216 -2 1 According to CCIR Report 696, an antenna gam of O dB can be used,
28、when calculating the interference power into the receiver input. 2 INTERFERENCE DUE TO SPURIOUS 2.1 Distance less than Io0 km For separation distances less than 100 km, the dominating propagation mechanism is diffraction rather than tropospheric scatter. It is considered probable that reflections fi
29、om overflying aircraft will account for a iarge proportion of the 10% so the interference level has been calculated for 1% of the time. Propagation estimates are taken from CCIR Recommendation 370-5, Figure 11. For various transmit antenna heights this plots against distance the field strength avaii
30、able for 1% of time at 50% of location, using a receiving antenna at a height of 10 m with the assumption that the AH between the terminals is 50 m. For the present purpose the plot for a transmit antenna height of 37.5 is chosen. This is typical for a RAS antenna, whilst the 10 m figure for the rec
31、eive antenna is typical of TPTS antenna. We should thus obtain the correct fieldstrength fiom the reciprocal path and also have the assurance that it is the 10 m high terminal which has a variable position as the figures assume. The plots of Figure 11 claim to be suitable for use over the band 450-1
32、000 MHz but in the 25 km region in which the example occur, the values given are less than those Figure 4a, the equivalent plots for 30-250 MHz, by only 2 dB, so there should be a small additional margin at 1670 MKz. Table I is for spectral measurements, for continuum measurement the mgh is 14 dI3 l
33、ess. Commentary In order to achieve greater attenuation the following methods could be considered: a) Out-of-band emission Recognising that most of the wide band emissions are caused by the power amplifier and that the current spdication (65 dI3 below the carrier), can be met by the aircraft transmi
34、tters, then a better performance can be expected from the ground station transmitters which are not subject to rigorous aviation requirements. So it should be possible to improve the out-of- band emission by 10-20 dB. *) Some RAS sites can accept -225, which gives in the following calculations an ex
35、tra margin of 12 dB *I ) It can be acceptable with -240 STD-CEPT ERC REPORT 11-EMGL 1372 2326434 00350b3 7TZ iils ERC REPORT 11 Page 4 b) e.i.r p. The calculations were made on the assumption that transmitters would operate at full power, it should be possible to arrange the system plan such that a
36、10 dB lower e.i.r.p. is sufficient near the critical RAS sites. c) Filters It is unlikely that more than 6 dB attenuation will be available even if TFTS transmission are restricted to above 1673 MHZ. There will inevitably be some loss in e.i.r.p. at the transmitted frequency, this will be made worse
37、 as more Nter sections are added to increase the attenuation. This approach also has an undesirable effect on frequency planning. Note that measurements on a PA may show that sideband power rolls off with increasing separation from the carrier, in which case some advantage could be gained by using t
38、he higher frequencies at critical sites even if filtering is not used. d) Siting There is much scope for improvement by choice of transmitter location. The calculations are for an average location at the distance chosen. Recommendation 370, Figure 12 indicates the variations, which may be found at v
39、arious locations. For AH equal to 50,1% of locations provide a further 20 dB attenuation. A ground survey is the only answer to a more accurate result. e) Antenna height Recommen thus the 100 dB bandwidth should be 33.3 kHz. Assuming that an observation is being made right at the edge of the band wi
40、th a 3 dB bandwidth between 1669.98 and 1670.00 MHz, the receiver will still have an FDR of 100 dB at 1670.033 MHz, so no problems should be encountered with spectral line observations. Many receivers can only achieve about 70 dB attenuation. In these cases sufficient site shielding must be provided
41、. 3.2 Continuum Assuming that the 3 dB bandwidth for continuum observations is 10 MHz (CCIR Report 696, Table l), the 100 dB bandwidth should be 16.67 MHz. With the 3 dB point at 1670 MHz. the -100 dB point will ocau at 1673.33 MHz, and the filter attenuation will be 2.9 dB/100 kHz. Taking this atte
42、nuation into account, the transmitted interfering power from 10 channels, spaced by 100 kHz, will be: I = lolog(lo 1.4 10-o.3-o.29n-o.29i 9 i=O (5) where n is the number of 100 kHz spaces not used in the TFTS transmitter. counting from the lower edge of the band. The maximum interference power in 10
43、 MHz for continuum observations is -205 dBW. Thus I = - 205 + Lb Inserting the values of Lb from section 3.1 (AH = 50), and solving for n, gives the following result: (6) Coast land I Land 25 Table VI11 These values are valid when no site shielding is afforded. In the worst case, as can be seen in T
44、able VIII, half the TFTS bandwidth must be kept free. If site shielding of 30 dB is present, the values are improved, see Table IX. Land 11 I Sea 14 Table IX The 70 dB attenuation point will be at 1672.3 MHZ, so even if the filter only achieves 70 dB attenuation, the above values essentially apply.
45、* In this case Lb is 189 dB, because of tropospheric scatter. STD.CEPT ERC REPORT LL-ENGL 3992 W 232b4L4 0035066 401 fl ERC REPORT 11 Page 7 4 NON-LINEAR EFFECTS WITHIN THE RECEIVER (RAS) Typical values for the intermodulation intercept point range from -55 dBW for a parametric amplifer to about 40
46、dBW for a transistor amplifier, both values being referred to the amplifer input. From Report CCIR 697-3, the following formula can be used to calculate the signal level which might cause interference by intemodulation. SJhq = (21P + APH)/3 dBW Where: APH is the input power that can cause harmful in
47、terference (dBW). IP is the input level to which the intercept point corresponds (dBW). With hpH = -220 dBW and Lp = -55 dBW, we get .Sm = - 110 dBW. The required attenuation from the FS is then: Lbm =44 dBm -30+110 = 124 dB. Compare this with the necessary attenuation due to out-of-band emission: Lb dBm -30-65 + 10 + 220 = 179 dB. Since we have come to the conclusion that it is possible to achieve an attenuation of 179 dB, the necessary attenuation to avoid intermodulation could also be achieved.
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