History of our INTRAC antenna controllers - Advantech

History of our INTRAC antenna controllersThe development of Smooth Step Track/INTRAC antenna controlIn 1977 the British Post Office (BPO, the predecessor to BT) initiated the development of a new type of antennacontrol system because it anticipated a large increase in the number of antenna controllers it would need and wasdissatisfied with the cost, complexity and relative unreliability of the monopulse controllers available at that time. Itsobjective was to develop an antenna controller having the accuracy, and the ability to operate with satellites ininclined orbits, which is characteristic of monopulse control, but with pricing similar to that of inexpensive Steptrackcontrollers.A radically improved control algorithm was developed to meet this objective. This control algorithm was called SmoothStep Track and the first systems were installed and successfully operated at Goonhilly Earth Station in 1978/79. Thealgorithm was based on developing a mathematical model of the satellite orbit and steering the antenna in accordancewith this orbital model. The orbital model is developed from a large number of separate steptrack measurements of theposition of the satellite. Each of these measurements is derived by making a step cycle movement of the antenna andestimating the position of the satellite from the way the received signal varies during the step cycle. These individualmeasurements are then fitted to a generalized 14-parameter model of the satellite orbit, as viewed from the ground, inorder to match the observed movements of the satellite. The orbital model is fully developed after the first 24 hours oftracking and is then kept updated by making step cycle measurements every ten minutes. This period is chosen so asto be sufficiently short to detect and correct for unexpected satellite station-keeping maneuvers.Because the antenna is steered in accordance with the orbital model and does not peak the signal on individualmeasurements, it has major advantages relative to basic step track. These advantages are described below.The INTRAC algorithm incorporates a number of intelligently adaptive features to cope with a wide range of differentconditions.The software that had been developed for the original trial/demonstration units was licensed to Signal Processors LTD(now SPL-ACT wireless Europe limited) by BT in 1983 and SPL then designed and manufactured commercial antennacontrol units using this software. Shortly after agreement of the license the specialist who had been responsible fordevelopment and coding of the original orbit modeling algorithm, Dr Martin Terrell, joined SPL and continued to designand develop improved antenna control systems based upon this algorithm. In order to differentiate the performance ofthe controllers from that of basic step track controllers the name of the algorithm was changed from Smooth Step Trackto INTRAC, for INtelligent TRacking Antenna Control.In addition to the basic advantages of accurate and reliable tracking even when satellites are in highly inclined orbits,INTRAC controllers also provide:• Tolerance of wind deflection of the antenna• Tolerance of satellite station keeping maneuvers• High tracking accuracy, particularly in conditions of fading and scintillationA large number of safety features which act to ensure that traffic is not lost by the antenna steering off track, whichcould otherwise occur under exceptional operating conditions or some types of equipment failureAn Advantech AMT TM Company

History of our INTRAC antenna controllersHistory of our INTRAC antenna controllersThe improved INTRAC control algorithm provides a great improvement in reliability of tracking relative to basic steptrackcontrollers. The improvement is particularly great in difficult propagation conditions and also when tracking satellites ininclined orbits. Basic step track controllers move the antenna in order to directly maximize or peak-up the receivedsignal. INTRAC controllers make step cycles and uses these step cycles to measure the angular position of the satelliteand construct a mathematical model of the satellite orbit. INTRAC controllers then control the antenna, continuously ifnecessary, in accordance with this model.Ordinary step track controllers are not able to operate properly if the signal is scintillating or fading while the step trackcontroller is seeking to maximize the received signal. In these conditions the steptrack controller will be unable to peakthe signal successfully and will continue 'hunting' to find the peak as the signal scintillates. In order to be able to 'tweak'the system to minimize the inherent tracking deficiencies of simple step track, such step track controllers allow theoperator to modify the step track parameters, such as step size, step track gain, scan threshold, tracking interval, etc.In the case of the INTRAC operating under the same scintillating signal conditions the measurement of the satelliteposition will be made more uncertain by the scintillation. However, because the INTRAC controller uses advancedoptimal estimation techniques, it is still able to make an accurate model of the orbit by combining a large number ofmeasurements, each of which can be subject to large uncertainty. Because of the design of the INTRAC algorithm noadjustment of operating parameters is needed because the controller itself characterizes the critical parameters of theantenna and automatically adjusts its operating parameters, such as step cycle frequency, to the correct values.A further point is that ordinary step track controllers, having maximized the signal, leave the antenna at that positionuntil they make another step track cycle. If the satellite is moving the signal strength will drop until the antenna ispeaked again. The result is that the antenna pointing always lags behind the position of the satellite and also that thereceived signal strength is less than the maximum except when it has just been peaked. Consequently, such controllersintroduce signal loss, which increases with increasing orbital inclination of the satellite being tracked.An Advantech AMT TM Company

History of our INTRAC antenna controllersSome of the basic features of orbit modeling control are now also claimed by other suppliers, who first introducedtheir own orbit modeling controllers into the market about eight years after SPL had introduced its Smooth StepTrack controllers. These competing controllers are relatively unsophisticated and lack several features incorporatedin the INTRAC controllers. These other controllers generally lack:• the INTRAC noise processing ability• self adaptive features, such as automatic compensation for reasonable levels of wear and backlash inthe antenna drives• capacity to correct automatically for wind distortions of the antenna structure• the ability to continue tracking, without need to reset or re-acquire, through all normal types of satellitestation-keeping maneuvers.By contrast, these advanced features have been incorporated in the INTRAC algorithm over many years ofcontinuous development. SPL-ACT wireless is unique as a manufacturer of antenna control systems in the depth ofits expertise in optimal estimation techniques and in signal processing techniques to extract the maximuminformation from noise-corrupted signals (an expertise also applied in the company's range of satellite modems). Asa result, competing controllers, which do not incorporate noise processing algorithms of the same power, operateless well in conditions of high scintillation, are less able to cope with unexpected station keeping maneuvers and areless able to cope with wind distortions or deflections of the antenna reflector which are not detected by the positiontransducers. These differences result in tracking performance which is less robust against such phenomena.Another result of our 19 year program of on-going development is that the INTRAC hardware and software areextremely reliable and well proven. We are not aware of any case where an INTRAC antenna control system haslost traffic after the control system has been commissioned except as a result of a hardware failure. In more than 16million operational hours to date INTRAC controllers have achieved an average MTBF of 128,000 hours, or onaverage more than 14 years between failures.The excellent performance of INTRAC control has been well established by its use on over 350 antennas, many ofwhich are tracking at very low angles of elevation in conditions of severe atmospheric scintillation and rain fading inthe tropics, and its use over many years for tracking military communications satellites with orbital inclinationsgreater than 7 o .Although most of the supplied control systems are installed by the end users or system integration companies, wehave personally completed over 70 antenna control retrofits at earth stations in the UK and Europe, North America,the Pacific, India, Thailand, Singapore, Hong Kong, the Caribbean and Middle East.Twelve of these retrofits have been on 30/32m diameter C Band antennas and one on a 17m wheel and trackantenna where INTRAC is the only means of control.An Advantech AMT TM Company

History of our INTRAC antenna controllersBased on these practical reliability figures, the estimated time to replace the sub-system and therefore the time lost peryear as a consequence of a failure are summarized below:Item% failure peryearTime toreplaceMinutes lost peryearINTRAC 505 ACU plus IBR-L Beacon 15.8% 30 minutes 4.78 minutesReceiverResolvers 0.7% 120 minutes 0.84 minutesC Band to L Band Block Down1.8% 20 minutes 0.4 minutesConverterMotor Control Unit 1.8% 90 minutes 1.6 minutesTotal7.7 minutesBased on the above estimated times to replace faulty sub-systems the estimated average period of traffic lost peryear as a result of tracking system failures is 7.7 minutes. This translates to a very high availability equal to99.998%.Please note that because the INTRAC systems can control the antenna in accordance with their internal orbitalmodel data, a considerable number of hardware failures, for example any failure in the Block Down Converter orthe integral beacon receiver in the ACU, or a failure in the ACU display circuits, will not affect the ability of thecontroller to continue to control the antenna. Therefore the availability for maintaining tracking and thereforeprotecting data traffic on the antenna, will actually be higher than would be assumed from the above conventionalsystem availability calculations.Intelligently Adaptive to Operating ConditionsAs noted in section 1.4.6 above, the INTRAC code incorporates many features to improve operational reliability andtolerance to non-ideal operating conditions. These special features include the ability to sense, and automatically correctfor, moderate levels of wear and backlash in the antenna drives and small nonlinearities in the resolver positiontransducers. This means that INTRAC controllers will continue to operate correctly even when there is progressive wear inthe antenna drive mechanism, provided that the wear is not excessive.The controller will also sense when there is a satellite station keeping maneuver, wind distortion of the antenna orexcessive scintillation or rain fading which is reducing the accuracy of its satellite position measurements, and willautomatically increase the frequency of its step cycle measurements to compensate.No requirement for parameter adjustmentBecause the INTRAC algorithm is intelligently adaptive to operating conditions there is no need for the operator to adjust ortry to optimize the operating parameters of the INTRAC control. This contrasts strongly with step-track based controllerswhich often require operator adjustment of parameters including size of step track movements, frequency of step trackcycles, step track gain, level of beacon change before new step track cycle is initiated, etc.An Advantech AMT TM Company

History of our INTRAC antenna controllersThe only parameters that need to be entered when a controller is installed on a new antenna are the antenna beamwidth,the resolver position offsets, if any, and the beacon signal frequency. All other parameter adjustment is automatic.Acceptance of IESS 412 or NORAD Track Data.Although INTRAC controllers can develop a fully accurate orbital model in 24 hours from switch-on and normally operateentirely autonomously, they can accept IESS 412 or NORAD Program Track data and use this information to immediatelycompute the corresponding INTRAC model and start tracking according to this model. They will then progressively refinethis model via the normal step cycle measurements to improve and maintain its accuracy.Minimization of Drive WearBecause INTRAC controllers do not attempt to peak the signal strength during the period when they move the antenna tomeasure the satellite position, and only move the antenna when necessary to maintain the pointing loss below 1/20 of abeamwidth or less, INTRAC controllers do not "thrash" in response to noise or scintillation on the beacon signal. Thisabsence of thrashing protects the drives from wear and helps to extend their life, in some cases by a factor of twenty ormore times relative to basic step track controllers.Ease of InstallationINTRAC antenna control systems are very easy to install on a wide range of different types of antennas (C band, X bandand Ku band) with a wide range of beamwidths. To date they have also been installed on antennas manufactured by atleast 25 different organizations.The only parameters in the INTRAC antenna controller which need to be set at the time of installation are the antennabeamwidth and the angular offsets (if any) of the azimuth and elevation position transducers. (Clearly it is necessary forthese to be properly mounted and coupled correctly to the azimuth and elevation axes of the reflector.)Therefore there is no need for other operator adjustable parameters to match the controller to the type of antenna and thesignal conditions, as is the case with signal peaking, step-track-based controllers. The controller is intelligently adaptive tothe signal conditions and also to the mechanical characteristics of the antenna drives.Provided that the antenna drives are mechanically sound and do not suffer from backlash or overrun which exceeds about1/40 of a beamwidth, the INTRAC ACU will automatically measure and compensate for the antenna drive characteristicsand will accurately control the antenna with a precision determined by the resolution of the position transducers.The result is that INTRAC control system are quick and trouble free to install, do not require any special expertise to "tune"them to the particular antenna being used, and are genuinely "point and track" units.Minimum Requirement for Routine MaintenanceThere is no requirement for routine maintenance of any parts of an INTRAC antenna control system.SPL-ACT wireless Europe Ltdoperates an ISO9001 compliantQuality Management System, whichis approved bySPL-ACT wireless reserves the right toalter this specification without noticeAn Advantech AMT TM Company