A Global Chinese Space Based Short Message System

Recent media reports indicate the US has developed a way to track the recent Chinese airships passing over North America. Here, a hypothesis is offered on how this may work and how observers can contribute.

In July of 2022, the Chinese commercial JILIN Earth imaging constellation of satellites was found to be emitting strange short burst data signals on spectrum reserved for the mobile-satellite (Earth-to-Space and visa-versa) radio service. This observation while curious at first caused the author to reconsider the meaning of that observation after the recent arrival of a large Chinese airship over North America and how it may communicate with its handlers as a bit more research revealed a clear connection to the Chinese BeiDou satellite navigation system and the emissions from JILIN.

Radio-determination vs Radio-navigation

Most everyone is familiar with a Radio Navigation Satellite Service (GNSS) like GPS, GLONASS, GALILEO etc. A GNSS service is a one-way service where a user listens for multiple satellite signals and from them is able to determine their location and the time accurately. There is no communication between the user and the satellite service.

A Radio Determination Satellite Service (RDSS) that not only allows the user to navigate but also communicate as the very service itself is two-way between the user and the satellite(s) it is navigating via. This uplink provision can be used to allow a user to send messages to orbiting satellites as well for purely communications purposes. Interestingly, the Chinese BeiDou system is capable of RDSS which means it’s a full blown two way communications system with a global reach.

The BeiDou Radio Determination Satellite Service (RDSS) communications flow structure. Note the inter-satellite links.
Source: Introduction to BeiDou-3 navigation satellite system, Yuanxi Yang, Weiguang Gao, Shuren Guo, Yue Mao, Yufei Yang, March 2019.

The BeiDou constellation is constructed with assets in three different orbit types. Three satellites in Geostationary (GEO), three in Inclined Geostationary (IGSO) and 24 in Medium Earth Orbit (MEO) as can be seen in the graphic below.

The BEIDOU-3 Constellation structure.
Source: Overview of the BDS III Signals, Mingquan Lu, Tsinghua University, November 8, 2018.

Let’s examine the public domain literature on the BeiDou message system. There are two messaging services that are used in the BeiDou system one is a regional system primarily using the GEO satellite arm of the system and is known as the regional short message communication service (RSMC). A summary of its properties are noted below.

BEIDOU regional short message communication service summary specifications.
Source: The Application Service Architecture of BeiDou Navigation Satellite System (Version 1.0), China Satellite Navigation Office, December 2019, page 4.

The RSMC service is limited to communication largely in regions in view of China and not a global system as that is were the BeiDou GEO assets are located. The BeiDou system does however, offer a true global communications system via it global short message communications (GSMC) service. The global system is more limited in the amount of data that can be sent in a single message to 70 bytes which is believed to be a civilian limitation and not one imposed on the People’s Liberation Army’s (PLA) use of the system. Specifications of the GSMC system as follows.

BeiDou global short message communication service summary specifications.
Source: The Application Service Architecture of BeiDou Navigation Satellite System (Version 1.0), China Satellite Navigation Office, December 2019, page 4-5.

Public source documents also discuss the frequency allocations for this service. The following text and table from the source documents is presented for context below. Pay particular notice to the uplink for the GSMC service being on L-band.

BeiDou system service plan.
Source: The Application Service Architecture of BeiDou Navigation Satellite System (Version 1.0), China Satellite Navigation Office, December 2019, page 2.

Researching ITU records for China’s COMPASS-MEO reveals that they have two current fillings under this name that appear to related to the versions two and three of the BeiDou series as can be noted by the filling below that relates to information obtained on the BeiDou-3 series of MEO spacecraft.

BeiDou-3 ITU filling under COMPASS-MEO.
Source: ITU Space Network List Online.

Pay particular attention to the reception of signals from 1610.5-1627MHz, this correlates to the emissions from JILIN. I.e. the JILIN’s or perhaps other ‘users’ of the BeiDou GSMC system emit at these frequencies and the BeiDou spacecraft hear them here.

The following graphic provides further information on the evolution of the BeiDou emissions over the three generations of spacecraft used. As the reader can see the system has evolved and has now begun using a global GNSS system capable of true global short message communication.

The BEIDOU-3 Constellation structure.
Source: Overview of the BDS III Signals, Mingquan Lu, Tsinghua University, November 8, 2018. Page 11.

Based on the open source information, China has a full blown global short message communication service with satellite inter-links that can allow telemetry, tracking and control of assets. The location of which the assets can be communicated with is provided in the excerpt below.

BeiDou RNSS performance standard.
Source: The Application Service Architecture of BeiDou Navigation Satellite System (Version 1.0), China Satellite Navigation Office, December 2019, page 2-3.

Here’s a summary of the specifications for the global short message system:

  • Service to any object on the surface of Earth or up to 1000km above it.
  • Uplinks on L-band and downlinks on S-band.
  • Civilian messages are 70 bytes long.
  • The system has a >95% success rate on message handling.
  • 300000 messages per hour up and 200000 per hour down.
  • Response times normally better than 1 minute.
  • Uplink terminal uses a power of <10 watts.
  • The system uses inter-satellite links.

Inter-satellite Telemetry, Tracking and Control (TT&C)

Over the past few years more a more western satellite operators have been using Globalstar to augment or provide TT&C. You can integrate a Globalstar transmitter (STX3) into small asset to track its moments around the Earth or even Low Earth Orbit (LEO) and have it send basic telemetry, position and other data at a periodic basis but regulations require the operator to be able to cease the transmission of the transmitter if on a satellite. Thus missions will integrate two-way communications by using the STX4 system which allows for transmission and reception of data of an asset on Earth or in LEO. The Globalstar system has been in use for a variety of missions over the past decade and has been increasing use as it reduces the ground infrastructure a small satellite would normally need to provide communications with an asset.

AMS uses a STX4 type modem to receive and transmit data via the Globalstar system. Note that the Globalstar system also uses the same frequency bands as BeiDou’s short message service on L and S-band.

Chinese nationals can buy and subscribe to the BeiDou short message service and purchase hardware to integrate their application with the system. The hardware closely resembles systems sold for the Globalstar system.

Unlike Globalstar, BeiDou allows for satellites out of view of ground stations in China to communicate via inter-satellite links. This means that the satellite can be communicated with via other satellites and relay it’s data via other satellites to Earth stations outside of it’s footprint. A helpful feature if you plan on a global short message communication system.

Case for JILIN and other ‘floating’ assets using BeiDou

The JILIN signals that were unexpectedly received last year on L-band are BPSK with about 4Msps, the signals appear encrypted as there was no cross-correlation peaks noted during the analysis. This correlates to published information on the signal type as noted below.

BeiDou RNSS performance standard.
Source: BeiDou Navigation Satellite System Open Service Performance Standard (Version 3.0) China Satellite Navigation Office, May 2021, page 22.

Given our understanding of the BeiDou GSMC system it appears entirely reasonable that JILIN is using the BeiDou GSMC system to communicate with their satellites on a global basis. This also provides the first confirmation that the BeiDou GSMC system is operational and being used for assets well outside the regional view of China and for objects in orbit around the Earth.

Further research also shows that the BeiDou system is being used for a similar purpose globally for ships at sea. Therefore, it’s not a stretch to consider that TT&C of an airship could be accomplished using the BeiDou GSMC and RSMC systems.

Readers should be aware China doesn’t operate any GEO satellites over North America and until recently has only just begun to operate two satellites (Shiyan-10 and Shiyan-10 02) in a Molniya class orbit that have loitering apogees over North America. The purpose of these two missions is not presently known in the public domain and no emissions from them have been detected as far as the author is aware. This implies they may be on signals intelligence missions as opposed to communications assets like the Russian Meridian system or the US SDS platforms in HEO. This underlines how important a system like the BeiDou GSMC would be for any airship mission with a global reach as China has limited reliable global communications options.

Therefore, I theorize that it is reasonable to assess that Chinese airships with a global mission would incorporate the BeiDou GSMC system for TT&C purposes as there is no other systemic domestic system known to be available to the Chinese. However, given the limited bandwidth of the GSMC and amount of data that could be sent, mission data may need another path. Based on a lack of known relay satellites over North America I would assess that high volume data is stored on such a payload for later downlink either to Chinese GEO assets over areas of the world covered by them or mobile ground/sea stations that can intercept the payload and perform a local download and forwarding of the airships data once it has travelled to an area accessible to such assets. It’s possible those assets could be within the target area too.

Observers looking for possible balloon objects using the BeiDou system should monitor the 1610.5-1627MHz band for emissions and characterize them by looking for the absence of Doppler and the duration the signal remains. Bear in mind that Globalstar uses this band when making assessments of what you are monitoring.


Author: Scott Tilley

Amateur visual and radio astronomer, radio amateur VE7TIL

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