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Instrument Host Overview
========================
For most Mars Exploration Rover (MER) experiments, data were
collected by instruments on the spacecraft. Those data were then
relayed directly to stations of the NASA Deep Space Network (DSN)
on Earth or indirectly using the Mars orbiters Mars Global Surveyor
(MGS) or 2001 Mars Odyssey (ODY). MER Radio Science observations
required that DSN and/or ODY hardware also participate in data
acquisition. The following sections provide an overview first of
the MER spacecraft, then of the DSN ground system, and finally of
2001 Mars Odyssey as each supported MER science activities.
Instrument Host Overview - Rover
================================
The Mars Exploration Rover (MER) mission sent two identical
spacecraft to two different landing sites on Mars. The hardware on
the first spacecraft is referred to as MER-2 (the rover named
Spirit which was sent to Gusev Crater), and the hardware on
the second spacecraft is referred to as MER-1 (the rover named
Opportunity sent to Meridiani Planum). The spacecraft design owed a
lot of its heritage to the Mars Pathfinder configuration for cruise
and entry, descent, and landing. For more detailed information on
the MER spacecraft, see [CRISPETAL2003].
Spacecraft Configuration for Entry, Descent, and Landing
--------------------------------------------------------
After separation from the cruise stage, the 840 kg entry vehicle
consisted of a backshell and heatshield enclosing the lander. The
550 kg lander had a tetrahedral structure that the air bags were
deployed from and surrounded, and that housed the gas generators
for the airbags, the RADAR altimeter, motors for the unfolding of
the tetrahedron's four sides or 'petals,' and the rover lift
mechanism for standing up the rover. On route to Mars, the rover
was stowed within the lander, and contained most of the power,
computing, and communication electronics for all phases of the
mission. The backshell and heatshield provided thermal protection
from the hyperbolic entry into the Martian atmosphere through the
use of ablating materials. Mounted inside the backshell was the
parachute, the deceleration and transverse impulse solid
rocket (TIRS) motors, a backshell inertial measurement unit, and
thermal batteries for the entry and descent phase. The Entry,
Descent, and Landing camera was mounted on the lander radar
bracket. The Descent Image Motion Estimation Subsystem (DIMES)
gathered the results of a real-time image correlation of surface
features contained in three successive Entry, Descent, and Landing
camera images of the Martian surface to compute the horizontal
velocity of the descending vehicle. TIRS used this horizontal
velocity measurement along with measurements of the attitude of
the backshell to compute a TIRS rocket firing solution.
Rover on the Surface of Mars
----------------------------
After landing, the instrument host for each spacecraft is just the
rover itself, which is a 6-wheeled drive, 4-wheel-steered vehicle
180 kg in mass, including the science package. At its wheel base,
the rover is approximately 141 cm long and 122 cm wide. At the
height of the solar panel, the rover is approximately 225 cm wide
by 151 cm long. In its deployed configuration with the Pancam
Mast Assembly (PMA) deployed, the rover is 154 cm tall.
The rocker bogie suspension system gave the rover the ability to
drive over obstacles approximately one wheel diameter (26 cm) in
size while providing a stable platform for instrument
measurements. The distribution of mass of the vehicle allowed the
vehicle to be stable even at a 45 degree tilt. Each wheel and
steering degree of freedom was independently actuated, which
allowed the vehicle to turn in place (turning diameter 1.9 m), to
skid steer to a tighter angle (turning diameter as small
as 0.9 m), to turn in gradual arcs, or to drag wheels that
effectively trench the Martian regolith. When moving on flat
terrain, the vehicle could achieve a top speed of 5 cm/s. Under
autonomous control using its hazard avoidance system, the rover
drove with an average speed of about 1 cm/sec.
The rover was powered by a combination of solar arrays and
rechargeable batteries. The solar panel provides 30 strings of
triple junction cells (gallium indium phosphorus, gallium
arsenide, and germanium) covering 1.3 square meters, which
produced about 800 to 900 W hours per sol at the beginning of the
MER mission. Each rover had two reference solar cells, one that
measures short circuit current and another that measures open
circuit voltage. Due to the change in season from late southern
summer to early southern autumn, and the degradation in
performance due to dust deposition, the energy produced by this
array dropped to about 600 W h per sol, 90 sols after landing.
Energy was stored in two 8 A h lithium ion rechargeable batteries
to provide over 400 W h of energy to support rover peak power
operations and provide auxiliary heating and operations overnight.
Temperature-sensitive electronics were housed in the rover warm
electronics box (WEB) which is a box built with honeycomb
composite material and insulated with 2.5 cm of opacified aerogel.
A combination of radioisotope heater units, waste heat from
electronics, and auxiliary heating by survival heaters ensured
that the internal electronics were maintained between -40 and +50
degrees C as the external Mars environment cycle ranged from 0 to
-97 degrees C. Survival heating in the WEB requires not more than
100 W h of energy during the coldest environment conditions. The
rechargeable batteries housed in the WEB supplied this energy.
The rover received commands and transmitted data to the Earth
through two distinct systems: a direct-to-Earth X-band system
supported by both a low-gain antenna and a steerable high-gain
antenna (HGA), and a UHF system supported by a monopole antenna
which enables relay communication to orbiters at Mars. Early in
the surface mission, the X-band system through the high-gain
antenna supported up to 28.8 kbps to a 70 m Deep Space Network
(DSN) station. Commands were received through the high-gain
antenna at a rate of up to 2000 bps. The X-band system through
the low-gain antenna provided a minimum capability of transmitting
telemetry at 40 bps and receiving commands at 40 bps throughout
the MER missions. The UHF system supported telemetry rates of up
to 256 kbps during orbiter passes which lasted up to 8 minutes
each. The UHF system also supported a command receipt capability
of 8 kbps through Odyssey only.
The computing, command, and data handling functions of the rover
were supported by a 20 MHz 32-bit RAD6000 processor housed in a
Versa Module Europa (VME) card cage. This radiation hardened
processor had access to 128 Mbytes of DRAM and 256 Mbytes of
nonvolatile flash memory that supported a multiprocess C-coded
software architecture. This system, supported by auxiliary
processing functions housed on boards within the VME card cage,
had the capability to acquire images from pairs of cameras, drive
up to 10 motors simultaneously from 35 motors located on the
vehicle, and process data from three spectrometers. The
multiprocess architecture allowed communication, image
acquisition, and operation of payload elements to proceed
simultaneously.
During the surface mission, the rovers communicated on X-band
typically once a day, reporting on status and the results of the
execution of commands transmitted that day. Data were also
relayed through the UHF communication system to the Mars Global
Surveyor and Mars Odyssey orbiters. Useful over-flights by these
orbiters at the landing site occurred as frequently as twice
per day per orbiter.
Each rover carried the Athena Science Payload consisting of two
remote sensing instruments that viewed the terrain from the top of
a mast 154 cm above the ground, four devices for in-situ analysis
on the end of a robotic arm and several magnets and calibration
targets. [SQUYRESETAL2003] describes this payload, the mast, the
robotic arm, and the plans for science investigation in more
detail. Azimuth and elevation actuators permitted the collection
of data sets for specific targets, regions, or full 360-degree
panoramas from the mast instruments, which are the stereo
multispectral Panoramic Camera (Pancam) and Miniature Thermal
Emission Spectrometer (Mini-TES). The five degree-of-freedom
robotic arm positioned the following devices on rocks and soils
for in-situ analysis or rock abrasion: Alpha Particle X-ray
Spectrometer (APXS), Moessbauer Spectrometer (MB), Microscopic
Imager (MI), and Rock Abrasion Tool (RAT).
In addition, the stereo navigation cameras (Navcams) mounted on
the top of the mast and the stereo hazard detection cameras
(Hazcams) pointed towards the ground beneath the solar panels in
the front and rear of the rover were required for engineering
purposes (rover navigation, hazard avoidance, and safe movement
purposes and positioning of the robotic arm), but were also used
for science analysis [MAKIETAL2003]. [ARVIDSONETAL2003] provides
more detail on the use of rover engineering sensors for assessing
terrain and soil physical properties, dust accumulation, and
other related investigations.
The MER rover coordinate frame is defined as follows:
-- +Zr axis is normal to the rover top deck plane and points down,
from the top deck toward the wheels;
-- +Xr axis is parallel to the rover top deck plane and points
from the center of the top deck toward the PMA assembly;
-- +Yr completes the right hand frame.
The origin of the MER rover 'navigation' frame is directly above
the middle wheels, as shown in the diagram below. A separate
MER rover 'mechanical' frame has its origin 29 cm toward the
front wheels (in the +Xr direction) but is otherwise identical.
UHF /\
HGA \/ PMA
.--. # ||
/ \ # ||
| | # ||
\ /=. # ||
`--' || # || Rover
======================= (deployed)
| =o=. |
| .' Yr `.__|o====o
.===o=== o------> Xr \\
.-. .|. `.-. ##o###
| o | | | | | o |
`-' `|' `-'
V Zr
Instrument Host Overview - DSN
==============================
The Deep Space Network is a telecommunications facility managed
by the Jet Propulsion Laboratory of the California Institute of
Technology for the U.S. National Aeronautics and Space
Administration (NASA).
The primary function of the DSN is to provide two-way
communications between the Earth and spacecraft exploring the
solar system. To carry out this function it is equipped with
high-power transmitters, low-noise amplifiers and receivers,
and appropriate monitoring and control systems.
The DSN consists of three complexes situated at approximately
equally spaced longitudinal intervals around the globe at
Goldstone (near Barstow, California), Robledo (near Madrid,
Spain), and Tidbinbilla (near Canberra, Australia). Two of
the complexes are located in the northern hemisphere while the
third is in the southern hemisphere.
Each complex includes several antennas, defined by their
diameters, construction, or operational characteristics:
70-m diameter, standard 34-m diameter, high-efficiency 34-m
diameter (HEF), and 34-m beam waveguide (BWG).
For more information see [ASMAR&RENZETTI1993].
Instrument Host Overview - 2001 Mars Odyssey
============================================
The 2001 Mars Odyssey (ODY) spacecraft was built by Lockheed Martin
Astronautics (LMA). Most spacecraft systems were redundant
in order to provide backup should a device fail. In addition
to transmitting data collected by ODY instruments and systems,
the telecommunications system was used to relay data from Mars
surface assets and measure their relative motion radiometrically
in the 400 MHz frequency range. For more information, see
[JPLD-16303].
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