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Low-Earth-orbit (LEO) satellites fly at different heights above Earth and can vary in the amount of time they are visible to a ground station (GS) to transferring data to and from the GS. For a typical LEO satellite in orbit 600 km above Earth, visibility to a GS may be less than 20 minutes per orbit, limiting the amount of data that can be exchanged. By recruiting a geosynchronous-Earth-orbit (GEO) satellite as part of the overall data link, however, it can be possible to increase the available LEO satellite data-transmission time (and amount of data).
Since the percentage of a LEO satellite’s visibility to a GS is less than 20% of the satellite’s orbital period, the time to download data in a satellite-communications (satcom) system is limited. To increase the imaging data capability and capacity of a remote-sensing LEO satellite, the imagery data must be stored onboard the satellite and downloaded at a higher data rate during such a limited visibility period. Furthermore, a greater number of networked GS sites around the Earth must be used.
Most of Earth’s surface is covered by water, so accomplishing this task would require that a large number of “ground” terminals be located on ships—at prohibitive cost. However, a GEO satellite is visible to a LEO spacecraft for more than one-half of the LEO satellite’s orbit, as a possible component in a data relay system. In theory, two widely spaced GEO satellites can provide continuous visibility/coverage for a LEO satellite.
1. This diagram attempts to illustrate the conditions for visibility of a LEO satellite from a stationary GEO satellite.
Figure 1 depicts the visibility of a LEO satellite from a GEO satellite and antenna tracking ranges. A GEO communications satellite is considered to be at point G in the GEO equatorial orbit, with zero inclination. Earth’s center is represented by point O, with line segments OC = OD = OH = OJ = R, the radius of Earth, which is approximately 6,380 km. A GEO orbit is represented by HG, at approximately 36,000 km. The height of a LEO satellite orbit is represented by HI (about 600 km for a typical remote-sensing satellite).
In the satellite diagram of Fig. 1, line segment OC is perpendicular to line segment ACG (where ACG is tangential to Earth at point C) and line segment OB is perpendicular to KBG (with KBG tangential to the LEO orbit at point B). The satellite at point G in the GEO orbit can have a maximum line-of-sight (LOS) visibility to point A or point E in the LEO satellite orbit. Arc ABIFE represents the maximum arc of the LEO satellite in which a GEO satellite can have LOS visibility with the LEO satellite. Angle BGF is the maximum angle a GEO satellite can have a LOS with the LEO satellite.
Wide-beam antennas are not practical onboard LEO or GEO satellites because of the need for the high gain required in support of transmitting signals at high data rates in excess of 100 Mb/s. The high-gain data links from LEO to GEO satellites call for antennas with narrow beam widths. In addition, to maintain LOS between satellites, the beams of LEO satellite antennas as well as GEO satellite antennas must be steered. An antenna on the LEO satellite intended for LOS with the GEO satellite must be mounted on the face of the satellite opposite the Earth’s surface.
The steering angle range of the antenna on a LEO satellite with respect to a GEO satellite is 180° or the angle denoted by OAG in Fig. 1. This indicates that an antenna on a LEO satellite shall have a beam-scanning capacity of more than ±90° while a GEO satellite antenna shall be able to scan over the angle represented by ±OGB in Fig. 1. Table 1 presents details for two typical LEO scenarios, with a LEO satellite in orbit 600 km above Earth and a GEO satellite for relaying data.
In principle, 100% visibility of a LEO satellite can be obtained by using two GEO satellites positioned 180° apart in GEO flight patterns. This calls for two ground stations (GSs) on opposite sides of Earth. The ground stations must be networked so that data is transferred to the required GS.
Two satellites separated, for example, by about 100° in GEO can be controlled by a single GS, with both satellites transferring data to the single GS. With these two GEO satellites separated by 100°, steering the LEO antenna by ±90° can cover the majority of its orbit. By steering the LEO antenna by about ±113o for maximum LEO satellite visibility, 100% of its orbit can be covered.
Such steering capability is not routine, however. The antenna on the LEO satellite must track over an angular range of ±XAG as represented in Fig. 1, which is more than 90° on either side of LOS contact with the GEO satellite from points A and E on the orbital map. Although this can provide maximum visibility of the LEO satellite to the GEO satellite, this can make the design of the LEO satellite’s antenna very complicated. That’s because it requires sufficient clearance from the satellite body to achieve a large field of view (FOV), and an antenna with a large boom.
Steering a beam either by dual-gimbal-controlled, high-gain antenna or by spherical phased-array antenna over ±90° is possible with minimum impact on the satellite. An alternative means of achieving the maximum possible visibility area is by employing such an antenna design as a quadrifilar helix antenna to provide a wide beam (±120°) with sufficient FOV of the LEO satellite and its antenna.
As an example, two stationary GEO satellites at 34°E and at 134°E (100° separation) can be controlled from a GS in India. Two types of antennas can be used on each satellite. A horn antenna with a relatively wide beam width of 18.7° can provide about 16 dBi gain. Alternatively, a higher-gain antenna with narrower beam width can be used, although it must be steered over a range of ±18.7° in elevation and 360° in azimuth. To better understand the results expected from the use of such different antennas, link analyses were performed with a LEO imaging satellite in 600-km-high orbit catering to low data rates of less than 10 kb/s, as well as payload data at rates to 150 Mb/s.
For the analyses, C- and Ku-band frequencies were not considered since these frequency bands are fully occupied. Rather, the use of links between satellites was considered for cases with bands specified by the International Telecommunications Union (ITU) at S-, X-, or Ka-band frequencies.
For intersatellite communications, when GEO tracking is limited to LEO arc FIB in Fig. 1, rain attenuation is not a concern since the radio links will not pass through the atmosphere. Under such conditions, 20/30-GHz Ka-band frequencies would be popular choices for intersatellite communications. However, for maximum visibility between satellites, the LOS between the GEO and LEO satellites will pass through Earth’s atmosphere; Ka-band frequencies are not well-suited due to rainfall attenuation.
In addition, for maximum visibility, the required antenna structure becomes complex. More than 85% orbital coverage is possible with a configuration of two satellites separated by 100° in the GEO satellite and antenna tracking limited to ±90° on the LEO satellite, As a result, Ka-band frequencies at 30 GHz were chosen as the operating range for a high-bit-rate link between LEO and GEO satellites.
Spectra at X-band frequencies (8025 to 8400 MHz) and Ka-band frequencies (25.5 to 27 GHz) were allotted for space-to-Earth data transmissions. Either of these bands can be used for high-bit-rate links from GEO satellite to GS. A link at X-band will be less affected by rain than one at Ka-band frequencies, but a Ka-band link can be used to transfer high-bit-rate data from a GEO satellite to a GS by adopting rain-loss mitigation techniques. By designing in sufficient link margin; using adoptive modulation, data-rate, and coding techniques; and using spatial diversity, it is possible to minimize the effects of rainfall attenuation on link performance.2. Shown are the various links between two different types of satellites and an Earth-based ground station (GS).
For lower-rate telemetry, tracking, and control (TTC) data, S-band frequencies can be used as a link between a GS and the GEO satellite since this link will be not be impacted by rainfall attenuation. When using a wide-beam antenna on a LEO satellite, almost 100% coverage can be obtained with two GEO satellites placed at 34° elevation and 134° elevation locations. S-band frequencies can also be employed for GEO-LEO and LEO-GEO inter-satellite communications. Alternatively, low-bit-rate data can be communicated to a GEO satellite from a GS using S-band frequencies and translated onboard the GEO satellite to Ka-band frequencies for transfer to a LEO satellite.
Making the Case via Link Analysis
To better understand the workings of inter-satellite communications, link analysis was performed for three possible cases, with various options. Each case assumed that high-bit-rate data was being communicated from a LEO satellite to a GEO satellite to a GS. The link from the LEO satellite to the GEO satellite occurred at Ka-band (30 GHz), while the link from the GEO satellite to the GS took place at Ka-band (26 GHz) or X-band (8.2 GHz).
In the first case, a wide-beam antenna was used on the LEO satellite with a high-gain antenna on the GEO satellite. The LEO antenna gain was 0 dBi with transmit power of 100 W. The GEO receive antenna had a diameter of 5 m and an antenna gain-to-noise-temperature (G/T) value of 31.5 dB/°K. This case was found to be capable of supporting a maximum data rate of 200 kb/s with about 3-dB link margin. It would not be practical to provide more transmit power to the LEO antenna, and a complicated mechanism was required for steering the 5-m antenna on the GEO satellite. As became evident, it is not easy to achieve a high-bit-rate data link with an omnidirectional, wide-beam antenna on the LEO satellite.
In the second case, a narrow-beam horn antenna with 18.7° beam width and 16-dBi gain was used on the GEO spacecraft to eliminate antenna steering. To send 100 Mb/s at 100 W Ka-band transmit power, a steerable parabolic antenna larger than 5 m in diameter would be needed on the LEO satellite, which is not very practical. For sustainable, high-speed data links, even the GEO satellite must be steerable and capable of high gain in this case.3. This block diagram represents the basic components of a GEO satellite system.
In the third case, to transmit data at 100 Mb/s between satellites, quadrature-phase-shift-keying (QPSK) modulation with 7.5-dB coding gain was used at Ka-band (30 GHz). A 0.6-m-diameter transmit antenna was incorporated on the LEO satellite with 40 W transmit power and effective isotropic radiated power (EIRP) of 57 dBW. A 1.5-m-diameter receive antenna was used on the GEO satellite with G/T of 21 dB/°K and a LEO-to-GEO-link carrier-to-noise (C/N0) ratio of 90 dBHz.
Several options were considered as part of these three satcom systems for analysis. For option 1, a link between the GEO satellite and the GS was assumed at Ka-band (26 GHz). The GEO satellite transmit antenna was 0.2 m in diameter with 20 W transmit power and 43.5 dBW EIRP. The receive antenna on the GS was 7.5 m in diameter with G/T ratio of 37.5 dB/°K. The downlink C/N0 level was 95 dBHz with an overall C/N0 of 88.8 dBHz. For 100-Mb/s data transfer with 10-6 bit-error-rate (BER) performance that maintains 7.5-dB coding gain, this setup provides about 3.5-dB margin for a clear sky.
For option 2, the link between the GEO satellite and GS was at X-band (8.2 GHz) using a 0.2-m-diameter GEO satellite transmit antenna with 40 W transmit power and 36 dBW EIRP. The receive antenna on the GS was 7.5 m in diameter with a G/T ratio of 32 dB/°K. The downlink C/N0 level was 92 dBHz with an overall C/N0 of 88 dBHz. For 100-Mb/s data transfer with 10-6 BER and 7.5-dB coding gain, this setup provides about 3.0-dB margin for a clear sky.
Suggested system configurations for achieving a Ka-band high-bit-rate data link from the LEO satellite to the GEO satellite requires a LEO satellite with a 0.5-m-diameter dual-gimballed antenna (DGA) that features 1.4° beam width and better than ±0.01° tracking accuracy. To achieve a better than 56 dBW EIRP, a 40-W Ka-band traveling-wave-tube amplifier (TWTA) is needed onboard the LEO satellite. For the GEO satellite, a DGA with 1.5-m diameter and 0.47° beam width can be used. It should have a G/T ratio of 21 dB/°K and better than ±0.04° tracking accuracy.
For the third case with option 1, Ka-band could be considered from the link from the GEO satellite to the GS if the satellite carries Ka-band payload transponders and the onboard antenna can be shared. To overcome at least 10-dB rainfall attenuation, a Ka-band antenna of at least 0.5 m in diameter should be used. The GS antenna should be 7.5 m in diameter with G/T performance of 37.5 dB/°K.
For the third case with option 2, less rainfall attenuation will occur when using an X-band link from the GEO satellite to the GS. To achieve an EIRP of 36 dBW from the GEO satellite to the GS, the GEO satellite should have a 0.2-m-diameter fixed antenna with 40-W-output TWTA or solid-state power amplifier (SSPA), or a 0.5-m-diameter antenna with 10-W-output-power SSPA. The GS should be 7.5 m in diameter with a G/T of 32 dB/°K. For low-bit-rate data links to 1 Mb/s at S-band, coding gain of about 9 dB is possible with turbo/LDPC coding.
For data links from the GS to the GEO satellite, a forward link from 2.0 to 2.1 GHz is possible by maintaining GS EIRP of 50 dBW via a 7.5-m-diameter antenna with 10 W transmit power and a GEO satellite with 0.5-m-diameter antenna and G/T of 13 dB/°K. For the return link from 2.2 to 2.3 GHz, the GEO satellite should have a 0.5-m-diameter antenna with 10 W transmit power and 27 dBW EIRP, as well as a GS with 7.5-m-diameter antenna and 12 dBW EIRP.
For data links from the GEO satellite to the LEO satellite (forward link) from 2.0 to 2.1 GHz, the GEO satellite should have a 2-m-diameter antenna with 50 W transmit power and EIRP of 47 dBW. The LEO satellite should have a 0.5-m-diameter antenna with G/T of –12 dB/°K. For the return data link from the LEO satellite to the GEO satellite, at 2.2 to 2.3 GHz, the LEO satellite should have a 0.5-m-diameter antenna with 50 W transmit power and the GEO satellite should have a 2-m-diameter antenna with G/T of +1 dB/°K. Analysis has shown that the link cannot be sustained with an omnidirectional or wide-beam-width antenna on the LEO satellite. For optimum high-bit-rate data communications at Ka-band, high-gain steerable antennas should be used on the LEO and GEO satellites.
Table 2 details the various options for the low-bit-rate forward and return links. For option 1, the antennas on the GEO satellite and on the two LEO satellites will be steered. For option 2, beamsteering is eliminated for the LEO satellites, but requires transmit power to be increased by 250-W power amplifiers (PAs) on the GEO satellite as well as on the LEO satellites, and a steerable 5.6-m antenna on the GEO satellite. Each 250-W PA requires more than 400 W dc power. Such added power consumption is difficult to justify in LEO satellites.
Since steering antennas on the GEO and LEO satellites is not a problem, option 1 is a more viable solution than option 2. Figure 2 shows a suggested satellite configuration, and Fig. 3 illustrates a block schematic of a GEO satcom system.
As this analysis has shown, the downlink data capacity of a LEO satellite can be enhanced by using GEO relay satellites. A single ground station can control the percentage of visibility with two satellites in GEO separated by about 100°, and enhance the data-transfer period to about 85% of the total orbital period of a LEO satellite.
V. Sambasiva Rao is a professor in the Department of Electronics and Communication Engineering (ECE), PES University, Bangalore, Karnataka, India 560085, e-mail: [email protected].
The author wishes to thank Dr. Chandar, HOD, E&CE Department, and Dr. KNB Murthy, Vice-Chancellor, PES University, for their encouragement.
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