TRACKING TECHNIQUES FOR INCUNED ORBIT SATELLITES

Transcription

1 TRACKING TECHNIQUES FOR INCUNED ORBIT SATELLITES KHALID S. KHAN Andrew Corporation, Richardson, Tx July 10, 1990 ABSTRACT Some of the international and domestic communication satellites have been placed in the inclined orbit. This decision is receiving wide support within the telecommunications industry. A satellite in an inclined orbit is expected to operate an additional five years. These additional operating capabilities of a satellite mean more revenue for the owners and less transponder cost for the users. This paper discusses and furnishes current information pertinent to satellite maneuvering in the inclined orbit along with various tracking techniques needed to access an inclined orbit satellite. It also suggests a hybrid tracking technique for an inclined orbit satellite. GENERAL The destruction of the space shuttle in mid air coupled with the loss of a few satellites and launching rockets have been blamed for the current difficulties of the satellite industry. On the other hand, satellite RF electronics life and reliability have increased from approximately seven years to almost twelve years. COMSAT has proposed a new scheme, called COMSAT Maneuver', to fully maximize the return on its investment. This maneuver is being examined as a technique to offset the problems associated with satellite industry. The fundamental principle of this technique is to conserve fuel and thus extend the life of a satellite. GEOSYNCHRONOUS ORBIT Most of the estern commercial communication satellites are located in a circular orbit called 'Geosynchronous Orbit.' The plane of this orbit coincides with the equatorial plane of the planet Earth. The revolution period of an object with respect to earth is directly proportional to the distance between the object and the planet Earth. The period of rotation of an object in a geosynchronous orbit matches that of the earth, and the object looks stationary. Kepler's laws govern the rotation of an object in an orbit. They also explain the nature of the curve (orbit), the revolution period, and the height of the geosynchronous orbit. Kepler's first law' following equation2: is given by the c = R (1) 1 + E cos 0 c = motion of the curve R = parameter of the second order E = eccentricity 8 = central angle Forgeosynchronous orbit, themotionof the curve 'cl equals to the parameters of the second order 'R. ' This condition occurs when the eccentricity 'E' of the curve equals to zero. Similarly, Kepler'sthirdlawdealswith the revolution period of an object in space. This law is mathematically described below:,2 = 4x2 x3 (2) IL T = revolution period p = Kepler's constant = 398, km3/sec2 x = Semi-major axis of elliptical orbit and the value of p = GM CH $1.OO IEEE

2 ~. G = Universal qravitational constant = 6.67 * 10 km3/kg/sec2 M = Mass of planet Earth = * kg and x = r + H r = Radius of the earth = 6,370 km H = Altitude of a satellite moving in a circular orbit By substituting the values of x in equation (2), it becomes as follow: h cc 0 0 v 03 c != J _J t- v, I ,2 = 4r * (r + H)3 (3) lj Similarly, the velocity of an object orbiting in geosynchronous circle can be determined by the equation given below: v = J gm/a (4) SATELLITE LONGITUDE(DEGREE) DIURNAL MOVEMENT OF AN INCLINED ORBIT SATELLITE OVER A 7 DAY PERIOD FIGURE 2.0 satellite revolution period as well as the velocity. This inclined orbit is an I J I & A I C ALTITUDE h(km) FIGURE I RELAllON 3ETEfN ALTIIUDI. REVOLUllON PERIOD AND ORBITAL VELOCITY OF A SATELLITE FIGURE 1.0 TIME IN YEAR5 PERFORMANCE PROFILE OF A SYNCHRONOUS ORBIT 5CLAR ARE FIGURE 3.0 Equations (l), (3), and (4) are the founding pillars of a geosynchronous orbit. Figure 1.0 shows graphically equations (3) and (4). To attain the revolution period of an approximately 24 hours, a suitable height of 22,236 miles was selected for satellites. If the eccentricity is gradually increased from zero to one then the orbit tends toward an elliptical shape. This change triggers the changes in the out growth of the geosynchronous orbit where inclination is approximately 0.85 per year, and this new orbit is called an Inclined Orbit. The orbital motion starts small and increases with time. The inclined orbit tends toward an elliptical shape over a period of time, and the major axis of this orbit is toward a north-south direction. There are several benefits in leaving the

3 satellite in the inclined orbit: a. To use as a storage orbit b. To save the expense of replacing satellites c. To provide back-up to primary satellites d. To provide additional service and revenue GRAVITATIONAL DISTURBANCES Disturbing forces such as the gravitational attraction of the sun and moon, the radiation pressure of the sun, and the earth's gravitational field constantly cause a satellite to move from its assigned position. The activity of keeping the satellite in the allocated position is ' called llstationkeeping.l* The stationkeeping requires the satellite to produce its own power. The propellant fuel which provides this power is sufficient for approximately seven years. For continued operation beyond this period, one of the following alternatives must be implemented: a. Carry more fuel in the spacecraft. This proposition will increase the weight of the spacecraft and may increase the cost for launching. b. Curtail some of the satellite stationkeeping activities which will have the least impact on the characteristics of the satellite. INCLINED ORBIT SATELLITE It is financially economical and technically feasible to curtail some of the stationkeeping activities. Most domestic satellite vendors have opted for east-west stationkeeping only. There are two basic reasons for this decision (a) compliance with the FCC rules and (1)) reduction of up to 96% in satellite fuel, which is in limited supply. The deletion of north-south stationkeeping activity approximately two months prior to the end of the seven years operational period allows the satellite to drift in a figure I8l shape. Figure 2 depicts a week of satellite path3 in an inclined orbit. SATELLITE PARAMETERS Movement of a satellite in the inclined orbit poses problems to an earth station in establishing a communication link with the satellite. The communication between the earth station and the inclined orbit satellite has the following significant drawbacks: a. Degradation in the performance of satellite components and equipment after a period of seven years b. Antenna beamwidth c. Extension of orbital inclination of the satellite d. Geographical location ofthe station Satellite EIRP (Effective Isotropic Radiated Power), flux density and the G/T (Gain to Total system Temperature) tend to decrease over time. This is also true with the solar power supply. Figure 3.0 shows a typical depletion of solar array poweroveraperiodof seven years. This reduction of satellite power supply capabilities has some impact on the performancedegradation ofthe satellite parameters. Generally, smaller antennas with larger beamwidth do not require tracking when accessing a geosynchronous satellite. This condition is not true for larger antennas which wish to track inclined orbit satellites. Thisdrawback requires frequent periodic corrections in the earth station antenna look angle. The larger inclination of a satellite poses greater problems forthe accessing ground station because the movement of thesatellite is not in synchronization with the movement of the earth. Please see equation 4.0. Other parameters, as such, polarization isolation and satellite beam coverage drastically change. To offset these problems a stringent tracking requirement is needed. Someofthe following international and domestic satellites presently orbitring in the inclined orbit are INTELSAT satellites at 338.5' E and 177' E, GSTAR 111, EUTELSAT F1 at 16' E t>tc. SATELLITE TRACKING As the satellites started crowding the geosynchronous orbit, trackingbecame an essential element of an earth station equipped with fairly large-sized antennas. The decision to incorporate tracking into an earth station is contingent upon antenna half-power

4 beamwidth, satellite orbit and cost. The signal strength of a transmit/receive carrier depends upon the alignment of antenna beamwidth with respect to the satellite. Therefore, it becomes necessary to implement a tracking system if the antenna has a narrow beamwidth. The simplest form of a tracking system could be a mechanical driver that moves the antenna boresight axis in a plane. Determination of the required movement could be achieved in three ways:!a) receive the satellite signal during entire communication period, (b) track the satellite when it is needed, and (c) beam an earth station toward a satellite based upon pre-determined position. In first category, Monopulse tracking, an expensive technique, utilizes a real time nulling process with Automatic Gain Control. Despite its expense, this technique could be used by an earth station to track an inclined orbit satellites. Step tracking can be a suitable candidate for above mentioned case b. Step tracking is less expensive than monopulse tracking. This technique utilizes scanning process where the satellite signal power is sampled in step. A simple step tracker can not take into account the inclined orbit satellite positions. The movements due to orbital inclination can be easily approximated by means of formulae which include the orbital inclination: the latitude of the station and the longitude difference between the earth station and the satellite. THE INTELSAT's approach to computing the positions of an inclined orbit satellite has been to sum up all the disturbing effects in three equations which take into consideration eleven parameters, callec' Ephemeris. Some of the significant parameters include mean longitude, drift velocity and its acceleration, longitude and latitude oscillation-amplification, sine and cosine terms of their rates of change in amplitude. These approximate data are obtained from least curve fitting techniques. This technique has an accuracy of +/ degree for a period of 7 days. Step tracking systems equipped with Trajectory Algorithm (TEA) are being used to track an inclined orbit satellite. The TEA approach takes into consideration INTELSAT's eleven parameters along with some additional parameters. Program tracking, an accurate way to follow a satellite at Ku-band frequency, is a suitable technique for above mentioned case c which requires prediction and computation of all physical effects acting on the satellite with respect to time. Pre-determined satellite positions are furnished by Telemetry Tracking and Control station to the transponder users. A simple program tracker is not capable of peaking an inclined orbit satellite. A program tracking system must be devised to take into consideration of parameters similar to INTELSAT's Ephemeris data. The Ephemeris data may be different for non- INTELSATls satellites. This solution is well-suited to smaller antennas. Larger antennas often utilize program in conjugation with step tracking. HYBRID TRACKING SYSTEM A Hybrid Tracking System (HTS) is a concept for accessing inclined orbit and geosynchronous satellites. It is a modified version of the conventional program tracking system. It is cost effective and technically acceptable. The basic principle of operation 0:' the HTS is very simple. Satellite owners, such as INTELSAT, supply their users with a set of computed data (pre-determined positions of an inclined orbit satellite) for seven days. An earth station user will enter this information in the HTS through a keyboard or remotely. Refer to block diagram provided in figure 4.0. The HTS will be equipped with software to refine these data and activate the positioningmechanismoftheantennato point it toward the inclined satellite at set times. Ephemerisdata, furnishedby INTELSAT, is updated weekly by the TT&C station operator. Once this data is entered into the HTS, and program tracking is enabled, a schedule of antenna coordinates over time is generated. Position command:; are sent tothe antenna controller according to the time schedule, maintaining the

5 I 3. Rory Chang and Les Veenstra, INTELSAT Report, March Electrospace Systems 'Communications SatelliteTracking Techniques forthe 1990's Rev. A February 16, 1989 FIGURE 4.0 antenna's lookanglewith the satellite. Feeding information to the HTS can be handled via a PC-based software package. The only additional requirements then are the PC, the software interface, and the cabling. Program tracking could be incorporated on a single axis drive system with small antennas. HTS can be used along with step-tracking. CONCLUSION The useful life of a satellite may be extended by shutting off the North- South stationkeeping. The satellite then goes into an inclined orbit, requiring earth stations to have intelligent tracking mechanisms. Usage of an inclined orbit transponder may be cost effective for the users. It can bring additional dollars to the satellite owners. The HTS technique is an adequate method to track a satellite operating in a geosynchronous orbit as well as one operating in an inclined orbit. This unit is economical and accurate. It can be used with small or larger antennas. ACKNOLEDGEMENT The author wishes to extend acknowledgement for technical support and guidance that he received from Tom Charlton, Scott alker, Dale Mowry, Tony Campbell, Barbara Hodge and Doug Pewterbaughof Andrew Corporation. REFERENCES 1. INTELSAT Document IESS 411 (Rev.1) 2. Miya, K, Satellite Communications Engineering, Lattice Company, Tokyo,