Operational Tests for Delay-Tolerant Network between the Moon and Earth Using the Korea Pathfinder Lunar Orbiter in Lunar Orbit

Abstract

The Korea Pathfinder Lunar Orbiter (KPLO) was launched on 5 August 2022, equipped on the SpaceX Falcon 9 launch vehicle. At present, the KPLO is effectively carrying out its scientific mission in lunar orbit. The KPLO serves as a cornerstone for the development and validation of Korean space science and deep space technology. Among its payloads is the DTNPL, enabling the first-ever test of delay-tolerant network (DTN) technology for satellites in lunar orbit. DTN technology represents a significant advancement in space communication, offering stable communication capabilities characterized by high delay tolerance, reliability, and asymmetric communication speeds—a necessity for existing satellite and space communication systems to evolve. In this paper, we briefly give an overview of the Korea Lunar Exploration Program (KLEP) and present scientific data gathered through the KPLO mission. Specifically, we focus on the operational tests for DTN-ION conducted for message and file transfer, as well as real-time video streaming, during the initial operations of the KPLO. Lastly, this study offers insights and lessons learned from KPLO DTNPL operations, with the goal of providing valuable guidance for future advancements in space communication.

1. Introduction

Unlike in past lunar exploration, many countries around the world recognize the Moon as an undiscovered area with infinite potential and are securing leadership in space exploration and development through lunar exploration [1,2,3,4,5,6]. The United States is pursuing the Artemis program, intended to land a human on the Moon again following the Apollo 11 mission, which first landed humans on the Moon in 1969 [7]. Not only countries advanced in space exploration such as Europe, China, Japan, and India but also private companies are participating in space exploration, specifically in lunar exploration [8]. These countries and companies are moving the area of space development from Earth to the Moon and Mars based on advanced space technology and are attaining more advanced space technology through space exploration [9,10].

South Korea has been operating a space exploration program since 2016, and its first-phase Korea Lunar Exploration Program (KLEP) ended in 2023. For the first step in this program, the Korea Pathfinder Lunar Orbiter (KPLO), also referred to as Danuri, was launched on 5 August 2022, carried on the SpaceX Falcon 9 launch vehicle. On 26 December 2022, the KPLO was successfully inserted into lunar orbit, and it is currently orbiting the Moon at an altitude of 100 ± 20 km above the lunar surface. It was promoted as a cooperative system in which the Korea Aerospace Research Institute (KARI) oversaw its system and main body and the Korean Deep Space Ground Station (KDGS), and domestic universities and research institutes supported payloads, deep space communications, and navigation technology along with National Aeronautics and Space Administration (NASA) in the United States [11]. The KPLO consists of a main body and six payloads measuring 1.82 m, 2.14 m, and 2.29 m in width, length, and height, respectively. The KPLO is carrying out its lunar exploration missions successfully and has sent invaluable scientific observations to Earth. Based on sufficient fuel capacity, the operation period was extended by two years from the end of 2023 to the end of 2025. Therefore, the goals of the first-phase KLEP to develop a test lunar orbiter based on domestic and international cooperation to secure and verify space exploration technologies were successfully achieved. South Korea now faces the second-phase KLEP, which aims to secure independent lunar exploration capabilities including successful landing a lander and rover on the Moon; it will operate for ten years from 2024 to 2033.

Among six payloads equipped on the KPLO, the delay-tolerant network (DTN) payload (DTNPL) was loaded to demonstrate space communications and perform operational tests for DTN technology from a satellite in lunar orbit to Earth. The space communication environment differs from the terrestrial communication environment in several key aspects, including intermittent connectivity, propagation delay, asymmetric data rates, and high error rates. These differences make it challenging to apply terrestrial internet protocols to space communications. The need for new communication protocols for space exploration has emerged as the scope of exploration expands beyond the Moon. DTN is an internet protocol designed for complete data transmission where communication delays occur frequently. Unlike terrestrial communication protocols such as the transmission control protocol/internet protocol (TCP/IP), which requires uninterrupted connectivity from the source to the destination for reliable data transmission, DTN prevents the loss of data by maintaining DTN nodes without discarding data packets thanks to store-and-forward capability. This allows data to be taken to the destination even in a situation where the route to the destination cannot be immediately established due to frequent interruptions; therefore, DTN technology has attracted attention for space internet protocols.

Some countries have already secured DTN technology through pilot projects. NASA’s Jet Propulsion Laboratory (JPL) successfully conducted its first DTN transmission test in October 2008 using the EPOXI spacecraft [12]. This DTN experiment, which was called the Deep Impact Network Experiment (DINET), transmitted 292 images from JPL nodes on the ground to the spacecraft, and the spacecraft relayed them back to the JPL nodes, confirming that DTN is a suitable protocol for space communications. In the following years, NASA tested DTN technology using the Earth Observing-1 (EO-1) satellite [13]. In particular, the test attracted attention as to whether the telemetry could be restored through the store-and-forward function. As a result of this function, telemetry was successfully received through DTN, and it was confirmed that the file was received normally even in a situation where 80% packet loss occurred due to an interrupted communication link in space. In 2012 and 2014, the European Space Agency (ESA) succeeded in controlling a Lego robot and the Eurobot in the European Space Control Centre (ESOC) from the International Space Station (ISS) through the DTN protocol [14].

South Korea is first to test the operation of DTN technology in space using satellites in lunar orbit through the KPLO mission. It ultimately aims to connect orbiters, landers, rovers, astronauts, and other equipment in space and on planets and to allow them to communicate with each other in the future. Therefore, the purpose of the DTNPL is to test the DTN-ION protocol with Licklider Transmission Protocol (LTP) and Bundle Streaming Service Protocol (BSSP) between the Moon and Earth for space communications, and the content for DTN-ION tests includes message transfer, file transfer, and real-time video streaming. Some operational tests were already performed in 2023. In addition, we conducted extensive tests to validate the inter-agency interface protocol on the ground, and we experienced numerous instances of trial and error along the way. Despite these instances of trial and error, we successfully carried out the actual mission without any major incidents. Finally, we provide insights and lessons based on these experiences, guiding the development of future deep space networks.

In this paper, we provide a brief introduction to the KLEP, outline the payloads aboard the KPLO, and present scientific research data acquired during the initial operations of the KPLO. The primary objective of this paper is to address the practical lessons learned from the KPLO’s DTNPL missions. As such, we delve into the DTN-ION test configuration, DTNPL hardware specifications, and operational test scenarios for the KPLO DTNPL mission. Lastly, we present the results of DTN-ION operational tests conducted since the KPLO’s launch and describe the invaluable lessons learned, highlighting the significance of DTN-ION protocols as a practical solution for space communications.

This paper is structured as follows. Section 2 presents the introduction of the first-phase KLEP, specifically detailing the six payloads on the KPLO and their scientific goals within the KLEP. In addition, research data provided by an individual payload are discussed. Section 3 explains the DTN protocol, detailing its specifications and describing the scenarios for DTN-ION operational tests. The test results are summarized in Section 4, and Section 5 discusses the lessons learned from the tests. Finally, Section 6 concludes this paper.

2. First-Phase Korea Lunar Exploration Program

The primary goals of the first-phase KLEP are to develop the KPLO and see its successful launch to and operation in lunar orbit. In addition, scientific instruments for lunar exploration were loaded on the KPLO to achieve individual scientific goals. In this section, we briefly provide scientific research data received from the KPLO and their invaluable insights for KLEP. The operational test for the DTNPL, which is a key topic of this paper, is presented in Section 3.

The KPLO project has outlined its mission objectives as follows: (1) it aims to acquire essential technologies for lunar exploration; (2) it seeks to investigate the lunar environment, which includes tasks such as generating a topographic map of the Moon to aid in selecting future landing sites, conducting a survey of lunar resources, and analyzing both the radiation and surface environments of the Moon; and (3) it aims to demonstrate and test space internet technology. Six payloads are part of the KPLO to achieve these goals, including the Lunar Terrain Imager (LUTI), polarimetric camera (PolCam), KPLO magnetometer (KMAG), KPLO gamma-ray spectrometer (KGRS), shadow camera (ShadowCam), and DTNPL, as shown in Figure 1. ShadowCam is supported by NASA, while the other five payloads were developed by Korean universities and research institutes, including KARI. The specifications, scientific goals, and supplier for individual payloads are summarized in

Table 1. Six payloads on KPLO.

Payload	Specifications	Scientific Goals	Supplier
LUTI	Two cameras (450–850 nm)		KARI
	Spatial resolution: 2.5 m/pixel	Produce observation images of the lunar surface.
	Observation width: 10 km	Explore candidate landing sites for future lunar lander.
	Mass: <15 kg
PolCam	Two identical cameras		KASI
	Spatial resolution: 70 m/pixel	Acquire polarimetric images of the lunar surface.
	Observation width: 35 km	Investigate the characteristics of lunar regolith.
	Mass: <3 kg
KMAG	Three triaxial fluxgate magnetometers	Measure the magnetic field of the lunar environment.	KHU
	Measurable range: ±100 nT	Investigate the properties of electromagnetic wave
	Resolution: <0.2 nT (10 Hz sampling rate)	near-Moon space.
	Mass: <3.5 kg
KGRS	Energy range:  30 keV to 12 MeV	Investigate the characteristics of lunar resources	KIGAM
	Energy resolution: < 5% @ 662 keV	including rare elements and minerals.
	Mass: <6.3 kg	Map the spatial distribution of the elements.
DTNPL	Processor: 32-bit LEON3	Demonstrate space internet communications	ETRI
	Operating system: RTEMS	between the Moon and Earth.
	Mass: <0.8 kg	Perform experimental test for DTN technology
	Power consumption: <5 W	(Bundle Protocol / Licklider Transmission Protocol).
ShadowCam	Spatial resolution: 1.7 m/pixel	Investigate the permanently shadowed regions.	ASU
	Mass: <10 kg	Map albedo patterns and interpret their nature.	NASA

.

2.1. Scientific Research Observations by KPLO

2.1.1. Lunar Terrain Imager: LUTI

The LUTI has taken high-spatial-resolution (less than 5 m) images of the lunar surface to explore candidate landing sites for a future lunar lander to land on the lunar surface in the second-phase KLEP [16]. Figure 2 is an image of the Drygalski crater taken by the KPLO LUTI, which is located near the lunar south pole and is not easily visible as it is located on the southern boundary of the Moon, which may not be observable from Earth. The diameter and depth of the crater is about 150 km and 4.6 km, respectively, and the central peak is approximately 2.5 km above the floor. Figure 2a is an entire image of Drygalski crater, and Figure 2b is an enlarged image of its central area. Ref. [17] investigated and suggested candidate landing sites for a future lunar lander based on three criteria: (1) the near side of the Moon, which faces Earth all the time and enables the lander to communication with Earth directly, (2) a region of scientific interest, and (3) flat ground where the lander can land safely. Because the specifications for the future Korean lunar lander have not been released yet, those criteria may not be appropriate to select a lander’s landing site. However, it could assist in the investigation for candidate landing sites by the LUTI and provide a guide for suitable criteria based on images from the KPLO.

2.1.2. Polarimetric Camera: PolCam

The PolCam will construct a lunar map of polarimetric parameters and titanium distribution with a polarization precision of less than 1% polarization within a phase angle range of $0°$ to 135° [19]. The PolCam utilizes three distinct wavelengths: a 320 nm wavelength designated for photometry, and 430 nm and 750 nm wavelengths allocated for polarimetry. The wavelength of 430 nm measures reflectance utilizing polarization filters of $0°$, $60°$, and $120°$ to capture both the degree and angle of linear polarization, while the wavelength of 750 nm performs this to ascertain the degree of linear polarization using polarization filters of $0°$ and $90°$. The reflectance ratio between 320 nm and 430 nm enables the construction of TiO₂ distribution maps. Hence, these specific wavelengths have been chosen for observation by the PolCam to facilitate comparative analysis with polarimetry-derived lunar soil maturity data [20,21]. In January 2023, the PolCam succeeded in observing polarization from lunar orbit for the first time in the world. By taking images at three wavelengths and various degrees of polarization, changes in surface particles and mineral components of invisible sizes were observed.

2.1.3. KPLO Magnetometer: KMAG

The KMAG was developed by Kyung Hee University and aims to delve into the magnetism of the Moon’s lithosphere and to evaluate the electromagnetic wave behaviors near the surface of the Moon [22,23]. KMAG comprises two components: the MAG unit and the fluxgate magnetometer control electronics (FCE) unit, as depicted in Figure 3. Within the MAG unit, there are three sets of identical three-axis fluxgate sensors housed in a 1.2 m carbon fiber-reinforced polymer boom including a hinge structure. The FCE unit, on the other hand, consists of four boards: a low-voltage power supply, an onboard computer, and separate analog and digital boards responsible for operating the fluxgate sensors. Since its launch on 5 August 2022, the KMAG has conducted a range of observations throughout the trans-lunar cruise phase. The KMAG has operated effectively, conducting observations of magnetic fields within the lunar crust and those induced in the lunar interior and monitoring various solar wind events. In [23], calibration and offset processes were carried out during the TLC phase, and the reliability of the KMAG’s lunar magnetic field observations were confirmed through comparisons with surface vector mapping data.

2.1.4. KPLO Gamma-Ray Spectrometer: KGRS

The KGRS is crucial for producing a comprehensive map of the Moon’s surface, highlighting elements such as water and helium-3. The natural background radioactive elements like uranium and thorium emit gamma rays on the lunar surface. In addition, when solar wind collides with the lunar surface, nuclear reactions occur, resulting in different gamma rays emitted by each element. The Korea Institute of Geoscience and Mineral Resources (KIGAM) has developed a precise gamma-ray spectrometer capable of measuring from low energy, 30 keV, to high energy, 12 MeV, in the world’s lightest weight of 6 kg to observe these gamma rays and to identify new elements on the Moon [25].

The top priority of the KGRS mission is to create a map of water on the Moon. While indirect evidence suggests that water (H₂O) exists in concentrations of over 10% in the lunar polar regions, there has been no direct observation using a gamma-ray spectrometer [26]. The KIGAM plans to identify regions where water is expected by observing hydrogen gamma rays on the Moon and confirming if the intensity of neutron radiation weakens, indicating the presence of water. Based on hydrogen distribution, a highly reliable water map can be created. Furthermore, the distribution of water on the Moon also becomes a significant criterion for selecting candidate landing sites for a future lunar lander. The existence of helium-3 (He-3), a next-generation energy source, and its map on the Moon are also gaining attention. Helium-3 exists in a gaseous state within ilmenite (FeTiO₃) and has been estimated based on the distribution of titanium on the lunar surface and in lunar rock samples [27]. By utilizing the gamma-ray spectrometer, precise measurements of iron, titanium, and other elements enable the development of a mineral map of ilmenite, enhancing overall accuracy. With estimates indicating the potential presence of over a million tons of helium-3 on the Moon, this research serves as a basis for future explorations of lunar resources.

2.1.5. Shadow Camera: ShadowCam

ShadowCam aims to observe water ice by collecting and analyzing images of permanently shadowed regions that receive little sunlight due to the revolution of the Moon with little inclination in its orbit. It was developed by Arizona State University and deployed aboard the KPLO under a collaborative agreement between NASA and KARI. Arizona State University also developed the narrow-angle camera mounted on the U.S. Lunar Reconnaissance Orbiter, which is currently operating in lunar orbit. ShadowCam surpasses the narrow-angle camera in terms of its sensitivity, which is 200 times higher, which accounts for the faintness of images taken by ShadowCam [28]. Sverdrup crater, whose enlarged portion is shown in Figure 4, is near the south pole of the Moon and has a diameter of about 35 km. Shackleton crater is located to the south of Sverdrup crater, and the boundary between the two craters is one of the candidate landing sites for the Artemis II, the plan for which is to land the first astronauts near the lunar south pole. In the center of Sverdrup crater, a large number of small craters were discovered through ShadowCam images.

3. DTN Payload on KPLO

The last equipment aboard the KPLO is the space internet payload. Efforts to apply terrestrial internet technology (e.g., TCP/IP) to space communication systems have been made since the 2000s. However, the space communication environment has several differences from the terrestrial communication environment such as an intermittent connectivity, a propagation delay, asymmetric data rates, and high error rates, which makes it difficult to utilize terrestrial internet protocols for space communications [29]. In the case of deep space communications, the propagation delay between Earth and the Moon is about 1.5 s, and it takes about tens of min to reach Mars when terrestrial internet protocols are used for space communications. To overcome these issues, DTN has been receiving attention for space communications; as a result, the DTNPL was selected as the space internet payload for the KPLO. The DTNPL was developed in accordance with the DTN specifications of the Consultative Committee for Space Data Systems (CCSDS) and will be directly utilized in communication between orbiters, landers, and rovers in future space exploration.

All payloads aboard the KPLO are fully developed, and their operational concepts are finalized by each payload’s supplier for specific purposes as described in Section 2.1. While the scientific observations of each payload may seem distinct, the transmission of these observations to Earth is crucial for lunar exploration. The DTNPL on future spacecraft must be meticulously designed, taking into account hardware limitations. As a result, the delivery of scientific observations by KPLO missions provides insights into the specifications and management of the DTNPL for the second-phase KLEP.

3.1. DTN Protocol

The concept of DTN originates from a generalization of requirements identified for interplanetary networking, specifically in order to make a Mars orbiter serve as a relay node for data transmission from a lander on Mars [30]. The DTN architecture introduces an overlay above the transport layer. The key benefit is that a delay and a disruption can be managed at each DTN node within the overlay along a path between a source and a destination. DTN nodes along a path are capable of storing application data before forwarding it to the next node. In addition, any necessary retransmissions in an automatic repeat request scheme may originate from an intermediate node, eliminating the need for end-to-end connectivity between a source and a destination. Consequently, protocols implementing DTN architecture offer a significant advantage as they do not require the continuous end-to-end connectivity demanded by TCP/IP or other protocols for reliable transmission.

The Bundle Protocol (BP) is designed for DTN implementation and stands as the most widely adopted DTN protocol. At its core, the basic unit of data is a bundle, which is a message containing application layer protocol data units, source and destination identifiers, and additional data essential for end-to-end delivery. BP can interact with different lower-layer protocols, typically transport protocols, via convergence layer adapters (CLAs), as illustrated in Figure 5. Various CLAs have been established, including those for TCP, User Datagram Protocol (UDP), and LTP. With BP, each DTN node along a path may select the most appropriate CLA for the next forwarding operation.

DTN architecture is well suited for serving as an overlay on top of a heterogeneous network comprising various segments, including wireless sensor networks, wired internet, wireless local area networks, satellite links, space internet, and so on. By deploying a BP agent on endpoints and nodes at the borders of homogeneous segments, the end-to-end path can be divided into multiple DTN hops. At each DTN hop, different CLAs can be utilized. In cases where the same CLA is used for a bundle on both inbound and outbound hops, which is a common scenario, different variants of TCP/IP can be employed. As a result, the TCP/IP protocol may not be entirely replaced; however, its role will change.

Another significant difference between DTN and TCP/IP protocols relates to information storage. In traditional networks where continuous connectivity between source and destination should be guaranteed, routers may need a temporary storage of information, but the information is stored only at an end node. However, in challenged networks where long round-trip times, channel disruptions, and the absence of end-to-end connectivity exist, the traditional way to retrieve the information directly from the source is not practical. Therefore, a DTN protocol that employs long-term storage of information at an intermediate DTN node, which is accomplished by a store-and-forward function, can be more robust against channel disruptions and end-to-end disconnectivity. These characteristics of DTN make it suitable to be applied to space communications.

3.2. Interplanetary Overlay Network

The interplanetary overlay network (ION) is an implementation of DTN functioning as an overlay network. This eliminates the need for a continuous end-to-end path from the source to the destination and allows DTN nodes to store and forward data as the next hops are accessible. The fundamental DTN protocols such as the BP, LTP, and bundle security protocols are key to the ION suite. The DTN nodes only need to implement these key protocols; however, establishing a network requires a comprehensive suite of tools and applications for testing, operating, and maintaining the network. The ION suite can provide these resources, thereby demonstrating the adaptability of DTN protocols for future interplanetary networking [31]. In 2008, NASA JPL successfully tested the DTN protocol with the DINET experiment on the EPOXI spacecraft, and it was the first test and implementation of the ION. A payload utilizing the new version of ION was released in May 2009, and a new gateway based on DTN2 was distributed by NASA Marshall Space Flight Center (MSFC) in 2012. Currently, ION components are incorporated into the deep space network, near-space network, and the MSFC’s TReK Toolkit for ISS payload developers. The ION open-source software developed by NASA is available for various operating systems such as Linux, Solaris, Windows, and macOS from both SourceForge [32] and GitHub [33].

3.3. Architecture for KPLO DTNPL Tests

The architecture for the KPLO DTNPL mission is configured as shown in Figure 6 to test the DTN protocol between the DTNPL on the KPLO and the ground station on Earth to demonstrate that DTN is a perfect solution for space communications. The DTNPL is deployed on the KPLO flying along a lunar orbit in space, and its hardware specifications are summarized in

Table 2. DTNPL hardware specifications.

Item	Specification	Image
Processor	32-bit LEON3
Computing power	49.3 MIPS
OS	RTEMS 4.11
Memory	MRAM & NAND
Power consumption	<5 W
Mass	0.8 kg
Size	120 × 170 × 39 mm

. The other nodes are located on Earth. The DTN Control Center (DCC) transmits a command (CMD) to the DTNPL and receives telemetry or mission data from the DTNPL, while the KARI DTN node (KDN), the Korea Mission Operation Center (KMOC) in the KDGS, and the NASA Deep Space Network (DSN) DTN node relay data between the DTNPL and DCC. The lander communication model (LCM) and the rover communication model (RCM) are emulations of a lander and rover, respectively, because a Korean lander and rover have not been delivered to the Moon and will be launched in the second-phase KLEP. The communication between LCM and RCM utilizes a Proximity-1 protocol, which is a short-range wireless communication protocol for space exploration established by the CCSDS [34]. The Electronics and Telecommunications Research Institute (ETRI) developed the Proximity-1 modem for ground testing between an LCM and RCM, which is expected to be in charge of communication between the lander and KPLO in future lunar exploration.

The KPLO utilizes both the NASA DSN and Korea Deep Space Antenna (KDSA) to test DTN protocols for space communications. Given the utilization of both the DSN and KDSA by the KPLO, extensive collaboration has occurred between the KARI and NASA DSN teams, starting from the early stages of KPLO ground system design and continuing through the establishment of operational concepts to real-time operations. The NASA DSN is located in California’s Goldstone Deep Space Communications Complex in the United States, Madrid in Spain, and Canberra in Australia, dedicated to supporting interplanetary missions [35].

3.4. Operational Test Methodologies and Scenarios for DTNPL

Several studies have predicted the performance of DTN in the space environment using a PC-based testbed [36,37,38,39]. However, practical and operational tests for DTN utilizing satellites in lunar orbit have been limited due to constraints in development budgets and time. Although DTN performance has been evaluated in space through various pilot projects [13], these tests primarily focused on verifying DTN performance for telecommand (TC) and telemetry (TM) purposes. Experiments involving data transmission critical for actual lunar exploration or broader space missions have not yet been conducted. This study was initially conducted to test the use of DTN in an actual space environment by utilizing the onboard DTN node, which is a key component of the KPLO project.

The DTNPL aboard the KPLO was implemented with the ION protocol, developed by NASA, and incorporated the CCSDS international standard protocols LTP, CFDP, and BSSP [40,41,42]. LTP is a satellite communication protocol designed to address the limitations of terrestrial network protocols such as TCP/IP and UDP, which are unsuitable for the space environment. It facilitates stable communication even under challenging conditions, such as sudden communication disconnections and high latency. The CCSDS file delivery protocol (CFDP) is used for transmitting files and is designed to handle metadata, file data, and end-of-file data, making it effective for the transmission of large volumes of data in space. BSSP is employed for video transmission, transmitting DTN bundles to the next node in a streaming manner, making it appropriate for real-time video transmission.

The primary objective of the DTNPL is to test the operation of DTN-ION with LTP and BSSP for space communications between the Moon and Earth. The maximum distance between the ground station and KPLO is 360,000 km, with S-band uplink transmission rates of 1 kbps and X-band downlink rates of 8.5 Mbps. In this setup, the KPLO DTNPL receives commands from the DCC and transmits messages, files, or video streaming data back to the DCC through the DSN DTN node and KDN. The process starts with the ETRI DCC node generating bundle data, which are sent to the KDN node at KMOC via UDP communication. Acting as a relay, the KDN node collects these bundles, reorganizes them, and forwards them to the Real Operation System (ROS) over UDP. The ROS converts the bundles into CCSDS TC format and transmits them to the DSN antenna via UDP. The Deep Space Operations Center (DSOC) then sends the CCSDS TC data received from the ROS to the KPLO DTNPL via the S-band communication link through the DSN antenna. For incoming signals, the KPLO DTNPL generates telemetry data as bundles, converts them to CCSDS TM format using the KPLO onboard computer, and transmits them to the ground station via the X-band communication link. The DSOC decompresses the CCSDS TM data to reconstruct the bundles and forwards them to the DSN DTN node. The DSN DTN node reassembles these bundles and sends them to the KDN over the TCP/IP network. Finally, the KDN delivers the received bundle data to the DCC. The test flow diagrams for both the forward and return paths are depicted in Figure 7. The communication protocols utilized at each node are detailed in Figure 8. Additionally, Table 3 provides a summary of the roles and responsibilities of each node within the DTN operational test scenarios. The contents of the DTN-ION operational tests include the following:

• Message transfer, which comprises uplink and downlink bidirectional message transmission and reception between the DCC and the KPLO DTNPL utilizing BP and LTP.
• File transfer based on CFDP, which is the downlink file transmission between the KPLO DTNPL and the DCC when the KPLO DTNPL receives a mission command from the DCC.
• Real-time video streaming based on BSSP from the KPLO DTNPL to the DCC when the KPLO receives a mission command from the DCC.

Figure 7. The flow diagram for KPLO DTN operational tests.

Figure 7. The flow diagram for KPLO DTN operational tests.

Figure 8. Communication protocol architecture for DTNPL operational tests.

Figure 8. Communication protocol architecture for DTNPL operational tests.

Table 3. Role of nodes in DTN operational test scenarios.

Content	Source	Relay	Antenna	Destination	Note
Message transfer	DCC	KDN, DTNPL	DSN	DCC	DTNPL is a relay node
File transfer	DTNPL	DSN node, KDN		DCC
Real-time video streaming	DTNPL	DSN node, KDN		DCC

Table 3. Role of nodes in DTN operational test scenarios.

Content	Source	Relay	Antenna	Destination	Note
Message transfer	DCC	KDN, DTNPL	DSN	DCC	DTNPL is a relay node
File transfer	DTNPL	DSN node, KDN		DCC
Real-time video streaming	DTNPL	DSN node, KDN		DCC

For message transfer, the process begins with the DCC on the ground, where the message is converted into a bundle and sent to a relay node at the KMOC ground station. At the ground station, this bundle is converted into CCSDS TC format using the ROS and transmitted to the KPLO via the S-band communication link. Upon receipt, the KPLO extracts the data from the CCSDS TC format back into a bundle and forwards them to the DTNPL. The DTNPL then converts the bundle into CCSDS TM format for transmission back to the ground station via the X-band communication link. The ground station processes the received data, extracts the DTN bundle, and sends them back to the DCC through the KMOC ground station relay node. Finally, the DCC compares the transmitted and received messages to confirm the success of the transmission.

The CFDP protocol was utilized to transmit large volumes of image files. The process begins with a command from the DCC on the ground, which is converted into bundles and transmitted to the ground station. At the ground station, these bundles are transformed into CCSDS TC format and sent to the KPLO via the S-band communication link. Upon receipt, the KPLO converts the data from CCSDS TC format back into bundles and forwards them to the DTNPL. The DTNPL then assembles the image files, stored in its memory, into bundles. These bundles are converted into CCSDS TM format by the DTNPL and transmitted back to the ground station via the X-band communication link. At the ground station, the ROS extracts the bundles and forwards the image files to the DCC through the relay node. The DCC then verifies the success of the image transmission based on the received files.

The DTNPL is responsible for managing video streaming. Upon receiving a command from the ground, it processes the stored video file by dividing it into bundles for streaming service. These bundles are sent to the DCC through the relay node using the ROS, which is designed for handling DTN missions at the ground station, over the X-band communication link. At the DCC, the streaming data are played back using a VLC player. The ground station networks are optimized for high data-transmission rates to effectively handle the real-time video streaming provided by the KPLO.

4. Operational Test Results of DTNPL

4.1. ION Software Configuration for DTN Nodes

Table 4. ION software configuration.

Location	Software	Version
DSN	TTC version	TTC V2.2 Build 9
	ION node name	DSN relay
	ION version	3.7.0
	ION node number	ipn:40
	Adaptation layer version(s)
	IP address for opsdtn 1a/b	137.xxx.xx.xxx/137.xxx.xx.xx
	IP address for opsdtn 2a/b	137.xxx.xxx/xxx/137.xxx.xxx.xxx
KMOC	ION node name	KDN
	ION version	3.6.1
	ION node number	ipn:31
	IP address	20.xx.xx.xx
ETRI	ION node name	DCC
	ION version	3.6.1
	ION node number	ipn:20
	IP address	10.xx.xx.xx
KPLO	ION node name	DTNPL
	ION version	3.6.1
	ION node number	ipn:19
KMOC	Node name	ROS
	IP address	20.xx.xx.xx/xx (backup)

represents the software configuration for ION open source, which was installed on the nodes for tests. The ION node number is a DTN node identifier, and ipn scheme [43] is used for DTN nodes such as DSN DTN node, KDN, DCC, and DTNPL. In addition, IP addresses for DSN DTN node, KDN, and DCC are also assigned for terrestrial networks; while ROS in KMOC has an IP address, it does not have an ION node number because it does not work as a DTN node.

4.2. Operational Test Procedure

Due to limited contact time between the KPLO DTNPL and the ground station antenna, the KPLO DTN test procedures were divided into four sections to cover all test scenarios. Additionally, the KPLO could not support long contact periods because it rotates around the Moon.

Table 5. Operational test procedures of KPLO DTNPL.

Step	Task	Successful Criterion
	Start ION all nodes—initialization steps
ION connection procedure between DTN nodes
1-1	{KDN1} bpsource ipn:40.4 “Any message here”.	Text message transfer between KDN1 and DSN opsdtn1a.
1-2	{DSN opsdtn1a} bpsink ipn:40.4.
	Type message on KDN1 and DSN opsdtn1a.
2-1	{DSN opsdtn1a} bpsource ipn:31.3 “Any message here”.	Text message transfer between KDN1 and DSN opsdtn1a.
2-2	{KDN1} bpsink ipn:31.3.
	Type message on KDN1 and DSN opsdtn1a.
3-1	{KDN1} bpsource ipn:20.3 “Any message here”.	Text message transfer between KDN1 and DCC.
3-2	{DCC} bpsink ipn:20.3.
	Type message on KDN1 and DSN opsdtn1a.
4-1	{DCC} bpsource ipn:31.3 “Any message here”.	Text message transfer between KDN1 and DCC.
4-2	{KDN1} bpsink ipn:31.3.
	Type message on KDN1 and DSN opsdtn1a.
	First DTN test at 40 min.
5	Start to run the XTXU, DTNPL APS.	KPLO XTXU, PDHU, SMBU operation procedure to be ready to DTN test.
Message transfer procedure
6-1	{DCC} bpsink ipn:20.3.	Display the sdakwer characters on the DCC.
6-2	{DCC} bpsource ipn:19.3 “#bpsource ipn:20.3 sdakwer”.
	Observe and record log file for each node.
File transfer procedure
7-1	{DCC} bpsource ipn:19.3 “#bpcp/ion/etri_spring.jpg 20:etri_spring.jpg”.	Display ETRI spring image on the DCC.
7-2	Check the downloaded file and checksum value with original data.
Train video streaming procedure
8-1	{DCC} bsscounter 90000 bssDB ./ ipn:20.99.	Display train stream data on the DCC.
8-2	{DCC} bpsource ipn:19.3 “#bssdriver ipn:19.99 ipn:20,99 90000 train.mp4”.
	Observe and record DTNPL receipt of video over BSSP.
9	Stop to run the XTXU, DTNPL APS.
10	Keep waiting on 1 hour.
	Second DTN test at 45 min.
11	Start to Run the XTXU, DTNPL APS.	KPLO XTXU, PDHU, SMBU operation procedure to be ready to DTN test.
Message transfer procedure
12-1	{DCC} bpsink ipn:20.3.	Display the sdakwer characters on the DCC.
12-2	{DCC} bpsource ipn:19.3 “#bpsource ipn:20.3 sdakwer”.
	Observe and record log file for each node.
File transfer procedure
13	{DCC} bpsource ipn:19.3 “#bpcp/ion/etri_winter.jpg 20:etri_winter.jpg”.	Display ETRI winter image on the DCC.
	Check the downloaded file and checksum value with original data.
K-pop video streaming procedure
14	{DCC} bpsource ipn:19.3 “#bssdriver ipn:19.99 ipn:20,99 90000 bts_dynamite_170K.mp4”.	Display K-pop streaming data on the DCC.
	{DCC} bpsource ipn:19.3 “#bssdriver ipn:19.99 ipn:20,99 90000 train.mp4”.
	Observe and record DTNPL receipt of video over BSSP.
15	Stop to run the XTXU, DTNPL APS.
16	Keep waiting for 1 hour.
	Third DTN test at 30 min.
17	Start to Run the XTXU, DTNPL APS.	KPLO XTXU, PDHU, SMBU operation procedure to be ready to DTN test.
Link interruption test procedure
18-1	Simulate outage during DTN LTP data flow.	Observe DTNPL attempt retransmission of ETRI spring image.
18-2	DSOT terminates the STOT feed to the TIS to induce X-band telemetry disruption.
18-3	{DCC bpsource ipn:19.3 “#bpcp/ion/etri_spring.jpg 20:etri_spring.jpg”.	Display ETRI spring image on the DCC after the reconnection.
18-4	Wait 1 min, then DSOT reestablish connection query.
	Check the downloaded file and checksum value with original data.
19	Stop to run the XTXU, DTNPL APS.

outlines the steps conducted for each section of the KPLO DTN-ION operational test procedures.

The first section, steps 1-1 to 4-2, requires all nodes on the ground to check the ION status. To initiate the DTN-ION protocol, nodes that are connected to each other should be activated to verify the ION protocol using the bpsource and bpsink commands.

The second section details the procedures for message and file transfer, as well as the transmission of a low-size train video stream from the KPLO DTNPL. First, the source node DCC issues the bpsink command to transmit the message then types the message into the bpsource command. Finally, the DCC checks the return message from the KPLO DTNPL (steps 6-1 to 6-2). Second, the DCC sends the bpsourceipn: 19.3 “#bpcp/ion/etri_spring.jpg20: etri_spring .jpg” command to transmit the file. The DCC then checks and displays the image received from the KPLO DTNPL (steps 7-1 to 7-2). Lastly, to transmit the low-size train video stream for the real-time BSSP test, the DCC issues the bsscounter 9000 bssDB./ ipn:20.99 command and types bpsourceipn: 19.3 “#bssdriver.ipn: 19.99 ipn:20.99 90000 train.mp4”. The result can be verified at the DCC (steps 8-1 to 8-2).

The third section of the procedure, steps 12-1 to 14, explains message and file transfer, as well as the transmission of a large-size video stream. The process for transmitting messages, files, and a video stream follows the same commands as outlined in the second section. To transmit the K-pop music video “BTS Dynamite” for the real-time BSSP test, the DCC issues the bpsource command. Subsequently, the KPLO DTNPL transmits the K-pop music video “BTS Dynamite”, which is stored onboard, to the ground station via the X-band link whenever it receives the command from the DCC.

The last section of this procedure, steps 18-1 to 18-4, considers the effect of link interruption, which frequently occurs in the deep space network, to validate the custody functionality of the ION protocol. In this test, the operator intentionally generates the link interruption due to onboard hardware limitations. The KPLO DTNPL should be able to reestablish the link after 1 min and subsequently transmit data to the ground station.

4.3. Operational Test Results

4.3.1. Message Transfer

The operational tests for the KPLO DTNPL were conducted on 29 March 2023 and 20 July 2023. Figure 9 illustrates the results of the message transfer test, where the DCC serves as both the source and destination nodes, with the KDN and DTNPL acting as relay nodes. Two messages were tested: in Korean, “five by five” and “It is going well. Wait, moon”. The DCC on the ground starts by converting the message into a bundle and then sends it to a relay node at the KMOC ground station. At the ground station, this bundle is converted into CCSDS TC format using the ROS and transmitted to the KPLO via the S-band communication link. Upon receiving a command, the KPLO converts the CCSDS TC format data back into a bundle and forwards the bundle to the DTNPL. The DTNPL then reformats this bundle into CCSDS TM format and sends it back to the ground station using the X-band communication link. Once the ground station receives the data, it extracts the DTN bundle and sends it back to the DCC through the KMOC ground station relay node. Messages shown in yellow boxes represent those initially sent from the DCC to the DTNPL, while messages in green boxes denote those successfully received by the DCC from the DTNPL.

4.3.2. File Transfer

Figure 10a,b show the received files, which are spring and winter landscapes, at the DCC from the DTNPL. Whenever the DTNPL receives a command from the DCC, the files stored in the DTNPL are transferred to the DCC via X-band link, and the DSN DTN node and KDN work as a relay. Figure 10c shows the packet rates of files incoming to and outgoing from the KDN. The blue line represents the packet rate of the file being delivered from the DSN DTN node to the KDN, while the red line indicates the packet rate of the file being transferred from KDN to DCC. Each peak indicates the volume of packet data, with the start time of each line closely matching the times when the bundles are received from the DSN DTN node and when they are forwarded to the DCC. This synchronization enables the KDN to process the transmission concurrently with data reception, thereby effectively reducing delay time. The packet transmission rate from the DSN DTN node to the KDN is higher than the rate from the KDN to the DCC, reflecting the different network characteristics between the nodes.

According to the internal and external communication configuration of the KDGS, the KMOC is connected to the NASA DSN TC/TM server through a dedicated line, while the tracking and DSN SPS portal servers are connected via a VPN internet connection. ETRI’s DCC and LCM are also connected to the VPN internet network, through which bundle data are transmitted. The KDGS external interface developed in the KPLO program was connected to the NASA DSN, NASA Johnson Space Center (JSC), and domestic participating organizations including ETRI. It was agreed that during the KPLO’s lunar orbit missions, NASA DSN antennas (California’s Goldstone, Canberra in Australia, Madrid in Spain) would be used in accordance with international agreements. Additionally, Australia’s Canberra DSN antenna, being on the same latitude as South Korea, would serve as a backup for KDSA. Therefore, KDGS was implemented with both a dedicated line and an internet network to ensure stable connectivity to the NASA JPL DSOC, while it was connected to ETRI and other domestic participating organizations via an internet network.The KDGS automatically selects either the dedicated line or an internet network to establish connectivity with the NASA DSN. However, domestic participating organizations are connected via the VPN internet network. During times of high user activity, network traffic increases, resulting in a lower packet transmission rate compared to the dedicated line. In addition, bundle data are transmitted from the DSN DTN node to the KDN using TCP/IP, with 1448 bytes per packet, and from KDN to DCC using UDP, with 1028 bytes per packet. Therefore, bit loss frequently occurred in the VPN internet network during the tests, causing the amount of data transmitted using UDP to be received at a lower rate than the amount of data transmitted using TCP/IP. For this reason, when a 4 MB image is transmitted using the BP, the number of TCP/IP packets is smaller than that of UDP, resulting in a higher transmission rate. This analysis corresponds with the results depicted in Figure 10c.

The packet transmission delay occurring after 310 s in Figure 10c is estimated to be the result of processing time involved in creating or splitting bundles within the KDN. Typically, despite these delays, it is anticipated that the KDN has effectively relayed the received packet to the DCC as long as the number of peaks is correct. In addition, the increase in packet rate observed between 235 and 245 s is attributed to network characteristics. The KDN is situated within the KMOC and is connected to an external node via a VPN internet network rather than a dedicated line. Consequently, variations in network traffic conditions may lead to significant fluctuations in packet rates, whether they increase or decrease abruptly.

4.3.3. Real-time Video Streaming

Figure 11 depicts the result of real-time video streaming from the DTNPL to the DCC. The K-pop music video “BTS Dynamite” is stored in the DTNPL, and the DTNPL initiates the transmission of the music video to the DCC upon receiving a command from the DCC. The ION offers bpdriver and bpcounter to support bundle stream services. The bpdriver generates a designated number of bundle streams at a specified bundle size. Figure 11a depicts the captured image of the BTS music video during its reception in the test. As the aim of the KPLO DTN-ION test is to transmit a video in real time, it is crucial that the transmission and reception rates align to facilitate seamless playback of the video stream at the DCC. This ensures that the KDN, acting as the relay node, is effectively designed and exhibits real-time transmission performance without delay. Therefore, Figure 11b demonstrates that the reception rate from the DSN DTN node and the transmission rate to the DCC are perfectly matched, indicating that the KDN is well designed for real-time transmission.

4.3.4. File Transfer under Link Interruption

Figure 12 shows the test result of a link interruption between the DSN DTN node and DTNPL during the transmission of the spring landscape image shown in Figure 10a. This test checks whether the ION protocol continuously attempts to send bundles even when the communication link is broken and ensures that the bundle is transmitted stably once the communication link is reestablished. The X-band transmitter unit mounted on the KPLO experiences performance issues when used continuously for more than 40 min due to heat generation problems, which critically impact KPLO mission operations. As a result, operational tests were limited to 30 min. These conditions make it impossible to test sending a bundle using the ION protocol; while transmitting a bundle from the KPLO, the communication link is cut off when the KPLO moves behind the Moon, and then the link is reestablished when the KPLO returns in front of the Moon. However, this process cannot be completed within 30 min. Therefore, the communication link had to be intentionally disconnected from the NASA DSN to create a space environment with the same conditions. On the ground, the bundle command is sent to the KPLO DTNPL via KDN, and the image bundle data are sent to the DCC via KDN through the DSN DTN node. This setup allows for monitoring the link disruption situation in KDN.

Figure 12a depicts the normal reception of an image when there is no communication link interruption. The green peak signal represents the signal received at the KDN node, indicating successful entry of the bundle command sent from the DCC to the KPLO DTNPL. Subsequently, the KPLO DTNPL sends this signal and starts transmitting image data to the DSN DTN node. The blue signal illustrates the KDN receiving bundle data of the image from the DSN DTN node. The absence of any time difference between the starting points of the green and blue signals indicates that the test was conducted under communication conditions without any interruptions in the communication link.

Figure 12b,c illustrate the results when a communication link interruption occurs for 60 s and 720 s, respectively. As previously explained, after the DCC sends a bundle command and receives confirmation from the DTNPL of successful receipt, the NASA DSN intentionally disconnects the communication link. Since the NASA DSN antenna cannot receive a signal, the DTNPL continuously attempts to transmit the bundle to the DSN DTN node. In Figure 12b,c, upon the communication link being restored in the NASA DSN, it can be observed that DTNPL begins transmitting the image bundle to the DSN DTN node after 60 s and 720 s, respectively. Subsequently, the image data are received normally at the DCC through the KDN.

5. Lessons Learned

5.1. Ground Station Setup to Support DTN-ION Functionality

The KPLO DTN tests were conducted using operational scenarios involving a DTN-ION node deployed in lunar orbit. The mission plan involved launching KPLO and undertaking a two-month journey to the Moon along the BLT-WBS trajectory. This approach minimized fuel consumption for lunar orbit insertion and ensured sufficient fuel reserves throughout the lunar mission. At distances of up to 1,250,000 km from Earth and during transit into lunar orbit, the KPLO DTNPL conducted tests on the performance of LTP, CFDP, and BSSP under the ION protocol. These tests utilized DTN nodes developed through collaborative efforts between ground stations as part of KARI-NASA international cooperation. This mission marks the first instance of conducting DTN-ION performance tests through actual installation in lunar orbit.

The KPLO DTNPL operating procedure was developed collaboratively by KARI, NASA JPL, and ETRI. Through various pretests on the ground, the KPLO DTNPL tests in space were successfully executed. During the tests, message transfer, image file transfer, and real-time video streaming were performed while maintaining communication between the ground station and the KPLO DTNPL. For message transfer, commands were sent from the DCC operator to the KPLO DTNPL using ION LTP. The message was then verified at the DCC upon receipt. To transmit images, the ground node used ION LTP to command the KPLO DTNPL, which then transmitted internally stored images to the DCC using CFDP. The DCC verified the received data and displayed the images. For real-time video streaming, the KPLO DTNPL stored two videos: “train.mp4” and the K-pop music video “BTS Dynamite”. Upon command from the DCC, the KPLO DTNPL used BSSP to stream the videos to the ground. Each ground relay node maintained a consistent real-time transmission rate and forwarded the stream to the DCC, successfully playing the video through real-time streaming.

This experiment confirmed the feasibility of real-time video streaming using the DTN-ION protocol. However, to maintain continuous real-time video streaming with the DTN-ION protocol in the future, other subsystems within the ground stations must be modified and redeveloped to support real-time data transmission. Existing deep space ground stations typically do not allow real-time data transmission and are accustomed to receiving data over extended periods. Consequently, development and testing are needed for NASA DSN and KARI KMOC ground station equipment to establish a real-time data transmission structure.

5.2. Heterogeneous Network Interfaces to Minimize Delay

Each node used in the KPLO DTN test mission utilized the ION protocol. There are two relay nodes in the ground segment: the KDN and DSN DTN node. The KDN is designed to be installed within the KARI KMOC and interfaces with three nodes: DCC, DSN DTN node, and KMOC ROS. It was developed to be compatible with each node and network protocol. Therefore, the KDN employed the TCP/IP and UDP methods provided by ION to connect with the DCC and KMOC ROS via UDP and with the DSN DTN node via both TCP/IP and UDP.

The DSN DTN node and KDN are integral parts of the ground station network operating the KPLO mission, designed for reliability through redundancy to ensure seamless communication with the KPLO DTNPL. The network interface between the DSN DTN node and KDN predominantly utilizes TCP/IP, supplemented by UDP. While the KDN acts as a ground relay node, the network faces significant traffic congestion due to numerous connected devices, resulting in reduced transmission rates. Even in this network structure, we successfully completed the experiment by adjusting the packet transmission volume on the ground station network to minimize end-to-end transmission delays. However, to establish a stable communication environment using the DTN-ION protocol in the future, it will be essential to implement a dedicated line between DTN nodes on the ground.

5.3. Ground Station Interfaces Among Different Agencies

The original plan for the KPLO DTNPL test involved utilizing the DTN node developed by ETRI without leveraging overseas ground stations. However, during the initial design phase, NASA JPL’s DTN team joined the project and designed a network structure incorporating DSN antennas and DTN nodes. This collaboration resulted in an asymmetric network structure in the forward and return paths for data transmission.

The domestic satellite communication ground commands require encryption, and therefore, they were formatted in CCSDS TC format with encryption by the KMOC ROS. This allowed them to be transmitted via S-band communication link without requiring packet regeneration by the NASA DSN. However, telemetry received from the KPLO DTNPL lacked encryption. Consequently, NASA DSOC processed the CCSDS TM data, regenerated them into DTN bundles, and then sent them to the DSN DTN node. This setup resulted in an asymmetric structure in the forward and return paths in the operational tests for the KPLO DTNPL, where only the return path utilized the DSN DTN node.

Therefore, it is essential to design the TC/TM processing equipment at existing ground stations of each agency to handle DTN bundle data transmission and reception without delays. To validate this capability on the ground prior to the KPLO’s launch, the JPL protocol test lab developed a simulator to emulate the KPLO node. Performance tests were conducted multiple times using the ground network setup. Through these tests, we were able to assess packet loss within the terrestrial network connected to terrestrial DTN nodes, evaluate the transmission speeds of the terrestrial network using TCP/IP and UDP for real-time streaming, and identify issues affecting real-time video playback at the DCC. This review allowed us to pinpoint problems and implement necessary improvements. While we successfully achieved real-time reception and playback of a video stream on the ground, we encountered frequent playback failures due to unexpected traffic from security equipment and routers within each agency’s terrestrial network. Consequently, adjustments were made to the design of each agency’s terrestrial network, leading to the successful completion of the operational tests for the KPLO DTNPL. Therefore, since the DTN protocol design incorporates different algorithms specific to each agency, it is crucial to share these differences in advance during design meetings when sharing bundle data through international cooperation. This will ensure that interfaces can be set up smoothly and effectively between different agencies.

6. Conclusions

The KLEP focuses on conducting scientific missions aimed at exploring the Moon. One of its pivotal missions is the KPLO. The KPLO conducted various operational tests using DTN-ION protocols to verify their feasibility in space communication. Specifically, the KPLO tested message and file transfers using DTN-ION protocols, along with real-time video streaming capabilities. These tests were crucial during the initial operations of the KPLO to ensure reliable communication between the KPLO DTNPL and ground stations on Earth. Insights and lessons learned from these DTNPL operations provide valuable guidance for future advancements in space communication systems. They highlight the importance of robust protocols and network designs tailored for deep space missions, as well as the need for seamless integration and compatibility among international partners’ systems during collaborative space exploration efforts.