1、 Rec. ITU-R S.1554 1 RECOMMENDATION ITU-R S.1554 Methodology for determining the overall accuracy of epfdmeasurements (Question ITU-R 236/4) (2002) The ITU Radiocommunication Assembly, considering a) that the World Radiocommunication Conference (Istanbul, 2000) (WRC-2000) adopted a combination of si
2、ngle-entry validation, single-entry operational and, for certain antenna sizes, single-entry additional operational equivalent power flux-density (downlink) (epfd) limits, contained in Article 22 of the Radio Regulations (RR), along with the aggregate limits in Resolution 76 (WRC-2000), which apply
3、to non-geostationary fixed-satellite service (non-GSO FSS) systems, that protect GSO networks in parts of the frequency range 10.7-30 GHz; b) that compliance of a non-GSO FSS system with the single-entry operational epfdlimits is not subject to verification by the Radiocommunication Bureau; c) that
4、administrations with assignments to GSO FSS and/or broadcasting-satellite service (BSS) networks that have been brought into use, as well as administrations with assignments to non-GSO FSS systems that have been brought into use, require reliable means of ascertaining that non-GSO FSS systems are in
5、 compliance with the single-entry operational limits referred to in considering a); d) that epfdlevels may have to be measured at operational GSO earth stations in order to determine if the operational limits are exceeded, noting a) that Recommendation ITU-R S.1558 is being developed to provide a me
6、thod for measuring epfdlevels, recommends 1 that the methodology described in Annex 1 can be used to determine the accuracy of the measurement procedure. 2 Rec. ITU-R S.1554 ANNEX 1 Methodology for determining the overall accuracy of epfdmeasurements 1 Introduction WRC-2000 adopted operational epfd
7、limits to protect GSO FSS links from suffering loss of synchronization or degraded performance due to non-GSO systems and to protect GSO FSS systems employing adaptive coding in 30/20 GHz bands. The non-GSO satellites, due to the geometry of their orbits with respect to the GSO ground antennas, may
8、exceed the operational limits for only short periods of time (s). The operational limits could be measured at earth station locations that suffer loss of synchronization events or degraded performance at an unexpected time (e.g. not obviously caused by a very high rain fade, a sun outage event, or a
9、n event caused by equipment failure and/or associated with switch over). The GSO satellite operator would determine whether the loss of synchronization or degraded performance was due to in-line interference from a non-GSO FSS system into the GSO network. If there is perceived to be a correlation be
10、tween loss of synchronization or degraded performance and a non-GSO system in-line event, a measurement system would be used at the GSO receive earth station site incurring the losses of synchronization or degraded performance to measure the level of non-GSO interference being experienced by the GSO
11、 earth station. The preferred embodiment of the measurement procedure requires a well-calibrated carrier system monitoring (CSM) earth station in the same beam as the affected earth station. The CSM station provides a calibrated reference level for the affected earth station. It allows the expertise
12、 required to perform calibration to be centralized. Additionally, it is desirable to set up an automated measurement system at the affected earth station to reduce the probability of human error affecting the results. In this Annex the overall accuracy of operational measurements of non-GSO interfer
13、ence is calculated for the preferred measurement procedure. Acronym list AWG : additive white Gaussian B : bandwidth CSM : carrier system monitoring DSP : digital signal processing IOT : in-orbit test LNA : low noise amplifier RSS : root squared sum S /N : signal-to-noise ratio T : time (s) Rec. ITU
14、-R S.1554 3 2 Measurement procedure realization using a DSP, spectrum analyser or power meter The critical concern in accurately measuring the non-GSO interference is system calibration. The calibration equipment may be integrated into the earth station affected by non-GSO interference; calibration
15、can be performed remotely from a CSM station, or a well-calibrated portable system can be brought to the affected earth station site. Additionally, the portable system could be implemented using a scanning antenna. Each system design involves its own set of trade-offs. In order to measure epfd accur
16、ately, one of the straightforward approaches could be to integrate the measurement and calibration equipment into the earth station receiving the interference. However, this approach will require disruption of the GSO service, while the system is integrated and calibrated. Service may be disrupted f
17、or several hours. Additionally, service would presumably be disrupted again if the measuring equipment is needed at another earth station site. A self-contained, portable, test set-up may be the most cost-effective approach. The system would not disrupt GSO operations and calibration measurements co
18、uld be performed ahead of time. The portable test set-up may require a smaller antenna than the earth station affected by the interference. In this case the wider beamwidth test antenna could receive higher epfd levels from the non-GSO system compared to the affected antenna at small off-axis angles
19、, as shown in Fig. 1. 1554-01TestantennaAffectedantennaNon-GSO interferenceFIGURE 1Measurements with a small test antenna and a large earth station antennaThere are two solutions to the problem in Fig. 1. If the direction of the non-GSO interference is known, then the gain difference between the tes
20、t antenna and affected earth station antenna can be accounted for if the respective antenna patterns are accurately known. Alternatively, the test antenna could track the non-GSO satellite. Additionally, if a large enough tracking antenna is used it will provide discrimination between the GSO signal
21、 and the non-GSO interference under test until the non-GSO is in the main lobe of the GSO antenna. Using a smaller test antenna will yield a lower S /N relative to a larger antenna that is receiving the same signal strength. The lower the received S /N the longer the measurement time required to get
22、 the same level of accuracy. Table 1 shows the half power beamwidths and S /N (assuming a 4 Rec. ITU-R S.1554 receiver temperature of 200 K), for several different antenna sizes. The S /N correspond to signal levels equal to the operational limit values for a 3 m (161.25 dB(W/(m2 40 kHz) and 10 m (1
23、66 dB(W/(m2 40 kHz) antenna, respectively. TABLE 1 Measurement characteristics for different size earth station antennas Finally, if a calibrated CSM earth station is available in the same beam as the affected earth station then it can perform the calibration for the affected earth station. The CSM
24、station is very accurately calibrated and can be used to determine the received signal level at the affected earth station. Figure 2 shows the CSM station and the affected earth station. Notice that the CSM receives the same signal from the satellite as the affected earth station but with a differen
25、t satellite gain. Since the relative satellite antenna gains in the direction of both earth stations are known (IOT measure-ments) the CSM station can calibrate the affected earth station receive signal level. This reference level can then be used by the affected earth station to determine its noise
26、 and interference levels. 1554-02CSMAffectedearth stationFIGURE 2CSM configuration providing calibration to an affected earth stationAntenna diameter (m) Half power beamwidth (degrees) (Signal level = 161.25 dB(W/(m2 40 kHz) S /N (dB) (Signal level = 166 dB(W/(m2 40 kHz) S /N (dB) 2 1.75 1.4 3.3 3 0
27、.58 4.5 0.2 10 0.175 15 10.3 Rec. ITU-R S.1554 5 This Annex focuses on the accuracy of the approach where a CSM station is used to calibrate the affected earth station receive signal level. The measurement procedure is as follows: Both the CSM and affected earth station could measure the satellite b
28、eacon. The beacon measurement provides a stable well-known signal level that can be used for calibration at the CSM station. It also provides an estimate of the differential between the atmospheric losses at the affected earth station compared to the CSM station. If the variations in the beacon leve
29、l due to stability and measurement error are greater than the variation due to atmospheric changes then the beacon measurement may not be valuable. The rest of this Annex assumes that interference measurements are performed in clear sky and the beacon measurements are not factored into the overall a
30、ccuracy estimate. It should be noted, however, that in the case of a low Earth orbit (LEO) system where the significant interference occurs near an in-line event the fading on the non-GSO signal can be assumed to be close to that of the GSO system. This implies that the carrier-to-interference level
31、 for the link is approximately the same in clear sky and during rain. Thus the true non-GSO interference level should take into account the fade level. At the CSM station measure a reference signal. Two sources for the reference signal have been investigated: the reference signal is the earth statio
32、n carrier affected by interference, the reference signal is a pilot signal transmitted in the guardband adjacent to the affected signal. Fig. 3 shows the affected signal under test. The guardband shows the pilot that can be used as the CSM reference signal. 6 Rec. ITU-R S.1554 1554-03N1N2Signal 2Sig
33、nal 1PilotFIGURE 3In-band versus out-of-band noise levelsNotice that because of the filter skirts the noise level in the guardband can be different than the noise at the signal frequency. This can cause an error if the affected earth station signal is used as the CSM reference. When the affected ear
34、th station signal is the reference it is used to determine noise level N1. However, the interference is measured relative to noise level N2. Therefore, if the affected earth station signal is used as a reference then the measured interference can be in error by the difference between noise levels N1
35、and N2. The pilot signal may provide a slightly more accurate measurement than using the affected earth station signal as the reference. The accuracy estimate calculated in this Annex does not address the error that may occur if the affected earth station signal is used as the CSM reference. At the
36、affected earth station measure interference plus noise and noise. The interference plus noise measurement is performed in the guardband next to the affected carrier. The noise level can also be measured in the guardband, but just before and after the interference event. All the measurements must be
37、performed close together in time so that the values do not have a chance to change. Additionally, the interference is present only for a short time, which limits the measurement time and accuracy. An automated procedure can be used to make the measurements. The set-up would require a measurement ins
38、trument (power meter, spectrum analyser, DSP) at the affected earth station. The output of the measurement device would be attached to a computer which stores the spectrum under test. An interference pulse would trigger interference plus noise and noise measurements on the stored data. At the same t
39、ime a signal could be sent automatically to the CSM in order to request a measurement of the reference carrier as soon as possible (more study is needed on the procedure to be employed here, and on the potential contribution to the error budget of any significant delay). Rec. ITU-R S.1554 7 3 Calibr
40、ation of the CSM RF path The measurement procedure relies on the accuracy of the CSM system. Fig. 4 shows a block diagram of a CSM station with the calibration equipment integrated directly into the RF path of the earth station. 1554-04FIGURE 4CSM block diagramDownconverterSpectrum analyser,DSP anal
41、yser, orpower meterComputer control and recording deviceSelectivepower meterFrequencysynthesizerLNAWaveguidedirectional couplerWaveguidedirectionalcouplersThe measuring instrument consists of a spectrum analyser, DSP analyser or power meter connected through a waveguide directional coupler after the
42、 LNA. Calibration equipment consists of a frequency synthesizer and power meter for calibrating the measurement equipment. The equipment is controlled from a computer interface. The computer controls measurement parameters, calibration times, and the recording of the measurement information. The fre
43、quency synthesizer provides a reference calibration signal. This is needed to minimize the effects of equipment drift and random variations on the measurements. Human error can be a significant source of error in setting up the calibration process, depending on the expertise of the earth station sta
44、ff. The monitoring system has to first be calibrated off-line. These calibrations remain static during operation. The second type of calibration is automatic and is performed during operation. Both types of calibration data are required to meet accuracy requirements. Off-line the instrumentation (i.
45、e. power meters, analysers, synthesizers), antenna, and components (i.e. waveguides, couplers, measurement filter, and LNA) are calibrated. These calibrations may be accomplished periodically, using standard laboratory procedure. Data files are maintained that include calibrations over a frequency r
46、ange depending on how the measurements are made. The frequency synthesizer and power meters are checked at regular intervals to ensure the accuracy of the calibration equipment. 8 Rec. ITU-R S.1554 Two automatic gain calibration measurements (GCand Gn) of the receive downlink chain from the antenna
47、headend to the measuring device must be performed periodically. A frequency synthesizer injects a signal into the LNA input. Accurate power meter measurements verify the signal level. The synthesizer level is constantly checked by a separate power meter measurement. The gains GCand Gnare both measur
48、ed at the non-GSO beacon frequency between the antenna feed and the measuring device. The choice of the injected carrier-to-noise ratio should be greater than 25 dB to allow the noise power to be neglected. The earth station receiver gain equation is given by: =CalImCnCLPPCWLGG)(1(1) where: Pm: powe
49、r at the power meter PI: injected power LC (CW) : loss of the measurement filter for the reference carrier LCal: calibration error between the reference carrier and the receive waveguide at the antenna feed. In Table 2 there is a breakdown of the error standard deviations associated with equation (1). These uncertainties are assumed to be Gaussian random variables. A Monte Carlo statistical evaluation of equation (1) by computer, using the uncertainties in Table 2, yields the receive gain uncertainty of 0.34 dB. TABLE 2 Earth station receive gain
