GEMS UAV Payload Integration
Data Explorer Tool
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The Sentek Systems GEMS payload relies on integrated accelerometers, gyroscopes, magnetometers, and
GPS. These sensors drive navigation algorithms to estimate image placement for automatic image
registration and mosaicing. It is essential for proper operation that these sensors not be jammed. Sentek
Systems is committed to making sure our customers get the best results possible with our payloads. We have
extensive experience designing and integrating UAV platforms, and we offer our assistance to our customers
to help with payload integration.
We have developed a tool that will aid UAV integrators in proper GEMS Sensor payload integration. This tool
allows integrators to explore raw data saved by the GEMS unit during flight and investigate the effects of
changes they make to payload integration.
This tool aids in correcting any potential integration issues that may degrade mosaicing quality and image
registration accuracy. This can be used to better inform the UAV integrator of changes that should be made
to optimize the performance and quality of the output of the GEMS system.
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• Download the GEMSDataExplorer1.0.zip
file
• Extract the entire folder to a location on
your computer
• Make sure all the files that come in the
extracted folder remain and are not
deleted
• To run the program double click on the
GEMSDataExplorer.exe
• Once launched it will prompt you to
select a file
• Navigate to your test flight data folder
from the GEMS unit and select the
Flightdata.bin file
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Single axis accelerometer Data with
acceptable noise envelope
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3- axis (x,y,z) raw accelerometer Data
Noise Envelope (x-axis within noise envelope bounds)
You can plot each axis separately with a target noise envelope. The signal
should stay on average within the light grey envelope.
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Noise Envelope (need to zoom in on data to view noise envelope)
Magnetometer data should be plotted on each axis and the noise should not
exceed the acceptable noise envelope
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Green – Good GPS Performance 42dBHz-52dBHz, Yellow – Fair GPS Performance
35dBHz – 42dBHz, Red – Poor/Unreliable GPS Performance < 35 dBHz
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Accelerometer Data:
Accelerometers are used to measure acceleration (rate of change in velocity). The GEMS payload measures
and reports accelerometer data in three different axis: x-axis, y-axis, and z-axis (these axes can be seen in
silkscreen on the GEMS PCB). Noise on accelerometer data is typically due to high vibrational environments.
On small UAV fixed wing platforms this tends to be less severe than on copters/multi-rotor platforms. Every
platform has its own unique resonance frequency which is dependent on where the center of mass of the
platform is located, the material the platform is made out of, and the motors/propellers being flown. We
highly recommend dampening vibrations on fixed wing platforms by using foam as an isolator. This can be
foam placed in between the payload and the motors or an encased cutout of foam that the payload sits on in
the payload bay. For multi-rotor platforms we highly recommend mounting the system in such a way that it is
not directly connected to the vehicle frame. This means mounting it on the bottom of the quad in between
the landing gear/skids with vibration isolators. Se tek’s custom enclosure has these incorporated into the
enclosure. (Refer to the hardware integration manual for pictures of install). These vibration isolators
dampen the x-axis, and y-axis substantially. Additional z-axis vibration isolation may be needed depending on
the platform characteristics.
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Magnetometer Data:
Magnetometers are sensors used to measure magnetic fields. The GEMS payload has a 3-axis magnetometer
onboard to sense the Earths geomagnetic field. This is an integral sensor for determining system orientation.
Certain materials will generate magnetic moments when they are placed in a magnetic field (i.e. they will act
as magnets themselves). Such materials are called ferromagnetic, and they locally distort the ambient
magnetic field by adding their own magnetic moment. If such materials are in close proximity to the
magnetometer on the GEMS system, they can severely corrupt its magnetic readings. Make sure there are no
ferro ag eti aterials i lose pro i it to the GEMS s ste separatio of 4” is a a solute i i u .
Iron and Steel are the most common ferromagnetic materials you may encounter. These are not commonly
used on air vehicles because they are heavy, but sometimes these materials are still used for small hardware
like screws and nuts. Fortunately, it is easy to determine whether or not a material is ferromagnetic: touch
the material with a permanent magnet; if the magnet is attracted to the material then it is Ferromagnetic
and you do not want it near the GEMS system. If the magnet does not stick to the material, then it is nonferromagnetic and is safe to use near the GEMS system. We recommend removing the GEMS payload from
your vehicle and performing a sweep of your vehicle using a permanent magnet.
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Magnetometer Data Continued:
Permanent magnets and electromagnets can also corrupt our measurements of the geomagnetic field, since
we end up measuring the sum of the geomagnetic field and the field produced by the magnet. Motors use
permanent magnets and electromagnets to convert electrical energy to mechanical energy. Some motors do a
good job confining magnetic fields to within the motor; these do not cause problems. In other motors, the
field can leak outside the motor and create interference. It is generally a good idea to keep the GEMS payload
a good distance away from motors, at least until it has been verified that the motors do not create
interference. In particular, this means that motorized gimbals are not the best way to mount the GEMS system
to a vehicle, since these can create magnetic interference. Since motors are rotating they typically produce
fast-changing magnetic fields. Because of this, interference caused by motors tends to look like noise added to
the magnetometer sensor data.
If you have removed nearby ferromagnetic materials and there are no motors or other magnets near the
GEMS system, but you are still getting less-than-ideal magnetometer data, it is possible that you are getting
interference from cables running nearby the payload. Current moving through a cable will create a magnetic
field around the cable. The field is stronger as the current running through the cable increases. We
recommend keeping cables not related to the GEMS system away from the payload during flight.
GPS Data:
Our GPS system has been extensively tested with multiple configurations and platforms. It has very high C/N0
(close to the theoretical maximum). We have taken extreme precaution on the signal integrity of the printed
circuit board layout and have included a 28 dBi gain antenna with the kit. If installed in a proper location on
the platform (i.e. on top no obstructions to sky) with a sufficient ground plane the system should lock on and
acquire GPS in 30-35 seconds. If the system is not achieving this it means there is likely an environmental
problem degrading GPS performance.
Sentek has found through extensive testing that you need a minimum of C/N0=28-30dBHz to lock on in a
coldstart environment. However, for C/N0 less than 35dBHz the GPS performance is not robust (meaning
increased satellite dropouts). An average C/N0 of 42 dBHz or higher gives significantly improved reliability and
performance when utilizing carrier phase GPS which the GEMS payload is using (increasing carrier phase hold
time).
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GPS Data:
Potential problems can include:
•
•
•
•
EMI from other onboard systems may be jamming GPS. Cables, OEM PCBs, dirty power sources, and other
hardware systems all radiate noise. If it is located sufficiently close to the GPS antenna it could be jamming
the system. Make sure our EMI shields are still on the payload itself. These have been designed to keep
other sources from jamming our system.
A radome or another material may have been placed on top of the antenna. If plastic or other objects like
metal are located in close proximity this can detune the antenna and degrade the received signal. Anytime
you place an object on or around an antenna the antenna needs to be re-tuned to 50 Ohms with the
surrounding environment.
The GPS cable attached to the antenna may have been severely crimped. It is OK to coil the RF coaxial cable,
but it is not OK to bend or crimp the cable at 90 degrees or more. If the cable does not take its original
shape after straightening it out, it has been damaged and will result in reduced GPS performance.
Se tek re o
e ds usi g a i i u of a 2” 2” grou d pla e u der the GPS a te a. This pla e should e
metallic (copper tape works very well and is easy to apply). Antenna designers like to shrink the size of the
antenna element to make them as small of form factors as possible. Most antennas require a ground plane
to radiate properly. The ground plane stabilizes the antenna performance (improving axial ratio) and allows
the signal to be properly radiated. When reducing the ground plane size the antenna performance is always
the first parameter to be degraded. The efficiency or gain of a patch antenna is highly dependent on ground
plane size. Additionally, the center frequency of the antenna will shift with the size of the ground plane.
These all negatively impact GPS signal to noise ratio degrading GPS performance. When in doubt add a
ground plane; it never hurts any antenna!
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Example of GPS Patch Antenna Performance versus Ground Plane Size:
Figure 1 Example of a Single Feed Patch
Antenna Gain versus Ground Plane Size
Figure 2 Example of a Single Feed Patch Antenna
Center Frequency versus Ground Plane Size
As ground plane decreases antenna gain decreases
As ground plane shrinks antenna detunes from center frequency degrading
performance
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Accelerometer:
• Ensure the payload is located off the main frame of the platform to reduce vibrations
• Ensure the payload has vibration isolators securing the payload to the main frame (if on a multi-rotor)
Magnetometer:
• Attempt to remove all ferromagnetic materials on the platform (perform a magnet sweep).
• If some ferromagnetic materials cannot be removed from the vehicle, then make sure they are at least
4” awa fro GEMS u it.
• Make sure the payload is not close to motors or magnets
• Make sure the platform has sufficient shielding of cables and unrelated cables are kept away from the GEMS
unit during flight
GPS Data:
• Ensure EMI shields are still on the payload (metal plates on bottom of GEMS unit)
• Make sure the GPS antenna is elevated above the rotors and on a GPS mast with a sufficient ground plane.
• Ensure proper shielding for noisy components (copper tape, EMI shields, ferrite beads)
• Ensure proper separation between payload and noisy hardware. Electromagnetic fields dissipate with
distance. The farther the distance between noisy hardware and the GPS antenna and the payload the better
the GPS signal strength will be.
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