The Space Architect's Diary #006
Artemis III: HLS - Gateway Communication Architecture
What is the communication architecture between the Human Lunar System (HLS) and Gateway during the different stages of the Artemis III missions? Below is an overview of our research on how the Artemis III communication architecture between various Cislunar Space Platforms(Earth, HLS, and Gateway) varies during different mission stages.
As a premise, when the Artemis III HLS mission starts, the Gateway station will already be orbiting the Moon in a polar Near Rectilinear Halo Orbit (NRHO). The communication architecture applies to both SpaceX‘s HLS and Blue Origin‘s HLS. SpaceX is more likely to have a single-tier HLS. In comparison, blue Origin will more likely have three-tier HLS – Transfer Vehicle Element (TVE), Descend Element (DE), Ascend Element (AE) – as it partners with Lockheed Martin, Northrop Grumman, and Draper, who will be designing the firs two Elements.
Launch - Cruise
During the Commercial Launch Vehicle’s(CLV) pre-Launch, launch, and ascend stages, the HLS comm systems will be nominally powered off. The CLV systems will communicate with the CSP-Earth (Cislunar System Platform) for critical communications. The CLV communication system is the primary source of communication with CSP-Earth while the CLV ascends to Low Earth Orbit (LEO), High Earth Orbit (HOE), and Cruising.
Note: At all times throughout the entire mission, CSP-Gateway and CSP-Earth maintain concurrent X-band (2Mbps Download, 5Mbps upload) and Ka-band (100Mbps download, 20Mbps upload) Radio Frequency (RF) links.
The HLS, also referred to as the Visiting Vehicle (VV), separates from the CLV after Translunar Injection (TLI). It then cruises along a translunar patched trajectory heading to rendezvous with the Gateway in its NRHO. Just after Separation, the HLS’s communication systems are turned on.
NASA has given SpaceX the choice of ground station(s), the number of links, and the characteristics of each link to use for the HLS/CSP-Earth communications, considering that a 2‐way encryption is mandatory on all links, as well as IP encapsulation. The links should also support command, voice (No latency), data, video, & tracking (Range & Range-Rate).
It is most likely that SpaceX will use X-band and Ka-band RF comm links. Both frequencies have higher throughput and require smaller equipment sizes than the S-band, and in case of severe weather conditions on earth, the HLS can rely on the X-band to link to CSP-Earth.
V V Range
The VV maintains concurrent link(s) with CSP-Earth throughout the translunar cruise and the entire mission.
When the HLS approaches the Deep Space Gateway’s (DSG) VV range (400km from the Gateway), it establishes and maintains an S-band (24 – 100 kbps upload/download) link with Gateway capable of 2-way command, voice, data, and tracking (range and range-rate) transmissions. This VV S-band link is maintained as long as it is within the VV range.
Crew Arrival, Transfer, and Checkout
Once the HLS is 200m away from the Gateway, a local Wi-Fi link is established between the VV and Gateway to support high-rate video and telemetry transfer, supporting the Rendezvous, Proximity Operations Docking (RPOD) operations.
Once the HLS docks with the DSG, a hard-line connection is established between the two CSPs through the docking port connector. At this point, the already established Wi-Fi link will supplement the connection.
The Orion Spacecraft, carrying the four crew members, would also dock, and its crew members transfer onboard the DSG into the Mini-Hab Module known as HALO (Habitat and Logistic Outpost).
During EVAs, the Wi-Fi link is used for communication between the astronauts and with Gateway Control.
Descend to Luna
As the HLS disengages from the DSG and sets on a descending trajectory, it will maintain the VV S-band link and the Wi-Fi link with the DSG ( as long as it is within the 200 m Wi-Fi range).
After the HLS crosses the VV range (400Km), it transitions from the VV S-band link with Gateway to an S-band Lunar Systems link for ranging, crew communication, and status. An additional Ka-band link is established with DSG for high-rate data transfer.
At Low Lunar Orbit (LLO), the HLS will cast its TVE (in the case of a three-element HLS design) and head down for a lunar approach trajectory. During descent, HLS maintains concurrent Radio Frequency (RF) communication link with earth (Direct-with-Earth Links) and the Gateway (LS VV S-band, & Ka-band link). RF comm links might be interrupted during attitude perturbation descent but will immediately be re-established.
Upon landing on the surface of the Moon, the HLS still maintain the RF comm links with earth and Gateway; however, because of a stable landing attitude, the HLS now switches from Omni/Hemi antennas to directional antennas for all links with Earth and DSG.
Lunar Surface - EVA
For EVA activities on the surface of the Moon, the astronauts use a UHF EVA link for voice and data links both between each other and the HLS.
The HLS also has an integrated Wi-Fi link to support high-rate EVA communications and data transfer in its proximity. The Wi-Fi link will have a 100Mbit/sec data rate in close proximity; however, the Wi-Fi has a max range of 500 meters as at that range, the data range drops to 1Mbit/sec.
Because the astronaut’s EVA suites do not have an integrated directional antenna, they will not have a direct link to either Earth or Gateway.
Ascend to Gateway
As the HLS (or the AE) ascends back to the Gateway, it maintains the RF comm links (S-band and Ka-band lunar system links). At the VV range (400 Km), it switches from the S-band Lunar System to the S-band VV link. As the HLS approaches the DSG for docking, it establishes a Wi-Fi connection link with the Gateway at the 200-meter range, then a direct Link once docked.
Trip Back to Earth
During Artemis III missions, NASA does not expect all elements of the HLS to be reused. Once back onboard the Gateway, all four astronauts will board the Orion Spacecraft, which will head back to earth maintaining concurrent X-band (2Mbps Download, 5Mbps upload) and Ka-band (100Mbps download, 20Mbps upload) Radio Frequency (RF) links.