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A ground data system consists of a network of ground stations and control centers, such as the Spacecraft Operations Control Center (SOCC), the Payload Operations Control Center (POCC) and the Mission Control Center (MCC). These networks and control centers may be located at the same geographical location depending on the type, size and complexity of the mission. However, for small spacecraft missions, there is often no distinction between MCC, SOCC and POCC, as these different networks support the overall objective of the spacecraft and the users of the data generated by the mission.
The ground segment supports the space segment (spacecraft and payload), relaying the mission data to the final users. To support the spacecraft mission, the ground data system must command and control the bus and payload, monitor their health, track the spacecraft’s position, and use ADCS sensor information to report the spacecraft’s attitude 1.
The author would like to highlight that the presented tables are not intended to be exhaustive but to provide an overview of current state-of-the-art technologies and their development status for this small spacecraft subsystem. There is no intention of mentioning certain companies and omitting others based on their technologies.
Small spacecraft ground data systems
The ground data systems architecture for small spacecraft missions will often take a different from classical architectures used for larger spacecraft missions. The low-cost paradigm shift and the accessibility of commercial-off-the-shelf (COTS) technology for the space sector have not only changed how designers think about spacecraft, but also the ground data systems architecture. To lower the costs of a small spacecraft ground data system, the entire small spacecraft mission is frequently managed from a single modified lab room. The ground station is either a fixed or mobile COTS antenna connected to mission control using standard cabling. Tracking, Telemetry and Command (TT&C) for both platform and payload is managed by a single computer.
The Figures above illustrate the variety of inground data system architectures that can be used for small spacecraft missions. The Figure (a) shows the Air Force Satellite Control Network (AFSCN) as an example of a classical ground data system setup. The topology of the AFSCN is hierarchical, with twelve nodes organized around a central master node at Schriever AFB, CO. Figure (b) depicts tdepicts the distributed network of ground stations used for the PhoneSat project as it was supported by 1,343 volunteer nodes organized in a distributed topology. Figure (c) illustrates the common small spacecraft ground segment topology, where a single node consists of a university ground station and control room.
Under stringent power and volume budget constraints, small spacecraft (primarily cubesats) missions typically use academic or amateur ground data systems with only one antenna, limiting the ability to communicate with more than one spacecraft simultaneously. This typically restricts cubesats to orbits below Geosynchronous (GEO) altitudes, as they are unable to carry far-ranging radio dishes or use more powerful antennae. However as of Spring 2018, Cubesats have begun to venture beyond Earth orbit with the two 6U MarCO spacecraft following advancements in transponders. Other disadvantages of using a single, small antenna include less bandwidth, lower data rate and less throughput capability for the entire mission.
Peer-to-peer topologies are also possible with a large number of ad-hoc nodes participating on a voluntary basis and, despite overcrowding of the frequency bands (typically UHF, VHF and S-band), the individual nodes in the topology can be interchangeable. For an exhaustive treatise on the characteristics of small spacecraft ground data systems, refer to Schmidt, 2011. Additionally, the services provided by cubesats ground stations generally do not provide the same security, reliability and latency as classical ground data stations. Larger and more complex spacecraft usually use Consultative Committee for Space Data Systems (CCSDS) standards based long-haul communication protocols. On the contrary, cubesats may use TCP/IP-based communication protocols, which provides lower data communication reliability and performance1.
Amateur and Non-Amateur communications bands
Traditionally, amateur radio bands have been the preferred means for cubesats to communicate with the ground, as frequency allocations from the International Telecommunication Union (ITU) for cubesat missions have been restricted to eliminate frequency conflicts with larger spacecraft. However, cubesats are increasingly shifting from low-performance missions to higher-complexity science or technology missions. The larger amount of data produced by these higher-complexity missions necessitates higher communication data rates than amateur bands can provide.
From a regulatory point of view, small spacecraft missions must adhere to the same radio spectrum regulations that apply to larger spacecraft. In the U.S. for example, these regulations are governed by the Federal Communications Commission (FCC). Amateur radio frequencies for communications have licenses that are simple and quick to obtain. Since this kind of license is not available to government entities, whose missions are regulated by the National Telecommunications and Information Administration (NTIA), a number of partnerships have emerged between government entities and academia. For instance, a number of cubesat missions developed by NASA Ames Research Center are operated from the MOC at Santa Clara University. Similar radio frequency regulations exist in other countries, and these regulatory issues can make small spacecraft partnerships increasingly difficult. It is the responsibility of the developers to ensure they follow the proper regulations as they build and operate their spacecraft.
In most administrations, unlike other RF spectrum users, radio amateurs may build or modify transmitting equipment for their own use within the amateur spectrum without the need to obtain government certification of the equipment, and this can be a big advantage in designing telecommunication systems for CubeSats. Licensed amateurs can also use any frequency in their bands (rather than being allocated fixed frequencies or channels) and can operate medium to high-powered equipment on a wide range of frequencies, as long as they meet certain technical parameters–including occupied bandwidth, power, and maintenance of spurious emissions. For example, the International Amateur Radio Union has allocated CubeSats in the spectrum between 437.100 and 437.575 MHz, with a maximum single satellite bandwidth allocation of 20 kHz. This was done to protect existing and future amateur radio voice satellites2.
While bands at 2.4 GHz and 5.8 GHz available for amateur spacecraft communication are increasingly crowded, higher frequency amateur bands require uncommon microwave parts to implement transceivers, and working with 10 GHz or higher frequencies requires electric power typically not available in CubeSats. Moreover, encryption is not generally permitted in the amateur radio service, except for the special purpose of spacecraft control uplinks. For these reasons, CubeSat missions are moving to higher, non-amateur frequency bands to support their data requirements. For instance, the 1.5U CubeSat Dynamic Ionosphere CubeSat Experiment (DICE), launched in 2011, used the 460-470 MHz meteorological-satellite band with L3 Cadet radios to produce a 1.5 Mbps downlink data rate to support its science mission3.As CubeSat missions abandon amateur radio bands for higher-speed frequencies, the radios and ground stations get more difficult and more expensive to build. Non-amateur radio licenses, on the other hand, prohibit autonomous beaconing of satellite data. This is a big disadvantage because the CubeSat teams can no longer rely on the existing network of amateur radio operators to downlink beacon data. Non-amateur satellite licenses are usually point-to-point, so all ground stations commanding and receiving satellite data must be on the same territory and must be licensed, which is an expensive and time-consuming process3.
CubeSat programs could use higher frequencies in either the C-band or X-band to reduce the volume and mass of both the transceiver and antenna, and to support increased power generation systems for three-axis stability requirements4. As this will also increase the bandwidth to support payloads that have a significant data downlink requirement, there is a need for highly precise pointing requirements. However, designers need to consider the utility of additional bandwidth with decreased size and mass against increased power requirements to close the link with the ground station, since the energy-per-bit is lowered for the same power consumption5.
As cubesat power generation systems become more effective and three axis stability is achieved, higher operating frequencies become increasingly feasible while permitting smaller components and increased antenna gain4. The user must carefully evaluate all the pros and cons that Amateur and Non-Amateur bands provide, in order to select and define the most appropriate telecommunication solution for mission requirements.
Delay/Disruption Tolerant Networking
As CubeSats are beginning to venture beyond Earth orbit, their networking design must be compatible with the challenging communications environments of deep space. Using Delay Tolerant Networking (DNT) protocols to enable solar system communication at low cost, could benefit CubeSat missions in multiple aspects. DNT is a communication protocol suite used for environments with long transmission delays, intermittent connectivity, and high bit error rate4. It is designed for environments where communication quality is not guaranteed, and for intermittent network connectivity. It works as an overlay network associated with Bundle Protocol (BP) and some convergence layer protocols like Licklider Transmission Protocol (LTP). Future space missions (swarms, constellations, spacecraft that need to communicate with a lander or orbiter) include features that cannot be accommodated by conventional link layer-based communications without intermittent connectivity and long light-time (idle) delays. Complex topology will require a network layer in the space communications protocol stack to provide reliable routing and forwarding of data, and DNT is an effort to solve this problem as TCP/IP (Transmission Control Protocol/Internet Protocol) cannot support this type of network6.
State of the Art
The ultimate goal for small spacecraft network ground stations is to relay all of its downlinked data as soon as it has commenced operations, and continue until all the intended data has been downlinked. Theoretically, data is downlinked to the different active ground stations during its entire pass. However, active ground stations are not always available for every pass, as there are a number of other spacecraft transmitting data to them7.
Ground station networks for small spacecraft have greatly improved in the last few years, as many companies are producing and developing new state-of-the-art systems. Some companies focus more on single products that have yet to be validated in space, others consolidate and extend their current services with turnkey solutions, which adds more capability and availability to their already well-developed ground data systems. This section focuses on the state-of-the-art of communication technologies of Ground Data Systems.
Turnkey solutions can be a good option for designers who want to focus more on the payload and systems engineering portions of the spacecraft. Table 11-1 lists some companies or organizations that develop and provide turnkey solutions for small spacecraft ground data systems.
|Table 11-1: Turnkey solutions for ground systems|
|ATLAS Global Network||ASAT||9 for ground infrastructure, TRL 8 for software integration||S-band, X-band, UHF (Ka-band in 2017)|
|KSAT Lite||Kongsberg Satellite Services||9||X-band and S-band D/L and S-band U/L. VHF, UHF, Ka-band D/L|
|Surrey Ground Segment||Surrey Satellite Technology Ltd.||9||S-band for U/L and D/L and X-band for D/L|
|ISIS Small Satellite Ground Station||ISIS B.V.||9||Amateur and non-Amateur protocols for VHF, UHF and S-band|
|Endeavour TT&C||Tyvak Inc.||8+||VHF, UHF and 2.2 – 2.29 GHz (S-band)|
|Open System of Agile Gorund Systems (OSAGS)||Espace Inc.||8||S-band for U/L and D/L. Additional HR/VHF/UHF receive capability|
|GAMALINK Ground Station Network||GAMALINK||7+||Provides VHF/UHF pack and S-band pack. Additional ranging and GPS support available.|
|Satellite Tracking and Control Station||Clyde Space||8||VHF, UHF, L-band and 2.4 GHz|
|Planet Labs Ground Station Network||Planet Labs||9||5+ terabytes of data downlinked per day|
|Spaceflight Networks Global Ground Station Network||Spaceflight Networks||9||Various bands, from UHF to X-band|
Assured Space Access Technologies (ASAT) is an affiliated corporation formed to develop the ATLAS global network of commercially available spacecraft ground stations, aimed at providing cloud based solutions for space access. It provides global TT&C operations systems using the Amazon Virtual Cloud, which interfaces connectivity for the user to the ground stations. The supported frequency bands in which ATLAS operates are mainly S, X and UHF8, however current status is unknown. The Figures below show how the ATLAS ground service works with the cloud service and the locations of the antennas around the globe.
KSAT Lite is a low-cost ground station antenna network designed to support different phases of small spacecraft missions. The company has launched 20 ground station sites across the globe. KSAT Lite is an extension of the existing KSAT network, but implements more flexible options and procedures in terms of priority allocation, availability and pass selection. The KSAT network has uniquely located polar stations in the Arctic and Antarctic regions, providing from 85% to 100% availability on passes for spacecraft in polar orbit. The network also operates mid-latitude ground stations, providing access for many other orbits. The baseline KSAT 3.7 m antennas provide X-band and S-band for downlink and S-band for uplink. In addition, KSAT Lite offers VHF and UHF capacities that support a variety of system configurations. Ka-band support for the small spacecraft market was integrated in 2016 9.
Similar to KSAT Lite but on a smaller, university scale, the Global Educational Network for Satellite Operations (GENSO) system, by the European Space Agency (ESA), is a software networking standard for universities which allows a remote operator to communicate with their small spacecraft using participating amateur radio ground stations around the globe 10. Data collection for this type of network allows several hours of data collection per day for any given spacecraft, as opposed to minutes per day with a single ground station.
Innovative Solutions in Space B.V. (ISIS) also offers turnkey ground station solutions, supporting CubeSats and small spacecraft in the UHF, VHF and S-band for amateur and non-amateur radio bands.
Spaceflight Networks is another established ground operations provider offering cost-effective solutions in development, launch, communications, and operations. They have partnered with a number of agencies and other small satellite companies, including Kratos/RT Logic, in order to provide powerful, low-cost hardware and services 11.
The Open System of Agile Ground Stations (OSAGS) supports high-frequency communications for small spacecraft. Owned by Espace, Inc., OSAGS is a low-cost network of three equatorial S-band ground stations located in Kwajalein, Cayenne, and Singapore, that are based on software defined radio12. The stations operate in S-band with a 2.025-2.0120 GHz uplink and 2.20-2.30 GHz downlink frequency. The agile system can support different spacecraft missions simultaneously and is readily available for any small spacecraft mission in need of low-cost ground segment support. Satellites are required to use dedicated software provided by Espace, Inc., and they must have the proper S-band capabilities to communicate with the system.
Government sponsored missions often use turnkey solutions offered by the Space Network (SN)13, Near Earth Network (NEN)14 and and Deep Space Network (DSN)15,
collectively known as Space Communications and Navigation (SCaN). Prior to May, 2018, the DSN offered the only existing TT&C service for beyond Earth orbit, but Analytical Graphics, Inc (AGI) has announced a commercial deep space radar tracking system. The Air Force Satellite Control Network (AFSCN) is even more tightly controlled than SCaN. However, the Air Force does make the services of the Joint Space Operations Center (JSpOC) available to the public, particularly for space situational awareness in the form of two-lined element sets (TLEs) for tracking satellites, and conjunction alerts for potential collisions. AGI has developed a similar system in the commercial sector called the Commercial Space Operations Center (ComSpOC).
Some companies can also provide specific individual components to users that want to assemble their own customized ground stations. For example, Helical Communication Technologies specializes in quadrifilar helical antennas, made of four helical filars or windings that support right and left hand circularly polarized signals. These antennas receive and transmit signals from the ground station to amateur radio satellites in LEO at frequencies between 300 and 3000 MHz, and are particularly useful when receiving small spacecraft signals shortly after launch without the need for tracking or positioning equipment and associated tracking software. Due to the nearly omni-directional pattern, the quadrifilar helical antenna provides good gain at low elevation.
KSAT and ISIS are also able to provide single antenna components that can interface with many different ground data systems. For example, Bring Your Own Device (BYOD) is a solution from KSAT which provides KSAT-rugged antennas that interface with a customer’s own back-end equipment.
Ground Data Systems Hardware and Software
Every ground station needs hardware and software components to operate and support spacecraft missions. There are a number of conceptual systems for the telemetry, tracking, and commanding of hundreds or thousands of small spacecraft. Emulation tools also play an important role for these types of missions and systems. Table 11-2 lists some companies that provide front-end and back-end hardware and software for ground stations.
|Table 11-2: Hardware and software for ground systems.|
|Product||Manufacturer||TRL Status||Type of Product|
|quantumGND||Kratos/RT Logic||9||quantumCMD: Command and Control (C2) software; qFEP: Front-End Processors for encryption of commands and decryption of telemetry; qRadio: digital IF front-ends and IP-Modem; T4: software framework|
|ISIS GSKit Ground Station||ISIS B. V.||9||UV Transceiver: contains the modem and the gain blocks; Rotator Controller: used to control the azimuth and elevation rotator|
|Soft FEP||AMERGINT||8+||Emulation ground systems software|
|Distributed Simulation & Test Environment (DSTE)||Celestia Satellite Test & Simulation||9||Hardware and software elements all operating within a single reference platform and environment|
|Gpredict||Alexandru Csete||9||Open source software that tracks satellites and provides orbit prediction in real-time. Radio and antenna rotator control for autonomous tracking.|
|GNU Radio||GNU Project||9||Free software development toolkit that provides signal processing blocks to implement software-define radios and signal processing systems|
|COSMOS||Ball Aerospace||9||Open source command and control system. Developed in 2006, and free as of 2015, COSMOS brings functionality that has previously been proprietary and expensive|
|SpaceCentre||Satellite and Airborne Radar Systems Laboratory||9||A web-based ground station application that enables effective mission planning and satellite operations|
QuantumGND is a turnkey ground data system solution offered by Kratos/RT Logic designed specifically for small spacecraft applications. It is a complete, turnkey small spacecraft ground data system package for Command and Control communication to Radio Frequency signal processing (C2-to-RF) that includes everything from the C2 system through the ground network, to the ground modem, giving a solution that is pre-integrated and easy-to-use. QuantumGND is comprised of quantumCMD for a small spacecraft command and control, qFEP for front-end processing, encryption and decryption, and qRADIO for network transport and RF signal processing. All these components are also available separately and independently for users who need only particular components for their customized ground data system.
For systems engineering and testing of a constellation of spacecraft, SoftFEP can emulate thousands of spacecraft in constellation with their ground networks. It dynamically exercises the constellation management, ground payload and TT&C software, and simulates the entire end-to-end, multi-node communication system. It has been used to model complex space-to-ground communication systems, and also to emulate thousands of data channels to test software applications that process data.
The Distributed Simulation & Test Environment (DSTE) is a family of standard products designed and developed by SSBV Space and Ground Systems to support simulation, assembly, integration and testing of spacecraft, subsystems and payloads. All the elements of DSTE are based on modular hardware and software architectures that use the latest technology to enable multi-purpose modules and components in a common, reconfigurable, spacecraft and instrument simulation and test environment.
Gpredict is a free application that offers fast and accurate real-time satellite tracking. It operates in tandem with the Ham Radio Control Libraries (hamlib), a standardized API to control any radio-oriented equipment through a computer interface. Gpredict is capable of providing information about future satellite passes along with autonomous tracking.
A possible alternative to using mission-specific ground stations altogether is to communicate with satellite phone data networks such as Iridium, Orbcomm and Globalstar. This section focuses on the state-of-the-art of alternative communication technologies for Ground Data Systems.
TechEdSat-1, a 1U CubeSat launched in 2012, investigated this alternative inter-satellite communication method. The spacecraft had Quake Global Q1000 and Q9602 modems onboard to test communications with both the Iridium and Orbcomm constellations16. Unfortunately, the spacecraft was forced to disable its modems before communications could occur due to a delay of the FCC license. In April 2013, another experiment including an Iridium modem flew as an additional payload attached to the outside of the Bell PhoneSat’s frame17. This experiment successfully communicated the spacecraft location to the Iridium constellation, which then sent the information to the mission team via email. The team saw improvements in data rate and signal quality as compared to communications with amateur radio ground stations. The experiment was also able to transmit ten hours of data to the Iridium constellation over a 24 hour period, which is a significant improvement over typical spacecraft-to-ground transmission durations for cubesats17.
Inter-satellite communication was tested again using TechEdSat-3p, a 3U CubeSat launched in 201318. After deployment, TechEdSat-3p successfully communicated with the Iridium satellite network using two redundant Quake Global Q9602 modems. TechEdSat-4, launched in 2014, built upon the success of TES-1, -2, and -3 and continues to demonstrate satellite-to-satellite communications along with a passive reentry device called the Exo-Brake 19. TechEdSat-5 and -6, which launched in 2016 and 2017 respectively, both feature improved hardware that continues to test the communications and Exo-Brake technology 20. While TES 7, 8, and 9 will continue the TES family line, they have yet to launch. They too will test improvements to previously flown technology, along with some additions, like a CubeSat Identity Tag (CUBIT) to help identify nanosatellites.
The Transformational Satellite Communications System (TSAT) funded by the USAF successfully tested a simplex Globalstar modem, the EyeStar, from NearSpaceLaunch. This test was repeated by the Globalstar Experiment and Risk Reduction Satellite (GEARRS), and GEARRS2 flights also successfully tested the EyeStar Duplex Globalstar modem21. LinkStar is another duplex radio being developed by sci_Zone that is still in the design phase and will also take advantage of the GlobalStar network. A NASA sounding rocket, the LCT2-b, tested a modem from LJT & Associates in August, 2008, as part of Sub-Orbital Aerodynamic Re-entry EXperiments (SOAREX-VI). The modem is intended to work with the Tracking and Data Relay Satellite System (TDRSS) 22. However, as the TDRSS system is administered by NASA, there might be regulatory complications for consumer spacecraft wishing to use it.
These missions are actively proving the value of using inter-satellite communications to relay data to the ground. Small spacecraft that use existing satellite phone constellations instead of ground stations may see potential cost savings and quality improvements.
On the Horizon
As the ground data systems and communication options for small spacecraft (particularly CubeSats) expand, engineers must consider the trade-off between data quality, data volume, and cost. In the past, several missions depended entirely on amateur radio ground stations to support spacecraft operation and communication, and the amateur radio community has proved to be invaluable to the CubeSat community. As mission complexity and data requirements increase, more projects are looking to non-amateur ground stations and other options like inter-satellite communications, laser optical communications, and phased array ground stations 23. These options, however, tend to present higher costs due to the need for associated radio frequency licenses and bespoke software specific to a given service provider. Further, the service itself may be priced based on data size or communication duration. Many factors can affect the cost, data quality and size of each communication method, and for some of these methods the factors are either only beginning to be understood in the context of small spacecraft operations, or they have yet to be encountered. The relationship between data quality, data size and cost for these communication methods must be studied over the coming years, as the various methods are analyzed by current and future small spacecraft missions.
Due to both the desire to speed up transmission of high-rate science data, and to the increasing demand for S-band and X-band telecommunications, the Ka-band is now considered the band of the future for NASA small spacecraft missions. Along with satellite hardware, BridgeSat Inc. is developing ground stations compatible with optical communications. They aim to create a worldwide network of stations that allow data downlink and uplink regardless of the optical terminal. They are planning a satellite-to-ground optical comm test for the near future that will demonstrate the feasibility of optical comms in consistently downlinking data from small satellites in LEO 23. Kratos is also looking at ways to minimize ground station costs, one of which is operating satellite constellations via a cloud. This could introduce significant cost savings by avoiding the maintenance of a traditional ground system24.
Planning & Scheduling and Data Management are two areas of ongoing research within the field of small spacecraft ground data systems software. The future will see an increasing number of small spacecraft missions involving not only single spacecraft, but swarms, constellations, and formations of spacecraft25. A distributed infrastructure of small spacecraft made up of dozens, if not hundreds, of units would allow low-cost, high-resolution Earth observation and science missions. However, the number of ground station networks that can accommodate constellations is restricted, as the scalability of mission operations is limited without significant automation. The number of operators typically scales linearly with the number of telemetry nodes required to monitor the spacecraft26. The Space Telecommunications, Astronomy and Radiation (STAR) laboratory from Massachusetts Institute of Technology presents a solution to scalability concerns regarding constellations. The Autonomous CubeSat Constellation Earth-observing Scheduling System (ACCESS) is designed to plan constellation operations using onboard and ground-based algorithms. This system would simplify data routing and offer better routing performance for inter-satellite data handling27.
Managing swarms of small spacecraft presents a unique cooperation challenge. In order to address the issue of scalable control of orbital dynamics, researchers at NASA Ames Research Center have introduced the Swarm Orbital Dynamics Advisor (SODA), a software tool that provides the orbital maneuvers needed to achieve the desired type of swarm relative motion 28. Ploschnitznig, McLaughling, and Falco propose that a constellation of hundreds of small spacecraft would require thousands of operators and thus an excessive operations budget, assuming a best case scenario. This number is determined by scaling up operations from a single small spacecraft, which requires roughly ten operators to ensure mission success (not including payload operators). In the CubeSat realm, they point out that conventional operations require an unrealistic commitment from the academic and amateur community. A novel solution to this legacy ground station approach is offered by Riverside Research, whereby a modification of existing cellular towers allows the integration of satellite communications, thus shifting the existing paradigm 29.
Moreover, to keep costs low and allow for the emergence of next-generation, distributed, small spacecraft platforms, it will become increasingly necessary for a spacecraft to perform certain operations autonomously in orbit or automatically from the ground. The challenges related to partially or fully autonomous operations and multi-mission operations centers for small spacecraft clusters are ongoing fields of research.
From the moment of launch, the only connection between the spacecraft and Earth is through the communication between the spacecraft and the ground data systems. The spacecraft sends scientific and engineering data through its antenna (or laser) back to Earth, and the ground data system receives that data, tracks the spacecraft, and commands the spacecraft.
Depending on the requirements and priorities of the user, different types of solutions to build and assemble a ground station are available in the market. If the user wants to focus more on the payload and the systems engineering of the spacecraft, some companies have pre-defined turnkey solutions, which provide full capability and support for the spacecraft ground communications. Other possible solutions are customizing the ground station with specific components (such as antennas, transceivers, modems and software) that can be provided by different manufacturers. The user can choose all the different pieces of hardware and software needed for this purpose, and have a customized ground station assembled. Finally, another valuable solution for small spacecraft to communicate with Earth is using an inter-satellite communications relay. Some CubeSat missions have already demonstrated these capabilities.
Whichever solution turns out to be the most reasonable and appropriate, the chosen ground system must provide cost-effective, accurate, and on-time space communications for the entire mission duration.