Problems of realization and implementation LEO-HTS systems

Analysis of various orbits, in particular low circular polar orbits to create a global communications and data transmission systems based on multi-satellite groups represented in a number of publications [1]. Implemented two systems using polar orbits. Personal satellite communication system Iridium and satellite data transmission system messenger Gonets. Today, of particular interest are new LEO-HTS global systems, since their effectiveness is assessed as higher even in comparison with HTS GSO systems [2].

The system LEO-HTS (in contrast to the HTS systems) provides a global service and the delay in the propagation of the signal is 30 times less than when using a satellite in geostationary orbit. However, the transition to the practical implementation of the LEO-HTS systems requires a preliminary analysis of a number of problems. Recall that back in the mid 90's attempts to create such systems have been made [3,4]. The most famous Teledesic system [5].

All projects are mid-90s of the last century [4], similar to the modern projects LEO-HTS, have been unsuccessful. The reason that investors have modern systems LEO-HTS note the high cost of creating the previous systems (such as Teledesic system cost about $ 9 billion, at an equivalent cost of one satellite approximately 10 million. $, on the original project of 7-8 mln. $ [5]) and a subscriber segment (https://www.linkedin.com/pulse/can-spacex-succeed-leo-constellation-phil-thomas). It was assumed that the cost of the most simple user Teledesic terminal will be 1000 $ (http://www.slideshare.net/abinas1/teledesic-14096612).

During the design of these systems, and work was begun to address EMC problems, but with the closure of projects, these works were stopped. While in the Radio Regulations were standards for permissible levels of interference flux density (Article 22 of the RR).

Pricing problems

In 2015. investors new LEO-HTS systems have stated that manufacturing cost of satellites LEO-HTS today can be reduced up to 20 times (ie 500 thousand. $ and lower the cost of a single satellite) and the cost of the subscriber terminal is planned within the $ 100-300 for the system SpaceX and $ 250 for OneWeb system [6]. uch a record decrease in the cost of the satellite is planned at the expense of their production seriality. Thus, the company OneWeb in April 2015 announced plans to build its plant for serial production of satellites (http://oneweb.world/press-releases/2016/oneweb-satellites-unveils-the-worlds-largest-high-volume-satellite-manufacturing-facility).

Hopes for a reduction in the cost of subscriber terminals are also connected to the serial production of phased array antennas for user terminals. And the decline in the value of the subscriber terminal to the $ 100-300 for investors statements is one of the most significant factors for the commercial success of LEO-HTS projects.

In [7] carried out the analysis system parameters OneWeb and SpaceX, to assess the possibility of creating a PA of subscriber terminals. The results of this analysis indicate that for the system OneWeb cost of the antenna system of the user terminal will amount to 7700 thousand. $ (PA Tx 18 x 18cm, APA Rx 36 x 36 cm), and for SpaceX system approximately $ 750 (PA Tx 13 x13 cm, APA Rx 22 x 22 cm). These data demonstrate that declared cost of the user terminal 100-300 $ unattainable in the foreseeable future.

This conclusion is indirectly confirmed by the message companies Kymeta, Phasor and C-Com Satellite Systems Inc., seeking to create a phased array antennas for user terminals in GEO and LEO-HTS systems in Ku and Ka-band.

From about 2013. there is an active search for technological solutions for the realization of PA and APA for satellite communication systems in the Ku-band and Ka-band. But as of 2016 real samples that meet investors' claims LEO-HTS systems are not represented. Basically posts concern prospects for the creation of a relatively expensive phased arrays of subscriber terminals for collective use for mobiles.

Company NSR predicts that the market for such phased array antennas to 2025 will reach annual sales volume of 710 million. $. However, information about creating a PA (APA) suitable for the individual user terminals not presented. For example, in the report [8] on the conference IEEE International Symposium on Phased Array Systems and Technology 2016 mentions that the company Kymeta is developing antennas phased array with electronic beam control, but the price for the Tx/Px mode not less than $ 2000 ($ 1000 per dish). Plans the company entering to the market 2017 (previously planned for 2015).

EMC problem for GSO systems

Satellites LEO-HTS create interference for earth stations operating in the FSS and BSS networks. Systems based on satellites in nongeostationary orbits are always secondary in relation to systems based on GSO satellites.

In a number of publications [7,9-11] marked EMC problem between the LEO-HTS systems (OneWeb in particular), with systems operating with satellites in geostationary orbit. This problem is especially notable in the Ku frequency band. The rules on the allowable interference to the protected earth station communications and broadcasting Radio Regulations set out in Article 22.

After many criticisms and the FCC requests a number of companies that satellites OneWeb and SpaceX will create significant interference to stations receiving information from geostationary satellites (above the norms permitted by the Radio Regulations), the company OneWeb proposed to solve this problem by turning the satellite pitch during their span in the equatorial zone.

Prior to the approach of the satellite to the equatorial zone of the spacecraft reoriented pitch of about 10 degrees in order to ensure the required angle α (Figure 1). Then, after passing the equator, the satellites are folded on the pitch in the opposite direction. At the moment of crossing the equator all satellite beams are switched off. Within the equatorial belt (15 ° N to 15 ° S), you can prevent a partial shutdown of rays that require additional modeling. This decision was made because each satellite generates a fixed OneWeb unmanaged group of 16-beams. The decision on the viability of this method for EMC between the system OneWeb and the systems operating with communication and broadcasting satellites in geostationary orbit, will be accepted FCC Commission after November 2016 (In 15 November 2016 the deadline for acceptance of claims https://apps.fcc.gov/edocs_public/attachmatch/DOC-341034A1.pdf).

EMC problem for HEO systems

Obviously, for systems designed on the basis of satellites in highly elliptical orbits, the EMC problem is no less important. But the problem is complicated by the large number of orbit types and changes in the angular direction of the receiving antenna of the user terminal, which is exposed to interferences. Equivalent EIRP spectral density of the interference generated by the satellite LEO-HTS, at a fixed time

Wi=Kri Pi Gt(Θi)/4πHi²

Kri = Gr(φi)/ Gr max - coefficient spatial selection of interference by the receiving antenna; θi = θi (t) the angle between the axial LEO-HTS beam radiation and the direction of the satellite receiving station HEO ; φi = φi (t) - the angle between the receiving station to the satellite HEO and direction of the satellite LEO-HTS ; Gt(θi) - gain of user beam LEO-HTS satellite; Pi - power spectral density of user beam LEO-HTC satellite in the normalized frequency range; Hi = Hi[t,(ψ; λ)]> - slant distance between the satellite LEO-HTS and receiving station of satellite HEO; Gr((φi) - the gain of the antenna receiving station operating to a satellite HEO, in the direction of the satellite LEO-HTS; Gr max - antenna gain at the maximum receiving station operating to a satellite HEO; (ψ; λ) - geographic coordinates (latitude, longitude) placing the receiving station operating to a satellite HEO; i - satellite number LEO-HTS; t - current time without taking into account the signal propagation delays at LEO-HTS lines and HEO.

The result of evaluating interference level in the Ku-band produced satellite oneweb, for earth stations receiving information from a satellite in orbit such as the Tundra, is shown in Figure 2.

Parameters OneWeb satellite and a receiving earth station operating to a satellite in orbit Tundra (approximate values taken on the basis of the data [12]) are presented in Table 1.

The envelope of the radiation pattern illustrated in Figure 3 for the receiving antenna 0.4m equivalent size at a frequency of 12 GHz (rec. S.1428 (for FSS) and BO.1443 (for the BSS)). Figure 2 shows the estimated signal level (C), received at the subscriber line VEO-Earth and interference level (I), generated by satellite OneWeb. Interference OneWeb even from one satellite to 12.6dB exceeds the level of the received signal, i.e. I / C = + 12.6dB. It is obvious that in such circumstances receiving information from the HEO satellite is not possibleIn order to receive information from HEO the interference should be reduced by 22 dB, i.e. ratio should be I / C <-10 dB. In this case, the reduction in C / N at the reception can be acceptable, no more than about 0.3dB. However, as follows from Figure 3, this is achieved if the angle between the axial OneWeb radiation and the direction of the receiving station (antenna 0.4m) HEO satellites will be more than 6 degrees. I.e θi ≥ 6 degrees to achieve Kri = Gr ((φi) / Gr max = 22dB.

Given the large number of satellites which have an angular spacing in the orbital plane and the spacing of orbital planes in space about 10 angular degrees to meet this condition can not at any point of time. Almost always, for any latitude and longitude of the receiving station operating with HEO will be observed for at least one satellite in OneWeb angular cone at the top of 12 degrees. And in the cone angle at the apex of 25.5 degrees, ie, for the receiving antenna scan angles (tracking satellites in orbit Tundra) will always be four satellites oneweb (interference value is little more than one satellite). Naturally spatial selection can increase by increasing the size of the receiving antenna, but this greatly reduces the attractiveness HEO systems based orbits.

For a comprehensive analysis of the EMC system LEO-HTS (OneWeb in particular) and HEO systems require multiple simulations of different situations, depending on the geographical coordinates of the receiving station, the size of its antenna and potential link badget (due to weather conditions, due to a variety of satellite boot options) .

Conclusion

1. One of the major problems at realization LEO-HTS systems is the lack of effective technological solutions create a cheap PA (APA) with electronic beam scanning. Moreover, in the foreseeable future there is no reason to hope for a solution to this problem in relation to the subscriber terminals of satellite communication systems in the Ku-band and Ka-band. But the search in this direction are necessary.

2. Simplified analysis of EMC problems LEO-HTS systems shows that there is no adequate solutions, which allow the use of these systems with other satellite networks in common frequency bands. For example, creating OneWeb system (as any other similar system) substantially closes the feasibility HEO systems in the frequency range of 10.7-12.7 GHz. Given that the Federal Space Program of Russia is considered more promising projects with the orbits of HEO ("Ekspess PB", "Polar Star", including at realization projects for the Arctic [13-15]), the problem of EMC with LEO-HTS designed system in Ku / Ka / Q / V band is the most relevant for Russia and requires international discussions on the level of ITU-R.

Literature:

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