Abstract:
In one aspect, the invention is the system for assessing a navigation augmentation environment. The system includes a reference station for receiving messages from a satellite. The reference station has a known position. The system also includes a simulator for simulating an augmentation system by determining corrections based on the messages received and the known position of the reference station. The system further includes a monitoring system to render the corrections to a user in a form enabling determination of the usability of the augmentation system in the navigation augmentation environment.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims priority to provisional application Ser. No. 60/636,778, entitled “SYSTEM AND METHOD FOR DETERMINING AUGMENTATION SYSTEMS USABILITY WITHIN GEOGRAPHICAL REGIONS OF THE EARTH”, filed Dec. 16, 2004, which is incorporated herein in its entirety. 
     
    
     TECHNICAL FIELD  
       [0002]     The invention relates to navigation systems, and more particularly, to global navigation satellite systems.  
       BACKGROUND  
       [0003]     A Global Navigation Satellite System (GNSS) transmits ranging signals, which are used by land, sea and air users to determine a three-dimensional position, a velocity and a time of day. The GNSS includes a network of satellites to transmit the ranging signals. One example of a GNSS is a Global Positioning System (GPS). The GPS includes 24 satellites, called GPS satellites, which operate in medium earth orbit.  
         [0004]     The GNSS may be accurate to no more than 10 meters due to errors such as from ephemeris data, satellite clocks, ionosphere, troposphere, multi-path and receiver noise. Thus, the GNSS by itself may not be accurate enough for certain users that require better accuracy, for example, aircraft. Other systems called augmentation systems adjust for these errors and broadcast them to users. A typical augmentation system for a GPS includes one or more reference stations having GPS receivers that receive GPS messages (i.e., ranging signals) from the GPS satellites. The precise locations of the reference stations are known. A master control station receives the GPS data from the reference stations and determines for each GPS satellite the bias based on the reference station&#39;s known location and the GPS satellite&#39;s determination of the reference station&#39;s position. The master control station determines a range error for each GPS satellite, and broadcasts the integrity data and corrections, which will eventually be received by a user.  
         [0005]     One known augmentation system is the satellite-based augmentation system (SBAS). In the SBAS system, the master control station sends integrity data and corrections to geosynchronous earth orbit (GEO) satellites for broadcast to the users on the same frequency and in a similar format to GPS satellites. This method allows both SBAS GEO signals and GPS signals to be received via the user&#39;s GPS receiver, and has the added benefit of providing additional satellite ranging sources to improve availability. Wide-area coverage is only limited by the footprint of the SBAS GEO satellite (which spans approximately ±76 degrees in longitude and latitude) and the quantity and location of the SBAS reference stations within the GEO footprint (i.e., range). Thus, a single SBAS may be used to service a nation or a continent.  
         [0006]     Another known augmentation system is the ground-based augmentation system (GBAS). A typical GBAS includes a series of reference stations having GPS receivers located around a local area (e.g., 20 or 30 miles). The GPS receivers forward the GPS data to the master control station, which processes the GPS data and broadcasts the information using a very high frequency (VHF) data broadcast to an aircraft. The GBAS can provide local area augmentation with integrity data and corrections for GPS satellites to users in the immediate vicinity of an airport via a VHF data link. Since the GBAS is utilized in a local area, a much higher degree of fidelity may be placed on the accuracy and integrity information provided to the aircraft for navigation, and a much more rapid response to fault conditions (i.e., time-to-alert) can be achieved.  
         [0007]     A further known augmentation system that does not use a reference station is the air-based augmentation system (ABAS). The ABAS typically uses the GPS signals from GPS satellites and user avionics to augment the GPS signal. The ABAS uses a Receiver Autonomous Integrity Monitoring (RAIM) to perform integrity monitoring. Since the RAIM is based on comparing the results of multiple combinations of GPS satellites, the ABAS requires more GPS satellites in view than the minimum four required for basic position fixing. Hence, the ABAS gains its additional integrity at the expense of reduced availability. The ABAS may also utilize an airborne inertial navigation system (INS) to improve availability and continuity.  
       SUMMARY  
       [0008]     When deciding to implement an augmentation system, a user is not able to determine whether the augmentation system will be usable in a particular geographic environment. The augmentation system is expensive and the errors introduced within a certain geographic region are difficult to determine. Even after purchasing and installing the augmentation system, the user may not be able to determine that the installed augmentation system performs as expected.  
         [0009]     Described herein is a novel way for the user to determine the usability of an augmentation system within a geographic area before purchasing the augmentation system. Once installed, the user may determine whether the augmentation system purchased performs as expected.  
         [0010]     In one aspect, the invention is a system for assessing a navigation augmentation environment. The system includes a reference station for receiving messages from a satellite. The reference station has a known position. The system also includes a simulator for simulating an augmentation system by determining corrections based on the messages received and the known position of the reference station, and a monitoring system to render the corrections to a user in a form enabling determination of the usability of the augmentation system in the navigation augmentation environment.  
         [0011]     In another aspect, the invention is a method for assessing a navigation augmentation environment. The method includes receiving messages from a satellite from a reference station having a known position. The method also includes simulating an augmentation system by determining corrections based on the messages received and the known position of the reference station, and rendering the corrections to a user in a form enabling determination of the usability of the augmentation system in the navigation augmentation environment.  
         [0012]     One or more of the aspects above may have one or more of the following features.  13 . One feature includes simulating the augmentation system including simulating a certified augmentation system using software having the same functionality as software from a certified augmentation system. Receiving satellite messages from a reference station may include receiving satellite messages from a mobile reference station. Receiving may also include receiving the messages from different sources using different formats and configuring the messages into a common format. Receiving may further include filtering messages from one or more different sources. Other features include storing the messages received and sending stored messages or live messages to the simulator.  
         [0013]     Further features include the reference station as a first reference station and receiving corrections from a host satellite-based augmentation system (SBAS), receiving messages from a second reference station associated with the host SBAS and rendering data for determining the usability of the second reference station. Still further features include receiving corrections from a host ground-based augmentation system (GBAS), receiving messages from a third reference station associated with the host GBAS and being connected to the receive manager and rendering data for determining the usability of the third reference station.  
         [0014]     Other features include sending corrections from the host SBAS and the host GBAS to an aircraft. The one or more different sources may be from the first reference station, the second reference station, the third reference station, the host GBAS and the host SBAS. Still other features include receiving corrections from a host GBAS, receiving messages from a second reference station associated with the host GBAS and rendering data for determining the usability of the second reference station.  
         [0015]     One or more of the aspects above may have one or more of the following advantages. The system provides a cost effective means for defining and evaluating augmentation architecture tradeoffs for GNSS service availability in any given area or region of interest. The system can be used to independently evaluate various combinations of stand-alone and expanded augmentation systems without impacting operations of host augmentation systems. The utility of the system does not end once an operational system is deployed. The system can continue to be used for training, terminal and approach/departure procedure development, and evaluations of new augmentation systems. The advantages listed are not intended to include each and every advantage. Other advantages will be apparent to one skilled in the art in light of the claims, drawings and description. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a diagrammatic view of a navigation environment.  
         [0017]      FIG. 2  is a diagrammatic view of a navigation system.  
         [0018]      FIG. 3  is a diagrammatic view of another embodiment of the system.  
         [0019]      FIG. 4  is a diagrammatic view of the system with a host SBAS.  
         [0020]      FIG. 5  is a diagrammatic view of a data flow in a receive data manager.  
         [0021]      FIG. 6  is a diagrammatic view of a data flow in a simulator.  
         [0022]      FIG. 7  is a diagrammatic view of the system with a host SBAS and a host GBAS.  
         [0023]      FIG. 8  is a diagrammatic view of the system with a flight test kit. 
     
    
     DETAILED DESCRIPTION  
       [0024]     Referring to  FIG. 1 , a navigation environment  10  includes Global Positioning System (GPS) satellites (e.g., GPS satellite  12   a  and GPS satellite  12   b ) and a geo-synchronous earth orbit (GEO) satellite  14 , which are in view (i.e., within range or line of sight) of a navigation system  20 . The system  20  is used to determine whether the navigation environment at a particular location is suitable for a satellite-based augmentation system (SBAS). For example, the system  20  determines, based on receiving GPS messages (e.g., ranging signals) from the GPS satellites  12   a  and  12   b , whether an SBAS may be installed at the particular location in order to determine corrections. As will be disclosed below, the system  20  may also be used to determine a preferred positioning of a reference station.  
         [0025]     Even though the system described herein initially describes a system for determining the usability of installing an SBAS into a particular geographic region, the system  20  may be used to test the usability of other augmentation systems. As will be described herein, the system  20  may be used to integrate other different or similar augmentation systems (e.g., air-based augmentation system (ABAS), ground-based augmentation system (GBAS)). The system described herein is in a GPS; however, any GNSS may be used.  
         [0026]     Referring to  FIG. 2 , navigation system  20  includes a navigation system reference station (NSRS)  22  and an evaluation system  24 . The NSRS  22  includes an omni-directional antenna  32  for receiving GPS messages (i.e., ranging signals) from GPS satellites  12   a  and  12   b  ( FIG. 1 ), a receiver  34  for decoding the GPS messages from a radio frequency (RF) message into a digital message, a processor  36  for filtering the digital message and a frequency clock  30  connected to the receiver  34  for synchronizing with the GPS satellites  12   a  and  12   b.    
         [0027]     In one particular embodiment, the NSRS  22  is a mobile reference station and may be set-up and operational in about two hours. The NSRS  22  also may be moved to different sites to determine which site is the preferred site in an augmentation system for a permanent reference station to be located and allows multiple sites to be evaluated in a relatively short time period without permanent installation delays and cost. The mobility of the NSRS  22  also allows for a quick response to multi-path errors when detected by easily moving the NSRS  22  until a position where minimal multi-path error is found. There are no facility requirements for the NSRS  22  so that the NSRS  22  may be quickly installed in an open area or on a facility. An installation of the NSRS  22  may be limited by the satellite visibility of antenna  32  and local multi-path factors.  
         [0028]     The evaluation system  24  includes a receive data manager (RDM)  42 , a simulator  44  and a monitoring system  46 . The RDM  42  receives the GPS messages from the NSRS  22 . The RDM  42  may also collect messages from multiple different sources, each having their own protocol. The RDM  42  converts the messages with the multiple protocols or message formats into a common format for use by the simulator  44 .  
         [0029]     The RDM  42  is configured to record the GPS messages and to playback stored GPS messages to the simulator  44 . This enables a user of system  20  to search for a particular event and play back the particular event in real time for analysis. The RDM  42  may also send to the simulator  44  stored GPS messages or live GPS messages.  
         [0030]     The simulator  44  simulates a certified SBAS by executing similar software that a certified SBAS would use. A certified SBAS system means that the SBAS software and hardware have been certified by an independent entity (e.g., Federal Aviation Administration (FAA)). However, the software executed by the simulator  44  is not certified for use with the hardware in system  20 , but executing the simulator software provides an accurate representation of what certified software behavior would be when it is running in the navigation environment. The simulator software contains all the common filters and all of the specifics associated with monitoring the GPS signals including adding integrity to the GPS signals. The simulator  44  produces the corrections for the ranging errors associated with each of the satellites that are being monitored. The ranging errors include but are not limited to horizontal and vertical positioning errors.  
         [0031]     The monitoring system  46  includes a graphical user interface (GUI)  52  and a display  54 . Using the GUI  52 , the user may select a presentation of the simulator output in human interpretable format, which is rendered on the display  54 . For example, the presentation may be Stanford integrity plots. The presentation may also be other plots, which graphically illustrate the usability of the augmentation system (e.g., plots depicting horizontal and vertical errors). In another example, monitoring system  46  renders graphs, which depict a percentage of the time a signal is broadcasting error messages.  
         [0032]     In other embodiments, additional reference stations may be added so that all the reference stations may be monitored in real-time.  
         [0033]     Referring to  FIG. 3 , in another embodiment, the system  120  includes a NSRS  122 , an evaluation system  124  and an SBAS  140 . The evaluation system  124  includes a receive data manager  142 , a simulator  144  and a monitoring system  146 . In this embodiment, the evaluation system  124  may receive SBAS data (e.g., corrections) directly from the SBAS  140 . The SBAS data may be received from a GEO satellite (not shown) or through a data link  160  from an SBAS ground station (not shown) within SBAS  140 .  
         [0034]     The RDM  142 , in addition to stop and playback capabilities, has the capability to filter-out specific sources in the output data stream. The sources may be one or more reference stations (e.g., NSRS  122 ) or data from an augmentation system (e.g., SBAS  140 ). For example, if an operator suspects a problem with a particular reference station, the operator can filter-out the particular reference station during an analysis to determine if the reference station was the actual cause of the problem.  
         [0035]     The monitoring system  146  renders real time displays of service availability contours and related metrics such as horizontal protection limits (HPL)/vertical protection limits (VPL) integrity metrics.  
         [0036]     Referring to  FIG. 4 , in another embodiment, the system  220  includes an evaluation system  224 , a NSRS  222 , SBAS secondary reference stations (e.g., SBAS secondary reference station  252   a  and SBAS secondary reference station  252   b ) and a host SBAS  254 . The evaluation system  224  includes a monitoring system  246  and a shelter  228 . The monitoring system  246  may be located separately from the shelter  228  and may be connected to the shelter  228  by a data line  250  (e.g., a T1line). The monitoring system  246  may be located inside a facility where technical personnel would conduct an evaluation of the usability of an augmentation system. The shelter  228  includes a RDM  242  and a simulator  244 .  
         [0037]     The local reference station  222  directly supporting the monitoring system  246  may be located with the shelter  228  to minimize communication line dependencies. In one embodiment, the shelter  228  is portable and environmentally controlled structure, which may be placed in any convenient location with power and adequate communication support.  
         [0038]     The system  220  may be used to determine the benefits of adding SBAS secondary reference stations  252   a  and  252   b  to the host SBAS  254 , which have a GEO footprint covering the navigation environment. This capability allows optimization and trade-off analysis to be conducted for determining the quantities and locations of SBAS secondary reference stations  252   a  and  252   b  within the navigation environment in a non-intrusive manner with the host SBAS  254 . For example, system  220  may be used to determine the best sites to position additional SBAS reference stations while not interrupting operations conducted at the host SBAS  254 . In one embodiment, the shelter  228  is capable of hosting up to as many as forty SBAS secondary reference stations. In one embodiment, the RDM  242  has the ability to selectively filter out one or more of the secondary SBAS reference station  252   a  and  252   b.    
         [0039]      FIG. 5  is a data flow for software processes and data storage for the RDM  242 . The processes and the data storage for the RDM  242  include an adaptation data store  256 , a read SBAS process  258 , a read NSRS process  260 , a raw NSRS data message store  262 , a GUI process  264 , a controller process  266 , a converter process  268 , an output SIM process  270 , a scheduler process  272 , a raw reference station (RS) data message store  274  and a RS data queue  284 .  
         [0040]     The adaptation data store  256  includes configurable parameters. One configurable parameter is a node database parameter. The node database parameter includes a list of node names, IDs, and active status that match the names and IDs of the simulator  244 . The node database parameter is used for start-up filtering configuration. Another configurable parameter is a recording files parameter which enables recording of raw NSRS data and RS data with converted NSRS data. A further configurable parameter is an archive period parameter, which is a period of time recording occurs before starting a new recording session and archiving the previous session to mass storage. Other configurable parameters include a recording filters parameter which is a list of node IDs to filter from recording files; a “NSRS receiver IDs to RS ID Mappings” parameter used when converting NSRS data which maps the NSRS node IDs to the RS node IDs used by the simulator  244 ; and a channel addressing parameter for channel addresses used to read the data from the Evaluation System local area network.  
         [0041]     The read SBAS process  258  reads the SBAS messages from the network based on the adapted channel addresses, filters out the adapted/user controlled messages and sends the data to the converter process  268 . The read SBAS process  258  includes read SBAS adaptation data which includes the adapted channel addresses used to establish communication and the adapted/user controlled filters based on the node IDs. The read SBAS process  258  also includes RS data messages which include SBAS messages from a fielded SBAS system. The read SBAS process  258  further includes RS data which includes RS data read from an RS channel (e.g., data line  250 ) which is sent to the controller process  266  for processing.  
         [0042]     The read NSRS process  260  reads NSRS data messages from the NSRS  222  based on the adaptable channel addresses and associates the adaptable simulator node IDs with the NSRS receiver IDs. If recording is enabled, then the read NSRS process  260  generates a raw NSRS data message store  262  to store the messages. A read NSRS adaptation data includes the channel addresses used to establish communication and adapted NSRS receiver ID to node IDs mapping. The NSRS data messages include NSRS data sent to the controller process  266 .  
         [0043]     The GUI process  264  allows the user to shutdown the RDM  242  and to change the filters associated with the data being sent to the simulator  244 . At startup, the filters rendered by the GUI process  264  are initially populated with the filters from the adaptation data store  256  read by the controller process  266 . As the filters are modified by the user, filter data is sent back to the controller process  66  to be distributed to the processes using the filters.  
         [0044]     The controller process  266  starts the RDM  242  and routes the data amongst all the processes. The controller process  266  reads SBAS adaptation data which includes the adapted channel addresses used to establish communication and the adapted/user controlled filters based on the node IDs. The data is taken from the adaptation data and sent to the read SBAS process  258 . The controller process  266  reads NSRS adaptation data which includes the channel addresses used to establish communication and adapted NSRS receiver ID to node IDs mapping. The data is taken from the adaptation data and sent to the Read NSRS  260 . The controller  266  filters data received from the adaptation data and sends the filtered data to the GUI process  264  for initial population of the filter dialog. The SBAS RS data is received from the read SBAS process  258  and put in a time ordered RS data queue  284  to await output to the simulator  244 . The NSRS data read from the read NSRS process  260  is sent to the converter process  268  for conversion into RS Data. The filter data read from the adaptation data and modified by the user through the GUI process  264  are sent to the converter process  268 .  
         [0045]     The converter process  268  converts the NSRS data received by the controller process  266  into RS data and filters out the converted NSRS data to be sent to the simulator  244 . The NSRS data read from the controller process  266  is converted into RS data, which is put in a time ordered RS data queue  284 . The filter data read from the converter process  268  is applied to the converted data to prevent output of filtered data to the simulator  244 . The converted RS data is sent to the time ordered RS data queue  284 .  
         [0046]     The output SIM process  270  establishes, at startup, the adapted communication channels for sending to the simulator  244 . The filters to apply for the adapted and selected RS Data Messages are read from the controller process  266 . While running, the output SIM process  270  sends the RS data messages to the simulator  244  from the RS data queue  284 , saving the unfiltered messages if recording is enabled. The output SIM process  270  receives output SIM adaptation data read from the controller process  266 , the adapted communication channels for communicating with the simulator  244  and the adaptable filters for recorded output traffic. The output SIM process  270  schedules the RS data (SBAS and converted NSRS Data) to be sent to the simulator  244 . The output SIM process  270  also sends RS data messages which are the RS data encapsulated in a simulator friendly format to the simulator  246 . If recording is enabled, the RS data messages are also sent to the raw RS data message store  274 .  
         [0047]     The scheduler process  272  reads the RS Data in the RS data queue  284  and at the scheduled epoch and sends the data to output SIM process  270 . The RS Data includes an epochs worth of RS Data from the RS data queue  284  and sent to output SIM process  270 .  
         [0048]     The raw RS data message store  274  is generated by the output SIM process  270  if recording is enabled. The raw RS data message store  274  saves a filtered set of RS data messages sent to the simulator  246 . The messages may be used for playback at a later time. The raw RS data message store  274  includes filtered RS data messages which are the adapted selected set of messages.  
         [0049]      FIG. 6  is the data flow diagram of the software process and data storage for the simulator  244 . The processes and data storage for the simulator  244  include a simulator front (SIMF) adaptation data store  276 , a node adaptation store  278 , a station exclusion data store  280 , a SIMF process  282 , a RS data queue  284 , a first correction process (CP 1 )  286 , a second correction process (CP 2 )  288 , a Safety Processor (SP) sequencer process  290  and a SP process  292 .  
         [0050]     The SIMF adaptation data store  276  includes configurable parameters. One configurable parameter is an input channel addresses parameter used to support the option of receiving from the RDM  242  a SBAS/NSRS mixture of data or receiving data directly from the SBAS reference stations. An output channel addresses parameter support sending to the monitoring system  246 . An input file parameter specifies a pre-recorded simulator friendly file to playback. A start/duration times parameters specify the start time within the input file and the amount of time to playback. A delay parameter defines the delay between receiving input data and outputting the data to the correction processes. A node names/IDs parameters define the SBAS and NSRS node IDs to be input into the simulator  244 . A filters parameter supports the filtering of data from each site, particularly useful for the option where the RDM  242  is not used and the data is received directly from the SBAS sites.  
         [0051]     The node adaptation data store  278  includes the configurable parameters such as node names/IDs parameters defining all possible node names (long and short) and IDs for all sites (SBAS and NSRS); locations parameters defining antenna locations for possible sites (SBAS and NSRS); and a service volume indicator parameter defining which sites participates in the defined service volume.  
         [0052]     The station exclusion data store  280  includes configurable parameters such as a site name/ID parameter designating a name of the site and the associated ID and a filter value parameter indicating a “True” or “False” value indicating whether to exclude the site from being part of the corrected solution.  
         [0053]     The SIMF process  282  reads the messages from the RDM  242 , SBAS sites, or recorded files and places the data into queues for the two correction processes (CP 1   286  and CP 2   288 ). The adaptable RDM channel addresses/recorded file location are read in at startup. The SIMF process  282  also reads the corrected user message and converts the data to RS data to be sent to the SMS on an adaptable channel address read in at startup. The SIMF process  282  also provides filtering based on the node IDs read in at startup; as well as, real-time filtering based on altering (adding or subtracting) the node IDs to filter when running the option without the RDM  242 . The RS messages for each epoch are read in from RDM  242  on adaptable channel addresses, SBAS sites, or from a pre-recorded file. The RS Messages are converted to RS data and placed into the RS data queue. RS messages matching the node IDs of the filters read in at run-time or modified real-time will be filtered and dropped before being placed in the RS data queue when running the option without the RDM  42 . The Corrected User Messages are read from the SP process, encapsulated in Corrected RS Messages, and sent to the monitoring system  246 . The corrected RS messages are sent to the monitoring system  246  to display the impact of the corrections in real-time.  
         [0054]     The RS data queue  284  includes RS data sorted by epoch. The SIMF process  282  inputs into the RS data queue and CP 1  and CP 2  output from the RS data queue.  
         [0055]     CP 1   286  and CP 2   288  take the ranging data and ephemeris data found in the RS Data and provides corrections associated with the latency of the messages from the GPS/GEO satellites and ranging/ephemeris data in the messages. CP 1   286  utilizes the data from threads A and C. CP 2   288  utilizes the data from threads B and C. In order to support system  220  need for real-time execution for processing the received RS Data, the process allows for orbit determination and iono threads to execute concurrently, much like certified software. To support the movement of the NSRS  222 , an adaptation file supplies, per thread, the simulator based node IDs, short and long node names, an antenna location, and service volume indicator. At startup, the node names, node IDs, antenna locations, and service volume indicators in the node adaptation data store  278  are read in to populate the site data bases.  
         [0056]     The RS data is read from the RS data queue  284  and processed to determine the corrections (errors) for the GPS signals. The RS data contains three sets, called threads, of similar data, labeled A, B, and C, from a particular station. The corrections are based on the antenna locations found in the node adaptation data store  278 , the latency of the messages and the ranging/ephemeris information within the data. The CP 1  and CP 2  processes monitor the three threads of data found in the RS data. If one of the threads is absent, then thread messages are sent between CP 1  and CP 2  to determine an agreed upon set of threads to get the ranging and ephemeris data. The corrections are based on two threads of data. If two out of the three threads of data are missing, then the remote station is considered unusable. The NSRS  222  data found in the RS data are considered in the thread processing.  
         [0057]     The validation message occurs once a second and confirms the previous message matches the previously sent message and there were no errors in transmission. In the simulator, the validation is always true and the message is used as a vehicle to get the corrected user message back to the correction process. The CP 1  process proceeds each time the message is received by the SP. Once again, the adapted NSRS  222  data is factored into the validation message and corrected message.  
         [0058]     CP 1  messages and CP 2  message contain the received user message and corrections from the iono and orbit determination threads with the correction process. Each message is sent to the SP Sequencer process  290  to merge the CP 1  message in with the CP 2  message.  
         [0059]     The SP sequencer process  290  takes two data streams (one from CP 1   286  and one from CP 2   288 ) and merges the streams into a single stream which is sent to the SP  292 . The SP sequencer process  290  also keeps the order in which the messages were sent from CP 1   286  and CP 2   288  in the same order sent to the SP  292 . For example, the SP sequencer merges CP 1  messages with CP 2  messages and sends the merged messages to the SP  292 .  
         [0060]     The SP  292  receives the merged CP 1  and CP 2  message. The data in the merged message is validated and a corrected user message is generated and sent to the monitoring system  246  through simulator  244 . At startup, data is read from a station exclusion data store. The configurable parameters are particular useful when the RDM  242  is not used to exclude data from sites. At startup, the data is read from the node adaptation data store  278  to populate the node database (SBAS and NSRS).  
         [0061]     CP 1  and CP 2  messages are read from the SP Sequencer process. The measurements from the two CPs are compared for validity and integrity and a new user message is created based on the data. The corrections in the new user message incorporate the data from the SBAS and remote mobile stations; as well as, any site data filtering from the RDM  242 , exclusion lists, or other filters. A validation message is sent to the CPs with the newly corrected user message and an indication of the messages matching the received message. A corrected user message, which incorporates the measurements from all unfiltered sites (SBAS and remote mobile stations), is sent to the simulator  244  for output to the SMS.  
         [0062]     Referring to  FIG. 7 , in a further embodiment, the system  320  includes a local reference station  322 , an evaluation system  324 , SBAS secondary reference stations (e.g., an SBAS secondary reference station  352   a  and an SBAS secondary reference station  352   b ), a host SBAS  354 , GBAS secondary reference stations (e.g., a GBAS secondary reference station  362   a  and a GBAS secondary reference station  362   b ) and a host GBAS  364 . The evaluation system  324  includes a monitoring system  346  and a shelter  328 . The shelter  328  includes a RDM  342  and a simulator  344 .  
         [0063]     The system  320  may be used to demonstrate and evaluate GBAS capabilities in conjunction with an existing or enhanced SBAS service to determining the benefits of either adding SBAS secondary reference stations  352   a  and  352   b  to the host SBAS  354  and/or to determine value of adding GBAS service to a given region including adding secondary reference stations  362   a  and  362   b  to the host GBAS  364 . This capability allows optimization and trade-off analysis to be conducted for determining the quantities and locations for GBAS secondary reference stations within the area or region of interest in a non-intrusive manner with the host GBAS.  
         [0064]     The secondary SBAS reference stations  352   a  and  352   b  and the GBAS reference stations  362   a  and  362   b  are portable and can be located at any geographic location of interest. If communication lines are not present within 300 meters of a desired site, a very small aperture terminal (VSAT) or other types of data communications are possible to support the low data rate communication (&lt;64 kb/sec.) between the secondary reference stations and the shelter  328 .  
         [0065]     Referring to  FIG. 8 , in a still further embodiment, the system  420  includes a local reference station  422 , an evaluation system  424 , SBAS secondary reference stations (e.g., an SBAS secondary reference station  452   a  and an SBAS secondary reference station  452   b ), a host SBAS  454 , GBAS secondary reference stations (e.g., a GBAS secondary reference station  462   a  and a GBAS secondary reference station  462   b ), a host GBAS  464  and a flight test kit  470 . The evaluation system  424  includes a monitoring system  446  and a shelter  428 . The shelter  428  includes a RDM  442  and a simulator  444 . The flight test kit  470  is located aboard an aircraft (not shown) and connected to avionics in the aircraft.  
         [0066]     In one embodiment, flight test kit  470  enables a flight demonstration and GBAS data link coverage evaluation capability. The system  420  includes a common VHF Data Broadcast (VDB) capability that contains both SBAS and GBAS corrections and integrity information.  
         [0067]     In other embodiments, a custom pallet is provided containing flight test kit  470  may be installed in the aircraft to receive and conduct both SBAS and GBAS based aircraft approaches. The flight test kit  470  merges both SBAS and GBAS capability into a single flight test kit entity. For example, when an aircraft is performing successive “touch-and-go&#39;s” (i.e., landing and takeoffs), the aircraft can select to perform a GBAS category (CAT)  1  (limited visibility) approach, an SBAS CAT  1  (limited visibility) approach, an SBAS vertical guidance type II (APV II) approach, an SBAS APV I approach, an SBAS Non-Precision approach, and an unaugmented RAIM based Non-Precision Approach without any equipment modifications or test setup reconfigurations.  
         [0068]     In other embodiments, flight test kit  470  may be used in a civil aviation environment to assess flight risk. In addition, the flight test kit  470  may be used to detect navigation signals in local areas so that a user may monitor the navigation information from the aircraft during the landing or approach or whatever phase of flight the aircraft is coding. In other examples, a user of system  420  may use flight test kit  470  to monitor the navigation signals received at the aircraft and reproduce that information at the evaluation system  424  much like reference station  422 , but in the aircraft using same signal the aircraft would receive while in flight.  
         [0069]     The system described herein is not limited to use with the hardware and software described above. The system can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations thereof.  
         [0070]     The system can be implemented, at least in part, via a computer program product (i.e., a computer program tangibly embodied in an information carrier (e.g., in a machine-readable storage device or in a propagated signal) for execution by, or to control the operation of, data processing apparatus (e.g., a programmable processor, a computer, or multiple computers)). A computer program can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program can be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network.  
         [0071]     Method steps associated with implementing the system can be performed by one or more programmable processors executing one or more computer programs to perform the functions of the system. All or part of the system can be implemented as, special purpose logic circuitry (e.g., an FPGA (field programmable gate array) and/or an ASIC (application-specific integrated circuit)).  
         [0072]     Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. Elements of a computer include a processor for executing instructions and one or more memory devices for storing instructions and data.  
         [0073]     Elements of different embodiments described herein may be combined to form other embodiments not specifically set forth above. Other embodiments not specifically described herein are also within the scope of the following claims.