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Angelfire now carries a GPS
radio downlink system in the nosecone. It transmits GPS (Global
Positioning System) coordinates to a base station for use in recovering
the rocket, as well as providing detailed flight information like
altitude, velocity and range. My particular implementation was built
by purchasing the key components and then integrating them together to
create the whole system. Alternatively, it is also possible to buy a
"ready to fly" system like the
BeeLine GPS
from BigRedBee
or the unit from
GPS Flight.
(The BeeLine
GPS is especially nice because it is much smaller than the unit I
built!)
I chose not buy one, simply because I wanted to "roll my own" for the fun of
it. Designing my own also allowed me to add a few enhancements. The
main one being that I wanted to use a GPS unit that transmits updates five times per second. This is five times faster than
standard GPS systems and gives better detail for the flight trajectory.
Obviously it would be overkill if the only objective was to just report
the landing coordinates.
The radio downlink part of
this system was built using two
MaxStream 9XTend RF
modules. These modules are spread spectrum frequency hopping
transceivers that have a serial port interface on one end and an antenna
connector on the other end. They can transmit up to 1 watt of power
when running on a 5 volt power supply. The range is advertised
to be as far as 40 miles line of sight with high gain antennas. They are
FCC approved in the USA and operate in the 902-928 ISM (Industrial,
Scientific & Medical) frequency band.
Click here to view the product manual. These units sell for $179 each at
Digi-Key. The
Digi-Key part number is XT09-SI-ND.
The GPS unit is the new
GPS
18-5Hz from Garmin.
It is a small round module that contains both the GPS antenna and the
signal processing electronics. It has a standard serial port
interface that can send out GPS position fixes five times per
second. It also requires a 5 volt power supply.
Click
here to view the product manual. The
Garmin price is $199
but I found one at
MegaGPS for $180.
To power these units I
selected a small 7.2V NiMH battery pack and created a 5V regulator on a
custom PC board. The
MaxStream radio
modules were also mounted onto this custom PC board. (All the details
are presented below.) The same PC board design was used for the
transmitter in the nosecone as well as the receiver on the ground.
However, the two are configured slightly differently by some jumper
settings. |
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of it. |
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This photo shows the
Angelfire nosecone and the electronics package that fits inside it.
The nosecone is 30 inches from the base to the tip and just over 5 inches
in diameter at the base. It is made from filament wound fiberglass.
Click here to see more details about the
mechanical aspects of the nosecone.
The electronics is sitting
on some small wood blocks because of a U-bolt that extends out that side
of the bulkhead. The wood blocks are not part of the design! |
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Here is a closer view of
just the electronics package. Click on the photo to get a good look at it.
The Walston transmitter holder is just a 35mm film canister with some pink
antistatic foam inside it to act as shock absorber padding for the little
Walston transmitter. The antenna for the Walston extends out through
a hole in the lid of the film canister and gets taped onto the long
vertical white plastic support. (Walston not shown in this photo.) |
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Close-up of the GPS unit
and the MaxStream transmitter along with its antenna. A simple
quarter-wave whip antenna was used on the first few flights of this
system. It will be upgraded to a crossed-dipole turnstile antenna in
the future. The GPS antenna is mounted about 4 inches high so that it is
above the shoulder on the nosecone and therefore is above the end of the
airframe body tube. This avoids requiring the GPS signals to
penetrate two thicknesses of fiberglass. The GPS unit plugs into a
connector on the PCA that holds the transmitter and the voltage regulator.
The PCA is mounted vertically by an aluminum right angle bracket at the
bottom of it. |
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A little different view of
the electronics package. From this view we see the backside of the
GPS module and the right side of the radio module. The voltage
regulator, battery pack and the power switch can also be seen. The
power switch is the same type I use for all my altimeter electronics.
It is a small rotary switch available
from Missile Works
for about $5. It is part number
MWC-SW-2. The battery
pack is from the local hobby shop. It is a 7.2V NiMH battery pack
intended for use in small radio controlled cars. The switch and the
battery pack are held in place by some custom aluminum brackets. One of
these brackets is held in place by the threads coming through the bulkhead
from the U-bolt on the other side. The battery pack plugs into the
voltage regulator PC board. The battery pack is unplugged and
connected to a standard NiMH charger prior to flight. |
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This is the
Garmin
GPS 18-5Hz module prior to mounting it. It is a sealed plastic module
about two inches in diameter. There is one cable with six wires that
exits the side of the module. Machine screw mounting bosses are
provided on the back of it.
View
Garmin GPS Manual (700 KB pdf
file) |
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The same battery pack was
selected for both the transmitter and the receiver. It is a
six cell, 1200 mAH, 7.2V, NiMH battery pack sold at the local Hobbytown
store. They are intended for small radio controlled cars. The
battery pack is part number VEN-1512 and comes from
Venom Racing.
The cells are "AE" size. They provide 90 minutes of transmit time
with the transmitter running at 1 watt output. They sell for about $27.
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The photo below shows the
MaxStream 9XTend radio module. It is a transceiver so it can be
used as both a transmitter in the nosecone and as a receiver on the
ground. On the right hand side, it has a 2x10 connector header
with pins on a 2mm pitch. On the left hand side, the gold colored
connector is a Reverse Polarity (RP) SMA connector for the antenna.
The module is built on a miniature blue colored PCA and all the
electronics components are inside the metal shield.
A complete manual for the device is available here. This
device is available from
Digi-Key as part number XT09-SI-ND for $179 each. |
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Top side of the MaxStream
9XTend radio module.
View manual
(2MB pdf file) |
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Bottom side of the MaxStream
9XTend radio module. |
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This photo shows both the top
side and the bottom side of the custom PCA that was designed for this
project.
The shape of the PCA was constrained by the
case used for the receiver. |
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A special PCA was designed
to connect up the MaxStream radio, the 5V regulator, the batteries, and
the GPS module. It also provides serial port connections for a host
computer. The same PCA was used in both the transmitter and
the receiver although the physical mounting schemes are totally different.
These PCA's were designed using the tool set provided by
ExpressPCB.
Their design tools are simple and very easy to learn, yet quite capable.
The layout tool will even send the final design to the
ExpressPCB fab
site for quick prototyping. Four days after submitting the
design, the boards were delivered and the quality was excellent! I
had six boards made. They are double sided with plated through holes and
precut to the final shape I needed. This cost $125 including the
second-day air shipping. That is just over $20 per board. |
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U1 in the schematic is an
LT1764A Low Drop Out (LDO) adjustable regulator. It is
made by Linear Technology
and sold by Digi-Key
as part number LT1764AET-ND for about $7.00. It has a very low
drop-out voltage spec of around 0.25V at 1.5A. This makes it ideal
for generating a 5V output from the 7.2V battery pack because the battery
cells themselves will be the limiting factor that set useful run time. Run
time will not be limited by the regulator overhead.
The other key component on
the schematic is the
MAX232A (U2) made by
Maxim. It is
also available from Digi-Key.
This part is a level translator and signal inverter that takes proper
RS232 signals from a host computer and interfaces them to the MaxStream
radio serial port. The MaxStream module plugs into connector J6.
It is a 2x10 pin header receptacle.
The
Garmin
GPS 18-5Hz module plugs into connector J2. The output from the
GPS already has the proper polarity for RS232 communication to a host
computer. Therefore, on the transmitter unit, the output from the
GPS must go through the MAX232 inverter to work correctly with the
MaxStream serial port. Two small jumpers must be installed on the
PCA used in the transmitter to route the GPS output to the MaxStream
radio. Note: The GPS must be unplugged from connector J2 if a
host computer needs to be connected to J4 on the transmitter to configure
the MaxStream radio.
Since the receiver does not
use a GPS module, the host computer on the receiver can always connect to
J4 to receive the output from the MaxStream radio module.
There are three LEDs on the
board. One indicates the MaxStream module is transmitting, one
indicates it is receiving, and one shows that power is on.
There is provision for monitoring the MaxStream Receive Signal Strength
Indicator (RSSI), but I have never used it.
Connector J3 was originally
designed into the board before I realized the GPS unit was already
compatible with an RS232 interface. Therefore, connector J3 and the
extra two inverters inside U2 are not really needed. |
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A simple quarter-wave antenna
was used on the first flights of the transmitter.
This antenna is part number
ANT-916-CW-QW-ND from
Digi-Key. It is made by
Linx
Technologies Inc.
It has an RP SMA connector built into the
base of it so that it can screw right onto the MaxStream radio. |
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Transmitter PCA is assembled
and ready for the MaxStream radio to be installed.
The long tab on the right hand side of the
PCA was cut off and a right angle aluminum bracket was added so that the
transmitter will mount vertically in the nose cone.
The 5V regulator is mounted on a black
multi-finned heat sink. |
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The MaxStream radio is now
installed onto the transmitter PCA. Four short standoffs and some
#4-40 machine screws hold it in position. The #4-40 nylon lock
nuts can be seen at each corner of the blue MaxStream board. |
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The completed transmitter
unit is mounted on the nosecone bulkhead with a couple of #4-40 machine
screws at each end of the right angle bracket. The battery pack is
then connected to J1 and the GPS is connected to J2. The two blue
wires from the power switch were hardwired to the PCA.
The baud rate on the GPS
was set to 38400. This is as high as it will go and is necessary in
order to allow it to stream out GPS readings five times per second. The
MaxStream radio in the nosecone receives the 38400 baud information from
the GPS but burst transmits that same information at 115K baud to the receiving
unit on the ground. (The MaxStream radio can be configured to
transmit at either 9600 or 115K baud. Obviously 9600 is too slow.) The MaxStream radio in the receiver on the ground receives the information at
115K baud but then sends it at 38400 baud to an
HP iPAQ hx2415 Pocket PC
that is capturing and displaying the data. (More about that below.) |
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This is the top side of the
completed receiver board. The MaxStream radio is in place.
Since the receiver requires far less power, the 5V regulator was simply
mounted flat onto the PCA. No heat sink is required.
The antenna shown here was only used for
testing the receiver. A higher gain Yagi antenna was used during
flight operations. |
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This is a view of the back
side of the receiver PCA. The left
hand side was laid out as surface mount prototyping area just in case some
modifications were necessary. It was not needed. |
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The power switch and the LEDs
on the receiver front panel are hardwired to the receiver PCA. |
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The receiver PCA is now
mounted into the case. The antenna connector extends out the top. An
RS232 connector will now be added to the bottom.
This case is part number PN-1321 from
Bud Industries.
It is sold by Digi-Key
as part number 377-1114-ND for about $10. |
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An extra blank PCA is mounted
over the top of the receiver PCA on four threaded standoffs. Then the
battery pack is placed on top of the blank PCA and held in place with two
nylon zip ties.
The battery pack plugs into connector J1 on
the receiver PCA. It can be unplugged and connected to a NiMH
charger when it needs to be recharged. |
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A 9-pin RS232 type connector is on
the bottom of the receiver and wired into the pins on connector J4 on the
PCA.
Only three pins on this RS232 connector are
used. Tx, Rx and ground. |
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Front panel of the receiver
after it was secured in place with four screws. The cable from the
antenna is connected to the top side.
The labels are custom
stick-on decals.
The power switch is a
locking toggle switch. It can not be actuated into the on or off position
unless the bat handle is first pulled upward. It is available from
Digi-Key as part
number CKN1125-ND.
The switch and the LEDs are
all moved to one side to make room for the internal battery pack that runs
the length of the case. |
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The receiver electronics
and the Yagi antenna are mounted together by a short section of aluminum
angle stock. The case on the receiver is bolted to the angle stock
with two machine screws through the bottom of the case. This
arrangement makes it all one solid assembly that can be held in the middle
and pointed in the direction of the rocket. It is well balanced and
easy to aim. This arrangement also minimizes the cable length
between the antenna and the receiver.
This photo also points out
the RS232 serial data connector that is at the opposite end from the
antenna. |
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An
HP iPAQ hx2415 Pocket PC
is connected to the receiver and is used to display and save all the GPS
data transmitted from the rocket. Alternatively, a laptop can be
used, however the iPAQ works great for this application. It is smaller,
more portable and unlike a laptop, the display can still be read in bright
sunlight.
A special RS232 adapter
cable is used to connect into the docking connector on the bottom of the
iPAQ. The adapter cable was purchased from
GoMadic for $20.
It is part number ISC-21-1700 and it is the short black cable in this
picture. |
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The
HP iPAQ hx2415 Pocket PC
sells for $399. It runs Windows Mobile 2005 (or 2003) for the Pocket
PC. It has an Intel PXA270 520MHz processor, 64MB SDRAM and 64MB
Flash ROM, with 70MB user accessible memory. There are also SD and
CF slots for adding lots more memory. It also has built in 802.11b
WLAN as well as Bluetooth wireless technology.
Eventually it should be
possible to completely eliminate the RS232 cable by adding a Bluetooth
interface into the receiver. (Future project!) |
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VisualGPSce is a software application that runs on the Pocket PC and
displays GPS data while also allowing the raw GPS NMEA strings to be
recorded into a file. Other features include a graphical satellite
azimuth/elevation view, a GPS signal quality bar chart, analog gauges and
statistical position averaging.
VisualGPSce can be download for FREE from
VisualGPS. They
also have other versions of the software that have more features and run
on laptops. Those are not free.
This
photo shows the VisualGPSce screen that displays the signal strengths from
all the satellites in view. It also displays the current GPS
coordinates and altitude above sea level. |
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This photo shows the
VisualGPSce screen with four different "analog gauges". These
gauges move in real time based on the GPS data. They show ground
speed, altitude, heading and vertical speed. It's pretty cool to
watch these gauges while the rocket is in flight.
A future project is to
replace the VisualGPS software with something custom written specifically
for this application. There are all sorts of special features that could
be added! |
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The
Angelfire flight at LDRS-24 really
demonstrated the potential benefits of having a GPS downlink. During that
flight, Angelfire accidentally deployed the main parachute at over 16,000
feet and the wind took it 3.5 miles down range into large farm fields with
very few roads. This was the maiden flight of Angelfire and there was only
a Walston radio transmitter
on-board. However, once the rocket touched down, the signal from the
Walston was lost. Driving around the few roads in the area and
searching for a signal did not yield any results. It became
necessary to hike cross-country following the last known line of sight
from the flight line until the signal could be acquired. This worked
fine but it took a long time and it was a very long hike. It could
have been avoided if we could have driven to the closest location and
started hiking from there. Without a GPS system on-board, there was
no way of really knowing where to begin the search from the roads. There
was no Walston signal on any of the roads because Angelfire landed in a
very low lying area with no line-of-sight to the roads. The moral of this
story is to use both! The GPS coordinates will normally take you
right to the rocket. However, if the GPS system should fail for any
reason, then the Walston is a very reliable backup. |
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