Ground Based Augmentation Systems (GBAS) for a navigation satellite systems (NSS) broadcast one or more parameters that can be received by a NSS receiver to correct for various errors in the satellite signals received. One such broadcast parameter is the Vertical Ionosphere Gradient standard deviation, also referred to as sigma-vig (σvig). σvig is used to protect against errors caused by anomalous ionosphere gradients in a Local Area Augmentation System (LAAS).
Typically, σvig is calculated for a future time based on the satellites that will be in view of the GBAS at the future time. Since satellites orbit the earth twice each sidereal day, over time, different satellites rise and set from the perspective of the GBAS. On every cycle, the calculation of σvig is performed for a subsequent time epoch, the next time interval in the future, for all predicted satellites which will be in view of the GBAS at the future time on all predicted sub-geometries. This calculation of σvig is done considering both the maximum horizontal distance (ddg
The larger of the values between the σvig calculated for one time step in the future, and the σvig value previously computed for what is now the current time step is broadcast to the NSS receivers. In some implementations, σvig is calculated at 1 minute intervals to minimize the occurrence of having more than one satellite rise and set in one time increment. At each interval, it calculates a σvig value. The σvig value that will be broadcast is the maximum between the most recently calculated σvig and the σvig that was calculated the previous minute.
In one embodiment, a method for real time subset geometry screening is provided. The method for real time subset geometry screening comprises the steps of determining a list of satellites in view of a ground based augmentation system in a navigation satellite system for a subsequent time interval in the future, defining at least one set of subset geometries from the list of available satellites, calculating a respective first σvig for each of the at least one set of subset geometries, setting a respective broadcast σvig for each set of subset geometries as the larger of the first σvig and a second σvig, wherein the second σvig was calculated for the previous time interval, saving the first σvig for a next iteration of the method, and selecting from the plurality of broadcast σvig to match an available broadcast constellation. As used in this embodiment, σvig is a vertical ionosphere gradient standard deviation.
Understanding that the drawings depict only exemplary embodiments and are not therefore to be considered limiting in scope, the exemplary embodiments will be described with additional specificity and detail through the use of the accompanying drawings, in which:
Problems can arise in the conventional determination of σvig when a satellite is included in the current calculation of σvig that was not included in the previous σvig, or when a satellite that was included in the previous avig calculation is no longer a part of the current calculation of σvig.
The subject matter described herein relates to a GBAS system that determines a set of future σvig values, and selects from the set of future σvig values to obtain a σvig value that protects an aircraft with a worst case geometry of two lost satellites on approach. Each σvig of the set of avig values is applicable to each respective sets of N number of satellites to N-2 satellites. This set of broadcast σvig values remains valid until a new set of broadcast σvig values are determined.
A geometry screening algorithm pre-calculates up to five σvig values (σvig
The satellite vehicle (SV) constellation geometry nominally varies slowly over time. However, the rising or setting of any SV creates a discontinuity in the constellation geometry. Each satellite which is to be included in the broadcast set must be a part of the screened set of satellites. Therefore, to ensure that the current set of satellites are immediately available for broadcast (present at the beginning and end of the screened interval) the set of screened satellites includes the newly risen or readmitted satellites in the interval prior to their arrival and the setting or excluded satellites remain in the screened set following their departure.
Therefore the sets of satellites screened provide inflation values for all potential sets of satellites resulting from rising, setting, excluded and readmitted satellites during any given time interval.
At block 101, almanac data is retrieved updated. Using the most recently collected and validated almanac, a set of healthy and usable satellites at the end point of the current interval is determined, at block 103. SVs that will either set or will enter a selective mask during that interval and SVs that will rise or leave a selective mask during the next interval are included in the list. In order to calculate the predicted rise and set of any given SV, the almanac is used to compute the SV position and velocity in Earth-Centered Earth-Fixed (ECEF) coordinates. SV ECEF position and velocity are used to calculate the range and the line of sight vectors in the E-frame. Using the line of sight vectors, the elevation and azimuth values for each satellite is determined, which are then tested to find if the satellite is available to the GBAS. The predicted rise and set times are also checked against any reference receiver selective masks to make sure they correspond to what the system and user will experience.
At block 104, the number of sets of subset geometries is defined. “S” defines the maximum number of satellites visible and tracked in either the current or previous inflation interval. S is limited to a minimum of 4, as the geometry screening algorithm is not performed if S is less than 4. S is constant over an inflation interval and does not change across sets of subset geometries. S is picked as the larger of the number of satellites for the current inflation interval end point, or the previous interval end point. This assures that σvig will cover any geometry change between the inflation interval end points. In one embodiment, the interval period is 150 seconds.
Given S satellites, only a single geometry includes all S satellites. There are an additional S number of geometries which contain all combinations of S-1 satellites. There are an addition (S2−S)2 geometries which contain all the combinations of S-2 satellites. Each of these geometries is defined to be a subset geometry, and a specified group of subset geometries is devoted to be a set of subset geometries.
N defines the number of satellites to be chosen from S visible satellites to generate a particular set of subset geometries. Each set of subset geometries is used to generate each σvig value (up to five values) for that inflation interval end point. Up to five separate sets of subset geometries are maintained for each interval period, where the number of sets of subset geometries is denoted as NScomb. For each set of subset geometries, one inflated σvig value is generated, meaning that a total number of NScomb separate σvig values are calculated. The current set of subset geometries is referred to as CS, and the number of combinations or geometries within that particular set is Ncomb.
In one embodiment, S≧10. The total number of sets of subset geometries is then NScomb=5. For N≧10 and N=S, the fifth of five sets of subset geometries is described by the following permutations of satellites:
σvig
For 10>N≧6, Ncomb for N-satellite combinations is selected from 9 down to 6 for the fourth through first sets of subset geometries are calculated. σvig
In another embodiment 10>S≧6. NScomb=S−5. Select all combinations of N for N=S down to 6. Ncomb is calculated for the fourth through first sets. σvig
In another embodiment, S=5, and NScomb=1. When the number of satellites being selected is S=5, both N and S are equal. A single set of subset geometries Ncomb is calculated. σvig
In another embodiment, S=4. NScomb=1. When the number of satellites is 4, both N and S are equal. A single set of subset geometries is calculated Ncomb. σvig
At block 105, σvig is calculated for each set of subset geometries. Global positioning system (GPS) satellite geometries vary slowly over time. However, a change in the usable set of satellites can occur quickly between inflation interval end points, thus requiring a new σvig value to be broadcast. Since σvig calculations are too CPU intensive to update in real time, the geometry screening algorithm pre-calculates up to five σvig values (σvig
At block 107, the σvig
The steps above can be seen in relation to specific time intervals as seen in
Calculations 214 describe for which sets each σvig is calculated. In this embodiment, σ1vig is calculated for sets
σ2vig is calculated for sets
σ3vig is calculated for sets
σ4vig is calculated for sets
σ5vig is calculated for sets
Thusly, σ1vig-σ5vig are calculated for time t3 during GSI1 210.
GSI2 220 spans from t2 to t3. During GSI2 220, σvig
At block 109, a σvig
In one embodiment, suppose S=10 at inflation interval end point T, and S=9 at inflation interval end point T+1. The likely scenario is that a satellite will descend out of view between time epochs T and T+1. However, the constellation may change abruptly due to Reference Receiver Optimization resulting from a broadband interference event. For this reason, the geometry screening algorithm will generate σvig
In some embodiments, real time subset geometry screening apparatus 310 is a differential global position system (DGPS) control unit 310 that includes a VDB cabinet 350 physically integrated into the same body of the DGPS 310, coupled to a RSMU 340. In some embodiments, DGPS 310 is the Honeywell SLS-4000 GBAS. DGPS 310 receives GPS information from RSMU 340 and status data from VDB cabinet 350, and performs σvig Inflation Algorithm 331 with the data. After calculating σvig inflation values, the σvig broadcast value is broadcast by VDB cabinet 350.
Example 1 includes a method for real time subset geometry screening comprising the steps of: determining a list of satellites in view of a ground based augmentation system in a navigation satellite system for a subsequent time interval in the future; defining at least one set of subset geometries from the list of available satellites; calculating a respective first σvig for each of the at least one set of subset geometries, wherein σvig is a vertical ionosphere gradient standard deviation; setting a respective broadcast σvig for each set of subset geometries as the larger of the first σvig and a second σvig, wherein the second σvig was calculated for the previous time interval; saving the first σvig for a next iteration of the method; and selecting from the plurality of broadcast σvig to match an available broadcast constellation.
Example 2 includes the method of example 1, wherein the next time interval is 150 seconds into the future, each time interval being 150 seconds.
Example 3 includes the method of any of examples 1-2, wherein if the list of available satellites is greater than 10 satellites, five sets of subset geometries are used.
Example 4 includes the method of any of examples 1-3, wherein five broadcast σvig values are calculated, one of the sets of subset geometries comprises all combinations of 8 to the number of available satellites.
Example 5 includes the method of any of examples 1-2, and 4, wherein if the list of available satellites is less than 10, but greater than or equal to 6, the number of sets of subset geometries used is the number of satellites minus 5.
Example 6 includes the method of any of examples 1-5, wherein broadcast σvig values are calculated, one for each of the respective sets of subset geometries.
Example 7 includes the methods of any of examples1-2, wherein if the list of available satellites is 5 or 4, only one set of subset geometries is used.
Example 8 includes the method of any of examples 1 and 7, wherein a broadcast σvig is calculated to protect the single set of subset geometries of all 5 and/or 4 satellite combinations.
Example 9 includes an apparatus for executing real time subset geometry screening comprising: a microprocessor; and a non-transitory computer readable medium; wherein, the computer readable medium is configured to provide instructions to the microprocessor to execute a real time subset geometry screening function; wherein the real time subset geometry screening function causes the microprocessor to: determine a list of satellites in view of a ground based augmentation system in a navigation satellite system for a subsequent time interval in the future; define at least one set of subset geometries from the list of available satellites; calculate a respective first σvig for each of the at least one set of subset geometries, wherein σvig is a vertical iono gradient standard deviation; set a respective broadcast σvig for each set of subset geometries as the larger of the first σavig and a second σvig , wherein the second σvig was calculated for the previous time interval; save the first σvig for a next iteration of the method; and select from the plurality of broadcast σvig to match an available broadcast constellation.
Example 10 includes the apparatus of example 9, wherein the microprocessor retrieves global positioning system (GPS) information from a GPS unit, wherein the GPS unit comprises a GPS antenna and GPS receiver, wherein GPS information comprises satellite health and satellite position parameters by which to compute satellite elevation and its corresponding visibility.
Example 11 includes the apparatuses of any of examples 9-10, wherein the GPU unit is separate from the apparatus for executing real time subset geometry screening.
Example 12 includes the apparatuses of any of examples 9-11, wherein the GPU unit is a remote satellite monitoring unit (RSMU).
Example 13 includes the apparatuses of any of examples 9-12 further comprising a very high frequency (VHF) broadcast unit, wherein the VHF broadcast unit comprises a VHF antenna and VHF radio, wherein the VHF broadcast unit is configured to broadcast the selected broadcast σvig.
Example 14 includes the apparatuses of any of examples 9-13, wherein the VHF broadcast unit is a VHF Data Broadcast (VDB) cabinet.
Example 15 includes the apparatuses of any of examples 9-14, wherein the apparatus for executing real time subset geometry screening is a differential global positioning system (DGPS).
Example 16 includes the apparatuses of any of examples 9-15, wherein the apparatus for executing real time subset geometry screening is the Honeywell SLS-4000.
Example 17 includes a system for real time subset geometry screening comprising: a local ground facility, wherein the local ground facility comprises: a real time geometry subset screening device; wherein, the real time subset geometry screening device is configured to: determine a list of satellites in view of a ground based augmentation system in a navigation satellite system for a subsequent time interval in the future; define at least one set of subset geometries from the list of available satellites; calculate a respective first σvig for each of the at least one set of subset geometries, wherein σvig is a vertical iono gradient standard deviation; set a respective broadcast σvig for each set of subset geometries as the larger of the first σvig and a second σvig, wherein the second σvig was calculated for the previous time interval; save the first σvig for a next iteration of the method; and select from the plurality of broadcast σvig to match an available broadcast constellation; a global positioning system (GPS) receiver coupled to the DGPS; a GPS antenna coupled to the GPS receiver; a very high frequency data broadcast (VDB), in communication with the DGPS; an encoder and transmitter configured to encode and transmit outgoing communications from the DGPS to the VDB; and a receiver and decoder configured to receive and decode incoming communications from the VDB to the DGPS.
Example 18 includes the system of example 17, wherein the LGF is part of a ground based augmentation system such as a Honeywell SLS-4000.
Example 19 includes the system of any of examples 17-18, wherein the LGF is a local area augmentation system ground facility.
Example 20 includes the system of any of examples 17-19, further comprising an aircraft, wherein the aircraft comprises a datalink device in communication with the VDB.