[meteorite-list] Mars Exploration Rover Update - June 6-12, 2008

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 27 Jun 2008 13:41:11 -0700 (PDT)
Message-ID: <200806272041.NAA09085_at_zagami.jpl.nasa.gov>

http://marsrovers.jpl.nasa.gov/mission/status.html

SPIRIT UPDATE: New Tricks for an Old Rover - sol 1574-1579,
June 6-12, 2008:

To conserve energy and protect one of the on-board spectrometers,
spacecraft operators have established the first major change to planning
for the Mars Exploration Rover mission since the end of the primary
mission, which lasted for 90 days in early 2004.

Spirit's scientists have declared that their highest priority for the
winter is preserving the miniature thermal emission spectrometer, an
instrument that identifies minerals in rocks from a distance. To do
this, the rover heats the instrument overnight and into the morning of
every sol. These heaters have been running longer as winter temperatures
have dropped and are now averaging about 55 watt-hours per sol.

Heating for Spirit's batteries has increased as well and is now
averaging 29 watt-hours per sol. Together, the two heaters account for
84 watt-hours or about 37 percent of Spirit's total energy usage.
Everything else, including on-board computers and memories, radios,
cameras, sensors and actuators, gets by on about 140 watt-hours --
enough energy to run a microwave oven for a scant 7 minutes.

In response, rover operators have further reduced Spirit's activity
levels. The rover now transmits data to Odyssey to be relayed to Earth
only every fourth sol. Instead of spending 20 minutes each sol using the
rover's high-gain antenna to listen for new instructions from Earth,
Spirit spends five minutes listening for instructions using the low-gain
antenna on all but two sols per week.

Rover operators create new activity plans once a week, on Fridays, that
cover seven sols at a time. Because Spirit isn't engaged in activities
that require rover operators to have new images or other data for
planning, the rover does not have to relay data to Odyssey just before a
planning day. Despite changes to multiple procedures and software tools,
the transition has been remarkably smooth.

For the time being, Spirit is basically just hanging out, charging the
batteries.

Recent Events

Initially, the uplink team deleted virtually all science activities
except for tau measurements of atmospheric dust. They then limited
transmission of rover data to NASA's Odyssey orbiter, which consumes
about 30 watt-hours, to every other sol. Still, the batteries continued
to use more energy than they could replenish.

Prior to this change, Spirit received new plans three times a week and
listened for new instructions for 20 minutes every sol. The 20-minute,
high-gain-antenna communication window was costing the rover a lot of
unnecessary awake time. On the other hand, engineers on Earth needed
Spirit to be awake for at least 15 minutes every sol. By changing some
of the unneeded 20-minute communication windows to shorter, five-minute,
low-gain-antenna communication windows, and by having the windows
overlap with the required awake time, Spirit's operators have shortened
the overall awake time from 39 minutes to 16 minutes and saved another
15 watt-hours per sol.

Engineers have also gotten more strategic about how they communicate
with Spirit. They send a new activity plan to Spirit every week (after
the Friday planning session). Because Odyssey downlinks happen only
every fourth sol, they can't guarantee they'll have a communication from
Odyssey showing whether an uplink actually made it to the rover. So they
send the same activity plan a second time. If the first uplink is
successful, software on the rover automatically changes the second,
high-gain communication window to a five-minute, low-gain window, saving
15 watt-hours. When this occurs, the second uplink fails and flight
software generates a bunch of warnings, known as event reports. The
warnings tell engineers that the plan is on board. If the first uplink
fails, the instructions to change the second uplink window don't take
place.

Why do engineers do the planning on Fridays? A new schedule of
communication opportunities, called a "strategic load," goes into effect
every other Friday. Rover operators plan activities on Friday so they
can include the strategic load in the uplink.

The new strategy is working. Battery states of charge are up about two
amp-hours (an amp-hour is a measure of electrical current flowing for
one hour) above the rather scary levels of two weeks ago and other
indicators of battery health are similarly improving. Spirit now has
some margin of protection against further increases in heating power or
unpleasant changes in the Martian environment. One concern is the
possibility that thin, water-ice clouds could form overhead during the
Martian winter. Such clouds are nearly invisible without image
enhancement but they're thick enough to noticeably lower solar array
energy. Fortunately, there's been no evidence of water-ice clouds so far.

Turning the Corner

The Martian winter solstice will be on June 25, 2008 (sol 1591). During
the winter solstice, the Sun is as low in the sky as it ever gets. From
there, it will rise higher each sol until the summer solstice in May
2009. For Spirit, solar power levels are expected to increase in a few
weeks. Unlike Earth, where the coldest temperatures arrive 4-6 weeks
after the solstice, Martian temperatures will begin to rise again almost
immediately -- but slowly, very, very slowly.

With little dust overhead, Spirit is seeing very little additional dust
settling out on the rover's solar arrays. The dust factor, a measure of
the proportion of sunlight penetrating the coating of dust on the solar
arrays, has remained almost unchanged at 0.349 (meaning that 34.9
percent of the sunlight, direct and scattered, that reaches the arrays
penetrates the dust layer to generate electricity).

Energy has been steady, averaging about 226 watt-hours each Martian day,
or sol, and varying by only a couple of watt-hours (100 watt-hours is
the amount of energy needed to light a 100-watt bulb for one hour). This
is due in large part to a clear and stable atmosphere.

Tau, a measure of dust in the atmosphere, has ranged from 0.178 to 0.207
and averaged 0.193. As a result, between 81 percent and 84 percent of
the sunlight reaching Mars continues down through the atmosphere to
Spirit's solar array. (The remaining 16 percent to 19 percent is either
scattered or absorbed by dust particles in the atmosphere. The portion
of sunlight that's scattered also contributes to Spirit's solar array
energy.)

A Tau this low means the skies above Spirit are remarkably clear. Not
only that, Tau has decreased by an average of about 0.01 per week over
the last month. (Though scattering and absorption are different and not
exactly comparable, a clear mountain day on Earth has a Tau of 0.1-0.2.)

Spirit is healthy and all subsystems are performing as expected as of
the Odyssey downlink on Sol 1578 (June 11, 2008). The next planned
Odyssey downlink will be on Sol 1582 (June 15, 2008).

What Do You Say When You Call Home?

One of the key ways engineers monitor Spirit is through "event records."
These are messages generated by the flight software -- basically, the
rover's operating system -- telling Earth how Spirit is doing and why.
Most modern operating systems store such information in log files;
Spirit transmits it over long distances.

Spirit's event records come in five "flavors." Activity event records
note that some event has occurred. Command event records log the
issuance and success or failure of commands. Warning event records
indicate unexpected events. Fault event records indicate more serious
problems the flight software must address, usually by disabling further
use of some device or capability. Fatal event records indicate problems
so severe they invoke the fault protection features of the operating
system. At that point, the whole rover is "disabled" and goes into
so-called "safe" mode by shutting down all activities while waiting for
instructions from home.

Whether such warnings indicate a problem depends on the context. For
example, every time a motor stalls (stops turning while still powered),
Spirit's flight software generates a warning event record. Sometimes,
the stall is intentional. For example, engineers calibrate the position
of an actuator by slowly driving it into a mechanical "hardstop" at a
known position. When the hardstop is reached, the motor stalls and
issues a warning event record. If one of the motors stalls unexpectedly,
that same event record could indicate a problem.

It's like easing into a parking space until your wheels hit the parking
bumper. The resulting jolt says you are correctly parked. On the other
hand, a similar jolt could mean a fender bender, depending on the context.

Spirit's operators are sending two copies of new command sequences and
then sending it twice again on a backup uplink one or two sols later.
Data relays are so sparse, they don't always know if the first attempt
succeeded and want to minimize the risk of not getting a new sequence on
board.

Typically, the first attempt is successful and the second is rejected.
Flight software generates event records telling engineers that the rover
received the files correctly and copied them into the primary and
secondary file systems. The software then rejects the same files later
because their names conflict with existing files of the same name.

Rover operators don't actually read all the event reports in text form.
An identification number encodes the "flavor" of the event report and a
time tag called SCLK for "spacecraft clock" translates the time into
readable format. Ground software decodes these and a few other
parameters and converts them into readable words.

Sol-by-sol summary

Spirit completed the following activities:

Sol 1574 (June 6, 2008): Spirit recharged the batteries, listened to
instructions from Earth for five minutes using the low-gain antenna,
measured atmospheric opacity caused by dust using the panoramic camera,
and relayed data to the Odyssey orbiter as it passed overhead.

Sol 1575: Spirit recharged the batteries and listened to instructions
from Earth for 20 minutes using the high-gain antenna.

Sol 1576: Spirit recharged the batteries, listened to instructions from
Earth for five minutes using the low-gain antenna, and measured
atmospheric opacity caused by dust using the panoramic camera.

Sol 1577: Spirit recharged the batteries and listened to instructions
from Earth for five minutes using the low-gain antenna.

Sol 1578: Spirit recharged the batteries, listened to instructions from
Earth for five minutes using the low-gain antenna, measured atmospheric
opacity caused by dust using the panoramic camera, and relayed data to
Odyssey during the overhead pass of the orbiter.

Sol 1579 (June 12, 2008): Spirit recharged the batteries and listened to
instructions from Earth for five minutes using the low-gain antenna.

Odometry:

As of sol 1578 (June 11, 2008), Spirit's total odometry remained at
7,528.0 meters (4.7 miles).
Received on Fri 27 Jun 2008 04:41:11 PM PDT