Chapter 16

GPS/Satnav

GPSGlobal Positioning System

Will eventually replace the older, radio/radar based systems of VOR, ILS and NDB.

The US system is GPS, Europe’s system is Galileo.

Global Navigation Satellite SystemGLONASS

Global'naya Navigatsionnaya Sputnikovaya Sistema

GPS Constellation

There are 24 satellites in orbit: 21 active plus 3 spares.They are 11,000 miles above the earth.They have 6 orbital planes which take 12 hours to complete one orbit.At least 5 or more satellites are available for navigational use at one time.

A 4B 5C 5 (6)

D 5E 6F 5

Number of satellites 30

http://www.navcen.uscg.gov/.

Schriever Air Force Base

3 GPS Segments

There are 3 major components of the GPS System.The Space Segment consists of satellites (SV’s for Satellite Vehicles).

The Control Segment is a station located in Colorado Springs that connects to 5 or more ground stations around the world.

Orbit information and clock corrections are uplinked to the satellite several times a day.

The User Segment consists of airplanes, ships, other vehicles and portable GPS.

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Satellite Signal Frames

The signal is broadcast in “frames” of information. Page 113

Signal Transmission

The signal from the satellite is transmitted as a pulse code, each satellite sends a unique code.The receiver in the airplane already knows the code patterns sent by every satellite.It searches until it finds a satellite signal that matches a stored pattern.By comparing this time with the time or arrival at the receiver, a time difference is calculated.This is multiplied by the speed of light (the speed of signal transmission) and the answer is distance.

A

B

On

Off

On

Off

On

Off

On

Off

On

Off

A

A+B

B

GPS Frequencies

L1 was shared between civil and military use.

L2 was exclusively military and used in conjunction with L1.

The advantage of using 2 frequencies is the ability of the receiver to measure and correct propagation error.

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L1 = 1575.42 MHzL5 = 1176.45 MHz

L5 L1 L5 L1

WAASWide Area Augmentation System

WAAS is a form of differential GPS designed to bring the accuracy up to Category I approach standards.

The 25 ground stations are spaced several hundred miles apart all over the US. These stations are precisely surveyed so their exact position is known.The ground stations pick up the signal and determine the error.

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LAAS Local Area Augmentation System

Receiver Autonomous Integrity Monitoring(RAIM).

When GPS equipment is not using integrity information from WAAS or LAAS, the GPS navigation receiver using RAIM provides GPS signal integrity monitoring.

RAIM needs a minimum of 5 satellites in view, or 4 satellites and a barometric altimeter (baro−aiding) to detect an integrity anomaly.

For receivers capable of doing so, RAIM needs 6 satellites in view (or 5 satellites with baro−aiding) to isolate the corrupt satellite signal

AC Advisory Circulars

 

AC 20−138,

Airworthiness Approval of Global Positioning System (GPS) Navigation Equipment for

Use as a VFR and IFR Supplemental Navigation

 

AC 20−130A, Airworthiness Approval of Navigation or Flight Management Systems Integrating Multiple Navigation Sensors, or equivalent. Equipment approved in

Introduction to EFISElectronic Flight Instrument

System

Electronic Flight Instrument System EFIS

Textbook page 124

Review Q&A Chapter 16 GPS/Satnav16.1 How many satellites are there in a GPS constellation?Answer: 24.16.2 How many GPS satellites are active? How many are spares?Answer: 21, 316.3 What is the European equivalent of GPS?Answer: Galileo16.4 Why don’t GPS receivers in airplanes require expensive atomic clocks (like those in satellites) to measure time with high accuracy?Answer: The receiver gets the clock time from the satellite and corrects for error.16.5 How does a GPS receiver identify a satellite?Answer: By it’s unique pulse code.16.6 What is the term for a satellite's identity?Answer: PRN Code, or pseudorandom code.16.7 How does a GPS receiver measure the time for the signal to travel from satellite to receiver?Answer: By comparing the transmit time with the time the signal was received.16.8 How is distance determined between the GPS receiver and the satellite?Answer: By taking the time it takes for the signal to travel and multiplying by the speed of light.16.9 GPS frequencies, or channels, are designated by the letter ____.Answer: L.16.10 How many satellites are required for a three-dimensional fix (latitude, longitude and altitude)?Answer: 416.11 How many satellite frequencies are required to perform propagation corrections?Answer: 216.12 What part of the satellite signal carries the satellite’s precise position in orbit? Answer: The Ephemeris16.13 Name the three segments of the GPS system.Answer: Space, User and Command16.14 The Wide Area Augmentation System (WAAS) uses ground stations and satellites to ____.Answer: Bring GPS up to Category I standards16.15 What is the advantage of LAAS (Local Area Augmentation System) over WAAS?Answer: More precise for that airport.16.16 What is the purpose of RAIM (Receiver Autonomous Integrity Monitoring)?Answer: To insure that GPS is functioning up to precision approach standards.