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SATCOM-On-The-Move—Why One Size Doesn’t Fit All
by Timothy Shroyer CTO, General Dynamics, SATCOM Technologies

Satcom-On-The-Move (SOTM) offers true broadband communications capabilities for civil and military users. While the ultimate desire is to use satellite Earth stations, which are as small and light as possible, several trade-offs affect the ultimate coverage areas and communications data rates these systems can provide. This article considers several of the factors that drive the selection of a specific SOTM Earth terminal configuration.

ShroyerFig1 SOTM Offers True Broadband
SOTM offers broadband satellite communications to mobile users, on land, at sea, and in the air. Due to vehicle mounting and other constraints, there is a desire to implement SOTM terminals with the smallest possible size and weight. If it was possible to provide very high data rate SATCOM service with an exceptionally small antenna aperture, there would be no need to use anything other than the smallest possible terminals.

As larger apertures are needed to support higher data rate services, SOTM system designers must optimize terminal size to satisfy the “best” trade of performance, size, weight, and power. Just as there is no single SOTM communications requirement, there is no single optimal SOTM terminal configuration.

This article considers several of the most significant factors affecting SOTM terminal implementation—maximizing communications performance with suitable terminal size, weight, power, and cost.

The fundamental problem facing all satellite communications systems is to provide suitable performance on the RF link through the desired satellite, while minimizing interference energy towards adjacent satellites, as shown in Figure 1 on the previous page early satellite communications systems used rather large Earth terminal antennas, which exhibit high discrimination of energy to and from adjacent satellites.

SOTM Earth terminal antennas are so small that they have exactly the opposite effect. The small aperture size offers relatively low gain towards the satellite of interest and simultaneously radiates a significant amount of energy towards adjacent satellites.

Adjacent Satellite Interference Constraints
While the effective function of any satellite communications Earth station is the radiation of RF energy to and from the satellite of interest, the principal limitation on RF power is not the energy radiated towards that satellite of interest. It is, instead, the limit of RF energy permitted to be transmitted towards adjacent satellites—Adjacent Satellite Interference or ASI. This limit is expressed in terms of EIRP power spectral density rather than absolute power and is intended to ensure that multiple satellites can effectively use the geostationary arc.

IET_ad_MSM05013 In satellite communications systems, a systems engineer performs a link analysis to determine the characteristics of the link through the desired satellite. The link analysis considers the data rate to be provided, the modulated signal structure, the propagation and atmospheric losses, and the required uplink power. It then determines required satellite power and bandwidth. Link analyses performed for large Earth stations typically then confirm that the ASI conditions are satisfied after designing the link. With earth terminal antennas as small as those used in SOTM links, it is often necessary to first consider the permitted ASI levels and then design the link modulation such that the on-satellite performance is sufficient under those constraints.

International treaties have been established which provide the ultimate limits for acceptable RF power density. On a global basis, these agreements are codified in the Recommendations of the International Telecommunications Union. For FSS Ku-band and FSS Ka-band operation, some of the most significant IRU Recommendations are ITU-R S.524-91—limits for FSS C, Ku, and Ka-band off-axis EIRP density and ITU-R S.728-12—limits off-axis EIRP density from VSATs.

Each country is responsible for administrating satellite communications transmissions from within their own territory, and they establish their own regulations to comply with the ITU Recommendations. In the United States, the Federal Communications Commission (FCC) is responsible for administering civil communications, including satellite communications on FSS Ku-band and FSS Ka-band.

Federal communications in the United States, including military X-and Ka-band satellite communications, are administered by the National Telecommunications Information Agency (NTIA). The FCC has recognized the need for SOTM systems and has implemented regulations for FSS Ku-band operations from land vehicles in FCC Regulation 25.2263—Blanket licensing for VMES, and from maritime vehicles in FCC Regulation 25.2224—Blanket licensing for FSS Ku-band ESVs.

Considering FSS Ku-band operation, for example, the VMES Regulations apply to SOTM operation from land vehicles in United States territory. The VMES Regulations are actually more restrictive than the ITU Recommendations, mostly due to the FCC’s desire to operate FSS Ku-band satellites spaced every 2 degrees over CONUS.

Figure 2 illustrates the EIRP power spectral density limits imposed on FSS Ku-band SOTM operation due to ITU S.524-9, ITU S.728-1, and FCC VMES Regulations. A SOTM terminal operating anywhere in the world will always have to satisfy the limits of S.524-9 or S.728-1. In the United States, an FSS Ku-band SOTM terminal must satisfy the ASI limits imposed by the VMES Regulations.

On other civil frequency bands, such as FSS Ka-band, there are other applicable regulations but similar ASI limits apply. U.S. DoD SATCOM operations are conducted per NTIA administration, which essentially results in compliance with the requirements of Mil-Std-188-1645 – Interoperability of SHF Satellite Communications Terminals. The Mil-Std-188-164B requirements contain different absolute levels for EIRP power spectral density, but provide similar constraints that limit SOTM ASI.

ShroyerFig2 SOTM Operational Trades

Aperture Size
The aperture size of the SOTM Earth terminal has a significant effect on both the uplink and downlink performance of the satellite communications link, just as with all satellite communications systems.

On the downlink, the aperture drives both the signal energy available from the satellite of interest as well as discrimination against downlink energy from adjacent satellites. On the uplink, the aperture size drives the EIRP transmitted towards the satellite of interest as well as discrimination of uplink energy radiated towards adjacent satellites that forms ASI.

To examine the effect of downlink aperture on power required from the satellite of interest on the link, a series of link analyses has been performed for a normalized 1 MBPS link.

Figure 3 provides a graphical representation of the downlink power required for different downlink aperture sizes on various typical satellite frequency bands. These link analyses were performed using typical satellite transponder characteristics operational in 2012. G/T of the various downlink apertures are driven mostly by the downlink aperture size and utilize the same typical LNA Noise Temperature for the appropriate frequency band.

The downlink EIRP requirements illustrated in Figure 3 are normalized to the same 1 MBPS data rate, assuming BPSK R-1/2 LDPC FEC operation. Scaling to higher or lower data rates effectively scales the EIRP required by the same ratio to 1 MBPS, assuming similar modulation characteristics. For fully-compliant operation it may be advantageous to change modulation format, which would effectively trade satellite transponder EIRP and bandwidth, but the scaling can be considered relevant.

In most frequency bands, as the aperture size exceeds about 3.8 meters, there is essentially no reduction in EIRP required as the link Noise level is dominated by satellite transponder noise floor instead of receive terminal G/T.

ShroyerFig3 As SOTM apertures are always relatively small to accommodate vehicle mounting, they require significantly more EIRP than larger Earth station apertures to provide the same data rate communications service.

From Figure 3 it can be observed that smaller apertures drive the requirement for higher EIRP from the satellite transponder. To satisfy specific link requirements and downlink power density requirements a trade in downlink aperture as well as satellite power and bandwidth will often be appropriate. Under some operating conditions higher data rate services will thus only be possible using larger apertures simply due to limits in downlink EIRP if no other factor.

On the uplink side a similar series of link analyses confirms the uplink EIRP required to implement the same 1 MBPS data rate link, assuming BPSK R-1/2 LDPC FEC operation. Figure 4 graphically illustrates the resulting uplink EIRP requirements using the same series operational transponder characteristics as the downlink evaluation described in Figure 3.

The uplink EIRP requirements illustrated in Figure 4 often have an even more dramatic impact on SOTM aperture selection than downlink EIRP. As ASI limits are expressed in terms of EIRP power spectral density, and not absolute EIRP, smaller apertures with higher EIRP requirements can easily exceed ASI limits with conventional PSK modulation. This situation directly drives a trade on the SOTM uplink signal which includes:

equation1 Link data rate
Modulation order and FEC rate
The use of spectrum spreading techniques

The ultimate limit in capacity of the satellite channel has been shown to be limited by the Shannon-Hartley theorem6, which states: Equation2

C = capacity of channel in bits per second
B = channel bandwidth in Hz

The bandwidth of the satellite transponder is always finite, and the combined transponder power and bandwidth is relatively costly. As SOTM antennas decrease in size, the same desired communications capacity can only be provided with the same power, or EIRP, yet the EIRP power spectral density must be maintained below the appropriate regulatory limits.

This process can be extended through FEC coding and spectrum spreading techniques until the limit of available transponder bandwidth is exceeded. Doing so is costly in terms of transponder resources.

ShroyerFig4 One can observe that at some point the available transponder power and bandwidth could no longer support a desired capacity. In such cases the only trade is to reduce the desired capacity or increase the power available on the link—by increasing the size of the SOTM aperture such that total EIRP can be supported with suitable EIRP power spectral density.

Aperture Pointing Accuracy
There is a further trade which also significantly affects the performance of the SOTM terminal on both the uplink and downlink paths—antenna pointing accuracy. On the downlink, decreased accuracy in pointing towards the desired satellite both decreases the effective received Signal energy and increases the downlink ASI. As downlink ASI levels depend upon the spacing of adjacent satellites and the presence of downlink RF energy on exactly the same downlink frequency and polarization, this may or may not be a problem in practice.

On the uplink side, the effect is similar in that uplink Signal energy towards the satellite of interest is decreased and additional ASI is generated towards other satellites. The loss of uplink Signal energy can be overcome by simply increasing the uplink EIRP. However, the small size of SOTM terminal apertures result in significant levels of energy radiated in directions other than those desired.

In an operational sense, suitable link performance could, theoretically, be obtained through any combination of gain towards the desired satellite, antenna pointing accuracy, and ASI limitation. If only one satellite were ever present on the desired operating frequency, the only potential interference source on the downlink would be terrestrial emitters and thermal noise. Without the need to worry about uplink ASI, the energy radiated in undesired directions would be inconsequential.

Of course, this is not the case. Not only are there ITU Recommendations which mandate uplink ASI limits and the tolerance of other satellite downlink energy, but no satellite frequency band can be considered exclusive.

Some of the Regulations governing the operation of SOTM terminals, such as the FCC VMES3 and ESV4 Regulations, combine limits on EIRP power spectral density with antenna pointing accuracy.

ShroyerFig5 In the FCC ESV and VMES Regulations satisfying the best specified pointing accuracy of +/- 0.2 degrees permits operation at the highest permitted EIRP power spectral density. Reductions in antenna pointing accuracy must be accommodated by a reduction in on-satellite EIRP power spectral density to ensure ASI limits are maintained or through coordination of uplink signals with adjacent satellites.

Other regulations, such as ITU Recommendations S.524-91 and S.728-12 specify only the absolute EIRP power spectral density limits and presume that steps are taken in antenna gain, pointing, and EIRP power control to stay within the limits.

To place this in perspective as to its impact on FSS Ku-band SOTM terminals, we can consider various potential SOTM terminal antenna gain patterns and how they affect ERIP power spectral density. Figure 5 illustrates the uplink radiation patterns of four different SOTM terminals and shows how their respective EIRP power spectral density can be controlled to satisfy the FCC VMES limits.

Figure 5 illustrates the EIRP density of four different SOTM apertures and the FCC VMES EIRP power spectral density limits in the region from bore sight toward the satellite of interest and 8 degrees. Each of the example antennas illustrated exhibit different gain and 3 dB beamwidth. Assuming the antennas are pointing within the specified accuracy of +/- 0.2 degrees, the EIRP power spectral density can be adjusted as illustrated to remain within the FCC VMES limitations.

As can be observed in Figure 4, a larger antenna, such as the 30-inch aperture shown, can then provide higher EIRP power spectral density towards the satellite and remain within the ASI limits. Smaller antennas not only have lower gain, but their EIRP power spectral density must also be reduced to remain below the ASI limits because their 3 dB beamwidth approaches the limit curve.

If the antenna pointing accuracy were to be reduced, such that a larger pointing error is permitted towards adjacent satellites, the EIRP power spectral density would have to be further reduced to remain compliant with ASI limits. The effect of a 1 degree pointing error on the 30-inch SOTM aperture is illustrated in Figure 6.

In this example, a pointing error on the order of 1 degree would induce a reduction of permitted EIRP on the order of greater than 5 dB. The combined effects of shifting the gain peak due to mispointing and reduction of permitted EIRP would result in a reduction of EIRP towards the satellite during off-pointing conditions of greater than 10 dB.

For normal SOTM operations this 10 dB reduction in on-satellite performance would have to be factored in to link performance either as expected outage or further changes in modulation format. Since the limitation is one of EIRP power spectral density rather than absolute EIRP it would not be possible to simply compensate with uplink power control.

The analyses described previously all considered apertures consisting of circular parabolic antennas. Other antenna architectures are certainly possible and offer additional advantages and disadvantages. If a non-circular parabolic antenna is utilized it can be considered to radiate with a beamwidth inversely proportional to the dimension along the related axis. Along the narrow axis, such an antenna will exhibit its widest beamwidth and vice versa. The use of this type of antenna in SOTM terminals can effectively reduce the size of the terminal itself, but all potential impacts on the energy radiated towards the desired and adjacent satellites must be considered.

ShroyerFig6 The FCC VMES Regulations, for example state, “For non-circular VMES antennas, the major axis of the antenna shall be aligned with the tangent to the arc of the GSO at the orbital location of the target satellite, to the extent required to meet the specified off-axis EIRP spectral-density criteria.”

Other types of antennas are also of interest for SOTM operation. Phased-array antennas, for example, offer much lower mounting height on a vehicle than parabolic antennas. This characteristic alone makes phased-array antennas worth evaluation for SOTM operation.

When considering the performance of such an antenna in SOTM operation, system designers must satisfy the same on-satellite gain and ASI limitation as previously described. This class of antenna offers the advantage of significant potential reductions in height on the vehicle but the disadvantage of potential complications in control of antenna gain towards the desired and adjacent satellites. Typical phased-array antennas exhibit gain which is a function of the antenna area perpendicular to the satellite, just as with parabolic and other antennas.

For the general case this can be modeled, to the first level of approximation, as a function of sine θ), where θ is the radiation angle, from parallel to the antenna surface. Figure 7 illustrates the modeled reduction in gain as the radiation angle diverges from perpendicular to the array surface. Terminal Hardware Cost

Terminal hardware cost is a significant factor in any satellite communications system and the same holds true in SOTM operation. Some applications may be driven by the communications link itself—to provide a suitable data rate link regardless of the implementation cost.

Most SOTM systems, however, will continue to be driven by a need to provide cost-effective communications. The cost trades for the RF equipment in SOTM terminals are similar to other types of Earth stations in terms of the antenna itself and High Power Amplifier, but must additionally consider the relative cost and complexity of the antenna pointing and tracking system.

High Power Amplifiers can be considered just as they are with other Earth stations. HPAs with higher output power are physically larger and heavier, consume more primary power, and generate more waste heat.

One of the first HPA cost trades driven by typical SOTM terminals is the appropriate size, weight, and power. If the required uplink power can be satisfied with an HPA mounted directly on the antenna itself, the terminal will exhibit higher efficiency and less total weight. HPAs mounted off the antenna result in more RF losses between the HPA and antenna feed—driving further increases in HPA output power as well as higher primary input power and more weight.

With current GaAs and GaN SSPA technology the relative cost of SSPAs suitable for SOTM operation can be considered roughly proportional to output power levels. This approximation remains valid until at some higher power level significant changes must be made in packaging and heat dissipation which results in a disproportionate increase in cost.

In general, the cost of a parabolic antenna itself changes little from the smallest to largest apertures that are practical for vehicle mounting. Larger apertures, however, drive the antenna pointing and tracking system costs.

A larger antenna has a narrower beamwidth and, therefore, must be pointed and tracked with better accuracy to satisfy link and ASI constraints. Larger antennas also have more mass so the pointing and tracking system must utilize more drive power to maintain the same or better pointing accuracy.

SOTM systems require some reference mechanism to both determine their operating location as well as terminal attitude, in terms of roll, pitch, and yaw, to acquire the desired satellite. GPS receivers provide a very cost-effective means for determining earth terminal location so they are used by virtually all SOTM systems.

Determining Earth terminal attitude, however, adds significant complication. Without a high accuracy terminal attitude reference system it may take an unacceptably long period of time for a SOTM terminal to search for the desired satellite. As SOTM operation experiences significant periods of signal blockage due to physical obstructions this process must then be repeated after every satellite signal interruption—possibly reducing link availability to an unacceptable level. The accuracy of the SOTM terminal attitude reference system is critical to acceptable operation and it can become a cost driver.

From this top level analysis, it is clear that SOTM terminals using larger antennas are more costly to implement than those with smaller antennas. If terminal cost were the only consideration the choice would be clear—use smaller antennas.

As can be seen from the earlier analysis, however, terminal cost is only one factor in the overall system design, and it is rarely the most significant factor in SOTM system implementation.

The Optimal SOTM Terminal
As can be observed through the analysis provided, the effective size of the SOTM antenna aperture has a significant impact on operation. There are advantages and disadvantages to each. At the system level one can consider general advantages and disadvantages such as:

ShroyerFig7 Larger Antennas
Lower satellite transponder costs
Improved overall efficiency
Supports higher data rates

More costly hardware
Must be pointed and tracked accurately
Less desirable physical size

Smaller Antennas
Less costly hardware
Easier to point and track
More desirable physical size

Higher satellite transponder costs
Reduced overall efficiency
Can be too small to support data rate

The optimal selection depends upon the specific system requirements. As smaller SOTM antenna apertures are physically incapable of supporting link data rates above a limited throughput, they are not a universal solution.

From the analysis above concerning Aperture Efficiency and Adjacent Satellite Interference, one could surmise that a significant increase in satellite transponder EIRP and G/T could drive the use of much smaller SOTM apertures. International agreements in place via the ITU establish a basis for satellite characteristics that permit multiple satellites to share the geostationary arc so this does not appear to be a practical alternative unless a new spectrum assignment is agreed.

Similarly, although larger antennas make more efficient use of satellite transponder resources, they are too large to mount or otherwise inappropriate in some situations.

SOTM system designers must continue to trade the relative merits of alternatives in selecting the optimal solution to specific requirements.


[1] ITU-R S.524-9 - Maximum permissible levels of off-axis e.i.r.p. density from Earth stations in geostationary-satellite orbit networks operating in the fixed satellite service transmitting in the 6 GHz, 13 GHz, 14 GHz and 30 GHz frequency bands.

[2] ITU-R S.728-1 - Maximum permissible level of off-axis e.i.r.p. density from very small aperture terminals (VSATs).

[3] 47 CFR 25.226 - Blanket Licensing provisions for domestic, U.S. Vehicle-Mounted Earth stations (VMESs) receiving in the 10.95-11.2 GHz (space-to-Earth), 11.45-11.7 GHz (space-to-Earth), and 11.7-12.2 GHz (space-to-Earth) frequency bands and transmitting in the 14.0-14.5 GHz (Earth-to-space) frequency band, operating with Geostationary Satellites in the Fixed-Satellite Service.

[4] 47 CFR 25.222 - Blanket Licensing provisions for Earth stations on Vessels (ESVs) receiving in the 10.95–11.2 GHz (space-to-Earth), 11.45–11.7 GHz (space-to-Earth), 11.7–12.2 GHz (space-to-Earth) frequency bands and transmitting in the 14.0–14.5 GHz (Earth-to-space) frequency band, operating with Geostationary Orbit (GSO) Satellites in the Fixed-Satellite Service.

[5] Mil-Std-188-164B - Interoperability of SHF Satellite Communications Terminals

[6] C. E. Shannon (January 1949). “Communication in the presence of noise” Proc. Institute of Radio Engineers vol. 37 (1): 10–21.

ShroyerHead About the author
Tim Shroyer is the Chief Technology Officer of General Dynamics Satcom Technologies, one of the world’s largest manufacturers of satellite earth station equipment and systems. He is a satellite communications systems engineer and has managed, designed, built, installed, and operated satellite earth stations around the world since 1975.

Mr. Shroyer’s first satellite communications experience was as an officer in the United States Navy and with the Defense Information Systems Agency. While at Stanford Telecommunications he served as the Program Manager for the development and installation of the Network Control system that still manages U.S. military communications satellites. Since that time he has managed the development and installation of satellite Network Control and TT&C (Telemetry, Tracking and Control) systems for many satellite networks around the world. He also was the founder and president of a VSAT (Very Small Aperture Terminal) transceiver manufacturing company.

He has designed and installed uplink stations of all types in many countries, in all the world’s continents, and developed new earth station architectures including pioneering the development of L-band IF (Intermediate Frequency) systems, now common in commercial and military satcom. Among his other responsibilities at General Dynamics, Mr. Shroyer is one of the principal architects of the Company’s Satcom-On-The-Move System and products. He has worked closely with the Federal Communications Commission in the United States to create a licensing class for this entirely new generation of satellite communications systems.

Shroyer holds a Bachelor of Science in Electrical Engineering Degree from the University of Southern California.