China's Space Situational Awareness Capabilities For Beyond GEOi

China Aerospace Studies Institute Sept 2022

China’s Space Situational Awareness Capabilities For Beyond GEO

Kristin Burke

This paper reviews the publicly available information on China’s existing and planned

ground and space-based systems for its beyond geosynchronous Earth orbit (GEO) missions.

This paper assumes that the PRC, like the U.S., has not needed a well optimized system for

discovering, tracking, and cataloging multiple objects between GEO and the Moon, but might,

like the U.S., consider how to leverage existing capabilities in anticipation of multiple actors

operating beyond GEO.

The PRC has many sensors to support its Lunar and Martian exploration programs but

does not currently have any publicly known dedicated ground or space-based sensors capable of

scanning the volume of space between the Earth and the Moon to discover unknown objects.

The PRC is taking steps to increase its capabilities over the next 5-10 years. China has only

recently begun to expand its systems for planetary defense, which could enable beyond-GEO

scanning for unannounced spacecraft. Even if the Chang’e 5 orbiter is still at the Lunar Distant

Retrograde Orbit (DRO), its updated camera, as discussed below, seems more tuned for close

ups and distant bright objects, rather than scanning for unknown dim objects. This deficit might

change as soon as 2023, but more likely by the end of 2024, when China will launch Chang’e 7,

which will include a Lunar obiter planned to last several years.

2,3,4

Introduction

Chinese astronomers’ interest in and ability to discover, observe, and track space objects

beyond GEO dates back to at least 185 B.C., based on historical records of comet observations.

In modern times, the Chinese Academy of Sciences (CAS) Purple Mountain Observatory (PMO)

first discovered an asteroid with an optical telescope in 1957, and Chinese military and civilian

ground-based radar and radio telescopes have been tracking the PRC’s Lunar missions since

2007.

6,7

This paper should be read as a supplement to CASI’s 2021 China’s Ground Segment: Building the Pillars of a

Great Space Power, which primarily covers China’s national Earth orbit SSA stations.

https://www.airuniversity.af.edu/CASI/Display/Article/2517757/chinas-ground-segment-building-the-pillars-of-a-

great-space-power/. Similarly, this report is about the sensors and their location, not the software and algorithms to

discover or track objects in non-repeating orbits. This report also assumes the reader has a basic understanding of

the sensors themselves, for example, this report does not explain that optical telescopes can be limited by cloud

cover whereas radio telescopes are not. It does not explain the difference between passive and active radio

telescopes.

China Aerospace Studies Institute Sept 2022

To scan space to find new objects and to track already identified objects requires different

sensors. Both scanning and tracking are needed for space situational awareness (SSA).

SSA

refers to the ability to find, identify and track objects in space, establish their orbits, and predict

their future positions and any natural or man-made threats to their operations. Countries with

Lunar and Martian exploration programs usually have the required beyond GEO SSA

capabilities to track their own systems, and some countries also have planetary defense sensors

which look beyond GEO. However, because specific sensors are tailored for particular tasks,

those geared for Earth orbit or deep space beyond the Moon are not always capable of

identifying or tracking objects in the space between the Earth and the Moon.

While the extent to which planned Lunar missions will materialize is unknown, countries

and companies will at least need to be able to better communicate with and track their own

spacecraft between the Earth and the Moon. Depending on the multitude of activities, countries

and companies may need to know the location of others for spaceflight safety. In absence of a

formal mechanism to share this data, individual country SSA systems optimized for beyond GEO

are an alternative.

The U.S. is in the process of learning how to leverage existing national,

commercial and academic sensors in anticipation of future demand for Lunar space traffic

management.

iii,9,10

Ground-Based Systems

Optical Telescopes

Optical telescopes passively collect light reflected off space objects and amongst their

benefits, they are well suited for detecting unknown objects in space, except when pointed in the

direction of bright objects like the Sun. The vast majority of Chinese ground-based optical

telescopes scheduled for SSA are focused on low Earth orbit (LEO) and GEO. Chinese military

academics have repeatedly indicated concern with the U.S.’s ability to conduct operations in the

graveyard orbit, which could indicate Chinese insecurity with their ability to persistently monitor

activity just above GEO.

11,12,13

There are however a few ground-based optical telescopes which

are primarily scheduled for astronomical studies that might potentially be capable of discovering

unknown objects between GEO and the Moon.

CAS’s National Astronautical Observatories (NAOC) and the affiliated PMO operate

ground-based optical telescopes in China that contribute to the Minor Planet Center’s database,

indicating they can observe objects beyond GEO.

PMO has six branch observatories across

China, and as of mid-2022, it was ranked 21

globally for the highest number of minor planet

PRC official and unofficial writings use the term space situational awareness (SSA) rather than space domain

awareness (SDA), and one Chinese space consulting firm states the reason is because the latter is a term used by the

U.S. to emphasize space is a warfighting domain.

iii

The U.S. Space Command, Joint Task Force-Space Defense, Commercial Operations Cell’s Public Satellite

Research Analysis team supported the referenced event and reviewed this paper.

The Minor Planet Center is the official body for observing and reporting on minor planets under the auspices of

the International Astronomical Union. Minor planets include asteroids, Pluto and other icy worlds beyond Neptune,

and other bodies. Minor planets do not include comets nor other planets.

China Aerospace Studies Institute Sept 2022

discoveries, which includes near-Earth objects (NEOs).

In order to discover an unknown

object, PMO has used its Xuyi Observation Station in Jiangsu Province to discover 1,855

objects.

NAOC’s Xinglong Station near Beijing in the 1990s participated in a NEO survey which

resulted in a new discovery, but has since then only recorded observations of known minor

planets.

Other NAOC facilities have been able to observe already discovered minor planets

from two additional stations: the Tibet Ngari (Ali) Prefecture Observatory and Yunnan Lijiang

Gaomeigu Observatory.

In addition to CAS, Chinese citizens and universities own and operate small and large

optical telescopes. One Chinese study found that while there are numerous university

observatories, no one has comprehensively reviewed them to date.

Most of China’s amateur

astronomers primarily use optical telescopes to view a solar eclipse, the Moon, or the Milky

Way, but some have been credited with asteroid discoveries through the Minor Planet

Center.

18,19,20,21

There is also a primarily independently funded observatory that has contributed

to the Minor Planet Center’s database: Xingming Observatory, Nanshan Station in Xinjiang

Province.

22,23

China has plans for many new ground-based optical observatories in its western regions

which have lower light and air pollution, and higher altitudes. NAOC is building new facilities

to include optical and infrared telescopes at Lenghu in Qinghai Province. PMO has considered

relocating Xuyi there for better observation conditions.

24,25

Some of these new facilities will be

useful for beyond GEO SSA.

26,27

Other new facilities include the 12 Meter Large

Optical/Infrared Telescope (LOT), the location of which might be at existing observatories in

Tibet, Xinjiang, or Sichuan provinces.

28,29

Laser ranging

Laser ranging uses light to actively pulse a known object in space and collect the returned

energy with optical systems to measure its relative distance. U.S., European, and Japanese deep

space missions have used this method to support orbit determination. China installed a laser

reflector on its Queqiao relay satellite at Earth-Moon Lagrange Point (EML) 2 to extend its

Lunar ranging capabilities, using at least its 1.2 meter aperture telescope in Yunnan Province.

China’s first successful lunar laser ranging experiment in 2018 was also conducted using the

Yunnan telescope, at that time with the Apollo 15 retro-reflector.

Radio Telescopes and Very Long Baseline Interferometry (VLBI)

Ground-based radio telescopes can passively and actively collect signals, which if

designed for collecting the radio frequency of a transmitting satellite beyond GEO, could be

useful for tracking known and even potentially finding unknown objects. VLBI is a technique

that combines measurements from several radio telescopes to simulate a much larger aperture,

A NEO is any small Solar System body whose orbit brings it into proximity with Earth.

China Aerospace Studies Institute Sept 2022

which is used to determine the precise orbit of a spacecraft. In a later section, this paper will

address space-based VLBI systems and their use for imagery.

China already has the ability to track and communicate with its own Lunar and Martian

probes. China’s Deep Space Network (CDSN) is composed of several components: at least

seven different locations in China which host various radio telescopes, Yuanwang satellite

tracking ships capable of uplink and downlink, as well as China’s two overseas stations in

Neuquén, Argentina and Swakopmund, Namibia.

vi,32,33

The CDSN also works in conjunction

with CAS’s VLBI network, which is also spread across at least seven locations.

34,35

When VLBI

is used for orbit determination in China’s Lunar program, the Beijing Aerospace Control Center

(BACC) transmits data commands to the primary Shanghai VLBI station, which then sends the

received information back to BACC.

China has tested extending its VLBI capability to its

overseas stations in Argentina and Namibia, but found that there was not enough bandwidth for

those stations to transmit real time tracking data and VLBI data back to Beijing, so the former

will continue as the priority.

vii,37,38

China’s Change’e 5 sample return mission provides an example of how the two types of

information work together for SSA. China used X-band tracking to communicate with its

orbiter, lander, and ascender, and combined that information with VLBI for precise docking of

the ascender to the orbiter.

39,40

As of at least 2018, Chinese VLBI could operate in at least six

bands: S-, X-, Ku-, L-, C-, Q-, and possibly K-, and plans for a space-based component will open

up four additional frequency bands from 30-1670MHz.

41,42,43

Radio telescopes can sometimes have SSA applications for identifying unannounced

spacecraft movements or even discovering unknown objects. Smaller passive radio telescopes

could be used to identify broad location changes of known objects beyond GEO. For example,

Western space enthusiasts were able to identify the X-band signal from Chang’e 5 as it moved

from Earth-Sun Lagrange Point (ESL) 1 to the Lunar DRO in early 2022.

A large passive

radio telescope could detect signals from beyond GEO if collecting in the same frequency band.

Notable is that while China hosts the world’s largest passive radio telescope in Guizhou Province

called the Five-Hundred-Meter Aperture Spherical Radio Telescope (FAST), it receives signals

in the L-band 70 MHz to 3 GHz, which could not pick up signals from S-, X-, and Ka-band

transmitting spacecraft beyond GEO without significant upgrades.

FAST’s size might also lead

The domestic facilities are located in Sanya in Hainan Province; Tianjin (70 meter); Jiamusi in Heilongjiang

Province (66 meter); Beijing (50 meter); Kashgar and Urumqi in Xinjiang Province (4x35 meter and 25 meter,

respectively); and Kunming in Yunnan Province (40 meter). The PRC regularly uses its overseas stations in

Argentina and Namibia for tracking its Lunar exploration program and may also use its Antarctic Kunlun Station,

when occupied. The PRC also coordinates with non-Chinese overseas tracking stations for additional data on its

beyond GEO missions.

vii

During China’s 2014 Chang’e 5 T1 mission VLBI data was not available to confirm the separation of the service

module and the return vehicle, according to Chinese researchers. Instead, they used a new GNSS measurement

technique. They installed a GNSS receiver on the spacecraft with a L-band C/A code receiver for GPS and

GLONASS tracking, with 2 receiving antennae mounted on opposite sides of the spacecraft to ensure that signals

are omni-directionally received.

China Aerospace Studies Institute Sept 2022

to over saturation of its sensors if used to listen to something too close, as it is designed for

listening to the whole solar system, far beyond the Earth’s Moon.

In terms of planned ground-based radio telescopes, China is also building the world’s

largest steerable radio telescope near Urumqi, Xinjiang, which once completed in 2023 will be

110 meters and operate in the 300MHz-117GHz range that includes but is not limited to S-, X-,

and Ka- bands.

China also plans a millimeter wavelength radio telescope at its Antarctic

Kunlun Station.

Radar

Since at least 2013, Chinese scientists and engineers have lamented China’s slow

development of ground-based deep space radar capabilities like the U.S. Goldstone, Haystack,

and formerly Arecibo radars.

During China’s 13

Five-Year Plan (2016-2020), Chinese

scientists and engineers evaluated different designs and proposed a ground-based distributed

coherent radar array, which can work in tandem with the CDSN and Chinese VLBI.

Researchers are particularly interested in the radar array’s ability to image the Moon, Mars and

NEOs.

51,52

CAS had previously faced challenges when trying to image the Moon with the

Kashgar radar of the CDSN, the Yunnan 40m radar, as well as the Qujing, Yunnan incoherent

scatter radar of China’s space weather monitoring system.

CAS has recently used the Sanya incoherent scatter radar to conduct preliminary Lunar

imaging experiments for algorithm development.

A separate effort between the Chongqing

Municipality government and Beijing Institute of Technology will also develop beyond GEO

radar capabilities. The university team has completed the first phase of a three-phase project to

build 400 20-meter S-band radars, with an estimated detection distance of 150 million

kilometers.

55,56,57,58

If the sensors can be properly correlated, this ground-based radar array

would be a good tool for SSA beyond GEO when pointed at key locations, like on and around

the Moon, and even possibly Lagrange Points.

Space-Based Systems

Optical and Infrared Telescopes

Through China’s Lunar Exploration Program (CLEP), the PRC has developed and tested

various optical sensors for the Lunar and deep space environment to perform local SSA, such as

for landing and system checks.

Chinese space engineers have tested those cameras for

different types of SSA, such as color imagery of the asteroid Toutatis during a 2012 flyby with

Chang’e 2, and photos of the Earth-Moon system from EML 2 with Chang’e 5 T1 in 2014. The

2018 Longjiang-2 micro-satellite that China launched with the Queqiao relay satellite also

carried an optical camera developed by Saudi Arabia and captured high-definition images of the

Moon.

China made significant improvements to Chang’e 5’s cameras, launched in 2020, for

more detailed imagery of close-up objects and distant bright objects.

In addition to enabling capabilities for CLEP, the PRC’s 2021 space activities white

paper stated that in the next five years China will, “[s]tudy plans for building a near-earth object

defense system, and increase the capacity of near-earth object monitoring, cataloguing, early

China Aerospace Studies Institute Sept 2022

warning, and response.”

This will require another set of SSA capabilities, to include wide-field

of view scanning to discover unknown, potentially hazardous objects. Towards this end, Chinese

scientists and engineers have proposed several beyond GEO optical and infrared satellite systems

to contribute to international monitoring of potentially hazardous NEOs.

In 2016, the China Aerospace Science and Technology Corporation’s (CASC’s) Qian

Xuesen Space Technology Laboratory proposed a seven satellite dispersed constellation with

satellites at a Lagrange Point and in a Venus-like orbit around the Sun for search and spectral

inspection of potentially threatening small celestial bodies.

viii,63,64,65

Since then, technical

experts in China have debated what sensor type to prioritize for planetary defense, not just the

location.

Such debates have spurred other proposals such as a mid- and long-wave infrared

sensor at ESL 1 to enable an integrated optical-infrared-microwave space- and ground-based

sensor network.

Scientists and engineers also proposed a heliocentric orbit for an Earth-trailing

optical telescope as well as EML 4 and EML 5 infrared telescopes.

68,69

Lunar-based Earth observation is also something China plans to further develop during

the 14

FYP, which could enable wide-field SSA. The Chang’e 3 lander on the Lunar near-side

has used its ultraviolet sensor to study the Earth’s ionosphere for space weather forecasting.

Since then, Chinese researchers have reviewed sensor types for a long-term Lunar-based Earth

observatory.

71,72

They seem to have decided to prioritize radar systems, according to a

solicitation for applications under the 14

Five-Year Plan.

Space-based Radio/Microwave, Radar, and VLBI

The Chinese plan for a potential Lunar-based radar system for Earth observation

mentioned above builds off the PRC’s existing space-based radio/microwave, radar and VLBI

capabilities. The 2007 Chang’e-1 carried a microwave sensor and recently released the first full

Lunar map after already releasing an updated stereo imagery map.

74,75,76

Microwave sensors are

used to determine the material composition below surfaces or behind walls. The Chang'e-2’s

stereo mapping capabilities reached 7 meters, a notable improvement compared to the 120-meter

resolution of the Chang'e-1 satellite.

Chinese experts said that the Chang’e-2 resolution can

reach 1 meter when the satellite is closer to the moon.

Chang’e-3 and -4 used optical and radio

sensors to perform local SSA, such as landing and localization updates.

China’s 2018 launch of the Longjiang-2 microsatellite to Lunar orbit initiated the PRC’s

first independent space-based VLBI tests and a continuation of its bilateral space-based VLBI

cooperation, this time with a joint Netherlands instrument on the Queqiao relay satellite.

80,81

Later, the Chang’e-4 lander also carried an antenna for VLBI tests. It is possible that the

Chang’e-5 orbiter’s extended mission to Lunar DRO is experimenting with the longest VLBI

baseline test yet.

China has several plans for future space-based VLBI systems which will

viii

The project is called the "Near-Earth Object Heterogeneous Monitoring Constellation" (CROWN), which plans to

equip “the "1+6" mother and child constellations” with a variety of payloads to carry out autonomous detection and

discovery of near-Earth objects. The Lagrange Point is said to the Lagrange Point 2, probably in the Earth-Sun

System, but could also be a typo, as similar Western proposals call for a Venus-like orbit and an ESL 1 combination.

China Aerospace Studies Institute Sept 2022

operate at least 3-5 years. The Chang’e-7 will include a Lunar orbiter with a 4.2 meter relay

antennae in X band for joint VLBI with ground based sensors.

Opinions, conclusions, and recommendations expressed or implied within are solely those of the

author(s) and do not necessarily represent the views of the Air University, the Department of the

Air Force, the Department of Defense, or any other U.S. government agency. Cleared for public

release: distribution unlimited.

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