Mars Pathfinder Navigation
Welcome to the WWW home page for the Mars Pathfinder navigation
system. This is the gateway for accessing various types of information
on MPF navigation for the general public.
Contents
The main responsibility of the Navigation Team is to maintain the
spacecraft on the planned trajectory for the duration of the mission. In the
development phase, the team helped to design the interplanetary trajectory
that achieves the mission goals within various constraints such as propellant
usage and planetary protection requirements. The team provides the project
with predictions of this trajectory for the spacecraft and with orbit data
for the planets and Martian satellites. In flight, the team is providing
best estimates for the actual past trajectory of the spacecraft along with
the predictions for the future trajectory. Based on these solutions, they
plan and generate the trajectory correction maneuvers (TCMs) required
to maintain the spacecraft on the desired trajectory.
For Mars Pathfinder, the navigation team will also provide information
used to execute a successful descent through the Martian atmosphere and
landing on the surface.
The primary responsibilities of the Mars Pathfinder Navigation Team are:
- determining the interplanetary flight path of the spacecraft from
tracking data collected via the DSN. This process of estimating the
spacecraft trajectory is called
orbit determination.
- calculating the magnitude, direction and commands needed for the four
trajectory correction maneuvers (TCM's) during cruise. This process is
called maneuver design.
- delivering trajectory and planetary orbit information to the flight
team, DSN, science teams and any other interested parties.
- determining the arrival conditions required to achieve successful
atmospheric entry and landing.
- calculating critical timing events associated with entry, descent and
landing.
- determining the actual landing site on the surface of Mars based on a
reconstruction of the flight path before and during atmospheric entry.
The Mars Pathfinder Approach to Navigation
To reduce costs, Pathfinder NAV has kept new software development to a
minimum. The majority of software is inherited from previous missions such
as Galileo, Mars Observer and TOPEX-Poseidon.
Previous navigation teams were sub-divided into specialists in orbit
determination, trajectory analysis and maneuver design, requiring a NAV team
of 6 people or more. Pathfinder has a NAV team of 3 people, each one
cross-trained in these three disciplines. The current members of the NAV
team are Pieter Kallemeyn, David Spencer, and Robin Vaughan. This team
will be aided by Bobby Braun from NASA's Langley Research Center during
the critical entry, descent, and landing preparations.
Pathfinder is using traditional radiometric data types (Doppler and range)
with an enhanced data filtering technique. This is done with the
MIRAGE-ODP, a program set developed by the Navigation Systems Section of JPL.
The flight path during Entry, Descent and Landing is being modeled using the
Atmosphere Entry Program (AEP), which simulates the atmosphere and gravity
of Mars with a model of the heatshield, parachute, and other components
of the Pathfinder flight system.
The maneuver process for past missions was typically 7-15 days from the
receipt of DSN tracking data to execution of the maneuver. By integrating
the navigation software with maneuver command software, Pathfinder has
reduced this turnaround time to 5 days.
Automated procedures are being used, where possible, to reduce and edit the
tracking data, relieving NAV analysts of these time-consuming tasks.
The navigation team uses some clever techniques from estimation and filtering
theory to determine the spacecraft's trajectory. A model of the forces acting
on the spacecraft, such as the gravitational attraction of the planets and the
Sun, is constructed from basic physical principles. Tracking data are obtained
from the spacecraft in flight and compared to predicted data from the model.
Orbit determination is the process of "tuning" the filter and model parameters
to obtain the "best fit" of the tracking data. The trajectory that best fits
the data is the best estimate of the actual trajectory of the spacecraft.
Two basic data types are obtained to locate the spacecraft precisely:
doppler and ranging. Each data type provides a different kind
of information; when used in concert, the accuracy of spacecraft position and
velocity relative to celestial bodies can be very high.
Doppler is a way to measure the speed at which an object is approaching
or receding from the Earth. In simple terms, a
Deep Space Network
antenna sends a radio signal up to the spacecraft which is then directly
returned. If the spacecraft is approaching or receding from the tracking
station, the signal is returned a tiny bit faster or slower, respectively.
If you've noticed how a car's beep, or the sound of an airplane engine sounds
lower after it passes you by, you understand how doppler works. Measuring
this difference in frequency can help pin down the spacecraft's speed in the
solar system, and therefore give navigators clues as to precisely where it
is headed.
Ranging uses the fact that light has a finite speed to determine the
distance from the Earth to the spacecraft. Signals sent to the spacecraft are
received and quickly returned, and the delay between when the signal is sent
from the Earth and when the same signal is received back on Earth is
proportional to the distance from the spacecraft to the Earth.
Ranging is similar to (but much more precise than)
mailing a letter to yourself to see how long the postal service takes for
delivery. When used together with Doppler, the spacecraft's position
and speed can be determined very accurately.
Like all other JPL interplanetary missions, Pathfinder uses antennas at
the 3 Deep Space Network complexes at Golstone, CA, Canberra, Australia, and
Madrid, Spain to obtain the Doppler and ranging data, as well as spacecraft
telemetry. Most of Pathfinder's tracking will be done with the 34-m antennas
at each site (34 m is the diameter of the antenna dish). Occasionally, these
will be supplemented by the 70-m antennas, the largest of the DSN antennas.
For most of the mission, the spacecraft will only be tracked 3 times each
week, typically once by each complex. Additional coverage occurs during
periods of critical activities. A summary of Pathfinder's tracking schedule
is shown below:
Launch to Launch + 30 days 3 passes per day (continuous coverage)
Launch + 30 days to Mars - 45 days 3 passes per week
3 days before to 3 days after each TCM 1 pass per day
Mars - 45 days to Mars arrival 3 passes per day (continuous coverage)
Despite our best efforts, the spacecraft will not follow its planned course
exactly. Small deviations in its flight path from the desired one can grow
into large errors at Mars arrival. Also, constraints imposed on the mission
prevent us from following the desired path directly from the beginning of
the mission. For these reasons, the spacecraft will
occasionally be commanded to fire its thrusters to change its velocity
at certain points during the cruise to Mars. These thruster firings, or burns,
are called trajectory correction maneuvers or TCMs. The velocity
changes caused by the thruster firings will alter the spacecraft's future
trajectory so that it returns to the desired path and arrives
at Mars with the proper geometry for atmospheric entry.
A total of 4 TCMs are planned for Mars Pathfinder. The first 2 of these are
scheduled in the first 2 months of the mission while the spacecraft is still
relatively close to Earth. The final 2 TCMs are scheduled near the end of the
cruise phase when the spacecraft is close to Mars. Contingency plans allow for
a fifth maneuver to be executed just a few hours before atmospheric entry, if
necessary. The table below gives a summary of the MPF maneuver schedule.
For a more technical discussion see the text under TCMs 3 and 4, and TCM 5.
| Maneuver | Time | Calendar Date |
Mean Velocity Magnitude | Comments |
| TCM 1 | Launch + 37 days | January 10, 1997
(delayed from January 4, 1997) |
33.3 m/sec | Remove injection bias, correct injection errors |
| TCM 2 | Launch + 60 days | February 4, 1997 |
2.08 m/sec | Correct TCM 1 errors |
| TCM 3 | Mars - 60 days | May 7, 1997 |
0.432 m/sec | Target to final Mars atmospheric entry point |
| TCM 4 | Mars - 10 days | June 24, 1997 |
0.138 m/sec | Correct TCM 3 errors |
| TCM 5 | Mars - 12 or 6 hours | July 4, 1997 |
0.2 -> 2.0 m/sec | Correct any remaining errors |
Mars Pathfinder must satisfy 2 NASA planetary protection requirements designed
to minimize potential contamination of the Martian environment. The first
requirement is that the probability of the unsterilized launch vehicle upper
stage impacting the surface of Mars be less than 0.0001 (or 1/10,000). The
launch vehicle stage could be carried to Mars with Pathfinder if there were
some failure in the separation procedure after the final burn that
places the spacecraft on target to Mars. For this reason, the targetted state
to be achieved by the launch vehicle, called the injection target, is biased
away from Mars - just enough so that the impact requirement is satisfied.
This means that the launch vehicle will not place Pathfinder on course for
its final desired arrival state at Mars. It's up to the spacecraft itself to
move toward this final aimpoint during its 7-month cruise. TCM 1 is the first
step towards achieving the desired entry conditions at Mars.
The second planetary protection requirement imposed on Mars Pathfinder is that
the probability of Pathfinder itself impacting Mars at a speed greater than
1,000 ft/sec be less than 0.001 (or 1/1,000). This requirement is met by
designing TCMs 1 and 2 to a target state that is also biased away from the
final desired arrival conditions - again, just enough to meet the requirement.
The target state is chosen so that if control of the spacecraft is lost
following either TCM 1 or 2, the spacecraft will enter the Martian atmosphere
with a shallow entry angle, allowing the atmosphere to slow it to below 1,000
ft/sec at impact (assuming the parachute does not deploy).
TCM 1 is designed to move from the biased launch injection target state to the
(still) biased Mars arrival state. The change required to move between these
two targets is known, so that the nominal size of TCM 1 can be computed
exactly. This type of maneuver is called "deterministic". However, there are
several types of uncertainties associated with the design and implementation
of maneuvers in flight. A second maneuver - TCM 2 - is planned to follow TCM 1
to correct any errors that occur in the design and/or execution of TCM 1.
If we knew the spacecraft trajectory exactly and the propulsion system could
execute the maneuver perfectly, there would be no need for TCM 2. This type
of maneuver is called "statistical" since its characteristics can only be
predicted by statistical analyses of the various error sources.
TCM 1 was originally scheduled for January 4, 1997, but was postponed so that
some changes in the attitude control software could be implemented. These
changes were made on the morning of Wednesday January 8, 1997. At that time,
the spacecraft also performed a turn to the required attitude for TCM 1
execution. TCM 1 had been rescheduled for January 10, 1997 02:00 UTC, or 6
PM PST on Thursday January 9. Maneuver execution was delayed slightly due
to some minor problems with hardware at the Deep Space Network complex in
Madrid. TCM 1 was successfully executed at 03:40 UTC, or 07:40 PM PST.
The thrusters were fired for about 1 and 1/2 hours, as predicted, to attain
the necessary velocity change. Following the burn at 06:18 UTC (or 10:18 PM
PST), another spacecraft turn was performed to point the spin axis at the
Earth.
The navigation team has produced an orbit determination
solution for the spacecraft's trajectory using data from launch through
January 1, 1997. A total of 26,605 Doppler and 6952 ranging measurements were
fit for this solution. Based on this trajecory, the team has calculated the
velocity change required for this maneuver. The velocity change is expressed
as a "delta-V vector", having both a magnitude and direction. This vector
is shown below in the Earth Mean Equator and Equinox of J2000 coordinate
frame:
delta-V vector for TCM 1: 17.300657 m/sec
-25.321327 m/sec
5.899257 m/sec
delta-V magnitude: 31.230 m/sec or 69.85952 mph
delta-V direction (unit vector): 0.55397
-0.81080
0.18890
The NAV team has been processing tracking data obtained since the execution
of TCM 1. Our current best estimate for the magnitude of TCM 1 is
30.091 m/sec or 3.6% lower than the design value. The estimated direction
is within 0.1 degrees of the design value.
For TCM 1, the
spacecraft was turned so that its spin axis was pointing along the delta-V
direction and the thrusters were fired to produce a force in that direction
only. This is called an "axial" burn. The spacecraft can also fire its
thrusters so that a force is produced normal to the spin axis. This is called
a "lateral" burn. For TCM 2, the delta-V
was implemented as a vector sum of an axial and a lateral burn.
The directions and magnitudes of the two burns were chosen so that the vector
sum of delta-V components for each burn equalled the overall desired delta-V
vector. Both segments of TCM 2 were successfully performed
on February 3, 1997.
The spacecraft was turned to point in the direction of the axial delta-V
on Friday January 31, 1997 at 8:25 AM PST. The axial burn was performed
on Monday February 3, 1997 at 23:00 UTC (3:00 PM PST). The burn
lasted 297 seconds, or just under 5 minutes. The lateral burn was then performed
about two hours later at 00:52 UTC on February 4, 1997, which is 4:52 PM
PST on February 3. Thrusters were fired in two pulses over
a 30-seconds interval. Following these burns, the spacecraft was
returned to an Earth-pointing attitude. The turn back to Earth was
performed on 02:08 UTC on February 4, or around 6:08 PM PST on February 3.
The navigation team had previously produced an orbit determination
solution for the spacecraft's trajectory using data from launch through
January 25, 1997. A total of 8596 2-way Doppler points and 10006 range
points from each of the 3 DSN complexes were used in this solution.
Based on this trajecory, the team had calculated the
velocity change required for this maneuver. The velocity change is expressed
as a "delta-V vector", having both a magnitude and direction. This vector
is shown below in the Earth Mean Equator and Equinox of J2000 coordinate
frame:
delta-V vector for TCM 2: 1.119292 m/sec
-1.123117 m/sec
0.035196 m/sec
delta-V magnitude: 1.58602 m/sec or 3.548 mph
delta-V direction (unit vector): 0.70573
-0.70813
0.02219
The selected attitude for TCM 2 resulted in the following axial and lateral
delta-V components:
AXIAL DELTA-V
axial delta-V vector for TCM 2: 1.16063 m/sec
-1.08413 m/sec
-0.04731 m/sec
axial delta-V magnitude: 1.5889 m/sec or 3.554 mph
axial delta-V direction (unit vector): 0.73045
-0.68231
-0.02977
LATERAL DELTA-V
lateral delta-V vector for TCM 2: -0.04134 m/sec
-0.03899 m/sec
0.08251 m/sec
lateral delta-V magnitude: 0.01002 m/sec or 0.02241 mph
lateral delta-V direction (unit vector): -0.41263
-0.38918
0.82357
The NAV team has been processing tracking data obtained since the execution
of TCM 2. Our current best estimate for the magnitude of the axial segment
of TCM 2 is 1.598 m/sec, 0.6% higher than the design value. For the lateral
segment, the magnitude estimate is 0.1020 m/sec, or about 1.8% higher than
the design value.
As explained in the preceding section, Pathfinder is not on target for its
final Mars arrival conditions after execution of TCMs 1 and 2. Another
maneuver is needed to remove the bias and target for the geometry necessary
to successfully enter the atmosphere, descend, and land on the surface.
TCM 3 is scheduled near the end of the cruise to Mars and is the first
maneuver to target to the desired Mars entry conditions. The dynamics of entry
impose strict constraints on the final target point. The spacecraft's position
and velocity must be maintained within a narrow corridor; the bounds of this
corridor represent regions where the spacecraft would either burn up in the
atmosphere before landing or "skip out" of the atmosphere and return to
interplanetary space. TCM 3, like TCM 1, is a deterministic maneuver since
the offset between the biased target for TCMs 1 and 2 and the desired final
target is known in advance. As for TCM 2, a second, statistical maneuver -
TCM 4 - is planned to follow TCM 3. TCM 4 will remove any errors in design and
execution of TCM 3, just as TCM 2 did for TCM 1.
We have executed our first maneuver to target the spacecraft for its descent
through the atmosphere and landing on Mars.
TCM 3 was performed on Wednesday May 7, 1997 00:00 - 04:00 UTC
which was Tuesday May 6, 1997 at 5-9 PM PDT. All of the scheduled events
executed successfully. Unfortunately, luck was not with us and the small
execution errors accumulated while performing the maneuver have conspired
to place the spacecraft on a trajectory that is just outside our
desired entry conditions - if uncorrected. So, we will be
doing TCM 4 in late June, ten days before arrival at Mars on July 4.
See the following paragraphs for more details.
The navigation team produced the orbit determination solution used to design
TCM 3 based on data through April 29, 1997.
A total of 1806 2-way Doppler points and 3259 range
points from each of the 3 DSN complexes were used in this solution.
Based on this trajecory, the team calculated the
velocity change required for the maneuver. The velocity change is expressed
as a "delta-V vector", having both a magnitude and direction. This vector
is shown below in the Earth Mean Equator and Equinox of J2000 coordinate
frame:
delta-V vector for TCM 3: 0.076190 m/sec
0.063028 m/sec
0.036165 m/sec
delta-V magnitude: 0.10529 m/sec or 0.2355 mph
delta-V direction (unit vector): 0.72364
0.59863
0.34349
TCM 3 was executed somewhat differently than either TCM 1 or 2.
For TCM 1, the spacecraft was turned so that its spin axis was pointing along
the delta-V
direction and the thrusters were fired to produce a force in that direction
only. This was an axial burn. This could not be done for TCM 2 due to
constraints on the spacecraft attitude. Instead, the spacecraft fired
thrusters to produce forces both along and nearly normal to its spin axis.
These axial and lateral delta-V components were sized so that their sum
equaled the overall desired delta-V vector for TCM 2.
As was the case for TCM 2, thermal and power constraints precluded doing TCM 3
as an axial burn by turning the spacecraft directly to the delta-V
direction. So, TCM 3 was implemented as a
combination of axial and lateral burns. But in this case, we
deliberately added an extra component to the delta-V breakdown.
The reason for this is that
our analysis for the late contingency maneuver - TCM 5 -
has shown that, if executed, this maneuver would require a velocity
change exclusively in the lateral direction. During the last 24 hours before
entry when TCM 5 would be executed, the spacecraft has turned to the
orientation required for safe descent through the atmosphere and cannot be
turned from that orientation to do the TCM. And the geometry is such that
the delta-Vs needed to target back to the desired
landing site are nearly perpendicular to the entry attitude direction,
so that TCM 5 will have to be performed as one or more lateral
burns. In the worst case, 5 sets of lateral burns would be performed
consecutively, each imparting a 0.4 m/sec velocity change.
So far, the spacecraft has only performed a lateral burn once as
part of TCM 2 and the velocity change for that burn was only
0.1 m/sec. The team decided to get some more data on lateral
burn performance at larger delta-V magnitudes during TCM 3.
The required delta-V for TCM 3 was broken down into 3 parts:
1. a lateral burn of 0.4 m/sec that tested TCM 5 execution
2. an axial burn
3. a 2nd lateral burn in a direction nearly opposite to that
of part 1. Its magnitude was 0.4 m/sec plus a little extra
that added together with the axial burn of part 2, gave the
total delta-V for TCM 3.
The selected attitude for TCM 3 resulted in the following axial and lateral
delta-V components:
1. LATERAL DELTA-V
lateral delta-V vector for TCM 3: -0.13925 m/sec
-0.33158 m/sec
-0.17511 m/sec
lateral delta-V magnitude: 0.4 m/sec or 0.89477 mph
lateral delta-V direction (unit vector): -0.348121
-0.828953
-0.437778
2. AXIAL DELTA-V
axial delta-V vector for TCM 3: 0.101451 m/sec
-0.02820 m/sec
-0.01259 m/sec
axial delta-V magnitude: 0.1060 m/sec or 0.23711 mph
axial delta-V direction (unit vector): 0.95666
-0.26592
-0.11872
3. LATERAL DELTA-V
lateral delta-V vector for TCM 3: 0.11424 m/sec
0.42342 m/sec
0.22419 m/sec
lateral delta-V magnitude: 0.4925 m/sec or 1.1017 mph
lateral delta-V direction (unit vector): 0.231940
0.859665
0.455170
Here's an outline of the events for TCM 3 as executed on the evening
of May 6, 1997:
Event PDT on May 6, 1997
--------------------------------------- --------------------
Turn to TCM 3/Earth-pointing attitude 4:45 PM
1st lateral burn (to test TCM 5) 5:31 PM
Axial burn 7:00 PM
2nd lateral burn 8:02 PM
The spacecraft executed all of its commands with no problems.
Since then, the NAV team has received 2 weeks of tracking passes
and has produced an assessment of the maneuver performance.
The first lateral burn was approximately 0.71% below the commanded
magnitude, while the axial burn was about 2% above the commanded magnitude.
Both of these burns appear to be very close to the commanded direction.
The second lateral burn was approximately 0.96% above the commanded
magnitude and it appears to be have been pointed about 0.9 deg
away from the desired direction. The predicted entry flight-path angle
is currently -14.18 degrees, which is within the required range.
However, the predicted landing site is outside of the desired
area on the surface, so
we will be doing TCM 4 in June to retarget the spacecraft
to the desired entry conditions. The NAV team will refine its assessment
of TCM 3 and the predicted entry and landing conditions
in the next few weeks and publish updates as they become
available.
Although TCM 3 was not quite optimal
in total, the first lateral burn executed according to expectations
and successfully validated our design and strategy for TCM 5 execution
(should it prove necessary).
We successfully executed our second maneuver to target the
spacecraft for its descent through the atmosphere and landing on Mars
on Wednesday June 25, 1997 around 17:00 UTC or 10 AM PDT.
The burn was performed as a sequence of lateral and axial burns followed
by an attitude turn to maintain the spacecraft antenna pointing at Earth.
All of these activities were performed with no problems.
The navigation team has now processed just under 24 hours of tracking data
taken during and after TCM 4. Our latest solution done on June 26, 1997
show TCM 4 has moved us back to our desired landing site :-) As of today,
it looks like we right on target! See the
picture in
our Trajectory Data (Technical) web page
for our current estimate of where we're landing.
The navigation team has produced the orbit determination solution
for TCM 4 based on data through June 23, 1997.
A total of 5776 2-way Doppler points and 5259 range
points from each of the 3 DSN complexes were used in this solution.
Based on this trajecory, the team calculated the
velocity change required for the maneuver. The velocity change is expressed
as a "delta-V vector", having both a magnitude and direction. This vector
is shown below in the Earth Mean Equator and Equinox of J2000 coordinate
frame:
delta-V vector for TCM 3: 0.001047 m/sec
-0.001375 m/sec
-0.006827 m/sec
delta-V magnitude: 0.01858 m/sec or 0.04156 mph
delta-V direction (unit vector): 0.56349
-0.73994
-0.36736
As you can see this was an extremely small maneuver, representing just a
small "tweak" of the spacecraft's trajectory.
As was the case for TCMs 2 & 3, thermal and power constraints precluded
doing TCM 4 as an axial burn by turning the spacecraft directly to the delta-V
direction. So, TCM 4 was implemented as a combination of axial and lateral
burns. The lateral burn was executed first.
The selected attitude for TCM 4 resulted in the following lateral and axial
delta-V components:
1. LATERAL DELTA-V
lateral delta-V vector for TCM 4: 0.0003454 m/sec
-0.014525 m/sec
-0.0071456 m/sec
lateral delta-V magnitude: 0.01619 m/sec or 0.0362 mph
lateral delta-V direction (unit vector): 0.021334
-0.897101
-0.441310
2. AXIAL DELTA-V
axial delta-V vector for TCM 3: 0.01013 m/sec
0.0007737 m/sec
0.0003181 m/sec
axial delta-V magnitude: 0.01064 m/sec or 0.0238 mph
axial delta-V direction (unit vector): 0.99661
0.07611
0.03129
From our June 26, 1997 navigation solution, there is less than
1% difference between the actual deltaV and the design value for the
axial burn and less than 4% difference between the actual lateral deltaV and
the design value. This still a preliminary assessmentl; the
solutions for the actual deltaVs may change a little
over the next few days as we get more tracking data following the
maneuver execution.
After execution of TCM 4, Pathfinder should be on course for a successful
landing on Mars. If the trajectory following TCM 4 moves outside of the
allowable corridor for successful atmospheric entry, an emergency maneuver -
TCM 5 - can be executed up to 5 hours before entry. This maneuver can also
be viewed as moving the predicted landing site back towards the target site
on the surface of the planet. Current plans call for
two "windows" for possible execution of a fifth TCM - one at 10.3 hours and one
at 5.3 hours prior to entry.
TCMs 1 - 4 are "custom designed" in the sense
that the navigation team computes the exact velocity change required to
achieve the desired target and the rest of the flight team designs and
executes new commands to implement each maneuver. Since this process
takes about 5 working days, it's not possible to do this for TCM 5.
Instead, a set of velocity changes will be chosen ahead of time
and commands will be developed to implement each of these. The NAV team will
choose whichever one is closest to the true velocity change indicated by
the latest orbit determination solution.
Mars Pathfinder Trajectory Data gives a short
summary of Pathfinder's current location.
More detailed information, including orbital elements, is given at
Mars Pathfinder Trajectory Data (Technical).
The last 48 hours before atmospheric entry will be a busy time for the
navigation team. The team will regularly generate updated solutions for the
spacecraft trajectory to determine the precise entry conditions. This will be
done hourly during the last 24 hours. This information will be used to
- determine if a 5th TCM should be performed to adjust the entry state and
landing site; select the velocity change and time for TCM 5, if needed
- update values used by the entry and descent flight software to determine
when to deploy the parachute and fire the RAD rockets
- update the predicted location of the landing site - latitude and
longitude - on the Martian surface
The need to execute TCM 5 will be officially evaulated first at 13 hours
before entry. If the trajectory solutions indicate it's necessary, it will be
performed at 10.3 hours before entry. If TCM 5 is not performed at this time,
another evaluation of the trajectory solution will be made. TCM 5 will be
performed at 5.3 hours before entry if the new trajectory solutions computed
between 14 and 7 hours out show that it is needed.
There are four designated times during
these last 24 hours when parameters for the parachute deployment and RAD
rocket firing and landing site location will be recomputed and sent to
the spacecraft: (roughly) 36, 22, 10, and 4 hours before entry.
In addition to the orbit determination software, this requires
using two different sets of software to model the trajectory once in the
Mars atmosphere. The Atmospheric Entry Program developed at JPL
and the POST software from NASA Langley are used to verify these
calculations. While similar, each of these programs has slightly
different capabilities. They are used in a complementary fashion
to insure that our calculations are as accurate as possible.
Revision date: 29 June 1997
Robin
Vaughan (rvaughan@mpfnav2.jpl.nasa.gov or robin.vaughan@jpl.nasa.gov)
Note: If you send e-mail during the week of June 30 to July
4, 1997, I may not answer you until after landing!
The NAV team will be very busy during this last week and I
must give priority to flying the spacecraft. Thanks in advance
for your patience during this time.