Global Navigation Satellite System
Global Navigation Satellite System (GNSS)
- Global Positioning System (GPS) is the most commonly used Global Navigation Satellite System (GNSS) in Civil Aviation
- GPS provides Standard Position Service of 30M for all users and Precision Position Service with an accuracy of 1M for US Military Forces
- Russian GLONASS, European GALILEO, Chinese BEIDOU and Indian INSAT are other operational GNSS systems
- Satellites called as Space Vehicles used in GNSS follow Kepler’s laws of planetary motion
- GPS Position Reference System is referenced to centre of earth in three dimensions based on World Geodetic Survey of 1984 (WGS-984)
- WGS considers earth to be perfectly spherical therefore height indications must be used with caution
- GPS System can be divided into Space segment consisting of satellites with atomic clocks, Control segment consisting of ground control stations and User segment consisting of GPS receivers
GPS Space Segment
- GPS Space Segment consists of 24 Space Vehicles including 21 Operational and 3 Spares
- SVs are positioned in 6 Orbital Planes with at the same Height of 20,180 Km and Orbital Period of 12 hours
- Orbital plane of all SV’s are equally spaced at an inclination angle of 55° to the equator
- The orbital planes are designed to ensure that an observer on earth has 5 to 8 Space Vehicles in his view
- Satellite in view means that the SV is at least 5° above the horizon that are considered usable for operation in the area
GPS Pseudo Random Noise Code (PRNC)
- Each Space Vehicles transmit a unique long coded message called Pseudo Random Noise Code that appears to be random numbers
- PRNC includes Satellite Vehicle Position, Clock Time, Clock Error, Ionospheric Condition, Almanac, Health Of Satellite as well as Command and Control
- The coded message is repeated at regular intervals with an epoch or the duration of transmission of 1 milli second
Standard Positioning (SPS) and Precise Positioning Service (PPS)
- Standard Positioning Service (SPS) is available to all users employs Coarse Acquisition (CA) Code
- Coarse Acquisition Code type of PNRC is uses L1 Frequency of 1575.42 MHz with a modulation of 1.023 MHz that is repeated every milli-second
- Precise Positioning Service (PPS) available to US military uses Coarse Acquisition and Precision Code (P-Code)
- Precision Code type of Pseudo Random Noise Code uses L2 Frequency of 1227.6 MHz with a modulation of 10.23 MHz
- Navigation and System Data is transmitted on P-Code with a Modulation of 50 Hz that is transmitted once in every 7 Days
- Anti-spoofing measures protect P Code from unauthorized users by encoding and the resultant code is called as Y Codes
GPS Space Vehicle Navigation Message
- GPS Navigation Message has one frame of 30 sec with 5 sub-frames of 6 seconds each
- Message contains one telemetry word (TLM) of 8-bits, Handover word (HOW) of 17-bits and Space Vehicle time of the week
- The message also transmits SV clock correction, Two parts of Ephemeris data, Ionospheric propagation model and the Corrected UTC
- An additional 25 frames are required to download the full Satellite Vehicle Constellation Almanac
Function of GPS Receiver
- Receiver obtains the Pseudo Random Noise Code from Space Vehicle as well as produces its own internal digital code like CA code similar to the one sent by Space Vehicle
- Distance between the Space Vehicle and Aircraft is calculated by the measured time delay between codes received from space-vehicle and receiver
- GPS Receiver receives Pseudo Range from four Space Vehicles to accurately fix an aircraft’s position
GPS Position Fix
- Range Position Lines or Pseudo Range from four satellites provide a 3D Position Fix of the aircraft
- One SV Pseudo-range provides a position line 10,900 nm while two SV Pseudo-ranges would provide a circular position line
- Three SV Pseudo-ranges would narrow aircraft to two possible positions in which one would be on earth and other in space
- Fourth SV Pseudo-range would be able to provide a single unique position fix
Control Segment of GPS
- Control segment comprises of Master, backup, Monitoring, operational and Future Control Stations
- Ground Control stations calculate the position errors due to the gravitational influence of Sun Moon Planets and Solar Radiation
- Master control station control the Monitoring and Operational Stations and Back-up control station is a back up for Master Control
- Monitoring stations check the Space Vehicle’s computed position and clock time every 12 hours and sends correct position and clock time to the SV
- Space Vehicles receive the Clock time error and adjusts its Broadcast PRNC
- Operational control segment (OCS) station Monitors the health of satellites
- National Geo-spacial Intelligence Agency (NGIA) monitors and provide precise orbital data
- Future monitoring stations are provided for future improvements
User Segment of GPS
- GPS Receivers on-board the aircraft could be either Sequential, Multiplex and Multi-Channel Receivers
- Sequential receiver scans satellite vehicles sequentially on 1 or 2 channels to obtain Range position line or pseudo-range one after another
- Multiplex receivers move quickly from one space vehicle to another providing faster time to first fix
- Multi-channel receiver is also called as an All-In-View receiver scans all space vehicles in view and selects the best suited for position fix
- Aircraft’s position with accurate latitude and longitude from GPS can be displayed on moving map display along with
- Date and time in UTC as well as track and ground speed of the aircraft are calculated and provided to the pilots
GPS Pseudo Range or Range Position Line
- Range position line is calculated from time taken for radio energy to travel from Space Vehicle to Aircraft
- Space vehicle time from two Caesium and Rubidium atomic clocks that are corrected by the ground control station is sent as Pseudo Random Noise Code
- Time monitored by receiver is not as accurate as SV Time and is hence further resolved by a signal process
- Final time obtained from PRNC is a number that provides months and weeks from 5 January 1980
- The time-delay is corrected to 1 micro second of UTC is used for calculation of Range Position line
Time Taken for First Position Fix
- If approximate position is known and almanac is up to date commence position update immediately since the receiver is aware of space vehicle in view
- The time to first fix will be 15 min that includes 12.5 Minutes to download almanac, 2.5 Minutes for sky search and select 4 suitable space-vehicles and 30 Seconds to obtain the first fix
Errors of GPS
- GPS errors include SV Geometry, Ephemeris, Clock Bias, Orbital Perturbations, Geometric Dilution of Position, Ionospheric Propagation and Delays
- Selective Availability, Receiver Noise, Multi Path Error, Receiver Manufacturing Error and Tropospheric Error adds to the GPS Position errors
- GPS errors are stated at 95 % probability level which means 19 out of 20 times the error should be within the specified limits
- ICAO accuracy requirement for navigation using GPS is 13m in horizontal, 22m in vertical and 40 nano seconds in UTC
Clock Bias Error
- Clock-bias error occurs due to difference between the clock time of space-vehicle and the receiver
- Clock bias is reduced by inducing a small error in receiver’s clock that produces a cocked-hat restricting clock-bias error to one direction
SV Geometry, Ephemeris, Clock Bias and Orbital Perturbations
- SV Geometry error occurs due to the space-vehicle not following their predicted orbit due to gravitational effects and solar radiation
- SV Orbital Perturbations occur due to other close by satellites
- SV Clock bias occurs due to error in SV clock that can alter position up to 1.5 M
Geometric Dilution of Position
- Geometric dilution of precision (GDOP) occurs due to poor cross cut as a result of satellites being close to each other
- Ideal cross cut range position lines can be obtained when 1 SV is overhead and 3 SVs equally spaced 120° spaced apart
Ionospheric Propagation Error and Delay
- Ionospheric Propagation Error is a result of variation of the current Ionosphere from the Average Ionosphere stored in the database
- Ionospheric Delay is Inversely Proportional to Square of SV Frequency which can be up to 5 M in a Single SV
- The delay can be calculated and corrected by comparison with other Satellites
Selective Availability
- Selective Availability error is an intentional error caused by US Airforce that reduces the accuracy of GPS at a specific location
- Accuracy is reduced by Dithering or changing the clock-time of a specific satellite up to 100 M
Receiver Noise, Multi-path, Manufacturing and Tropospheric Errors
- Receiver noise error is a result of internal noise in the receiver electronics that could be up to 0,3 M
- Multipath error is caused due to reflection from ground or other objects
- Receiver manufacturing errors are due to manufacturing imperfections
- Tropospheric Error is a result of Pressure, Density, Temperature or Humidity in the troposphere