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Robert Bowen, Ali Shoamanesh (Telesat Canada), and Arvind Bastikar (CSA) SPACEOPS 2004 May 17-21, Montreal

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## Robert Bowen, Ali Shoamanesh (Telesat Canada), and Arvind Bastikar (CSA) SPACEOPS 2004 May 17-21, Montreal

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Operational Electromagnetic Compatibility Study between Non-Geostationary Earth-Exploration Satellite Networks (Interference/Sharing Analysis)

Robert Bowen, Ali Shoamanesh

(Telesat Canada), and

Arvind Bastikar (CSA)

SPACEOPS 2004

May 17-21, Montreal

The Interference Environment of CSA’s NGSO Satellite Networks at 2 GHz and at 8 GHz

- CSA’s NGSO satellite networks such as RADARSAT, SCISAT, and MOST operate in limited frequency ranges at both 8 GHz and at 2 GHz.
- The large number of such NGSO satellites worldwide prevents dedicated frequency bands for each operating satellite network.
- Because of this shared use of the available spectrum, inter-network interference is a necessary consideration in the design and operation of such networks.
- This paper discusses the interference environment of such satellite networks, and describes a method of evaluating the seriousness of that interference.

Differences in the Characteristics of Interference in GSO and in NGSO Satellite Networks

- The interference between two GSO satellite networks is “static” or relatively time-invariant, because the relative position of the satellites is stationary.
- In contrast, the interference between two NGSO satellites, or between an NGSO and a GSO satellite network is a series of short high-interference bursts separated by long intervals of no appreciable interference.

Characteristics of the Interference at an NGSO Earth Station or Space Station Receiver

Characteristics of Interference in NGSO Satellite Networks (continued)

- Recognized characteristics of interference in NGSO include;
- Maximum levels of interference
- Interval between interference bursts, and
- Probability that the interference exceeds a specific magnitude.
- Criteria of interference into EESS NGSO networks in ITU-R recommendations is based on this third criteria, the probability that the interference from another network exceeds a specified level a specific percentage of time.

Interference Between NGSO Downlink Networks

- The work that is described in detail in this paper is the interference between the RADARSAT and another satellite network at 8 GHz.
- ITU-R Recommendation SA.1026-3 specifies that the interference into such a network should not exceed -117 dBW per 100 MHz bandwidth more than 0.025 % of the time.

Ways to Determine and Meet the Conditions in SA.1026-3

- Determination whether the limit of -117 dBW per 100 MHz bandwidth more than 0.025 % of the time limit is met is not easy.
- This determination may be necessary as part of the ITU coordination between two networks.
- Determination can be by either analysis or by simulation.
- Use of the Information obtained, to meet the required interference limit where necessary, can be either through either system design changes or by implementation of operational measures.

Method Described Here to Estimate the Interference Between Two Networks

- The method described here to determine the interference probability is analytic, rather than a simulation tool.
- Result is in terms of the sum of a finite number of terms, determined by modeling the interference as a stochastic process.
- The resulting algorithm can be as accurate as is desired by increasing the number of terms in the finite sums involved.
- The algorithm has been computerized in an EXCEL program. This EXCEL program is being modified to be faster and more flexible.

The Cause of an Interference BurstBetween Two Downlink Networks

- The interference into the earth station receiver of an 8 GHz satellite network exceeds the ITU limit only when both the desired transmitting satellite and the interfering satellite are simultaneously in the main beam or the near sidelobe of the antenna of the receiving earth station
- This condition occurs with very low probability, resulting in the interference limit being exceeded with a similarly very low probability.
- The task is to determine as accurately as possible the magnitude of that very low probability.

Diagram of the Interference Condition (Figure 10 of ITU-R Rec. S.1325-2)

Interference Estimation Process

- The basis for determining the probability that the interference exceeds a specified level is determination of the probability that the interfering satellite is in the main beam of the tracking interfered-with earth station.
- This has to be determined for every location of the interfered-with and the interfering satellites, in the region visible from the interfered-with earth station antenna beam.

Key to Determining the Probability of Significant Interference

- The key to determining the probability of significant interference is determining the probability that a satellite is in a specified small segment of its orbital “shell”.
- That key is found in ITU-R Rec. S.1257. It specifies the probability that a satellite in a circular LEO orbit is in a small solid angle of A steradians.

Key (continued)

- Rec. S.1257 specifies that the probability that a satellite in a LEO circular orbit is in an area defined by its solid angle of A steradians is

P = (A/2π2){sin2(i) - sin2(L)}-1/2 ….. (1)

where i is the latitude of the area of interest, and L is the maximum latitude of the satellite’s orbit.

Conversion Factors for Different Area Latitudes and Inclination (Fig. 4, ITU-R Rec. S.1257-3)

Assumptions Made in Determining the Probability of Significant Interference

- Assumption #1: The two satellites have asynchronous orbits, so the probability that Satellite #1 is in small area Ai is statistically independent of the probability that satellite #2 is in the same area Ai.
- Assumption #2: Interference times are only counted when the interfered-with satellite is visible from its earth station.
- Assumption #3: Higher-order statistics such as clusters of interference bursts are not taken into account.

Interference Probability of Interest

- The probability of interest is P(I/K), given in Equation 2 of the written paper, is

P (I/K) = j {P (Kj ) / i P (Ki )} * P (Rj ) …….. (2)

where Kj is the event that the interfered-with satellite is in the small area Aj,

K is the event that the satellite is in area K,

P (Kj ) is the probability of event Kj,

Rj is the area over which the interfering satellite causes significant interference when the interfered-with satellite is in the sub-area Aj, and

P (Rj ) is the probability that the interfering satellite is in the sub-area Aj.

Calculation of P (I/K)

- The first step is to divide the area A that is visible from the interfered-with earth station into a large number of sub-areas {Aj, j = 1,2,…N}. The Aj must be small enough that the function {sin2(i) - sin2(L)}-1/2 does not vary significantly over Aj. These Aj must be smaller at higher latitudes, and must also be such that the power-flux-density from the interfering satellite does not vary significantly over Aj. The Aj are determined using Equation 1.

Calculation of P (I/K) (continued)

- The Rj, j = 1,2,…N are then determined, taking into account the earth station antenna pattern, the EIRP of the satellite, and the distance from the satellite to the earth station. Note that Rj, like Aj, is measured in steradians.
- In the example calculations described below, the area A was divided into 202 sub-areas Aj, with Aj being smaller at higher latitudes, as shown in the following figure.

Example Calculation (continued)

- In the example calculation done using a rudimentary EXCEL computer program
- The latitude of the earth station was that of Fairbanks Alaska, a worst-case high-latitude location;
- Both interfering and interfered-with satellites had an 81.4° maximum latitude and a 790 km altitude;
- The transmitting satellite had the characteristics of RADARSAT-2A or RADARSAT-2B;
- In the hypothetical worst-case example studied, both satellites had receiving earth stations in Fairbanks; and
- The receiving earth stations had the characteristics of the RADARSAT earth station in Prince Albert.

Example Calculation Results

- Results of the calculations based on a 202 sub-area quantization, the interference probability was
- 5.9 % of the 0.025 % ITU limit when the interfering satellite was RADARSAT-2A; and
- 28.3 % of the 0.025 % ITU limit when the interfering satellite was RADARSAT-2B.

Example Calculation Results (continued)

- In an earlier simpler calculation with a smaller area-quantization, with only 80 sub-areas Aj, the results were:
- 5.9 % of the 0.025 % ITU limit when the interfering satellite was RADARSAT-2A; and
- 28.3 % of the 0.025 % ITU limit when the interfering satellite was RADARSAT-2B

Discussion of Results

- The above results indicate that:
- 1. Choice of how the area visible from the interfered-with earth station is divided into {Aj} has a significant effect on the results obtained. This is because of the of the detail of the function {sin2(i) - sin2(L)}-1/2;
- 2. The probability estimate for RADARSAT-2B is greater than that for RADARSAT-2A, simply because of its 3 dB higher EIRP, but neither satellite exceeds the ITU-R recommended limit when 10 meter earth stations are used; and
- 3. The interference-analysis tool described here can be used to better understand the capabilities and limitations of frequency reuse of the 8 GHz EESS space-to-Earth allocation.

Extension to 2 GHz of the 8 GHz Analysis Tool Described Here

- As described above, the analysis tool described here is useful in exploring the capabilities and limitations of the use of the 8 GHz EESS band.
- The tool can be readily extended to exploration of the capabilities and limitations of the use of the 2 GHz EESS bands 2025-2110 MHz and 2200-2290 MHz.
- The major differences in extending the 8 GHz analysis tool to 2 GHz is the relevant ITU-R interference-probability limitations. At 2 GHz recommendations SA.5214-3; SA.1160-2 and SA.1163.2 apply rather than SA.SA.1026-3.

Operational Electromagnetic Compatibility Study between Non-Geostationary Earth-Exploration Satellite Networks (Interference/Sharing Analysis)

Robert Bowen, Ali Shoamanesh

(Telesat Canada), and

Arvind Bastikar (CSA)

SPACEOPS 2004

May 17-21, Montreal

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