Patent Publication Number: US-6670913-B1

Title: Self-calibrating electronic distance measurement instrument

Description:
This application is a continuation-in-part of application(s) application number 09 122,265 filed on Jul. 24, 1998 now abandoned “and which designated the U.S.” 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to surveying instruments, and more particularly to devices and methods for using reference signals from a satellite navigation receiver to automatically and precisely calibrate electronic distance measurement instruments, and for servo-driving the telescopes in electro-optical total stations. Such calibration specifically includes hardware techniques for aligning a reference clock or oscillator, and/or software techniques for measuring local clock and frequency offsets and then subtracting such offsets out in the final calculations. 
     2. Description of the Prior Art 
     Electronic distance measurement (EDM) equipment became commercially available after World War-II and has since become very important to the surveying, navigation and scientific communities. Since the introduction of EDM, the instrument size and power consumption have been reduced, and the precision and speed of measurement have been improved. Because the miniaturization of EDM equipment became possible, it made good sense to mount EDM&#39;s on theodolites which have telescopes that can precisely sight a horizontal and vertical angle to a target. Such combinations are electro-optical hybrids called “total stations.” 
     Combination electronic theodolite and EDM instruments allow surveyors to find the “space vector” from the instrument to a distant target. When a total station is connected to an electronic data recorder, field information can be quickly gathered and used to generate maps and plans in the office. 
     Flexible tapes, leveling staves, electro-optical distance meters, and other surveying equipment are calibrated to a legal standard and calibration certificates are issued, e.g., a “Regulation  80  Certificate,” as is issued in Western Australia. Such calibration is especially important where a legal purpose is in mind, e.g., an inspection to enforce a law or to be used as evidence in a court action. A flexible tape calibration laboratory in Midland is registered by the National Standards Commission of Australia for calibration of 1-100 meter lengths. 
     There are two certified baselines in Western Australia against which EDM instruments can be calibrated. The aim of EDM calibration is to ensure that it measures in accordance with the internationally recognized definition of length, as set forth by the Conference Generale des Poids et Measures (CGPM—the General Conference on Weights and Measures). Other governments in the world provide similar baselines and certification opportunities. When a Regulation  80  Certificate is required for the purpose of legal traceability to the Australian Standard for length, the EDM instrument is submitted to the Surveyor General for calibration. The Director of the Mapping &amp; Survey Division is the verifying authority for length and is appointed by the National Standards Commission. The Surveyor General now provides a software application program, called BASELINE, to assist surveyors with their regular calibrations of EDM instruments. 
     The accuracy of electronic distance measurement equipment is derived from an internal reference frequency source, e.g., a crystal oscillator. But such crystal oscillators can drift over time and with age. Exposure to extreme environments can also upset delicate calibrations of the reference frequency source, both short term and long term. Therefore, EDM equipment should be regularly calibrated by using it to measure a known length. 
     Long-range electronic distance meters, e.g., ranges over five kilometers, typically use microwave signals for measurement. Short range electronic distance meters often use infrared light. See, Rueger, J. M., Electronic Distance Measurement-An Introduction, Springer Verlag, Berlin, third edition, 1990. Both the long-range and short-range EDM&#39;s use pulse or phase comparison methods to determine the distance between instrument and a remote target. However, the phase comparison method is more commonly used for survey instruments. 
     The pulse technique is based on timing the signal travel time to and from a distant reflector. The velocity of the signal is assumed to be known. For phase comparison, the phase difference of signals is observed at several frequencies. The unambiguous distance between the target and the instrument is resolved using phase difference observations. But in all cases, the basis for measurement precision depends on the accuracy of the stand-alone reference frequency source. 
     One of the present inventors, Nicholas C. Talbot, described a combined satellite positioning/electro-optical total station system in U.S. Pat. No. 5,471,218, issued Nov. 28, 1995. One candidate satellite positioning system that can be used effectively is the Global Positioning System (GPS) operated by the United States. Such patent is incorporated herein by reference. 
     The combined satellite positioning/electro-optical total station system allows rapid instrument orientation and positioning in the field. Another integrated surveying system that combines electro-optical instrumentation with a satellite position measuring system is described by Ingensand, et al., in U.S. Pat. No. 5,233,357. 
     SUMMARY OF THE PRESENT INVENTION 
     It is therefore an object of the present invention to provide a combined satellite positioning and electro-optical total station system in which the electronic distance measurement is automatically and precisely calibrated. 
     It is another object of the present invention to provide a combined satellite positioning and electro-optical total station system that avoids duplicating components between its satellite positioning portion and its electro-optical total station portion. 
     Briefly, a combined satellite positioning and electro-optical total station system embodiment of the present invention includes a reference oscillator that provides local oscillator signals for a satellite navigation receiver and a precision frequency source for use by an electronic distance meter. When the satellite navigation receiver is locked onto and tracking orbiting navigation satellites, the highly precise cesium-rubidium clocks in the navigation satellite system can be used as standards to control the reference oscillator in the combined satellite positioning and electro-optical total station system. Baseline measurements made by the electronic distance meter are therefore not subject to mis-calibrations and drift as long as the satellite navigation receiver is locked onto and tracking the orbiting navigation satellites. 
     An advantage of the present invention is that a combined satellite positioning and electro-optical total station system is provided that includes an electronic distance meter that remains automatically calibrated. 
     Another advantage of the present invention is that a combined satellite positioning and electro-optical total station system is provided that is less expensive to manufacture and maintain than the separate instruments it replaces. 
    
    
     These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment that is illustrated in the drawing figure. 
     IN THE DRAWINGS 
     FIG. 1 is a functional block diagram of combined satellite positioning and electro-optical total station system embodiment of the present invention; 
     FIG. 2 represents a plot of short-term oscillator drift and the effect of the present invention to correct long-term oscillator drift; 
     FIG. 3 is a functional block diagram of a total station which uses an external reference oscillator that is stabilized by a timing signal obtained from a GPS receiver; 
     FIG. 4 is a functional block diagram of a 10 MHz reference oscillator in a generic product that is locally stabilized or disciplined by a GPS receiver with zero-crossing comparisons at one pulse per second; 
     FIG. 5 is a functional block diagram of a 10 MHz reference oscillator in a generic product that is remotely stabilized or disciplined by radio transmissions it receives from either a GPS receiver or government time-standard broadcasts such as from WWV; 
     FIG. 6 is a schematic diagram of a GPS receiver useful in the configurations shown in FIGS. 1-3; and 
     FIG. 7 is a functional block diagram of a combined satellite positioning and electro-optical total station system embodiment of the present invention with software correction. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates a combined satellite positioning and electro-optical total station system embodiment of the present invention, referred to herein by the general reference numeral  10 . A global positioning system (GPS) part of the system  10  includes a microwave patch antenna  12  for receiving L-band transmissions from orbiting GPS satellites, a preamplifier and downconverter  14 , a code mixer  16 , an in-phase carrier mixer  18 , a quadrature phase carrier mixer  20 . The carrier mixer outputs are each sent to low pass filters  22  and  24 . A numerically controlled oscillator (NCO)  26  is driven by a bi-phase locked loop filter  28  and a multiplier  30 . The NCO  26  produces a corrected frequency output that tracks the GPS-satellite carrier being tracked plus any Doppler effects. 
     The low pass filter  22  produces a fifty Hertz navigation message that is input to a navigation computer  32 . An adder  34  combines a squared in-phase signal (I{circumflex over ( )}2) and a squared quadrature-phase signal ({circumflex over (Q)}2) to produce a signal-power signal (I{circumflex over ( )}2+Q{circumflex over ( )}2)  36  that is proportional in magnitude to the despreading code, correlation. The I{circumflex over ( )}2+Q{circumflex over ( )}2 signal  36  is used to control the code-phase of a PRN-code generator  38 . A GPS-master reference oscillator  39  receives correction signals from the navigation computer  32  that maintain the satellite tracking. A precision reference frequency is then made available to drive a clock  40  and the downconverter  14 . A buffer driver  41  allows the reference frequency to be brought external from the GPS portion and isolates the reference oscillator from external load variations. 
     FIG. 2 represents a plot of short-term oscillator drift and the effect of the present invention to correct long-term oscillator drift. 
     Referring again to FIG. 1, code measurement, time measurement, and the navigation message are used by the navigation computer  32  to compute the current three-dimensional position of the system  10 . The GPS system time, e.g., in Universal Time Coordinated (UTC), is also determined by the navigation computer  32 . Such UTC is typically accurate in absolute terms to better than one hundred nanoseconds. It is better than that on a relative basis, over a short term. 
     Once the location of the instrument station is determined either from GPS or other means, a minimum of only one satellite is required to calibrate the time base of the instrument. 
     An electronic distance meter (EDM) part of the system  10  includes a phase comparator and charge pump  42  that servo controls a slave oscillator  43 . When the GPS navigation receiver part is tracking enough satellites to obtain a position fix, a highly accurate estimate of time and local oscillator frequency is available and used to precisely fix the operating frequency of oscillator  43 . Inexpensive crystal oscillators can be used throughout and for the local oscillator in the GPS receiver, and their absolute frequency accuracy is relatively unimportant because once signal lock is obtained with the GPS satellites, phase locked loops can be used to establish a precision frequency reference that is almost as accurate as the cesium-rubidium clocks in the GPS system. 
     An EDM phase measurement subsystem  44  is connected to a transmitter  46  that sends an out-bound signal  47  through a telescope  48  to a distant target  50 . The target  50  may include a prism corner-cube reflector, or active repeater for microwave EDM, to return an in-bound signal  51 . The signals  47  and  51  may be infrared or other laser light, or microwave signals. The EDM phase measurement subsystem  44  can conduct either pulse time-of-flight or carrier phase measurements to determine the line-of-sight distance to the target  50 . Conventional methods and equipment can be used to do this. A target range measurement  54  is output that can be presented on a local display, recorded electronically, or transmitted to a user that is at the target and is moving the target around to mark a particular range from the system  10  location. 
     A theodolite part of the system  10  includes the telescope  48  mounted to an angle measurement instrument  56  connected to a servo actuator  58 . A theodolite measurement  60  includes an elevation and azimuth output that can be presented on a local display, recorded electronically, or transmitted to a user that is at the target  50  and is moving the target around to mark a particular vector angle from the system  10  location. A space vector to target signal  62  is computed by the navigation computer  32  from a target position seed input  64 . 
     The navigation computer  32  is able to compute the current position of the system  10  and outputs this as a position estimate  66 . From this position estimate, it is possible to determine the altitude and azimuth vector to the target  50 . The space vector to target signal  62  commands the servo  58  to move the telescope  48  so that it is roughly pointed at the target  50 . A conventional search and tracking mechanism can then be used to find and keep the target  50  locked in. For example, the Geodimeter SYSTEM-500 is a commercially marketed system that is a servo-driven survey instrument in an automatically pointed electro-optical total station. The target location seed can be computed using differential satellite position calculations relative to the EDM reference station. 
     FIG. 3 illustrates a system  70  in which a total station  72  inputs a 10.00 MHz precision reference oscillator  74  that is stabilized by a timing signal  75  derived from a GPS receiver  76 . For example, GPS receivers marketed by Trimble Navigation Limited (Sunnyvale, Calif.) outputs a utility one-pulse-per-second (1PPS) that can be used by a phase comparison and frequency control circuit  78  to make minor corrections in the operating frequency of oscillator  74 . Such reference oscillator may be a voltage-controlled oscillator (VCO) or a numeric controlled oscillator (NCO) type. For the VCO type, the control signal from circuit  78  is a variable analog voltage or current. For the NCO type, the control signal from circuit  78  is a digital value. 
     FIG. 4 shows a precision reference system  80  in which a 10.00 MHz reference oscillator  81  is a generic product that is stabilized or disciplined by zero-crossing comparisons at one pulse per second. A divider  82  is used to reduce the output of the oscillator  81  to 1.00 Hz. A local GPS receiver source  83  provides a reference 1.00 Hz signal that is exceedingly precise and stable because it is derived from the atomic clocks used in the GPS system time standards. A phase comparator  84  provides an error signal  85  that is applied to an integrating filter  86  that drives the static phase error to zero for synchronization. A control signal  87  is returned via a buffer  88  to the oscillator  81 . The overall effect is to reduce the accumulation of errors over the long term to an average of zero, as in FIG.  2 . 
     FIG. 5 shows an alternative embodiment of a precision reference system  90  in which a 10.00 MHz reference oscillator  91  within an otherwise standard commercial product is stabilized or disciplined, e.g., with one-pulse-per-second signals. A divider  92  reduces the 10.00 MHz output of the oscillator  91  all the way down to 1.00 Hz. A radio receiver  93  is tuned to a 1.00 Hz remotely transmitted signal that is exceedingly precise and stable. A phase comparator  94  provides an error signal  95  that is applied to an integrating filter  96  that drives the static phase error to zero for synchronization. A control signal  97  is returned to closed-loop lock in the frequency of operation of oscillator  91 . A GPS receiver and radio transmitter combination  98  or a government time-standard broadcast transmitter  99 , e.g., WWV, are examples of sources used by the receiver  93 . Such a configuration would be helpful in the total station system  10  of FIG. 1 in areas with intermittent GPS coverage due to tree canopies or urban-canyon effects. For example, receiver  93  could comprise a commercial product such as is marketed by ESE (El Segundo, Calif. 90245), as the ES-180A master clock. The ES-180A receives and synchronizes to time data broadcast from the NIST via short-wave radio, WWV in Fort Collins, Colo., and WWVH in Hawaii, and provides a time-code output (TC89), ASCII time output (queried RS232), and a 1-PPS (pulse-per-second) output. 
     Time bases that use radio transmissions from the WWV and WWVH stations operated by the United States Government typically provide a usable received accuracy of one part in ten million for frequency, and about one millisecond for timing. The frequencies as transmitted, however, are accurate to one part in a billion because they are based on the primary NIST Frequency Standard and related NIST atomic time scales in Boulder, Colo. The difference in transmitted and received accuracy is due to various propagation effects. 
     FIG. 6 is a schematic diagram of a GPS receiver  100  useful in the configurations shown in FIGS. 1-3. The GPS receiver  100  incorporates a microprocessor control unit (MCU) and digital signal processor (DSP) combination  102 , e.g., a “SCORPION” integrated circuit designed by Trimble Navigation Limited (Sunnyvale, Calif.). The radio frequency tuning, downconversion, and digital sampling are done with a radio frequency circuit  104 , e.g., a “SURF” integrated circuit designed by Trimble Navigation Limited (Sunnyvale, Calif.). A 10.00 MHz ovenized crystal oscillator (OCXO). 106  provides a precision reference frequency output  108  that can be used by the EDM&#39;s and total stations described in FIGS. 1-3. Such reference frequency output  108  has very high frequency precision, both short term and long term. Signals from orbiting navigation satellites are used as references and locked on to by tracking loops within the SCORPION  102  and SURF  104  combination. The MCU/DSP  102  samples the OCXO  106  at its XCLK input and the SURF  104  uses an RO input to generate its local oscillator signals. Alternatively, an external 10.00 MHz source maybe connected to input  110 . The SCORPION  102  and SURF  104  combination computes frequency errors and controls a digital to analog converter (DAC)  111 . A DAC output  112  is then able to discipline the external 10.00 MHz source. A utility 1PPS output  114  is provided that can be used as shown in FIGS. 2 and 3. 
     In FIGS. 1-4, the EDM and GPS oscillator are discussed as being physically distinct and separate units. The GPS oscillator is assumed to be aligned with GPS system time by virtue of its tracking the signals of the visible GPS satellites. But in many GPS receivers, e.g., some of those marketed by Trimble Navigation (Sunnyvale, Calif.), the GPS receiver oscillator is not steered or physically aligned with GPS time. Rather, the clock and frequency offsets are calculated and used later in “software” to arrive at accurate solutions. This software technique is extended in embodiments of the present invention to EDM and other surveying equipment. Such is represented in FIG.  7 . As a consequence, such surveying equipment need not be periodically calibrated by a standards laboratory nor certified by government authority. Each measurement in the field is corrected in computer calculations in real-time to approximately the absolute accuracy of the satellite navigation system master clocks. The opportunity for long-term drift to creep in is eliminated as well as the measurement uncertainty that would result. 
     A system  200  in FIG. 7 is similar to that of FIG. 1, except that the oscillator  43 , its control  42 , and buffer  41  (all of FIG. 1) are no longer needed. Also the tracking correction from navigation computer  32  to reference oscillator  39  is not used. The navigation computer  32  ( 232  in FIG. 7) computes the clock and frequency offsets which are used later in “software” to arrive at accurate solutions. 
     FIG. 7 illustrates a combined satellite positioning and electro-optical total station system embodiment of the present invention, referred to herein by the general reference numeral  200 . A global positioning system (GPS) part of the system  200  includes a microwave patch antenna  212  for receiving L-band transmissions from orbiting GPS satellites, a preamplifier and downconverter  214 , a code mixer  216 , an in-phase carrier mixer  218 , and a quadrature phase carrier mixer  220 . The carrier mixer outputs are each sent to low pass filters  222  and  224 . A numerically controlled oscillator (NCO)  226  is driven by a bi-phase locked loop filter  228  and a multiplier  230 . The NCO  226  produces a corrected frequency output that tracks the GPS-satellite carrier being tracked plus any Doppler effects. 
     The low pass filter  222  produces a fifty Hertz navigation message that is input to a navigation computer  232 . An adder  234  combines a squared in-phase signal (I{circumflex over ( )}2) and a squared quadrature-phase signal (Q{circumflex over ( )}2) to produce a signal-power signal (I{circumflex over ( )}2+Q{circumflex over ( )}2)  236  that is proportional in magnitude to the despreading code correlation. The I{circumflex over ( )}2+Q{circumflex over ( )}2 signal  236  is used to control the code-phase of a PRN-code generator  238 . A GPS-master reference oscillator  239  provides a precision reference frequency is then made available to drive an EDM phase and measurement device  244 . The GPS code measurement, time measurement, and the navigation message are used by the navigation computer  232  to compute the current three-dimensional position of the system  200 . Once the location of the instrument station is determined either from GPS or other means, a minimum of only one satellite is required to calibrate the time base of the instrument. 
     The electronic distance meter (EDM) part of the system  200  includes the EDM phase measurement device  244  connected to a transmitter  246 . An out-bound signal  247  is directed through a telescope  248  to a distant target  250 . The target  250  may include a prism corner-cube reflector, or active repeater for microwave EDM, to return an in-bound signal  251 . The signals  247  and  251  may be infrared or other laser light, or microwave signals. The EDM phase measurement subsystem  244  can conduct either pulse time-of-flight or carrier phase measurements to determine the line-of-sight distance to the target  250 . Conventional methods and equipment can be used to do this. A target range measurement  254  is output that can be presented on a local display, recorded electronically, or transmitted to a user that is at the target and is moving the target around to mark a particular range from the system  200  location. 
     A theodolite part of the system  200  includes the telescope  248  mounted to an angle measurement instrument  256  connected to a servo actuator  258 . A theodolite measurement  260  includes an elevation and azimuth output that can be presented on a local display, recorded electronically, or transmitted to a user that is at the target  250  and is moving the target around to mark a particular vector angle from the system  200  location. A space vector to target signal  262  is computed by the navigation computer  232  from a target position seed input  264 . The navigation computer  232  is able to compute the current position of the system  200  and outputs this as a position estimate  266 . 
     The clock and frequency offsets that exist in the hardware are corrected for in software of navigation computer  232 . 
     From a position estimate, it is possible to determine the altitude and azimuth vector to the target  250 . The space vector to target signal  262  commands the servo  258  to move the telescope  248  so that it is roughly pointed at the target  250 . A conventional search and tracking mechanism can then be used to find and keep the target  250  visually locked in. For example, the Geodimeter SYSTEM-500 is a commercially marketed system that is a servo-driven survey instrument in an automatically pointed electro-optical total station. The target location seed can be computed using differential satellite position calculations relative to the EDM reference station. 
     Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that the disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the invention.