IRIDIUM-NEXT COMMUNICATIONS SATELLITE

Table of Contents

Introduction

Iridium-NEXT Mission

Spacecraft Overview

Bus Design

Payload

Antennas

Orbit Design

Hosted Payloads

Compact Radiometer

Compact Total Electron Content Sensor

MicroCam Multispectral Imager

Radiation Dosimeter

Compact Earth Observation Spectrometer

Micro-Electro-Mechanical Systems (MEMS) Accelerometer

Conclusion

Bibliography

Communication is the corner stone of our society. We can communicate through several different means; hands, voice, written, and body language. It helps define who we are and separates us from the other species. Each form of communication is unique and has its own rules. The form of communication we tend to take for granted is verbal or voice. Ever since Alexander Graham Bell developed the first telephone system, we have been developing ways to make voice communication easier to access for everyone across the globe. The first communications satellite was launched in 1962 and it was the first-time telephone or television was broadcasted via satellite from North America to Europe (History Channel, unknown).

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Iridium launched its first communication satellite constellation between 1997 and mid-1998. This constellation went into service in November of 1998. The company name is derived from the elemental number of Iridium, which is 77, because early calculations pointed to this number as the minimum number of satellites in a low earth orbit that would be able to provide complete global coverage. Because of the early high cost of satellite communication services, the original Iridium filed for bankruptcy just a year after offering services. This left the original 66 satellite constellation that were still active to be placed up for sale. In 2001, services were reestablished by a private group of investors and re-named Iridium Communications, LLC. As of the middle of 2016 only 64 of the original satellites launched by Iridium where still functional. As the aging original Iridium satellites began to fail due to service life limits causing gaps in coverage and service outages, the need for the next generation of low earth orbit communication satellite constellation needed to be designed (Spacecraft & Satellites, 2018). This paper will attempt to examine the next step in satellite mobile telecommunications by taking a closer look at the next generation of communications satellite constellation to be launched by Iridium Communications, the Iridium-Next constellation.

 Iridium Communications is a mobile satellite services provider that offers worldwide voice and data communications. Iridium Communications provides this service through mobile or stationary transceiver units and satellite phones to provide a truly global communications package that includes coverage of both polar regions, the oceans and airways (Spacecraft & Satellites, 2018). In June of 2010, Iridium Communications announced a comprehensive plan for the deployment of Iridium-NEXT. This plan included details on funding, building and deployment schedules for the next generation communications satellite constellation. Iridium-NEXT is being designed and deployed to replace the aging original Iridium communication satellite constellation. The Iridium-NEXT constellation will consist of 66 cross-linked Low Earth Orbit (LEO) satellites that will form a global network (Krebs, 2018). There will also be six in-orbit spares launched with six spares in hangar storage. The satellites have a 10-year design with a 15-year mission maximum life expectance (Kramer, Unknown).

 The Iridium-NEXT constellation will be launched to an altitude of 780km in a polar orbit. The orbital period will be 101 minutes with an orbit inclination of 86.4° (Kramer, Unknown). The SpaceX Falcon 9 rocket will serve as the launch vehicle to deliver the Iridium-NEXT satellites into orbit. Up to ten Iridium-NEXT satellites can be launched at one-time from the SpaceX Falcon 9 rocket platform. Thales Alenia Space will provide the bus structure and Northrop-Grumman will handle assembly, integration, test and launch support (Iridium Communications, Inc., 2018). Iridium-NEXT will also offer the ability for other companies to have payloads hosted, making this a unique partnership of Public and Private companies. The hosted payloads offered by Iridium-NEXT is a first for a commercial satellite constellation that provides a unique opportunity for the hosted companies to have access to a cost-effective means of getting Earth observation equipment into orbit that may not have been possible as stand-alone satellite program (Spacecraft & Satellites, 2018).

 The Iridium-NEXT satellite will operate of the ELiTeBus-1000 (Extended LifeTime Bus) satellite platform designed and manufactured by Thales Alenia. Each one of these satellite bus platforms weighs approximately 450kg. The Iridium-NEXT will weigh almost 900kg at launch and will measure 3.1 by 2.4 by 1.5 meters in the stowed launch configuration. The ELiTeBus has been used for both Globalstar and O3b communications satellite, thus making this a proven satellite bus platform. The attached payload will consist of pointing stability, propulsion, stable power supply and data connections. All payload components will be attached to an Earth-pointed panel that measures 3 by 1.5 meters. This panel a will be oriented in a nadir-facing direction for nominal flight phases (Spacecraft & Satellites, 2018).

 The spacecraft will be powered by two post launch deployable two-segmented solar arrays. Both solar arrays can follow the track of the sun in order to maximize power generation.  The solar arrays are made up of gallium-arsenide solar cells that are capable of an average of 2,200 watts of power distributed via a 28-volt power bus for the payload. Energy will be stored using lithium ion batteries.

 Iridium-NEXT will employ a combination of things to provide stability, attitude, and telemetry. First, the satellite will employ three-axis stabilization by using precise pointing technology from data received from Star Trackers, Earth & Sun Sensors along with an inertial measurement unit. Other satellite momentum stabilization will be accomplished using a combination of torque rods and reaction wheels. GPS will be used to determine its orbit position allowing for pass geometry calculations as it passes over ground terminals. The AA-STR Star Tracker System by Selex Galileo will provide the primary means of attitude determination to keep Iridium-Next in the correct orientation. This tracker system can make orientation calculations autonomously using a pre-programed catalog of the known star constellations without having to ground station input to make attitude corrections (Spacecraft & Satellites, 2018).

 The satellite bus will be outfitted with a 141kg propellant load of Hydrazine monopropellant propulsion system. This will provide propulsion to the eight 1-Newton thrusters. These thrusters will provide orbit adjustments for maintenance and attitude control in all modes. The fuel load is designed to provide the required fuel for the expected 12.5 years of service and should have enough reserve fuel that satellite service life can be extended to 15 years (Spacecraft & Satellites, 2018).

Bus Design

 Thales Alenia Space will be responsible for the design and manufacture of the spacecraft bus. The spacecraft bus platform chosen is the ELiTeBus-1000. The spacecraft bus will be a trapezoidal shape made with rigid aluminum honeycomb panels attached to an aluminum tubular sub-structure (Spacecraft & Satellites, 2018) (Thales Alenia Space France, Unknown). The satellite bus has a lifetime of seven to 12 years depending on the radiation exposed to, with a reliability of >0.9 at 12 years. In a LEO environment the bus has a 1000W average power capacity. The ELiTEBus-1000 touts a 55 arcsec accuracy, a 10 arcsec/sec stability, a 70°/min roll slew rate, and a pointing knowledge of 22 arcsec bus pointing performance.

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 The ELiTeBus is powered by two solar arrays that generate 2.7kW, and supplies power to the 28V power bus and the 126 Ah lithium-ion batteries. When the solar arrays are deployed after reaching orbit the ELiTeBus has a “wing span” of 9.4m. The solar arrays provide power to the satellite payload via a LEON 3 processor and a 1553 data bus. The LEON 3 processor offers a data processing speed of 40 MIPS to handle large data processing requirements (Thales Alenia Space France, Unknown).

 Thermal control will be accomplished by using both passive and actives means. Some of the thermal technology that will be employed on the Iridium-NEXT include secondary surface mirror (SSM) radiators, heaters, heat pipes, multi-layer insulation, and paints. The use of passive and active thermal control measure will ensure the onboard avionics payloads will have a stable operating temperature environment. Thermal design helps ensure the satellites can achieve a maximum service life (Thales Alenia Space France, Unknown).

Payload

 The payload that makes up the Iridium-NEXT consist of a high-power supply and state of the art avionic architecture that provides a flexible ground to satellite link communications satellite platform. The power supply consists of an unregulated 28V power bus that draws its power from a two axes Solar Array that can produce a maximum 2.7 kW and a Lithium-ion battery with a 126 Ah battery capacity (Thales Alenia Space France, Unknown).  The avionics payload consists of a LEON 3 microprocessor capable of 40 MIPS that will handle the large data processing requirements. Along with the microprocessor the avionics payload has a 1553 data bus, a star tracker, and 3-axes Reaction Wheel control for stability. All satellite payload systems are routed through an OBC system for command and control. The 1553 data bus connects all of the systems to the OBC system and allows for interfacing with the satellites communication system (Spacecraft & Satellites, 2018).

 The main part of the Iridium-NEXT payload is the L-Band phased array antenna. The phased array antenna is mounted on the Earth-facing panel and can generate 48 spot beams for transmitting and receiving communication signals. Along with the L-Band phased array antenna there are also Ka-Band antennas for inter-satellite link communications. The Ka-Band antennas are central to providing the crosslink communications that the Iridium-NEXT platform will provide to improve the ease of signal hand-off. The last set of antennas included in the payload is an omni-directional antenna that will be used for telemetry and commanding the satellite as it orbits the earth (Spacecraft & Satellites, 2018).

 The propulsion part of the payload is important as this will not only deliver the satellite into the orbit injection point but will also help the satellites maintain its orbit and any deorbiting needs during the satellites mission life. There will be a total on nine 1N catalyst thrusters included in the payload. One of the nine thrusters will be used to achieve orbit insertion, the remaining eight will be used once in orbit to maintain satellite orientation and spacing. The propulsion fuel source selected for use is the industry standard of hydrazine (Thales Alenia Space France, Unknown).

Antennas

 Iridium-NEXT will be outfitted to provide communications via L-Band, Ka-Band antennas and omni-directional antennas that will provide telemetry, tracking and command (TT&C). L-Band communications will be handled by a single 48-beam phased array antenna that will handle transmit and receive functions. This will be accomplished using time-division duplex (TDD) architecture (Kramer, Unknown). The L-Band antennas will operate at 1616 to 1626.5 MHz for up and downlinks. This will provide voice data transfer rates of 2.4kbps, up to 64kbps if using L-Band Handset Data Services & Short Burst Data, and 512kbps to 1.5Mbps with the use of the Iridium OpenPort Terminals (Spacecraft & Satellites, 2018). The 48-beam phased array antenna will provide a coverage diameter of 4,700km cellular pattern on Earth’s surface, this will help the 66-satellite constellation achieve full global coverage (Krebs, 2018). The L-Band phased array antenna will have a dedicated control module unit and beamforming network with 168 transmit and receive modules inside the body of the satellite (Spacecraft & Satellites, 2018).

 The Ka-Band antennas will consist of two 20/30 GHz steerable feeder links, and four 23 GHz crosslink, two steerable and two fixed antennas with TDD architecture. The 20/30 GHz steerable feeder links will connect the satellites to the terrestrial gateways. They will provide an uplink speed of 20 GHz and with a downlink speed of 30 GHz (Kramer, Unknown). The four crosslink antennas will provide the satellite the means to connect to the neighboring satellites. This crosslink will connect a satellite to the satellites in front of and behind it in the same orbital plane and via the two fixed antennas. The two steerable antennas will connect the satellite to the neighboring satellites in the adjacent orbital planes. The crosslink communication via the four 23 GHz antennas will have a data rate transfer of 12.5Mbps, that will be half duplexed. TT&C will be accomplished using 20/30GHz omni-directional antennas, that will provide 20 GHz uplink and 30 GHz downlink. This will allow of command and control of the satellites through all spacecraft orientations (Spacecraft & Satellites, 2018).

Iridium-NEXT will have an identical constellation orbit as the original Iridium constellation system. The constellation will consist of 66 operational satellites in a polar oriented LEO. The constellation orbit will be circular at an altitude of 780km with an orbit inclination of 86.4 degrees to achieve global coverage. The satellites will have an orbital period of about 100 minutes (Boiardt & Rodriguez, 2010). The 66 satellites in the constellation will be divided among six different planes with each plane having 11 total operational satellites (Spacecraft & Satellites, 2018) (Kramer, Unknown). LEO satellite orbits are more desirable for voice communications and real-time data transfer applications, such as high-speed internet, due to the short transmission latency and lower signal attenuation that can be achieved (Tang, Feng, Yu, Zhao, & Wu, 2014).

 The LEO polar orbit used by Iridium-NEXT will have an angle between planes with the same direction of movement (∆Ω) of 31.6°. The phase angle of satellites in the adjacent planes (ωf) of 16.36° (Tang, Feng, Yu, Zhao, & Wu, 2014). This orbital design will have the greatest overlap in the polar regions and the greatest separation when the satellites pass over the equator. This gives Iridium-NEXT constellation the ability to support 3,168 spot beams for communication services when only 2,150 simultaneous beams are required for global coverage (Spacecraft & Satellites, 2018).

Iridium-NEXT will be the first communications satellite constellation to offer space for hosted payloads from other companies. This unique approach will give smaller companies who are looking to get earth-monitoring payloads into space that lack the infrastructure or funding to develop a stand-alone satellite system. Iridium-NEXT will be offer or be able to accommodate hosted payloads weighing less than 54kg with overall payload dimension of 30 x 40 x 70cm limited to a power supply average of 90W and 200W peak. The hosted payloads that have been targeted by Iridium-NEXT to provide the following mission sets: altimetry, broadband radiometry, multispectral imaging, and GPS radio occultation (Kramer, Unknown).  This data from the hosted payload is provided in near real-time to the operators. The hosted payload size is to allow for maximum use of available space to offer the greatest payload hosting opportunity. The hosted payloads will be provided by the following companies Aireon, LLC., Harris Corp. and exactEarth Ltd., and the National Science Foundation (Spacecraft & Satellites, 2018).

Aireon, LLC hosted payload is an Automatic Dependent Surveillance-Broadcast (ADS-B) system as part of the global ADS-B system to provide real-time aircraft surveillance services for the various Air Navigation Service Providers (ANSP) and Air Traffic Control (ATC). This hosted payload will be like no other system that currently exist or that are planned, and provided aviation stakeholder with new opportunities (Aieron, 2018). This will use a Mode-S transponder operating at 1090MHz frequency. The data that will be contained within the signal will be aircraft identification, current position, altitude and intent. This is a joint venture between the FAA (Federal Aviation Administration), NAV CANADA and Aireon, LLC. to help bring improvements to air traffic control by increasing efficiency and flexibility to aircraft routing saving fuel and reducing aircraft separation requirements (Spacecraft & Satellites, 2018). The ADS-B receiver payloads will operate independently of the Iridium-NEXT primary mission of providing satellite communications and will be deployed aboard all Iridium-NEXT satellites (Kramer, Unknown).

Like ADS-B, Automatic Identification System (AIS) is another hosted payload that will provide global tracking of commercial and private ships. This system allows for sea-going vessels to send and receive messages that contain information on other ships positions, current course and speed that will help with collision avoidance and planning vessel movements. The AIS system will be operated by Harris Corporation and exactEarth LTD. This hosted payload will mark the first time that AIS terminals will be capable of providing real-time global coverage (Spacecraft & Satellites, 2018).

When the Iridium-NEXT communications satellite constellation introduces services through Iridium Certus, the constellation will be able to provide data transfer speeds of up to 1.4mbps. Iridium Certus will be the program around which the terrestrial satellite terminals will built upon. Iridium Communications has picked four companies to manufacture and sell these terminals. Those companies are Cobham, PLC.; Rockwell Collins; L-3 Communications; and International Communications Group. The terminals created by these companies will be available to both aviation and maritime markets (Werner, 2015).

Another hosted payload is GEOScan, which is a multiple-sensor suite provided by the National Science Foundation as a grassroots effort to benefit a large portion of the scientific community. Deployment aboard Iridium-NEXT will provide scientist with data sets and observations that were previously unavailable due to cost barriers (Kramer, Unknown). The GEOScan payload is made up of six different earth-monitoring sensors. The sensors included in the GEOScan suite are: 1) a compact radiometer that will measure Earth’s total outgoing radiation; 2) a GPS Compact Total Electron Content Sensor to monitor Earth’s plasma environment and gravitational phenomena; 3) a Microcam Multispectral imager that will deliver instantaneous imagery of Earth, monitor cloud cover, vegetation, land use and auroral phenomena; 4) a Radiation Dosimeter system to measure the distribution of energetic electrons and protons; 5) a Compact Earth Observing Spectrometer to measure atmospheric properties and monitor vegetation; and 6) MEMS Accelerometers to deduce non-conservative forces, contributing to gravity measurements and assessing neutral drag systems (Spacecraft & Satellites, 2018).

GEOScan will measure 20 x 20 x 14 centimeters and weigh 5kg, making this a very compact payload. The power requirements average 5W with a peak of 10W. Data will be generated at an average of 10kbps to a peak of 100kbps. The compact design of GEOScan was intended to minimize cost and to make manufacturing easier (Spacecraft & Satellites, 2018). The GEOScan payload makes up the bulk of the hosted payload mission sets.

Compact Radiometer

The compact radiometer of GEOSCan will measure the Earth’s outgoing radiation via a two-channel radiometer system. The radiometer has a weight of 0.6794kg and uses a power average of 0.257W with a peak power demand of 5W. It measures 10 x 9 x 10 centimeters and has a data transfer rate of 60 bit/s. The sensor will have a field of view (FOV) of 127° that will collect the data on the Earth’s Total Outgoing Radiation (TOR) every 2 hours have better than 0.15% accuracy. The two-channel system consist of a shortwave channel and total channel. The shortwave channel has a wavelength of 0.2-5μm and the total channel has a wavelength of 0.2-200μm and is calibrated with an accuracy of 0.3Wm-2 and precision of 0.09Wm-2 in accordance with the National Institute of Standards and Technology.  The wavelength range includes the UV spectrum into the infrared spectrum to collect data on the Earth’s TOR (Kramer, Unknown) (Spacecraft & Satellites, 2018).

Compact Total Electron Content Sensor

The Compact Total Electron Content Sensors (CTECS) uses a modified commercial off the shelf receiver and firmware. CTECS will monitor and provide for a continuous snapshot of the global ionosphere and plasmasphere. The sensor will weigh less the 200g and require 1.5W of power, 1.2W for the receiver and 0.3W for the antenna. The sensor will also provide a view temporal and spatial evolution both the ionosphere and plasmasphere with a temporal resolution of 5 minutes and resolution height of 10km having a measurement error of less than 3 TECU (total electron content unit). The sensor will have a maximum data downlink of 1.44MB per day for TEC at 0.1Hz when the elevation is greater than 0° and 1Hz when the elevation is less than 0° at 200 occultations a day per satellite with an average of 15 minutes per occultation when the elevation is greater than 0° and 5 minutes when the elevation is less than 0° (Kramer, Unknown) (Spacecraft & Satellites, 2018).

MicroCam Multispectral Imager

The MicroCam Multispectral Imager (MMI) uses commercial off the shelf imaging software STAR-1000 that is capable of 1024 x 1024-pixel imaging. The MMI less power than the other similar cameras at 0.55W. It has a FOV of 33° and will capture images at an interval of 29 seconds and total global imagery coverage every two hours. With this FOV the ground footprint of the MMI is 465km x 465km. The MMI camera has a visible to near-infrared spectrum that offers a wide field of view (Spacecraft & Satellites, 2018).

Radiation Dosimeter

Radiation Dosimeter payload will provide a Radiation Belt Mapping System that will allow for the studying of the Earth’s radiation belts that will include relativistic electron micro-burst, loss of atmosphere, and the variations in geomagnetic solar energetic particles. The payload consists of one pair of micro-dosimeters manufactured by Teledyne, one electron dosimeter and one proton dosimeter. Each dosimeter is capable of measuring thresholds of 100 keV an 3MeV respectively. The shielding for the dosimeter will allow for electron and proton energy resolution at 0.3 to 5 MeV and 10 to 50 MeV. The dosimeter has a weight of 20g, and measures 36mm x 26mm x 1mm. The power consumption has an average demand of 280mW with a peak demand of 400mW. Data transfer from the dosimeter is at the rate of 1 byte/s (Kramer, Unknown) (Spacecraft & Satellites, 2018).

Compact Earth Observation Spectrometer

The Compact Earth Observing Spectrometer (CEOS) will provide spectral data which is used to calibrate climate models. The sensor will have a 1° FOV and will take measurements in the spectral range of 200 to 2000nm. The sensor will be capable of providing a spectral resolution of 1nm from 200-1000nm and 3nm from 1000-2000nm. This will allow for the sensor to generate a ground resolution of 14km.

Micro-Electro-Mechanical Systems (MEMS) Accelerometer

MEMS Accelerometer for Space Science (MASS) will be used for studying the variations in Earth’s gravitational field as well as the drag forces on the Iridium-NEXT satellites to determine neutral density. The MASS is made up of silicon-based accelerometers that require less power than accelerometers used on other satellite platforms. The accelerometers chosen for the MASS payload are low-noise and have a white noise factor of 10-11g2/Hz. They also have an excellent sensitivity threshold of around 10ng/√Hz with the potential to reach 1ng/√Hz. This performance makes them very suitable for use in measuring gravitational and non-gravitational forces.

This paper has taken a closer look at the next generation of communications satellite constellation, the Iridium-NEXT. The first Iridium communications satellite constellation was launched in the late 1990’s. Iridium-NEXT constellation like the original Iridium constellation will consist of 66 satellites divided over six orbital planes. Iridium-NEXT will also be the first LEO constellation to provide platform space for hosted payloads from other companies and entities. The hosted payloads will allow Earth-monitoring projects a cost-effective means to get them launched and makes this program a unique partnership of Public and Private industries in search of a common goal.

Iridium-NEXT communications satellite will provide LEO mobile satellite services will into the year 2030. With satellite launches set to be finished by early 2019, Iridium-NEXT will be providing high-speed data services across the globe very soon. With a lower service cost and improves signal coverage Iridium-NEXT should be very successful. The hosted payloads will offer the scientific community large amount of data sets that will help them develop solutions to the environmental problem that face humanity today.

It was interesting to learn how the company name was developed, based on the early theory that 77 satellites were needed to provide complete global coverage. Iridium has an elemental weight of 77, which is how the company name was developed.  Satellite communications have come a long way since the first telecommunications satellite, Telstar was launched in 1962, which only provided telephone and television broadcast from North America to Europe. Without Alexander Graham Bell inventing the first telephone, who knows where we would be as a society. The telephone has made verbal or voice communication over great distances much easier. And since the invention of the telephone we have continued to improve upon Bell’s invention to make voice communication easier and more accessible. Humans can communicate via several methods verbal, non-verbal body language, written and with our hands. Communication is the corner stone of our society and without the free follow of information and ideas would be impossible.

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