Aero mobile Satellite communication technologies for Aircrafts and UAVs

Aero mobile Satellite communication technologies for Aircrafts and UAVs

IDST – International Defence, Security and Technologyby Rajesh Uppal 41m

The future of aviation worldwide is one of significant continuing growth in air travel, air cargo, and private general aviation. Passenger services communications are expected to generate revenue for airlines and service providers. They will require a “critical mass” of users to justify costly avionics installation and operating costs, hence they will need to be broadband services.

However, passenger aircraft remains one of the few places where ubiquitous data connectivity cannot be offered at high throughput, low latency and low cost. Airline and business jet passengers are demanding Internet connectivity as they travel across the globe. A survey by Honeywell revealed that nearly 75% of airline passengers are ready to switch airlines to secure access to a faster and more reliable Internet connection on-board and more than 20% of passengers have already switched their airline for the sake of better in-flight Internet access.

They will therefore evolve in a market-driven way, in fits and starts, with successes and failures dependent upon the quality and usefulness of service, customer acceptance, and service cost.  While the quality of service improves and costs are reduced, user demand for on-board communications connectivity will increase as the general public grows accustomed to ubiquitous access to wireless access in flights.

This is leading to increasing demand for high bandwidth data access on commercial aircraft for both business jets and major airliners. Until now such high bandwidth data links have been predominantly provided when the aircraft is over land, using a system of ground-based installations to provide the link to the aircraft. For full transcontinental coverage, SATCOM is the only effective way of providing connectivity with Inmarsat’s L-band coverage, for example.

In the future, to achieve the required bandwidths, the frequency of operation must move to the K_u-band or K_a-band to support data rates up to 1000 Mbps. New satellites are being launched that support higher frequencies to enable this increase in bandwidth.

SATCOM systems traditionally have utilized geostationary Earth orbit (GEO) satellites—satellites that relative to the Earth’s surface will stay in a fixed location. To achieve geostationary orbit, the satellite must be at a very high altitude—over 30 km from Earth’s surface. The benefit of such a high orbit is that very few satellites are needed to cover a large area of ground, and transmitting to the satellite is simplified because it has known, permanent coordinates. Due to the launch cost of these systems, they are designed for long lifecycles, resulting in a stable but sometimes antiquated system.

Because of altitude and radiation challenges, additional device screening or satellite shielding is often needed. Furthermore, because the satellite is so far away, there will be significant loss with the user on the ground, impacting signal chain design and component selection. The long ground-to-satellite distance also results in high latency between the user and the satellite, which can impact some data and communication links.

The proliferation of UAVs in the defense (and soon the commercial world), has created a new arena of SATCOM links. UAVs face similar challenges. The advanced defense-focused UAVs are required to operate around the globe with remote piloting, possibly from a different continent. These requirements drive a need for high bandwidth datalinks to support video, control, and advanced payload data, potentially saturating the existing communications infrastructure. With commercial UAVs also set to have expanded coverage in the future, global network high bandwidth connectivity will pose the same SATCOM challenges as in commercial aviation.

At the same time, supporting legacy data links, minimizing size, weight, and power (SWaP), and reducing investment in system development is driving a need to develop flexible architectures and maximize system reuse. Recently, many alternatives or complementing systems to GEO satellites have been proposed, with UAVs and low Earth orbit (LEO) satellites being considered. With lower orbits, these systems mitigate most of the challenges described with GEO-based systems, but at the expense of reach, with many more satellites or UAVs required for similar global coverage.

Challenges

Terminals in the aeronautical market work under one of the most difficult environments. Antennas carry very sophisticated requirements in terms of form factors (aerodynamic limitations), weight, pointing accuracy, reliability and many other technical requirements. Similarly, RF equipment has strong limitations in size and weight, while skyrocketing throughputs continuously demand more power. Modems orchestrate the link offering high-quality, high-speed communications in this challenging ecosystem.

Modems and baseband equipment operating in this vertical have very sophisticated requirements. The global nature of demand involves high initial CAPEX investments in baseband equipment. Routes and traffic patterns change depending on the season, and anytime of the day, which
requires very flexible networks. Consequently, requirements for the network management system are very sophisticated and include ensuring advanced Service Level Agreements for individual aircrafts and fleets running applications at different priority levels with planes constantly migrating to different beams and satellites — all this with end-users enjoying the highest standards of connectivity.

Ultimately, beam hopping and beam forming are capabilities that ground platforms and the space segment need to coordinate to optimally utilize capacity. Equipment must serve all these requirements, while meeting the stringent, expensive, and exhaustive certification process involved for any equipment used in the aeronautical sector.

Aeronautical channel model for satellite communications

Designing a satellite airborne technology for the military is significant technology advancement, yet it presents several distinct, technical challenges. Propagation effects of aeronautical channels differ from maritime and land mobile propagation because of the high velocity of aeroplanes, their distance from ground and influence of the aircraft body on antenna performance.

Aeroplane manoeuvres can affect signal under conditions when aircraft antenna is shadowed by the aircraft structure. When considering helicopters, the rotation of the rotor blades causes a cyclic interruption to the signal path.

An aeronautical channel model for satellite communications needs to consider two main contributions:
• a strong line of sight (LOS) component that is present all the time (except during maneuvers where the satellite signal might be blocked by the aircraft body) affected by ionospheric and tropospheric effects, and possibly by specular and/or diffuse scattering from the aircraft;
• a surface (ground, sea, ice, snow) scattering component which arrive with a certain delay, phase and attenuated power with respect to the LOS component.

The overall channel is considered time-varying due to various conditions: atmospheric dynamics, geometry changes due to aircraft flight (affecting not only the LOS, but also the fuselage scattering at each instant), dynamics of elements on the fuselage (e.g. blades for helicopters), time-variation of the surface scattering, and dynamics of the surface (e.g. sea waves). The effects of antenna pattern must not be neglected, the type of antenna may mitigate ground and fuselage multipath effects, but in cases such as banking maneuvers, multipath may arrive as strong contribution impacting the communications signal.

Researchers have designed a software tool for propagation analysis of satellite to aeronautical communications, including more physically based models, valid from VHF to Ka-bands, and which could take into account the following technical issues:
• modifications induced on the antenna pattern by its installation on the platform,
• geometry of the trajectory and platform dynamics during movements including the attitude,
• surface scattering effects for sea and ground surfaces, including vegetation layers,
• atmospheric effects due to troposphere at different altitudes and ionosphere.

Signal shadowing and multipath influence system design in a number of ways: signal blockage affects link reliability; and multipath noise causes errors in digital transmission.  frequency-selective fading causes inter-symbol interference when signal bandwidth exceeds the coherence bandwidth;

Airborne COTM networks must be able to efficiently and reliably support a wide range of throughput rates from basic email, to flash override Voice over IP (VoIP), to high-definition video.

These applications must operate on fast-moving aircraft, using a very small antenna and overcoming issues like the Doppler Effect and rain fade. An airborne COTM network must also support near seamless global coverage, track deployed units and manage beam switching while meeting stringent security and budget requirements.

The Doppler Effect is the change in frequency of a wave, as perceived by a receiving station, as either the transmitter or the receiver moves. Doppler frequency jitter appears as noise-causing detection errors, and Doppler frequency shift has to be compensated.

Historically, the Doppler Effect in satellite transmission has been a secondary consideration arising from the satellite’s motion in its station-keeping box. With highspeed vehicles such as aircraft, the Doppler Effect has a great impact on the effectiveness of demodulators. Therefore commercial aircraft systems such as  iDirect include built-in Doppler Compensation features that handle satellite frequency shifts to ensure seamless availability and performance.

LEO satellites potentially offer some relief. These operate at a much lower altitude—roughly 1 km off earth’s surface—but at this altitude they are not stationary, and in fact sweep across Earth’s surface, with an orbital cycle of roughly 30 minutes. The low altitude reduces the launch cost, and with a less harsh environment potentially less screening and shielding is needed. And critically, the low altitude means less propagation delay. But the primary difficulty for a LEO system is that the satellite is only within range of the user for fairly short bursts, necessitating the use of handoffs.

Technologies

Providing high-speed connectivity through small mobile antennas is one of the toughest challenges of airborne COTM. Sub-one meter antennas required for COTM have low gain characteristics. Higher power is required to ensure the receiving terminal hears the remote over the background noise created when the satellite boosts the signal. These high-rate signals coming from small antennas often cause interference with adjacent satellites that may be using the same frequency and polarization.

Advances in satellite manufacturing and directional earth-station technology, particularly the development of multi-axis stabilized earth-station antennas capable of maintaining a high degree of pointing accuracy, while stationary or on rapidly moving platforms, have made earth stations with very stable pointing characteristics both available and practical.

Network engineers must provide broadband connectivity to moving platforms without causing, or being impacted by, adjacent satellite interference. The solution is spread spectrum technology. Spread spectrum is a satellite router feature that diffuses high rate signals by “spreading out” the transmissions to minimize the interference to adjacent satellites without limiting connectivity to the target satellite. Yet, this can come at high bandwidth cost.

Automatic Beam Switching

Recent years have also seen several advances in satellite systems and networks, allowing better efficiency, reliability, increased data rates, and new applications. New paradigms such as mega constellations are manifest, triggering significant investments in future constellations. By 2020-2025 there will be more than 100 High Throughput Satellite (HTS) systems using Geostationary (GEO) orbits but also mega-constellations of Low Earth Orbit (LEO) satellites, delivering Terabit per second (Tbps) of capacity across the world.

Multibeam satellite systems have been specifically developed to allow efficient frequency reuse and high-throughput broadband rates across the coverage area, not unlike their terrestrial cellular counterparts. Satellite broadcasting via geostationary satellites will remain in widespread use. It will be a main source of revenue for satellite operators for the foreseeable future. But technology evolution in the coming years will also offer the possibility of new services via “very high throughput satellites (VHTS)” and “multispot” geostationary satellites, says ITU.

Military aircraft typically travel across multiple satellite beams. This presents an important service continuity challenge as an onboard remote must maintain a connection across these beams. iDirect handles this through a technology called Automatic Beam Switching (ABS). With ABS, iDirect remotes can travel across satellite footprints and maintain seamless connectivity without the need for manual intervention.

K_u-Band/K_a-Band

These systems often required two, and sometimes even three, stages of analog upconversion and downconversion, each requiring a synthesizer, amplification, and filtering that drives up system SWaP. However, to fit within the existing airliner infrastructure and power distribution system incorporating such signal chains for all the possible data links may be untenable.

The large number of components, power consumption, and isolation challenges means the printed circuit board (PCB) will be large. And because of the high frequency routing, more RF appropriate PCB material may be needed, significantly impacting cost. With a need to continue to support the L-band frequency of operation, the SWaP and design effort challenges are compounded.

Antenna requirements and technologies

Providing high-speed connectivity through small mobile antennas is one of the toughest challenges of airborne COTM. Sub-one meter antennas required for COTM have low gain characteristics. Higher power is required to ensure the receiving terminal hears the remote over the background noise created when the satellite boosts the signal. These high-rate signals coming from small antennas often cause interference with adjacent satellites that may be using the same frequency and polarization.

Global Network Management

To achieve global coverage, airborne remotes need to traverse networks on various transponders and satellites, controlled from a variety of hubs and networks. This poses a number of challenges for IP networks and Network Management Systems (NMS) regarding how to track and authenticate remote units, monitor service reliability and manage Service Level Agreements (SLAs). Security is a top priority for military operations. For mobile remotes on an IP satellite network, this means secure channel activity, control channel information, unit validation, physical security and data encryption.

References and Resources also include:

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