The Design and Engineering of Curiosity
Page 19
Curiosity’s daily commanding is scheduled for approximately 10:00 a.m. local time because Earth is always above the horizon at that time. The high-gain antenna sits above the rover’s deck, but its view of Earth can be blocked by the rover mast or the hardware that sticks up from the back end of the rover (see Figure 4.7). To avoid this problem, rover drivers sometimes finish a drive with a turn designed to provide the high-gain antenna an unobstructed view of Earth for the next morning’s uplink window.
At some times, Earth can be quite low on the eastern horizon during the usual communications window. This happens a few months before Earth-Mars opposition, when Earth is at its maximum elongation in Mars’ sky and rises long after the Sun does in the morning (Figure 4.10). During these times, local topography and/or rover tilt can block the high-gain antenna’s view of Earth. For instance, when the rover crossed Dingo Gap around sol 535 – a time when Earth was already rising late – the rover finished the drive tilted downhill to the west, causing the RTG to obscure the high-gain antenna’s view of Earth. The mission rescheduled their morning command windows later in the day, when Earth was higher in the sky, compressing the time they had available for the day’s activities.23
Figure 4.10. Positions of Sun (colored circles) and Earth (squares) in Curiosity’s sky at 10 a.m. local time during the first Martian year of operations. Diagram by Emily Lakdawalla using Sun positions from the NASA GISS “Mars24” software.
4.5.2.2 Low-gain antenna
The low-gain antenna provided direct-to-Earth information on rover status throughout the landing. Since landing, it is used every rover morning when Curiosity finishes execution of its master sequence and starts execution of the next master sequence, an event called “hand-over.” At the start of the new master sequence, the rover sends a “beep” from its low-gain antenna; receipt of that beep on Earth indicates that all’s well with the new sequence. Other than that, Curiosity has only used its low-gain antenna when it is in safe mode. Because the rover may not know Earth’s position well enough to point the high-gain antenna, it awaits instruction from Earth through the low-gain antenna at a rate of 15 bits per second. One of the first actions in a safe mode recovery is to tell the rover where to point the high-gain antenna in order to increase data transmission rates.
4.5.2.3 UHF antenna
The single UHF quad-helix antenna is connected to redundant Electra-Lite transceivers, whose design is based upon the Electra transceiver in Mars Reconnaissance Orbiter. There are also Electra transceivers on NASA’s MAVEN and ESA’s ExoMars Trace Gas Orbiter. The NASA Odyssey and ESA Mars Express orbiters use older types of transceivers. Compared to the orbiter Electras, Curiosity’s Electra-lite is less capable, but it is also less massive and consumes less power. When Earth visibility is limited, the UHF system can also be used to receive commands. The UHF link can operate on one of three frequencies, but in practice Curiosity almost exclusively uses 401.585625 megahertz, the same as the fixed frequency of the Mars Exploration Rover and Phoenix radios.
4.5.3 Orbiter relays
Characteristics of all the Mars orbiters capable of communicating with Curiosity are listed in Table 4.2.24 Almost all of Curiosity’s data has passed through two of them, Mars Odyssey and Mars Reconnaissance Orbiter. When Curiosity landed, both orbiters traveled in near-polar, sun-synchronous orbits, with local time on the ground beneath the orbiter being about 3:00 a.m. and p.m. for Mars Reconnaissance Orbiter, and 4:00 a.m. and p.m. for Odyssey. On February 11, 2014 (sol 540), Odyssey began an orbit adjustment that would shift its orbit to 6:00 a.m. and p.m. The orbit shift was complete on November 10, 2015 (sol 1160).25 Table 4.2. Telecommunication capabilities of Mars orbiters. After Edwards et al 2013a and 2013b .
Mars Odyssey
Mars Express
Mars Reconnaissance Orbiter
MAVEN
ExoMars Trace Gas Orbiter
Agency
NASA
ESA
NASA
NASA
ESA
Launch/arrival dates
7 Apr 200124 Oct 2001
2 Jun 200325 Dec 2003
12 Aug 200510 Mar 2006
18 Nov 201322 Sep 2014
14 Mar 201619 Oct 2016
Orbit
400 km circular 93° inclination sun-synchronous 118 min period ~4:00 a.m. ascending node later moved to~6:00 a.m. ascending node
330 × 10,530 km 86.9° inclination non-sun-synchronous 7.5 hr period
225 × 320 km 93° inclination sun-synchronous 112 min period ~3:00 p.m. ascending node
150 × 6200 km 75° inclination non-sun-synchronous 4.5 hr period
400 km circular 74° inclination non-sun-synchronous 118 min period
HGA diameter
1.3m
1.65m
3m
2m
2.2m
UHF Transceiver
CE-505
Metacom
Electra
Electra
Electra (dual-string)
Forward link: Frequency Data rate
437.1 MHz8, 32 kbps
437.1 MHz8 kbps
435–450 MHz8, 32, 128 kbps
435–450 MHz8, 32, 128 kbps
435–450 MHz8, 32, 128 kbps
Return link:Frequency Data rate
401.585626 MHz8, 32, 128, 256 kbps
401.585626 MHz2, 4, ..., 128 kbps
390–405 MHz1, 2, 4, ..., 2048 kbps adaptive data rates
390–405 MHz1, 2, 4, ..., 2048 kbps adaptive data rates
390–405 MHz1, 2, 4, ..., 2048 kbps adaptive data rates
Curiosity’s communications system was designed to a goal of an average 75 megabits of data per sol through Odyssey and 250 megabits per sol through Mars Reconnaissance Orbiter. Particularly favorable passes can achieve 150 megabits through Odyssey and more than 500 megabits through Mars Reconnaissance Orbiter. In its slower orbit, MAVEN can relay even more data in a single pass, though less frequently.
Electra is a software-defined radio, which means that modifications to its programming can introduce new capabilities. After Mars Reconnaissance Orbiter launched, software engineers upgraded its radio to support adaptive data rate capability, where the transceiver monitors the signal-to-noise ratio of Curiosity’s transmission in real time, and commands the rover’s radio to increase the data rate when possible, making the most of every contact.
Because Odyssey and Mars Reconnaissance Orbiter are in sun-synchronous orbits, Curiosity can rely upon the availability of communications sessions with both of them twice a day, once before sunrise and once in the afternoon. The actual time of a communications pass depends on how far to the east or west of the rover the ground track passes. Successive Odyssey ground tracks are separated by about 29.5°, while Mars Reconnaissance Orbiter ground tracks are separated by about 27°. So the best ground track on any given sol may be as much as about 14° east or west of the zenith, which pushes the center of the contact time earlier or later in the day by nearly an hour. Passes that are not overhead are also of lower quality because of the greater distance separating the rover and orbiter and because the orbiter is above the horizon for a shorter duration. Some days may have two useable passes, both with poor data rates. There is a roughly 5- to 6-day cycle for each orbiter, affecting the quantity of data that Curiosity can deliver and the time of day at which the rover must be prepared to deliver the data.
Odyssey is an old orbiter, and its communication rate with Curiosity is limited to 256 kilobits per second. Conservative use of Odyssey’s remaining fuel should keep it going until around 2020. However, one of its four reaction wheels failed in 2012. If a second reaction wheel fails, it will have to transition to thruster-only attitude control, which will burn fuel at a much more rapid rate, ending the mission in 1 to 3 years. Its new, later orbit is less convenient for mission planning, because data relay comes much later in Curiosity’s day, limiting the time available to prepare sequences before they need to be uplinked.
Mar
s Reconnaissance Orbiter’s fuel could last until 2035 at current usage rates. But it has had important equipment failures. One of the two redundant traveling wave tube amplifiers for its radio system failed early in the mission, and it had to switch to its backup inertial measurement unit in 2013. The lifetime of Mars Reconnaissance Orbiter is likely to be limited to the lifetime of one or the other of these backups.
ESA’s Mars Express demonstrated relay capability several times early in its mission, on sols 13, 24, 30, and 59. They now test relay capability four times per Earth year.26 However, Mars Express is also aging. It has experienced some serious anomalies with its solid state memory and is running very low on maneuvering fuel. It could do emergency backup communication but is not likely to ever become a major participant in Curiosity data relay.
NASA’s MAVEN demonstrated Curiosity relay using adaptive data rates on November 6, 2014 (sol 800). The two missions began formally testing regular communications on April 3, 2016 (sol 1301) with a 10-part plan testing both forward and return links between the two spacecraft.27 As of 2017, MAVEN performs routine relay passes roughly once every other week. Exercising the relay communications between MAVEN and Curiosity is a high priority for JPL and NASA, because the future Mars 2020 rover has to plan to rely on MAVEN for telecommunications.
ESA’s ExoMars Trace Gas Orbiter carries two NASA-provided Electra radios. It performed a data relay test at a fixed rate with Curiosity on 22 November 2016. The orbiter will begin testing adaptive data rates and forward linking in 2018.
NASA is in discussions with the Indian Space Research Organisation (ISRO) to include Electra hardware on India’s second Mars orbiter, currently planned for launch in 2022.28
4.5.4 Issues affecting communications
During solar conjunction, when the Sun lies directly between Earth and Mars, reliable uplink can’t be counted on, so all Mars spacecraft are placed into a low-activity mode. Solar conjunction does not affect their ability to function, but if an activity placed a spacecraft in danger, Earth engineers couldn’t reliably uplink commands to resolve the problem. Solar conjunctions happen once every 26 months (roughly 760 sols), and the period of uplink blackout lasts for 3 to 5 weeks. Table 4.3 lists conjunctions during the Curiosity mission so far. During the 2013 conjunction, orbiters did not relay data to Earth. However, in 2015 and 2017, Curiosity spent conjunction uplinking data to orbiters, and the orbiters successfully relayed much of it to Earth.29 Table 4.3. List of solar conjunctions during the Curiosity mission to date.
Sols
Date
Location
236–261
April 2013
Yellowknife Bay
1005–1026
June 2015
Marias Pass
1759–1779
July-August 2017
Base of Vera Rubin Ridge
Occasionally, an Earth weather-related issue affects uplink or downlink; these problems are infrequent, but expected. However, the DSN has been embattled during the rover’s time on the Martian surface, with budget cuts stressing maintenance and staffing.30 The DSN has continued to meet its official targets of 95% uptime, but is suffering compared to historically overachieving performances of more than 99% uptime. For Curiosity, lost data is usually recoverable, but lost communications sessions can result in lost opportunities to acquire new data. If an uplink session is lost, Curiosity sits idle for at least a day, and the team has to choose whether to retry the same plan the next day. The loss of Friday uplinks results in the loss of two or three sols of activity. If there is a problem with the downlink of images after a drive, Curiosity can’t point at specific targets, drive, or use its arm in the next sol’s plan because the engineers don’t know the rover’s position. (Effectively, an unrestricted sol is turned into a restricted sol when a downlink session is lost.)
4.5.5 Performance on Mars
Mars Reconnaissance Orbiter returns the lion’s share of Curiosity’s data, though not as much as it might, because of interference from a spurious 400 megahertz tone generated by the orbiter’s CRISM instrument. When the rover landed, the orbiter shut off its science instruments temporarily in order to test the communications link.31 For the first two weeks, they tested varying frequencies and fixed data rates. Curiosity achieved a transmission rate of 2048 kilobits per second overnight on sol 17.
In the early morning of sol 18, they tested a new capability of adaptive data rate transmissions, in which Mars Reconnaissance Orbiter diagnosed the strength of the signal it detected from the Curiosity radio link, and commanded the rover to the optimal data rate as the signal strength changed. The pass had a maximum elevation angle of only 36° – not the best geometry – but the orbiter was able to command Curiosity to return data at high enough rates to receive 479 megabits of data, the largest-ever amount of data returned in a single communications pass from the surface of Mars by a wide margin.32 They began using adaptive-data-rate transmissions for all Mars Reconnaissance Orbiter passes on sol 22. With the new transmission protocol, Curiosity routinely exceeded predicted downlink volumes by factors of 2.33
From sol 27 through 62, they powered on the orbiter’s science instruments one at a time to assess the impact of interference on the quality of the signal. Operating CRISM introduces interference that mostly prevents Curiosity from achieving 500-megabit relay sessions, reducing the maximum nearer to 400 megabits. The effect is most pronounced at higher elevation angles. Nevertheless, the link still averages 225 megabits per downlink.34
Periodically, one or the other orbiter experiences an anomaly that sends it into safe mode, interrupting relay communications. As of this writing, there has never been a sol when both Odyssey and Mars Reconnaissance Orbiter were in safe mode. Should one of these two orbiters fail, MAVEN will be called upon to do more frequent communications sessions in order to ensure the Curiosity mission continues with as little interruption as possible.
4.6 MOBILITY SYSTEM
Curiosity’s mobility system comprises the wheels, their motors, and a system of linkages called a rocker-bogie suspension (Figure 4.11).35 The rocker-bogie suspension system permits the rover to traverse obstacles more than one and a half times the height of one wheel, while keeping all six wheels firmly in contact with the ground, distributing the weight of the rover evenly among the wheels, and limiting the tilt of the rover body.
The suspension system is connected to ten motors, which drive and steer six wheels. (The middle wheels do not steer.) Ongoing damage to the wheels has been a source of trouble for the mission, but careful driving has reduced the rate of damage, and Earth testing has verified that the rover will be able to complete planned mission extensions – all the way onto the highest unit that Curiosity can reasonably be expected to reach on Mount Sharp – even with the ongoing rate of damage.
Figure 4.11. Engineers demonstrate the obstacle-climbing capability of the rocker-bogie suspension system on the “Scarecrow” test bed rover, June 19, 2007. Scarecrow is a stripped-down model designed to exert the same force on Earth’s surface that the actual rover does on Mars under lower Martian gravity. Note that Scarecrow’s body is nearly level and all wheels are in contact with the ground despite the fact that three of the wheels are scaling obstacles similar in height to a wheel. Photo by Emily Lakdawalla.
4.6.1 Rocker-bogie suspension system
The front wheels attach to a long rocker arm. The middle and rear wheels are linked together to form a bogie, which connects to the back end of the rocker arm through a passively rotating pivot that can tilt forward and back by as much as 45°. The rocker arm is connected to the rover body at another passive pivot, which can tilt forward and back by about 20°. (In practice, much tighter limits are usually set on these pivots such that the rover will autonomously stop driving if unexpectedly higher angles are reached.) If that were the end of it, the rover body would flop forward or backward on the two rocker pivots, but a differential mechanism connects the left and right sides of the rocker-bogie suspension system
to keep the rover body nearly level. A vertical swingarm connected to the rocker rises above the rocker pivot and connects through a link assembly to a horizontal swingarm that crosses the back of the rover. The horizontal swingarm is attached to the rover body at the center differential pivot, another passive pivot.
If one front wheel climbs an obstacle, it pushes the horizontal swingarm backward on that side, resulting in an equal and opposite downward motion of the front wheel on the other side. The opposing vertical motions of the front wheels ensure that they maintain contact with the ground, and the rover body stays level. Meanwhile, the passive bogie pivot allows the middle wheel on the same side as the obstacle to drop, staying on the ground, as the front wheel climbs.
The rover is robust to local tilt, designed to be stable on a slope of up to 45°. For safety, rover drivers set tight limits on rover tilt based upon their expectations for the terrain. They rarely set limits above 7° of tilt for the rockers and 17° for the bogies, which are the angles they expect when traversing a 40-centimeter-tall obstacle sitting on flat terrain.36