Global Positioning System (GPS)

Question 1

GPS advantages

Answer 1

•Highly cost effective
•Available 24 hours a day
•Unlimited number of users
•Very accurate
•Not affected by weather

Question 2

Four Different GNSS Systems

Answer 2

•Russian GLONASS
•European Galileo
•United States GPS
•China BeiDou

Question 3

GPS space segment

Answer 3

•Over 30 NAVSTAR Satellites (constellation)
•Orbit 11,000 nm above Earth– 60° apart
•6 orbital planes/ 4 satellites in each
•Positioned so that 5+ satellites will be in view of user
•Each satellite has an atomic clock used to send out their time as well as location that will help with trilateration

Question 4

GPS control segment

Answer 4

•Ground stations
•Monitor satellite performance and make adjustments to maintain accuracy
•Location of Master Ground Station
•Schriever (Falcon) Air Force Base in Colorado Springs

Question 5

GPS user segment

Answer 5

•Airborne GPS unit
•Must meet certain standards to operate IFR
•Handheld GPS units do not meet this requirement
•Must review NOTAMs prior to IFR flight

•Different Manufacturers
•Examples
•Garmin
•Bendix-King

Question 6

How GPS works

Answer 6

•GPS satellite transmits code on two frequencies
•Carries both status message and Pseudo Random Code for timing
•Each satellite transmits a unique complicated Pseudo Random Code
•Course Acquisition (CA code)
•Speed – 186,000 miles per second
•Receiver measures Travel Time and also relates it to the satellites exact position
•Uses trilateration to measure position

Question 7

GPS trilateration

Answer 7

•Trilateration involves measuring distances
•Triangulation measures angles
•Satellites broadcast a signal picked up by your receiver with time and distance.
•Distance forms a circle equal in all directions
•You are anywhere on this circle at this radius
•Do this three times

Question 8

Why do we need a fourth satellite

Answer 8

•The first three satellites are able to determine our possible position (2D)
•The fourth satellite gives us a 3D picture of our location
•With this, they are able to calculate your exact position
•(X,Y,Z and TIME)

Question 9

Receiver Autonomous Integrity Monitoring (RAIM)

Answer 9

•System to verify the authenticity and accuracy of the GPS signal
•Needs minimum of 5 satellites in view, or 4 with a barometric altimeter
•Baro-aiding: Enter current altimeter setting into receiver to aid
•6 satellites to isolate a corrupt satellite and remove it from the navigation solution
•Two types of RAIM error messages
•Not enough satellites
•Detection of possible error
•Without RAIM we cannot ensure accuracy of GPS signal

Question 10

GPS in IFR

Answer 10

•Must meet standards
•Technical Standard Order (TSO) C-129
•Must have appropriate data base for operations
•Must be equipped with an approved alternate means of navigation appropriate to the flight (includes the alternate).
•If you have RAIM you do not have to monitor other navigation sources

Question 11

GPS restrictions

Answer 11

•Proper install
•RAIM (enroute at a minimum)
•Current database and fixes retrieved from database
•Data base must be updated every 28 days
•RAIM outage alternative plan
•CDI in terminal mode (1NM) in terminal area
•Non-GPS approach must exist at alternate airport
•If DME/ADF is required for the non-GPS approach the aircraft must be equipped with it
•GPS cannot be used to meet principle instrument approach procedure requirements (the source)

Question 12

GPS substitution

Answer 12

Substitute for ADF and DME to include:
•Determining position over a DME fix (includes FL240)
•Flying a DME arc
•Navigating to/from a NDB/compass locator
•Determining position over NDB/compass locator
•Holding over a NDB/compass locator

Question 13

GPS errors

Answer 13

•30 satellites normal, down to 24 and GPS may not be available in certain geographic locations
•Antenna shadowed in bank
•Deep valleys with high mountains
•Exchange errors
•Between the receiver and satellite
•GPS ‘LOI’ Indicates Loss of Integrity of GPS information

Question 14

Differential Global Positioning Systems (DGPS)

Answer 14

•Measures changes in variables to provide a more accurate signal
•Provides a reference signal correction from a reference receiver placed at a known location
•Two Types
•Wide Area Augmentation System (WAAS)
•Local Area Augmentation System (LAAS)

Question 15

Wide Area Augmentation System (WAAS or SBAS)

Answer 15

•Space Based Augmentation System
•WAAS is designed to improve the accuracy, integrity, and availability of GPS signals.
•WAAS service is available for all classes of aircraft in all phases of flight, including: en route navigation, airport departures, and airport arrivals.
•This includes vertically-guided instrument approaches in IMC at all qualified locations throughout the US national airspace system.
•More and more aircraft are WAAS-capable, allowing them to fly more precise GPS instrument approaches.
•WAAS corrects GPS satellite orbit and clock drift and signal delays caused by the atmosphere and ionosphere (ephemeris errors)

Question 16

Local Area Augmentation System (LAAS)

Answer 16

•LAAS mitigates GPS threats in the Local Area to a much greater accuracy than WAAS and therefore provides a higher level of service not attainable by WAAS.
•Allows for a much precise approach