In June 2022, the Electra Relay Operations team began conducting radiometric phase and power data collection during relay overflights of the InSight Lander by the ExoMars Trace Gas Orbiter (TGO). This mode of the Electra radio has potential applications for both navigation and radio science, but is relatively untested in flight. Here, we present both one-way (non-coherent) and two-way (coherent) Doppler observations conducted during 32 TGO relay overflights. Then we explore the ability to find the surface position of the lander with each data set and compare this against other methods.Looking forward to Mars Sample Return, the capability to do one-way Doppler tracking would be essential for monitoring the launch performance of the Mars Ascent Vehicle and determining the final trajectory of the Orbiting Sample container. Testing these positioning capabilities now will help us to prepare for that anticipated need.

The Mars Relay Network (MRN) has successfully demonstrated how a communications network at another planet can be assembled from dissimilar spacecraft over many years to provide relay services to smaller Mars surface missions. Consisting of Mars orbiters from NASA and ESA, NASA's Mars surface missions, NASA's Deep Space Network (DSN), ESA's deep space tracking network (ESTRACK), and the various operations centers that manage each of these elements (among other entities), this highly successful example of an inter-agency collaboration is a case study in how relay networks at other locations around the solar system could be instantiated.As exploratory interests in other planetary bodies (such as at the Moon or at Venus) continue to increase, the likelihood of needing to implement a relay network at those locations similarly increases. Though much study has been done on the link budgets, communications protocols, and overall network architectures between the various nodes in such networks, many of the lessons learned from the MRN are instead related to managing operations challenges.This paper outlines specific solutions that have been developed to manage the challenges that have arisen from the heterogeneous nature of the MRN, interfacing missions with different spacecraft designs, data handling architectures, operations paradigms, and institutional cultures. The adoption of data exchange standards has been instrumental in ensuring interoperability between the spacecraft and between their human operators. On this latter point, the critical role of the Mars Relay Operations Service (MaROS), which provides a centralized database and platform for mission operators to schedule and coordinate relay opportunities within this node-limited network, is summarized.This paper also describes the ongoing challenges of the ever-evolving MRN as it continues to provide relay services to the Mars surface missions and prepares to provide support for future Mars missions. Finally, this paper makes some recommendations regarding how future, similar networks may improve upon what has been achieved by the MRN.

The Next-Generation Mars Telecommunications Orbiters, a.k.a. Mars ComBirds, (MCB), are intended to serve as a deep-space relay hub that provides high-performance links to Earth at extreme data rates and to increase data return from a variety of Mars rovers, landers, aerobots, and science orbiters. As the current science orbiters used for relay are aging, the needs for these MCBs are becoming realistic, justified, and increasingly urgent. The message is further echoed when demands for Earth return data for next-decade missions to Mars, such as ExoMars, Mars Ice Mapper, Mars Sample Return, and future human exploration missions to the Red Planet, continue to increase. In addition, by communicating directly to these MCBs instead of Earth, communications systems for future science missions can be reduced. Thus, the costs can be lowered, or more science equipment can be added. Furthermore, MCBs' fields of view with Earth are much longer; therefore, an appropriate choice of orbits, a network of MCBs with cross-link capability can connect any users at Mars with Earth almost continuously. In this paper, we primarily provide a trade study on the design of the MCB orbits, which include the number of orbits, sizes, shapes, and orientations. Special attention is also given to a class of orbits that provides daily repeating ground tracks. These orbits can facilitate surface operations because they rise and set daily over a specific area at constant revisiting times. In addition, there is another class of orbits where a spacecraft would tug a science orbiter to a sun-sync Mars orbit and then raise its altitude and serve as a relay orbiter. More particularly, we will consider different orbit types such as (1) circular equatorial, (2) circular sun-sync, (3) Apoapsis at Constant time-of-day Critically Inclined (ACCI), (4) Apoapsis at Constant time-of-day Equatorial (ACE), and (5) SEP-Tugs. Mars surface users are assumed to be global and of any longitude and latitude. For users in orbit, we assume their orbital parameters similar to the typical low-Mars sun-sync orbits such as Mars Odyssey and Mars Reconnaissance orbiters. JPL-developed Telecom Orbit Analysis and Simulation Tool (TOAST) software is used to compute the contacts between the orbiters and users. The performance of these orbit constellations can be assessed through several metrics of interest, which include the maximum latitude, number of contacts per sol, contact duration, total contact time per sol, and maximum communication gap. Recommendations for the optimal orbital constellation choices (3-planar and coplanar variations) will be provided based on comparing the weighted means of each metric calculated at latitude-longitude coordinates during a simulation duration of 1 sol. The chosen orbits will then be further investigated in greater depth to weigh the pros and cons regarding a satellite's operational capabilities and limitations at that orbit.

On February 18, 2021, NASA's Mars 2020 Rover “Perseverance” conducted the Entry, Descent, and Landing (EDL) sequence to land safely on the surface of Mars at Jezero Crater. Due to the criticality of the EDL sequence, redundant telecommunications strategies were utilized to observe the event. The rover transmitted an X-band (8.4 GHz) signal encoded with Multiple Frequency Shift Keying tones direct-to-Earth and a UHF (401.5 MHz) signal with 8 kbps encoded telemetry to orbiters, which relayed the signal back to Earth at X-band. NASA's Deep Space Network complexes at Goldstone, California and Madrid, Spain received the direct-to-Earth X-band signal. Two of the largest steerable radio telescopes in the world, the 100-meter Green Bank Telescope in West Virginia and the Effelsberg 100-meter Radio Telescope in Germany also received the UHF carrier signal on Earth. During the EDL event, full spectrum recordings of the UHF and X-band signals were captured at these observatories using open-loop receivers. Real-time and post-processing signal analysis to determine power level, received frequency, and Doppler shift allowed determination of the spacecraft's health and safety state during critical sequences, including event detection such as atmospheric entry, plasma blackouts, and parachute deployment. Simultaneous observations of the UHF and X-band radio links allow for redundancy in the event of a fault and greatly improve confidence in Doppler event detection.

The Perseverance rover represents NASA's latest achievement in Mars exploration. Landing successfully on 18 Feb 2021, the rover's transmitted data during its entry, descent, and landing (EDL) were captured by the Mars Reconnaissance Orbiter (MRO) and the Mars Atmosphere and Volatile Evolution (MAVEN) orbiter. This data, broadcast in near-realtime to the world, allowed everyone to share in the excitement (and “terror”) of the day. The images returned thereafter included the first images of the new landing site, video of the landing itself taken from a variety of vantage points, and eventually the historic images of the first powered flight on another planet. Behind the scenes, the return of that data to Earth was accomplished via Mars orbiters operated by NASA and ESA, using three different ground tracking networks. Considered together, this Mars Relay Network (MRN) enabled the successful, timely, and unobtrusive return of the rover's data. This paper describes the preparations taken by the participants of the MRN in anticipation of the arrival of Perseverance at Mars. These were not only focused on successfully acquiring the rover's critical event telemetry during its EDL, but also on readying the network to return the rover's data on an ongoing basis as it pursued its mission objectives. Included is a brief description of the MRN, which represents a highly successful international collaboration and continues as critical infrastructure for NASA's and ESA's ongoing Mars exploration. Also summarized are the activities performed prior to EDL, including landing site reconnaissance and mission test and training activities; those activities performed on EDL day, especially the recording, return, and processing of the rover's critical event telemetry; and those activities that are now being performed on an ongoing basis during the rover's surface operations, including an outline of the planning processes that enable relay services. Finally, a description of the performance of the network to-date on behalf of the Perseverance rover is given, summarizing the success of the network to provide support to both it and other spacecraft on the surface of Mars.

This paper describes the implementation of Low-Density Parity-Check (LDPC) channel codes in the Mars Relay Network; specifically in the NASA MAVEN orbiter and the ESA ExoMars Trace Gas Orbiter. LDPC codes, also known as Gallager codes, were invented in the early 1960s. At the time, available computational hardware was not powerful enough to implement these codes, so they became “forgotten.” They were later “rediscovered” in the 1990s, whereupon it was demonstrated that practical LDPC decoders performed substantially better than standard convolutional codes. This spurred research and development of LDPC codes at JPL, which culminated in the implementation of a rate 1/2 LDPC decoder and encoder in the Electra proximity radio delivered to MAVEN in 2012 and subsequently to other Mars spacecraft. The paper starts with a brief history of LDPC code development and then goes on to describe the development and validation of recent firmware and software updates to facilitate LDPC code operation with adaptive data rate control. The LDPC codes developed for Electra were verified to produce approximately 3 dB coding gain relative to NASA's standard convolutional code, increasing data return by up to a factor of two. The paper concludes with an assessment of in-flight performance of LDPC-enabled relay passes between MAVEN and the Mars 2020 rover.

A highly successful international collaboration, the "Mars Relay Network" (MRN) leverages the combined NASA and ESA orbiter capabilities to transfer data to and from Mars surface missions. The MRN has admirably flight-demonstrated the benefits of a relay network and validated how international protocol standards may be used to ensure interoperability. However, principally designed for science missions, these orbiters addressed relay requirements as a secondary function, which introduced limitations to what can be achieved with the network. Next-decade missions are expected to have significantly greater communication needs than can be accommodated by the aging MRN. This paper reports the results of a broad study that evaluated the potential of a next-generation relay network, referencing the current MRN as a benchmark. A wide variety of orbital altitudes, surface latitudes, and mission scenarios were evaluated around a specific set of assumptions regarding the telecommunications payloads included. The study outlined how current day technologies could be applied to greatly enhance the data throughput to and from Mars on behalf of future science and reconnaissance missions. The instantiation of such a network would be enabling for a variety of missions and mission classes that have been heretofore unachievable, including both large and small orbiters, and landed vehicles representing new mission types (i.e. climbers, diggers, drones, etc.), and further argues that such a network would be instrumental in advancing human exploration interests at Mars.

A Mars Sample Return (MSR) campaign would involve a series of three flight missions to acquire and cache Mars samples, retrieve those samples and launch them into Mars orbit, and then capture these samples and return them to Earth. Relay communications would be crucial for supporting this campaign, characterized by multiple critical events, complex surface operations, and an on-orbit Mars rendezvous. The existing Mars relay network offers significant capability, and efforts are underway to maximize the likelihood that one or more of these current assets will still be operational in the timeframe of an MSR campaign. In addition, the Earth Return Orbiter (ERO) element of a campaign could serve as a primary relay asset, if it can achieve a useful relay orbit by the time of arrival of the Sample Retrieval Lander mission. We describe key operational challenges of the MSR campaign that would drive the required relay capabilities, and characterize the performance of the existing relay orbiters as well as ERO itself in meeting those relay needs.

In anticipation of increased demands on Mars relay services, including planned arrivals of NASA's InSight Lander in November 2018, and of NASA's Mars 2020 rover and ESA's ExoMars Rover & Surface Platform mission in Feb-Mar.2021, we have assessed a number of potential modifications and upgrades to Mars relay orbiters and quantified their impacts in terms of key relay support metrics, to support NASA and ESA programmatic decisions. Specific areas of investigation include: 1) implementation of "split-pass" relay capability for support to collocated landers, 2) modifications of the extended mission orbit for the Mars Atmosphere and Volatile EvolutioN Mission (MAVEN), and 3) introduction of Low-Density Parity Check (LDPC) coding. We report here on each of these potential upgrades, quantify the performance implications each would have on future Mars relay services in the context of future mission support scenarios, and provide a status on implementation.

The European Space Agency's ExoMars Trace Gas Orbiter (TGO) arrived at Mars on October 19, 2016, three days after releasing the Schiaparelli Lander on a ballistic trajectory to Meridiani Planum. During the separation event, and subsequently during Schiaparelli's Entry, Descent, and Landing (EDL), the NASA-provided Electra Ultra-High Frequency (UHF) payload onboard TGO was used to record signals from the Schiaparelli Lander for post-processing on the ground to recover both tracking of the lander's carrier signal and reconstruction of the lander's 8 kb/s telemetry. In addition, ESA's Mars Express orbiter recorded the Schiaparelli signal, with ground post-processing providing independent tracking of the lander carrier signal, and the Giant Metrewave Radio Telescope near Pune, India was configured to provide real-time detection of the lander carrier signal. While an anomaly in the latter stages of EDL led to loss of the lander, these critical event data sets, and in particular the telemetry reconstruction enabled by the TGO Electra recording, proved essential in enabling detailed diagnosis of the anomaly. While the loss of the lander during EDL precluded the planned surface relay operations, the preparations for that activity provide important lessons learned for future Mars relay support scenarios.

The Mars Atmosphere and Volatile EvolutioN (MAVEN) mission will launch in late 2013 and, following a 10 month cruise to Mars, will study the upper atmosphere of the planet. In addition to the science instruments, the MAVEN spacecraft is equipped with an Electra UHF transceiver to support relay communication with landed assets. This paper describes how UHF relay service is provisioned by MAVEN. The discussion includes a description of the Electra payload, the process by which relay activities are coordinated and accounted for, the process of a typical relay session, including uplink and downlink, as well as special commands to calibrate and verify relay performance. The operational processes for providing these services are inherited largely from prior Mars missions and take advantage of existing infrastructure and lessons learned from those missions. Preliminary data volume return capabilities using adaptive data rates and low-density parity check channel coding are presented.

Two landers are scheduled to arrive at Mars in the fall of 2016. Telecommunication relay support from the network of Mars relay orbtiers will be crucial in supporting these landers' mission objectives. Arriving first, NASA's InSight Lander mission is scheduled to arrive at Mars in September 2016. Coverage of InSight's Entry, Descent, and Landing (EDL) requires re-positioning of the Mars Reconnaissance Orbiter's orbit plane. After landing, InSight relay support is complicated by the proximity of its landing site to Gale Crater, where Curiosity may well be continuing its surface mission. We present strategies for addressing the relay needs of both landers. Three weeks later, the ESA/RSA ExoMars Trace Gas Orbiter will arrive, deploying an EDL Demonstrator Module (EDM) to demonstrate Mars landing technologies and carrying out a short four-sol surface mission. NASA relay orbiters will be critical for support of the EDM surface mission. TGO will continue its science operation while also providing relay services to ESA and NASA landers using a NASA-provided Electra UHF transceiver.

The Mars Science Laboratory (MSL) mission landed the Curiosity Rover on the surface of Mars on August 6, 2012, beginning a one Martian year primary science mission. The UHF relay link from Curiosity to the Mars Reconnaissance Orbiter (MRO) incorporates new features enabled by the Electra and Electra-Lite software-defined radios on MRO and Curiosity, respectively. Specifically, the Curiosity-MRO link has for the first time utilized frequency-agile operations, increased data rates from 256 kbps up to 2048 kbps, employed suppressed carrier modulation and a new Adaptive Data Rate algorithm in which the return-link data rate is varied to match the observed channel condition. During the first 200 sols, the telecom operations team has been able to tune the radio and protocol parameters to maximize return-link data volume, which is now averaging roughly 500 Mbits per sol or twice the design requirement of 250 Mbits per sol. The telecom team has also derived new predict models that reduce data volume prediction errors and that quantify the impact of operational modes and link parameters, providing further planning insight for MSL mission operations team.

The Mars Relay Operations Service (MaROS) has been deployed by NASA's Mars Program Office and the Multimission Ground Systems and Services (MGSS) Project into mission operations to aid in the coordination of relay activities at Mars. This live system presents standardized interfaces and a centralized infrastructure to current and future participants in the Mars Relay Network for the purpose of reliably and securely exchanging and storing all relay-related planning and operations data. The initial development of this system leveraged over eight years of experience performing relay operations between the various spacecraft at Mars. Now, four years after its initial deployment, MaROS continues to undergo further refinement to better meet the needs of the Mars Relay Network. The most substantial, recent update was focused on providing capabilities needed by the Mars Science Laboratory project, which landed on Mars in August of 2012. This paper will describe the nature of that update and describe additional features being added to the system to better serve the needs of current and future Mars missions.

Over the past decade, several NASA Mars orbiters - Mars Global Surveyor, 2001 Mars Odyssey, and the Mars Reconnaissance Orbiter - along with ESA's Mars Express orbiter, have provided telecommunications relay services to a series of Mars landers, including the Mars Exploration Rovers (Spirit and Opportunity), the Phoenix Lander, and the Mars Science Laboratory's Curiosity Rover. For each of these missions, relay communications has demonstrated significant benefits, including greatly increased data return from the Martian surface, reduced energy-per-bit cost of communication, and capture of high-rate critical event engineering telemetry during entry, descent, and landing. The orbiters in this relay network, however, are operating well beyond their original design lifetime. To replenish this aging infrastructure, two additional science/relay orbiters are slated for launch in this decade, both equipped with Electra UHF relay transceivers with the plan to provide relay services in addition to each mission's primary science objectives. On November 18, 2013, NASA successfully launched the Mars Atmosphere and Volatile Evolution Mission (MAVEN). And in January 2016, ESA plans to launch the ExoMars/Trace Gas Orbiter mission, with redundant Electra payloads provided by NASA. Key aspects of each mission relating to its relay service characteristics will be reviewed.