31-12-2009, 06:09 PM
ABSTRACT
This paper outlines some of the techniques being developed to provide affordable, reliable satellite communications suitable for a wide range of military aircraft, from agile platforms such as jets and helicopters to surveillance, tanker and transport aircraft. It also gives an overview of airborne SHF (Super High Frequency) and also EHF (Extremely High Frequency) satcom techniques. Although presently used UHF (Ultra High Frequency) satellite communication are relatively simple to install and comparatively inexpensive, suffer from very limited capacity and are prone to multipath and unintentional interference due to their poor antenna selectivity. Whereas, the SHF satcoms offer significantly increased bandwidth for high data rates or increased use of spread-spectrum techniques, together with localized (spot) coverage and adaptive antenna techniques “ for nulling unwanted signals or interference.
INTRODUCTION
The beginning of new millennium sees an important milestone in military aviation communication with the introduction of the first Super-High-Frequency (SHF) airborne satellite communication (Satcom) terminals, which are due to enter service on Nimrod maritime reconnaissance aircraft (MRA4). Satcom terminals using the Ultra-High-Frequency (UHF) band have been fitted to larger aircrafts for a number of years. Although relatively simple to install and comparatively inexpensive, UHF satcoms(240-270 & 290-320 MHz bands) suffers from very limited capacity (a few 25 KHz channels per satellite) and are prone to multipath & unintentional interference due to their poor antenna selectivity. SHF satcoms (7.25 “ 7.75 & 7.9-8.4 GHz) offer significantly increased bandwidths (hundreds of MHz) for high data rates or increased use of spread-spectrum techniques, together with localised coverage and adaptive antenna techniques for nulling unwanted signals or interference.
For airborne platforms, the advantages of SHF satcoms come at the expense of a significant additional burden in terms of antenna siting and pointing, particularly for smaller, highly agile aircrafts. Antenna should be large enough to support the desired data rate and to provide enough directivity to minimise interference with adjascent satellites and avoid detection by hostile forces. Another feature of satcoms, unique to aircraft, is the effect of unwanted modulation from moving parts such as helicopter rotor blades, propellers and jet engines.
This paper gives an overview of development of airborne SHF and also Extremely-High-Frequency (EHF) satcom techniques, and terminal demonstrators by DERA (Defence Evaluation and Research Agency). This research is aimed at providing affordable, secure and robust satcoms to a range of military aircraft, supporting ground attack and reconnaissance roles to surveillance, transport and tanker aircraft.
MILITARY PERSPECTIVE
The UK Ministry of Defence (MoD) currently operates a constellation of three geostationary military communication satellites collectively known as skynet. The current generation of skynet 4 satellites provides satellite communication for all types of armed services at both UHF and SHF bands. Demand for all types of military satcom is rising rapidly, due principally to the need for even more timely information to prosecute operations effectively. Another factor is the growing expectations of service personnel familiar with a world of instant global communication and rapid availability of information via internet-type services.
Satcoms can span distance, terrain and hostile forces to provide a global reach for dispersed mobile platforms such as aircraft, submarines, surface ships, vehicles and man-packs. Fig.1 illustrates schematically, the breadth of military aerial roles that satcom may be required to support in future, providing global beyond-line-sight communications between aircraft and commander in theatre. Information carried by satellite could include:
¢ Near-real-time command & control “ tasking, position, reporting etc.
¢ Data from reconnaissance & surveillance aircraft.
¢ Targeting data for stand-off weapons.
¢ Situation awareness.
¢ Transfer data from dissimilar and/or geographically separated line-of-sight (LOS) networks.
Fig. 1 Potential use of satcom by military aircraft.
In some case, direct communication with aircraft will not be necessary. For eg: air superiority fighters operates in the vicinity of an E3-D Sentry air-borne early-warning (AEW) aircraft with which they are in LOS communication. For these fighter aircrafts it would be more efficient to communicate via their LOS page link to the E3-D and then via satcom from E3-D, with the transfer between the two links operating seamlessly.
Typical rates for some of these military roles are given in Table-1. At the other end of the scale, unprocessed reconnaissance data demand large amount of bandwidths - the amount increasing year-on-year as the resolution of imaging and radar sensor technologies improves.
Scenario Data type Typical data rates
Tasking and position reporting
Verbal communication
Reconnaissance
Receive-only direct broadcast short messages
Voice
images/video
video/multimedia 50 “ 300 bit/s
2.4 “ 4.8 kbit/s
16 kbit/s “ 20 Mbit/s
2 “ 20 Mbit/s
Table 1: Typical data rates for various air missions
USE OF CIVILLIAN SATELLITE SYSTEMS
The use of non-military satellite systems, principally INMARSAT, by the military to supplement capacity for non-mission-critical communications is already common. Civilian mobile satcom is a $ 2.5 billion global industry and can provide significant additional capacity using the latest state-of-art technology.
Use of commercial satellite systems is bound by international treaties which typically place severe constraints on the use of these systems by military. Commercial satellites also have no specific built in resiliance to jamming and limited protection against information warfare.
Until recently INMARSAT was the sole provider of commercial aeronoutical satellite services, using 10MHz of spectrum in L-band (~ 1.6 GHz). Iridium system developed by Motorola provides truly global coverage using 66 satellites in polar orbit with 48 beams per satellite. This revolutionary system uses L band for communication with mobiles and Ka-band (20/30 GHz) for links to ground stations. Iridium is the first commercial satellite system to use on-board switching with inter satellite links (in the 23 GHz water vapour absorption band) so that links between mobiles need not use terrestrial gateways. Although Iridium system is primarily intended to support personnel communications aeronautical terminals with one to eight channels are being developed for the Iridium system by AlliedSignal Aerospace. Other Low Earth Orbit (LEO) personal communication systems (PCSs) such as Global star and Medium Earth Orbit (MEO) systems such as ICO are due to come on-line in near future.
Fig. 2(a) AlliedSignal Iridium terminal currently undergoing trials
(b) An RAF Hercules aircraft.
THE AIRFRAME “ ANTENNA PLACEMENT
One of the first problems encountered when considering an airborne satcom terminal is where to site the antenna on the airframe so as to gain an uninterrupted path to the satellite. Helicopters in particular present a unique challenge due to their rotor and body shape, which make it difficult to find an uninterrupted view of the satellite for all orientations. Fig. 3 shows the blockage for two hypothetical antenna location on the Apache attack helicopter. The blockage has been computed by projecting the silhouette of the airframe and main rotor, as illuminated from the antenna site, onto a hemisphere. Most apparent from these diagrams is the extent of the shadowing by the main rotor blade, which causes unwanted amplitude and frequency modulation. With the antenna mounted on the helicopter spine just behind the engine, the silhouettes of the cockpit (fore) and tail (aft) are very apparent. Moving the antenna to one of the two stubby `wingsâ„¢ which support the armament provides better fore and aft coverage but suffers blockage on one side due to the fuselage. Two antenna, on each wing would provide the necessary hemispherical coverage. Surprisingly, the best location for a satcom antenna on the Apache is above the centre of the main rotor on a de-spun mast.
Fig. 3
THE MODEM
In addition to engine/rotor modulation, the modem (modulator/demodulator) in an airborne satcom terminal has to be designed to accommodate the effect of Doppler shift of the RF signal due to the aircrafts speed and acceleration. This speed could be upto 600m/s relative to satellite. The amount of Doppler shift increases with increasing radial velocity and RF carrier frequency.
Parameter Performance
Data rate C/N
Sensitivity Mode 1
(asynchronous data)
Sensitivity Mode 2
(synchronous voice)
Max. multipath
Max. Doppler offset
Max. Doppler rate of change
Max. rotor modulation degradation in C/No 50 bit/s 23 dB HZ
75 bit/s 25 dB HZ
300 bit/s 31 dB HZ
2.4 kbit/s 40 dB HZ
4.8 kbit/s 43 dB HZ
-10 dB
8 kHZ
500 Hz/s
1 dB
Table 2: Required SHF modem performance for large and rotary wing aircraft
An airborne tactical modem has the following requirements:
¢ To be efficient interms of carrier-to-noise density (C/No) for a
given bit error ratio (BER).
¢ To operate under high Doppler offsets & rates of changes due to
platform motion.
¢ To operate with minimal degradation in the presence of
multipath propagation & occasional signal loss due to airframe
blockage & antenna switching.
¢ Rapid acquisition of communications following signal loss.
¢ To operate with minimal degradation in the presence of rotor
modulation.
¢ To comply with international regulation on power spectral
density & adjacent satellite interference.
¢ To implement electronic protection measures as required.
To illustrate the combined effects of Doppler, rotor modulation and multipath, we show Fig.4, the scattering function for communication to a medium-sized helicopter. Here the satcom antenna is mounted behind the main engines on the central fuselage below the main rotor.
Fig. 4
The scattering function is based upon the cross-ambiguity analysis, a tool commonly used in radar. It is essentially the output of a `filterâ„¢ that is matched to the uncorrupted transmitted signal (Sref). The signal filtering and processing incorporated in a satcom receiver is normally a very close approximation to that required to perform matched filtering of a received signal from the satellite. The magnitude of the scattering function illustrated in Fig.4, is a measure of the interference present in the received signal (S), which comprises the vector sum of several signal paths, each simultaneously offset in frequency (fD) and delayed in time (). Ideally the matched filter output for zero delay (multipath) and frequency shift (Doppler) would be a `spikeâ„¢ centered upon zero Doppler and zero delay (broadened by the finite time-bandwidth product). Significant signal loss arises when the received signal scattering function is `smearedâ„¢ in time and frequency by Doppler and multipath smearing is non-symmetric due to aircrafts relative motion.
The received signal undergoes both amplitude and phase modulation as a consequence of rotor modulation. Periodic fading results from rotor blade obscuration, and deep nulls arises due to complex coupling of phase and Doppler shift. The resultant scattering function in this case depends upon many factors, including the aspect angle to the rotors and aircraft from terminal and receiver, the number of rotor blades and structural composition, pitch angle, flexing and hub rotation rates. Rotor modulation thus poses serious problems for conventional satcom page link acquisition, RF carrier tracking and page link retention. With correct signal processing, the effect of rotor modulation can be reduced to <1dB power penalty.
THE ANTENNA
After determining the required communication data rate and the available satellite power to support it, the aircraft antenna (aperture) has to be determined. Antenna requirements for use with geostationary satellite fall into three categories:
¢ Low gain for position reporting & messaging.
¢ Medium gain for voice & low-data-rate communication.
¢ High gain for applications such as file-transfer, surveillance/reconnoissance data etc.
The requirements for low-gain and medium-gain antennas are summarised in Table 3.
Parameter Low-gain antenna Medium-gain antenna
Transmit gain
Receive gain
Transmit polarization
Receive polarization
Frequency
Axial ratio
Power handling
Rx/Tx isolation
Steerable
Packaging 6 dBi
6 dBi
RHCP
LHCP
Rx7.25 “ 7.75 GHZ
Tx 7.9 “ 8.4 GHZ
6 dB
40W
N/A (different antennas)
switched
aircraft blade radome
1 Tx and 1 Rx blade 16 dBi
15.5 dBi
RHCP
LHCP
7.25 “ 8.4 GHZ
3 dB
30 W
20 dB
fully
aperture less than 10 cm
fully contained within 12cm
Table 3: Required SHF antenna performance (R (L) HCP = right (left) handed circular polarisation)
For satcom applications, the size, performance (aperture efficiency noise temperature) and cost of current phased-array technologies severely limit the usefulness of `holy grailâ„¢ of airborne antenna which is thin, conformal phased array which take the shape of the aircrafts skin. To reduce the cost, DERA has developed compact proof-of-concept low-gain and medium-gain airborne SHF satcom antennas using more conventional technologies.
Low Gain Antenna
A number of approaches have been investigated for a low gain antenna with the additional goal of reducing the complexity of the antenna pointing method to further minimise the cost. In particular the use of a number of static, switched-beam antennas with azimuth-independent gain has been pursued. The minimum number of independent beams required to cover a given solid angle can be inferred from the gain required for a single beam and Table 4, gives the theoretical directivity of switched beam antennas for various number of beams. In practice, the antenna gain will be lower than the directivity due to feed losses. The poor directivity of low gain antenna necessitates the need for alternative technique to reduce the unwanted interference, notably spectral spreading.
Beams per hemisphere Directivity per beam
1
2
3
4
5 3 dBi
6 dBi
8 dBi
9 dBi
10dBi
Table 4: Minimum antenna directivity for different numbers of switched beams
A practical realisation of a switched three beam low-gain antenna is shown in Fig 5. Each beam is omni directional in azimuth with overlapping elevation coverage. The three elements (high, middle & low) are switched according to the aircrafts attitude. The preferred configuration is to have one transmit and one receive antenna, switched in tandem, each containing three beams in a so called `bladeâ„¢ housing (radome).
Fig. 5 Axial-symmetric, switched 3-beam, SHF low-gain antenna: (a) schematic diagram; (b) antenna patterns; © practical realisation
Medium Gain Antenna
The bidirectional medium-gain antenna for voice/low data-rate communication originally employed a corrugated circular horn Fig 6a, which has good cross polarisation and low sidelobe performance. The swept volume of this antenna is determined by its length and not its aperture, with the consequence that a relatively large radome is required “ with a resultant impact on aerodynamic drag. Alternative solution is the short `back-fire™ antenna Fig 6b, appear to offer a more compact mechanical solution. These antennas operate in X band with 15% fractional bandwidth. The back-fire antenna has a very high aperture efficiency (the ratio of antenna gain to that of a perfect antenna of the same aperture area) of around unity, partly as a result of its semi resonant structure. It also has a very small swept volume “ essentially limited by its aperture.
Fig. 6 Medium-gain antennas: (a) corrugated horn; (b) compact `short back-fireâ„¢ antenna
PLATFORM DYNAMICS & ANTENNA POINTING
Having sited the antenna on airframe it is necessary to ensure that it continues to maintain the transmitted and received signals over a given page link within specific limits, regardless of the motion of the aircraft. Signal loss is an unwanted consequence of the inherent uncertainty in the tracking process and is affected by:
¢ Angular pointing offsets & drift.
¢ Antenna beamwidth.
¢ Platform stability.
¢ Static & dynamic loading.
¢ Beam squint resulting from near-field distortions & multipath.
¢ Attitude-sensor data accuracy & update rate.
¢ Time & frequency dependent collimation errors.
For a given positional accuracy, the signal loss due to antenna pointing is intrinsically proportional to the antenna beamwidth. This in turn is inversely proportional to the square root of the antenna gain. High gain antennas therefore require high accuracy pointing or tracking systems.
Most modern aircrafts incorporate an inertial navigation system (INS) or similar integrated navigation system that can potentially provide the information on the aircraft attitude to allow the antenna to be steered towards the satellite. The data obtained from the INS is not always suitable for satcom antenna pointing due to inadequate update rates or flexing of the aircraft structure between the INS sensor and the antennas location.
Here the approach is to exploit INS data where available but to concentrate on the use of autonomous attitude and heading reference system (AHRS), built into the satcom terminals as close to the antenna as is feasible. The two low-cost AHRS systems are:
The Watson C304 unit, Fig 7a is a small, lightweight strap-down AHRS based on a solid-state three axis gyrocluster, a pair of liquid angle sensors and a three axis magnetometer. The attitude is derived by integrating the signals from the triaxial gyro. These are then referenced to the earth axes via the angle sensors, which are employed as the gravitational reference and the triaxial magnetometer.
Fig. 7(a) Watson AHRS
The Trimble Advanced Navigation Sensor (TANS) vector, Fig 7b is a solid state attitude-determination and position-location system which uses the carrier phase difference of global positioning system (GPS) signals from upto six satellites, received at four separate antennas. The TANS vector requires no attitude or position initialisation.
Fig. 7(b) Trimble TANS Vector
The complete autonomous antenna pointing sub-system, Fig 7c comprises a GPS receiver, AHRS, antenna positioner and computer based antenna control circuit unit. The basic subsystem employs an open-loop pointing system, which in its simplest form predicts the antenna pointing angles based on satellite ephemeris data, air platform location and attitude.
Fig. 7 © control system schematic diagram
The accuracy and update rate of these low-cost AHRS systems with medium-gain antennas are, in fact, considered adequate for all the most demanding air platforms. Fig.8 shows the relative pointing losses associated with a range of platforms for 1 and 10 antenna (3 dB) beamwidths with both low-cost and high-grade AHRS. Pointing errors are most significant for satcoms with narrowest beamwidths on-board the most agile platforms. This impacts most markedly on the use of EHF satcoms on fast jets.
Fig. 8 Relative pointing antenna errors for various manoeuvres on different platforms: (a) 1 antenna beamwidth, high-grade AHRS; (b) 1 antenna beamwidth, low-cost AHRS; © 10 antenna beamwidth, low-cost AHRS
LOW-DATA RATE & VOICE-CAPABLE SHF SATCOM TERMINAL
DERA has developed two variants of a proof-of-concept airborne SHF satcom terminals, Fig 9 based around a modular (plug & work) approach. The terminals can be configured to operate in various modes by connecting modules to a common controller. These terminals are fully autonomous requiring only DC power from the host platform. The principal operating modes of these terminals are:
¢ Mode 1: World-wide position reporting & messaging.
¢ Mode 2: Voice communication (in satellite spot beams).
¢ Mode 3: Voice plus position reporting & messaging.
Fig. 9 Proof-of-concept airborne SHF satcom terminal control unit
The SHF terminal modules are:
¢ The attitude & heading unit, comprising the AHRS.
¢ The antenna unit, comprising either the switched low-gain or steerable high-gain antenna together with a low-noise amplifier (LNA), high-power amplifier (HPA) & diplexer filters.
¢ The terminal control unit, developed in conjunction with Delta communication which contains the RF up & down-converters, the GPS receivers, the voice coder-decoder (vocoder) & a PC 104 embedded Pentium PC.
¢ The remote display unit, which provides the operator interface and comprises embedded PC with a touch-sensitive LCD display.
MILLIMETER WAVE SATCOMS: THE FUTURE
The existing skynet 4 constellation is due to be replaced by 2007. The next generation of satellites, skynet 5, due to be introduced from 2005, is currently the subject of a series of `Public Finance Initiativeâ„¢ studies. One of the options being actively considered for skynet 5 is the provision of an on-board processed, Extremely-High-Frequency (EHF) payload similar to that used on US Milstar constellation.
The Milstar EHF system operates in the 20.2-21.2 GHz (down-link)/ 43.5-45.5 GHz (up link) bands and was originally developed to provide post-nuclear strike command and control facilities. Milstar was designed to provide:
¢ Enhanced resistance to jamming.
¢ Enhanced immunity to ionospheric scintillation (such as that
following a high-altitude nuclear explosion).
¢ Reduced probability of interception/detection.
Milstar achieves this through combination of four factors:
¢ Use of an EHF for earth-to-satellite links.
¢ Use of highly robust communication waveform.
¢ Use of sophisticated on-board processing.
¢ Use of cross-satellite links.
Recent Milstar satellites have been adapted to provide tactical users with up to 1.5 Mbits of high-data-rate communications each. The use of time division multiplexing (TDM) on the down-link contributes to increased efficiency for high-data-rate services and removes the problem of intermodulation with frequency division multiplexing (FDM). On-board regenerationswitching also allows direct communication between so-called disadvantaged users, such as aircraft, without the need to use a large ground station as a hub.
High antenna gains are often cited as a benefit of operation at EHF frequencies; however the beamwidth of satellite antennas are generally determined by the geographical coverage required, with the consequence that all antennas with similar coverage also have similar gain, regardless of frequency.
The principal appeal for the higher frequency bands is therefore largely due to the increased spectrum available. Wider spectrum allows either accommodation of more users/services or greater exploitation of spread-spectrum techniques to counter jamming and to reduce the probability of interception through the use of spread-spectrum is usually characterised interms of the coding gain “ the ratio of the spread bandwidth to the unspread bandwidth.
For airborne users the primary advantage of EHF frequencies is reduced likelihood of interception due to smaller antenna beamwidths for a given data rate. Deep-strike aircraft and attack helicopters can operate hundreds of kilometers behind enemy lines to disable the enemyâ„¢s infrastructure and supply lines. Low probability of detection (LPD) is vital on such missions. Increased spectrum for use by spread-spectrum techniques work in favour of the aircraft and against those trying to detect its transmission since a detector must search a larger spectral space to detect the aircraft signal.
Fig. 10 Integrated atmospheric attenuation against frequency at 90 elevation for an aircraft near sea level. The lower line is for relatively dry, rain- and cloud-free conditions; the upper line is for 12 mm/hr rain.
The most obvious disadvantage of operating at EHF frequencies is increased atmospheric attenuation in the troposphere, as shown in Fig 10. The familiar absorption peak occurs at ~60 GHz and ~118GHz due to gaseous oxygen and at ~23 GHz and ~183 GHz due to water vapour. Continuum attenuation above 10 GHz also increases significantly in the presence of rain and rain-bearing clouds as the signal wave-length becomes comparable with the size of water droplets. One advantage of the increased atmospheric attenuation at sea level is likely to be that terrestrially based interception will be more severely affected by atmospheric attenuation than airborne satcoms. At sea level, the effects of attenuation are greatest, and at about 20000ft (6096m) the attenuation is almost negligible.
Atmospheric attenuation is a major concern for users of EHF, but it is not the only problem to affect operation at these frequencies. Antenna pointing and airframe integration are key issues which are likely to become even more difficult as we move towards more agile, stealthy aircrafts such as the future offensive air system (FOAS). Providing highly robust, Milstar compatible, EHF satcoms to a highly agile, stealthy, air platform is a significant challenge. By developing and evaluating key technologies in airborne EHF terminal design, DERA is able to investigate the practical issues associated with these and other potential EHF options (such as use of military Ka band) for skynet 5 and minimize the risks associated with the procurement of the MoDâ„¢s future airborne satcom terminals.
CONCLUSION
For airborne users the primary advantage of EHF (Extremely High Frequency) is reduced likelihood of interception due to smaller antenna beamwidths for a given data rates. Increased spectrum for use by spread- spectrum techniques work in favour of the aircraft and against those trying to detect its transmission since a detector must search a larger spectral space to detect the aircraft signal. The combination of wider spread-spectrum bandwidth and narrower antenna beamwidth at EHF result in a significant improvement in aircraftâ„¢s LPD (Low Probability of Detection). Providing highly robust, Milstar compatible, EHF satcoms to a highly agile, stealthy, air-platform is a significant challenge and making it affordable is an even greater one.
REFERENCES
1. Electronics and Communication Engineering Journal, IEE
2. Link margin for UHF airborne satellite communications operation. (FRANKE, E., and VAAL, G.) IEEE
3. An overview of aeronautical telecommunications in Europe and worldwide, IEE
4. Fixed and mobile terminal antennas, IEE
5. Rationale for future use of EHF for military SATCOMâ„¢s, IEE
6. Fundamental performance characteristics that influence EHF MILSATCOM systems, IEEE.
ACKNOWLEDGEMENT
I extend my sincere thanks to Prof. P.V.Abdul Hameed, Head of the Department for providing me with the guidance and facilities for the Seminar.
I express my sincere gratitude to Seminar coordinator
Mr. Manoj K, Staff in charge, for his cooperation and guidance for preparing and presenting this seminars.
I also extend my sincere thanks to all other faculty members of Electronics and Communication Department and my friends for their support and encouragement.
Syed Arafath
CONTENTS
¢ INTRODUCTION 01
¢ THE MILITARY PERSPECTIVE 02
¢ USE OF CIVILIAN SATELLITE SYSTEMS 04
¢ THE AIRFRAME-ANTENNA PLACEMENT 06
¢ THE MODEM 07
¢ THE ANTENNA 10
¢ PLATFORM DYNAMICS AND ANTENNA POINTING 15
¢ LOW-DATA-RATE AND VOICE-CAPABLE SHF SATCOM TERMINAL 20
¢ MILLIMETRE-WAVE SITCOMS: THE FUTURE 21
¢ CONCLUSION 25
¢ REFERENCES 26
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