Unique operations for a highly inclined, elliptical, geosynchronous satellite

Acta Astronautica 55 (2004) 285 – 290 www.elsevier.com/locate/actaastro

Unique operations for a highly inclined, elliptical, geosynchronous satellite Patrick T. Anglin∗ , Robert D. Briskman Sirius Satellite Radio Inc., New York, NY, USA

Abstract The +rst space segment devoted to a Digital Audio Radio Service (DARS) for the Continental United States (CONUS) was established when the last satellite of a three satellite constellation (Flight Models FM-1, FM-2 and FM-3) was launched in November 2000. Each satellite is in a highly inclined, elliptical, geosynchronous orbit that is separated by 120◦ in Right Angle of the Ascending Node (RAAN) from the other two satellites’ orbits. This results in an 8 h phasing in ground track between each satellite. These distinct orbits provide superior look angles and signal availability to mobile receivers in the northern third of the United States when compared to geostationary satellites. However, this unique orbital constellation results in some particular performance and operational di8erences from geostationary orbit satellites. Some of these are: Earth Sensor noise, maneuver implementation and power management. Descriptions and performance improvements of these orbit speci+c operations are detailed herein. c 2003 International Astronautical Federation. Published by Elsevier Ltd. All rights reserved. 

1. Introduction The DARS satellite constellation delivers over 100 channels of talk and music to subscribers in CONUS, with the emphasis being on service for mobile users. The satellite constellation described herein is the +rst commercial system to use this highly inclined, elliptical geosynchronous orbit. FM-1 was launched on June 30, 2000, FM-2 on September 5, 2000 and FM-3 on November 30, 2000, all using the Proton Block DM launch vehicle from the Baikonour Cosmodrome in Kazakhstan. Since the Space Systems/Loral FS-1300 bus used for these DARS satellites was designed for geostationary operations, there were several changes to the satellite design as well as its intended operation completed prior to launch. Further, as the ∗

Corresponding author.

program has matured, new operations have been established to improve the satellites’ performance. These operations include Earth Radiance Gradient Disturbance (ERGD) mitigation procedures, Slanted Angle Maneuvers (SLAM) and Special Power Management (SPM). These special operations arise from the satellites’ unique orbits. 2. Orbits The constellation’s orbits were designed to provide high elevation angles for broadcast service over CONUS [1]. The high inclination angle of the satellites’ orbits require that the satellites change their yaw angle during a majority of the year to maintain the solar angle closely perpendicular to the solar arrays. This is done to maximize solar array output power. This

c 2003 International Astronautical Federation. Published by Elsevier Ltd. All rights reserved. 0094-5765/$ - see front matter
doi:10.1016/j.actaastro.2004.05.043

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P.T. Anglin, R.D. Briskman / Acta Astronautica 55 (2004) 285 – 290 Table 1 Orbital parameters Semi-major axis (km) Eccentricity Apogee altitude (km) Perigee altitude (km) Inclination (deg) Apogee longitude (deg) Argument of Perigee (deg) RAAN (deg) FM-1 FM-2 FM-3 Period (min)

Fig. 1. The ground track of the three satellites.

variation in yaw angle leads to two modes of satellite operation: a yaw steering mode where yawing is necessary to allow the solar arrays to maintain perpendicularity to the sun and an orbit normal mode where no yawing is necessary. The majority of the maneuvers required to maintain the orbit are performed during the orbit normal seasons. Having each of the satellites in a consistent orientation twice per year throughout both of the one and a half month long orbit normal seasons simpli+es the orbital calculations associated with maneuvers. Each geosynchronous satellite has the same ground track, and the RAANs cause satellite separation from each other going around that track by 8 h. Each satellite spends 16 h of its ground track north of the equator and the remaining 8 h south of the equator (see Fig. 1). The constellation always has two satellites above the equator providing broadcast service to CONUS. The speci+c orbital parameters of the satellites are listed in Table 1. Both of the satellites transmitting to the coverage area broadcast at a distinct frequency, timing and location di8erent from the other satellite. This provides the users’ receivers with frequency, time, and spatial

42,164 0.2684 47,102 24,469 63.4 96 270 285 165 45 1436

signal diversity. The implementation and advantages of these diversi+cation methods are elaborated upon in full elsewhere [2]. All of these features coupled with the high elevation angles of the satellites to mobile users throughout CONUS, particularly in its northern third, signi+cantly decrease the likelihood of blockage, multipath and foliage attenuation, all of which degrade or eliminate signal availability to the users. The satellites’ orbits enable this robust system of signal delivery but also require some di8erent orbital operations from traditional geostationary satellites. From experience with the unique orbits, distinctive and inventive solutions to the operation of the satellites were developed and are described subsequently. 3. ERGD During the northern hemisphere summers, the satellites have occasionally experienced Earth Sensor (ES) noise, which is termed ERGD. These disturbances can be signi+cant, causing errors as large as 2◦ in pointing (see Fig. 2) and lasting up to an hour in duration. The most severe cases could cause the attitude control actuators (typically the reaction wheels) to change the satellite attitude to the perceived attitude brought about by these ERGD e8ects. These pointing errors are typically in excess of the allowable pointing errors permitted for the broadcast service. However, these ERGD e8ects have only occurred while the satellite is in the southern hemisphere where the service is not transmitted. The ES produces two scan lines that measure the infrared signal radiated by the earth from which the

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Fig. 2. Example of ERGD noise.

attitude control subsystem determines the correct earth pointing based on a comparison of these two lines traverse of the earth’s disc. The ERGD is believed to be the result of a sudden change in the infrared signal reading of the ES causing the determination of the earth/space boundary to become corrupted. This signi+cant change in the earth radiance gradient is most likely caused from one of the ES scan lines crossing the Antarctic during the southern hemisphere winter, drastically reducing the infrared radiation received by the sensor at that location. Prior to the development of the mitigation methods detailed subsequently, the satellite’s attitude reference was commanded from Earth Sensor to the Digital Integrated Rate Assembly (DIRA) reference once ERGD e8ects were seen. Utilizing the DIRA prevents corrupted ES data from entering the attitude control loop. The DIRA uses three gyroscopes to measure the inertial movement of the satellite in the 3-axes of inertial space. The satellite’s attitude control system uses this information to maintain proper pointing towards the earth. The DIRA does not provide as accurate an attitude reference once ERGD e8ects have already affected the satellite’s control loops. The pointing reference the DIRA will attempt to maintain could be invalid if the satellite attitude has already been moved away from its proper pointing orientation. Moreover, the drift errors of the DIRA gyroscopes could be more pronounced if the yaw attitude is not well maintained prior to the switch to the DIRA for attitude reference.

Even neglecting the prior pointing accuracy problems, the DIRA can only be used for satellite pointing over a limited amount of time due to the inherent drift that all gyroscopes possess. Understanding and compensating for these pointing errors makes DIRA operation, when used in response to ERGD events, demanding on the satellite control center operators. Consequently, another preventive procedure using a bounding process described subsequently was developed for use during the 2003 ERGD season. Following the 2002 ERGD season, all of the ERGD events were recorded and, from these empirical data, a bounding method for these events was established. The two parameters that suGciently describe the geometry of the satellite during an ERGD event and used for the ERGD activity area boundaries are the Argument of Latitude (Arg Lat) and the Reference Yaw Angle (Ref Yaw). Each Arg Lat value has a ground track location throughout one revolution of the orbit describing uniquely where the satellite is positioned above the earth, while the Ref Yaw gives the absolute rotation about yaw relative to earth north. Since the ES are +xed on the satellite body, any rotation of the satellite in yaw will rotate the ES scan lines as well. Therefore, the ES scan line locations on the earth’s disc directly correspond to the satellites’ orientation in Arg Lat and Ref Yaw. As can be seen from Fig. 3, all three of the satellites have a distinct region of Arg Lat and Ref Yaw where ERGD events occur during the period of yaw

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100 Orbit normal pre-perigee

Arg Lat versus Ref Yaw beginning of ERGD event Arg Lat versus Ref Yaw end of ERGD event

50 Arg Lat (deg) X-Axis 180

0

200

220

240

260

280

320

340

360

orbit normal postperigee

-50

FM-1yaw steering

300

-100 FM-2 yaw steering -150 FM-3 yaw sterring -200 Ref Yaw (deg) Y-Axis

Fig. 3. Illustration of ERGD regions of activity as a function of satellite orientation.

steering operations. The three regions exist because each satellite has its own orbital RAAN value, which causes each satellite to be subject to di8erent solar angles. The variation in solar angles will result in different satellite orientations in relation to earth as each satellite will be yawed to keep its solar array close to perpendicular with the sun. Conversely, all three satellites view the same regions of ERGD activity during orbit normal operations since there is no yawing out of the orbit plane during orbit normal mode. Both FM-1 and FM-2 have exhibited ERGD events within these regions while in orbit normal operations during the southern hemisphere winter. FM-3 has not yet experienced ERGD events during orbit normal operations because it does not have its orbit normal seasons during the southern hemisphere winter. From the clustering of these ERGD events, well-de+ned regional boundaries were determined (see Fig. 3). One procedure developed to mitigate the ERGD effects is the biasing of the satellite’s yaw angle, using the satellite’s actuators and attitude control system at the appropriate time to move the satellite’s ES

scan lines orientation outside of the ERGD activity region (see Fig. 4). This procedure is performed during yaw steering operation ERGD events. The biasing of the yaw positive or negative at the appropriate time will move the satellite’s ES scan lines out of the region where ERGD events have been previously experienced. Further, since the satellites have previously traversed the areas to which the ES scan lines will be biased with no resulting ERGD events, ERGD events during this yaw biased period are unlikely. The second mitigation method is the use of the DIRA for attitude reference in a similar manner as described earlier. For ERGD events during orbit normal operations, yaw biasing is not a viable preventive option. This is because there are no data to con+rm that the satellites will not experience ERGD e8ects if they are yawed positively or negatively since the satellites’ earth scans have never traversed those biased paths in the past. The simplest mitigation method for orbit normal ERGD events is the transition of the attitude control system’s reference to the DIRA prior to entry into the ERGD boundary areas. The satellite attitude

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Fig. 4. Rotation of the ES scan lines away from an ERGD area by yawing the satellite.

system’s reference is then reverted back to the ES once the ERGD exit boundary has been safely crossed a short time later. These two mitigation procedures used with the boundary analysis greatly reduce the demand on satellite operations. 4. SLAM The SLAM procedure was implemented as a means of reducing the complexity of satellite maneuver operations to correct apogee longitude. In the past, Small In-Plane maneuvers (SIPs) were performed during yaw steering operations to add minor corrections to the orbital period for maintaining the longitude of apogee within the required limits until that satellite’s next maneuver season. The original SIP maneuver consisted of a reorientation of the satellite such that the selected thruster set is aligned to +re in the direction of the plus or minus X -axis of the satellite. The reorientation involves a yawing of the satellite prior to the maneuver +ring, which were executed at perigee for maximum

fuel eGciency. The reorientations to and from the +ring attitude are time consuming components of the SIP maneuver and also have a minor e8ect on the accuracy of the satellite pointing. The potential for reaction wheel speed saturation as well as the potential lack of yaw reference create a chance for a slightly increased pointing error during SIPs. Under the SLAM maneuver, the reorientation is completely removed. Rather the +ring is performed at the maxima or minima of the satellite yaw pro+le. At these orbital points, the yaw rate of angular change is lowest. Since the SLAMs burn occurs over a short period of time compared to the yaw rate at these times, the maneuver +ring can be considered to occur at one particular yaw angle, simplifying orbital calculations. SLAMs are shorter in duration and produce less potential pointing errors when compared with SIPs; however, SLAMs consume slightly more fuel. There is more fuel used during a SLAM maneuver than a like SIP because the maneuver +ring is not performed at perigee, and some of the intended satellite velocity change from the thruster +ring is imparted into a non-desired direction.

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Since SIPs account for less than 1% of the overall fuel budget, there is only a small increase in expended fuel from the use of SLAMs. Moreover, the reduced operational complexity resulting from the simpli+cation of the maneuver operations outweighs the minimal increase in fuel use. To date there have been 3 SLAMs executed with each meeting performance requirements. 5. SPM Each satellite does not broadcast during its 8 h southern hemisphere transit. Thus, there is a large difference in the power consumption between northern and southern hemisphere operation, since operating the broadcast payload traveling-wave tubes at no drive reduces the overall DC (direct current) power consumption by 2500 W. At beginning of life, the satellite solar arrays generate 10; 800 W of DC power, which reduces to 8800 W in the 15th year of operation (for a satellite with solar arrays perpendicular to the Sun at summer solstice) due to normal radiation degradation e8ects. Therefore, the satellites’ have an excess of generated energy and DC power over the load requirements during southern hemisphere operation. This energy surplus may be stored in the batteries and later utilized during northern hemisphere transit. The procedures and algorithms that enable the storage and utilization of this power are referred to as SPM. The daily energy excess that is stored using SPM operations can provide many hundreds of watts of power to supplement the solar array energy during northern hemisphere operations. The batteries can then be returned to full charge within the 8 h spent in south hemisphere transit. SPM can be utilized to o8set solar array degradation due to radiation e8ects and extend the number of years that full broadcast power can be radiated. For instance, near end of life, after the solar arrays have degraded to such a point that the array power

is not adequate for housekeeping plus payload power requirements, SPM can be used to overcome these power de+cits. SPM’s use of the satellites’ southern hemisphere power surplus extends satellite operation past the classical point of zero array power margin so that the spacecraft may operate at full broadcast power for additional years. This creative use of the satellites’ unique orbit and payload operations allows for either a signi+cant extension of the satellites’ life, or a means to mitigate against unanticipated solar array power output degradation. 6. Conclusion The previously described broadcast system utilizes a unique orbital constellation to provide CONUS satellite radio service. There were many anticipated advantages of this type of orbit; however, additional advantages resulted after some operational experience. Those described, ERGD, SLAM and SPM help extend the satellite’s life and/or improve the ease and safety of operations. Acknowledgements The contributions of Ronald W. Bounds, Paul W. Crawford, Christopher A. Croom, Theodore Hessler III and Alicia F. Mejias to this paper are gratefully acknowledged. References [1] EGcient High Latitude Service Arc Satellite Mobile Broadcasting System, US Patent and Trademark OGce, 6223019B1, April 24, 2001. [2] R.D. Briskman, R.J. Prevaux, S-DARS Broadcast from Inclined, Elliptical Orbits, 52nd International Astronautical Congress, IAF-O1-M.5.05, 1–5 October, 2001.