# Engineering Constraints for MSL Landing Sites

## EDL engineering constraints: Introduction

The Mars Science Laboratory Mission will land a long-range rover
equipped with a sophisticated suite of scientific instruments on Mars.
The entry, descent, and landing (EDL) system is designed to land the
rover within a 10 km radius circle at elevations as high as 2 km with
respect to the MOLA (Mars Orbiter Laser Altimeter) defined geoid.

The rover, inside the entry vehicle, will separate from the de-spun
cruise stage and within approximately 10 minutes, after turning to a
proper attitude, will encounter the martian atmosphere at hypersonic
velocity of approximately 6 km/s (Figure 1). The entry vehicle's heat
shield will slow the spacecraft. Peak heating will occur after
atmospheric entry, and within a few (3-4) minutes the vehicle will
decelerate to a supersonic velocity of approximately Mach 2. The vehicle
will then deploy the parachute for further deceleration at an altitude
of \~10 km. The heatshield will then be released, the radar activated
and separation from the parachute will occur allowing the start of the
powered descent phase. During powered descent, the vehicle will use the
radar and its propulsion system to control position and velocity. At
approximately 35 m above the surface (as measured by the radar), the
rover will be lowered on a tethered system from the propulsion system in
a "sky crane" configuration and placed directly on the martian surface
with its mobility system fully deployed (wheels deployed). The
propulsion system (known as the descent stage) then flies away from the
rover. It should be noted that the design and specifications of the
landing system are still being worked, so all performance
characteristics identified below should be considered working
assumptions subject to future review and revision.

![](media/image1.jpeg){width="6.122916666666667in"
height="3.7756944444444445in"}

Figure 1. Illustration of EDL events.

## Latitude

MSL EDL is designed to land at any latitude between 60°N and 60°S.

## Altitude

Previous landing sites have generally been at low elevation for an
adequate atmospheric density column to provide enough drag and
consequently enough time to allow completion of all the events needed
for a safe landing. For MSL, the EDL System is designed to provide
guidance during descent, thereby allowing the landing site altitude to
be as high as +2 km with respect to the MOLA defined geoid.

In order to achieve this performance, MSL deploys a large 19.7 m
diameter supersonic parachute at approximately Mach 2.0. Since there are
some potential risk issues with this deployment, which are currently
being evaluated, a possible contingency plan (which is not currently
baselined) of deployment at lower Mach number could reduce landing site
altitude from +2.0 km to -2.0 km. The Mars Program is therefore also
interested in identifying potential landing sites that meet this
\"contingency\" requirement and specifically request additional
submissions of candidate sites for this -2.0 km landing site altitude
scenario.

## Landing Ellipse

MSL EDL is designed to allow precision landing with errors no greater
than 10 km radially. It is likely that additional analysis will slightly
change the size and shape of the landing ellipse (more ellipsoidal with
a slightly shorter cross track direction).

## Slopes

A Doppler velocimeter/altimeter, known as the terminal descent sensor,
on the descent stage uses multiple radar antennas to measure the
distance to the surface and the descent velocity (both vertical and
horizontal components). The first measurement is taken while still on
the parachute before backshell separation, with continuous measurements
up to rover release. Over the range of the vehicle's trajectory during
this time, slopes in the landing site are important for accurate
altitude estimation, fuel consumption, and vehicle safety. These
considerations result in the following slope constraints within the
landing ellipse: three degrees or less over a 2 to 5 km length scale to
avoid altimetry errors in the start of powered descent; fifteen degrees
or less over a 20 to 40 m length scale to avoid altimetry errors in
preparation for the sky crane landing maneuver; and fifteen degrees or
less over a 5 m length scale to ensure landing stability and
trafficability of the rover. Slopes at the kilometer length scale can be
addressed with MOLA point-to-point elevation data. Slopes at the 20-40 m
length scale can be addressed by extrapolation from MOLA point-to-point
elevation and pulse-spread data as well as via stereo image data and
photoclinometry. Slopes at the 5 m length scale require high-resolution
stereo images or photoclinometry. Intermediate and small length scale
slopes are expected to be determined for only the highest priority
sites. Although not all of the landing ellipse must meet all of these
slope requirements, the more area that exceeds them, the less likely the
site would meet all the safety criteria thereby increasing the chances
for failure and selection of a different landing site.

## Rocks

The area below the rover must be free of rocks capable of damaging the
rover's lower structure, or "belly pan," which is 0.6 m above the
ground. The rover mobility system (section 2.1.4) can accommodate rocks
that are 0.5 m high. The probability of damaging the rover via landing
on high rocks must be a small fraction of the allowable failure
probability being book-kept for EDL. This allocation implies the
probability that a rock taller than 0.6 m occurs in a random sampled
area of 4 m^2^ (the area of the belly pan) should be less than 0.25%. If
the rock size-frequency distribution is assumed similar to models based
on measured distributions at the existing landing sites, this translates
to a rock abundance (cumulative area covered by rocks) of around 10%.
However, given the expected acquisition of very high-resolution images
of high-priority landing sites during the selection process, potentially
damaging rocks may be characterized directly to estimate this hazard
more accurately.

## Winds

Steady horizontal and vertical winds over the last few kilometers of the
descent result in the landing system's velocity potentially matching the
wind speed and can increase the total velocity of the landing system
limiting control over the landing error ellipse and rover velocity at
touchdown. The entry system and low rover touchdown velocity presently
constrain steady horizontal and vertical wind speed to be less than 30
m/s and 10 m/s, respectively, at altitudes between 0 and 10 km above the
surface. No particular constraints on wind gusts are presently known or
anticipated, although testing and models during development may yield
future constraints. Models of the atmosphere at the highest priority
sites will be used to compare with tested configurations.

Radar Reflectivity and Thermophysical Properties

The surface material at the landing site must: i) be radar reflective
(sufficient radar backscatter cross-section) to enable measurement of
altitude and velocity during descent, ii) bear the load of the rover at
landing, iii) be trafficable by the rover (next section), and iv)
experience a range of temperatures within the limits of the rover
design. These requirements constrain the radar and thermophysical
properties of the surface materials, including albedo, thermal inertia
(and bulk density, through the latter), radar backscatter cross-section
and reflectivity (and inferred bulk density).

The doppler velocimeter/altimeter requires Ka band radar echoes from the
martian surface to properly measure altitude and velocity of the descent
vehicle. This requires that the landing site have sufficient radar
backscatter cross-section (\>-20 db at Ka band) and a radar reflective
surface. The requirement will be addressed via X-band, S-band, and UHF
radar returns and models that relate their backscatter and reflectivity
to Ka band.

Broad tracts of Mars have very low thermal inertia and high albedo and
have been interpreted to be surfaces dominated by loose dust that could
be meters thick. Experience and extrapolation from the existing landing
sites argues that loose dusty material is not load bearing. In addition,
at least one such dusty surface is not radar reflective. Global thermal
inertia and albedo data show a mode with thermal inertias less than 100
J m^-2^ s^-0.5^ K^-1^ and albedo higher than 0.25 that corresponds with
very dusty surfaces. Further, the rover is designed for temperatures
between approximately 145-310K, with a maximum diurnal range of 145K.
Although thick, dusty surfaces are unlikely to violate these temperature
constraints, they will fail to meet the other requirements listed above.

Surfaces with these characteristics (thermal inertias less than 100 J
m^-2^ s^-0.5^ K^-1^ and albedo higher than 0.25) are not suitable for
landing spacecraft or driving rovers (next section) and the dust would
curtail science operations. Large temperature extremes at low thermal
inertia, high albedo sites would also reduce surface operations through
the diversion of available energy to rover thermal maintenance and
reduced hazard avoidance (from CO~2~ frost coverage).

# MSL Trafficability Considerations

## Vehicle Performance Characteristics

The following sections outline the intended mobile capabilities of the
MSL surface system. Constraints derived from the mobility system are
separate from those derived from EDL, but are levied on the entire
landing ellipse and any traverse planned outside the ellipse as might
occur for a "go to" site. It should be noted that the design of this
vehicle is still in its preliminary stages, so all performance
characteristics identified below should be considered working
assumptions subject to future review and revision.

### Traverse Rate and Distance

Rover traverse speed is affected by several variables, both operational
and environmental. The vehicle's mechanical speed is determined by the
rotational rate of its drive and steering actuators, while the system
speed is a combination of mechanical speed and required computational
time for navigation and hazard avoidance. Finally, vehicle speed is
greatly affected by the terrain in which it traverses, both in slope
incline and slip rate.

Currently, the rover is being designed for a mechanical ground speed of
4.2 cm/s (approximately 2.5 m/min) on hard, flat terrain. When the
vehicle is using hazard avoidance and onboard path planning, the
effective traverse rate would be 50% of the mechanical speed, or 2.1
cm/s (approximately 1.25 m/min). When visual odometry, a technique using
engineering cameras to determine actual vehicle traverse progress, is
utilized, the resultant traverse rate would be 25% of the mechanical
speed, or around 1 cm/s (approximately 0.6 m/min).

As part of its primary mission, the MSL rover includes the capability
for traversing long distances. Currently, the system is being designed
for a total actual traverse distance capability of no less than 20 km.
For purposes of hardware life and cycle evaluation, it is assumed that
this traverse occurs over a terrain with an average rock abundance of
15%, an average slope of 5 degrees, and an average slip rate of 10%.
Under these conditions the rover would travel on average about 100-150
m/sol.

### Vehicle Maneuverability

The vehicle's mobility system is a 6 wheel drive, 4 wheel steer
rocker-bogie configuration, similar in architecture to MER. Given this
configuration, the vehicle has the capability to perform three types of
traverses: 1) straight line motion, forward or reverse; 2) turn-in-place
motion, pivoting the vehicle about a position in the center of the
vehicle at the midpoint between the two center wheels; and 3) arc turn
motion, with a minimum arc turn radius capability of 1.5 m.

### Static Stability / Slope Access

Vehicle stability is a key characteristic of both a successful skycrane
touchdown and surface accessibility. The MSL rover is being designed to
a static stability of no less than 45 degrees tilt in any direction. Of
course, vehicle slope access will also likely be affected by the
composition of the local terrain itself, so the static stability limit
should only be seen as an upper bound for vehicle safety. Nominal
vehicle operations will usually be kept at vehicle tilt angles below 30
degrees. For testing the slope of the surface is grouped into three
types: Low slope ([\<]{.underline}5 degrees); Moderate slope (5\< slope
[\<]{.underline}15 degrees); and High slope ([\>]{.underline}15
degrees).

### Hazards and Rock Field Trafficability

One key feature of the rocker-bogie suspension system is its ability to
traverse obstacles larger than the vehicle's wheel diameter. Coupled
with a high ground clearance, this gives the rover a significant
capability to traffic areas of the surface populated by dense rock
fields. Currently, the MSL rover is being designed to successfully
traverse a protrusion or hole obstacle of less than 0.5 m in height /
depth. This compares to a 0.2 m allowable obstacle height / depth on
MER.

One measure of the effect of increased obstacle-climbing capability is
in a parameter called vehicle mean free path. Specifically, mean free
path is a measure of the total straight line distance the rover could
traverse without encountering a hazard that would have to be avoided.
The MSL vehicle's mean free paths in a terrain covered by a 20% rock
distribution is 48 m.

While the above information highlights the MSL rover's increased
trafficability in rocky terrain, it should also be noted that other
types of protrusion / hole hazards, particularly sand ripples, may
affect the overall climbing capability. This, of course, can and will
vary as a function of the size and shape of the feature. Ripples of a
size similar to the vehicle's own wheelbase or trackwidth may more be
appropriately be categorized as sandy slopes, and therefore performance
would be dictated by the limitations outlined in the next section.
Surface roughness that could limit trafficability can be assessed from
radar backscatter data and derived root-mean-squared slope.

### Slope Traversability -- Granular Material

The vehicle's capability to access science targets at high terrain
angles will be driven by the soil's own material properties as much as
the vehicle's own performance criteria. One important characteristic of
vehicle design that affects traversability in granular media is ground
pressure, which is a function of vehicle weight and wheel size. MSL is
currently designing to an average ground pressure that is less than or
equal to that of MER. Given this design requirement, it is a reasonable
assumption that MSL will have similar traversability in granular
material to the twin MER rovers.

The ground pressure of the rover requires the surface to be load
bearing. Very low density, very fined grained materials may not be load
bearing. Dusty material did not support a footpad of Viking Lander 1 and
experience with Mars Pathfinder and the Mars Exploration Rovers
indicates that deposits of dust are not load bearing. As a result as for
landing, surfaces dominated by fine-grained dust are not suitable for
rover traversing. Thermal inertia, albedo and radar reflectivity will be
used to assess dusty and non-load-bearing surfaces.

Increased terrain angle in granular or aggregate materials will
predominately affect the rover's slip rate performance. In general, slow
speed vehicle traverse in sandy / granular terrain can be separated in
three distinct categories:

-   Low / Moderate slope angle (\< 10 degrees tilt) = 0 -- 25% upslope
    slip

-   Transitionary slope angle (10 -- 17 degrees tilt) = 25 -- 80%
    upslope slip

-   High slope angle (\> 17 degrees tilt) = \> 80% upslope slip

In granular terrains of low to moderate slope, wheel traction and local
tread / terrain interaction determines the degree of vehicle slip. As
slope angle increases through the transitionary regime, slip rate will
increase dramatically as the slope material begins to fail out from
under the rover wheels, limited by the material's own bearing and shear
strength. This transition can be quite abrupt, occurring over as little
as 2 -- 3 degrees tilt, and is obviously a function of the slope
material's own mechanical properties. In high slope angles, granular
material failure dominates the slip regime, and vehicle forward motion
is as much an exercise in material transport as it is in rolling motion.
In terrain with slope angles over 20 degrees, vehicle upslope motion can
effectively be arrested, with slip rates well over 90% quite common.

### Slope Traversability -- Solid Surface

While access to high terrain angles in granular material can be quite
limited, this is not necessarily the case when faced with planning a
route through sloped terrain consisting of predominately solid surfaces.
The most relevant example of this type of terrain is the wall material
of Endurance crater, where favorable surface interaction allowed vehicle
access up to and exceeding the system's own operational limit of 30
degrees. In this type of surface interaction, access to high angles is
limited by the frictional holding limit between the surface material and
the wheel and the rover's own static stability limit. It is expected
that the MSL rover will meet or exceed the capabilities of MER in
terrain of this type. However, while this capability may exist, it is
very likely that a traverse including terrain in this category would be
approached with caution, requiring special operational restrictions and
/ or testbed evaluation.

## Nominal Terrain "Design-To" Cases

This section provides the nominal "design-to" terrain cases. The
following small set of terrain examples represent the type of surface
compositions the MSL vehicle is being designed to successfully traverse
(Table 1).

Rather than specify terrains based solely on assignment of
characteristics defined in the above subsections, selected terrains from
the MER missions are utilized. This not only provides reference imagery
to better understand terrain, but also actual in-flight
experience/performance with the MER rovers.

**Table 1: "Design-To" Mobility Terrain Cases**

  -------------- -------------- -------------- --------------- --------------
  Case           Material       Slope          Structure       Rock Abundance
                 Analog                                        

  Rim of         Beach Sand     5-10 degrees   N/A             20%
  Bonneville                                                   
  crater                                                       

  Columbia hills Beach sand     10-15 degrees  N/A             10%
                 with embedded                                 
                 rocks                                         

  Meridiani      Beach sand     N/A            z =             N/A
  Ripples                                      0.3sin(4x/pi)   
                                               in meters       
  -------------- -------------- -------------- --------------- --------------

### Vehicle Capabilities in Extreme Conditions

The above sections outline vehicle performance under nominal and,
sometimes, fringe environmental conditions. It is expected that the
rover will have reduced capabilities in other, more extreme
environments. In these cases, nominal vehicle operations may be hindered
or restricted, but overall vehicle safety is still maintained. These
terrains should be avoided within landing ellipses and reduced
operations expected in traverses with these terrains. The best example
of this type of vehicle condition is MER Opportunity's experience at
Purgatory drift. In this case, vehicle operations were hampered for
several weeks, but overall vehicle safety and eventual functionality
were not compromised. Examples of such conditions include, but are not
limited to, the following:

-   Partial or complete burial of one or more wheels

-   Access to terrain in excess of 30 degrees

-   Obstacle climbing in excess of 0.5 meters height / depth

-   Belly pan contact with terrain

### Trafficability Go To Requirements

The small landing ellipse and long traverse capability of the rover
allow the possibility of considering \"go to\" sites. The MSL rover is
designed to traverse at least 20 km over a terrain with an average rock
distribution of 15%, an average slope of 5 degrees, and an average slip
rate of 10%, and so is capable of driving out of the landing ellipse in
any direction. "Go to" sites have a safe landing site adjacent to the
target of science interest and require traversing outside of the landing
ellipse to sample the materials of highest interest. In this case, the
area that must be traversed to get into the region of highest science
interest (required to accomplish the science objectives of the mission)
must be trafficable from anywhere within the ellipse. This requires that
the trafficability requirements be satisfied in the area being roved
through that is outside the landing ellipse.

Traversing out of the ellipse would take time out of the beginning of
the nominal mission. For terrain with the characteristics described
above, the rover could travel on average about 100-150 m/sol. At these
rates, traversing 10 km would take 65-100 sols, which is roughly 10-15%
of the nominal mission. This suggests that "go to" sites should be
especially compelling scientifically.

# MSL Operational Considerations

## Vehicle Performance Characteristics

MSL is being designed to operate over a broad range of latitudes.
However, the design is optimized for "average surface conditions" over
the accessible latitude range, as opposed to optimized for high latitude
performance. As the latitude increases (North or South), several
conditions begin to develop that will naturally reduce rover operational
efficiency. When the latitude approaches 50-60°N or S, these can in some
cases be significant. For example, reduced illumination and the presence
of CO~2~ frost may degrade the quality and interpretablity of images
used for science or rover operations such as arm motions or driving.
Persistent cold temperatures may reduce the energy available to operate
the science instruments. Finally, the short duration or absence of Earth
view will impede direct to/from Earth communications.

For these reasons, the science value of a high latitude site must
therefore be great enough to outweigh possible operational efficiency
reduction. For high northern latitude cases, this case is even stronger,
since MSL is arriving at an L~s~ between 120 and 150, meaning austral
spring heading into austral summer. Thus, far northerly landing sites
will be heading into a reduced capability phase shortly after landing,
which is less favorable from an engineering and operations viewpoint.

A table (Table 2) with all discussed landing and roving engineering
constraints is on the last page.

## Table of Engineering Requirements 

+-----------+-----------+-------------+-------------------------------+
| *Eng      |           | *R          | *Notes*                       |
| ineering* |           | equirement* |                               |
|           |           |             |                               |
| *P        |           |             |                               |
| arameter* |           |             |                               |
+-----------+-----------+-------------+-------------------------------+
| Latitude  |           | 60°N to     |                               |
|           |           | 60°S        |                               |
+-----------+-----------+-------------+-------------------------------+
| Altitude  |           | ≤ 2 km      | MOLA-derived elevation        |
+-----------+-----------+-------------+-------------------------------+
| Landing   |           | ≤ 10 km     | Excluding uncontrolled wind   |
| ellipse   |           |             | effects during parachute      |
| radius    |           |             | descent                       |
+-----------+-----------+-------------+-------------------------------+
| Slopes    | 2 to 5 km | ≤ 3 degrees | Radar altimetry errors to     |
|           | length    |             | start powered descent         |
|           | scale     |             |                               |
+-----------+-----------+-------------+-------------------------------+
|           | 20 to 40  | ≤ 15        | Radar processing -- spoofing  |
|           | m length  | degrees     |                               |
|           | scale     |             |                               |
+-----------+-----------+-------------+-------------------------------+
|           | 5 m       | ≤ 15        | Rover tilting                 |
|           | length    | degrees     |                               |
|           | scale     |             | Trafficability in loose       |
|           |           |             | granular material             |
+-----------+-----------+-------------+-------------------------------+
| Rock      |           | ≤ 0.6 m     | Probability that a rock       |
| height    |           |             | higher than 0.6 m occurs in a |
|           |           |             | random sampled area of 4 m^2^ |
|           |           |             | should be less than 0.25%.    |
|           |           |             |                               |
|           |           |             | Suggests low to moderate rock |
|           |           |             | abundance                     |
+-----------+-----------+-------------+-------------------------------+
| Winds     | Steady    | ≤ 30 m/s    | At altitudes of 0 to 10 km    |
|           | state     |             | above the surface             |
|           | h         |             |                               |
|           | orizontal |             |                               |
+-----------+-----------+-------------+-------------------------------+
|           | Steady    | ≤ 10 m/s    | At altitudes of 0 to 10 km    |
|           | state     |             | above the surface             |
|           | vertical  |             |                               |
+-----------+-----------+-------------+-------------------------------+
| Wind      |           | Evaluate    | At altitudes of 0 to 10 km    |
| gusts     |           | case by     | above the surface             |
|           |           | case        |                               |
+-----------+-----------+-------------+-------------------------------+
| Radar     |           | Ka band     | Adequate Ka band radar        |
| ref       |           | reflective  | backscatter cross-section     |
| lectivity |           |             | (\>-20 db)                    |
+-----------+-----------+-------------+-------------------------------+
| Load      |           | Not         | Thermal inertia \>100 J m^-2^ |
| bearing   |           | dominated   | s^-0.5^ K^-1^ and albedo      |
| surface   |           | by dust     | \<0.25; radar reflectivity    |
|           |           |             | \>0.01 for load bearing bulk  |
|           |           |             | density                       |
+-----------+-----------+-------------+-------------------------------+

Table 2: Summary of engineering requirements