Emily Lakdawalla
Page 21
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 commu-
nications 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 down-
hill 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
23 Ashwin Vasavada interview, February 6, 2014
4.5 Telecommunication 157
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
158 How the Rover Works
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
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 engi-
neers 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 communi-
cations 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
24 Edwards et al (2013a and 2013b) describe orbiter relay support for Curiosity 25 Lakdawalla (2016)
4.5 Telecommunication 159
Gas
string)
race
(dual-
ve data rates
km circular
min period
ExoMars T
Orbiter
ESA
14 Mar 2016
19 Oct 2016
400
74° inclination
non-sun-synchronous
118
2.2m
Electra
435–450 MHz
8, 32, 128 kbps
390–405 MHz
1, 2, 4, ..., 2048 kbps
adapti
v 2013
ve data rates
VEN
sun-
ASA
MA
N
18 No
22 Sep 2014
150 × 6200 km
75° inclination
non-
synchronous 4.5 hr
period
2m
Electra
435–450 MHz
8, 32, 128 kbps
390–405 MHz
1, 2, 4, ..., 2048 kbps
adapti
.
2013b
2005
ve data rates
synchronous
and
min period
ASA
Mars Reconnaissance
Orbiter
N
12 Aug
10 Mar 2006
225 × 320 km
93° inclination
sun-
112
~3:00 p.m. ascending
node
3m
Electra
435–450 MHz
8, 32, 128 kbps
390–405 MHz
1, 2, 4, ..., 2048 kbps
adapti
2013a al
ds et
sun-
After Edwars.
Mars Express
ESA
2 Jun 2003
25 Dec 2003
330 × 10,530 km
86.9° inclination
non-
synchronous 7.5 hr
period
1.65m
Metacom
437.1 MHz
8 kbps
401.585626 MHz
2, 4, ..., 128 kbps
s orbiter
min period
y
ved to~6:00 a.m.
2001
km circular
ASA
Mars Odysse
N
7 Apr
24 Oct 2001
400
93° inclination sun-
synchronous 118
~4:00 a.m. ascending node
later mo
ascending node
1.3m
CE-505
437.1 MHz
8, 32 kbps
401.585626 MHz
8, 32, 128, 256 kbps
y
ver
Telecommunication capabilities of Mar
val dates
y Data rate
y
ranscei
ard link:
Table 4.2.
Agenc
Launch/arri
Orbit
HGA diameter
UHF T
Forw
Frequenc
Return link:Frequenc
Data rate
160 How the Rover Works
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.
Mars 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 per-
formed 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
26 Ashwin Vasavada, personal communication, email dated January 11, 2017
27 Ashwin Vasavada, personal communication, email dated January 11, 2017
28 Bagla (2017)
29 Lakdawalla (2015)
4.5 Telecommunication 161
Table 4.3. List of solar
Sols
Date
Location
conjunctions during the
Curiosity mission to date.
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 prob-
lems 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 staff-
ing. 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 trans-
missions, 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 mega
bits 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
30 Voosen (2016)
31 Edwards et al (2013a)
32 Sol 18 Mission Manager’s report, MSL Curiosity Analyst’s Notebook
33 Sol 17 Mission Manager’s report, MSL Curiosity Analyst’s Notebook
162 How the Rover Works
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 intro-
duces interference that mostly prevents Curiosity from achieving 500-megabit relay ses-
sions, 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.