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Final Report
Marine Corps Warfighting Laboratory Commercial Hunter Program
GPS Weapon Guidance System
Josh Wood, Max Tonsi, Gavin Goodson
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Caruth Institute for Engineering Education
Lyle School of Engineering
SMU, Dallas, Tx
May 134, 2010
1. Background
In Fall 2009, the Marine Corps approached engineering students at several universities
with an intriguing proposition. Students of multiple disciplines and backgrounds were
split up into teams andtold asked to disrupt a specific military scenarioin essence,to
play the bad guys. Using only an internet connection, a basic engineering background,
and assuming an unlimited budget, students were encouraged to think outside the box
and use consumer off the shelf products to create sources of mission interference. A
group of SMU students proposed a GPS missile guidance system.
Essentially, tThe students used a frequent and currently used terror tactic as the source
of their COTS disruption. Insurgents in hostile areas are using basic trigonometry and
ballistic physics to aim unguided rockets at specific targets. Due to a number of factors,
including wind and propulsion inconsistencies, these attacks very rarely hit their
intended targets. If this accuracy could be increased even slightly by adding a relatively
inexpensive GPS guidance process, these attacks could instantly become more
devastating. With information about GPS and autonomous flight abundant on the
internet, it is not difficult to understandwhy this is a very possible threat. A cheap
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solution to retrofitting existing missile tech in hostile areas could be devastating, so it isvery viable information to know whether this capability is possible.
The initial report made by these students was a generic application of GPS technology; it
did not include design details or an in depth analysis of the processes behind the
technology. In January of2010, SMU engineering students were once again asked to be
of service by prototyping this technology.
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2. ResearchOne of the key ideas of this project is the influence of the internet not only as a source
to purchase materials from, but also as a knowledge base. Therefore, the research
conducted for this project was performed using only the internet(and limited to
websites that did not require any login credentials or any other registry information).
The research required for this project can be categorized into two components. The
first issue to tackle is the notion of autonomous flight using GPS. as the key guidance
component. In correlation with the recent surge in interest of autonomous electronics,
there exist a number of websites that are dedicated to this burgeoning technology.
DIYDrones.com was the most novice-friendly of these websites, offering beginners
guides, links to simple hardware platforms and sample software, and discussion boards
with troubleshooting. The community support of this website and the hardware it
promotes was probably the key factor in our decision to go with one of their platforms.
The ArduPilot autopilot hardware board was chosen based on several key factors. It
boasted a feature list that encompassed all of the functionality we required:
y Programmable 3-D waypoints the ability to pre-program destinations was key,as we can then translate destination points into targets
y Altitude control via elevator/throttle the elevators can be remapped to thehorizontal control canards allowing for elevation assistance and potential range
extension
y GPS interface for EM406A GPS the board includes the desired GPS interfacey Battery powered a must for portable electronics such as a rockety Small (30mm x 47mm) easily fits into the body tube of our chosen commercial
model rocket
y Extensive sample code repository code variations to assist in our debuggingprocess
The feature set on this hardware platform gave us a cost-effective autopilot system.
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and roll stabilization to the rocket were somewhat clear, so initially just the GPShardware required interfacing and extensive modification. Therefore, we took as many
design elements from the pre-existing R/C controlled rocket as we could.
Due to the nature of the testing that would be required of this system (approximately
15 unrecoverable launches would require the construction of at least 15 of these
rockets) we chose to apply these control surface elements to an existing model rocket.
One of the potential issues with attempting to navigate a rocket in mid-fl ight is the
possibility of roll along the longitudinal axisif the rocket rotates 180 degrees, any up
command will translate to down, and rudder controls will be reversed as well.
Eliminating this roll is the key to a successfully guided flight, and the online plans we
discovered had an elegant solution for solving this problem.
The method we use to gain roll stability is simple and works well. It
requires launching only on a clear day with the sun low on the horizon. A
control circuit is used to sense the sun and generate pulses sent to a
model servo to correct any roll. Then the operator can concentrate on
guiding the rocket by the joystick on the transmitter. The circuit that
changes the CDS photocell resistance into different pulse widths for the
servo is a simple 555 timer thatcan be assembled on an experimenter's
circuit board.
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Figure 3.1. Photo roll stabilizer circuit
There are several boards to choose from at Radio Shack. Other parts are
.01, .22 and 10 mf capacitors, a silicon diode (276-1122), a CdS cell (276-
116), a 68k, 1/4 watt resistor, and the proper connectors for your make of
servo.
Figure 3.2. Roll stabilizer circuit diagram
If you are firing the rocket vertically it is best to shoot when the sun is low
on the horizon during morning or evening. Set upthe launch pad so the
sun shines onto half the photocell thru the window in the body tube. As
the rocket turns, the servo must move the control surfaces so as to turn
the rocket back to keep the sunlight on half the cell. If light totally floods
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the cell, it should roll back to be half-covered. If it is shaded entirely, itshould roll to be half-lighted again.
Figure 3.3. Roll stabilizer window and control surfaces
Due to the age of the website containing these instructions, some of the parts theyused
in their design were discontinued. The CDS cell we used to replicate this system had a
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smaller internal resistance, which had to be accounted for in the circuit design. Otherthan that issue, this circuit was copied and initially verified as functional.
Figure 3.4. Modified design using existing model rocket
Figure 3.5. Light-sensing roll stabilizer circuit
With roll stabilization addressed, attention then turned to recreating the RC rocket and
modifying the ArduPilot hardware. For the control surfaces, we decided to emulate the
RC rocketit was a proven design (according to the websites author), and would
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therefore possibly save us some development time compared to devising our ownsystem. We developed the autopilot software while assuming we had functional radio
control of the control canards; this allowed for the a lignment of the development
schedules of both systems.
The website for the R/C rocket lacked detail when describing the linkages between the
front servo motors and the control canards. The best approximation can be obtained by
simply looking at the photo the author provided. It is these linkages that would prove to
be the most complex mechanical design element, as the non-uniformities of our
manufacturing process became more and more evident.
Figure 3.6. R/C rocket layout
The software modifications we decided to implement on the ArduPilot board were
intuitive for the application of model rockets as weapons. Using a fully functional
sample code provided on the DIY drones website (originally configured to an
inexpensive Styrofoam model aircraft), we simply remapped the rudder controls on the
autopilot to the vertically-oriented control canards on the rocket. The same process
was applied to the elevators, remapping them to the horizontal canards.
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There is one last item ofnote regarding the design of our modified rocket system. Thenotion of testing this illegal guided weapon system raises several logistical
complications, most notably testing locations. By raising our maximum potential flight
range, it would severely limit the number of testing sites within a reasonable geographic
distance. Therefore, we decided to power the unguided and guided rockets with a D
size engine capable of20 Newton seconds of thrust, which is one size smaller than what
the rocket was originally designed to handle. This engine/rocket combination was rated
for a 600 ft. maximum altitude when launched straight up. Since our launches were
going to be around 50 degrees from the ground, we needed a radius of approximately
1000 ft in all directions. An increase in motor size would significantly increase the
required testing real estate, so the decision to use the D engines was made.
4. Navigation Control and Testing
The first phase of testing consisted of confirming the functionality of the initial
hardware purchase. For this, we constructed a basic electronics tray much like what
would eventually go inside the body tube of the rocket. From there, we wired the board
up as per the assembly instructions page on the ArduPilot website. Two servo motors
were added, but not attached to any control fins; for this test, we were only concerned
with accurate motor turning and proper direction of turn. These were connected to the
rudder/elevator-OUT ports on the GPS-interface ArduPilot board, with the R/C control
coming through the proper IN ports.
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Figure4.
1.
Wiring for RC/
S Tes
Our original thought was to si ly obtaina GPSlockan walkaroun relati e to a pre-
progra e waypoint an obser e theser o motor movement for therudder. This
testing method was quicklyrevised upon furtheranalysis of thecode, as thelocation
resolutionand refresh rate of the GPS wasnot high enough to obtaina quality bearing
determinationat walking speeds. To solve this, we programmed anew waypoint and
hit the freeway to obtainsolid GPS data. At highwayspeeds, the GPS output our
coordinatessuccessfully.
5. Wi
T
el Te
i
Due to the unrecoverablenature of our design, simulationisakeycomponent in testing.
In order to realistically test the two mainareas of our design (in-flight longitudinalroll
stabili ationand controllable flight viacanards) weneeded to simulate the high speed
flight of therocket. This would require theconstruction ofa wind tunnel.
The originalconstruction planconsisted ofa 6x2x2chamber with a transparent side
and lid foreasy observationand to eventually test therollstabiliersunsensitivity. The
source of wind was originallyanarray ofseven ducted fans powered byDC power
supplies. The feasibility of thisconstruction was quicklynixed, as the powerneeded for
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the fans to output the required wind speed (approx. 175
mph) would require enoughpower supplies to fill a room.
Figure 5.1. Original wind tunnel design
In the spirit of the commercial of the shelf theme of this project, we opted instead to
power the wind tunnel with readily available electric leaf blowers. The model we chose
boasted a 210 mph wind speed, a 2 way speed selector switch, and a standard 120V AC
plug.
In an attempt to avoid significant power-related issues in our wind tunnel construction,
we hit a minor snag. The electric leaf blowers each draw 10 amps, which would blow
any normal circuit if two or more were connected at start-up. To circumvent this issue,
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Figure 5.2. Roll Stabilizer test setup
We reconfigured the tail section to be further toward the front of the wind tunnel
chamber, thus increasing the wind exposure of the control flaps. We again experienced
little to no rotational influence cause by the control flaps. Instead, we noticed a pattern,
which is explained by the fact that our rocket had three stabilizer fins (compared to the
four in the template we were following). This creates a weight imbalance, creating a
natural tendency to have two fins on at the bottom and on at the top. To combat this,
we added weight at the tip of the bottom fin, as to maximize the effect of the weight on
limiting the roll of the rocket.
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Figure5.3. Repositioned tail fins
Figure5.4. Originalstabilier fins
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Figure5.5. Weighted fin.
With this weight added, and byshifting the batteries to the bottom of the tray, we
significantlylowered thecross-sectionalcenter of gravity. In the high wind conditions,
the weight influencecaused therocket to stay properly oriented effectively. From
there, we decided that wecould conquer theroll-stabilityissue with asimplere-
distribution of weight.
Our final wind tunnel test was to confirm theability of the front canards to steer the
flight of therocket. For this, weconfigured thecanard control motorsinto the RC
mode, so that wecould manually drive therocket whilein the wind tunnel. Thoughwe
experienced some dead zones when trying to maneuver therocket through the
airstream, thecanards functionality wasconvincingproven. There was potential for
more wind tunnel testing before westarting our production, but the generator used to
power theleaf blowers wasrented, and therefore we had alimited window to do our
testing.
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Figure 5.6. Free-moving wind tunnel guidance
6. Flight Tests
Our flight test plans began with choosing an appropriate location to perform these
potentially dangerous activities. We settled upon a ranch in rural Oklahoma, with
enough flat land to safely test and keep our rockets in a reasonable range of visibility.
We set up our launch pad and anemometer and determined a launch direction. For the
first set of tests, we were simply attempting to gain unassisted ballistic launch data.
Using D engines in unmodified rockets (save for weighting the nosecones to ensure a
strong ballistic trajectory), we launched a rocket at a 45 degree angle from the ground.
It was assumed that this would maximize range, but with the extended burn of the
rockets, we felt that a higher trajectory would extend the range and flight time
considerably. At 60 degrees, a noticeable jump in range is achieved. Therefore, we
continued with nine more (10 total) launches with this configuration in relatively similar
wind speeds. The results can be found below in the test results section.
We then attempted a guided launch with the same setup and a GPS destination set to
the right of where the average unguided launch landed. The results were
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underwhelming, as we had grossly underestimated the impact of the added weight ofthe electronics tray, control canards, and batteries. The first guided flight managed
only 25 yards or so. We decided to return the next week with larger engines.
Figure 6.1. Unguided launch w/ anemometer
The largest off the shelf available rocket engines is size E which has a maximum thrust
of40 Newton Seconds., which is what o Our rockets were originally intended to use E
size rockets for propulsion. We reestablished our range difference (betweenguided and
unguided) by launching two unguided, unloaded rockets with size E motors. With this
data recorded, we attempted four more guidance test launches. The purpose of these
was originally to establish a maximum loaded rocket range as well as fine-tune the
control loops involved in the guidance process.
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The team came to the conclusion that the system was not operating properly, but it wasour last day of testing, and we needed to acquire some comparable data. Of the ten
remaining launches for the fully configured guided rockets, we experienced mechanical
(improper alignment whenglue dried) and electrical (ArduPilot board simply would not
power on) issues on two of them. This left us with eight rockets that were available for
a viable launch. The data from these launches is below.
The point repeatedly confirmed throughout our fully loaded rocket launches is that
these rockets, for their new weight, are still significantly underpowered. The flight
times we recorded were in the range of four to six seconds; this truncated in-flight
duration allows for only 3-4 potential course corrections, which is far lower than our
original prediction of30. It is also important to know that, in their current
configuration, the ArduPilot boards were unreliably switching between operating
modes. Our rockets were only designed to operate when the autopilot board is in
Waypoint mode; unfortunately, these boards were not holding their states as reliably
as the initial 3 boards we purchased from the manufacturer.
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7. Unguided Flight Results
The following table shows the GPS location information for the launch station and the
subsequent 11 unguided launches. The firstlaunch behaves as an outlier because it was
before we adjusted the launch angle from 45 degrees to 60 degrees.
Table X7.1. Unguided Launch Info
Below is the application of these GPS landing coordinates into the GoogleEarth
program. The CoM tagged icon is the center of mass of the landing zones. The standard
deviation of this landing data is 27.45 meters.
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Figure 7.1. Unguided rocket landing zone spread
8. Guided Flight Results
The following table denotes the landing GPS coordinates for the guided test launches.
Note the significant difference in the range of an unburdened rocket with a D size
engine vs. the fully equipped guided rocket with the more powerful E engine. Also,
several of these flights experienced inconsistencies that are worth noting.
Flight F one of the tail fins became detached loose in the transportation process.
Flight G one of the more mechanically sound rockets we produced, the launch stand
began to fall down at the exact moment G was being launched. This resulted in a
roughly 30 degree ballistic angle vs. the ideal 60 degree angle.
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Flight I Mechanical issues plagued flight I; the control linkages and canards wereinterfering with one another prior to launch, and the rocket was missing its top flight rail
guide
Flight M in an attempt to compensate for the wind issues, we ended up having the
launch rod angle too high,basically launching Mnearly vertical directly up.
.
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Figure 8.1. Guided rocket landing zone spread Formatted: Font: +Body
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Figure 8.2. Cumulative test data
10. Design Critique
As a complete system, the GPS guided rocket design was susceptible to several sources
of error. The wind-tunnel testing earlier in the schedule provided a certain degree of
confidence that a simple weight r e-distribution could eliminate or severely reduce
longitudinal roll during flight; however, this characteristic was inconclusive during our
test flights. On multiple occasions, the rocket rotated almost immediately after leaving
the launch guide rod, but did not cover enough distance to self-correct. From the
testing done so far, the flight times have not been long enough to determine if the
weight distribution effectively limits the body roll.
Another significant factor in the discussion of sources of error lies with the variation in
manufacturing of these rockets. From the electronics tray to the control canards and
steering linkages, custom fabrication was required to successfully modify these model
rockets. Unfortunately, variability was impossible to limit given the number of rockets
constructed and the complexity of the control system we were attempting to create.
This led to inconsistencies in axel placement, control arm throw distances, tray locking
mechanisms, and other areas.
Our decision to modify an existing rocket system for our prototyping was also
potentially detrimental to the design. The cardboard tubes we were using had a
significant amount ofgive and flex, and the holes cut for the canard axels did not stay
round for very long. Working with these existing rockets also made construction very
tedious.
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The last significant source of potential design flaw is linked to the GPS update rate. TheGPS that the ArduPilot board was designed for, the EM406A, has a 1 Hz update rate.
This We originally did not find issue with a 1Hz update rate, as the desired flight time
was to be around 30 seconds. This would provide roughly 30 course corrections over
the flight of the rocket. Unfortunately, due to the severe overestimation of flight time,
the rockets were unable to receive more than a few course corrections per flight.
11. Next Steps
At this point in the project, we have determined what has and has not worked, and
what areas of improvement are required. The most significant of these is the overall
platform from which to prototype this guidance system. From the teams perspective,
the next logical step in the design process is to design a rocket from scratch. The hobby
rocket platform was severely limiting in what designelements we could integrate into it.
A custom designed rocket would solve a significant amount of the manufacturing issues
(primarily the repeatability and interchangeability of processes).
Building upon a custom platform, the roll stabilization system needs an overhaul.
Where our initial wind tunnel tests provided a reasonably high confidence level in the
weight/balance modification solution, it became clear that addition systems were
needed. Our next approach would be to actually incorporate the sun-sensing roll
stabilizer with a modified weight distribution.
Though it has been mentioned repeatedly throughout this report, the flight time still
stands as a large barrier to our progress. It is difficult to pinpoint the sources of error
when the test sample (in this case, a test flight) is so small. To counter this, the future
plans should most certainly include a more powerful engine.
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The last notable redesign element is to improve the electronics configuration. Thecurrent system was rigged to be toggled by an RC controller, with no internal timer or
mechanism to initiate guidance at a specific point in flight. Some of the inconsistencies
we experienced on the hardware/software side of this project can be addressed through
the use of hard-coding and persistent connections.
12. Feasibility of Deployment
Our experiences with developing this system led us to a series of conclusions about the
feasibility of retrofitting GPS guidance onto existing missile systems. Our team strongly
believes that this capability is in fact possible, but that it is also a very difficult
accomplishment. The notion of adding such a precise level of control to an existing
platform calls for a great deal of ingenuity, patience, and a great deal of resources. With
more testing and design revisions, this is a distinct possibility.
13. Countermeasures
One of the aspects that makes GPS guided weapons so troublesome is the ability to fire
and forgetthis basically means that once the trigger is pulled, there are very few
avenues to go about stopping a weapon of this type. To be able to disrupt or confuse a
GPS guided autonomous missile, GPS signals would need to be blocked in the area of
interest. This action would render all GPS useless though, both friendly and foe.
Instead of focusing on how to disrupt this system after it has been launched, we suggest
interfering before the first prototype is even built. This would require the monitoring of
large purchases of hobby gear (servo motors, engines, autopilot hardware), as well as
screening the community interactions for any suspicious conversations.
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Lastly, the nature of this type of system lends itself to significant amounts of testing.Therefore, the surveillance of potential testing sites could potentially thwart this threat.
It is difficult to hide a missile launch, much less 15 launches.
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14. Acknowledgements
We would like to acknowledge the following individuals and organizations for their
contributions:
Marine Corps Warfighting Laboratory- for providing this opportunity to assist them in
their missions to defend and protect this Nation
Nathan Huntoon for his guidance and support throughout all phases of this project
Kristine Reiley for the constant trips to the hardware store and judicious use of her
Chevrolet Extended Cab Pickup
Mel Lively for the use of his land, as well as his home, as a rocket testing range
Mikes Hobby Store for prompt hobby component orders and great customer service
Hobby Town USA for providing timely rocket engine restocking and lessons in
removing the ejection charge of said engines
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