GPS Theory and Operation

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					   B737 GPS/FMS



Part 1: GPS Theory and Operation
Week 13
 Avia 160
                                 Topics
    •   GPS Background
    •   GPS Signals and Ranging
    •   GPS Components
    •   GPS Accuracy
    •   World Geodetic Survey 84 (WGS 84)
    •   Receiver Autonomous Integrity Monitoring (RAIM)
    •   Fault Detection and Exclusion (FDE)
    •   Step Detector
    •   Barometric Altimeter Aiding (baro-aiding)




3
                   GPS Background
          The Global Positioning System (GPS) is a satellite based
    navigation system offering precision navigation capability. Originally
    designed for military use, civilian access has been permitted to specific
    parts of the GPS.
          GPS offers a number of features making it attractive for use in
    aircraft navigation. Civilian users can expect a position accuracy of 100
    m or better in three dimensions. The GPS signal is available 24 hours
    per day throughout the world and in all weather conditions. GPS offers
    resistance to intentional (jamming) and unintentional interference. The
    equipment necessary to receive and process GPS signals is affordable
    and reliable and does not require atomic clocks or antenna arrays. For
    the GPS user, the system is passive and requires a receiver only
    without the requirement to transmit.



4
         GPS Signals and Ranging
           In its most basic terms, GPS determines the position of the user
    by triangulation. By knowing the position of the satellite and the
    distance from the satellite; combinations of satellites can be used to
    determine the exact position of the receiver.
           The fundamental means for GPS to determine distance is the use
    of time. By using accurate time standards and by measuring changes
    in time, distance is computed.




5
               A simplified GPS system illustrates the concept of satellite ranging. A
    satellite transmits a time signal, as shown. The receiver is stationary and has an
    absolutely accurate clock, perfectly synchronized to GPS time. By measuring the
    difference in time from when the signal left the satellite to when it is received by
    the aircraft, the distance from the satellite to the user can be calculated. This is
    the product of the time difference and the speed of light (300,000 km/sec).




6
           With one satellite, and knowing the position of that satellite, the
    location of the user would be anywhere along an arc. If three satellites
    were used, the location of the user would be at the intersection of the
    three arcs created by the satellites, as shown. Stated mathematically,
    in order to solve for the three dimensional position (with three variables:
    latitude, longitude and altitude), three equations (or satellites) are
    needed.




                                                Signal left the satellite at time = 100 sec
                                    Satellite     Reception Time   Time Difference (sec)   Distance (km)
                                       1             100.0855            0.0855                25650
                                       2             100.0900            0.0900                27000
                                       3             100.0875            0.0875                26250




7
           This example assumes a receiver clock in perfect synchronization
    with the satellite and exhibiting the same accuracy. It is impractical and
    prohibitively expensive for GPS receivers to use atomic clocks as those
    used on the satellites to maintain an accurate time. As a result,
    receiver clocks are not perfectly synchronized satellite time. For every
    microsecond (one-millionth of second) difference between the satellite
    clocks and the receiver clock, a 300 meter error is introduced. This
    error is known as a clock bias.




8
          The location of the receiver is somewhere in the area defined by the
clock bias for each satellite, as shown. Because of this bias, an extra satellite is
required to resolve this error. For example, with three satellites, only a two
dimensional position can be determined (clock bias, latitude and longitude). In
order to determine a position in three dimensions, a fourth satellite is required.
Stated mathematically, in order to solve for the three dimensional position (three
variables: latitude, longitude and altitude) and the time bias, four equations (or
satellites) are needed.




9
            The electromagnetic radio waves or signals broadcast from the
     GPS satellites form the means for a GPS receiver to perform the timing
     and distance calculations. GPS receivers are passive devices meaning
     that signals are received only with no requirement or means to transmit.
            GPS ranging signals are broadcast on two frequencies: L1
     (1575.42 MHz) and L2 (1227.6 MHz).
            The L1 frequency is available for civilian use. The frequency has
     two modulations:
            1) The Clear Acquisition Code or C/A: this is the principal civilian
     ranging signal and is always broadcast in a clear or unencrypted form.
     The use of this signal is sometimes called the Standard Positioning
     Service or SPS. This signal may be degraded intentionally but is
     always available. The signal creates a short Pseudo Random Noise
     (PRN) code broadcast a rate of 1.023 MHz. The satellite signal repeats
     itself every millisecond. The C/A code is also used to acquire the P
     Code.
            2) Protected Code or P Code: this is also known as the Precise
     Positioning Service. This signal has been encrypted and is not
     available to civilian users.


10
           Both the C/A and P code use the same principle to measure the
     time taken for the satellite signal to reach the receiver. The GPS signal
     modulation consists of a repetitive binary signal that receivers use to
     determine the time at which the code was sent from the satellite, as
     shown. The waveform from the satellite is matched with an internally
     generated waveform within the receiver. The time difference between
     matching waveforms is used to compute the distance from a satellite.




                                     Satellite Wave


                                     Receiver Wave

                                      Time Difference

               The binary information found on the L2 frequency is
     reserved for military use and is thus not available for civilian access.
     Civilian users can access the L2 frequency and its carrier, however.
11
                Both the L1 and L2 frequencies broadcast a satellite message as
          part of their signal. This low frequency (50 bits per second) data
          stream provides the receiver with a number of critical items required in
          determining a position. This data stream is broadcast continually and is
          repeated every 30 seconds. This data stream is broken down into five,
          six-second subframes:

     Subframes 1 through 5 each provide a synchronization, hand over word
     and a C/A code time ambiguity removal. The remainder of the data is
     formatted as follows:

     Subframe 1: satellite clock corrections, age of data and various flags
     Subframe 2 and 3: ephemeris (exact satellite orbit description)

     Subframe 4: ionospheric model, UTC data, flags for each satellite
     indicating whether anti-spoofing is on, almanac (approximate satellite
     ephemeris allowing the receiver to select the best set of satellites or to
     determine which satellites are in view) and health information for satellite
     number 25 and greater

     Subframe 5: almanac and health information for satellite number 1 to 24

12
            The reception and decoding of the data stream is performed
     automatically by a receiver without any intervention by the operator.
     The information within this data is critical to GPS operation. The
     almanac and ephemeris provides the description of the satellite orbit.
     With this information, the receiver can determine the satellite’s position
     at any time and combine this with the receiver distance from the
     satellite, yielding a GPS position. The health information is critical to
     prevent a receiver from using the ranging information from a satellite
     that has been declared unfit for navigation purposes. The remainder of
     the information found in the data stream – clock corrections,
     ionospheric model, UTC data – are used to resolve potential sources of
     GPS position errors. These will be discussed later.




13
                   GPS Components
         The Global Positioning Systems consists of three major
     components: satellites, control segment, and the user.
      Satellites
            The GPS constellation is designed for a minimum of 24 satellites
     (21 active satellites and three orbital spares) orbiting the earth. GPS
     satellite orbit is designed to be circular however some eccentricity (non-
     circular orbit) can be present. The satellites orbit the earth at an
     altitude of 20,163 km above the earth’s surface or 26,562 km from the
     center of the earth. The orbital velocity is 3.87 km/sec. The orbital
     plane is inclined at 55 degrees with reference to the equator. The
     satellites complete two orbits each sidereal day. To a viewer located
     on the surface of the earth, the satellites are in constant motion (non-
     geo-synchronous orbit) with satellites rising and setting.




14
            Six orbital planes are in use, each spaced equally around the
     earth, separated by 60 degrees (360 degrees/6 planes=60 degrees).
     The planes are named A to F.
            Each orbital plane hosts four satellites. These satellites are not
     spaced evenly on each plane, however. Spacing between adjacent
     satellites varies from 31.13 degrees to 119.98 degrees. Each plane
     exhibits a different angular spacing for the satellites resident to it. A
     computer model was used to determine the satellite spacing to
     accommodate a single satellite failure and still maintain optimal satellite
     geometry. Satellite geometry and its affect upon GPS accuracy are
     discussed later.




15
            The primary mission of GPS satellites is the transmission of
     precisely timed GPS signals and the data stream required to decode
     the signals to produce a position. The timing signals are referenced to
     atomic clocks, either cesium or rubidium.
            With the GPS satellites in constant motion, the number of
     satellites in view and their relative location is dynamic. A 24-satellite
     configuration provides adequate satellite coverage to perform three-
     dimensional position fixing. Failures of satellites and/or the
     requirement for more than four satellites (as discussed later) may result
     in inadequate satellite coverage.
            The following slide shows the motion of nine satellites. The
     ground tracks show the movement of these satellites over a twelve
     hour period and the position of the satellites at one moment in time.
            The ground tracks show a number of features. Each satellite
     follows a unique path over the ground. Also, every satellite operates
     between 55 degrees North and 55 degrees south.
            The snapshot of satellite positions show that a point on earth will
     see a different set of satellites compared another point on the surface.
     Also, as these satellites move in their orbits, the satellites in view at
     each location changes with time.

16
17
            The equatorial and polar regions enjoy the best satellite
     coverage. Receivers located near the equator are able to view
     satellites on both sides of the equator and at the limits of their orbits.
     Receivers in the polar regions are able to view satellites towards the
     equator but also satellites on the other side of the earth. Satellite
     coverage and probability distribution for a 24 satellite constellation and
     a 5 degree mask angle are provided. “Mask Angle” is a term describing
     the angle from the horizon below which a receiver is unable to track
     satellites. This value is determined by the capabilities of the antenna
     and receiver as well as any local terrain.




18
               60                                                                                   35
               50                                                                                   30
               40                                                                                   25
 Probability




                                                                                      Probability
                                                                                                    20
               30
                                                                                                    15
               20                                                                                   10
               10                                                                                    5
               0                                                                                     0
                    4   5   6       7      8      9      10      11   12   13                            4   5   6     7      8       9       10    11    12        13

                                 Number of Satellites in View                                                        Number of Satellites in View

                                0 Degrees Latitude - Equator                                                           35 Degrees Latitude




               35                                                                                   50
               30
                                                                                                    40
               25
 Probability




                                                                                Probability
               20                                                                                   30
               15
                                                                                                    20
               10
                5                                                                                   10
               0
                    4   5   6       7      8       9      10     11   12   13                       0
                                                                                                         4   5   6     7          8       9    10    11        12        13

                                  Number of Satellites in View                                                       Number of Satellites in View

                                    40 Degrees Latitude                                                              90 Degrees Latitude- Pole



19
                 Control Segment
           Five monitoring stations are located throughout the world (Hawaii,
     Colorado Springs, Ascension Island, Diego Garcia and Kwajalein
     Island) provide continuous surveillance of the GPS constellation. Four
     of these stations (all except Hawaii) have the ability to upload
     information to the GPS satellites.
     The objective of the GPS control segment is to:
           • Maintain each of the satellites in its proper orbit through
             infrequent, small commanded maneuvers,
           • Make corrections and adjustments to the satellite clocks and
             payload as needed,
           • Track the GPS satellites and generate and upload navigation
             data to each of the GPS satellites, and
           • Command major relocations in the event of satellite failure to
             minimize the impact.




20
            The monitoring stations record a number of parameters including
     satellite position, clock errors and GPS signal. This information is
     transmitted to the Operational Control Center at Falcon Air Force Base,
     Colorado Springs, Colorado. The data is processed to determine
     ephemeris (orbit) errors, clock error, satellite health for each satellite,
     etc. Navigation data packages are then prepared for uploading to the
     satellites via the ground antenna stations for storage and use on the
     satellites. Although uploads generally occur once per day, fresh
     uploads can be provided up to three times daily. Uploaded data can be
     used for up to 14 days – this feature provides the satellites with a
     degree of autonomy should there be difficulties in uploading data for an
     extended period of time.




21
                          GPS User
           The antenna receives the GPS signals and amplifies them for
     further processing. A filtering eliminates signals or noise from adjacent
     frequency bands. The signal is then sampled and fed to parallel sets of
     delay locked loops where multiple satellites can be tracked
     simultaneously. The pseudorange, carrier phases and navigation data
     is then estimated. A signal generator replicating the signal produced by
     the satellites is used to determine the time difference between when
     the signal was transmitted by the satellite and received by the user.
           Using the navigation data provided in the data message, the
     pseudorange and phase information is then corrected for satellite clock
     errors, earth rotation, ionospheric delay, tropospheric delay, relativistic
     effects and equipment delays. This information is then processed with
     other sensory data (if available) to produce a position and velocity
     output. The coordinates are then converted by the appropriate
     geodetic transformation to the local coordinate set.




22
            World Geodetic Survey
           A number of geodetic coordinate systems have been developed
     and used to describe a position. A World Geodetic Survey (WGS) is a
     “consistent set of parameters describing the size and shape of the
     earth, the positions of a network of points with respect to the center of
     mass of the earth, transformations from geodetic datums and the
     potential of the earth”. The World Geodetic System of 1972 (WGS-72)
     has been traditionally used by air navigation systems and Aviation
     Information Publications (AIP’s) have used the North American Datum
     of 1927 (NAD-27).
           WGS-84 and NAD-83 are now in use in Canada and the United
     States. The difference between these two is less than 100 feet within
     the US, however the difference between these two datums and other
     international datums can exceed more than two nautical miles. GPS
     uses WGS-84 – a Cartesian earth-centered earth fixed (ECEF) –
     reference system.
           If some countries do not publish AIP data in WGS-84 compatible
     coordinates, navigation accuracy is limited. Enroute operations will not
     be affected by this inaccuracy however approach operations and
23   accuracy is severely restricted.
                          Error Sources
           GPS is vulnerable to a variety of errors that serve to degrade its
     accuracy. Adjustments are required to allow for imperfections of GPS
     ranging. These are:


      Ionospheric


           The ionosphere is a region of ionized gases beginning at 75 to
     100 km above the earth’s surface and varies in thickness from 200 to
     400km. The size and shape of the ionosphere experiences wide
     fluctuations from day to day, between night and day (diurnal effect) and
     with solar conditions.




24
            The ionosphere path delay can have a significant effect upon GPS
     timing. The extra time required for the GPS signal to pass through the
     ionosphere can vary between 2 and 50 nanoseconds, creating a distance
     error of between 0.67 m and 16 m, respectively. Further compounding the
     path delay error is the obliquity factor - the angle at which the GPS signal
     passes through the atmosphere. A GPS satellite passing overhead (90-
     degree angle) experiences the least effect as the signal passes through the
     smallest amount of ionosphere. With a lower elevation angle the obliquity
     factor increases by a factor of 3 with a satellite on the horizon. An
     ionospheric delay is therefore over 3 times the nominal value for satellites
     with large elevation angles. What was a 16m error for a satellite located
     above the receiver becomes a 48m error for the same satellite located just
     above the earth’s surface.
            The ionospheric delay can be mitigated by a number of techniques.
     Receivers with access to both the L1 and L2 frequencies can compare the
     time differences for the same timing signal to reach the receiver on the two
     different frequencies. The ionospheric error can be calculated from this
     time difference and adjusted for in determining the satellite range.
            For users without access to the L2 frequency a mathematical model
     is used to simulate the ionosphere. The necessary terms in the equation
     vary with time and are uplinked to the satellite. These corrections are
     transmitted to the user as part of the data modulation carried on the GPS
     signal.
25
      Tropospheric


           The troposphere is the region of dry gases and water vapor
     extending from the earth’s surface to an altitude of approximately 50
     km. The characteristics of the troposphere make it easier to model than
     the ionosphere.
           The time delay of a GPS signal passing through this region of the
     atmosphere normally results in a position error of 2.6 m for a satellite at
     the zenith (vertical) and can exceed 20 m for a satellite at elevation
     angles less than 10 degrees. Modeling the effects of a dry atmosphere
     are relatively simple and can eliminate 90% of the error. Dealing with a
     wet atmosphere is more difficult and only 10% of the error can be
     compensated for mathematically.




26
      Multipath


           Multipath is the effect of the same satellite signal reaching the
     GPS antenna more than once. The first signal to reach the antenna
     takes a direct path from the satellite. The multipath signals are
     reflected by either ground or water surfaces, as shown.




     Aircraft are particularly vulnerable to this effect. Satellite signals
     reflecting off the ground or sea present multipath errors. An antenna
     design shield the multipath is not a viable option since satellites at
     moderate or low elevation angles would also be shielded.
27
      Selective Availability


            “Selective Availability (SA) is the intentional degradation of the
     GPS signal with the objective to deny full position and velocity accuracy
     to unauthorized users”. SA was not part of the original design of GPS.
     During its initial testing in the 1970’s, accuracies were much better than
     expected using C/A code (20-30 m position accuracies compared to the
     expected greater than 100 m accuracy). The US Department of
     Defense decided to intentionally degrade the accuracy to 500m (95%
     probability) then modified it to 100 m (95%) to make it comparable to a
     VOR used for non-precision approaches.
            Two techniques are used to degrade GPS position using SA.
     Manipulation of the satellite navigation orbit data degrades the
     accuracy of the calculated satellite positions. The actual satellite
     positions in space are unaffected but the parameters describing the
     satellite orbits (ephemeris and almanac) are corrupted. This type of
     error is slowly varying (periods measured in hours).
            The second technique used to effect SA is clock dither. In this
     case, the actual satellite clocks are manipulated to produce position
     errors. This affects both C/A and P code (military) users. In addition,
     this type of error is produces rapid changes and its period is in the
28
     order of minutes.
                  Ranging Accuracy or GPS Error Budget

                  Ephemeris Data      Satellite Clock          Ionosphere   Troposphere
                                                               GPS Measurement Error Sources Multipath   Receiver Measurement   Other



                                                                       Length (meters)




                  S/A




              No S/A




                        0.0           5.0                     10.0           15.0           20.0          25.0           30.0           35.0
                                                   No S/A                                                        S/A
Other
Receiver Measurement                                    0.5                                                      0.5
Multipath                                               1.4                                                      1.4
Troposphere                                             0.7                                                      0.7
Ionosphere                                              4.0                                                      4.0
Satellite Clock                                         2.1                                                      20.0
Ephemeris Data                                          2.1                                                      2.1




                              GPS receiver position accuracy is directly related to the error sources
  29
                              described earlier. These errors and their typical values are shown.
           With Selective Availability turned off the dominant error is
     ionospheric delay followed satellite clock errors and ephemeris data.
     The combination of all of the errors totals a UERE of 5.3 meters (the
     effects are not added but are squared, added and then the square root
     is taken).
           With S/A, the satellite clock error becomes dominant error source.
     The combined UERE becomes 20.6 meters.
           For aviation purposes, the assumed UERE is 33.3 meters for all
     error sources.




30
                      GPS Accuracy
           GPS position accuracy is the product of the ability of the GPS
     system to accurately measure its pseudorange (User Equivalent Range
     Error, UERE) and the effect of satellite geometry in degrading the
     position accuracy (Dilution of Precision, DOP).
           UERE represents the combined effects of ephemeris
     uncertainties, propagation errors (ionosphere and troposphere), clock
     and timing errors and receiver noise. This is typically expressed in a
     measurement of length such as feet or meters.
           DOP is an expression of how the satellite geometry contributes to
     or degrades the position accuracy and is expressed as a scalar (non-
     dimensional) number. A number of different terms are used to
     pseudorange error including UERE and Figure of Merit (FOM)
           Position accuracy represents the end state capability of a GPS
     receiver. This is related to but not the same as ranging accuracy. The
     quantity linking ranging accuracy to position accuracy is Dilution of
     Precision (DOP).
     Satellite position accuracy is defined as follows:

31   Position Accuracy = (Ranging Accuracy) x (Dilution of Position)
      Dilution of Precision (DOP)


            The position of GPS satellites in relation to the receiver – satellite
     geometry - forms the critical component of the DOP. The value of DOP
     is also influenced by the number of satellites in view, the capability of
     the receiver to simultaneously track satellites (number of channels) and
     the minimum reception angle that an antenna can track a satellite
     (mask angle).
            A two dimensional position requires three satellites for a position
     solution. In this case, the optimum value of DOP is achieved with the
     satellites spaced equally at 120 degrees apart, producing a Horizontal
     Dilution of Precision (HDOP) of 1.1547. A different geometry of three
     satellites will lead to an increase in HDOP and a resulting decrease in
     position accuracy.
            With more than three satellites available for the two dimensional
     solution, the value of HDOP can improve. In the ideal case with the
     five satellites spaced equally at 72 degrees, the value of HDOP
     becomes 0.8944.
            The following illustrates the changes in HDOP and Vertical
     Dilution of Precision (VDOP). Four satellites are used however their
     position has shifted to reflect the movement of the satellites over time.
32
             An infinite combination of satellites and their relative positions
     exist. Moreover, with the satellites in constant motion, the DOP values
     are also constantly changing.




                In these examples, four satellites are provided. The example
       on the left has four satellites at a 45 degree elevation and equally
       spaced around the horizon yielding a Horizontal DOP (HDOP) of 2 and
       a Vertical DOP (VDOP) of 162.2. Moving the same four satellites as
       shown on the right changes the HDOP to 1.5 and the VDOP to 3
33
            The position accuracy can now be determined as the product of the
        UERE and the DOP. For example, with a UERE of 20 meters with a
        HDOP of 3, the position accuracy is:

     Position Accuracy = (Ranging Accuracy) x (Dilution of Position)
     Position Accuracy = (20 meters) x (3)
     Position Accuracy = 60 meters

              For aviation purposes the assumed position error for enroute,
        terminal and non-precision approaches is 100m or 0.054 nautical miles.




34
           These pages from the
     CMA 900 MCDU illustrates the
     accuracy measurement
     capabilities of the Flight
     Management System.
           Different terminology is
     used. Figure of Merit (FOM)
     equates to ranging accuracy
     and HOR INT (Horizontal
     Integrity) is the position
     accuracy.
           The value of HOR INT is
     also the the Actual Navigation
     Performance (ANP) value
     found on the following page.
           These will be discussed
     in more detail later.




35
     Receiver Autonomous Integrity
          Monitoring (RAIM)
            A unique aviation requirement of GPS avionics is RAIM. While
     GPS provides the user with unparalleled levels of accuracy, one
     significant deficiency of GPS is integrity, that is, the ability of the system
     to provide a timely warning if the navigation solution is inaccurate or
     erroneous. Navigation systems prior to GPS, particularly aviation
     applications, provided a means to warn the aircraft that the signal was
     outside certain limits. For example, a Category I ILS provides this
     warning within six seconds.
            The only means available for the GPS system itself to provide the
     user with a warning of system unreliability is through the data message
     forming part of the GPS signal. The “health” flag found in subframe 4
     and 5 will alert the receiver to a failure of a GPS satellite.
            The time lag from the beginning of the failure to when it is
     incorporated in the health flag – up to eight hours - represents an
     unacceptably long period of time for aviation.



36
             To overcome this, RAIM was developed and is a mandatory
      feature of all aviation-grade receivers. RAIM uses combinations of
      satellites to determine the receiver position. Should a large
      discrepancy between position solutions occur, a RAIM alert is created
      rendering the GPS navigator unreliable.
             Different phases of flight use different values of “integrity alarm
      limits” prior to issuing a RAIM alert. These are as follows:
      Phase of Flight                                      Alarm Limit   Time to Alarm


      Enroute (oceanic, domestic, random and J/V routes)    2.0 n.m.        30 sec


      Terminal                                              1.0 n.m.        10 sec


      RNAV approach, non-precision                          0.3 n.m.        10 sec

               The ability of a receiver to perform RAIM computations is dependent
     upon the number of satellites in view, their geometry and the mask angle
     which is dependent upon the ability of the antenna to track satellites near the
     horizon and any local terrain. Whereas GPS needs a minimum of four
     satellites to produce a three-dimensional position, a minimum of five satellites
     are required for RAIM. For this reason, RAIM may not be available in
     circumstances of poor satellite coverage or poor satellite geometry.
37
      Avionics certified under
Technical Standard Order (TSO)
C129 also provide the crew with
a number of other RAIM
capabilities. Upon transition
from terminal to approach
integrity satellite geometry is
automatically verified to ensure
RAIM availability at the Final
Approach Fix and Missed
Approach Point
      A RAIM availability
prediction can be performed at
any time using any waypoint or
the destination and an ETA.
This provides a prediction for
ETA +/- 15 minutes in 5-minute
intervals. This also known as
Predictive RAIM (PRAIM).




38
     Fault Detection and Exclusion
                 (FDE)
            A RAIM integrity warning – the identification of one or more errant
     satellites - will render the GPS system unusable for the intended phase
     of flight and will require the aircraft to revert to another form of
     navigation.
            Fault Detection and Exclusion (FDE) takes a RAIM alarm and
     performs further analysis to identify the faulty satellite(s). The faulty
     satellite(s) is (are) excluded from any navigation computations and the
     GPS receiver is declared operational. This is particularly important for
     uses of GPS as “primary means” and “sole means”. FDE occurs
     automatically without any pilot input or annunciations. A minimum of
     six satellites is required for FDE.




39
                    Step Detector
            A GPS step-detector is another form of integrity check. In this
     test, unreasonable pseudorange differences between consecutive
     measurements are detected. This serves to monitor pseudorange step
     failures and should a failure be detected that satellite will be removed
     from the solution.
            For example, if consecutive pseudorange measurements produce
     a change of 10 meters per second and the change suddenly jumps to
     50 meters per second, a ranging error is evident and the satellite gets
     excluded from the position and velocity solution.




40
       Barometric Altimeter Aiding
             (baro-aiding)
            A barometric altimeter altitude can be introduced into the GPS
     solution. This serves three important purposes: improved vertical
     position accuracy, the elimination of one variable in the GPS solution
     (altitude) and an improved level of RAIM and FDE availability as the
     baro input serves to act like a satellite in the position computation.
            The input of the barometric altitude is performed automatically in
     aviation grade GPS receivers. Normally the pressure altitude is
     provided with a requirement for the input of the local barometric
     altimeter setting for terminal and approach operations. This is normally
     performed in two ways: the crew is alerted by the GPS receiver to input
     this altimeter setting or the barometric setting is automatically derived
     by the altimeter setting of one of the altimeters.




41
Note: in the case of the
   Canadian Airlines B737
   installation, the local
   barometric altimeter
   setting is required to be
   inputted manually into the
   FMS.
   This feature is found on
   Progress page 4/4, shown.
   An upcoming modification
   (Fall 1999) will
   automatically provide the
   local barometric setting by
   using the Captain’s
   altimeter setting.




  42

				
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