Lunar communications equipment must handle long signal delays, changing Doppler shifts and interruptions caused by orbital motion and terrain—all before it leaves Earth. The RLS-2100 Radio Link Simulator from Square Peg Communications lets engineers recreate these conditions in the laboratory while testing real radios, modems and terminals. It combines RF channel impairments, orbital dynamics and transmitter-to-receiver signal-path modeling in one instrument, with up to 2.4 GHz of processing bandwidth, allowing teams to evaluate individual links and end-to-end communications performance before deployment.
A lunar mission depends on local links between surface assets, communications between a rover and an orbiting relay, and onward connections from that relay to Earth. Each segment has different propagation conditions, visibility windows, and timing requirements. Testing must establish whether the equipment can maintain communication as those conditions change, and how it behaves when a connection becomes unavailable.
Distance is one part of the problem. A radio signal takes roughly 1.25–1.3 seconds to travel one way between Earth and the Moon, introducing delays into commands, acknowledgements and data transfers. Orbital motion adds changing Doppler shifts, propagation delay and received signal levels. Visibility creates another constraint: the lunar far side has no direct line of sight to Earth, while craters, ridges and other terrain can obstruct local or surface-to-orbit links. A receiver therefore needs to be evaluated under changing conditions rather than only at a fixed signal level.
Recreating Lunar Link Conditions with Real Communications Hardware
The RLS-2100 brings those conditions into a test setup through hardware-in-the-loop testing. Engineers connect the RF input and output ports of actual communications equipment to the simulator, then configure the stations, signal paths, and impairments. Signals passing through the instrument experience the selected conditions in real time. This allows teams to observe how their equipment acquires a signal, maintains a connection and recovers after an interruption, using repeatable laboratory scenarios.
The modeled path can include the RF characteristics of the transmitter, uplink, satellite or relay, downlink, and receiver. Supported impairments include path loss, delay, Doppler, additive noise, phase noise, interference, multipath fading, blockage, shadowing, antenna patterns, and high-power-amplifier nonlinearity. These functions allow a test to combine several effects, for example, declining received power and changing Doppler as a relay moves toward the edge of coverage. Atmospheric and rain-fading models are also available for applicable Earth-side links.
For lunar applications, the supplied scenarios range from local surface communications and rover-to-orbiter links to Earth-to-lunar-relay and satellite-to-satellite cislunar connections. The important distinction is that these paths have different behavior: a short local connection cannot be represented by the same delay and motion profile as an Earth-Moon link. The simulator supports fixed, position-based, and user-file-driven path parameters, allowing the test conditions to follow the intended scenario.
Configurable RF Channels, Handover Testing and Automation
The device’s bandwidth is configurable across multiple channels. Supported arrangements include two channels of 1,200 or 1,000 MHz, or four channels of 600, 500, 400, 300, 200, 150 or 100 MHz. The standard independent input/output frequency range is 140–2,450 MHz, with optional coverage of 400–7,125 MHz and 10.7–31 GHz, the latter using an external converter. Up to four cooperating units can provide 16 channels, supporting larger test configurations.
Multiple channels also support tests in which a terminal changes its serving beam or satellite. The RLS-2100 can model make-before-break handovers using multiple channels and break-before-make transitions using a single channel. Its real-time orbital modeling covers LEO, MEO, HEO, GEO, and mixed constellations, with independently configurable impairments. This gives engineers a way to examine both the transition itself and the changing RF conditions surrounding it.
For test automation, the RLS-2100 provides an Ethernet-based Python API, with station-position interfaces including Ethernet and ARINC 429, and antenna emulation through OpenAMIP or custom interfaces. Integrated GPS L1 simulation operates at 1,575.42 MHz. Spectrum, power profile, and link parameter displays help engineers relate equipment behavior to changes in range, loss, delay, and Doppler, supporting investigation and repeat testing before the communications hardware is deployed.
LunaNet Interoperability and Delay-Tolerant Networking
Shared lunar infrastructure adds an interoperability requirement. NASA, ESA and JAXA jointly develop the LunaNet Interoperability Specification, which defines common standards and interfaces for cooperating communications and navigation services. The objective is to let compatible spacecraft, surface terminals and relay networks work together across providers, reducing the need for each mission to build an independent infrastructure. Ground testing must therefore examine the behavior of connected equipment as well as individual RF links.
Delay/Disruption Tolerant Networking (DTN) addresses the networking side of interrupted connectivity. Its store-and-forward approach lets a node retain data until the next connection becomes available, rather than requiring an uninterrupted end-to-end path. In a test bed containing DTN-capable equipment, RF link emulation provides a way to examine how the network responds to delayed or unavailable connections. The simulator supplies the link conditions; the connected networking equipment implements the data-storage and forwarding behavior.
This is becoming more important as lunar exploration moves toward repeated missions and shared infrastructure. Artemis II carried four astronauts, including Canadian astronaut Jeremy Hansen, around the Moon in April 2026, and NASA plans an Artemis III demonstration in Earth orbit in 2027 to test operations involving Orion and commercial lunar landers, followed by the first Artemis lunar landing on Artemis IV, targeted for early 2028, and another surface mission on Artemis V later that year. These missions create a continuing need to validate communications between equipment supplied by different organizations.
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