Relay Architecture

Almost nothing a Mars surface vehicle collects goes directly to Earth. It goes to a UHF radio, to an orbiter passing overhead for a few minutes, into that orbiter’s memory, and to the Deep Space Network hours later. For Spirit and Opportunity’s primary missions through September 2005, about 92 percent of total data return went through Odyssey, 5 percent through Mars Global Surveyor, and 3 percent over the X-band direct-to-Earth link [1]. Curiosity’s downlink is UHF relay only [2].

Why relay

The rover UHF return link runs at 8, 128 or 256 kbit/s to Odyssey or Mars Global Surveyor against an X-band direct-to-Earth downlink that falls to 10 bit/s in the fault configuration [1]. The link exists only while an orbiter is above the horizon, so the downlink becomes a scheduling problem rather than a bandwidth problem, and more than 60 percent of MER mission data return was planned through it.

Pass geometry

	MER, Odyssey or MGS	Phoenix, Odyssey and MRO	Curiosity, MRO
Passes per sol per orbiter above 20 degrees elevation	1.8 average, minimum 3 per 2 sols, maximum 4
Passes per sol, both orbiters	up to 4	more than 5 average over 157 sols	2 planned, morning and afternoon
Pass duration	2 to 8 min
Pass times, local solar	MGS about 01:30 and 13:30, Odyssey about 04:30 and 16:30 at the Spirit site	forward link between 02:00 and 09:00, return between 12:00 and 19:00	afternoon pass 15:00 +/- 1 h at the equator

Sources: [1] for MER, [3] for Phoenix, [2] for Curiosity. Phoenix’s Odyssey and MRO supported 860 relay passes over the 157 sol landed mission, Odyssey providing 71 percent of the contacts; Phoenix limited itself to no more than two MRO passes per sol because MRO was still in its primary science mission [3]. Curiosity’s MRO pass start times range from 01:45 to 04:00 local mean solar time for the morning pass and 14:00 to 16:15 for the afternoon pass at 30 degrees north or south [2].

Geometry translates directly into volume. A higher maximum elevation gives both a longer pass and more time at short slant range, so the MER project preferentially scheduled high elevation passes [1]. Rover azimuth matters more than tilt, because the UHF antenna gain pattern is asymmetric while orbiter range dominates the tilt term: the same pass returning 50 Mb at a favorable azimuth can return half that if the high gain antenna assembly blocks the view. Rovers were eventually oriented deliberately in azimuth after a sol’s science to increase return, and the prediction program was later extended to handle tilt as well. On sol 278, 4 November 2004, Opportunity was tilted 31.04 degrees during the Odyssey afternoon pass, and the no-tilt against 31 degree predictions were 57.4 Mb against 41.5 Mb [1].

How much actually gets down

Mission	Planned or required	Achieved
Spirit and Opportunity	more than 60 percent of mission data return by relay	about 56 Mb/sol per rover via Odyssey, about 49 Mb/sol per rover via MGS
Phoenix	60 Mb/sol return, 30 Mb in a single afternoon overflight; 1 Mb/sol forward in a single morning overflight	more than 38 Gb over 157 sols, an average 242 Mb/sol, more than four times the requirement; 574 Mb of command data delivered, 97 percent via Odyssey
Curiosity	250 Mbit/sol prime via MRO, 100 Mbit/sol optional via Odyssey, planning assumption 125 Mbit per pass; decisional data capped at 100 Mbit/sol	modeled worst case no less than 125 Mbit/sol, average 687 Mbit/sol

Sources: [1], [3], [2]. Curiosity’s model assumes MRO performs a roll maneuver of up to 30 degrees every pass to point its UHF antenna at the rover, a 10 degree elevation mask, the rover at the equator, adaptive data rates, the best morning and evening passes, and 2 sigma return link margin [2]. Relay capacity follows a 5 or 6 sol pattern of two low volume passes followed by three or four high volume ones.

Phoenix’s forward link exceeded its requirement in the same way: the commanding requirement was 1 Mb per sol, but once the link was characterized the uplink bundles regularly ran 1 to 2 Mb, and raising the Odyssey morning pass to a 32 kbit/s forward rate gave the science team about 12 Mb of forward capability that was rarely fully used [3].

The orbiter buffer is part of the budget

Odyssey allocated 100 Mbit, 12.5 MB, of onboard memory to both MER rovers and to Beagle 2 under the pre-landing relay operations plan, later raised to 120 Mbit per rover for the primary mission [1]. What a rover can send in a pass is therefore constrained by what remains in the orbiter buffer from the previous pass, which in turn depends on the Deep Space Network coverage allocated to the orbiter [1]. When the MER return rate was raised to 256 kbit/s for some Odyssey passes, the allocation could be overflowed by a single overflight.

Prediction accuracy

Relay planning runs on predicted volumes, so the useful measurement is prediction error rather than throughput. For Phoenix and MRO over a 30 day period, with a 2 dB margin policy [3]:

Elevation mask	Average data volume delta	Passes more than 10 percent below predict
10 degrees	-0.22 percent, -0.262 Mbit	25.00 percent
15 degrees	+13.13 percent, +6.84 Mbit	3.75 percent

The Phoenix science team was shown this and chose to keep the 10 degree mask and leave the model alone, on the reasoning that the Odyssey model was conservative enough to guarantee the planned volume, with the rule that any critically needed data through MRO would be planned to a 15 degree mask instead [3].

Forward link and contingency

Relay carries commands as well as data. MSL uplinks sequences, flight software and other data through the relay orbiters, and expects to use the 03:00 MRO pass for sequence uplink more than half the time, with large non-time-critical data cleanup commands sent in that pass [2]. The Electra-Lite radio can wake the rover avionics on hearing a hail forward link from MRO, which is a contingency path independent of the rover’s own schedule.

Mars Express provided backup relay for Phoenix. It received the one-way link during part of entry, descent and landing, and a demonstration pass on the third surface sol verified relay capability at an 8 kbit/s forward and 32 kbit/s return rate [3]. Fifteen contact opportunities were booked for the critical first week, invokable at short notice by sending a direct operations request file to the Mars Express project, with the orbiter reusing a lander pointing mode tested with Opportunity in 2008 that boresights the Melacom antenna within 35 degrees of the landing site. Three of those passes were exercised.

Lunar relay

Lunar relay is at the specification stage rather than the operational one for NASA. The LunaNet interoperability specification defines the service interfaces for lunar relay and position, navigation and timing services rather than a specific spacecraft [4]. The Chinese Queqiao and Queqiao-2 relay satellites that served Chang’e-4 and Chang’e-6 have no open technical description in English.