GB2516066A

GB2516066A - Portable device for determining azimuth

Info

Publication number: GB2516066A
Application number: GB1312368.2A
Authority: GB
Inventors: John Keith Sheard; Nicholas Faulkner
Original assignee: Atlantic Inertial Systems Ltd
Current assignee: Atlantic Inertial Systems Ltd
Priority date: 2013-07-10
Filing date: 2013-07-10
Publication date: 2015-01-14
Anticipated expiration: 2033-07-10
Also published as: DE102014109592A1; JP2015017984A; US20150019129A1; GB2516066B; US9395187B2; JP6339429B2; GB2516066A8; GB201312368D0

Abstract

A device for determining azimuth comprising a MEMS Inertial measurement unit (IMU), a GPS system and a processor configured to receive data from said IMU and from said GPS system, said processor being configured to derive a true north reference. The device may be handheld and may be moved in an oscillatory or periodic manner and may further be inverted. The two systems IMU and GPS may be rigidly connected at a distance of less than 5cm. The MEMS IMU may comprise a 6 degrees of freedom unit, 3 accelerometers or 3 gyroscopes. The device may be encased in a shockproof material which may be a synthetic resin.

Description

PORTABLE DEViCE FOR DETERMiNiNG &ZiMUTh

FIELD OF TECHNOLOGY

A new compassing device for determining azimuth is described herein, and more par cularly an azimuth determination device that may be portable and/or hand held.

BACKGROUND

Azimuth is an angular measurement in a spherical coordinate system. The vector from an observer (origin) to a point of interest is projected perpendicularly onto a reference piano; the angle between the projected vector and a reference vector on the reference plane is caVed the azimuth. A position ol a star in the sky can be measured using this concept. In such an example, the reference plane is the horizon or the surface of the sea beneath the point of interest being measured (i.e. the star), and the reference vector points to the north. The azimuth is then the angle between the north vector and the perpendicular projection of the star down onto the horizon. Azimuth is usually measured in degrees () and the reference plane [or an azimuth in a general navigational context is typically true north, measured as a azimuth. For example, moving clockwise on a 360 degree circle, a point due east would have an azimuth of 9Q0, south 180', and west 2700.

The concept may be used in many practical applications including navigation, astronomy, engineering, mapping, mining and artillery. Present hand held azimuth determination (i.e. compassing) devices and methods reiy on magnetic sensors to determine azimuth.

Some known devices and techniques for determining azimuth utilise data provided by an inertial measurement unit (IMLJ), An MU comprises three accelerometers and three gyroscopes and optionally three magnetometers. The accelerometers are placed such that their measuring axes are orthogonal to each other. They measure inertial acceleration, also known as G$orces. Three gyroscopes are placed in a similar orthogonal pattern, measuring rotational position in reference to an arbitrarily chosen coordinate system. The accelerometers and gyroscopes therefore produce inertial data and in known azimuth determination devices, a processor would normally process the inertial data from the IMU, via strapdown and inertial navigation algorithms, to dedve a navigation solution. Other known devices for determining azimuth use GPS systems. Some known handheid devices use magnetic sensors to determine azimuth,

SUMMARY

A device is described herein for determining azimuth comprising a MEMS inertial measurement unit (IMU), a GPS system comprising a GPS antenna and receiver, a processor configured to receive data from said MU and from said GPS system, said processor b&ng configured to process said IMU data and said GPS data to derive a true north reference based on said IMU data and said OPS data.

A method for determining azimuth is also described herein comprising providing a device that comprises a MEMS inertial measurement unit (IMU), a OPS system and a processor, wherein said GPS system comprises a GPS antenna and receiver, said method further comprising the step of moving said device, and transmitting said IMU data and said OPS data to said processor, and said processor processing said IMU data and said GPS data and deriving a true north reference based on said MU and GPS data.

The device may be configured to be hand held and may be configured to produce s&d IMU data and said OPS data due to movement of said device. The movement may comprise oscillatory translation of said device. In some embodiments, the movement may comprise inversion of said device and oscWatory translation of said device.

The 1MU data and the GPS data may correspond to first and second independent measurements of the same movement of said device.

The GPS antenna and MU of the device may be located relafive to each other so that they experience the same motion when the device is moved. In some embodiments, they may be located in/on said device.

The GPS antenna and MU of the device may be located relative to each other and rigidly connected to each other and/or the device so that they experience the same motion when the device a moved.

The OPS antenna and MU may be located at a distance of 5cm or less from each other, so that, in use, they experience the same motion when the device is moved.

In some embodiments described herein, the distance between the IMU end the OPS antenna is less than 10% of the oscillatory translation distance and the residual leverarm effects of any superimposed rotation being corrected.

In some embodiments described herein, the device is configured to produce said MU data and said GPS data due to movement of said device and wherein said movement comprises osciatory translation of said device, and further wherein the MU and the GPS antenna are located reiative to each other at a distance that is less than 10% of the oscillatory tran&ation distance.

In sonic embodiments described herein, the device is configured to produce said MU data and said OPS data due to movement of said device and wherein said movement comprises inversion of said device and oscillatory translation of said device, and further wherein the IMU and the OPS antenna are located r&ative to each other at a distance that is less than 10% of the oscUlatory translation distance.

In some embodiments, the device may comprise only one GPS antenna. .3

The GPS data may comprise OPS phase carrier measurements. In some embodiments, the OPS data may also further comprise satellite code data.

The processor may be configured to compare the IMU data with the SF3 data to derive the true north reference, in some embodiments, the processor may be configured to integrate the iMU data and compare the integrated IMU data with the SF3 data to derive the true north reference.

The MU of the devices or methods described herein may comprise a six degree of freedom MbMS. in sonic embodnhnts, the MU may ompnse three MEMS accelerometers and the data received from the MU may comprise inertial accelerometer data.

The IMU of the devices and methods described herein may further comprise three MEMS gyroscopes and the data received from the MU may comprise angular rate data.

In some embodiments, the IMU, OPS receiver and processor of the device described herein may he encapsulated within a first materiaL In some embodiments, the first material may be shock resistant. The first material may comprise synthetic re&n.

En some embodiments described herein, the step of moving said device may produce said IMU data and said GPS data. In some embodiments, the step of moving said device may comprise creating oscillatory translation of said device. In further embodiments, the step of moving said device may comprise inverting said device and creating oscmatory transiation of said device.

n some embodiments described herein, the processor may compare the IMU data with the GPS data to derive the true north reference. ln further embodiments, the processor may integrate the EMU data and compare the integrated MU data with the GPS data to derive the true north reference.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of an improved azimuth determination device and a new method for detecting true magnetic north are herein described with reference to the accompanying drawings, in which: Figure 1 shows a side view of the internal features of an azimuth determination device as described herein.

Figure 2 shows a part of the dece of figure 1 after 1 has been encapsulated.

DETAILED DESCRIPTION

Figure 1 shows a side view of some of the internal features (Le. the device before S encapsulaUon, as described later) of a first embodiment of a new azimuth determination device, ID, as described herein. The device may be sized and shaped so that it is configured to be handh&d. The device may also be used in other portable situaUons other than being handhold. For example, it may be positioned within a moving boat or vehicle, In the embodiment shown in figure 1, the azimuth determination device. 10, comprises a six degree of freedom Microelectromechanical System (MEMS) inertial measurement unit (IMU), 11, comprising three MEMS acceierorneters, 20, and three MEMS gyroscopes, 30, as are known in the art. The accelerometers are placed such that their measuring axes are orthogonal to each other. They measure inertial acceleration, also known as G4orces. The gyroscopes are placed in a similar orthogonal pattern, measuring rotational position in relerence to an arbitrarily chosen coordinate system.

The device, 10, further comprises a GPS system, such as a high dynamic Global Positioning System (GFS) system, which comprises a GPS receiver, 12, as well a corresponding integral OPS antenna, 13. An example of such a system is the OinetiQ 020 High Dynamics GPS Receiver Module, which is a high performance and ultracompact GPS unit, The GPS receiver may incorporate a high stability time reference to enable the required measurement precision.

The device, 10, further comprises a processor, 14. The processor has means for receiving data from the IMU, 11, which may include inertial data from the accelerometer, 20, and/or angular rate data from the gyroscope, 30. A 32 bit floating point DSP processor as is known in the art may be used for this, In the embodiment shown in figure 1, the device has a first end, 15, and an opposing second end, 16, and the processor, 14, is provided at the first end, 15, whereas the OPS antenna, 13, is provided at the opposite end, 16. TheIMU, 11, and the GPS receiver, 12, are provided therebetween. Piternative positioning of these features may be used, however, and this is just one example of how the features may be arranged.

In some embodiments, at least a part ci the device, e.g. the IMU, GPS receiver and processor may be encapsulated in a shea to thereby provide a device that is shock tolerant or resistant, so as to prevent damage if dropped or subjected to impact, and to ensure the directional stability of the sensor measurement axes. It may also be constructed to be shock resistant to such an extent that it is capable of withstanding an artillery launch shock. For example, figure 2 shows a part of the device of figure 1 after it has been encapsulated in a synthetic resin material. In this figure, the GPS antenna, 13, is not shown, however, this would still be present in the device, as in Figure 1. The accelerometers are also not visible in this figure.

but would stiu be present. Many other different shock resistant materials may alternatively he used to encapsulate the internal features of the device including epoxy resin and polyurethane.

Due to the fact that a MEMS inertial measurement unit is used, the device may be made small enough to have a volume of approximately 35 cubic centimetres and a mass of 75 grams. Devices could of course be made having pther volumes and masses and this is just one example of how small and lightweight the device may be. The device can therefore be easily be held in a persons hand and so is very

portable.

In use, a simple inversion foUowed by an oscillatory translation of approximately 0,5m distance and 1Hz frequency, which may be readily achieved by hand due to the low volume and mass of the device, provides a sufficient motion input to the IMU, 11. This input may only need be appUed temporarily, typically for a period of up to about 10 seconds, in order for the device to provide accurate pointing to be maintained by the unaided 1MU for a period of several minutes. Other types of movement may also be used in order to provide an input to the device. For example, in one embodiment only osculatory translation may be used, with no inversion.

The processor of the device described herein further comprises means for receiving data from the OPS system. The GPS data may comprise carrier phase measurements but may also additionally comprise GPS satellite code measurements. The processor of the device described herein may then compare the MU data with the GPS data to accurately determine the orientation of the IMU, 11, with respect to true north.

Spedficay, th is achieved by using two independent measures of the oscifiatory motion introduced by the user: the IMU sensed accelerations are integrated to derive velocity in the MU frame of reference, whHst concurrent UPS carrier phase data are used to measure position changes, with successive measurements being differenced to derive the assodated vebcify with respect to true North; the distance between the MU and the UPS antenna being less than 10% of the oscillatory translation distance and the residual everarm effects of any superimposed rotation being corrected. The difference between the two measures of the same osdilatory motion in the different frames of reference indicates the relationship between these frames of reference and hence provides an estimate of the error in the system azimuth angle. The system azimuth angle is otherwise maintained by continuous integration of the IMU gyro data, with progressive corrections applied from the error estimates to refine its accuracy.

In some embodiments described herein, the UPS antenna and MU of the device may be located relative to each other in such a way that, in use, the UPS antenna and 1MU experience the same motion when the device is moved. In sonic embodiments, this may be achieved by rigidly connected these to each other and/or the device so that they experience the same motion when the device is moved.

The close physical relationship between the inertial sensors and the UPS antenna, combined with the use of a stable frequency reference for the UPS receiver, ailows precise carrier phase UPS measurements to be used; this then permits the correlation process to converge supported only by the sma scale motion described above.

This differs from known techniques of determining azimuth in that known devices normally operate by processing the UPS measurements of range, with an accuracy of a few metres, and range rate, with an accuracy of about a tenth of a metre per second. Although these measurements work well to detect most of the system error states, the mechanism for azimuth error detection requires a horizontal acceleration generating a displacement which is significant in relation to the UPS errors, ic, several metres per second of velocity or several tens of meters of position. The device described herein is not reliant upon such motion due to the short distance between the MU and the UPS antenna, and the stability of the frequency reference, allowing OPS Carrier Phase measurements to supplement GPS range and range rate data.

As described above, the processor of the device described herein may combine and compare IMU data with GPS carrier phase data to accurately determine azimuth. Carrier Phase measurements provide a very high relative position resolution, in the order of a few millimetres. In known systems, multiple antennas are normally used to simultaneously measure the phase difference from at east two points, and therefore determine the angle of those points in relation to the reference datum, Le. due north. In an embodiment of the devices and techniques described herein, however, the data (eg. carrier phase data) from the GPS system may be combined and compared with the IMU data and applied as Kalman fllter measurements to the inertial navigabon system. In such an embodiment, there is therefore no longer a reliance upon simultaneous measurements from two points as the IMU of the device deschbed herein provides accurate short term position tracking of the single antenna as it is displaced.

In addition to the above, the device described herein exploits the close physical relationship between the IMU, 11, and the GPS antenna, 13 to provide the advantageous effects. For example, the antenna and the IMU of the device described herein are provided within/on the same small, handheld device and are located only a few cm apart from each other. In one embodiment, the GPS antenna phase centre may only be located 2 cm from the inertial centre of measurement of the IMU, 11. In other embodiments the distance between the IMU and the GPS antenna may be greater, but preferably less than 5 cm. This has significant advantages in that the antenna and IMU therefore experience the same motion, which results in any potential errors due to differential motion being very smalL In some embodiments, in use, the device may be attached to a larger host system, and may be temporariy removed to allow motion to be used to condition the Kaiman filter the required niotion being a single or repeated inversion to observe accelerometer errors, then a single or repeated dispacement of approximately 05 metres to observe the azimuth error.

In view of the above, the device and method has significant advantages over known devices and methods, such as those that rely on magnetic sensors to determine azimuth, This is because the accuracy of such magnetic sensors is limited by magnetic dechnation as they are susceptible to interference from nearby magnetic S influences as we as variation between magnetic and true north.

The new device and method described herein also has further advantages in that it is sma enough to be handheld and operates due to simple hand movements. The processing of the combination of GPS data, and in particular carrier phase data with inertial data also further provides an accurate true north reference, using only a single GPS antenna. This is unlike known systems which use multiple GPS antennas or require more significant vehicleborne motion, as described above.

The device is also able to provide wireless communication for unpowered data transfer using an ISO 15693 vicinity protocol; this allows any necessary mission data to be transferred to the device before any external power is applied.