Access once encountered TT&C mode based on space–air–ground integration network

2.1 Current status of spacecraft TT&C network

China’s space TT&C system has experienced a development process from scratch, from weak to vigorous, and has gradually formed a space TT&C system in line with China’s national conditions. It could provide TT&C support for the launch and in-orbit operation of human-crewed spacecraft and various application satellites in different orbits, as well as for recoverable satellites. The primary development trend of China’s space TT&C network is to optimize ground-based TT&C network, build and develop space-based TT&C network, build deep space exploration TT&C network, and build the SAGITN.

The construction and development of a space-based TT&C network can fundamentally solve the problem of high coverage of TT&C and communication and the problem of high-precision orbit determination and large-capacity information transmission. This is the development trend of the world’s aerospace TT&C system. A high-level communication architecture for future space-to-ground CubeSat communication was proposed within NASA Goddard Space Flight Center. This architecture addresses CubeSat direct-to-ground TT&C and communication, using tracking data relay satellite system (TDRSS) (Sobchak et al. 2018, Wong et al. 2016, NASA space communication architecture working group SCAWG 2006). Europe has developed the European data relay satellite system (EDRS) within the advanced research in telecommunications system program (ARTES-7). EDRS became the first commercial European data relay system providing a wide range of operational services (both optical and Ka-band based) (Laux et al. 2012). In order to meet the demand of real-time data transmission of earth observation satellites, Japan launched a new data relay satellite in 2020 successfully, which adopts optical inter-satellite communication technology, the first user satellite, named ALOS-3 which is planned to be launched in 2022 (Shiro et al. 2022). Russia has also developed the “Torrent” series military relay system and the “Ray” series civil relay system. In 2017, the Russian geostationary segments of the COSPAS-SARSAT system built on the base of the relay satellites Louch-5A and Louch-5V of the multifunctional relaying space system Louch and ground stations for reception of the data from EPIRBs tests the fight performance characteristics of the relay satellites. The results show that the fight performance are considerably greater than similar equipment (Arkhangel’skiy et al. 2017). Currently, the space-based TT&C network mainly includes data relay satellite systems and navigation satellite systems. The data relay satellite system has high orbit coverage, solid real-time performance, strong data transmission capability, and high system cost-effectiveness. Navigation satellite systems can provide autonomous navigation, real-time positioning, attitude determination, and high precision time synchronization with excellent performance and simplicity for middle and low orbit spacecraft.

As mentioned above, space-based TT&C has tremendous advantages and construction value, but due to the following limitations, it cannot wholly replace ground-based TT&C in the foreseeable time:

1. Relay satellites work in geosynchronous orbits and are far away from micro-satellites operating in low orbits. Micro-satellites are limited by size, antenna gain and transmission power are generally small. Space-based TT&C system cannot completely manage all the satellites.

2. Although the current utilization rate of relay satellites is not high, with the rapid increase in the number of orbiting aircraft, its limited TT&C capabilities will become prominent. More than 120 information acquisition satellites are operating in orbit, and data need to be received nearly 1000 times a day. The first and second generation relays can only meet more than 200 laps, which is less than a quarter of the need.

Therefore, it is an inevitable development way to construct a ground-based TT&C network, integrated with space and earth, flexible and efficient, and capable of disaster tolerance and destruction resistance, mainly based on space, supplemented by ground (sea-based equipment will be considered in the ground TT&C network, which will not be explained separately below).

2.2 System composition and framework

Affected by various factors, aerospace engineering needs have long dominated the construction and development of the aerospace TT&C network. Therefore, long-term planning is insufficient. Considering the existing pattern and the frontier direction of future aerospace development, the SAGITN should have the following characteristics:

1. Open. The system design should be based on a long-term perspective, with substantial flexibility and scalability, and provide standard interfaces, allowing a variety of aircraft or equipment to flexibly access the network, to better meet future space rendezvous and docking, space confrontation, deep space exploration, and on-orbit scientific experiments and other diversified task requirements.

2. Intelligent. As the number of aircraft increases, the amount of TT&C management tasks will increase exponentially. Traditional TT&C business based on planned management, conflict resolution, and some equipment-side operations that require manual intervention have become complicated or challenging to complete. Intelligent system operation is the only way to solve it.

3. Low cost. As the commercial market enters the aerospace field, low cost is an inevitable development trend.

In summary, the idea of building an integrated space–ground TT&C network is shown in Figure 1.

Figure 1

Conceptual diagram of SAGITN.

As shown in Figure 1, the SAGITN can be divided into four levels according to a top-down system: space-based TT&C system, target for TT&C, ground-based TT&C equipment system, and system control center. The relay satellite system and the ground-based TT&C network implement the TT&C business of various aircraft. Navigation satellites provide spatial positioning information to various aircraft, allowing subsequent autonomous orbit adjustment. At the same time, navigation satellites and relay satellites operate in a self-organizing network mode, significantly improving system openness and survivability.

3.1 Encountered TT&C mode

3.1.1 Definition and advantage

The concept of AOE TT&C mode comes from “random access,” which originated from computer networks, and is now widely used in cellular mobile communication systems (Niu et al. 2012, Tutschku 1998). It refers to the process of the user sending a random access preamble to attempt to access the network to establish an essential signaling connection with the network. As shown in Figure 2, the basic process includes the following four steps (Dahlman, Parkvall, and Sköld 2011).

Figure 2

Random access process of mobile cellular network.

Step 1: The transmission of the random access preamble sequence. Its primary purpose is to notify the base station that a terminal is trying to access and let the base station estimate the delay. This value will be used in the second step to adjust the terminal’s uplink transmission time.

Step 2: Random access response. The base station sends a response message to the terminal to notify the terminal of its resources and sends it a temporary identifier for further communication.

Step 3: The terminal sends radio resource control (RRC) signaling (terminal identification) to the base station, and after that, it sends the mobile terminal identification information to the base station.

Step 4: The base station sends RRC signaling (contention resolution) to the terminal. The base station notifies the terminal to resolve the contention and confirm the connection.

Learning from the random access mechanism of mobile cellular networks, satellites and TT&C stations are regarded as mobile terminals and base stations, respectively, (Manlio et al. 2018), and the single-lap TT&C process is regarded as a data service between mobile terminals and base stations. Then, random access can be introduced into the TT&C network to form the TT&C mode with “on-demand autonomy, access on encountered, and seamless handover” characteristics.

AOE TT&C refers to the condition that the space-based or ground-based TT&C mode has full airspace beam coverage. Once the spacecraft enters the beam range, it can access the TT&C equipment network for TT&C business without a predetermined plan.

Compared with the traditional planned access TT&C mode, the advantages of the AOE TT&C mode are summarized as follows:

1. High support for simultaneous TT&C of multiple satellites. The spacecraft can be allocated channel resources if it enters the beam range and sends an access request. Using code division multiple access (CDMA) to distinguish multiple spacecraft within the beam range can significantly increase the communication capacity of the system. Compared to a single beam with a single target, it can effectively solve the problem of insufficient TT&C resources caused by the future surge in the number of satellites.

2. It can reduce the complexity of resource scheduling. The mode of single cycle tracking TT&C task driven by schedule is changed into satellite autonomous AOE, which changes the situation that traditional resource scheduling is difficult to adapt to the increasingly intensive TT&C business volume.

3. Higher access efficiency. For AOE mode, once the spacecraft enters the range of beam of the TT&C mode, a two-way handshake is carried out to establish the control channel connection, and it is always maintained until the range of action of the node airspace is exceeded. When the TT&C demand is generated, it can quickly launch the business without going through the capture link under the traditional planned access mode, and the response speed is fast.

4. High reliability. The satellite can simultaneously establish a control link with more than one TT&C station and perform rapid switching when necessary. To a certain extent, it realizes multi-station redundancy backup and improves the reliability of TT&C.

5. Increased flexibility. The choice of satellites or equipment accessing the network can be dynamically and automatically adjusted, which can better adapt to the needs of various tasks. At the same time, it is also conducive to flexible, customized TT&C management services for commercial satellites.

6. High efficiency-to-cost ratio of long-term TT&C management. Compared with the method of realizing multi-spacecraft TT&C by building more new ground stations, the integration of multi-spacecraft TT&C based on AOE mode, which saves the cost of construction, management, and maintenance, can significantly improve the ratio of efficiency to cost and reduce the cost of TT&C management.

3.2 Basic idea of realization

It is easy to see that to realize AOE TT&C, the primary problem is meeting the coverage to ensure a gain. According to the basic theory of antenna gain, the beam width and gain of a single antenna cannot be guaranteed simultaneously. The solution is to use multi-beam antenna technology.

For the space-based TT&C system based on China’s TDRS, we can refer to the communication satellite also in the geosynchronous orbit to realize AOE TT&C based on S-band multiple access through multi-beam panoramic coverage of the ground (Zhang et al. 2021, Zhan et al. 2020). Related research has analyzed the method’s basic idea and system capacity (Liu et al. 2019, Zhang et al. 2021, Zhu et al. 2021), and the analysis results believe that the system can support 500 spacecrafts simultaneously. The shortcoming of this method is that the angle measurement cannot be realized, so the positioning and orbit determination can only be realized by the receiving terminal of the spacecraft’s onboard navigation system. The principle of implementing space-based AOE TT&C using TDRS is described in Figure 3.

Figure 3

Schematic diagram of three TT&C equipment under two access modes: (a) traditional access mode, (b) ground-based AOE mode, and (c) space-based AOE mode.

For the ground-based TT&C system, the recent research on constructing multi-target TT&C equipment with hemispherical all-aerospace coverage has provided a method for AOE TT&C. Although the unified S-band (USB) system in the form of parabolic antenna is still adopted as the mainstream in China’s space TT&C network, the basic idea and implementation scheme of using phased array antenna to realize multi-beam and multi-target TT&C have been mature and developed. Currently, the direction of TT&C equipment construction in China is the research and deployment of multi-beam equipment in the whole airspace. It uses a hemispherical columnar array antenna with a beam that can cover the entire airspace. The working principle of the two devices is also described in Figure 3.

Although the hemispherical all-aerospace coverage equipment can track multiple spacecraft simultaneously, spacecraft cannot be detected immediately once it enters the visible range. Therefore, it is necessary to add a fixed airspace coverage beam based on the original TT&C tracking beam of the multi-spacecraft equipment in the entire airspace. The two can be integrated and designed in the dome hemispherical cylindrical array. The split form realizes the extensive airspace coverage to complete the target detection function, and most of the sub-arrays form the TT&C tracking beam to complete the TT&C communication function (Du et al. 2021, Li 2019). The scene is shown in Figure 4.

Figure 4

Schematic diagram of AOE TT&C coverage scene.

In the follow-up discussion of this article, only the ground-based TT&C system is used as the research background of the AOE TT&C mode. The related concepts are also applied to space-based TT&C, and this article will not discuss the improvement of its adaptability.

3.3 TT&C process based on AOE mode

According to the current single-lap TT&C process, the equipment of the TT&C ground station is driven by the work plan and the number of orbits issued by the satellite control center. System tasks are generally divided into five stages: system self-check, pre-task calibration, satellite capture, tracking, and post-processing.

In implementing TT&C based on AOE mode, to achieve the purpose of parallel access control and data transmission, a control channel and a business channel are separately designed. Figure 5 shows the TT&C process based on the AOE mode. The typical workflow is:

1. The satellite enters the range of the ground-based TT&C network, and the program-controlled access system is powered on and initialized, ready to be accessed.

2. The access system sends a downlink inquiry signal to search TT&C ground stations and selects TT&C stations to be accessed according to the preset priority rules. The query signal contains the satellite identification code.

3. Send the access request to the selected TT&C ground station.

4. After receiving the request, the TT&C station will issue an inquiry application to the control center of the ground-based TT&C network to verify the satellite’s identity and confirm its TT&C demand level. After verification and confirmation, if the station has TT&C resources, it will directly agree to the access request and continue to meet the access process. If the channel of the station is full, it can choose to interrupt the current non-priority service or refuse the request depending on the demand level.

5. If the access is successful, the business channel will start the routine TT&C process, and the control channel will continue to search for other TT&C stations for switching at any time.

6. After the current lap TT&C service is completed (instructions are sent and executed for confirmation, or telemetry reception is completed), or when the coverage of the currently connected TT&C station is exceeded, the satellite initiates the disconnection procedure. and the satellite enters the search state or directly switches to the preselected TT&C station. The TT&C station releases channel resources to access other targets, the TT&C station can also initiate a service interruption program as appropriate.

7. The satellite performs the next phase of the AOE TT&C process. By continuously sending inquiry signals to all stations within the range, the satellite control center can grasp the dynamic information of the satellite’s exit and entry into the TT&C network.

Figure 5

TT&C process based on AOE mode.

4.1 TT&C mode

4.1.1 Access process control

The strategy of optimization selection for spacecraft access is needed to study. The coverage of TT&C stations is generally overlapped, especially in the domestic area. The spacecraft faces multiple visible TT&C stations in some business orbit section. If the coverage of TT&C stations can be reached, it is necessary to make an optimal choice. In order to consider the optimization of resources allocation and emergency response efficiency, the following rules should be followed:

1. For daily business, spacecraft should prioritize the measurement with long coverage orbit and close distance.

2. When the channel quality is required to be higher, the station with strong measurement and control ability is preferred.

3. The center regularly evaluates the channel saturation of the TT&C stations, and the evaluation results are uploaded to the satellite as a reference for its access decision.

Since the coverage of a single TT&C station is always limited, soft handover should be carried out to ensure uninterrupted TT&C business when the aircraft flies through the coverage area of multiple TT&C stations.

Figure 6 shows soft handover across 2 TT&C ground stations. The control center can control the handover process using the soft handover method. Besides, during the overlapping period of coverage area, downlink data is received by two ground monitoring and control stations simultaneously and then merged at the control center. As a result, the two ground stations act as backup to each other, increasing system reliability.

Figure 6

Diagram of handover across two TT&C ground stations.

Handover requires satellite and ground control stations to relay information over a control channel. The control channel is separated from the traditional TT&C business channel. Multichannel division can be carried out in three ways: time division, frequency division, and code division.

Since the control channel is required to be independent of the business channel and always in a state of connection, the real-time requirements of spacecraft TT&C are very high, time division is unsuitable. In the frequency division mode, different working frequencies are used for different targets, which requires increasing the bandwidth of the ground receiving system. While spectrum resources are limited, when the number of targets increases, there will be a limited situation. Therefore, code division is the most feasible way to reference the ground cellular mobile communication.

4.2 Design of protocol stack

Whether it is the TT&C business for satellite platform or the operation for payload, whether it is ground-based or space-based, their technical systems are quite different. With the construction process of SAGITN, “the integration of survey, operation, and control, the gradual integration of space-based and ground-based TT&C resources, and the IP-based networking of all nodes” are the inevitable development trend.

Based on the open system interconnection model (Li et al. 2011, Saxena 2014) of the Internet and the research on the protocol stack of the existing data link (Sharma et al. 2018, Fahmy 2021, Gagandeep and Kumar 2012), this work designs the AOE TT&C protocol stack into four layers: application layer, network layer, media access control (MAC) layer, and physical layer.

As shown in Figure 7, the protocol stack of the AOE TT&C mode is shown. The information interaction between different layers through user-defined packets is helpful to the realization of the whole protocol mechanism.

Figure 7

The protocol stack of the AOE TT&C mode.

The application layer is the function implementation layer, which completes the access decision, channel management, data management, and other functions – the database and integrated monitoring management system run in this layer and interacts with various users.

The network layer is the implementation layer of networking, and the processing object is IP packets. It mainly completes the encapsulation and unsealing of IP packets, IP address management, and routing selection and realizes the information interaction between the application layer and MAC layer. IP integration of different spatial protocols is completed in this layer.

The MAC layer is the communication implementation layer. The processing object is a data frame, which mainly completes the data frame group. Frame solving, frame synchronization, data buffer, error control, and channel switching mechanism are mainly implemented in this layer.

The physical layer is the signal implementation layer, which is mainly responsible for providing channels for the reliable transmission of information. Select appropriate frequency points, signal forms, and coding methods according to the channel characteristics, performing appropriate modulation, amplification, transmission, reception, demodulation, and other signal processing, and cooperate with the MAC layer to complete channel switching.

4.3 Unification of TT&C frequency spectrum resources

It is necessary to consider the unity of the spectrum resources used for either space-based or ground-based TT&C (von der Ohe 2020), to realize the spacecraft’s AOE TT&C. There are two aspects to this problem. One is that the frequency band used for spacecraft TT&C is not entirely unified. Second, the measured target may be equipped with multiple sets of different frequency bands of TT&C equipment. The analysis is carried out in turn.

Currently, there are two ground-based aerospace TT&C networks in China. The medium and low-orbit aerospace TT&C network adopts the S-band frequency, commonly used in international space operations, with an uplink frequency range of 2,025–2,110 MHz and a downlink frequency range of 2,200–2,290 MHz. The geosynchronous orbit satellite TT&C network is mainly used for geosynchronous satellite missions. The frequency band used is the international standard C-band. The uplink frequency range is 5,850–6,425 MHz, and the downlink frequency range is 3,625–4,200 MHz. With the development of technology and changes in demand, the S-band has become the main frequency band for spacecraft TT&C in recent years. At the same time, VHF/UHF, C, X, Ka, and other frequency bands are also used in aerospace in different scenarios (Zhang et al. 2021, Wei 2020, Dong et al. 2018, Li et al. 2013, Babuscia and Angkasa 2021, Xue et al. 2020). The details are shown in Table 1.

On the other hand, some TT&C targets are equipped with multiple sets of equipment and are limited by the cost of long-term management, especially for private commercial small satellites. They often do not rely on the existing S-band aerospace TT&C network, and self-built TT&C stations for long-term control, which requires them to load at least two sets on the satellite TT&C equipment of different frequency bands; S-band TT&C equipment is used for launching into orbit section, and VHF/UHF frequency band (or other unlicensed frequency bands) is used for long-term management section. If considering the need for higher-rate data transmission, X or Ka-band TT&C equipment must be installed, which causes redundancy and waste. If the onboard TT&C spectrum can be unified, it will save precious onboard load resources and effectively reduce costs.

In summary, the S band has become the standard frequency band of aerospace TT&C networks, and from the domestic perspective, the S band will still be used as the main TT&C frequency band for a long time. At the same time, with the engineering application of the AOE TT&C mode, the long-term satellite management cost will be significantly reduced, and commercial satellites will not need to install additional frequency band TT&C equipment. Therefore, this article is designed to be the universal S band specified by the ITU, which is used for TT&C management and low-speed data transmission, and high-speed data transmission is used to build a separate receiving station. If the follow-up plan is to upgrade the TT&C frequency band as a whole, the existing AOE TT&C framework can still be used, and only the working frequency band of the equipment needs to be increased.

5.1 Comparisons of management resource consumption

For a general TT&C system, the daily operation and management cost is represented by the consumption of management resources, which can be defined as the human resources needed for the system to work in a fixed period (Pacheco et al. 2003, Yin et al. 2022, Soma et al. 2004). The calculation methods for the two modes are as follows.

1. The TT&C network consists of hemispherical coverage multi-target TT&C equipment for AOE mode. Since the equipment is in operative mode all the time, there are satellites in the visible range at any time. If one person is needed to be on duty for single equipment, then the consumption of management resources in a time period of T can be expressed by T, which is a fixed value.

2. For the planned access mode, the TT&C network consists of USB equipment, which adopts a narrow beam parabolic antenna, and only supports single spacecraft mode. Thus, at any time, N equipment is required for N spacecraft simultaneously, while every single equipment requires one person on duty. Since the number of visible satellites is a value that changes with time, statistical methods are used. Assuming that the time period with duration T is divided into N short time intervals, there are S _n satellites in the visible range in the n-th hour period (n = 1,2,…N), then in this mode, the management resource consumption is:

The length of each short time interval represents the statistical resolution ratio of the comparison method. In order to illustrate the station’s visibility with multi-spacecraft, assuming that there are four satellites within the visible range of the station at the current moment, the flight path schematic diagram is shown in Figure 8.

Figure 8

Flight path schematic diagram of four satellites in visible coverage.

If we want to calculate the management resource consumption of the planned access TT&C mode, we first need to know the number of satellites in the visible range of the site. Therefore, the following simulation is used to obtain this value.

In order to make the simulation environment close to the working situation of the real TT&C system, the scene of multi-satellite in-orbit operation is designed first. Taking the Walker constellation as the basic model, a complex constellation composed of two groups of satellite constellations with different orbital altitudes is established. Further, to investigate the effect changes in AOE mode compared with planned access mode under the total number of satellites in different constellations, we established two such complex constellations. We configured them according to the parameters shown in Table 2.

Next the TT&C equipment system is established. In order to compare the two access modes based on different ground-based TT&C equipment more accurately, the space-based TT&C system is not considered here. Therefore, considering the influence of latitude on observability (Tang et al. 2019), three ground stations at different latitudes were designed, which were located in Jiamusi, Xiamen, and Sanya, and their location coordinates are described in Table 3.

The statistical results are shown in Table 4. Among them, the calculation method of improved efficiency is shown in formula (2), where 3,600 is the management resource consumption of AOE mode in 1 h.

As can be seen from Table 4, the AOE mode has apparent advantages over planned access for improved efficiency. Otherwise, we can see:

1. The higher the latitude, the better the improved efficiency. That is determined by the parameters of the complex constellations in the simulation scene close to the actual situation. Therefore, TT&C stations should be built at higher latitudes in general.

2. The bigger the constellation, the better the improved efficiency. This indicates that the AOE mode is more suitable for the future management of giant constellations.

5.2 Comparison of real-time performance

During the execution of the TT&C business, since the distance of the communication link is fixed, the real-time performance of the two modes is consistent. However, in the period before TT&C business is carried out, the real-time performance of the two modes is different when emergency TT&C demand (for example, the ground needs to conduct rapid orbit change of satellite, satellite payload instrument switch, and other operations) arises suddenly, which is also one of the advantages of AOE mode. The comparison assumes that the satellite is within the visible range with an elevation of more than 5°.

For the AOE mode, as described above, since the control channel keeps the connection state, it is always available for business channel access immediately. All the information needed for satellite access is known in advance. It only needs to send the access instruction to the satellite through the control channel and allocate the array resources of TT&C equipment in the ground station. Only the time delay caused by one-way link distance is considered in real-time.

For the planned access mode, in addition to the one-way link distance time delay mentioned in the previous paragraph, two additional processes should be added: the capture and tracking and the manual resource scheduling process (Cui et al. 2019, Li et al. 2014, Si and Huan 2020). The typical capture and tracking process is described in Figure 9, and Table 5 shows the empirical statistics of time delay for each operation in this process. The manual resource scheduling process includes schedule generation, audit and confirmation, and plan delivery. The time they spend is also listed in Table 5. The data in the table are based on the historical business of the China satellite control center.

Figure 9

The capture and track process of planned access mode.

Therefore, according to the data statistics of the current TT&C system, when dealing with emergency TT&C tasks, the AOE mode saves about 193 s compared with the planned access mode, showing significantly better real-time performance.