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RIGHTS OF THE GOVERNMENT 
     The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to the field of laser aided land surveying instruments and methods as applied to the precise azimuth disposition of a subterranean pipe/conduit/tunnel or similar object in the earth. 
     Laser inclusive instruments are often used to establish the azimuth and grade for man made surface objects such as roads and buildings. Under certain conditions such instruments are also usable for accurately laying pipe and conduit in the earth and for tunneling and other below grade construction tasks. With such instruments the azimuth alignment of a pipe/conduit/tunnel can be accurately controlled with minimized expenditure of measurement and earth movement assets. In the pipe/conduit/tunnel azimuth alignment instance these instruments generally consist of a visible spectrum energy-emitting laser physically oriented by instrument azimuth and pitch-determining elements and mounted in a housing appropriate for field use—use at the bottom of a hole or trench for example. In such instruments azimuth or azimuth and elevation pointing of the instrument may be achieved with steering optics that also expand and collimate the laser beam. 
     According to present-day typical practices, after disposing a subterranean course alignment instrument at the bottom of an appropriate hole in the earth or at the bottom of an initially commenced trench, the instrument is brought to designating the desired pipe/conduit/tunnel azimuth by locating a transit or similar device on the earth surface directly above the instrument and aiming the transit at a surveying reference, e.g. a stake in the ground or a positioned surveyor&#39;s rod, identifying the desired pipe/conduit/tunnel course. While maintaining the transit&#39;s azimuthal alignment, the transit telescope is then pointed downward into a hole in the earth or into a commenced trench, toward a wall surface. The laser alignment instrument beam is then aimed at the same wall surface and brought into the desired alignment with a point determined by using the transit telescope reticule. 
     Moreover according to current practice, the transit or similar device can alternatively be used to position an alignment target on the wall within the hole or commenced trench and then bringing the laser into the desired alignment with the target. Present day subterranean course alignment instruments typically include apparatus enabling remote control of the azimuth and elevation pointing of the laser beam upon command from an operator person. A single person, using such remote aim-point control of the instrument, can usually accomplish an initial setup of an instrument in 10 to 15 minutes after excavating the trench and placing the instrument at the bottom. Subsequent realignment requires a comparable length of time. Work site equipment movement and the associated vibration however often require the instrument to be realigned frequently. The thus-described conventional alignment technique therefore requires an extensive open trench in the earth; this requirement is often considered a safety concern for people and equipment. A faster alignment capability not requiring an open trench is therefore highly desirable. The present invention addresses this need. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved subterranean course alignment laser instrument and a system for rapid and convenient calibration or setup of this instrument to obtain accurate definition of a sub surface course for a pipe/conduit/tunnel or the like. Instrument setup is made in response to an above-ground course definition marker such as a surveyor&#39;s stake or surveyor&#39;s rod, without use of a surveyor&#39;s transit and while the instrument is disposed in a subterranean location. Optical elements selected in recognition of a limited error budget available in such equipment are added to the instrument to provide separate output beams for setup and for pipe/conduit/tunnel alignment uses. 
     It is an object of the present invention therefore to provide a subterranean course alignment instrument capable of convenient, fast and accurate setup. 
     It is another object of the invention to provide a subterranean course alignment instrument having single person setup capability. 
     It is another object of the invention to provide a subterranean course alignment instrument having desirable accuracy characteristics. 
     It is another object of the invention to provide a subterranean course alignment instrument capable of desirable accuracy operation under field use conditions. 
     It is another object of the invention to provide a subterranean course alignment instrument capable of operation within a desirably small error budget. 
     It is another object of the invention to provide a subterranean course alignment instrument having a two beam initial alignment procedure. 
     It is another object of the invention to provide a laser alignment instrument employing curved three dimensional optical elements in its initial alignment procedure. 
     It is another object of the invention to provide a subterranean course alignment instrument having above ground initial alignment input capability. 
     It is another object of the invention to provide a subterranean course alignment instrument having a pseudo periscope underground to above ground optical communication arrangement. 
     It is another object of the invention to provide a subterranean course alignment instrument in which complementary optical elements are used to reduce setup error characteristics. 
     It is another object of the invention to consider the error sources encountered in a concave beveled mirror and convex beveled mirror arrangement of a subterranean course alignment instrument. 
     These and other objects of the invention are achieved by a surface-referenced pipe/conduit/tunnel subterranean azimuth course-determining laser apparatus comprising the combination of: 
     a first laser element disposed in a subterranean receptacle and generating subterranean receptacle-contained horizontally directed radiant energy emission at an output port thereof; 
     a desired pipe/conduit/tunnel azimuth course marker element disposed at a surface reference location distal of said subterranean receptacle; 
     first curved mirror apparatus selectively coupled to said output port of said first laser element and selectively directing said subterranean receptacle-contained horizontally directed radiant energy emission upward and out of said subterranean receptacle; 
     second curved mirror apparatus disposed above said subterranean receptacle and said first laser element and orienting said upward and out-directed first laser element radiant energy parallel with said first laser element horizontally directed radiant energy emission away from said subterranean receptacle in an above ground selected azimuth direction toward said desired pipe/conduit/tunnel course marker element; 
     said subterranean receptacle-contained first laser element horizontally directed radiant energy emission selectively designating a desired subterranean azimuth course for said pipe/conduit/tunnel in response to prior optimized receipt of said first laser radiant energy at said desired pipe/conduit/tunnel course marker surface reference element via said first curved mirror apparatus and said second curved mirror apparatus. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows laser alignment instrument according to the invention in an initial alignment and field use situation. 
     FIG. 2 shows a more detailed top view of a laser alignment instrument according to the present invention. 
     FIG. 3 shows a more detailed side view of a laser alignment instrument according to the present invention. 
     FIG. 4 shows a side view of a convex beveled mirror apparatus usable in the invention. 
     FIG. 5 shows a bottom view of a convex beveled mirror apparatus usable in the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 in the drawings shows a simplified diagram of a subterranean course alignment instrument according to the present invention in both an in-the-field initial instrument alignment and in a subsequent instrument use condition. In the FIG. 1 drawing the subterranean course alignment instrument and associated above ground reflector apparatus  100  are shown in a course-determined or aligned condition with an above ground course determining marker  140  and its associated apparatus at  102 . In the FIG. 1 drawing the subterranean course alignment instrument and associated apparatus at  100  can be observed to include a first laser member  110  disposed in a receptacle or hole  104  in the earth  103 . 
     The output energy from the laser  110  flows along the horizontal path  120  in FIG. 1 to a movable, two position mirror element  116  mounted on a pivot member  126 . In the mirror position shown in FIG. 1 the output energy of laser  110  is reflected upward along the path  118  by this mirror  116 . In the alternate and downward rotated position of mirror  116  (as appears at  310  in FIG. 3) the laser  110  output energy along path  120  is allowed to continue along path  122  to a spot  124  located on a side of the receptacle  104 . As will be better understood from subsequent discussion herein the path  122  and the spot  124  define the desired course of the path being arranged in the FIG. 1 drawing, i.e., the course indicated generally at  150  in FIG.  1 . The course  150  may represent the path of a sewer line, a water line, an electro-optic conduit line, a petroleum product line, a ditch or a tunnel and other structures for examples. 
     In the FIG. 1 illustrated upward rotated position of mirror  116  the laser output energy from path  120  is reflected along path  118  to another mirror element  130  mounted on a tripod  106  or similar device received over the earth receptacle  104 . The mirror element  130  is a part of a mirror assembly  108  carried on a tripod  106 -mounted platform assembly  134 . The mirror element  130  accomplishes a second corner turning function for the laser energy of paths  120  and  118  and directs this energy along the generally horizontal path  132  toward the course-determining marker apparatus  102 . 
     Connected to the laser  110  in the FIG. 1 drawing is a second laser  112 , a laser generally orthogonally disposed with respect to the laser  110  and its related paths  120  and  122 . Output energy from this second laser  112  is directed along the upward path  114 , a path which is also orthogonally disposed with respect to the laser  110  and the paths  120  and  122 . The second laser  112  in the FIG. 1 apparatus provides an optical signal usable in achieving the desired vertically aligned dispositions of the reflecting surfaces of mirrors  116  and  130  during the instrument setup sequence described herein. To facilitate this alignment the mirror assembly  108  is preferably provided with an optical and semiconductor-based alignment mechanisim employing output of the second laser  112  to horizontally align the upper and lower elements  108  and  110  using a pair of horizontal servomechanisms in the mirror assembly  108 . The second laser  112  may be of the semiconductor or solid state variety since its distance of operation is generally limited to the depth of the receptacle  104 . The primary laser  110  may be of the semiconductor type or of the gaseous laser type if needed for greater energy output and spectral selection considerations. 
     The above ground course determining marker and its associated apparatus at  102  may be observed in the FIG. 1 drawing to include the elected pipe/conduit/tunnel terminal point marker  146  (a mark disposed on or in the earth surface), the surveyor&#39;s rod  140  received on this point  146  and the rod target  142  all of which may be attended by the person  144 . The terminal point of marker  146  may of course be identified with use of a wooden stake, an earth-driven rod or other marking devices known in the art (which may be unattended) in substitution for the surveyor&#39;s rod  140 . The distance between subterranean course alignment instrument and associated apparatus at  100  and the above ground course-determining marker and its associated apparatus at  102  is indicated at  152  in FIG.  1 . 
     The possible length of this distance  152  in an instrument use situation may be limited for example by conditions such as terrain irregularities, interfering objects and atmospheric conditions. The above mentioned selection of laser wavelength and laser type, whether semiconductor or gaseous, and laser operating power level can influence these characteristics. The maximum length of this distance may also be influenced by a tendency of the laser energy beam to spread with increasing lengths of distance  152 . Notwithstanding such spreading however it is often feasible to detect the center point of a Gaussian-spread laser beam energy distribution to a degree enabling satisfactory instrument alignment. Typical lengths of distance  152  in the heretofore used instrument alignment practice discussed above are fifty to five hundred feet, these distances are practical for the present invention instrument also. 
     Generally the above ground instrument setup alignment path  132  connecting the above ground reflector assembly  108  with the above ground course determining marker and its related apparatus at  102 , and the subterranean laser energy paths  120 - 122 , are parallel disposed. It is the above ground instrument setup alignment path  132  which is used by the FIG. 1 apparatus to define the desired alignment for the subterranean pipe/conduit/tunnel or other structure (hereinafter referred-to generically simply as a pipeline) paths  120 - 122  and  150 . In other words during a setup cycle of the FIG. 1 apparatus the invention contemplates adjusting the physical position of laser  110  in the receptacle  104  causing the laser energy along path  132  to fall on the appropriate portion of the above ground course determining marker  140  and its associated apparatus at  102 . With this instrument setup alignment accomplished, a pivotal position change of the mirror  116  causes the laser output energy on path  126  to continue along the path  122  and designate the point  124 , i.e., the commencement point for the pipeline path  150 . As the trenching or tunneling for the pipeline ensues from the receptacle  104  the laser energy point  124  of course moves to the left in the FIG. 1 drawing to fall on each new earthen receptacle sidewall as it is exposed. The moving point  124  therefore continuously designates the subterranean pipeline course and provides the desired trenching or tunneling azimuth guidance to assure the pipeline will pass below the point marker  146  and the marker  140 . 
     FIG.  2  and FIG. 3 in the drawings shows additional details of the laser  110  and the mirror  116  appearing in FIG.  1 . The FIG. 2 drawing represents a cutaway top view of a housing  206  for the laser  110  and the mirror  116  and shows refinements attending each of these elements not represented in the FIG. 1 drawing. FIG. 3 represents a cutaway side view of the housing  206  and shows details of yet additional attending refinements. Identification numbers used in the FIG. 1 drawing are repeated to the best degree possible in the FIG.  2  and FIG. 3 drawings and in the subsequent drawings herein. Of particular interest in the FIG.  2  and FIG. 3 drawings is the laser mounting or supporting structure providing a needed degree of yaw axis and elevation axis freedom for the laser  110 . Generally this supporting structure includes the pivot-topped mounting pedestal  314 , the elevation adjustment jack  300  and its extension screw  302  and the yaw angle-anchor member  200  together with its adjustment screw  202 . (Both FIG.  2  and FIG. 3 merit consideration in reaching an appreciation of these elements and their function.) 
     The mirror  116  in FIG. 1 is shown, particularly in the FIG. 2 drawing, to be of a curved and three dimensional nature; this showing is in supplement or additional clarification of the simplified planar nature of mirror  116  represented in the FIG. 1 drawing. Additional detail of this mirror appears in the FIG. 3 drawing where one segment of the beveled or forty-five degree angle-disposed reflecting surface of the mirror becomes particularly visible again. As may be appreciated from the FIG.  2  and FIG. 3 views of mirror  116  this mirror may be described as having a “concave beveled surface” in its active portion. Preferably this “concave beveled surface” is covered by a front-side reflection coating which serves to reflect radiant energy from laser  110  to the oppositely curved or convex mirror, represented in simplified form, at  130  in FIG.  1 . 
     The pivot  126  which was shown in FIG. 1 also appears in the FIG. 3 drawing. In FIG.  2  and FIG. 3 the mirror  116  is shown to be in an instrument setup position wherein laser output energy along path  120  is reflected to a right angle path  118  in order to travel in an upward direction out of the earthen receptacle  104  of FIG.  1  and ultimately illuminate the surveyor&#39;s rod  140 . In an instrument use position of mirror  116 , as shown at  310  in FIG. 3, this mirror  116  is removed from the laser energy path by way of mirror rotation about pivot  126  and then laser energy travels along the extension of path  120  represented at  122  in FIG. 1, FIG.  2  and FIG.  3 . Appropriate “stops” to provide precise positioning of mirror  116  in each of its FIG.  3 -illustrated positions  310  and  314  may be arranged according to mechanical techniques known in the art. The yaw axis pivot point at  113  in the FIG. 2 top view is shown in dotted form in response to the fact that the laser  112  covers the pivot point in this view. 
     In view of the subterranean course alignment instrument being intended for placement in an earth receptacle as shown at  104  in FIG. 1, it is convenient for the laser mounting elements in FIG.  2  and FIG. 3 of the drawings to be remotely controllable in order that positioning of the instrument, as needed to provide alignment with the surveyor&#39;s rod  140 , can be accomplished from an above-ground and possibly receptacle-removed location. To this end the threaded screw members shown at  202  and  302  in FIG.  2  and FIG. 3 may be provided with electric motor or other remotely energizable driving members, motors disposed for example in the support members  200  and  300 . In a somewhat related manner the FIG.  2  and FIG. 3 subterranean course alignment instrument is contemplated to include a self leveling apparatus, preferably of the servomechanism-operated and substantial accuracy type. Self leveling devices of this type are available as an off the shelf package in the commercial marketplace and may for example be obtained from The Fredericks Company of Huntingdon Valley, Pa. as part number 0717-2201 and from others. In the receptacle  104  or other in-the-field use locations the housing  206  may rest on the attached feet  312  which can be provided with coarse leveling capability in order to manually assist the above described self leveling apparatus. 
     Windows for transmission of the radiant energy from lasers  110  and  112  are provided at  306  and  308  in the FIG. 3 drawing. These windows may be made from glass or plastic or other suitable materials and are characterized by a need for spectral compatibility or having a reasonably efficient energy transmission window located at the wavelength of the relevant laser energy. These windows  306  and  308  are preferably arranged to be easily cleaned in view of their exposure to in-the-fleld conditions. Protective cup-like shields may be used to surround each window  306  and  308  to exclude soil particles and other earth receptacle-related debris. 
     Laser radiant energy communicated along the path  118  in FIG. 1 is directed at the surface of the mirror  130  where it is reflected to communicate along the path  132  to for example the surveyors rod  140 . The mirror  130  is also curved in configuration and in fact may be described as having a “convex beveled surface”. The mirror  130  is disposed in a rain hat-inclusive assembly identified generally at  108  and this assembly is mounted on the tripod  106  by way of the platform apparatus  134 . Located above the platform apparatus  134  is a second self leveling device of the type described above or a related type which serves to dispose the mirror  130  in vertical alignment with its input optical axis aligned with the output optical axis of the subterranean course alignment instrument concave mirror  116 . This second leveling device is shown in enlarged form at  404  and discussed in connection with FIG. 4 below. The output energy communicating along the FIG. 1 path  114  from the second laser  112  provides an optical reference for two axis horizontal alignment of the mirror  130  with the optical axis of this subterranean course alignment instrument concave mirror  116  as indicated previously. 
     The concave and convex mirrors  116  and  130  in the FIG. 1 apparatus are used to provide a desirable maximum degree of alignment criticality freedom for the “pseudo periscope” formed by the mirrors  116  and  130 . The nature and perhaps the extent of this alignment criticality freedom may be appreciated for example by considering the characteristics of the FIG. 1 apparatus obtained if the mirrors  116  and  130  were of a planar rather than the disclosed concave beveled surface and convex beveled surface types. With a planar mirror located at  116  for example it may be appreciated that angular misalignment between the laser  112  and the mirror  116 , i.e., selection of a non perpendicular radius between the laser  112  and the laser-intercepted horizontal chord of a planar mirror at  116 , would result in the laser beam path  118  being provided with an angular component tilting the path  118  into or out of the plane of the FIG. 1 drawing. Such tilting of the path  118  is however largely absent when the mirror is configured as the disclosed concave beveled surface. A similar advantage prevails with the convex beveled surface of mirror  130 . 
     As a further exploitation of this concave beveled surface advantage the curvature radius of the concave beveled surface of mirror  116  may be made equal to the distance between the mirror surface and the pivot axis  113  for angular positioning of the laser  112  i.e., the pivot point used by the motor driven screw  202  discussed in connection with the FIG.  2  and FIG. 3 drawings above. In addition it is desirable for the radius of curvature of the concave beveled surface mirror  116  and the convex beveled surface mirror  130  to be of equal magnitudes. Such equal radii are found to provide a degree of compensation for distortions introduced into the beam along path  118  for example as a result of the three dimensional curvature of the mirror  116 . Such distortions tend to be offset in the beam from mirror  130  communicating along path  132  to the surveyor&#39;s rod  140 . This distortion removal mechanisim does not of course preclude a tendency of the beam along path  132  to broaden or expand slightly as discussed above. This tendency is sufficiently small as to be acceptable with moderate values of the distance  152  between apparatus  100  and apparatus  102  in FIG.  1 . 
     FIG.  4  and FIG. 5 in the drawings show additional details of the mirror assembly  108  of the FIG. 1 apparatus  100 . FIG. 4 is a larger side view of this mirror assembly  108  and FIG. 5 a bottom view. As may be observed in the FIG.  4  and FIG. 5 drawings the mirror  130  is preferably shielded from sun, rain and physical abuse by an overhanging cap  400  which may be made of plastic or coated metal materials for examples. The cap  400  and reflector mirror  130  are mounted on the second leveling device  404  of the FIG. 1 apparatus which is in turn carried on the tripod  106 -supported platform assembly  134 . As implied in the FIG. 4 drawing the platform assembly  134  and leveling device  404  include centrodial apertures suitable for communicating the position-determining output beam  114  of the second laser  112  to a receptor and two axis vernier apparatus usable to dispose the mirror  130  directly over the mirror  116 . The vernier apparatus is not shown in FIG. 3 or FIG. 4 but may comprise a part of the platform assembly  134 , the leveling device  404  or the mirror/sun and rain cap elements shown in FIG.  4  and FIG.  5 . The FIG. 3 platform assembly  134 , leveling device  404  and mirror/sun and rain cap elements are preferably shaped as shown in FIG. 4 in order to avoid interference with the path  118  of the primary laser  110 . 
     Error Budget Considerations 
     Notwithstanding the concave-convex beveled surfaces distortion compensation tendency discussed above certain error producing mechanisms do remain possible in the FIG. 1 apparatus. Generally for an instrument of the FIG. 1 type to be usable for the described purposes it should provide overall alignment errors between marker  146  and determined pipeline course  150  that total less than one half milliradian or less than ±0.06 degrees. Another statement of the desired accuracy is that the realized error should be less than one part in ten thousand and preferably less than one part in two thousand. 
     One of the error mechanisms attending the FIG. 1 apparatus concerns for example the instrument accuracy obtained if the mirrors  116  and  130  are laterally displaced along the left to right horizontal direction in FIG. 1, e.g., the error resulting from imperfect mirror alignment accomplished with the laser  112 . If such misalignment occurs it may be appreciated that instrument error will arise because the curvature of the upper mirror  130  will cause the laser beam along path  132  to be deflected at some azimuth angle with respect to its desired location. Generally displacement of mirrors  116  and  130  along the left to right horizontal direction in FIG. 1, when using mirrors of 410 millimeters radius of curvature, results in a beam azimuth error of 2.39 milliradians per millimeter of mirror displacement. 
     Another error source possible with the FIG. 1 apparatus involves lateral displacement of the mirrors  116  and  130  along the fore and aft or into and out of the page direction in FIG.  1 . Generally displacement of mirrors  116  and  130  along this fore and aft horizontal direction in FIG. 1, when using mirrors of 410 millimeters radius of curvature, results in a beam displacement error of 0.125 milliradians per millimeter of mirror displacement. 
     Additional errors may involve tilting of either the roll or pitch variety in the laser  112  or the mirror assembly  108  in FIG.  1 . In this latter case with mirrors of 410 millimeters radius of curvature at  116  and  130  and a mirror separation of three meters (i.e., a depth near two meters for the receptacle  104 ) pitch error in the laser  112  results in beam deflection along the path  132  of two milliradians per milliradian of laser pitch. Roll tilting of the laser  112  or the mirror assembly  108  in FIG. 1 results in a beam error of 7 milliradians per milliradian of tilt. 
     Alternate arrangements of the present invention are possible while remaining within the spirit of the invention. These arrangements may include for example a disposition of the mirrors  116  and  130  in a common tube-like member to form a rigid periscope and assure accurate mirror alignment. Additionally the optical reflectors  116  and  130  may be replaced with holographic elements performing similar functions. Such holographic elements may for example have transmissive or reflective characteristics and be disposed to deflect at any desired angle. 
     The disclosed invention therefore is believed to provide a subterranean course alignment instrument of desirable accuracy, reliability and setup speed capabilities. 
     While the apparatus and method herein described constitute a preferred embodiment of the invention, it is to be understood that the invention is not limited to this precise form of apparatus or method and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

Summary:
A subterranean course-defining laser instrument and a system for initial calibration or setup of this instrument to provide accurate definition of a sub surface course for a pipeline or the like. Instrument setup is made in response to an above-ground course definition marker such as a surveyor&#39;s stake or surveyor&#39;s rod. A double mirror pseudo periscope arrangement is used to enable optical signal communication from the subterranean location of the laser instrument to the above ground marker with each of the mirrors being of a curved and error-minimizing nature. Details regarding two major components of the apparatus and quantitative consideration of achieved error budget are included.