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3 Answers

Number of GPSes for downgrade to occur during GPS approach

Asked by: 440 views Instrument Rating

Hi everyone,

I went through the old post http://www.askacfi.com/10753/lpv-or-lnavvnav.htm

and came up with questions that I want to clarify.

  1. How many satellites make the approach mode to downgrade?(For example, LPV to LNAV) 
  2. There are total 3 WAAS satellites available over North America, do we need all 3 of them?


3 Answers

  1. John D Collins on Mar 27, 2017

    The number of satellites in view do not directly determine if an LPV is downgraded to LNAV. Two other WAAS provided integrity values are used to determine if LPV is downgraded to LNAV. With WAAS, RAIM is not used, HPL and VPL are controlling. These are respectively the Horizontal and Vertical Protection Levels. They represent the 5 sigma containment radius of the actual position horizontally and vertically (99.99999% probability). Depending on the LPV DH, the VPL must not exceed 30 (DH< 250 feet) or 50 meters (DH=>250 feet). The HPL must not exceed 40 meters for any LPV or LP procedure. A determination of the HPL and VPL is made one minute prior to reaching the FAF and if either are above the limits, the approach is downgraded to LNAV. LNAV only requires an HPL of 556 meters. If it is worse than that, then the approach is waived off.

    Yes and no. If you are in the service volume of one of these WAAS satellites, then WAAS should be available. You only need one, but before there were three WAAS satellites, the original system had two. The more western one of the two failed and could no longer be controlled from the ground, so it slowly drifted eastward (still transmitting WAAS signals) and some of Alaska went outside of the WAAS coverage. This lasted almost a year until the third Satellite was launched. Eventually the second satellite lost orientation and powered itself off. When it came back on, control from the ground was restored and software was updated to prevent this from happening in the future. After many months, the second satellite was back on station and added back into the system. You can see the footprint of the three WAAS satellites at http://www.nstb.tc.faa.gov/DisplayGEOStatus.htm

    The black footprint is the one of the errant satellite that is now back working. Note that if it goes down, the Alaska northern slope loses WAAS coverage.

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  2. connor on Mar 27, 2017

    Thank you sir,

    So if the values for HPL, and VPL( which I saw on System status page of G1000) exceeds, LOI comes up on HSI correct?

    If you do not mind, could you please post the link for this information?

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  3. John D Collins on Mar 28, 2017

    Usually HPL and VPL are not displayed values. The TSO call for displaying HFOM and VFOM, which are figures of merit and are the 95% containment area. I don’t believe that the Integrity annunciator will be on for a downgrade, but I am not positive.

    With respect to references, the specification for the TSO C145/146 is found in RTCA DO 229D. It may be purchased from the RTCA for around $400. Other information comes from the various GPS pilot guides.

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