COMPATIBILITY STUDY BETWEEN RADIO NAVIGATION SATELLITE SERVICE IN THE MHz BAND AND FIXED SERVICE OPERATING UNDER RR S5.355 AND S5.

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1 SE-28, Paris July 1998 SE-28(98) Doc103-Rev1 COMPATIBILITY STUDY BETWEEN RADIO NAVIGATION SATELLITE SERVICE IN THE MHz BAND AND FIXED SERVICE OPERATING UNDER RR S5.355 AND S5.359 Presented by Institute of Navigation, University of Stuttgart and German Air Navigation Services (DFS) The purpose of this document is to assess the impact of Fixed Service transmitters to the radio navigation satellite systems GPS, GLONASS and ENSS. 0 General WRC-2000 Agenda Item calls to consider the status of allocations to services other than the radionavigation-satellite (RR S5.355 and S5.359) in the MHz band; The band MHz is allocated on a primary basis to the Aeronautical Radio Navigation Service (ARNS), Radio Navigation Satellite Service (RNSS) in the space to earth direction and Fixed Service (FS) operating under RR S5.355 and S This document presents the results of a compatibility study concerning a possible sharing between Radio Navigation Satellite Service and Fixed Service. 1 Introduction Radio Navigation Satellite Service is recognized by ITU as a safety service. RR S4.10 applies: Members recognize that the safety aspects of radionavigation and other safety services require special measures to ensure their freedom from harmful interference; it is necessary therefore to take this factor into account in the assignment and use of frequencies. RR S5.359 gives an additional allocation to Fixed Services on a primary basis in the following States: Germany, Saudi Arabia, Armenia, Austria, Azerbaijan, Belarus, Benin, Bulgaria, Cameroon, Spain, France, Gabon, Georgia, Greece, Guinea, Guinea-Bissau, Hungary, Jordan, Kazakstan, Kuwait, Latvia, Libya, Mali, Mauritania, Moldova, Mongolia, Nigeria, Uganda, Uzbekistan, Pakistan, Poland, Syria, Kyrgyzstan, the Democratic People s Republic of Korea, Romania, Russia, Senegal, Swaziland, Tajikistan, Tanzania, Turkmenistan, Ukraine, Zambia and Zimbabwe. 1

2 RR S5.355 allocates the bands MHz and MHz to the Fixed Service on a secondary basis within the following States: Saudi Arabia, Bahrain, Bangladesh, the Congo, Egypt, the United Arab Emirates, Eritrea, Ethiopia, the Islamic Republic of Iran, Iraq, Israel, Jordan, Kuwait, Lebanon, Malta, Morocco, Niger, Oman, Qatar, Syria, Somalia, Sudan, Sri Lanka, Chad, Togo, Yemen and Zambia. The Fixed Services comprise mainly military authorities which are using the spectrum for the transmission of digital and voice messages via fixed communication links. The fixed service stations utilized by German Army for tactical relay stations can make use of the frequency range 1400 MHz to 1660 MHz. Based on national regulations the frequency range MHz +/-14 MHz will not be utilized in order to avoid interference with GPS. Whether other countries have similar arrangements is unclear. Therefore we distinguish between two cases with and without a guard band of 28 MHz for GPS. Fig. Error! Unknown switch argument. shows the spectrum usage in the satellite navigation frequency band. The spectra of GPS, GLONASS and ENSS can be compared with the guard band of 28 MHz that is kept free in Germany. Fig. Error! Unknown switch argument.: Spectrum usage of satellite navigation systems and Fixed Services 2 Background In the satellite navigation receiver every interference signal, almost independent of it s frequency and bandwidth is attenuated by the so-called processing gain. If the frequency of the interference signal is not identical to the center frequency of the satellite navigation signal it is additionally attenuated by the 2

3 pre-correlation filter. If the bandwidth of the interference signal is not small compared with bandwidth of the PN-Code spectrum (2 code clock frequency), there is also an attenuation due to the filtering effect of the correlation process. To describe the maximum tolerable power (narrow band signals) or power density (broadband signals) for the interference signal a so-called interference threshold can be indicated. For broadband signals it is common to indicate the maximum tolerable interference power density at center frequency of the satellite navigation signal in W/MHz units. Here, we assume the interference signal to be a broadband signal with constant power density in the band f a to f b and negligible power density outside: B/ 2 J SINC ( T Eq. Error! ( )) df [ W eff a f J 2 = ( ) π J C f f ] f J Unknown switch B/ 2 c argument. To calculate the maximum tolerable power density the frequency range f a to f b the so-called frequency dependent rejection has to be introduced: B/ 2 J SINC ( T Eq. Error! ( )) df [ W eff a f J 2 = ( ) π J C f f ] f J Unknown switch B/ 2 c argument. Where: FDR: Frequency Dependent Rejection in db, i.d. attenuation of the interference signal under investigation, compared with a reference interference signal at the center frequency of the satellite navigation signal that has a bandwidth of 1 MHz f: frequency in MHz f HF : center frequency of navigation signal in MHz (e.g MHz for GPS-L1) f a, f b : lower and upper edge of interference frequency band f C : code clock frequency of the satellite signal in MHz T C : code chip duration T C = 1/f C in µs a(f): frequency response of the pre-correlation filter, Butterworth filter of order 5, a(f) = 1 for in-band interference SINC(x): sin(x)/x-function For the pre-correlation filter a Butterworth filter of order 5 is used, which is very common. The equation for the filter transfer function is rather complicated and therefore is omitted here, but plots of it for the C/A-Code and P-Code reception of GPS are shown in Fig. Error! Unknown switch argument. holds for interference signals with a bandwidth (f b -f a ) >>1 khz, which is true for all cases analyzed in this paper. 3

4 Fig. Error! Unknown switch argument.: GPS-spectra and filter transfer functions in the case of GPS With a given interference threshold at center frequency the threshold for the actual interference signal can be calculated as follows: N = N 0 B Eq. Error! Unknown switch argument. With: T c : interference threshold at the center frequency of the satellite navigation signal in dbw/mhz (e.g. given by a standard like ICAO GNSS SARPs) T: interference threshold for the interference signal under investigation in dbw per given bandwidth (f b -f a ) in MHz FDR: frequency dependent rejection in db for the interference signal under investigation To determine the minimum required distance to avoid harmful interference the received interference power density as a function of the distance to the transmitter has to be compared with the interference threshold: N = N 0 B Eq. Error! Unknown switch argument. With: d min : minimum required distance in m T: interference threshold in W/(bandwidth in MHz), defined for a reference interference signal in a standard for a satellite navigation receiver EIRP: equivalent isotropically radiated transmitted density in dbw/(bandwidth in MHz) λ I : wavelength of interference signal in m at it s center frequency (f b -f a )/2 G r : receive antenna gain in db towards the interference source 3 Characteristics of Fixed Service Transmitter The Error! Unknown switch argument. summarizes characteristics of German Army communication equipment operating in the Fixed Services Frequency Band. (Source: 1) ERC Report 4

5 29, Brussels, June 1994, Compatibility study between Mobile Satellite Service in the MHz band and Fixed Service operating under RR 730; 2) Siemens Data Sheet for Radio Relay System FM1000 ) Tab. Error! No text of specified style in document.-error! Unknown switch argument.: Characteristics of German Army Communication Equipment Frequency range: 1400 MHz to 1660 MHz!! with exception of MHz +/- 14 MHz Transmit Power: Transmit Antenna Gain: 1.25 W 1 dbw 22 db EIRP: 23 dbw = 200 W!! Half Power Beamwidth of the Transmit Antenna: Bandwidth: Modulation: Worst Case Scenario khz to 1030 khz FDM, 12 to 24 channels, PCM, 256 to 1152 kbit/s Bandwidth = 600 khz, EIRP = 23 dbw/0.6 MHz 4 Scenarios As mentioned above the minimum required distance to prevent harmful interference can be calculated by comparing the received interference power with the interference threshold that is correct for the bandwidth and frequency offset of the interference signal by means of the frequency dependent rejection (FDR). In the case of GPS one has to discriminate between out-of-band interference (28 MHz guard band for GPS in Germany) and in-band interference (no guard band for GPS in other countries). Since there are not yet any guard bands defined to prevent interference from Fixed services to GLONASS and ENSS, for these systems the analysis is performed for the in-band interference case. For all systems two cases are investigated: horizontal incident of the interference signal (-4.5 db antenna gain towards interference source) and incident from the lower hemisphere (-10 db antenna gain). 4.1 Interference to GPS Assumptions: Signal Frequency: Nominal Signal Power: MHz -160 dbw 5

6 (according GPS-ICD) Code Clock Frequency: Pre-correlation Filter: MHz (C/A-Code), MHz (P/Y-Code) 5 th order Butterworth Filter with bandwidth: MHz (C/A-Code), MHz (P-Code) Calculation of minimum required distance d min : GPS-L1 Interference Frequency Range: Interference EIRP: Interference Threshold T C : out-of-band interference: f a = MHz 14 MHz 0.6 MHz to f b = MHz 14 MHz in-band interference: f a = MHz 0.3 MHz to f b = MHz MHz 23 dbw/0.6 MHz dbw/mhz for acquisition (according to ICAO Draft GNSS SARPs) FDR w.r.t. 1 MHz bandwidth centered at MHz G r = -4.5 db Horizontal incident out-of-band: db d min = 0 km C/A-Code in-band: 1.6 db d min =2233 km Out-of-band: 30.4 db d min =37.4 km P/(Y)-Code in-band: 2.2 db d min =2084 km G r = -10 db Incident from below d min = 0 km d min =1186 km d min =19.9 km d min =1107 km * In the case of the in-band interference, GPS-receivers a very susceptible. Because of the very high required minimum distance the range of the interference is rather limited by the radio horizon than by the transmit power. The results of the analysis show that the guard band of 28 MHz that is applied in Germany is sufficient to protect the C/A-Code-reception while the P(Y)-Code reception is still influenced. That does not necessarily mean that military P-Code receivers will have problems, since it can be expected that they a much higher interference resistance than civilian ones due to direct P-Code acquisition, multi-bit A/D-converters and other features. Despite the fact that the P-Code can tolerate higher interference power, for a civilian P-code receivers has to acquire the C/A-code prior to acquiring the P-Code and therefore the interference thresholds for C/A-Code receivers and P- Code receivers are the same. 6

7 The analysis shows that without a guard band (in-band interference) the minimum required distance is several thousand kilometers. In this case the range of the interferer is not limited by it s transmit power but by the radio horizon. 4.2 Interference to GLONASS Assumptions: Signal Frequency: 1602 MHz => in-band interference to channel 0 (representative for all GLONASS channels) Nominal Signal Power: Code Clock: Frequency: Pre-correlation Filter: -161 dbw (according GLONASS-ICD) MHz (C/A-Code), 5.11 MHz (P-Code) no attenuation due to in-band interference Calculation of minimum required distance d min : GLONASS-L1 C/A-Code Interference Frequency Range: 1602 MHz +/- 0.3 MHz Interference EIRP: Interference Threshold: FDR w.r.t. 1 MHz bandwidth centered at 1602 MHz G r = -4.5 db horizontal incident 23 dbw/0.6 MHz -152 dbw/mhz for acquisition (according to ICAO Draft GNSS SARPs) 0.4 db d min = 4750 km 7

8 G r = -10 db incident from below d min = 2522 km * Due to the in-band interference, GLONASS-receivers are very susceptible. Because of the very high required minimum distance the range of the interfere is rather limited by the radio horizon than by the transmit power. 4.3 Interference to ENSS Assumptions Two out of tree frequencies of the proposed ENSS-system can be influenced by Fixed service transmitters. The E1- band ( MHz) and the E2-band ( MHz) are located near the edge of the 28-MHz guard band for GPS in Germany while the E3-band ( MHz) lies far apart. Although the frequency bands of the Fixed Services and the E1, E2-band are overlapping only by approximately 50 % (see Fig. Error! Unknown switch argument.) one can expect that the spectrum of the Fixed Services have a roll-off with a restricted slope. Therefore we do not distinguish between in-band and out-of-band interference here and do assume only in-band interference. Signal Frequencies: Nominal Signal Power: Code Clock Frequency: Pre-correlation Filter: E1: MHz => in-band interference E2: MHz => in-band interference E3: MHz => no within range of FS (according to ITU-R 217/8) dbw (according to ITU-R 217/8) MHz (E1, E2), MHz (E3) (according to ITU-R 217/8) no attenuation due to in-band interference for E1 and E2 Calculation of minimum required distance d min : ENSS-E1 ENSS-E2 ENSS-E3 Interference Frequency Range: MHz +/- 0.3 MHz MHz +/- 0.3 MHz Interference EIRP 23 dbw/0.6 MHz 8

9 Interference Threshold: FDR w.r.t. 1 MHz bandwidth centered at E1 or E2 G r = -4.5 db horizontal incident dbw/mhz for acquisition (according to ITU 8/D/31 E) 2.1 db 2.1 db N/A d min = 7245 km d min = 7378 km N/A G r = -10 db incident from below d min = 3846 km (Radio Horizon) d min = 3917 km (Radio Horizon) N/A * Due to the in-band interference, ENSS-receivers are very susceptible. Because of the very high required minimum distance the range of the interfere is rather limited by the radio horizon than by the transmit power. 4.4 Calculation of the Radio Horizon As mentioned above, if the minimum required distance that is calculated from the link budget is several thousand kilometers the range of the interference source is limited by the curvature of the earth surface. The Radio horizon is calculated as follows: d = 2 R ( h t + h r ) Eq. Error! Unknown switch argument. With d: distance between the transmit antenna of the interference source and the radio horizon in m R: modified earth radius: (4/3) R Earth = (4/3) m (the factor 4/3 takes the extension of the range by atmospheric refraction into account) h t, h r : height of the transmit and receive antennas in m If we assume for an worst case assessment a transmit antenna height of 10 m and a height of the receive antenna mounted on an aircraft a height of m we yield a radio horizon of km. 5 Summary and Conclusion The investigation shows that the guard band for GPS applied in Germany of MHz +/- 14 MHz is sufficient to rule out interference to C/A-Code receivers. But it seems to be not sufficient prevent interference to civilian P-Code receivers that need to acquire C/A-Code. The use of GPS in aviation will be jeopardized if such a guard band is not applied in countries listed under RR S5.355 and S The impact of the interference from the Fixed Services to GLONASS and ENSS is very severe. The analysis in this paper demonstrates that the required geographical separation necessary to protect RNSS from interferences produced by Fixed Services is only limited by radio horizons. It is 9

10 therefore concluded that in order to protect present and future GNSS applications sharing of the band MHz between RNSS and Fixed Service is not feasible. Subsequently, the allocation to Fixed Services in the band MHz based on provisions RR S5.355 and RR S5.359 has to be deleted. 10