Compatibility interference criteria

Interference is potentially received from several sources from multiple services simultaneously. The value listed in Recommendation ITU-R RS.1029-2 (for a specific band) is the maximum permissible interference level for the passive sensor from all sources of interference. This compatibility level of –166 dBW in any bandwidth of 200 MHz should be used for the band 23.6-24 GHz.

7.4 Interference assessment

Dynamic simulations were conducted to determine the impact of unwanted emissions from inter-satellite links in the HIBLEO-2 and TDRSS systems into the passive sensors described in § 7.1.4. In addition a semi-static analyses were done for the Hibleo-2 and DARTS satellite systems. 7.4.1 Semi-static analyses

7.4.1.1 Semi-static interference analysis of HIBLEO-2

The chart in Fig. 7-15 illustrates a worst case configuration involving one HIBLEO-2 19 MHz channel. Only the Nadir beam of the data acquisition antenna is considered. The amount of interfering power received may vary depending on the sensor antenna beam considered and on the orientation of the ISS antenna.

FIGURE 7-15

Configuration used for the dynamic analysis (co-frequency case, one HIBLEO-2 19 MHz channel)

Depending on the geocentric distance between the ISS and the EES orbits, three interfering events are occurring:

– At 0° angular distance between the two orbits, the Nadir beam of the data acquisition antenna is aiming at the ISS antenna. Interfering path ISS antenna far lobes to one push- broom antenna beam is established. The interference level produced by one ISS channel is

57.6 dB above threshold. It is proportional to the number of transmitting channels activated on the ISS satellite. Because the discrimination between adjacent beams is not infinite (see Table 7-4), all antenna beams are affected by interference.

Because all beams of the data acquisition antenna are permanently activated, similar situation exists for all of them within angular distances ± 10.25° between the two orbits. – Around angular distance 6° between the two orbits, the cold space calibration antenna is

aiming close to the ISS. The interference level is 33.7 dB above threshold. In such situation, all sounding data acquired by the sensor are invalidated.

– Around angular distance 47° between the two orbits, the EES is in the main-lobe of the ISS antenna. Interfering path ISS antenna main-lobe to push-broom antennas far lobes is established. The worst interference level (cold space calibration antenna) is 21.2 dB above threshold. All sounding data are invalidated.

The dynamic analysis should carefully consider each specific situation. In all cases, each beam which is contaminated by interference counts for one interfering event.

7.4.1.2 Semi-static analysis by DRTS to conically scanned sensor (including the attenuation evaluation of umwanted emission)

A semi-static analysis is performed to check if the EESS (passive) sensor receives the threshold level given in § 7.3. The results are shown in Fig. 7-16 for MEGHA, AMSR-E, AMSR, and CMIS sensors.

As the figure shows, the power received by the conically scanned sensors from the DRTS ISS forward link does not exceed the threshold level at any geocentric distance between the EESS sensor and DRTS satellite.

FIGURE 7-16

Power received by the conically scanned sensors (semi-static analysis)

7.4.1.3 Semi-static analysis by DRTS to Nadir sensor (including the attenuation evaluation of unwanted emissions)

The sensor antenna does not point towards the DRTS satellite because the angle with respect to the direction of the Nadir sensor field of view is ± 50° as given in Figs. 7-6 and 7-7.

The Nadir direction of the cold space calibration antenna is 83° as given in Table 7-3, making the calibration antennas more vulnerable than the sensor antennas.

Besides, judging from the Nadir sensor beam patterns of the AMSU-A and the push-broom, the push-broom sensor receivers receive more interference from the DRTS satellite than the AMSU-A sensor. Therefore, the push-broom sensor is to be chosen for this analysis as representative of the Nadir sounder.

The results of the semi-static analysis, with respect to the interference power level, are shown in Fig. 7-17.

The impact of interference to the sensor has been assessed for the DARTS system considering both the scanning and calibration modes of the sensor. Although the interference criteria was not exceeded for either mode, the calibration mode was identified as being more vulnerable and may warrant further analysis.

FIGURE 7-17

Power received by the push-broom sensor (semi-static analysis)

One interference scenario occurs when the DRTS ISS antenna directs towards the Nadir calibration antenna. This scenario occurs when the geocentric distance between the sensor and DRTS satellite is from 85° to about 90°. In this case, the excess level above the interference threshold for the push-broom sensor is calculated as 23.7 dB.

Table 7-9 shows the minimum off-axis angle of the EESS calibration antenna when the unwanted emissions of the DRTS satellite into the push-broom sensor exceed the interference threshold of –166 dB(W/200 MHz). When the angle between the direction of the calibration antenna and the direction of the DRTS satellite link is greater than 4.91°, the interference level does not exceed the criteria threshold level.

TABLE 7-9 Link budget

Parameters Values Total e.i.r.p. of DRTS in the worst 200 MHz in the band

23.6-24 GHz (dBW) 31.40

Free space loss (dB) 212.40

Off-axis angle of the EESS calibration antenna (degrees) 4.91 EESS calibration antenna gain (dBi) 14.98 Power received by the EESS calibration antenna –166.0

A semi-dynamic simulation is conducted to determine the probability of occurence when the angle between the calibration antenna and DRTS satellite becomes greater than 4.91°, using a time increment of 2 s during 9 days (EESS orbital period). This step is sufficiet for the simulation because the time step of 2 s corresponds to 0.000257% of 9 days, which is smaller than the threshold 0.01%.

The results of this simulation show that there is no occurence of time when the angle between the direction of the calibration antenna and the direction of the DRTS satellite link becomes less than 5.9°, hence it is always greater than 4.91°. Therefore, the DRTS ISS link in the 23 GHz band satisfies the interference criteria of the EESS (passive) in the 23.6-24 GHz band.

7.4.2 Dynamic analyses

7.4.2.1 HIBLEO-2 simulation model and results methodology used to assess interference level

Two dynamic simulations were considered. One simulation simulated interference from the HIBLEO-2 system to the AMSU-A, AMSR-E and CMIS passive sensors. The other considered interference into a push-broom passive sensor.

7.4.2.1.1 Simulation for AMSU-A, AMSR-E and CMIS passive sensors

Figure 7-18 illustrates the Iridium deployment model used in this simulation. Based on the national regulations applicable to the HIBLEO-2 system, integrating the unwanted emission mask over the lowest 200 MHz of the EESS (passive) band yields an unwanted emission transmit power of –32.8 dB(W/200 MHz), and a corresponding unwanted emission EIRP of 3.8 dB(W/200 MHz). At each time step, interference calculations are performed for each of four possible inter-satellite links from the HIBLEO-2 satellite. Simulations for this deployment model were conducted for 2 000 000 km2 measurement areas in North America between 32.524° N and 45.476° N and between 89.966° N and 106.034° W as illustrated in Fig. 7-18.

FIGURE 7-18 HIBLEO-2 deployment model

Simulations were run to produce CDFs over a simulation run of 16 days with a 200 ms step size when the passive sensor was able to sample points within the measurement area. The CDF of the interference from the Iridium inter-satellite links into the passive sensor in the 23.6-24.0 GHz band is presented in Fig. 7-19. It should be noted that the levels of interference received by the passive sensor from the HIBLEO-2 system unwanted emissions do not exceed the permissible interference criteria of Recommendation ITU-R RS.1029-2.

FIGURE 7-19

Interference CDF with HIBLEO-2

Additional simulation runs were produced to examine a hypothetical situation when the HIBLEO-2 orbit altitude is assumed to be same as the passive sensor orbit altitude. Figure 7-20 shows this hypothetical case assuming AMSU-A and HIBLEO-2, AMSR-E and HIBLEO-2, and CMIS and HIBLEO-2 have the same orbit altitude. It should be noted that the levels of interference received by the passive sensor do not exceed the permissible interference criteria of Recommendation ITU-R RS.1029-2.

FIGURE 7-20

Interference CDF with HIBLEO-2 at the same altitude as the passive sensor altitudes

7.4.2.1.2 Dynamic simulation for a push-broom passive sensor

The following Fig. 7-21 shows the result of a dynamic simulation for a single where one constellation HIBLEO-2 within only one active channel is in operation. According to the characteristics of HIBLEO-2, the bandwidth of one active channel is 19 MHz. The time increment selected for the present simulation is 0.5 2 s in order to get sufficient accuracy.

The EES orbit parameters used for the simulation (altitude, inclination and period) are those given in the Table 7-3 and the push broom sensor is used within the proposed dynamic simulation.

The ISS constellation orbit parameters are given in Tables 7-5 and 7-6.

All the results are expressed using a co-frequency approach. Afterwards, the attenuation provided by the Recommendation ITU-R SM.1541 should be applied, as no other method is currently proposed. However, due to the calculations performed within the Appendix of Annex 1, it should be emphasized that the mask provided by Recommendation ITU-R SM.1541 for MSS and FSS systems is unrealistic since it significantly overestimates the unwanted emission power.

The dynamic simulation based on the HIBLEO-2 constellation is based upon a simple assumption: each satellite tries to communicate with the nearest four within the constellation.

However, this dynamic simulation does not take into account the effect described in § 7.1.4.2 about the discrimination provided by the antenna pattern of the push broom composite antenna gain, as this dynamic simulation considers the push broom antenna in its composite form.

FIGURE 7-21

Cumulative distribution resulting from the dynamic analysis between HIBLEO-2 and a passive sensor push broom type

TABLE 7-10

Dynamic analysis between one inter-satellite link of the non-GSO system HIBLEO-2 and one EESS sensor push-broom type

Cumulative distribution (%) 0.0021 0.01 0.10 1.30 2 160 Push-broom: corresponding interference

power received by EESS (dBW) within a (200 MHz reference bandwidth)

–111 –117 ₋158 –164 –166 –178

According to Table 7-10, there is a high probability that the EESS satellite experiences interference when the inter-satellites links are in operation (the amount of data contaminated by interference is above the threshold required).

For a cumulative distribution of 0.01%, corresponding to the percentage of time or area the interference threshold may be exceeded, the interfering power indicated in Table 7-10 is –117 dBW for a 200 MHz reference bandwidth of the passive sensor of 200 MHz. However, as the active system is transmitting within a bandwidth of 19 MHz only, it would imply that the threshold is exceeded by a factor of _∆P₌166 – 117 = 49 dB.

These results, in particular the 49 dB excess above threshold for 0.01% were obtained with the HIBLEO-2 system described in the Table 7-4, assuming that one transmitting channel yields a total EIRP of 39.6 dBW within the passive band.

These values are to be compared with the level of 57.6 dB above threshold determined in the semi-static analysis, when one unique 19 MHz transmitting channel radiating 40.2 dBW is considered.

Therefore the maximum value applicable to a constellation similar to Hib-Leo2 (provisional) to be adopted for further consideration is in the co-frequency case when applying the technical appendix of the methodology in § 2.3:

e.i.r.p. (one channel) = –9.4 dBW (39.6 dBW –49 dB) when one link of the ISS constellation is in full operation on a 200 MHz reference bandwidth

The current dynamic analysis assumes that only one constellation will be in operation within the frequency band indicated in Table 7-5: the aggregation issue has not been considered. However, it is to be noted the aggregation of multiple constellations will have an impact on the result of the compatibility analysis.

If the unwanted emission power radiated within the passive band by one single satellite similar to one in the HIBLEO-2 system does not exceed –9.4 dBW for a reference bandwidth of 200 MHz, compatibility would be achieved.

7.4.2.1.2.1 Attenuation provided by Recommendation ITU-R SM.1541 and RR Appendix 3

If the following chart (see Fig. 7-22) shows the attenuation provided through the usage of Recommendation ITU-R SM.1541 for a bandwidth of 19 MHz. It is assumed that one single channel is activated. One considers a single active channel with an emission power of 3 dBW on a 19 MHz necessary bandwidth, the passive band will belong to its spurious domain.

Hence, the required attenuation is given by the RR Appendix 3 and is equal to the less stringent value between 43 + 10 log (P) or 60 dBc in a 4 kHz reference bandwidth in the special case of

space services.

It results an absolute value for the spurious power within the passive band of 3 – (43 + 3) = –43 dBW in a 4 kHz reference bandwidth. This value will be 4 dBW in a 200 MHz reference bandwidth.

It remains a shortage of 50 dB (4 – (–9.4 – 36.6)) to protect EESS from HIBLEO-2.

Nevertheless, it is generally well known that current regulatory limits for the spurious emissions are too important in some cases. It is particularly the case in this study.

It is then expected that unwanted emission will present a roll off aspect higher than those required by Recommendation ITU-R SM.1541 and RR Appendix 3.

Therefore, it is then proposed to apply the formula (3) in § 7.1.1.1.2 described in the technical appendix to the methodology in § 2.3 considering that unwanted emission mask provided by Recommendation ITU-R SM.1541 is available in the passive band. The unwanted emission domain mask is not limited to 250% of the necessary bandwidth and is extended within the passive band. It results from this assumption that the required attenuation within the passive band has to be greater than: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + ∆ = N ref B B P C 10log 2 = 59 dBsd, with: Bref2 = 200 MHz BN = 19 MHz.

Using this methodology, the computed attenuation equals 49.7 dBsd if the ISS HIBLEO-2 active channel is closer to the EESS (passive) band (with f0 = 23.367 GHz) or 57.4 dBsd if the ISS HIBLEO-2 active channel is further to the EESS (passive) band (with f0 = 23.192 GHz). It is to be

noted that this computation does not contain any post modulation filtering which is generally always the case. Therefore, it is expected that the unwanted emission is attenuated at least down to –59 dBsd within the passive band 23.6-24 GHz, which is enough to ensure the compatibility.

7.4.2.2 TDRSS simulation model and results

Figure 7-23 illustrates the TDRSS deployment model with the TDRSS satellites at 174° W (186° E) and 41° W (319° E). Based on the national regulations applicable to the TDRSS system, integrating the unwanted emission mask over the lowest 200 MHz of the EESS (passive) band yields an unwanted emission transmit power of –20.3 dB(W/200 MHz).

The simulation includes other satellites that could be served by the TDRSS forward links. For the purpose of this simulation, these satellites include the space shuttle, the international space station, and a constellation of 20 satellites in randomly distributed orbits. At each time step, interference calculations are performed for each of four possible inter-satellite links from a TDRSS satellite to the space shuttle, the international space station, and the two closest satellites of the 20 satellite constellation. At each time step, the interference levels from the two links to each TDRSS satellites producing the highest interference level into the passive sensor are combined to calculate the interference from the inter-satellite links of that TDRSS satellite. This TDRSS simulation model was run for the AMSU-A, AMSR-E and CMIS passive sensors.

FIGURE 7-22 TDRSS deployment model

Simulations for this deployment model were conducted for 2 000 000 km2 measurement areas in North America between 32.524° N and 45.476° N and between 89.966° W and 106.034° W as illustrated in Fig. 7-23. Simulations were run to produce CDFs over a simulation run of 16 days with a 200 ms step size when the .passive sensor was able to sample points within the measurement area. The CDF of the interference from the TDRSS inter-satellite links into the AMSU-A, AMSR-E and CMIS passive sensors in a 200 MHz reference bandwidth is presented in Fig. 7-24. It should be noted that the levels of interference received by the passive sensors from TDRSS unwanted

emissions do not exceed the permissible interference criteria of Recommendation ITU-R RS.1029-2.

FIGURE 7-23 Interference CDFs

7.4.3 Summary of study results

One set of studies dynamic simulations were conducted for the non-GSO HIBLEO-2 and the GSO TDRSS and DRTS systems in the ISS and several types of current passive sensors in the EESS (passive). These simulations indicate that the aggregate EESS (passive) permissible interference criteria of Recommendation ITU-R RS.1029-2 will be satisfied for current sensors. Consequently, no mitigation techniques need to be applied to either the active or passive service in order to achieve compatibility.

Another study indicated that if the unwanted emission power radiated in the passive band by one single satellite similar to one of the HIBLEO-2 system does not exceed –9.4 dBW in a reference bandwidth of 200 MHz, compatibility would be achieved for future passive sensor. The HIBLEO-2 studies have been based on some but not all of the characteristics of the system, and assume that it is operating 100% of the time at full capacity. In practice the HIBLEO-2 capacity varies a great deal in regards to time of day and in regards to different parts of the world. This unwanted eirp within the passive band can be mitigated through the application of an unwanted emission mask and a post modulation filtering.

7.5 Mitigation techniques 7.5.1 EESS (passive)

Current and future passive sensors integrate the signal received at the satellite and it is not possible to differentiate between the natural and the artificial emissions. For current passive sensors, no mitigation techniques need to be applied since the permissible interference criteria specified in Recommendation ITU-R RS.1029-2 are not exceeded. However, for future push broom passive sensors, there is risk of getting corrupted measurements from several areas that may impact reliable weather forecasts over the world.

7.5.2 ISS

Currently operational ISS systems are compatible with current passive sensors without the need for any additional mitigation. However, for future passive sensors, the analyses have shown that the required overall attenuation to protect the passive band can be easily met by active services. Therefore no specific mitigation techniques are need.

7.5.3 Potential impact 7.5.3.1 EESS (passive)

No adverse impact on the EESS (passive) is expected since the EESS (passive) interference criteria are satisfied by currently operational ISS systems.

7.5.3.2 ISS

No adverse impact on the EESS (passive) is expected since no mitigation techniques need to be applied to currently operational ISS systems in order to satisfy the EESS (passive) interference criteria.

7.6 Conclusions