Abstract:
A network distributed seismic data acquisition system comprises seismic receivers, connected to remote data acquisition modules, receiver lines, base line modules base lines, a central recording system and a seismic source event generation unit. A Global positioning system antenna is positioned at many or all seismic receiver take-out points. Each antenna is supported by minimal antenna signal processing circuitry for transmitting antenna reception to a base GPS receiver having full GPS signal processing capability for determining the distinctive global position of each antenna.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The priority date benefit of Provisional Application No. 60/877,181 titled PseudoRover GPS Receiver filed Dec. 26, 2006 and of Provisional Application No. 60/880,688 titled PseudoRover GPS Receiver filed Jan. 16, 2007 is claimed for this application. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention relates to seismic survey equipment. In particular, the invention relates to seismic equipment assembly combinations and the logistics of seismic equipment deployment. 
         [0005]    2. Description of the Related Art 
         [0006]    Utilization of a land/transition zone seismic data acquisition system such as the ARAM ARIES system described by U.S. Pat. No. 6,977,867 entails the distribution of seismic sensor groups over a wide geographic area. A precisely located and timed seismic event such as an explosion or Vibroseis™ discharge releases shock (seismic) energy against and into the earth. Each sensor in a group detects the magnitude of such seismic energy received by the sensors and converts the detected energy magnitude to a corresponding electrical signal, either analog or digital. The sensor groups are connected to remote data acquisition modules (RAMs) which are joined to other RAMs and to other data processing/communication modules such as base line units (BLUs) or line tap units (LTUs) by communication signal carriers such as electrical cable, optical fibers or radio linkages that are further connected by appropriate signal carriers to a Central Recording Unit (CRU). As appearing herein, a sensor “group” may comprise one or more geophones, hydrophones or other pressure sensor type (vertical or multi-component) that remains in one position for a period of time, typically at least several days. Such a distributed data acquisition system is disclosed in U.S. Pat. No. 6,977,867. 
         [0007]    After processing, sensor signal amplitude data is indicative of subsurface seismic conditions related to the geology and fluid content of the geologic formations. To facilitate correct processing of the acquired seismic data and enable optimum subsurface imaging, resolution analysis of the sensor data requires knowledge of the geographic position coordinates (X, Y and Z or longitude, latitude and altitude) for each sensor group and of the seismic event. 
         [0008]    Conventional survey means comprise many available methods and may include the application of GPS, or other satellite system means such as GLONASS, in various forms to calculate position coordinates of sensor groups and seismic source points. A geometric plan of the seismic survey activity is formulated prior to field operations. In one conventional procedure, the planned locations of sensor groups are staked by surveyors. Implementation of real time GPS and combined GPS/inertial navigation systems in mobile units (called ‘Rovers’) may be done to assist the surveyors in placing (identifying and recording) the seismic source and sensor group locations. These portable GPS receiver systems are characterized as “Rover” systems due to the functional characteristic of transportability to a desired location for computation of its present position based on contemporaneously acquired data. The computed positions are used to facilitate the location of sources and sensor groups which may be staked for later deployment of equipment; or they may be used concurrently (without staking) to place the equipment, such as vibrator seismic sources, at their correct operational positions on the ground. 
         [0009]    The term, GPS receiver, as defined according to industry practice and as used in this specification, is an integrated unit comprising an antenna, a data processor with a clock, a memory, input/output capability, a power supply and software which is capable of driving the data processing essential for acquiring GPS satellite signals utilizing the antenna and converting received GPS satellite signals to calculated positions and/or time of reception of the satellite signals and current time. 
         [0010]    According to industry practice any device which is defined as a GPS receiver must be capable of receiving GPS satellite signals and processing them to compute position and/or time. A device comprising only an antenna and antenna signal-conditioning processor is not a GPS receiver but may be a component of a GPS receiver. If a GPS receiver can receive GPS satellite signals and process them to compute geographic position and/or time without receiving assistance from any other GPS receiver (such as a Master GPS receiver) it is a fully capable GPS receiver. If a GPS receiver receives assistance to perform these functions it is an assisted or slave GPS (aGPS) receiver. 
         [0011]    The term GPS is defined in this document to encompass all present and all future satellite-based global positioning systems including the US NavStar Global Position System, the Russian GLONASS and the future European Galileo global positioning system. 
         [0012]    One implementation process for GPS technology within land seismic acquisition systems requires an operative combination of GPS receivers with the RAMs and/or other distributed modules as components of the seismic data acquisition network. A Master GPS receiver in communication with the CRU may be used in combination with the aGPS receivers in the distributed modules. Differential GPS position analysis may be utilized wherein the Master GPS receiver is at a known location. Also, assisted GPS (aGPS) receivers may be implemented to receive tracking assistance information over the communication network from the Master GPS receiver. The aGPS receivers provide range data to the master for its processing and receive the resultant position calculations back from the master and can compute current time utilizing this location information. See U.S. Pat. No. 7,117,094 for a description of aGPS in a networked seismic data acquisition system. 
         [0013]    All of the prior art disclosures and implementations of GPS positioning and synchronization for a distributed data acquisition network (whether for seismic or any other type of sensor data acquisition) call for a GPS receiver to be linked to its own individual GPS antenna and for the GPS antenna to be in physical proximity or incorporated together with other GPS receiver components in a tightly coupled manner. 
         [0014]    The availability of a communication network connecting remote sensor groups within a data acquisition site and the fact that a sensor group may occupy a single physical position for an extended period of time (such as in one class of seismic data acquisition systems) provide an opportunity to utilize a single fully capable GPS receiver with a multitude of distributed GPS antennae to determine exact positions of the remote sensor arrays. Prior art has not recognized this opportunity and has required a GPS receiver at each GPS antenna, whether fully capable or, alternatively, requiring assistance from a Master GPS Receiver. Prior art has not recognized this opportunity and has required a GPS signal processor, i.e. receiver at each GPS antenna location, whether fully capable or, alternatively, requiring assistance from a Master GPS Receiver. 
       SUMMARY OF THE INVENTION 
       [0015]    The present invention is of a novel method and apparatus for acquiring GPS signals in a distributed sensor data acquisition system (such as a land/transition zone seismic data acquisition system) to better determine locations (including vertical and horizontal coordinates) and time of acquisition of sensor data. The disclosed invention is characterized by the implementation of one or more Base GPS Receivers (called PseudoRovers) which receive GPS signals from a multiplicity of remote GPS antennae which are static for an extended period of time (such as one or more days) as the GPS signals (and sensor data) are acquired. A PseudoRover processes selects portions of the remote GPS antennae data to determine individual antenna positions. A PseudoRover also processes GPS data from its local antenna. The time determinations from GPS signals may be used to synchronize a system Master Clock. 
         [0016]    In this document, the term PseudoRover refers to a GPS receiver having full GPS signal (L1 and L2 frequency) processing capacity (fully capable). Distinctively, the PseudoRover may be supplied via a communication network with digital or analog signals from additional antennae. The additional antennae may be positioned over a wide area. The PseudoRover processes the signals from all of the antennae with which it is in communication 
         [0017]    Selected GPS signal data received at the remote antenna stations may be communicated in analog or digital form to a PseudoRover via a network communication pathway connecting the sensor groups. Such a network communication pathway may consist of appropriate signal carriers passing through a series of RAM, BLU and TAP modules to the CRU and thence to the PseudoRover module. Alternatively, the antenna data may be recorded on removable media at the antenna location and physically moved to a PseudoRover or communicated to it by independent means. One or more PseudoRovers may be located within one communication network. 
         [0018]    A PseudoRover may also be moved from one position in a communication network to another position in the network. The purpose of such a move may be to bring the PseudoRover into network proximity of a different group of antennae or simply for operational convenience. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The advantages and further aspects of the invention will be readily appreciated by those of ordinary skill in the art as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference characters designated like or similar elements throughout. 
           [0020]      FIG. 1  schematically typifies a field layout of the invention. 
           [0021]      FIG. 2  is a component assembly schematic of an antenna module according to a preferred embodiment of the invention. 
           [0022]      FIG. 3  is a schematic for battery power supply to the antenna module for one embodiment of the invention. 
           [0023]      FIG. 4  is a schematic for line power supply to the antenna module for one embodiment of the invention. 
           [0024]      FIG. 5  is a schematic for power supply over communication cable to the antenna module for one embodiment of the invention. 
           [0025]      FIG. 6  schematically represents a digital data packet and memory structure for time-stamped antenna data. 
           [0026]      FIG. 7  represents a software algorithm for the invention to calculate position and time. 
           [0027]      FIG. 8  is a schematic of a base GPS receiver module. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    Satellites of the NAVSTAR Global Positioning System orbit the earth at an altitude of approximately 20,000 km and transmit signals at a center frequency of 1575.42 MHz known as L1, and 1227.60 MHz known as L2. The signals are modulated such that nearly symmetric upper and lower sidebands are transmitted with the carrier completely suppressed. The L1 signal can be represented with the equation: 
         [0000]        s   L1 ( t )= m ( t )cos(2 πf   c   t +θ)+ n ( t )sin(2 πf   c   t +θ)  eq. 1 
         [0029]    where:
       f c =L1 frequency, and   m(t) and n(t) are pseudo-random functions with zero mean. Both functions are mutually orthogonal.
 
Each of the satellites making up the NAVSTAR constellation of satellites transmit unique m(t) and n(t) functions which are all mutually orthogonal. The bandwidth of n(t) is exactly ten times m(t).
       
 
         [0032]    The L2 signal can be represented with the equation: 
         [0000]        s   L2 ( t )= n ( t )sin(2 πf   c   t +θ)  eq. 2 
         [0033]    where: 
         [0034]    f c =L2 frequency 
         [0035]    m(t) is transmitted at the L1 frequency only, but n(t) is transmitted at both L1 and L2 frequencies. In the GPS literature, m(t) is known as the “Clear/Acquisition” code or C/A code and n(t) is known as the “Precision” code or P code. n(t) has exactly 10 times the bandwidth of the m(t) function. 
         [0036]    The power spectral density of the m(t) modulating function is: 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0037]    The power spectral density of the n(t) modulating function is: 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0038]    As represented by  FIG. 1 , one or more RAMs  16  are connected serially by sections of receiver line cable  19   a  and  19   b . Typically, each section of receiver cable  19  comprises a data transmission conduit  14   a  or  14   b  and four seismic sensor signal carriers called “takeouts”  13   a-d  and  13   e-h . Each takeout  13  respectively connects a seismic sensor group connection point  12  to a corresponding RAM  16 . There may be, for example, more than three thousand groups  12  and takeouts  13  in a 3-dimensional seismic survey layout. One or more sensors (not shown separately) may be connected to a single takeout  13  at a respective group connection  12 . The RAMs  16  are operatively linked to each other in a series and the series to a BLU  17  or to a LTU not shown by respective data transmission conduits  14   a  and  14   b . Base line cable sections  24  (without sensor group takeouts) join successive BLUs and LTUs and ultimately connect to a CRU  30 . Network telemetry is utilized to transmit commands and control information from the CRU  30  to the RAMs  16 . Similarly, network telemetry is used to transmit acquired seismic data and other information to the CRU  30 . Such other information may include GPS antenna signals. 
         [0039]    In a preferred embodiment of the invention, GPS antennae  10  and respective antenna signal processing modules  20  are located within a close proximity zone  22  at or very near the location of each sensor group connection points  12 . The present invention embodiment provides no direct connection or interaction between an antenna module  20  and a sensor group connection point  12 , however. 
         [0040]    Preferably, a GPS antenna  10  is semi-permanently affixed to the cable section  14  at the location of each takeout  12 . When the cable section  14  is deployed for data acquisition, the GPS antenna may require manipulation by the layout technician to orient it in the optimum position for reception of typical GPS signals, normally vertical or near vertical. 
         [0041]    Each remote GPS antenna  10  is operatively connected to a signal conditioning processor  26 . The antenna assembly together with the power supply and signal conditioning processor and ancillary incorporated items are collectively called the antenna module (AM)  20 . See  FIG. 2 . 
         [0042]    The signal conditioning processor receives power either from its own battery supply  27  ( FIG. 3 ) or from conductors  29  supplied by a power source located elsewhere in the network, such as the nearest RAM  16  ( FIG. 4 ). 
         [0043]    The functionality of the signal conditioner  26  comprises reception of antenna-gathered GPS satellite signals and either transmitting them as analog or digital signals to the PseudoRover to which they are assigned. In the case of digital, transformation of the received signals in analog form from their original frequency band (1.57542 GHz) to the lowest available band (20 to 30 MHz approximately) and digitizing (A/D converter  32 ) these re-modulated signals in the lower band. Data compression means may be applied to the re-modulated signals. Transformation to a sign bit representation may be a possible means of data compression. 
         [0044]    Signal-to-noise ratio enhancement processes may be applied in the signal conditioning processor  26 . The sign bit transformation represents one potential means of signal-noise ratio enhancement, as it will mitigate the otherwise desultory effects of high amplitude noise bursts that temporarily obscure GPS signals. 
         [0045]    The signal conditioning processor  26  also receives via the network a timing signal from the nearest local clock. Preferably, the nearest local clock is in the RAM that acquires the seismic data from the sensors connected at the takeout where the AM is located. The local clock is synchronized to the master clock at the CRU. The methods of U.S. Patent Application Publication US-2004-0105341-A1 are preferred for synchronizing the local clock to the master clock time. Preferred for stabilizing the rate of time measurement by the local module clocks is the method described by Timothy D. Hladik and Alan R. Phillips in their U.S. Provisional Patent Application No. 60/880,597 titled STABILIZING REMOTE CLOCKS IN A NETWORK. 
         [0046]    After the GPS signals are processed by the signal conditioning processor within the AM  20 , they, together with corresponding timing signals, may be stored in local memory  34  and recorded on removable media  36  by a controller  38 . The removable media  36  is preferably a memory of very high storage capacity. 
         [0047]    The signal conditioning processor  26  is capable of receiving and responding to commands originated at the CRU  30 . Additionally, the signal conditioning processor  26  is capable of transmitting conditioned antenna data, timing data and other information, such as status information, via the network to the CRU  30 . 
         [0048]    Preferably the PseudoRover  40  controls the timing and duration of acquisition of GPS signals by each AM  20 . Each AM may acquire GPS signals as directed by commands formulated by the PseudoRover  40  and implemented by the CRU  30 . Commands may prescribe multiple time periods of varying duration for acquiring GPS signals. 
         [0049]    Each AM  20  is independently controlled by addressed commands originated by the PseudoRover  40  (and implemented by the CRU). AMs operate independently of each other and may be activated simultaneously, sequentially, or any combination thereof by the PseudoRover acting through the CRU  30  via Rover interface  18 . Because the multi-antenna GPS receiver of the present invention can determine positions of multiple points as normally accomplished by a mobile Rover—without physical movement of any equipment—it is called a PseudoRover. Moreover, the PseudoRover may interface at a RAM  16 , BLU  17  or LTU as well as the CRU  30 . 
         [0050]    The PseudoRover  40  also acquires signals from a Base GPS antenna  42 , which is located in proximity to it. See  FIG. 8 . This Base GPS Antenna  42  has a clear view of the sky and may be mounted on a tower to ensure this. The PseudoRover may process fully all GPS signals received via the Base GPS Antenna  42  as long as the supporting receiver is in operation and is selectively commanded to do so by the CRU  30 . 
         [0051]    As a full capacity GPS receiver, the PseudoRover  40  includes an antenna controller  43  for receipt of base antenna  42  signals and a communication processor  48  for receipt of AM  20  signals from the CRU  30  via interface connectors  18  and  49 . See  FIG. 8 . Other constituents of the PseudoRover  40  comprise a signal conditioner  44 , a signal processor  47 , a memory  45  and a clock  46 . 
         [0052]    The Antenna Modules  20  are not fully capable receivers but are only dispersed elements of the PseudoRover. Neither are the Antenna Modules  20  slave receivers. The PseudoRover  40  has adequate processing capacity and is programmed to simultaneously process GPS signals from a multiplicity of AMs  20  while also processing signals from the Base GPS Antenna  42 . 
         [0053]    The communication network utilized by the AMs  20  is preferably one based on 100 BASE-TX Ethernet protocol. A four wire carrier may be contained in the cable  15  to support an implementation of this embodiment. TCP/IP is the preferred communication protocol to be used by the AMs  20 . 
         [0054]    One alternative embodiment of the invention may use digital or analog fiberoptic communication from each antenna location to another network location such as a RAM. If analog signals are transmitted by optical fiber, the signals could be digitized in the nearest RAM and then transmitted via the network to the PseudoRover  40 . If digital signals are transmitted by optical fiber from the antenna location, the analog-to-digital conversion may take place at the antenna location. 
         [0055]    Another embodiment of the invention may comprise a radio frequency communication link between the PseudoRover  40  and the AMs  20 . In such an embodiment, the PseudoRover  40  may be mobile for reasons such as operational convenience or to obtain signal proximity with a selected group of AMs  20 . Moreover, a radio frequency communications link may liberate the PseudoRover  40  physical unit from the physical unit of the CRU  30 . As in the case of a wire or optical fiber signal carrier medium, a radio signal communication between the PseudoRover and the several AMs may be analog or digital. 
         [0056]    Power, if supplied to the AMs from the RAM locations, may be carried by the Ethernet carrier wires using the Power Over Ethernet (POE) methodology represented schematically by  FIG. 5 . 
         [0057]    The AMs  20  have a communications transceiver function as well as the functions previously described. They receive communications originated by the PseudoRover Unit  40 , transmitted by the CRU  30  and relayed to them by intervening transceiver units. They relay such communications to the next more remote transceiver unit. They receive communications coming from the opposite direction (from a more remote transceiver unit and relay them onward toward the PseudoRover Unit. They also originate and transmit their own communications toward the PseudoRover Unit which ultimately receives them. 
         [0058]    In terms of GPS antenna capability, in the preferred embodiment the AMs  20  are L1 capable only and are not designed with the additional complexity required for L1 plus L2. The PseudoRover  40  preferably has both L1 and L2 full capability. 
         [0059]    Carrier phase utilization is not a requirement in the Preferred Embodiment although it could be incorporated for applications (possibly non-seismic) in which sub-meter or even sub-centimeter accuracy is desired. 
         [0060]    PRN code from L1 GPS signals when effectively received over an extended period of time and processed by the PseudoRover  40  can provide accuracy to 30 cm which is considerably better than required for normal petroleum seismic data acquisition. Accuracy to within one meter is the objective of the invention in the seismic application of the preferred embodiment. 
         [0061]    When addressed commands (formulated by the PseudoRover Unit and communicated to the CRU  30 , for example, via a Rover interface  18  for implementation) require an AM  20  to transmit a selected time window of its data ( FIG. 6 ) to the PseudoRover Unit, the AM  20  transmits the data via the Ethernet link to the next closest Ethernet transceiver unit  52 , which transmits it onward toward the PseudoRover Unit. In a cable implementation of the preferred embodiment, the receiving transceiver unit, which may be another Antenna Unit  20  or a RAM  16 , BLU  17  or LTU, further transmits the received data to the next closest unit along the network pathway toward the PseudoRover Unit  40 . This process is repeated until the data is received by the PseudoRover Unit, possibly via the CRU  30  in the last stage of reception and transmission before the PseudoRover Unit is reached. 
         [0062]    The data is then fully processed by the PseudoRover Unit  40  to determine the position of the AM  20 . Referring to the Antenna Module  20  of  FIG. 2 , an L1 signal is first filtered with a SAW bandpass filter centered at the L1 frequency and amplified with a Low Noise Amplifier (LNA).  FIG. 2  has a one stage RF to baseband demodulation step but it may be advantageous to add more demodulation steps to remove the carrier f c . More stages can improve receiver sensitivity by increasing the ability to remove jamming signals. 
         [0063]    The satellites are moving relative to the Antenna Module  20  which results in the carrier f c  being Doppler shifted by a maximum of approximately 4.5 Hz. The demodulation stage, or stages, should leave an allowance for the Doppler frequency when demodulating the modulating signals to baseband. The baseband signal is lowpass filtered and an analog-digital converter digitizes the analog signal. The digitized signal is stored in memory  34  by the controller  38 . In addition, the controller  38  adds timing stamps to memory, approximately every millisecond. Refer to  FIG. 6 . The timing stamps are derived from a very accurate oscillator. 
         [0064]    The controller  38  receives commands through two Ethernet transceivers  52  and transmits the data in memory through the Ethernet transceivers. 
         [0065]    An external memory  36  interface is present which allows an external collection device to be connected to the Antenna Module  20  and the contents of memory in the Antenna Module to be written to the collection device. 
         [0066]    The Antenna Module  20  can receive power from a few different methods. Refer to  FIGS. 3 ,  4  and  5 . The Antenna Module  20  may derive its power from batteries ( FIG. 3 ), or a Power-over-Cable method ( FIG. 5 ). 
         [0067]      FIG. 7 , illustrates an algorithm in the Pseudo-Rover to calculate position and time. The digital data collected in the Antenna Module is input into two tracking loops. The first loop correlates the received C/A sequence with the Pseudo Random/Number (PRN) sequence unique to each satellite. A strong correlation is tracked and the Doppler frequency is calculated and removed from the input data stream. The C/A phase is used to calculate Pseudo Range and is input in the Position and Time calculation. The Navigation Message is decoded from the PRN correlation and used to calculate Position and Time. The Navigation Message is used to calculate the current position in the P-Code sequence. The expected P-Code is correlated with the generated P-Code and the result is used as Pseudo Range data in the Position and Time calculation function. If the P-Code is encrypted, additional steps must be taken using the encrypted P-Code, commonly called Y-Code, in calculating Pseudo Range data. 
         [0068]    Once the Pseudo Range data is calculated, it is used to generate a Position and time solution for the current epoch. During each epoch, the position and time may not exist because of lack of signal. The generated positions and times are filtered and a best probable position and time is calculated using methods such as least squares methods. The probable position is sent to the seismic survey system. 
         [0069]    In this processing, the PseudoRover Unit may utilize pre-programmed position information such as the expected location of the AM based on the survey plan and a topographic model or other relevant position information. Utilization of such pre-planned information will optimize the determination of proper position coordinates for each AM upon reception of the GPS signal information from said AMs by the PseudoRover Unit. 
         [0070]    The PseudoRover Unit uses the timing data concurrently acquired with the GPS signals by the AM to confirm time of GPS data acquisition and correctness of library procedures in GPS data identification. 
         [0071]    The PseudoRover Unit may use the corresponding L2 data it has received and recorded to improve the positioning accuracy of the AM. 
         [0072]    Multiple time epochs of GPS data from a single AM may be processed independently or combined in one processing execution to improve the positioning accuracy using calculation methods familiar to those experienced in GPS processing. 
         [0073]    The PseudoRover evaluates the positioning results in terms of consistency and reliability using known methods and decides whether the position has been adequately determined. Adequacy of determination is based on survey accuracy goals input by the user to control the PseudoRover decision making as well as consistency of repeated observations. 
         [0074]    If the position of the AM is deemed by the PseudoRover to be adequately determined it may command discontinuation of any further GPS data acquisition by that AM as long as it remains in the same position. However the communication transceiver functions of the AM will be continued so long as there are further remote AMs that have yet to be adequately positioned. If all further remote Antenna Units in that branch of the network have also been adequately positioned, the transceiver functions of that AM (and the further AMs) is also shut down to conserve power. 
         [0075]    In terms of quality control of physical positioning of sensor groups, the PseudoRover is initially pre-programmed using information from a project plan that contains intended locations of all planned sensor groups and source points. The user prescribes quality control criteria that quantify the maximum deviation of actual location of each planned position to be tolerated. When the PseudoRover Unit calculates that a particular sensor group is more than the specified maximum deviation away from the planned position, it notifies the user immediately. The ‘Red Flag’ raised by the PseudoRover Unit may be acted upon or ignored by the user. If he desires he may halt survey operations to reposition the wrongly placed sensor group and its RAM, or he may continue with the survey operations with a documented change in the position of that RAM with respect to its original pre-planned position. 
         [0076]    Having fully disclosed a preferred embodiment of our invention,