LEO Communications Satellites: The IRIDIUM Constellation

DeAnn Redlin
ASEN5050

Submitted to:
Prof. Nerem
December 14, 2000

Abstract

    During 1998, the last satellites of the 66+ satellite IRIDIUM constellation were launched into orbit. These LEO communications satellites are the backbone of the Motorola and Iridium LLC satellite project boasting voice, data, fax, and pager transmission capabilities from any point on the planet to any other point. The satellites all reside in circular polar orbits and are spaced around the planet in order to provide full Earth coverage at all times.

    Iridium LLC filed for bankruptcy early in the year 2000 but has recently been bought by Iridium Satellite LLC. Iridium Satellite LLC intends to continue providing commercial satellite communications to the U.S. Government and plans to launch more affordable satellite communication services for industries that have need for satellite communication.

    As an extension to this mission, a case where the satellites are moved to sun-synchronous orbits in order to provide continuous exposure to sunlight and thus reduce dependability on battery power will be analyzed.

    The orbital dynamics of the system of satellites will be further discussed throughout this report as well as the possibility for an extension to the mission beyond its original financial limit.

Introduction
    Background
      In 1990, the IRIDIUM concept of a satellite constellation that would provide constant global communication from any point to any other point on the Earth was conceived. It was designed to revolutionize the way humans communicate today. The original design called for 77 satellites, thus the reason for its name. The element iridium has an atomic number of 77. After a few design iterations, the constellation was trimmed to a mere 66 satellites and is how it appears today.

      Launches commenced in the early nineties with the last launch carried out in 1998. Once the system came on line in November of 1998, continuous global satellite communication became a reality. However, with this technology came a few hurdles to overcome. Because the satellites all reside in near polar orbits, the cost of launch was huge due to the large amounts of fuel needed to obtain such and orbit. This along with the costs of operation drove up the price of the handheld phone as well as the calling service to a somewhat ridiculous $3,000 per phone with service reaching $7 per minute. These high prices, coupled with the fact that at the time of IRIDIUM’s release, cellular technology had advanced to cover the majority of the locations that people wanted to call from, caused the company to file for bankruptcy in early 2000. It seemed that the satellites were doomed to be de-orbited before their time because of a near failure in finding a possible buyer for the system.

      Just recently, in early November 2000, it was announced that the satellites would not meet the fate that was predicted by so many. Iridium Satellite LLC bought all the assets of Iridium LLC and will continue to provide satellite communications to the U.S. government with plans to re-launch affordable satellite communications services to certain industries that have a need for satellite communications.

    Constellation Layout
      The 66 main satellites reside in 86.4 degree near circular (e = 0 0002939) orbits at an altitude of 780 kilometers above the Earth’s surface. There are six orbital planes spaced 30 degrees apart with 11 satellites in each. The dynamics of this constellation provide full Earth coverage at all times. The mission design takes into account phase angles between the satellites in each plane and the angles between the planes in order for full coverage to occur. The north – south distance between the satellites in one plane remains constant while the east – west distance between satellites in differing planes is constantly varying, becoming smaller as the satellites approach the poles. Figure 3.1 shows and approximate layout of the orbital planes while Figure 3.2 has a graphical representation of the constellation’s Earth coverage.

      Figure 3.1. Plane separation.

       

      Figure 3.2. Layout of IRIDIUM constellation showing global coverage [7].

      In Figure 3.2, notice that the vehicle footprints overlap greatly near the poles. In order to reduce interference between so many spacecraft in one area, the constellation begins to shut down specific cells on individual spacecraft in these areas.

      Six spare satellites are also included in the constellation. One spare orbits in each plane but at a slightly lower altitude of about 650 km. When a spare satellite is needed, the inoperative spacecraft is de-orbited and the spare is maneuvered into the open position in that plane.

Hardware

    The hardware of the IRIDIUM system consists of the handheld devices used by the customer as well as the satellites themselves. The phones are advertised as being "pocket sized," but at about 6 inches tall, it probably wouldn’t fit in an average-sized pocket. Each handheld device weighs about a kilogram. Along with the phones are pagers and the possibility for devices to be hard-wired into cars and airplanes. Data, such as email and fax, can also be sent through the system.

    Figure 3.3 IRIDIUM handheld phone.

    Data is transmitted between the satellites and from the ground stations to the satellites on Ka-band frequencies while data is transmitted from the satellites to the handheld devices using L-band frequencies.

    The size of the satellites themselves was driven mainly by the size of launch vehicle available and introducing the capability to launch more than one spacecraft at a time. The final design of the satellites yielded a triangular craft with sides about 3.5 feet long and an overall length of about 14 feet. Each satellite weighs about 1,460 pounds fully fueled. The crafts have two large solar arrays and three mission antenna panels. A schematic of the satellite can be seen in Figure 3.4. The satellites have a 0.58 probability of success for a 5-year mission.

    Figure 3.4. IRIDIUM Spacecraft.

    Three different launch vehicles were chosen for raising the 72-satellite constellation into orbit. The U.S. Delta II rocket launches five satellites at a time, the Russian Proton rocket launches seven at a time and the Chinese Long March IIC/SD launches two at a time.

    STK was used to plot the ground track of a single IRIDIUM satellite from the NORAD two-line element sets (TLEs) [10]. The TLEs seen below are for IRIDIUM 8 with an epoch of December 7 2000 22:25:20.81. A plot of the ground track of the IRIDIUM 8 satellite can be seen in Figure 3.5.

    IRIDIUM 8
    1 24792U 97020A 00341.93426864 -.00000293 00000-0 -11161-3 0 4143
    2 24792 86.3964 321.0910 0002939 81.7917 278.3696 14.34210923188012

    Figure 3.5. STK rendering of IRIDIUM 8 satellite obtained from the NORAD TLEs [10].

Extension

    Putting the IRIDIUM satellites into sun-synchronous orbits will reduce their dependency on battery power by providing continuous exposure to the sun. Calculations will be done for a circular orbit since the eccentricity of each orbit is only 0.0002939. Using the following equations, the inclination needed is found.

    (4.1)

    (4.2)

    (4.3)

    Inclination for a sun-synchronous circular orbit at an altitude of 780 km is calculated with sunsync.m to be 98.52 degrees. See Figure 4.1 for an STK rendering of the groundtrack of this new orbit.

    Figure 4.1. Groundtrack of sun-synchronous orbit.

    In order to make the plane change from an inclination of 86.4 degree to that of 98.52 degrees Equation (4.4) is used.

    (4.4)

    Where the flight path angle is zero since the orbit is nearly circular (e = 0.0002939). The change in inclination needed is the difference of the initial inclination and the inclination found with Equation (4.4) and is 12.12 degrees. The initial velocity of the orbit can be found using Equation (4.5), the velocity of a circular orbit.

    (4.5)

    Where r is simply the semi-major axis of the orbit is circular. With these parameters, the change in velocity needed to move the satellite into a sun-synchronous orbit is 1.576 km/s. This is quite a large change in velocity and requires a lot of fuel to implement.

    Realistically, it probably wouldn’t be beneficial to make this change to the orbits. These satellites were designed to reside in the orbits that exist now and may not be able to function as well in a sun-synchronous orbit. One reason for this is the thermal regulation on the crafts. They are designed to have the time in shadow to release some of the heat accumulated during time in sunlight. If they were to be in continuous sunlight, some of the systems may overheat. It could also reduce the spacecraft’s lifetime due to a smaller fuel reserve for orbit maintenance. However, the exercise of calculating the inclination and change in velocity needed for sun-synchronous was beneficial because principals learned in class were applied to a real-world scenario.

Conclusions

    The IRIDIUM satellite constellation consists of 66 main satellites and 6 spares dispersed in a 6 equally spaced planes. Each operational plane holds 11 satellites orbiting at 780 km and one spare orbiting at 648 km. The planes are inclined at an angle of 86.4 degreed from the equator and are very nearly circular with an eccentricity of 0.0002939. Each satellite is triangular with side lengths of 3.5 feet and an overall length of 14 feet weighing about 1460 pounds fully fueled. Two large solar arrays provide solar power to the craft when it is exposed to the sun. Three main mission antennae provide the communications needed. Satellites transmit to each other and the ground stations through Ka-band frequencies and to customers through L-band frequencies.

    The extension of the constellation proposed was to put the satellites into sun-synchronous orbits in order to reduce dependency on battery power. In order to reach an inclination of 98.52 degrees a change in velocity of 1.576 km/s is needed. This would probably not be an efficient way to use the fuel on board and my reduce lifetime because of a smaller fuel reserve for orbit maintenance. It could also cause problems such as overheating on the crafts.

References

    [1] Garrison, T.P.; Ince, M; Pizzicaroli, J; Swan, P.A. "Systems engineering trades for the IRIDIUM® constellation." Journal of Spacecraft and Rockets. Vol 34. Iss 5. Pp 675-680. 1997.

    [2] Kleiner, K; Graham-Rowe, D. "Cash or Burn." New Scientist. Vol 165. Iss 2231. Pp 9-9. 2000.

    [3] Malakoff, D. "Communications satellites – Iridium’s loss is astronomers’ gain." Science. Vol 287. Iss 5461. Pp 2135-2135. 2000.

    [4] Rossi, A; Valsecchi, G.B.; Farinella, P. "Collision risk for high inclination satellite constellations." Planetary and Space Science. Vol 48. Iss 4. Pp 319-330. 2000.

    [5] Swan, P.A; Cloutier, P.N. "Global Personal Communications this Decade with Iridium™." Space Technology-Industrial and Commercial Applications. Vol 13. Iss 4. Pp423-425. 1993.

    [6] Vallado, D. A. Fundamentals of Astrodynamics and Applications. McGraw-Hill. New York, NY. 1997.

    [7] www.ee.surrey.ac.uk/Personal/L.Wood/constellations/iridium.html

    [8] www.flatoday.com/space/explore/stories/2000b/111600e.htm

    [9] www.apspg.com/whatsnew/iridium/iridium.html

    [10] www.celestrak.com