Gravity on the move

GT-1 Technical Description

The GT-1 gravimeter is a vertical sensor, GPS-INS, scalar gravimeter with a Schuler-tuned three-axis inertial platform.  Both the airborne (GT-1A) and marine version (GT-1M) consists of three basic units as illustrated below.

The main gravimeter unit, shown in Fig. 1 with its access panels removed from all four sides, and the rotation-table together measure 40 x 40 cm by 70 cms high. They are mounted on a shockmount having a cut-off frequency of 7 Hz. The shockmount is 60 cm in diameter and 22 cms high, giving an overall height to the gravimeter of 92 cms.

 

 

 

 

 

 

 

 

 

 

 

 

             Fig. 1 The GT-1 gravimeter mounted on its shock-mount.

The main unit houses most of the electronics in the top third, while the gravity sensor is held vertically by the inertial platform in the bottom; the rotation-table provides the platform’s Z or azimuth axis.

Connections to the gravimeter are: a 27 Vdc power source drawing approximately five amps; a GPS RS-232 serial link providing data used to assist in aligning the platform vertical; and a second serial connection to any laptop or field data acquisition computer. This computer acts as the control and display unit (CDU) for the gravimeter, and also collects and stores survey data.

The vertical accelerometer, or gravity sensing element (GSE) has an axial design with a reference mass on a spring suspension, a photoelectric position pickup and a moving-coil force feedback transducer. The GSE suspension design minimizes the effect of cross-coupling, an undesirable effect which contaminates gravity measurements with components of horizontal accelerations induced by aircraft motion. This feature allows the GT-1A to collect data in the presence of large horizontal accelerations, such as during aircraft turns or during periods of high turbulence.

The GSE, with a bandwidth of 100 Hz, is installed on the gyro-stabilized platform contained in a double-axis gimbal suspension (black cylinder above). The platform also holds two horizontal accelerometers, a dynamically tuned gyro with a vertical angular momentum, a fibre optic gyro for azimuth control, and two gravimeter calibration devices (GCD). The GCD’s are designed to calibrate the gravimeter by means of inclination without dismounting the GSE.

The gravimeter also contains a microprocessor, input/output interfaces, and secondary power supplies, all within the electronics bay. The GSE is placed in a double-loop constant-temperature environment on the inertial platform. The other elements installed on the platform, plus the current regulator of the code-to-current converter within the gyro control circuit, are individually temperature controlled.

The gravimeter is fully automated – no operator is required on board the aircraft while collecting data on survey lines. All systems including stabilization servo systems, temperature control systems and gyroscopic correction systems are controlled by the built-in microprocessor. The computer also takes control of actuation, reference measurements, balancing, and measurements during survey mode. A vertical gyro correction system using GPS-derived information on latitude and aircraft speed provides vertical gyro stability. An optimal Kalman filter is implemented in control algorithms for both the stabilization servo system and the vertical gyro correction system.

Two dynamic ranges are measured and recorded simultaneously: +/- 250 Gals (0.25 g) and +/-500 Gals (0.50 g). The fine channel, which also has the higher resolution, is used during periods of calm to moderate flying conditions, while the coarse-range channel allows measurements to continue in more turbulent conditions.

Data is acquired through short periods of accelerometer saturation in high turbulence by the automatic application of a reduced-order Kalman filter, enabling platform misalignment to be computed and hence controlled.

The internal microprocessor carries out a large number of measurement and control functions.

Measurements of platform orientation, rotation rates and accelerometer outputs are taken at 300 Hz, the same rate at which critical platform control and other internal functions are carried out. The high sampling rate eliminates aliasing of the high frequency vibrations encountered in most vehicles. This data is passed through an anti-aliasing filter and is subsequently processed at a rate of 18.75 Hz. The main control loop also runs at this frequency. As mentioned above, velocity and latitude data from the GPS receiver provide GPS-aided platform levelling.

The gravimeter is operated by programs running on the CDU. The easy-to-use menu-based programs include calibration and diagnostic functions in addition to the normal data logging capability.

The calibration program has two modes for carrying out automatic GSE calibration.  A 3-hour measurement period provides GSE calibration coefficients, while over a period of 5.5 hours, the program determines the above coefficients plus the non-perpendicularity angles between the GSE and the platform surface. This program is normally run once before each survey project and does not need to be run at the completion of the project.

The diagnostic program maintains a running check on the serviceability of all gravimeter sub-systems. Warnings regarding data quality are presented to the pilot (in the case of the GT-1A) through a remote light: solid green means all systems are okay; flashing red means turbulence is approaching upper limits for the acquisition of high quality data; and solid red means high quality data cannot be recovered from the data being collected under present flight conditions.

The GT-1A’s GSE design provides for a highly linear drift which is easily compensated by recording a reference measurement at the same location immediately prior to and following each flight. These reference measurements are then used to compensate data collected during the flight for drift with a linear interpolation. Four internal temperature monitors are recorded and are also used during post-processing to correct the accelerometer's inherent drift to within 0.1 mGals/day.

The GT-1A provides a wide range of spatial resolutions through the application of filters with varying cut-off frequencies.

For the GT-1A, filters are chosen on the basis of aircraft ground speed, flight conditions and target dimensions. Filters might vary from 80 to 120 seconds or longer for more turbulent flights, which provides half-wavelength resolutions of 2.0 and 3.0 kms respectively with an aircraft ground speed of 50 m/sec. A helicopter flying at half this speed would provide resolutions of 1.0 km and 1.5 kms respectively with 80 and 120-second filters.

Click here to download this Technical Description with diagrams in pdf format (312 KB).

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