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GWR Instruments, Inc. is the exclusive manufacturer of the Superconducting Gravimeter (SG). The SG uses persistent supercurrents, which are trapped in superconducting magnets, to produce an ultra stable magnetic field which levitates a superconducting test mass (sphere). The SG consists of two basic components: 1) the Gravimeter Sensing Unit (GSU) which includes: the superconducting magnets, the sphere, circuitry for energizing the coils, temperature control circuitry and magnetic shielding; and 2) the liquid helium tank (Dewar) and refrigeration system which keeps the GSU close to 4.2^OK to maintain the superconducting state. In 1993, GWR introduced the CT Compact Tidal Gravimeter which dramatically decreased the overall size of the SG and simplified field installation. The standard support equipment for the Compact Tidal Gravimeter includes a pair of cryogenic tilt meters, an automatic tilt compensating system, a gravimeter electronics package, a current supply/heater pulser, a helium level sensor and a helium transfer kit. The Compact SG is routinely equipped with the CD-125 Standard holdtime Dewar refrigeration system. This allows the gravimeter to operate for approximately 500 days between liquid helium refills.

Some users of the SG have found that it is necessary to remove small infrequent offsets from the gravity data to preserve the baseline (zero) on a continuous record. However, geophysical signal and noise sources often mask the offsets and make them difficult to measure and remove precisely. In response to this need, GWR now manufactures a new Dual Sphere SG GSU that incorporates two levitated test masses into a single sensor (See section I.A.2). Since it is unlikely that offsets occur on both spheres simultaneously, the differential signal gives a precise measurement of any offsets that may occur in the record. This makes offset identification and removal a straightforward procedure during data processing.

In response to customers' requests, GWR has also developed a second high performance refrigeration system. This Ultra Long Holdtime (ULHD) Dewar (See section II.B) allows indefinite operation after a single filling of liquid helium.

Figure 1. Superconducting Gravity Sensor Unit (GSU)
Figure 2. The GWR CD-125 liter Compact Refrigerated Dewar
Figure 3. The GWR CD-125 liter Compact Refrigerated Dewar Footprint
Figure 4. GEP-3 and GEP-3DAnalog Controller chassis dimensions
Figure 5. GEP-3 Power Supply chassis dimensions
Figure 6. AMI135 Liquid Helium Level Monitor dimensions
Figure 7. HC-2 Helium Compressor dimensions
Figure 8. Gas Line end fitting dimensions
Figure 9. 100AC Water Chiller dimensions
Figure 10. Druck RPT 301 dimensions.

As shown in Figure 1, the GSU contains a 2.54 cm diameter spherical proof mass. The sphere is levitated by the forces produced by magnetic fields generated from a pair of superconducting coils. Since the sphere is superconducting, it behaves as a perfect diamagnet so that surface currents are generated which exactly cancel and exclude any applied magnetic field from its interior. It is the interaction between the sphere's surface currents and the applied magnetic field that produce the levitation force. Both the position of the sphere and the vertical force gradient (spring constant) are optimized by adjusting the ratio of the currents in the two coils. Superconducting/normal heat switches are used to "trap" the supercurrents in the magnetic coils. This allows the external power supply to be disconnected from the magnet coils after the trapped currents have been adjusted to their final values. The use of trapped persistent supercurrents to produce an ultra stable levitation force accounts for the unprecedented long term stability of the superconducting gravimeter in comparison to mechanical spring type gravimeters.

The position of the sphere is sensed by a capacitance bridge network consisting of three spherical capacitor plates positioned around the sphere. The upper and lower plates are driven by precisely matched AC signals that are 180 degrees out of phase. The sphere capacitively couples these excitation signals to the center plate of the bridge. When the sphere is equidistant from the upper and lower plates, the drive signals cancel and the resulting signal on the center plate is zero. When changes in gravity cause the sphere to move from its null position, it produces an error signal that is linear in displacement. During operation, the position of the sphere is held close to its null position by a feedback circuit which applies a magnetic force through a separate feedback coil. Since the force from feedback coil is linear with current, measuring the current through the feedback coil provides a linear measurement of the force of gravity.

The gravity sensor is surrounded by a superconducting magnetic shield to eliminate effects from changes in the external magnetic field. It is also enclosed in a vacuum can and temperature regulated to a few m ^OK. This makes the sensor completely insensitive to environmental effects such as changes in external temperature, humidity and barometric pressure.

The vacuum can is sealed inside the liquid helium Dewar during assembly at the factory. Electrical leads are brought out through the top end of the Dewar through a fiberglass neck tube and terminated at the head of the instrument. Great care is taken in design and manufacturing of the neck and signal leads to insure that heat leaks into the liquid helium reservoir are minimized. High quality environmental connectors allow interfacing the gravity sensor and its subsystems to an electronics package that resides external to the Dewar.

The CT-D Dual Sphere GSU operates on the same principal as the single sphere sensor. However, this sensor has two superconducting spheres and capacitance detection bridges separated by a spacing of 20 cm. The spheres are levitated by two separate sets of superconducting coils wound on the same copper magnet form. By adjusting the currents in each set of coils, each sphere is independently levitated and centered in its own magnetic field. After levitation and adjustment of the force gradient, additional superconducting side coils are used to apply small horizontal forces on one of the spheres. This allows the tilt minimum positions of the two sensors to be precisely aligned.

The Dual Sphere SG operates in the same Dewar as the single sensor SG. Therefore, its Dewar can be equipped with either of the two refrigeration options described below in Section III.

The CD-125 Compact Superinsulated Dewar is the result of more than 19 years of development of high efficiency Dewars at GWR Instruments. It is optimized to operate in conjunction with a cryogenic refrigeration system which allows the system to operate approximately 500 days between fillings of liquid helium. Un-refrigerated, the Dewar maintains a holdtime of about 60 days after each filling of 125 liters of liquid helium.

The CD-125 measures 1 meter in height, 0.7 meters in diameter, and weighs approximately 100 kg with the GSU installed (see Figures 2 and 3). This small size makes it ideal for applications where the gravimeter will be operated in tunnels or vaults at existing geophysical observatories. The Dewar is supported at three points which mate with a reinforced flange welded around the outside of the Dewar. The heights of two of the support points are adjustable using precision micrometers and can be precisely controlled by using thermally controlled levelers which are an integral part of the tilt compensation system. The height of the support points coincide with the height of the sphere. This minimizes the effects of ground vibrations on the gravimeter.

The Dewar is constructed mostly of aluminum with two radiation shields situated between the outer shell and the inner liquid helium reservoir. Many layers of aluminized mylar (superinsulation) are wrapped inside the Dewar to minimize radiative heat from entering the system. During manufacturing, the GSU is first operated in a test Dewar. After extensive testing the GSU is installed into the CD-125 through a large port in the bottom of the Dewar. The port is then sealed and the Dewar's vacuum space is evacuated. This "sealed in" design allows the use of a 5 cm diameter neck instead of a 13 cm diameter neck tube which would be required to insert the GSU from the top of the Dewar. This is of major importance in reducing heat conducted into the Dewar and in extending the Dewar's holdtime. In order to strengthen the mechanical system, radial stiffening spokes are incorporated into the design. These allow the use of an extremely thin neck tube assembly while maintaining a rigid system that is not excited by ground vibrations.

The CD-125 is filled with liquid helium at the factory, then air-shipped cold to the installation site. During installation it is refilled with helium and the refrigeration system is installed. With the refrigeration system option, most users refill with liquid helium every 300 to 500 days. Their refilling schedule depends on the reliability of electrical power and their preferred safety margin. Other than refilling, no regularly scheduled maintenance is needed for the Dewar.

Two tilt meters are mounted orthogonally inside the GSU. These sensors are mounted as close as possible to the gravity sensor. Tilt meters mounted externally will introduce tilt artifacts due to tilt differences between the point at which the tiltmeter is mounted and the position of the gravity sensor. These effects include changing temperature profiles in the neck of the Dewar and changing stress in the Dewar system as the liquid helium boils away.

The tiltmeter sensor uses a rectangular pendulum paddle which hangs from a thin metal foil. Capacitive sensing techniques similar to that used in the gravity meter are used to sense the paddle position and its motion in response to tilts. In this case the excitation voltage is applied to two side plates, located on either side of the hanging paddle. When the paddle is perfectly centered, the resulting signal sensed on the hanging paddle is zero. When tilts cause the paddle to move from its null position, an error signal is produced. The tiltmeters have a dynamic range of greater than 60 mradians and a sensitivity of 0.1 m radians.

The tilt compensation system consists of two thermally activated leveling units and their control electronics. As shown in Figure 2, the leveling units bolt directly to the CD-125 Dewar, and are part of the structure that support the Dewar. The tilt of the Dewar is controlled by varying the lengths of the two leveling units relative to the third fixed point upon which the Dewar sits. Inside each leveling unit case, two sliding cars travel on precision crossed roller bearings. The bearings allow extremely smooth movement without transmitting noise or vibrations to the gravimeter. The lower car rests on a hardened steel ball which is captivated by a carbide insert. The inserts are bonded to aluminum feet which are bolted to granite blocks. At locations where earthquakes are likely to occur, the blocks can be secured with anchor bolts; alternatively, they can be cemented in place. The upper car is positioned by the micrometer mounted at the top of the leveling unit case. A stainless steel bellows filled with oil separates the two cars. The weight of the Dewar presses the two cars and bellows between the support point and the end of the micrometer. The lengths of the leveler can be adjusted in two ways: 1) rotating the micrometer moves both the upper and lower cars vertically along the bearing; and 2) since the volume of the oil is temperature dependent, changing the temperature of the oil moves the lower car position with respect to the upper car.

During installation of the instrument, the gravity meter is aligned precisely with the gravity vector by manually adjusting the height of the levelers with the micrometers. This "tilt minimization" procedure is performed by observing the gravity signal while tilting the instrument in pre-determined steps. This is done while the active portion of the actuators are controlled at a constant temperature and length. After installation is complete, the levelers are placed in automatic mode where their length is controlled by a signal derived from the output of the tiltmeters. In this configuration the gravimeter is held to within 0.1 m radians of its tilt minimum position. The thermal levelers have a range of about 1 mm, providing ample compensation for most sites. Control electronics for powering the leveling units reside in the GEP-3 gravimeter electronics package.

The tilt compensation system is necessary to counteract changes in local tilts common even at seemingly stable locations. Tilts can be introduced from settling of the underlying substrate, varying substrate density, heating and cooling of the support platform, and changes in humidity or water table.

There are several advantages to maintaining the system at its tilt insensitive position, rather than measuring tilts and calculating their effects. Since the response of the gravity meter is proportional to the square of the tilt error, small tilt changes that cannot be compensated for have minimal effect when the gravimeter is held close to the tilt insensitive position. The closed loop operation of the system also improves the precision of tilt sensing. Since the tilt meters are operated at their null position, electronic changes inside the feedback loop which effect the measurement are minimized. Actively controlling the tilt also eliminates problems associated with calibration of tilt effects on the gravity signal, and removing these effects in post processing.

The Gravimeter Electronics Package (GEP-3) includes electronics that sense and control the gravimeter, temperature, and tilt subsystems. The package includes an Analog Control chassis that houses the analog circuit cards, a separate Power Supply chassis that houses ultra low noise power supplies, pre-amplifiers that mount to the head of the gravimeter, and all necessary cabling. A data recording system is not included in this package but may be purchased separately. An Integrated Electronics and Data Acquisition Package (IEDP) is also now available from GWR that includes the Gravimeter Electronics (GEP-3), Data Acquisition System (DDAS-3), Temperature Controlled Electronics Enclosure (TREE), an Un-interruptible Power Supply (UPS), and all necessary hardware for a turnkey solution to operation of the gravimeter. This package is described separately.

The Gravity circuit card is mounted in the Analog Control chassis. This card generates precisely matched drive signals that are applied to the upper and lower capacitor plates surrounding the sphere. The resulting signal from the center capacitor plate indicates the sphere’s displacement from its null position.

The signal from the center plate is connected to a preamplifier via a cryogenic triax cable. This low-noise high input impedance preamplifier is mounted directly on the head of the gravimeter external to the Analog Control chassis. By minimizing the path of the low level signal, noise pickup and losses are greatly reduced. To further reduce the capacitance of this very low level signal to ground, the central shield on the cryogenic triax cable is driven at the signal potential.

The signal from the output of the preamplifier is carried to the gravity circuit card through a coaxial cable. There, it is detected using a phase sensitive lock-in amplifier. This signal is then applied to an integrator which generates a feedback current through the feedback coil in the GSU. Using this technique, the position of the sphere can be detected to within a few angstroms. The closed loop design linearizes the response of the instrument and minimizes effects of gain changes inside the feedback loop that may occur due to temperature effects in the electronics or component aging.

On the Gravity card, an eight pole Bezel filter with a corner frequency of 8 seconds is provided as an anti-aliasing filter for digitizing the gravity signal. This filter, named the GGP1 filter, is intended for sampling at 1 second intervals. It was designed to meet the specifications of the Global Geodynamics project (GGP)¹. An additional 2 pole Bezel filter with a corner frequency of 5 seconds is provided for sampling the gravity signal at faster rates. The characteristics of the GGP1 filter are listed below:

The gravity card also includes circuitry so that the system can be modulated by a voltage source. The "Feedback Modulator" circuit allows the frequency response and phase characteristics of the instrument to be precisely characterized in the field.

The tilt sensing electronics operate on the same principle as the gravity sensing electronics. However, the tilt circuit also includes circuitry for adjusting the ratio of the excitation signals applied to the side plates. This allows the user to adjust the null position of the tilt signal to coincide with the optimum tilt position defined by the gravity sensor.

The tilt circuit cards also contain low-pass anti-aliasing filters for monitoring the tilt signals.

The temperature sensing circuit uses a germanium thermometer in a wheatstone bridge to sense the temperature of the superconducting elements within the GSU. The signal is detected using a phase sensitive lock-in amplifier, then fed back to control the temperature of the GSU slightly above the ambient temperature of the liquid helium bath. Using this technique, the core of the sensor is controlled to within a few m ^OK.

An auxiliary circuit card provides excitation and conditioning for several sensors including various cryogenic temperature sensors. A step function generator is included to measure the step response of the gravimeter or other sub-systems. This circuit accepts a TTL pulse and outputs a step function with variable gain and offset.

The Dual Sphere GEP-3D electronics package is very similar to the single sphere GEP-3 electronics. The tilt and temperature control electronics are identical. However, the GEP-3D package is outfitted with a second set of Gravimeter Control Electronics to operate the second sensor. This consists of a plug in module, an additional pre-amplifier and necessary interconnects to accommodate the second sensor.

The Current Supply with Heater Pulser (DPS-3) is used to energize the levitation coils in the GSU. This unit combines a very stable dual current supply with circuitry for controlling the "persistence switches" used for precisely varying the supercurrents in the levitation magnets. By varying the main currents and the width of the pulses applied to the "persistence switches", the user can levitate the sphere and place it precisely at the desired position. The current supplies are capable of delivering 0 to 6A continuously and are stable to one part in 10⁵.

The DPS-3D option provides a control box for easily switching the DPS-3 power supply between the upper and lower gravity sensors coils during the levitation process. A secondary power supply is included to provide currents to the side tilt coils in the upper sensor. These are activated and adjusted to precisely align the tilt insensitive positions of the two sensors.

A separate electronic component is required for monitoring the amount of liquid helium in the Dewar. A four terminal measurement is used to determine the resistance of a superconducting element mounted inside the Dewar. When power is applied, the portion of the level sensor above the liquid becomes resistive while the section emersed in liquid remains superconducting. The unit then converts this resistance value to % He remaining in the Dewar. Input power can be selected to accept 50/60 Hz, 100/120/220/240 VAC. Outputs include a digital panel meter and a chart voltage output capable of driving a chart recorder. A RS232 interface is added when the DDAS-3 option is specified.

The Helium transfer kit contains all supplies necessary to easily transfer liquid helium from a supply Dewar into the CD-125L Dewar. It includes: 1) a flexible transfer tube constructed with a superinsulated vacuum sheath; 2) a low pressure gauge scaled with recommended pressures used during different stages of the transfer process; 3) a helium exhaust tube used to direct cold gas away from critical components of the gravimeter sensor; 4) a helium dip stick (flutter stick) used for measuring the contents of storage Dewars; 5) various hoses and fittings needed during the transfer process. Special flex joints and bayonet fittings can be added according to the operators requirements (e.g. limited space or ceiling height). Contact the factory for details.

The CDR-125 refrigeration system consists of an APD Cryogenics HC-2 helium compressor with DE-202 expander (coldhead), flexible interconnect hoses, heat exchange interface flanges, vibration isolation bellows with sealing flange, and a vibration isolation coldhead support frame. The system extends the holdtime of the helium Dewar by cooling two thermal shields inside the Dewar. The cooled shields intercept the conductive and radiative heat load from the outside of the Dewar. By removing this energy before it reaches the liquid helium, boil off of the liquid is reduced.

The refrigeration unit is a closed cycle system consisting of a compressor attached to the coldhead via flexible hoses. The coldhead is mounted inside the neck of the CD-125 Dewar. The coldhead uses a modified Gifford McMahon cycle for cooling and is supplied high pressure helium gas by the compressor. The gas entering the coldhead is first pre-cooled by exhaust gas and then cools further when it is allowed to expand. The DE-202 coldhead has two cooling stages. After equilibration, the first stage cools the outer shield to about 70^OK and the second stage cools the inner shield to about 10^OK. Heat exchange between the coldhead and Dewar shields is made through a helium gas filled heat exchanger. No mechanical contact is made at the point of heat exchange. This isolates the gravimeter from the vibrations of the coldhead and compressor which otherwise would add noise to the gravimeter signal. The only connection between the coldhead and the gravimeter is through a soft rubber bellows that prevents air from entering the Dewar.

Use of the refrigeration option is desirable from both economic and scientific perspectives. Operating costs are reduced by more than 10 times by increasing the efficiency of the Dewar with the refrigerator system. From a scientific standpoint, the refrigeration system allows collection of uninterrupted data over much longer periods. Although it is not necessary to cease data collection when the Dewar is refilled with liquid helium, the process does disturb the gravity signal during the transfer process. In addition, if the user mechanically shocks the gravimeter during the transfer, an offset may occur in the data which must be removed during data processing. Reducing the frequency of these events is therefore desirable.

The coldhead is supported inside the gravimeter neck on a vibration isolation frame which stands on rubber feet. A sliding mount allows the coldhead to be easily removed and inserted while the gravimeter is operating. This is necessary when refilling the Dewar with liquid helium and during coldhead service which must be performed periodically at one to two year intervals. The CSS-1 is designed for the 11 Kelvin cryogenic coldhead and the CSS-2 is designed for the 4 Kelvin cryogenic coldhead.

Flexible stainless steel hoses six meters in length allow the compressor to be mounted close to the gravimeter. When it is desirable to move the compressor further away from the gravimeter, extension hoses can be supplied (see option CEH-XXXM).

Coldhead Service: Seal replacement at 10,000 - 20,000 hour interval or when indicated by diminishing performance.

Several options exist for removing heat from the helium compressor’s cooling water. If the ambient air temperature is within permissible limits, either of two Dry Cooler heat exchangers can be used. If the ambient air temperature is not within permissible limits, a Water Chiller that uses compressed freon should be used. See the document entitled "Pre-Installation Preparations" for help in determining the appropriate type of cooler.

Either the Cool-Pak 4 or the AW75 can be supplied under option WDC-2KW. The Cool-Pak 4 is intended for indoor use while the AW75 can be used outdoors. These coolers are appropriate in situations where the ambient air temperature does not exceed 21^OC.

The APD Cool-Pak 4 provides a simple means of removing the 1.7 kW of heat generated by the HC-2 refrigeration compressor and discharging it to another location. This cooler takes warm water from the refrigeration compressor, passes it over a finned heat exchanger, then pumps the cooled water back to the compressor inlet. Four fans provide air flow through the heat exchanger to remove heat from the system. Adequate provisions must be made to allow the heat emitted from the Cool-Pak 4 to dissipate. The Cool-Pak requires indoor installation and must be protected from rain and dust. This closed cycle cooling system provides a reliable, clean source of cooling water independent from changes in the local water supply.

The AW75 functions in a similar manner as the Cool-Pak 4. This unit is larger, consumes slightly more power and is rated for outdoor use. A single vertically mounted high capacity fan provides air flow through the heat exchanger.

NOTE: If conditions are not suitable for installation of the Cool-Pak 4 or AW75, option WCH-2KW may be suitable. For its description, see option C below.

The ULH Dewar refrigeration system consists of a newly designed Dewar interfaced with a cryocooler capable of obtaining temperatures below the vaporization point of liquid helium. The system is based on the KelKool 4.2 GM cryocooler manufactured by Leybold Vacuum Products Inc. This Gifford-McMahon type cryocooler uses a mechanically driven piston and offers superior cooling performance, as well as greatly reduced noise and vibration compared to previous gas driven cryocoolers.

The lower stage of the cryocooler can cool below the vaporization temperature of liquid helium. This allows boiled off helium gas to condense at this stage. Therefore, during normal operation the system consumes no liquid helium and will operate indefinitely.

GWR supplies a water refrigeration system for cooling the helium compressor. The water cooler can be located remotely so that heat generated by the compressor can be exported to a remote location. This water refrigerator is a compressed freon system that cools the water to acceptable limits as required by the high performance system.

If the WDC-2KW dry cooler cannot provide sufficient cooling for the temperature extremes expected at the intended site of operation, a Water Chiller should be used. Water Chillers operate in a similar fashion to a home refrigerator or air conditioner using compressed freon to allow the water to be cooled below the ambient temperature.

A model GWR-100AC Water Chiller is provided when option WCH-2KW is selected. This model provides 1 ton (12,000 BTU/Hr) of cooling capacity and is rated for outdoor use. The chiller can be configured for single or three phase power at 50/60 Hz 200-460 VAC. The Type of available power must be specified so that the unit can be properly configured at time of order from the manufacturer.

At many installation sites the superconducting gravimeter is installed as part of an array of geophysical instrumentation. Such instruments may be compromised by heat and vibrations from the refrigeration compressor. In cases where heat is a concern, use of the water cooler to transport heat out of the observation area is a simple solution. If vibrations are also a problem, the compressor can be moved as far away from the observation site as is practical. For this purpose custom extension hoses can be provided in lengths up to 100 meters or more. These hoses are ultra clean, vacuum baked, and shipped pre-charged with ultra pure helium gas. Addition of these hoses will not degrade refrigeration performance. To order this option specify the length of the hoses in meter in place of the XXX in the part number. For example to order 100 meter extension hoses order part number CEH-100M

The Integrated Electronics and Data Acquisition Package (IEDP) is a complete system that allows operators to record the highest quality data from a Superconducting Gravimeter (SG). It fully integrates a data acquisition system (DDAS-3), temperature regulated electronics enclosure (TREE) and uninterruptible power supply (UPS-3) with the gravimeter electronics package (GEP-3). The core of this system is the DDAS-3 which is carefully designed to meet both the precision and timing specifications recommended by the GGP. The system includes the following features:

The IEDP is designed to address and eliminate many sources of offsets, drift, and gaps in the gravity data record. These include: ground loops, DVM aging, sensitivity to temperature and humidity variations, and variations in ac power. The IEDP provides remote access to all gravimeter subsystems which allows the operator to quickly and easily verify optimal performance on a daily basis. When occasional problems do arise, the IEDP allows the operator to rapidly determine the cause of failure without traveling to the installation site. A GWR engineer in San Diego can also retrieve data from the system in a standard format that enables rapid analysis and consultation with the operator. In this way, problems can be rapidly diagnosed and repaired. This will improve long term data quality and will reduce the manpower required to operate an SG. The IEDP provides the operator with a complete, tested, and integrated system designed specifically for use with the GEP-3 electronics for maximizing long term gravity data quality.

The DDAS-3 interfaces with the GEP-3 Gravimeter Electronics Package and various environmental sensors. The system's primary function is to record uninterrupted data from gravity and barometric pressure sensors. Additional signals are logged to verify system health and for maintenance purposes. Data sampling time is referenced to UTC through a GPS receiver which communicates via a precision time pulse and a serial interface. The system is designed to comply with all GGP specifications. Data is automatically backed up to a removable mass storage media which allows rapid transfer of large data files.

The Data Acquisition Controller contains a processor and ten (10) serial I/O ports. These ports communicate with two voltmeters (three for dual sphere system), a GPS receiver, barometer, liquid helium level monitor, host PC, voltage reference, and UPS. The controller program resides on a PROM which improves reliability compared with a hard disk based system. The large buffering capacity of the controller allows maintenance to be done on the host PC without interrupting data collection.

Precise sample timing is provided through the generation of hardware trigger pulses presented directly into each DVM trigger input. During normal operation a precision timing pulse from the GPS receiver is used to initiate a controller routine that results in independent hardware triggers asserted for each DVM. During the unlikely loss of GPS synch, triggers are generated by the controller’s internal clock. Since the DVM trigger initiates an acquisition cycle on the gravity DVM which integrates over several hundred milliseconds, the center of the acquisition cycle is slightly offset from the trigger pulse. The DDAS-3 controller compensates for this offset so that the acquisition time is truly synchronous with UTC. Strict compliance to this GGP specification is essential when comparing or "stacking" records from different data acquisition systems. Most other systems ignore this offset which may produce a variable and undetermined time delay depending on the hardware used.

Two digitizers are used on systems with single sphere gravity sensors. An HP34420A 7 ½ digit digital voltmeter (DVM) is dedicated to the gravity channel. An HP34970A 6 ½ digit Data Acquisition / Switch Unit with 20 channel multiplexer is used for all other signals. On dual sphere sensors, an additional HP34420A is used for the second gravity channel.

In order to verify and measure the stability of the DVMs, a high precision transportable Voltage Transfer Standard provides a stable voltage that is sampled periodically. The accuracy of the standard is better than 20 ppm/year and temperature coefficient is better than 2 ppm/^OC. It is input to channels number 2 (and number 4 for the dual sphere system) which are multiplexed inside the 34420A DVM. The standard can be disconnected and sent out for calibration at a NIST qualified calibration site as part of regular system maintenance. Removal of the standard does not effect logging of all other channels. This important feature allows the user to periodically calibrate the DVMs without interrupting the data record. This need for long term periodic calibration is often overlooked in other data systems. The voltage standard allows one to correct data for changes in DVM calibration from aging, or for the exchange of DVMs in the event of a failure.

Descriptions of the channels and their uses is shown in the table below. Both the balance and feedback signals for the gravity control, temperature control, and tilt control cards are recorded to ensure proper operation of these important subsystems. A digital barometer is also logged by the controller. Since its signal is output in digital form a it does not require a channel from either of the DVMs.

Ch #	Channel Name	Input Signal	DVM Channel	Sample Rate
1	GGP-GRAV1	+/- 10VDC	34420A #1 chan-1	1 sample / sec.
2	STD-GRAV1	+/- 10VDC	34420A #1 chan-2	TBD
3*	GGP-GRAV2	+/- 10VDC	34420A #2 chan-1	1 sample / sec.
4*	STD-GRAV2	+/- 10VDC	34420A #2 chan-2	TBD
5	Barometer	serial	N/A	1 sample / sec.
6	Grav #1 Balance	+/- 10VDC	34970A chan-1	1 sample / 60 sec.
7*	Grav #2 Balance	+/- 10VDC	34970A chan-2	1 sample / 60 sec.
8	Tilt-X Power	+/- 10VDC	34970A chan-3	1 sample / 60 sec.
9	Tilt-X Balance	+/- 10VDC	34970A chan-4	1 sample / 60 sec.
10	Tilt-Y Power	+/- 10VDC	34970A chan-5	1 sample / 60 sec.
11	Tilt-Y Balance	+/- 10VDC	34970A chan-6	1 sample / 60 sec.
12	Temp Balance	+/- 10VDC	34970A chan-7	1 sample / 60 sec.
13	Heater Power	+/- 10VDC	34970A chan-8	1 sample / 60 sec.
14	Neck Therm. #1	+/- 10VDC	34970A chan-9	1 sample / 60 sec.
15	Neck Therm. #2	+/- 10VDC	34970A chan-10	1 sample / 60 sec.
16	Body Therm	+/- 10VDC	34970A chan-11	1 sample / 60 sec.
17	Belly Therm	+/- 10VDC	34970A chan-12	1 sample / 60 sec.
18	Helium Flow	+/- 10VDC	34970A chan-13	1 sample / 60 sec.
19	Grav 1 PCB Temp.	resistance	34970A chan-14	1 sample / 60 sec.
20	Controller temp.	resistance	34970A chan-15	1 sample / 60 sec.
21	Room Temp.	resistance	34970A chan-16	1 sample / 60 sec.
22	Inlet Air temp.	resistance	34970A chan-17	1 sample / 60 sec.
23	Inlet water temp.	resistance	34970A chan-18	1 sample / 60 sec.
24	Outlet water temp.	resistance	34970A chan-19	1 sample / 60 sec.
25	Water flow	frequency	34970A chan-20	1 sample / 60 sec.
26	UPS	serial	N/A	1 sample / 60 sec.
27	LHe Level	serial	N/A	user selectable

Timing accuracy is maintained by synchronization with a Trimble Palisade GPS receiver. This receiver is capable of tracking eight satellite vehicles enabling rapid time synchronization after power up.

In order to maintain precise synchronization the 1PPS signal from the GPS is used to generate a hardware interrupt in the controller’s processor. This allows timing accuracy to be maintained to within a few milliseconds of UTC. Accurate timing facilitates comparison of gravity data from gravimeters located at distant locations and is essential to achieve goals set forth by the GGP.

The Host PC communicates with the data acquisition Controller through a serial RS422 port. This allows the PC to be located up to 1200 meters from the gravimeter electronics. The host PC is used for data storage as well as a user interface. Data is backed up from hard disk onto optical media at regular intervals. The host PC is equipped with a telephone modem and Ethernet card allowing easy access from a remote location. All user interface and remote functions can be accomplished without degrading the timing accuracy or interrupting the Data Acquisition Controller.

The user interface includes numeric and graphical displays of all data channels. A User’s log allows the operator to enter notes when visiting the system. A comprehensive alarm system allows the operator to be notified based on several selectable criteria.

Ample temperature sensors are provided to insure complete monitoring of subsystem status. This includes monitoring of water cooler performance, room temperatures, and electronics temperature. In addition, the helium flow rate and water flow rate are monitored directly. Monitoring the status of all subsystems allows the user to operate the system remotely with greater confidence.

Digital lines from the GPS receiver and the host PC are optically isolated. In addition, lightening arrestors are provided for the GPS receiver to minimize the possibility of damage to equipment during electrical storms.

The Temperature Regulated Electronics Enclosure (TREE) provides a stable environment for the GEP-3 Analog Control chassis, the absolute pressure transducer and analog to digital converters. It consists of a sealed rack with a thermostatically controlled heat exchanger. While operating, external temperature fluctuations are attenuated by a factor of more than 10 inside the case. Rubber gasketing provides protection against contaminates that may be present in a harsh environment.

The enclosure is designed to operate slightly above the ambient temperature. This prevents the possibility of condensation appearing on the electronics which can cause changes in their characteristics. The system is cooled by two isolated air circulation systems. The external system sucks in air, then passes it over a finned heat exchanger. The internal system circulates the internal air over the opposite side of the heat exchanger and throughout the enclosure. In this manner dust and other contaminants are not allowed to enter the enclosure. To regulate the internal temperature, a thermostat is used to vary the speed of the external circulating system depending on the internal temperature.

When ambient temperature fluctuations are greater than ±5 ^OC, a heater option is available to extend the useful operating conditions of the enclosure. Consult GWR for pricing and recommendations as required for your specific needs.

Since changes in temperature effect all electronic components, it is important to minimize these effects when possible. Although care is taken in the design and manufacture of the gravimeter electronics, at some level, changes in temperature and humidity will effect the gravity signal. At sites where ambient temperature fluctuations are greater than 2 or 3 ^OC or where unusually high or variable humidity exists, use of the TREE is recommended.

A high quality enclosure is supplied to house items that do not require temperature regulation. This includes the GEP-3 power supply, Data Acquisition Controller, Liquid Helium Level Monitor, and user’s Chart Recorder.

A level indicator with serial interface is supplied to allow remote control of the Liquid Helium Level Monitor by the Data Acquisition Controller. This unit can be controlled manually or by the DDAS-3.

An Uninterruptible Power Supply (UPS) is provided in order to maintain proper system operation during power failures. This unit also provides some isolation from noisy power lines or spikes induced by electrical storms. The UPS is provided with 10 minutes of backup capability. If power failures of longer duration are expected, additional battery modules can be added without interrupting operation.

The Druck RPT 301 Resonant Pressure Transducer operates on a resonant silicon principle. The technology incorporates a resonator and pressure sensitive diaphragm, micro machined from a single piece of silicon. The resonator is bonded to a silicon wafer containing the drive and pickup system under vacuum. This isolates the resonator from the pressure media, thereby maintaining accuracy regardless of the density of the pressure media. This eliminates humidity effects often associated with resonant transducers. The sensor and electronics are housed in an environmental enclosure. An internal microprocessor calculates the pressure in user defined units and provides temperature compensation. Data is output via a serial port which can be configured for RS232 or full duplex RS485. As with any pressure transducer, in order to maintain specified accuracy, this system should be periodically calibrated either at the factory or by running in conjunction with a calibration transfer meter.

Precise measurement of atmospheric pressure is essential for proper analysis of gravity data. Changes in atmospheric density result in real changes in gravity observed at the earth's surface. These changes are a direct effect of gravimetric attraction from the varying overhead mass. The effects can be as large as 10 m Gal or more.

Note: Alternate components of equal or greater value or serviceability may be substituted without notice.