Field of the Invention
This invention relates to an orientation measurement unit and in particular, this invention
relates to an orientation measurement unit which consists of an attitude and heading
reference system having Micro Electro Mechanical Systems (MEMS) sensor modules to
determine the attitude of a set of unguided instruments and payloads. More particularly,
this present invention also relates to an orientation measurement unit which is used to
measure the attitude of a payload on-board a weather balloon. Furthermore, this
invention also relates to an orientation measurement unit having the exact orientation of
the detectors facing the sky in order to infer the nature of the obtained data and their
respective sources. Moreover, this invention also relates to orientation measurement unit
having the exact orientation of the detectors facing the sky in order to infer the nature of
the obtained data and their respective sources and a software identifies the source of each
signal detected at every instant of time.
Background of the invention and the related Prior Art
Gyroscopes are being used for direction measurements for a long time. Similarly
magnetic compasses are also used for direction measurements. Even certain birds are
known to fly using the knowledge of magnetic field direction. In Astronomical satellites
and large balloons instruments are sent to space to measure properties of certain specific
sources, such as the Sun and other stars. For this, very accurate direction measurement
techniques are required. They are often very massive and are capable to slewing heavy
payloads or optical/X-ray telescopes to the direction of observation. These instruments
slew from an object to another object systematically and carry out continuous observation
for a considerable time. Sometimes for rockets specially programmed inertial measure
units are attached so as to follow a specific trajectory.
In the document US6380526, a payload launching method measures the attitude of a
booster during a launch with an inertial measurement unit on the payload to compare the
actual booster trajectory with a desired trajectory to control the payload at the
deployment location, the inertial measurement unit on a booster carrying the payload
using said desired trajectory to reach the deployment location. According to the
invention, the payload IMU computer contains the trajectory plan used by the booster's
IMU. The computer also includes the comparison routines, e.g. Kalman filter, for that
task during the flight the payload IMU measures the motion of the booster and compares
the measurement to the planned trajectory so that payload, in effect emulates, the booster
IMU in so far as it knows the trajectory to its deployment point to compute the attitude
solution for the mission. In other words as done on the booster IMU, the payload IMU
uses a payload attitude determination algorithm that operates on the trajectory difference
or error to separate the component of trajectory dispersion from that of navigation error
caused by payload IMU's attitude error. This is accomplished, in the payload IMU during

flight, with the use of a math model governing the statistics of the trajectory dispersion
and a math model governing the effect of attitude error in trajectory. These math models,
which are known, are used in the payload IMU processor, as the trajectory dispersion
covariance matrix and the navigation error covariance matrix, which exhibit totally
different characteristics. The attitude determination algorithm takes the difference
between the payload IMU computed trajectory and the stored, desired trajectory and use
statistical models to separate the trajectory dispersion from the effect of the attitude error.
Once separated, the effect of attitude error is processed to back out the payload IMU
attitude error and complete the operation of attitude determination, in the form of a
Kalman filter.
According to the document US8212195, a guided projectile may include a projectile
body. An inertial measurement unit may be disposed within the projectile body, the
inertial measurement unit including sensors to measure motion parameters relative to
first, second, and third mutually orthogonal axes. Each of the first, second, and third
mutually orthogonal axes may form an oblique angle with a longitudinal axis of the
projectile body. A controller may be configured to control a trajectory of the guided
projectile in response, at least in part, to measurement data received from the inertial
measurement unit.
According to the invention which provides a guided projectile may include a projectile
body. An inertial measurement unit may be disposed within the projectile body, the
inertial measurement unit including sensors to measure motion parameters relative to
first, second, and third mutually orthogonal axes. Each of the first, second, and third
mutually orthogonal axes may form an oblique angle with a longitudinal axis of the
projectile body. A controller may be configured to control a trajectory of the guided
projectile in response, at least in part, to measurement data received from the inertial
measurement unit which has been stated in the document US20120025008.
The present disclosure relates to a payload measurement system for a machine. A weight
of the payload on the machine is required to be estimated to have an optimal loading of
the machine. The payload measurement system provides an accurate estimation of the
weight of the payload on the machine which has been stated in the document
WO2015000665.
The patent document KR20030087681 describes a method and an apparatus for
processing remote measurement data and commands in a communication satellite
payload are provided to execute the control and state monitoring of the interior of a
payload by using an interface unit and an associated algorithm in the inside of the
payload. An apparatus for processing remote measurement data and commands in a
communication satellite payload is comprised of an on-board computer, a payload, and a
PIU (Payload Interface Unit). The on-board computer receives and processes a command
transmitted from a transmitting earth station or transfers it to the PIU. Also the on-board
computer receives remote measurement data collected by the PIU and transmits the
received data to the transmitting earth station.; The PIU is composed of a power control
board, a remote measurement and command processing board, a data control board, a

serial board, and a heater control board. The PIU analyzes a command received from the
on-board computer and transmits an analyzed result to the payload through an associated
interface board. Also the PIU receives remote measurement data, collected from the
payload, through a relevant interface board and transmits the received data to the on-
board computer.
According to the document CN203287552, a height measurement device, and specifically
used for measuring a terrain clearance of an onboard mission payload of a small-sized
ship-based unmanned helicopter. The measurement device mainly comprises a contact
plate, a jack and a base seat, wherein the contact plate is in parallel with the base seat,
and the jack is disposed between the contact plate and the base seat; the jack is fixed with
the middle of the bottom surface of the contact plate by welding, and the lower end of the
jack is fixed with the base seat by welding; and the jack is engraved with a scale, and the
measurement precision is 1mm. The measurement device is small in size and relatively
high in stroke, the lowest point from the ground of the mission payload can be detected
rapidly and accurately, and therefore the terrain clearance of the mission payload is
detected and measurement efficiency is largely increased; the measurement device is also
suitable for measuring other mission payloads with complex and different shapes,
universality is good, and cost is reduced effectively; and the measurement device is also
characterized by reasonable structure, simple shape and easy movement.
There is an urgent need to measure orientation of any object which is not intended to be
guided in the first place but it is essential nevertheless to know its instantaneous
direction. Specifically, when instruments are flown on an unguided balloon, the balloon
moves and rotates on its own depending on the direction of winds and shear, but
knowing the instantaneous direction of the instrument would allow one to associate the
data with objects in that direction post-facto. In small balloons small instruments could
be sent to near space at about 40km altitude for atmospheric and space measurements and
it is advisable to know from which object the data is being collected at every instant of
time. An Attitude and Heading Reference System (AHRS) is thus essential for these
missions. Here, the AHRS is built using the commercially available 9 Degrees of
Freedom (DOF) Inertial Measurement Units (IMUs). The obtained data from the AHRS
used in the mission are then further used to calculate three Euler's angles with time
stamps. Once the Euler's angles are known, we can know the orientation of the payload at
the corresponding time instant and hence the stars those were in the Field of View (Fo V)
of the detectors by converting their Right Ascension (RA) and Declination (Dec) into
horizontal coordinate system (Azimuth and Altitude).
Summary of the invention
This invention relates to an orientation measurement unit and in particular, this invention
relates to an orientation measurement unit which consists of an attitude and heading
reference system having MEMS based sensory modules to determine the payload
attitude. More particularly, the present invention also relates to an orientation
measurement unit which is used to measure the attitude of a payload on-board a weather
balloon. Furthermore, this invention also relates to an orientation measurement unit

having the exact orientation of the detectors facing the sky in order to infer the nature of
the obtained data and their respective sources. Furthermore, this invention also relates to
an orientation measurement unit which is used to measure the attitude of a payload on
board weather or any balloon or any unguided floating object with detectors getting data
any source in the sky or earth at every instant of time in order to infer the source of the
corresponding data at every instant of time. Moreover, the invention includes software in
the form of Graphics User Interface (GUI) to view the object plane with time from the
detector's perspective.
Detailed description of the invention with accompanying drawings
One skilled in the art may make modifications, in whole or in part, to a described
embodiment of the invention, its components and their functions without departing from
the true scope and spirit of the invention and many of its advantages should be readily
understood and appreciated. The drawings are in four sheets.
Generally in every costly cell phone now-a-days there exists a measurement unit to
determine directionality for navigational purposes. However, imagine that a set of space
instruments are floating with balloons which are deployed to obtain X-rays and gamma-
rays from sources in the sky. Since the balloon randomly rotates and moves in any
direction, it is difficult to point those instruments towards any given object since the
pointing instrument could be heavy and could not be lifted by small balloons. So this
invention includes a very light weight orientation measurement unit which, post-facto
determines the direction from which the observation (Electromagnetic radiation at all
wavelengths) is being made every instant of time thereby knowing the properties of many
sources precisely even specifically pointing to any one of them.
The principal embodiment of the invention is to provide an orientation measurement unit.
The other embodiment of the invention is to provide the orientation measurement unit
wherein payload AHRS (Attitude and Heading Reference System) uses a 3-axis
accelerometer and a 3-axis magnetometer.
The other embodiment of the invention is to provide an orientation measurement unit
which consists of an attitude and heading reference system having MEMS based sensory
modules to determine the payload attitude.
The other embodiment of the invention is to provide an orientation measurement unit
which is used to measure the attitude of a balloon-borne payload containing instruments
for scientific study.
The other embodiment of the invention is to provide an orientation measurement unit
having the exact orientation of the detectors facing the sky in order to infer the nature of
the obtained data and their respective sources.

The other embodiment of the invention is to provide an orientation measurement unit
which is mounted on the payload carrying the scientific instruments which measures X-
rays/Gamma-ray or any electromagnetic wave or acoustic wave (collectively designated
as the 'signal') and this unit gives the direction from which the signal is coming at every
instant of time to identify the source.
These and other advantages of the present invention will become better understood from
the following description read in conjunction with the drawings, of which:
FIG. 1 illustrates the Block diagram of the components and their interconnections for the
payload orientation measurement unit of the present invention;
FIG. 2 illustrates the flow chart of the software design of the payload orientation
measurement unit of the present invention;
FIG. 3 illustrates the Bias calibration of accelerometer and magnetometer of the payload
orientation measurement unit in the present invention;
FIG. 4 illustrates the Visualization software algorithm of the payload orientation
measurement unit in the present invention;
FIG. 5 illustrates the Graphical user interface of the visualization software of the
orientation measurement unit of the present invention.
The payload AHRS (Attitude and Heading Reference System) uses a 3- axis
accelerometer and a 3-axis magnetometer. The accelerometer and the magnetometer
readings change with the orientation of the PCB on which they are mounted. The pitch,
roll and yaw of the device, defining the PCB orientation is measured from a tilt-
compensated algorithm. Using this orientation information, the payload orientation can
be determined of the platform/ payload on which this device is mounted.
Figure 1 shows the overall architecture of the system in block diagram form. The
peripheral measurement units are connected with a central computing unit which is a
microcontroller. Along with main 9DOF Inertial Measurement Unit which docs the
accelerometer, magnetometer and gyroscope sensors, other peripheral modules involve
Real Time Clock (RTC) module and a GPS Unit. The acquired data is finally being
stored in a data storage unit (microSD/SD card). For additional data of temperature and
pressure we also connect a pressure-temperature sensor module with the processing unit.
The accuracy of the device is highly dependent on the calculation and subtraction (in
software) of stray magnetic fields both within, and in the vicinity of, the magnetometer.
The error induced in calculation due to the presence of fixed magnetic fields is termed as
Hard-Iron effects and those induced by the presence of ferromagnetic substance thereby
distorting geomagnetic field are termed as Soft-Iron effects. Any zero fields offset in the
magnetometer during the manufacture are normally included with the Hard-Iron eftects
and is subtracted at the same time. The accelerometer also suffers some gravity offset
which also have to be removed.

The system for the attitude measurement has a hardware part to gather the data of the
payload orientation and a software part to calculate payload attitude and visualize the
objects seen by the detector. The key instrument used in this system is a 9 DOFchip. The
chip contains a 3-axis accelerometer, 3-axis magnetometer and a 3-axis gyroscope. The
accelerometer is ADXL345, magnetometer HMC5883 and the gyroscope is ITG3200.
Here in this application the raw data of accelerometer is used to measure the elevation
(tilt) angle with respect to ground by measuring the gravitational acceleration (g) in each
of the three axes. The magnetometer is a MEMS electronic compass module containing
three magneto resistive sensors orthogonal to each other. The sensor measures Earth's
geomagnetic field components along its each axis. So, from the raw data of
magnetometer axes the heading of the sensor with respect to the geomagnetic North Pole
can be obtained. The heading depends on the local magnetic declination of a particular
place. But this magnetic declination is not really necessary as it is canceled out in the
calculation.
The accelerometer readings provide pitch and roll angle information which is used to
correct the magnetometer data, this in turn allows for accurate calculation of the yaw. The
tilt-compensated device will neither operate under free fall nor low-g conditions nor
while subjected to high-g accelerations.
We tested several types of processor boards to acquire the sensor data. Micro-controller
based architecture works well. The micro-controller, Atmel's AVR ATMega32 -an 8 bit
RISC processor has been used. The maximum clock speed is 16 MHz. It offers byte-
oriented Two-Wire serial Interface (TWI) useful to interface the 9 DOF IMU using 12C
communication protocol. The voltage required for ATMega32 is 4.5 to 5V. This
processor provides an easy way to interface different devices. It provides up to 16 MIPS
throughput at 16 MHz clock speed.
Another tested processor architecture is from ARM 11 family. Raspberry Pi board uses
700 MHz clock for operation. This board offers an array of associated peripherals,
making it a standalone, multipurpose box for many applications. Besides having the
GPIO (UART, SPI, 12Cetc), it also supports video and audio processing. The main
advantage from a programmer's point of view is that this board supports a number of
operating systems. Some of them are Arch Linux ARM, Debian GNU /Linux, Gentoo,
Fedora, FreeBSD, NetBSD, Plan9, Raspbian OS, RISC OS, Slackware Linuxetc,
Archlinux is used here for programming the Raspberry Pi. It has two USB ports where
one can connect keyboard, mouse etc. for programming. This board also provides HDMI
support and video output for interfacing with VDUs. It is particularly useful for the light
weighted balloon-borne science as the size of the board is just as the size of a credit card
(85.60mm X 56mm) and the weight is only 45g.

We have developed a software architecture to make use of the data obtained from the
Attitude Measurement Unit (AMU). The functional flow chart of the software is
shown in Figure 2. The main outcomes from the software are:
• Calculation of the payload attitude, from which one calculates which part of the
sky is inside the detector or FoV.
• Given a co-ordinate of an object, (e.g., Sun or any X-ray or any electromagnetic
wave emitting candidate) in the sky or any radiation or acoustic wave emitting
object on earth (collectively terms as 'signal') one can separate out its data from
the entire data set recorded by the detector.
• The objects can be studied and may be identified to see if any scientifically
important behaviour is seen.
With the huge improvements in the field of semiconductor fabrication technology, the
IMU sensors are now available in a very small IC package. But there are number of
factors working behind erroneous data resulting from these sensors. The errors are
generally due to some imperfections in the process technology, hysteresis in the sensor
materials, piezoelectric effects etc. These imperfections result in some errors in the
generated data like shifting, scale error etc.
For the accelerometers and gyroscopes the main error is the offset or bias error. For
magnetometer the calibration is much critical while measuring the Earth's geomagnetic
field. Apart from the design imperfections and electronic noise, there are hard and soft
iron effects that have to be addressed while dealing with the compass module. The hard
iron effect refers to the distortion of the geomagnetic field in presence of a constant
additive source of magnetic flux such as a magnet. Soft iron effect arises in presence of
a ferromagnetic element.
To illustrate the effect it can be said that if no permanent magnet or ferromagnetic
materials are present, the 3D plot of the magnetometer axes data would result into a
perfect sphere. The soft iron effect makes the sphere into an ellipsoid, as the field is
distorted by the ferromagnetic materials. Moreover, the hard iron effect shifts the center
of the ellipsoid from (0, 0, 0).
For the accelerometer there is no effect equivalent to soft iron effect in magnetometer,
but there is always the offset problem which can be treated in the similar way of treating
the hard iron effect.
To nullify these problems of stray magnetic fields and bias offsets, a large amount of data
can be collected from the device by orienting it in all possible directions. In ideal case of
zero distortion and no offset,these data will populate over the surface of a sphere. But

due to those effects, for the magnetometer, the data would be distributed on an ellipsoidal
with a shifted center from the origin. To calibrate the magnetometer, the data can be
fitted with an ellipsoid; transform the ellipsoid into a sphere of equivalent volume and
then calculate the center offset of the sphere as the bias offset. After that, use the
transformation matrix to correct the real time magnetometer data from the distortion. For
the accelerometer, data is fitted with a spherical surface and we get the center offset as
the bias offset. The calibration data fitted with spheroid and ellipsoid are shown in Figure
j.
There are several applications of our invention. One can identify stars or celestial bodies
from which signals (e.g., optical, X-ray, Gamma-rays etc.) are picked up by detector units
(payloads) attached on a floating platform such as a balloon. For this application, a
Graphics User Interface or GUI has been developed also by us which use the payload
orientation data and star position information. We have shown the functional flowchart of
the visualization software in Figure 4. To calculate the payload attitude we have to first
calculate the pitch, roll and yaw of the device. Any orientation of the device can be
modeled as resulting from rotations in yaw, pitch and roll applied to a starting position
with the device flat and pointing northwards. We use tilt-compensated algorithm to
calculate the device heading in RA and DEC from the pitch, roll and yaw of the device
and are subsequently used in further calculations. The GUI software used to provide a
graphical view of the sky where the detectors point at any instant; uses a database for
stars. The altitude and azimuth (Horizontal Coordinate system) of those stars are
calculated for a particular date (the date of experiment) from their respective positions in
the Equatorial Coordinate system (RA and DEC) obtained from the attitude measurement
unit. Thus the computed positions for the stars in the horizontal coordinate system along
with the star names are stored in a star data file which is used in the GUI software for
display. The software created for the purpose is implemented using ROOT and
MATLAB. The GUI has been implemented to interface a graphical viewing window,
buttons for loading the star data and the device orientation data in case we are interested
in the sky object. A screenshot of the GUI showing its functionality is shown in Figure 5.
The star data file comprises names of the stars and their angular positions in horizontal
coordinate systems. The angle data file contains time stamp, yaw and pitch and rolls all
expressed in degrees. A play/pause button is there to start the video view of the sky with
time. A time seek bar similar to the ones used in the media players is also present is the
GUI developed by us. A window is there to indicate the time stamp of the current video
frame. Another pop-up graphics window is available and gets invoked when the program
starts running. This window can be made to be full-screen and it gives the view as per the
set aspect ratio. Of course, one can always identify sources from where the signals are
picked up by the detector attached on an unguided floating platform from the RA/DEC
obtained from our attitude measurement unit. This makes the data useful for astrophysical
or earth science purposes.
The software basically gives a view of stereo-graphic projection of the grid lines of the
horizontal coordinate system and the stars are plotted on the gridlines as per their angular
positions in the sky of the given date. The grid lines can be turned on/off through the user
interface provided in the GUI.

The FoV i.e., the extent of the visible part in the graphics window can be adjusted. The
software records all stars which appear within the FoV in a text file with their respective
time stamps. The software also records the frames in avi video file format with given
frame rate.
The two input files to the system are device orientation data and star data file. The angle
data file contains three columns: the first one is the UT (Universal Time) in seconds;
second one is the Azimuth angle in degrees and the third one is the altitude angle in
degrees. These angles are calculated from the raw data obtained using the 9 DOF IMU
chip discussed previously of a specific balloon flight.
The star data file also contains three columns. The first one is the Azimuth angle in
degrees, second is the altitude angle in the same unit and the last column contains name
of the star. The visualization program that we developed at first brings all the data files
and loads the data into RAM using array data structures. The arrays containing the star
names and star angles form a structure. Here, another MATLAB script which we
developed takes the star catalog as input where the angles are in RA and Dec i.e. in
elliptic coordinate system and then transforms them into azimuth and altitude (Horizontal
coordinate systems). This script also incorporates calculation of Local Sidereal Time
(LST) using J2000 convention (Julian Date).
Once a flight with a payload attached with our invention is made by a floating unguided
system, such as a balloon, each 'signal', be it electromagnetic or acoustic in nature, which
is procured by the detector attached to the floating system, will have a direction also
associated with it. From the standard map of the sky or the earth, we are in a position to
identify the source of the signal. Thus from a given flight, we can identify several objects
from where the signals have come. They can be object-wise grouped together and their
properties could be studied post-facto. The sources can also be from the earths surface if
appropriate map is used.
Although specific embodiments of the present invention have been described above, it
will be readily apparent to one skilled in the art that the teachings of the invention may be
carried over to other embodiments and other fields. Therefore, the present invention
should not be regarded as limited to the embodiment just described in detail, but should
be accorded the full scope of the following claims.
Advantage over prior Art:
i) So far in satellites or large balloon borne instruments floated for
measurements of signals from specific sources are accompanied by very
massive orientation measurement units which use either distant stars, sun and
earth to fix their coordinate system, or have fly wheels and gear systems.
ii) The existing systems are very massive and are unsuitable for lighter balloons,
particularly for meteorological balloons which can carry at the most a few
kilograms of loads.

iii) The balloons are unguided and they move and rotate according to the wind
direction and shear. As such the payloads cannot be used to obtain data from a
specific object with existing technologies.
iv) Instruments used to identify stars in the night sky do not extract the direction
information instantaneously and thus are not useful for scientific platforms on
floating platforms.

We Claim:
1) An orientation measurement unit which consists of;
a) Attitude and Heading Reference System,
b) software platform
c) detector
2) An orientation measurement unit as claimed in claim 1 wherein Attitude and
Heading Reference System consists of a) 3- axis accelerometer b) a 3-axis
magnetometer and c) 3-axis gyroscope.
3) An orientation measurement unit as claimed in claim 1 wherein the raw data of
accelerometer is used to measure the elevation (tilt) angle with respect to ground
by measuring the gravitational acceleration (g) in each of the three axes.
4) An orientation measurement unit as claimed in claim 1 wherein the raw data of
magnetometer with the raw data of accelerometer are used together to compute
the tilt compensated Yaw (Azimuth) angle.
5) An orientation measurement unit as claimed in claim 1 wherein the accelerometer
readings provide pitch and roll angle information which is used to correct the
magnetometer data and allow to accurate calculation of the yaw of the detectors
in the payload.
6) An orientation measurement unit as claimed in claim 1 produces data after a
balloon flight which is collected and post-processing is done to calculate the Roll,
Pitch and Yaw angles accurately with time information and these information are
passed to a software tool for graphical representation of the payload for that flight.
7) An orientation measurement unit as claimed in claim 1 incorporating the software
platform provides a convenient Graphical User Interface (GUI) for easy control
and different options for data analysis including performing stereographic
projection of the grid lines of the Horizontal Coordinate systems where stars are
plotted as per their angular positions for a particular date.
8) An orientation measurement unit as claimed in claim 1 has a software platform
which presents a graphical view of a portion of projection of sky enclosed by a
given field of view from the orientation data.
9) An orientation measurement unit as claimed in claim 1 has a software platform
which has a provision to draw an outline view of a collimator
(rectangular/circular) on the graphics to visually identify the objects which were
within the collimator view at the time when signal enters the detector.
10) An orientation measurement unit as claimed in claim 1 has a software platform
with a user interface control to play the sequence of generated graphics of the sky
view with play/pause options and the frame sequences can be recorded in an avi
video file with given frame rate.