The document summarizes the main global navigation satellite systems (GNSS) and their components. It discusses the two main types - GPS and GLONASS, with two more systems becoming operational in the future - Galileo and Beidou. GNSS consist of satellite constellations that receivers can use to determine position. The key components are the space, control, and user segments. Sources of error and different augmentation systems to improve accuracy are also outlined.

1. TWO MAIN TYPES :-
- USA NAVSTAR GPS (NAVigation System with Timing And Ranging Global Positioning System)
- Russian GLONASS (Global Navigation Satellite System)
• Two more will become fully operational in the coming years:-
- European GALILEO
- Chinese BEIDOU
• WHAT THEY DO AND ARE?
- Consist of a constellation of satellites which can be used by suitably equipped receivers to
determine position
- Are interoperable

2. • To define satellite track we need to know
the track of Earth. Based on Keplers laws
:-
• First Law : Orbit of each planet is an
ellipse with Sun as its focii.
• Second Law : Line joining the Earth and
sun sweeps equal area in equal time.
• Using these laws, and given a starting
point, the satellites - space vehicles (SVs)
calculate their positions at all points in
their orbits. The SVs’ orbital position is
known as ephemeris.

3. • GNSS use an earth referenced three
dimensional Cartesian coordinate system
with its origin at the centre of the earth.
• Common Model of earth used :-
1. GPS : WGS 84 (The World Geodetic Survey of
1984)
2. GLONASS : PZ 90
3. GALILEO : ETRS 89

4. • 1. Space segment: constellation of satellites
transmitting radio signals to users.
• 2. Control segment: consists of a global network of
ground facilities that track the GPS satellites,
monitor their transmissions, perform analyses, and
send commands and data to the constellation.
• 3. User segment: consists on L-band radio
receiver/processors and antennas which receive
GPS signals, determine pseudoranges (and other
observables), and solve the navigation equations in
order to obtain their coordinates and provide a
very accurate time.
• GPS time is measured in weeks and seconds from
00:00:00 on 06 January 1980 UTC. An epoch is
1024 weeks after which the time restarts at zero.

5. • Comprises 24 SVs. (Currently the USA has 31
SVs).
• Average height of 10898 NM (20180 km).
• Orbital period of 12 hours.
• orbital planes have an inclination of 55° and
are equally spaced around the equator.
• An observer on or close to the surface of the
earth will have between five and eight SVs in
view, at least 5° above the horizon.An SV will
be masked if its elevation is less than 5° above
the horizon.

6. • Each satellite broadcasts ranging signals on two UHF frequencies
- L1 1575,42 MHz
- L2 1127,60 MHz
• GPS can operate in two different modes:
- SPS (Standard Positioning Service): civilian users
- PPS (Precise Positioning Service): authorised users
• SPS is a positioning and timing service provided on L1 frequency
• PPS uses both L1 and L2 frequencies(why?,standby)
The C/A code is a pseudo random noise (PRN) code sequence
Repeats every millisecond. Is unique and therefore provides the
mechanism to identify each satellite

8. • The control segment comprises:
• A master control station (plus an alternative master
control station)
• 12 command and control ground antennas
• 16 monitoring stations
• The main tasks of the control segment are:
• - Managing SPS performance
• - Navigation data upload
• - Monitoring satellites

9. • All the GPS receivers using the space segment to determine position on and close to the
surface of the earth
• Sequential receivers : scan the SVs sequentially to determine the pseudo-ranges.
• Multiplex receivers : able to move quickly between SVs. Faster.
• Multi-channel receivers: monitor all the SVs in view and select the best 4 to determine
position. Preferred type for aviation.
• The initial distance calculated to the satellites is called “pseudo range” as it is biased by
the lack of time synchronisation between GPS satellite and receiver clocks. Also biased by
other effects such as ionosphere, troposphere and signal noise.

10. • • Each range defines a sphere with its centre at the
satellite
• • Three spheres (hence three satellites) are needed to
determine a two-dimensional position
• • Four spheres (hence four satellites) are needed to
determine a three dimensional position
• • The GPS receiver synchronises to the correct time base
when receiving four satellites
• The use of 4 SVs provides a 3D fix and an accurate time
reference, i.e. a 4D fix, at the receiver. The X, Y, and Z
coordinates can now be transposed into latitude and
longitude.

11. 1. Ephemeris Errors : caused by the gravitational effects of
the sun, moon, planets and solar radiation.
2. SV Clock Error
3. Ionospheric Propagation Error : interaction of the radio
energy with the ionized particles in the ionosphere causes
the radio energy to be slowed down as it traverses the
ionosphere.The higher the frequency is, the smaller the
delay and the higher the levels of ionization, the greater the
delay.The ionospheric delay is inversely proportional to the
square of the frequencies. Most significant error upto 5 m.
As two different frequencies will experience different
delays, by measuring the difference in arrival time of the
two signals we can deduce the total delay experienced
hence minimising the error and calculate a very accurate
range. MOST SIGNIFICANT ERROR. ANSWER:)
4. Tropospheric Propagation Error : Variations in pressure,
temperature, density and humidity affect the speed of
propagation.
5. Receiver Noise Error
6. Multipath Reception : Reflections from the ground and
parts of the aircraft.

12. • The ICAO specification requires an accuracy
(95%) of the SPS to be:
• • Horizontal: ± 13 m
• • Vertical: ± 22 m
• • Time: 40 nanoseconds (10-9)
• If the SV information degrades, the GPS
receiver has no means of determining the
degradation. Consequentially the safety of
flight may be seriously endangered. DGPS is a
means of improving the accuracy of GPS by
monitoring the integrity of the SV data and
warning the user of any errors which occur
• . There are three kinds of DGPS currently in use or
under development:
• Air based augmentation systems (ABAS)
• Ground based augmentation systems (GBAS)
• Satellite based augmentation systems (SBAS)

13. • If any of the data from any of the SVs is in error requires the use of a fifth SV. By comparing positions
generated by the combinations of the five SVs it is possible to detect errors in the data, and hence
which SV is in error. The rogue SV can then be deselected.
• However, once the system is back to 4 SVs the facility is lost. The CAA recommend that a minimum of 6
SVs are available, so that if a SV is deselected the integrity monitoring continues to be available. The
GPS term for this is “receiver autonomous integrity monitoring” (RAIM)
• 5 SVs : Fault detection only
• 6 SVs : Fault detection and exclusion
• ABAS does not provide improved position accuracy.

14. • Sometimes called LAAS: Local Area Augmentation System
• provide both failure warning and enhancement of the
GPS receiver position by removing ephemeris and SV
clock errors and minimizing ionospheric and tropospheric
errors.
• requires a precisely surveyed site on the aerodrome and
a means of transmitting the corrections to aircraft
operating close to the aerodrome. On the site is a GPS
receiver which determines the GPS position and
compares it with the known position of the site. The error
in the X, Y and Z coordinates is determined and specially
formatted to be transmitted to approaching aircraft.
• The data is transmitted to aircraft via a dedicated VHF
link. A pseudolite (pseudo-satellite) is also provided to
give range to the runway threshold using GNSS
techniques.
• The coverage of the GBAS station is of about 30 km. The
LAAS has the potential to provide the necessary accuracy
to achieve category IIIC type operations.

15. • Measure on the ground the signal errors transmitted by GNSS satellites
and transmit differential corrections and integrity messages through
geostationary satellites(height 35800km,24 hr orbital period).
• SBAS can provide:
1. Approach and landing operations with Vertical guidance (APV)
2. Precision approach service
• SBAS include:
• - EGNOS in Europe
• - WAAS in USA
• - MSAS in Japan
• - GAGAN in India
• - SDCM in Russia
• - SNAS is China
• (The best decision height achieved to date is about 300 ft, and this is
unlikely to be improved upon in the near future