Satellite Navigation (GNSS) Flashcards
GPS Constellation
● The design GPS Constellation contains 24 satellites, orbiting the earth twice a day
● 4 satellites are required for 3D navigation.
● 3 satellites are required for 2D navigation
When satellites are widely spread, geometry and accuracy are better.
GPS Augmentation
● Augmentation of GPS is required to meet the accuracy, integrity, continuity and availability requirements for aviation.
● There are currently three types of augmentation:
○ Aircraft-Based Augmentation System (ABAS)
○ Satellite-Based Augmentation System (SBAS)
○ Ground-Based Augmentation System (GBAS)
ABAS Aircraft-Based Augmentation System
● RAIM - Receiver Autonomous Integrity Monitoring.
● FDE - Fault Detection and Exclusion
RAIM Receiver Autonomous Integrity Monitoring
● It usually takes 4 satellites to compute a navigation solution, and a minimum of 5 for RAIM to function.
FDE Fault Detection and Exclusion
● FDE allows the avionics to detect which satellite is faulty, and then to exclude it from the navigation solution.
● FDE requires a minimum of 6 satellites with good geometry to function
Satellite-Based Augmentation System SBAS
● A network of ground-based reference stations that monitor navigation satellite signals and relay data to master stations, which assess signal validity and compute error corrections.
● The master stations generate two primary types of messages: integrity, and range corrections
● The information from the ground stations are then sent to satellites
● This information is then broadcasted to the aircrafts receiver, which can then apply the corrections
● Vertical guidance provides safer stabilized approaches and transition to visual for landing
● The other advantage is lower approach minima at many airports, as a result of greater lateral accuracy
SBAS = WAAS
● The first SBAS, the U.S. FAA WAAS, was commissioned in 2003.
● WAAS – Wide Area Augmentation System.
● WAAS currently uses 3 GEO satellites located over the equator
● Pilots in Canada can use GNSS (GPS, or GPS augmented by WAAS), to fly IFR in the en route, terminal, and NPA phases of flight.
● Approach procedures with vertical guidance (APV) classified as LPV (localizer performance with vertical guidance) and lateral navigation / vertical navigation (LNAV/VNAV) approaches may be flown using WAA
● Sat Nav capability may be provided by a GPS or WAAS sensor
GPS Airway Navigation
● When using GPS to navigate along airways, VOR or ADF reception is not a requirement. (After all, you won’t be using them, except as a backup!)
● This means that pilots using GNSS for navigation can file or request an altitude below the MEA to avoid icing, optimize cruise altitude, or in an emergency.
○ However, we must never fly or file below the MOCA
RNAV Minima
● RNAV (GPS) approach charts will, in many cases, depict three sets of minima:
○ LPV (localizer performance with vertical guidance – APV)
○ LNAV/VNAV (lateral/vertical navigation – APV)
○ LNAV (lateral navigation only – NPA non precision approach)
RNAV Approaches – LNAV Only
● RNAV (GPS) “LNAV only” approaches do not define a vertical path through space; as such, each approach segment has a minimum step-down altitude below which the pilot may not descend.
● These were once flown using the “level-descend-level” method familiar to most pilots
● The pilot is required to remain at or above the MDA unless a visual transition to landing can be accomplished, or to conduct a missed approach at the missed approach waypoint
Satellite Navigation (GNSS) Part 2
GPS Overlay Approaches
● GPS overlay approaches are traditional VOR- or NDB-based approaches that have been approved to be flown using the guidance of IFR approach-certified GNSS avionics
● GPS overlay approaches are identified in the CAP by including “(GNSS)” in after the runway designation [e.g. NDB RWY 04 (GNSS)]
● In many cases, the pilot can bypass the procedure turn (PT) and fly directly to the FAF (Final Approach Fix) for a more efficient approach, as long as MSAs are respected, and the approach plate or ATC authorizes the routing.
● Unless required by the aircraft flight manual (AFM), it is not necessary to monitor the underlying navigation aid, and it is even permissible to fly a GPS overlay approach when the navigation aid is temporarily out of service
GNSS may be used to substitute:
➢ DME during departure/enroute/terminal/approach operations;
➢ VOR or NDB
Vertical Guidance on RNAV Approaches
● Aircraft with WAAS avionics, may fly RNAV (GPS) approaches to LNAV/VNAV minima with vertical guidance in a similar manner to the way they fly an ILS approach: with both a lateral course deviation indicator (CDI) and a vertical deviation indicator (VDI).
● The lateral guidance must be based on GPS or WAAS.
● The vertical guidance may be based on WAAS, or on barometric inputs (BARO VNAV)
● The final approach vertical flight path angle for LNAV/VNAV and LPV approaches is 3 degrees
● The LNAV/VNAV and LPV minima depict a decision altitude (DA), which requires the pilot to initiate a missed approach at the DA if the visual reference to continue the approach has not been established
RNAV Approaches with BARO VNAV
● The vertical guidance for these systems has been derived from a barometric altitude input; hence BARO VNAV
RNAV Approaches with Vertical Guidance Based on WAAS
● RNAV (GNSS) approaches with vertical guidance-based on WAAS are entirely dependent on the WAAS signal.
● WAAS meets essentially the same navigation performance requirements (accuracy, integrity and continuity) as ILS
● WAAS avionics continuously calculate integrity levels during an approach, and will provide a message to the crew if alert limits are exceeded, analogous to ILS monitors that shut down an ILS signal when its accuracy does not meet the required tolerances
GNSS Instead of DME
● If the DME associated with an ILS/DME approach is unserviceable, traditional aircraft would be denied the approach; however, under these rules, the pilot of a GNSS-equipped aircraft may request and fly the approach
Non-Overlay Program Approaches
● For NDB or VOR approaches that are not part of the GPS overlay program, pilots shall use ADF or VOR as the primary source for final approach track guidance.
● For these approaches, and for approaches based on a localizer (LOC) for lateral guidance, pilots shall not use GNSS as the primary source for missed approach guidance when the missed approach procedure requires flying a published track to or from an NDB or VOR
Reporting DME Distance
● When reporting DME distance, the pilot includes “DME” in the report (eg. “30 DME from Sumspot VOR”).
● This enables ATS to allow for the DME slant range.
