Abstract:
A low cost instant Real-Time Kinematic (RTK) positioning system and method are disclosed. The system comprises at least the following elements: a base station and a rover unit, each equipped with a Satellite Positioning System (SATPS) receiver and a generally license-free radio link transceiver. Such system has the distinctive feature of having no carrier integer cycle ambiguity to solve, thus allowing low cost single frequency SATPS receivers to be used for instant centimetre level relative positioning.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    There are no cross-related applications. 
       FIELD OF THE INVENTION 
       [0002]    The present invention principally relates to surveying, but also extends, and does not limit, to high-precision navigation, searching, marking and distance measuring using satellite navigational equipment. 
       BACKGROUND OF THE INVENTION 
       [0003]    A Satellite Positioning System (hereinafter “SATPS”) such as, but not limited to, the Global Navigation System (GPS), the Global Navigation Satellite System (GLONASS) and the yet to be deployed Galileo system in Europe and Compass system in China, generally allows civilian users world-wide to position themselves free of charge. Those civilian users usually benefit of an absolute positioning precision of about 2 to 20 meters using SATPS. It is also possible to achieve decimetre to meter levels of precision by relative positioning techniques, better known as Differential GPS (DGPS). 
         [0004]    Both absolute and relative positioning techniques rely on the measurement of the ranging codes transmitted in the SATPS satellites signals. Those codes usually have a wavelength of tens of meters to hundred of meters, resulting in relatively coarse measurements. Instead of using the code phase, it is however possible to precisely measure the signals carrier phase. Because the carrier wavelength is much smaller than the code wavelength (about 19 centimetres for the GPS L1 carrier, which frequency is 1545.75 MHz), centimetre precision can thus be achieved by carrier phase-based relative positioning. 
         [0005]    Carrier phase-based relative positioning in real-time is also known as “Real-Time Kinematic satellite navigation” (hereinafter “RTK”). Because it operates in real-time, RTK requires a radio frequency transmitter and receiver to send measurements from a base station SATPS receiver (a ground fixed receiver used as a reference point) to a rover unit (the position measurement unit itself). RTK has been used for many years for surveying but still suffers from many problems which prevent it from being used in consumer devices. 
         [0006]    The first and main problem of RTK is that integer carrier cycle ambiguities have to be solved. In opposition to the ranging codes, it is actually impossible to distinguish between one carrier cycle and another. Thus, it is necessary to test multiple integer carrier cycle combinations before obtaining a centimetre level position fix, which usually takes several minutes with low-cost equipment. For this reason, new methods have been developed in order to accelerate the integer ambiguity solving process. However, such methods often rely on expensive high-precision, multiple frequencies, receivers that are unaffordable to most consumers. 
         [0007]    The second problem of RTK is that bulky and heavy equipment has to be carried out. The equipment is also complex as every RTK unit usually has separate radio transceivers, SATPS receivers, antennas, handheld user interfaces and battery packs, plus a tripod or a survey pole. For those reasons, RTK is generally restrained to trained professionals. Lighter and smaller equipment would thus simplify the use of RTK. 
         [0008]    The third problem of RTK is that powerful radio transmitters are usually used to transmit data from a base to a rover unit. Thus, specific frequencies must be used, requiring a special permit to operate RTK equipment. However, such expensive radios might not be necessary for short baselines (that is the distance vector from the base to the rover). Cheaper and less powerful radios operating at open frequency ranges would help to reduce the price and the size of RTK receivers. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention describes a low cost instant RTK system and method to solve the above problems. The system generally consists of a base station and a rover unit, both incorporating a preferably low cost SATPS receiver and a preferably low cost, low power and license-free radio transmitter and/or receiver in order to reduce the overall price and weight. This also means smaller components, which could help in the integration of the base station and the rover unit into smaller and more compact devices. 
         [0010]    The disclosed invention generally targets short baseline measurements. Short baseline measurements are usually measurements of varying distances which vary according to the conditions in which the system is deployed (e.g. open rural area vs. dense urban area). Typically, but not exclusively, short baseline measurements vary between ˜0 and ˜2 km. The targeting of short baseline measurements has allowed the development of a new method to instantly remove the carrier cycle ambiguities. This method consists of bringing into close proximity the base and rover SATPS antennas on start-up. By placing the center of phase of the antennas close enough from one to the other (closer than one carrier wavelength), then no integer ambiguity exists. Therefore, there is no need of using complex ambiguity solving algorithms or high precision, multi-frequencies, SATPS receivers. Low cost single frequency receivers could be directly used instead. 
         [0011]    Another method also includes backup points in order to remove the integer ambiguities once the rover is away from the base. Those backup points, previously stored by the rover, allow the RTK algorithm to instantly resume in case of SATPS signal outage or cycle slips. Therefore, there is no need to go back to the base every time the signals are lost or corrupted. 
         [0012]    The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The above and other objects, features and advantages of the invention will become more readily apparent from the following description, reference being made to the accompanying drawings in which: 
           [0014]      FIG. 1  is a simplified block diagram illustrating one embodiment of elements of the low cost RTK system in accordance with the present invention. 
           [0015]      FIG. 2  is a perspective view of one embodiment of the base station mounted onto a tripod and one embodiment of the rover unit placed in close proximity to that base in accordance with the present invention. 
           [0016]      FIG. 3   a  is a simplified illustration of one embodiment of the rover unit located at a backup point in accordance with the present invention and  FIG. 3   b  illustrates an alternate embodiment of the rover unit located at a backup point in accordance with the present invention. 
           [0017]      FIG. 4  is a simplified flow diagram illustrating the proximity initialization process to be performed with respect to one embodiment of the low cost RTK system in accordance with the present invention. 
           [0018]      FIG. 5  is a simplified flow diagram illustrating the remote initialization process to be performed with respect to one embodiment of the low cost RTK system in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]    Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well known methods, procedures, components and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention. 
         [0020]      FIG. 1  depicts an exemplary embodiment of a low cost RTK system in accordance with the present invention. The system  10  includes a base station  20 , a rover unit  40  and a radio link  60  between the base station and the rover unit. 
         [0021]    The base  20  is generally located at a fixed position. The rover  40  is a moving device. The complete system  10  is designed to provide position fixes: (1) in real time, almost instantly, and (2) very precisely, that is with centimetre accuracy, after integer ambiguity is removed. Those position fixes represent the measurement of the baseline that is the distance vector from the base  20  to the rover  40 . 
         [0022]    Both the base  20  and the rover  40  include a SATPS receiver  22  and  42  respectively along with a SATPS antenna  23  and  43  respectively. Both base and rover SATPS antenna and receivers are designed to receive the satellite signals  80  emanating from a Satellite Positioning System (SATPS)  100 . 
         [0023]    The base  20  and rover  40  also include a radio transmitter  24  and receiver  44  respectively, along with a radio antenna  25  and  45  respectively. The main purpose of those radio transmitter, receiver and antennas is to transfer, in real time, the measurements made by the base SATPS receiver  22  to the rover unit  40 . One has to note that such data transfer task from the base  20  to the rover  40  could also be accomplished by means other than radio communications. For example, optical (e.g. laser), Infra-Red (IR) or sonic communication devices could be used to transfer the base SATPS receiver measurements to the rover; the present invention is not so limited. 
         [0024]    The base  20  has a system controller  26 , which main purpose is to relay the measurements from the base SATPS receiver  22  to the base radio transmitter  24 . Another task of this controller is to verify the close proximity of the rover unit  40  to the base. This is typically achieved by using a proximity sensor  28 . The proximity sensor  28  can be of various types: magnetic, IR, sonic, radio, optical or contact sensitive. The proximity sensing could also be directly done via an input from an operator or a user. The latter case will be further referred to as proximity detection means based on an operator or a user intervention (e.g. indication of proximity via an actuator or a voice command). If rover proximity is detected, the proximity sensor (or actuator or voice command device) sends a signal to the base system controller  26 , which in turn transmits the base radio channel number (a radio channel specifically used by the base radio transmitter  24 ) to the rover, preferably, but not exclusively, with the use of an Infra-Red (IR) transmitter  30 . One has to note that every single base station  20  has a different radio channel number, thus allowing multiple base stations to operate in a same area without interfering with each other. 
         [0025]    If the rover  40  is close enough from the base  20 , it receives the base radio channel number through an IR communication link  70 , thanks to an IR receiver  50 . This IR receiver directly sends the base radio channel number to the rover system controller and navigation computer  46 . The controller and navigation computer then tunes the rover radio receiver  44  to the right channel in order to receive the base SATPS receiver  22  measurements. Because the base  20  and the rover  40  must be in close proximity at this moment, no integer ambiguity exists (explained below) and the rover system controller and navigation computer can immediately start computing a RTK navigation solution. This RTK navigation solution can be directly stored in a data storage device  52  or transferred, for instance to a computer, through Input/Output (I/O) ports  54 . The RTK navigation solution could also be examined and manipulated in real time by a user, thanks to an appropriate user interface  56 . 
         [0026]    The rover unit  40  can also incorporate a dead-reckoning (DR) unit  58 . This DR unit has the purpose of increasing the precision of the RTK navigation solution as well as increasing its robustness. 
         [0027]    One has to note that the IR transmitter  30 , the IR receiver  50  and the IR communication link  70  stated above have been chosen with the sole purpose of explaining the present invention. Therefore, those transmitter, receiver and communication link could also be radio (preferably license-free), optical or sonic transmitters, receivers and communication links; the present invention is not so limited. 
         [0028]      FIG. 2  shows a perspective view of the base  20  and rover  40 . The base is represented on a tripod  200  and the rover is represented as a handheld device. The base and rover antennas  23  and  43  respectively are represented as small enclosed patch antennas. Other forms of antennas such as, but not limited to, helical antennas, could also be used. 
         [0029]    As  FIG. 2  suggests, the base  20  and rover  40  are held in close proximity. As explained above, the base detects the rover by using its proximity sensor  28 . The base then sends its channel number using an IR transmitter  30 . This channel number is received by the rover thanks to an IR receiver  50 . Traditionally, this process would be followed by the execution of an algorithm in order to solve the integer carrier cycle ambiguity. However, the present invention is designed so that the base and rover SATPS antennas  23  and  43  center of phase are spaced apart  250  by less than a SATPS signal carrier wavelength at that moment. In that particular case, no integer ambiguity exists. It is thus possible to proceed directly with a RTK solution without having to solve the ambiguities. 
         [0030]    According to the second edition of “Understanding GPS: Principles and Application” by E. D. Kaplan, published by Artech House in 2006, the single difference (SD) observation equation for a single measurement of SATPS satellite p is: 
         [0000]        SD   p =φ p   +N   p   +S   p   +fτ   (1) 
         [0031]    where φ p  is the satellite p carrier phase measurement difference between the base and the rover, N p  is the SD integer ambiguity of satellite p, S p  is the phase noise of satellite p due to all sources (e.g., receivers, multipaths),f is the carrier frequency and τ is the clock bias between the base and the rover. 
         [0032]    Because the base and rover SATPS receivers are running on two different clocks, it is difficult to anticipate the clock bias τ. For this reason, it is preferable to compute the double differences (DD). According once again to the second edition of “Understanding GPS: Principles and Application” by E. D. Kaplan, the DD observation equation for a single measurement of SATPS satellites p and q is: 
         [0000]        DD   pq =φ pq   +N   pq   +S   pq   (2) 
         [0033]    where φ pq =φ p −φ q , N p  is the DD integer ambiguity of satellites p and q and S pq  is the DD phase noise of satellites p and q due to all sources. 
         [0034]    By placing the center of phase of the base and rover SATPS antennas in close proximity (this is closer than one SATPS signal carrier wavelength), we can suppose a near-zero baseline, thus DD pq ≈0. It is then possible to directly remove the integer ambiguity by computing (the noise term is dropped to simplify the expression): 
         [0000]        N   pq   =FIX (−φ pq )  (3) 
         [0035]    where FIX is an operator that rounds to the nearest integer toward zero. 
         [0036]      FIG. 3   a  shows an embodiment of the rover unit  40  located over a measurement point  320 . A weight  340  is attached to the rover by a chain, a cable, a cord or a piece of string  360  in order to precisely indicate the location of the measurement point  320 . 
         [0037]      FIG. 3   b  shows another embodiment of the rover unit  40  located over a measurement point  320 . A pole  380  is attached to the rover in order to precisely indicate the location of the measurement point  320 . 
         [0038]    If the integer ambiguity is removed, it is then possible for the rover  40  to store the measurement point  320  coordinates with centimetre accuracy. Suppose that the SATPS signals were lost, corrupted, or that carrier cycles slips could not be repaired, integer ambiguity would have to be removed once again. By previously storing a backup point, that is a measurement point  320 , one could directly go back to that backup point to remotely and instantly remove the integer ambiguity. Therefore, this prevents the necessity to go back to the base every time a SATPS signal problem occurs. One could also directly measure, by using for example a laser or sonic range finder and a compass, a backup point coordinates relative to the base station and thus remotely and instantly remove the integer ambiguity from this newly measured backup point. 
         [0039]    According to the second edition of “Understanding GPS: Principles and Application” by E. D. Kaplan, published by Artech House in 2006, the DD computation equation for a single measurement of 4 different SATPS satellites is: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0040]    where DD cppq  is the carrier phase measurements double difference of satellites p and q (previously referred to as φ pq ), e pqx , e pqy  and e pqz  are the line of sight differences between satellites p and q on all three axis, that is x, y and z, b x , b y  and b z  are the baseline vector components on all three axis, N pq  are the double differences integer ambiguity and λ is the SATPS signal carrier wavelength. 
         [0041]    By moving back the rover to a backup point, one knows precisely the baseline vector components as they were previously stored by the rover or precisely measured at that moment. Moreover, the line of sight matrix can be computed from one of the SATPS receivers coarse position fixes. Therefore it is possible to remotely remove the integer ambiguity by manipulating equation (4): 
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         [0042]    Equation (5) can finally be expressed as a matrix equation: 
         [0000]        N=FIX {( DD   cp   −E·B )λ −1 }  (6) 
         [0043]    where N is the integer ambiguity vector, DD cp  is the carrier phase measurements double difference vector, E is the line of sight matrix, B is the baseline vector, X is the SATPS signal carrier wavelength and FIX is an operator that rounds each elements of a vector to the nearest integer toward zero. 
         [0044]      FIG. 4  is a simplified flow diagram. It summarizes the proximity initialization process, which is the process explained above to remove the integer ambiguity by placing in close proximity the base and the rover. 
         [0045]    The proximity initialization process begins by bringing the base and the rover into close proximity  400 . If the base proximity sensor detects the rover, the process may continue, otherwise previous step must be retried  410 . Afterward, the base sends its radio channel number through the IR communication link  420 . The rover then tunes to the correct radio channel and picks up the base SATPS receiver measurements from the radio link  430 . This allows the rover to instantly remove the integer ambiguity according to a zero baseline  440  as explained above. Finally, the rover can start computing a RTK solution  450 , which can be further processed, stored or displayed in real time by means of a user interface. 
         [0046]      FIG. 5  is also a simplified flow diagram. It summarizes the remote initialization process, which is the process explained above to remove the integer ambiguity by moving the rover to a backup point. 
         [0047]    The remote initialization process begins by moving the rover to a backup point  500 . If the rover already knows the base radio channel number, which mean that a proximity initialization has been already performed, and that the base radio signal is detected and is in range, then the process may continue  510 . Otherwise, the rover must be moved again in order to detect the base signal, or a proximity initialization must be performed  520 . If the process is allowed to continue, the rover then picks up the base SATPS receiver measurements from the radio link  530 . This allows the rover to instantly remove the integer ambiguity according to the baseline vector at backup point  540  as explained above. Finally, the rover can start computing a RTK solution  550 , which can be further processed, stored or displayed in real-time by mean of a user interface. 
         [0048]    Because the present invention can achieve instant centimetre precision without the need for complex signal processing and integer ambiguities resolving, low cost, single frequency, SATPS receivers can be used. Because the present invention also targets short baseline measurements, that is, for example, measurements of distance in the order of 2 km or less depending on the type of area (e.g. urban, rural, etc.) in which the system is deployed, the radio transmitter and receiver can as well be chosen to be low cost and low power. For convenience, such radio transmitter and receiver can also be chosen to operate on license-free frequency bands. This means important cost reductions of the present invention compared to the prior art. It also means weight, size and complexity reduction. 
         [0049]    While illustrative and presently preferred embodiments of the invention have been described in detail hereinabove, it is to be understood that the inventive concepts may be otherwise variously embodied and employed and that the appended claims are intended to be construed to include such variations except insofar as limited by the prior art.