Antenna Communications in the Lunar Environment

Most satellite landers and orbiters today use either a dish or a single fixed antenna. This article will examine the benefits of using different antenna designs in the lunar environment. Specifically, the article will focus on a lunar orbit on the far side of the moon, where Vulcan Wireless has contributed to the development of advanced lunar communication devices for upcoming missions. Using a typical lunar orbit, the article examines the link performance for a particular operating scenario. The operational scenario that will be considered has three devices on the lunar surface, where all three devices are in a band and trying to exfiltrate sensor data back to Earth.

Landing and surviving on the moon require careful attention to both radio and antenna design. As an example, environmental conditions include extreme temperature changes ranging from -410°F (-246°C) to 250°F (121°C) on the lunar surface. Several upcoming missions will be utilizing both Vulcan Wireless’s software-defined radio (SDR) and cryogenic antenna for S-Band. This article compares the performance of this system with a Vulcan Wireless phased array antenna that is currently in development and available for future deployments. The basic metric that will be utilized for link performance is the data exfiltration rate. This is the amount of data, typically sensor data, that the lander or rover can transmit back to Earth on an average Earth day. The article will describe ways to maximize this data.

Vulcan Wireless is producing multiple SDRs for lunar operations within NASA Commercial Lunar Payload Services (CLPS) programs. Shown in Figure 1 are past and upcoming CLPS missions. Specifically, Vulcan Wireless has SDRs in Firefly’s Blue Ghost Mission 1, Firefly’s Blue Ghost Mission 2 and Firefly’s Lunar Orbiter. These missions are depicted in Figure 1 as item numbers 3 and 4.

Figure 1 Lunar landers and lunar orbiters in the NASA CLPS program.

Note that some near-side lunar missions can communicate directly with the Earth without the use of a lunar orbiter. However, far-side missions require an orbiter for communication. To communicate to a far-side lunar lander/rover, a basic approach involves the use of a directional satellite dish on the orbiter. This article will examine the communication performance in this extreme lunar environment. The key communication performance metric that is used is the number of data bits that the lunar lander can exfiltrate per Earth day. A larger number of exfiltrated bits means more sensor data and more images can be captured and analyzed back on Earth. The communication performance in the presence of interference will be discussed. The article will show how Vulcan Wireless’s phased array antenna can be used to combat interference and significantly increase the exfiltration rate.

For the communications protocol, a number of different communication waveform protocols can be used. The Vulcan SDR, shown in Figure 2, supports many different Consultative Committee for Space Data Systems (CCSDS) protocols that are used in space and lunar applications. Specifically, it supports CCSDS telecommand (TC), CCSDS telemetry (TM), CCSDS proximity and CCSDS DVB-S2, which are critical for ensuring reliable and standardized data transmission. The SDR has been used for uplinks, downlinks and crosslinks. The SDR also has support for precision navigation and timing. Software configurations allow the radio to be used for both time transfer and ranging applications. The SDR exceeds expectations in radiation testing at the NASA Goddard facility and it is also available with military-grade top secret and below encryption.

Figure 2 Vulcan Wireless SDR.

Figure 3 Vulcan Wireless cryogenic antenna.

The simulations use the antenna profile of the Vulcan Wireless cryogenic S-Band antenna shown in Figure 3. NASA has approved this antenna to withstand the lunar night, which can reach -410°F (-246°C). The lunar surface is particularly challenging due to the temperature extremes accompanying the change between lunar day and lunar night. A lunar day and lunar night are equal to one Earth month, which is 30 Earth days.

SINGLE ANTENNA POINTING AT THE LUNAR LANDER

To understand data exfiltration on the moon, a lunar orbit derived from the expected ephemeris of upcoming launches is used as an example. The tracks are shown in Figure 4. The upper image in Figure 4 shows the track for one Earth day and there are three distinct tracks. These are three passes that would occur during a 24-hour Earth day. The lower picture has many tracks, which correspond to all the passes within a 30-day Earth month. The yellow markings on both images in Figure 4 indicate the locations used in the simulation analysis.

Figure 4 Lunar orbiter track and simulation analysis locations.

Figure 5 Single orbiter pointed at Rover 0, other rovers not transmitting.

Figure 4 identifies three hypothetical rover locations to be used in the simulations. The location of Rover 0 is at the proposed location for Lunar Surface Electromagnetics Experiment Night. The location of Rover 1 is at the South Pole at an upcoming planned lunar mission site and the Rover 2 location is on the near side in Mare Crisium.

Figure 5 shows the simulation results for Rover 0 pointed at a single orbiter with no other rovers transmitting. The top subplot shows the carrier-to-noise density over time, the middle subplot shows the data rate over time and the bottom subplot shows the total amount of exfiltration data over time. Figure 6 shows the same data presentation for Rover 1 pointed at a single orbiter with no other rovers transmitting. Figure 7 shows the data presentation for Rover 2 pointed at a single orbiter with no other rovers transmitting.

Figure 6 Single orbiter pointed at Rover 1, other rovers not transmitting.

Figure 7 Single orbiter pointed at Rover 2, other rovers not transmitting.

In this hypothetical comparison, the orbital satellite and the rovers all have consistent and realistic link parameters, such as transmit power and antenna gain. They are kept constant across the data examples to make realistic comparisons. From the data plots, it can be observed that Rover 1 has the lowest exfiltration of the three. That is, Rover 1 can exfiltrate 0.37 Mb/day on average, while Rover 0 is able to exfiltrate almost 10x that, at 3.3 Mb/day and Rover 2 is able to exfiltrate the most, at 12.3 Mb/day.

As the data shows, the location of the lunar lander can have a significant effect on the amount of exfiltration data that can be captured. Note that for some rover locations, the amount of data per pass can change significantly, as in the case of Rover 0. But for some rover locations, a pass is very consistent. This is the case for Rover 1, located at the South Pole. The three hypothetical rover locations used in the simulations and the resulting average exfiltration rates shown in Figures 5 to 7 are summarized in Table 1.

SINGLE ANTENNA POINTING AT THE LUNAR LANDER WITH INTERFERENCE

To illustrate the effect of unintentional co-channel interference, this section considers the case when the orbiter is communicating with Rover 1, but Rover 0 is transmitting in the same band. That is, Rover 0 is causing interference with Rover 1 communicating to the orbiter. This may be the case when Rover 0 is deployed from a country that does not participate in widely accepted spectrum allocations and standards. For example, in this scenario, the country may not have gotten approvals from the National Telecommunications and Information Administration (NTIA).

Figure 8 Rover 0 interfering with Rover 1 with the orbiter pointed to Rover 1.

In this case, Rover 0 degrades Rover 1’s performance. This is illustrated in Table 2. The instantaneous data rate and carrier-to-noise density are shown in Figure 8. Note that the interference from Rover 0 degrades the performance significantly on some passes and insignificantly on others. The overall performance reduces the exfiltration rate by over 50 percent of the non-interfering exfiltration rate. Specifically, without interference, Rover 0 was able to exfiltrate 0.37 Mb/day, but in the presence of interference, that result gets reduced to 0.18 Mb/day.

PHASED ARRAY ANTENNA POINTING AT THE LUNAR LANDER WITH INTERFERENCE

The other interesting result shown in Table 2 is the improvement in the data exfiltration rate when the orbiter has a phased array antenna, even when Rover 0 is interfering with Rover 1. For this application, Vulcan Wireless has developed the S-Band phased array shown in Figure 9. The phased array antenna is a smart antenna that autonomously determines the direction of arrival of both the desired source and the interference. The performance curves for Rover 0 interfering with Rover 1 when the orbiter uses a phased array radar are shown in Figure 10. Comparing the results of Figure 10 with the results of Figure 8, it is clear that the phased array improves the performance by more than an order of magnitude over the single antenna case in the presence of interference. The hardware for the phased array SDR and smart antenna leverages the flight-proven technology of a previous generation of phased array antennas.

Figure 9 Vulcan Wireless S-Band phased array antenna.

Figure 10 Rover 0 interfering with Rover 1 and orbiter using phased array antenna.

CONCLUSION

This article has discussed data exfiltration from the lunar surface back to Earth. It has looked at several cases to illustrate how the data exfiltration rate depends upon the location of the rover relative to the orbiter. A significant degradation in data exfiltration rate has been observed when a second rover is broadcasting its data to a secondary orbiter. However, introducing a phased array antenna on the orbiter increases the gain to the desired user and helps to mitigate the effects of the in-channel interferences. Even in the presence of interference, this architecture has been shown to increase the amount of data exfiltration by a lunar rover by more than an order of magnitude versus the best-case performance of a single antenna with no interference.