Abstract:
A method and system for leveraging available navigation frequencies for determining position by creating a flexible navigation signal architecture is disclosed. Such an architecture is designed for customizing a navigation system by combining, adding, replacing or removing one or more removable RF section modules. At least one of the RF section modules has a mutually complementary frequency set with respect to a frequency set of one or more of the other RF section modules in the system.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   The present application is related to the concurrently filed application entitled, A NTENNA  C OMBINATION  T ECHNIQUE  F OR  M ULTI -F REQUENCY  R ECEPTION , U.S. application Ser. No. 11/499,544, filed on Aug. 4, 2006, by inventors, Paul Montgomery and David Lawrence, the entire contents of which are hereby incorporated by reference as if fully set forth herein. 
   TECHNICAL FIELD 
   The present invention is directed to precise local positioning systems, and more specifically to the design of multi-frequency Global Navigation Satellite System receivers. 
   BACKGROUND 
   Global positioning satellites are on track to offer new frequencies, such as unencrypted L2 and L5 signals for use in global positioning applications. Future Global Navigation Satellite System (GNSS) constellations are expected to offer more frequencies than are available from present day GPS constellations. For example, Galileo will offer signals at 1278.75 MHz and 1207.14 MHz. 
   In general, the more frequencies that a receiver is capable of tracking, the more complex the receiver design. It is more than likely that such receivers will be considerably more expensive than conventional receivers due to their manufacturing complexity and in part due to market forces, such as supply and demand. 
   Thus, in view of the above problems, there is a need for a method and system for providing multi-frequency capability in a positioning system in a cost effective manner. 
   SUMMARY OF THE INVENTION 
   According to one aspect of certain non-limiting embodiments, a plurality of RF section modules is provided in a system for determining position. The RF section modules provide signals that are combined for determining position. At least one of the plurality of RF section modules is designed for convenient removal from the system and has a mutually complementary frequency set with respect to a frequency set of one or more of the other RF section modules in the system. 
   According to another aspect of certain non-limiting embodiments, available navigation frequencies for determining position can be leveraged by creating a flexible navigation signal architecture. Such an architecture is designed for customizing a navigation system by combining, adding, replacing or removing one or more removable RF section modules. At least one of the RF section modules has a mutually complementary frequency set with respect to a frequency set of one or more of the other RF section modules in the system. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a high-level block diagram that illustrates an upgrade of a single frequency base station to a multi-frequency base station, according to certain embodiments. 
       FIG. 1B  is a high-level block diagram that illustrates an upgrade of a dual frequency base station to a three-frequency base station, according to certain embodiments. 
       FIG. 2  is a high-level block diagram that illustrates an upgrade of a dual frequency receiver of a rover that is installed on farm equipment to a three-frequency rover, according to certain embodiments. 
       FIG. 3  is a high-level block diagram that illustrates a systems architecture with simplified RF section modules in relation to components for sampling down-converted data, for correlation of sampled data and for processing of correlated data, according to certain embodiments. 
       FIG. 4  is a high-level block diagram that illustrates a systems architecture with one type of complex RF section module in relation to a high-level receiver navigation processor, according to certain embodiments. 
       FIG. 5  is a high-level block diagram that illustrates a systems architecture with another type of complex RF section module in relation to a high-level receiver navigation processor, according to certain embodiments. 
   

   DETAILED DESCRIPTION 
     FIG. 1A  is a high-level block diagram that illustrates an upgrade of a single frequency base station to a multi-frequency base station, according to certain embodiments.  FIG. 1A  shows a base station system  100   a  that can be upgraded to system  100   b . System  100   a  comprises a single frequency radio frequency receiver  102  with a single-frequency antenna  101 . Receiver  102  is serially connected to a radio modem  104 . The radio modem is used to broadcast differential correction data to rovers. 
   System  100   a  can be upgraded to a corresponding system  100   b  by adding another single frequency Radio Frequency (RF) section module  116  that is capable of receiving a frequency that is different from that of single frequency receiver  102 . In other words, RF section module  116  has a mutually complementary frequency with respect to RF section module  102 . 
   A first RF section module is said to have a mutually complementary set with respect to a second RF section module or receiver if the first RF section module can receive signals in at least one frequency that the second RF section module or receiver in the system is incapable of receiving and vice versa. According to certain embodiments, the complementary frequency set consists of one or more frequencies selected from a set of frequencies that can be provided by global positioning constellations, such as Galileo and GPS constellations. Such constellations are expected to offer more frequencies than are available at the present day. The embodiments are not limited to single frequency RF section modules. 
   Returning to  FIG. 1A , system  100   b  comprises single frequency RF section module  116  with a corresponding RF section, correlator and processor (not shown in  FIG. 1A ) and a single frequency antenna  114 . RF section module  116  is serially connected to single frequency receiver  102  and radio modem  104 . Single frequency RF section module  116  intercepts the serial connection between single frequency receiver  102  and radio modem  104 . The phase measurements from receiver  102  and single frequency receiver  116  are combined for determining a position solution. 
   According to certain embodiments, an RF section module is a simple device that comprises an RF section, as described in greater detail herein with reference to  FIG. 3 . According to certain other embodiments, an RF section module includes an RF section, an Analog-to-Digital converter (ND), and a correlator for correlating data. Such an RF section module is described in greater detail herein with reference to  FIG. 4 . According to certain embodiments, an RF section module includes an RF section, an Analog-to-Digital converter (ND), a correlator for correlating data, as well as a low-level central processing unit (CPU). Such an RF section module is described in greater detail herein with reference to  FIG. 5 . Further, an RF section module is a modular unit or a modular set of components that is designed to be easily removed from a system or added to a system, according to certain embodiments. The modular unit or modular set of components may or may not include a hardware enclosure for encapsulating the components of the RF section module. 
   With reference to  FIG. 1A , for purposes of explanation, assume that the single frequency receiver  102  is an L1-only receiver and that the single frequency RF section module  116  is an L5-only RF section. Single frequency RF section module  116  combines the L1 correction information from single frequency receiver  102  with L5 measurements to generate L1/L5 correction data, preferably in a standard L1/L5 correction message, according to certain non-limiting embodiments. The L1/L5 correction data is sent to radio modem  104 . Radio modem  104  can then broadcast the L1/L5 correction data to rovers in the system. 
   According to certain embodiments, a correction to account for the offset between the antenna phase center of antenna  101  and that of antenna  114  can be applied either by rovers or the base station. If the rovers are to apply the antenna offset correction, then the offset information between the antenna phase center of antennas is supplied to the rovers. 
   There are various non-limiting techniques for correcting antenna phase center offsets. Such non-limiting techniques may vary from implementation to implementation. For example, the antennas of the RF section modules can be mounted on a single ground plane, and where the ground plane is in a known orientation. As another example, an independent L1/L5 system is used to calibrate the antenna phase center offsets between antenna  101  and antenna  114  as a one-time set-up procedure. 
     FIG. 1B  is a high-level block diagram that illustrates an upgrade of a dual frequency base station to a three-frequency base station, according to certain embodiments. In  FIG. 1B , system  160   a  comprises a dual frequency antenna  161  with a dual frequency receiver  162  that is serially connected to a radio modem  166 . The radio modem is used to broadcast differential correction data to rovers. System  160   a  can be upgraded by adding either a single or dual frequency RF section module  180  with corresponding correlator and processor (not shown in  FIG. 1B ) and with antenna  178  that is capable of receiving a frequency that is different from at least one frequency of receiver  162 . RF section module  180  intercepts the serial connection between receiver  162  and radio modem  166 . 
   To illustrate, assume that dual frequency receiver  162  is an L1/L2 receiver and that RF section module  180  has an L1/L5 RF section. RF section module  180  combines the L1/L2 correction information from receiver  162  with L5 measurements to generate L1/L2/L5 correction data. The L1/L2/L5 correction data is sent to radio modem  166  for broadcast to rovers associated with the system. The offset between antenna  161  and antenna  178  can be calibrated by using traditional techniques based on the frequency common to both antennas, such as L1 in the foregoing example. 
   The embodiments are not limited to the frequencies described with reference to the figures herein. For example, in  FIG. 2 , the frequencies L1/L2 and L5 are chosen for purposes of explanation.  FIG. 2  is a high-level block diagram that illustrates an upgrade of a dual frequency receiver of a rover that is installed on farm equipment to a three-frequency rover, according to certain embodiments. 
   In  FIG. 2 , existing position-determining equipment  200   a , such as a farm tractor steering system, comprises a roof array that includes two L1/L2 antennas  204 , and an L1/L2 receiver  206 . Antennas  204  are on ground planes connected to receiver  206 . Equipment  200   a  can be upgraded to a three-frequency system  200   b  by adding an RF section module  208  to the array. RF section module  208  includes an RF section  212  and is connected to an L5-only antenna  210 . Antenna  210  is added in line with the existing antennas  204 . Phase outputs from the L5 RF section  212  are routed to a serial port on the existing L1/L2 receiver  206 . The firmware on L1/L2 receiver  206  is upgraded to merge the L5 signals from the RF section module  208  with the L1/L2 signals of receiver  206 . The offset between the L5 antenna and the L1/L2 antenna can be accounted for by using known attitude information associated with the system, as an example. 
     FIG. 3  is a high-level block diagram that illustrates a systems architecture with simplified RF section modules in relation to components for sampling down-converted data, for correlating sampled data and for processing of correlated data, according to certain embodiments.  FIG. 3  shows a position determination system  300  comprising RF section modules  306  and  308 , each of which are operatively connected to subsystem  310 . RF section modules  306  and  308 , each includes an RF section ( 306   b ,  306   b ) and a corresponding antenna ( 306   a ,  308   a ). Subsystem  310  includes components for sampling downconverted data received from the RF section modules, components for correlating the sampled data, and components for processing the correlated data, according to certain embodiments. The RF section modules of  FIG. 3  have mutually complementary frequency sets relative to each other. For example, RF section module  306  may be capable of receiving L1 signals and RF section module  308  may be capable of receiving L5 signals. For purposes of simplicity, only two RF section modules and one subsystem are shown in  FIG. 3 . The number of RF section modules and subsystems may vary from implementation to implementation. 
     FIG. 4  is a high-level block diagram that illustrates a systems architecture with one type of complex RF section module in relation to a high-level receiver navigation processor, according to certain embodiments.  FIG. 4  shows system  400  that comprises RF section modules  402  and  408 , each of which is connected to navigation processor  406 . RF section module  402  comprises an antenna  402   a , an RF section  402   b , an Analog-to-Digital converter (A/D)  402   c , and a correlator  402   d . Similarly, RF section  408  comprises an antenna  408   a , an RF section  408   b , an Analog-to-Digital converter (A/D)  408   c , and a correlator  408   d . The Analog-to-Digital converter (A/D) converts the signals received by the receiver to digital data that is passed to a correlator for correlating data. The correlator output is then passed to the processor, such as processor  406 , for processing of the correlated data. For purposes of simplicity, only two RF section modules and one processor are shown in  FIG. 4 . The number of RF section modules and processors may vary from implementation to implementation. 
     FIG. 5  is a high-level block diagram that illustrates a systems architecture with another type of complex RF section module in relation to a high-level receiver navigation processor, according to certain embodiments.  FIG. 5  shows system  500  that comprises RF section modules  504  and  506 , each of which is connected to high-level navigation processor  508 . RF section module  504  is connected to an antenna  504   a , and includes an RF section  504   b , an Analog-to-Digital converter (A/D)  504   c , a correlator  504   d , and a central processing unit  504   e . Similarly, RF section module  506  is connected to an antenna  506   a , and includes an RF section  506   b , an Analog-to-Digital converter (A/D)  506   c , a correlator  506   d , and a central processing unit  506   e . The Analog-to-Digital converter (A/D) converts the signals received the receiver to digital data that is passed to a correlator for correlating data. The correlator output is passed to the CPU, such as CPU  504   e , for processing of the correlated data to produce phase output data. The phase output data is passed to the navigation processor for further processing. For purposes of simplicity, only two RF section modules and one processor are shown in  FIG. 5 . The number of RF section modules and processors may vary from implementation to implementation. 
   Techniques for combining the phase information from complementary RF modules can be divided into two categories:
     1) Techniques that require all system components in need of the phase data to compensate for any non-traditional characteristics of the phase data from the modular design. Examples of such characteristics are rapid clock divergence and significantly non-collocated phase centers.   2) Techniques that attempt to create phase messages or differential corrections that are indistinguishable or that are functionally equivalent to messages output from a traditional multi-frequency receiver. Such phase messages, assuming that they adhere to legacy phase message standards, can be used in systems with hardware components running legacy firmware. Examples of phase message standards include open standards such as RTCM SC-104 and CMR, and proprietary phase messages used by different GNSS receiver manufacturers.   

   For the first category of techniques, any method that conveys the phase data or phase correction data along with any necessary other information such as the relative position of various antennas is adequate. The other system components can manipulate the phase data and other information after reception to compensate for any non-traditional characteristics of the modular design. For example, an L1/L5 RTK rover that is aware that the base station consists of an L1 receiver and antenna and an L5 receiver and antenna can receive L1-only phases from the L1-only receiver and L5-only phases from the L5-only receiver and can combine that information with the relative position between the antennas. 
   The second category of techniques may be required to compensate for antenna phase center offsets and/or clock divergence between RF modules before generating phase messages or differential corrections. In some cases, two mutually complementary RF modules may share an antenna that has frequency capabilities of the union of the RF modules (for example, an L1/L2 RF module and an L1/L5 RF module may share an L1/L2/L5 antenna). In such cases, no antenna phase center compensation is required. Similarly, sometimes two mutually complementary RF modules may share a common clock (one module may receive a clock signal from the other module, or they both receive clock signals from an external source). In such cases, no dynamic clock compensation between frequencies is required. In general, there are six cases to consider:
         1) Common clock, common antenna   2) Independent clocks, common antenna   3) Common clock, independent antennas with a shared frequency   4) Independent clock, independent antennas with a shared frequency   5) Common clock, independent antennas with no shared frequency   6) Independent clock, independent antennas with no shared frequency       

   For cases 3 through 6, the system needs to compensate for phase center offsets between the independent antennas before generating the phase messages or differential correction. If the antennas are static (for example, a permanent base station installation), the relative positions can be calibrated once and used thereafter to compensate for position offsets. Otherwise, a dynamic estimate of position needs to be derived. In cases 3 and 4, the relative position between the antennas can be calculated in real-time by using traditional carrier phase RTK processing techniques on the shared frequency. In cases 5 and 6, an independent estimate of the relative position may be used. For example, if the antennas are mounted on the same rigid body for which the orientation is known (based on inertial sensor, independent GPS attitude or other data), the relative position of the antennas can be calculated. Given the relative position between the antennas, the phases from one antenna can be projected to the other antenna location:
 
φ′=φ+ ê·Δ             

   Where: 
   φ′ is the projected phase. 
   φ is the original phase 
   ê is a unit line of sight vector towards the satellite
         Δ          is the relative position vector       

   Alternatively, the phases from each antenna can be projected to the same reference point using similar techniques. 
   For cases 2, 4, and 6, it may be necessary to compensate for the different clock offsets between RF modules. It should be noted that most RTK systems have some inherent robustness to slow common mode (affecting all satellites equally) carrier phase drift between frequencies. For example, an RTK system should be robust to the different rates of circular polarization windup observed as an antenna rotates. However, due to dynamic range considerations or other implementation-specific reasons, it is often undesirable to have a significant common-mode drift between frequencies. For example, the CMR phase correction standard expresses the L2 carrier phase as a scaled offset from the 1.1 code phase. If there is a significant drift between L1 and L2 phases, the offset will eventually exceed the dynamic range provided in the L2 offset field. Therefore, it is desirable to make at least a coarse correction for clock drift between RF modules. For case 2 (if the RF modules share a common frequency) and case 4 (after compensating for antenna offset), it is possible to measure the phase on a common frequency relative to each of the clocks to directly observe the clock drift. For case 2 (if the RF modules do not share a common frequency) and case 6, there is no common frequency to compare. However, if the phases from one frequency are scaled up or down to nominally match the phases from another frequency, any mismatch is mainly due clock drift and ionosphere drift between frequencies. In most cases, it is acceptable to ignore the ionosphere drift because it will not cause dynamic range problems. In such cases, one satellite or an average of several scaled satellite measurements can be used to estimate the clock drift between frequencies. If other considerations require a better estimate of clock drift than one corrupted by the ionosphere, an estimate of the ionosphere drift can be found several ways. For example, if there exists a dual frequency antenna connected to a dual frequency RF module in the system, traditional techniques can be used to calculate an ionosphere drift estimate that can be removed from the scaled phase measurements between different RF modules to provide an uncorrupted clock drift estimate. Alternatively, models of the ionosphere or wide area ionosphere corrections (such as from WAAS or Omnistar, for example) may be used to estimate the ionosphere. For case 1, no clock or antenna phase offset compensation is necessary. 
   In the foregoing specification, embodiments of the invention have been described with reference to numerous specific details that may vary from implementation to implementation. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. The invention is intended to be as broad as the appended claims, including all equivalents thereto.