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Radio Navigation - Global Navigation Satellite Systems (GNSS)

34 important questions on Radio Navigation - Global Navigation Satellite Systems (GNSS)

4 main Global Navigation Satellite Systems (GNSS)

USA NAvigation System with Timing And Ranging Global Positioning System (NAVSTAR GPS)
Russian GLObal NAvigation Satellite System (GLONASS)
European GALILEO
Chinese BEIDOU

GPS health message

The GPS health message is used to exclude unhealthy satellites from the position solution. Satellite health is determined by the validity of the navigation data.

Two codes transmitted on L1 frequency

C/A code and a Precision (P) code. The P code is not used for SPS.

C/A code

the C/A code is a Pseudo Random Noise (PRN) code sequence, repeating every millisecond. Each C/A code is unique and provides the mechanism to identify each satellite.
Satellites broadcast the PRN codes with reference to the satellite vehicle time which are subsequently changed by the receiver to UTC.

Satellite time reference

Satellites are equipped with atomic clocks, which allow the system to keep very accurate time reference.

Master control station

The master control station is responsible for all aspects of the constellation command and control.

Tasks of control segment

— managing SPS performance;
— navigation data upload;
— monitoring satellites.

GPS receiver used in aviation

The GPS receiver used in aviation is a multichannel type.

GPS position fix

GPS supplies three-dimensional position fixes and speed data, plus a precise time reference.

GPS distance determination

GPS receiver is able to determine the distance to a satellite by determining the difference between the time of transmission by the satellite and the time of reception.

Determining a three-dimensional position

Four spheres are needed to calculate a three-dimensional position, hence four satellites are required.

Receiver ground speed determination

The receiver is able to calculate aircraft ground speed using the SV Doppler frequency shift and/or the change in receiver position over time.

NAVSTAR GPS integrity

‘Receiver Autonomous Integrity Monitoring (RAIM)’. A technique whereby a receiver processor determines the integrity of the navigation signals. RAIM is achieved by consistency check among pseudo-range measurements.
Basic RAIM requires five satellites. A sixth is for isolating a faulty satellite from the navigation solution.
When a GPS receiver uses barometric altitude as an augmentation to RAIM, the number of satellites needed for the receiver to perform the RAIM function may be reduced by one.

Three components of GLONASS

— space segment, which contains the constellation of satellites;
— control segment, which contains the ground-based facilities;
— user segment.

Space segment GLONASS

— 24 satellites in 3 orbital planes with 8 equally displaced by 45° of latitude;
— a near-circular orbit at 19 100 km at an inclination of 64.8° to the equator;
— each orbit is completed in 11 hours and 15 minutes.

Control segment GLONASS provides

— monitoring of the constellation status;
— correction to orbital parameters;
— navigation data uploading.

Navigation message GLONASS

Navigation message has a duration of 2 seconds and contains ‘immediate’ data which relates to the actual satellite transmitting the given navigation signal and ‘non-immediate’ data which relates to all other satellites within the constellation.

Immediate data GLONASS

— enumeration of the satellite time marks;
— difference between onboard time scale of the satellite and GLONASS time;
— relative differences between carrier frequency of the satellite and its nominal value;
— ephemeris parameters.

Non-immediate data GLONASS

— data on the status of all satellites within the space segment;
— coarse corrections to onboard time scales of each satellite relative to GLONASS time;
— orbital parameters of all satellites within the space segment;
— correction to GLONASS time relative to UTC (must remain within 1 microsecond).

Integrity monitoring GLONASS

— Continuous automatic operability monitoring of principal systems in each satellite. If a malfunction occurs, an ‘unhealthy’ flag appears within the ‘immediate data’ of the navigation message.
— Special tracking stations within the ground-based control segment are used to monitor the space-segment performance. If a malfunction occurs, an ‘unhealthy’ flag appears within the ‘immediate data’ of the navigation message.

Timing section GALILEO

In the ‘timing section’ two clocks have been developed, a Rubidium Frequency Standard clock and a more precise Passive Hydrogen Maser clock.

Navigational signals GALILEO

The navigation signals consist of a ranging-code identifier and the navigation message.

Navigation message GALILEO

The navigation message basically contains information concerning the satellite orbit (ephemeris) and the clock references.

Navigation antenna GALILEO

The navigation antenna has been designed to minimise interference between satellites by having equal power level propagation paths independent of elevation angle.

GPS, EGNOS and GALILEO

GPS, EGNOS and GALILEO are compatible, will not interfere with each other, and the performance of the receiver will be enhanced by the interoperability of the systems.

Most significant factors affecting accuracy

— ionospheric propagation delay;
— dilution of position;
— satellite clock error;
— satellite orbital variations;
— multipath.

Inonospheric Propagation Delay (IPD)

Can almost be eliminated by using two frequencies.
In SPS receivers, IPD is currently corrected by using the ionospheric model from the navigation message, but the error is only reduced by 50 %.
Ionospheric delay is the most significant error.

Errors in the satellite orbits are due to:

— solar wind;
— gravitation of the Sun, Moon and planets

GBAS ground subsystems provide two services

Precision approach service
GBAS positioning service

Precision approach service GBAS

The precision approach service provides deviation guidance for final-approach Segments

GBAS positioning service

The GBAS positioning service provides horizontal position information to support RNAV operations in terminal areas.

SBAS consists of three elements

— the ground infrastructure (monitoring and processing stations);
— the SBAS satellites;
— the SBAS airborne receivers.

SBAS approach and landing operations

SBAS can provide approach and landing operations with vertical guidance (APV) and precision approach service.

Satellite-Based Augmentation Systems include:

— EGNOS in western Europe and the Mediterranean;
— WAAS in the USA;
— MSAS in Japan;
— GAGAN in India.

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