GNSS Technology & Radio Navigation

• GNSS stands for Global Navigation Satellite System
• GPS is the most popular GNSS, and has become a system for everyday life
• GPS = Global Positioning System

Old Navigation Systems

• Before GNSS, navigation used Celestial bodies
• Sextants were used
• Navigational charts/maps were used
• Radar/Sonar navigation was used
• Iter-Avto was an innovative navigation system

Radionavigation Principle

• Position calculation uses Trilateration through time of arrival measurements
• The formula rk = c * tk is used
• Distance (r) to position (x, y) is calculated as rk = √(xk - x)² — (ук – y)²
• Minimum of three distance measurements are needed for 2D position calculation

GNSS Principle

• Pseudoranges are used for measurements.
• Satellite Positions (x(k), y(k), z(k)) are known
• The formula to calculate position is p(k) = (x(k)-x)² + (y(k)-y)² + (z(k)-z)² - b
• When K≥ 4, the formula can solve for user position (x, y, z), and receiver clock bias b

GNSS by Names

• There are multiple GNSS systems with different names, which are:
• NAVSTAR GPS*
• It is a Navigation System with Timing and Ranging Global Positioning System
• It has been around since 1995
• It is controlled by the US MoD
• It has 31 satellites in operation
• GLONASS*
• Is Globalnaya Navigazionnaya Sputnikovaya Sistema
• It has been around since 2012
• It is controlled by the Russian MoD
• It has 24 satellites in operation
• GALILEO*
• It is a European Global Navigation Satellite System
• It has been around since 2016
• ESA and 9 non-EU countries use it
• It has 22 satellites in operation
• BDS*
• It is a BeiDou Navigation Satellite System
• It is a Chinese system
• It has been around since 2000
• It has 35 satellites in operation
• IRNSS*
• Is an Indian Regional Navigation Satellite System
• It has been around since 2013
• It has 7 satellites in operation
• QZSS*
• Is a Quasi-Zenith Satellite System
• It is a Japanese system
• It has been around since 2010
• It has 5 satellites in operation

GNSS Components

• Has three segments
• Space Segment*
• Uses L1-carrier for
  • Time impulse
  • Ephemerides
  • Almanac
  • Status
  • Date, time
• Control Segment*
• Sends information from the ground to control stations
• User Segment*

Space Segment - Satellite System

• Consists of GNSS satellites constellation
• Each satellite includes up to 4 atomic clocks
• Each satellite transmits its precise position and time as a radio wave
• Signal needs ca. 67.3 ms to reach the measurement point on earth at nadir

Satellite System Data

Space Segment - Signal Transmission

• Signals transmit Satellite time, synchronisation signal, ephemerides, time correction procedure, almanac, correction signal for calculation of run time, data on Ionosphere, technical status of satellite → duration 12.5 min
• Signal is transmitted with C/A-Code (Coarse/Acquisition-Code) on frequency of 1,023MHz (L1-channel)
• Each satellite has unique code of 1023 numbers (binary code)
  • pattern has a length of 1 ms and is used for identification and measurement of run time

Control Segment - Ground Stations

• Master stations
  • e.g., GPS has two Master stations
  • Adjust satellite orbit parameters and has a high precision clock
• Data uploading and Monitoring stations
  • e.g. GPS has 16 monitoring stations
  • Installed over broad geographic area
  • Monitors satellite's signals and status

User Segment

• User Segment
• Equipment processes the received signal from the GNSS satellite;
  • Antenna
  • Receiver
  • Processing units

Error Sources

• Selective Availability*
• Prior to May 2000 precision of GPS signal degraded artificially
• Latest generation of satellites do not have this capability
• Dilution of Precision (DoP)*
• DoP is a measure of satellite geometry and indicates the quality of GPS measurement
• Error Sources*

• Atmosphere effect*
• Atmosphere causes delays to the satellite signals
• There is a 4 to 5m difference due to atmospheric disturbances
• Multipath*
• Smooth surfaces (e.g., water) or high objects (e.g., buildings) can reflect the GNSS signal
• Leads to higher inaccuracy
• Other errors*
• Satellite clock and orbit error
  • Continuously controlled and corrected
• Faulty antenna and/or receiving devices
• Measurement noise
• How to increase Position Accuracy*
• Two-frequency measurement (L1/L2): Phase shift between different frequency bands (L1 und L2) used
• Geophysical correction models: only useful for confined areas
• Differential-GPS (DGPS)
  • Post processing
  • Real time
• With DGPS Differential correction can be done real-time (RTC) or afterwards (post processed)