Chapter 16
GPS/Satnav
GPS
Global 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 System
GLONASS
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
B
C
D
E
F
4
5
5 (6)
5
6
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).
Page 114
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.
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.
On
A
Off
On
Off
B
On
Off
On
Off
On
Off
A+B
A
B
GPS Frequencies
Page 110
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.
L1 = 1575.42 MHz
L5 = 1176.45 MHz
L5
L1
L5
L1
WAAS
Wide Area Augmentation System
page 115
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.
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 EFIS
Electronic Flight Instrument
System
Electronic Flight Instrument System
EFIS
Textbook page 124
Review Q&A Chapter 16 GPS/Satnav
16.1 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, 3
16.3 What is the European equivalent of GPS?
Answer: Galileo
16.4 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: 4
16.11 How many satellite frequencies are required to perform propagation corrections?
Answer: 2
16.12 What part of the satellite signal carries the satellite’s precise position in orbit?
Answer: The Ephemeris
16.13 Name the three segments of the GPS system.
Answer: Space, User and Command
16.14 The Wide Area Augmentation System (WAAS) uses ground stations and satellites to ____.
Answer: Bring GPS up to Category I standards
16.15 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.