Global Navigation Satellite System (GNSS)

The global navigation satellite system (GNSS) is a worldwide position and time determination system that includes one or more satellite constellations, aircraft receivers and system integrity monitoring, augmented as necessary to support the required navigation performance for the intended operation. The Global Positioning System (GPS) is the name given to the American GNSS satellite constellation. The design GPS constellation contains 24 GPS satellites, orbiting the earth twice a day at an altitude of 10,900 NM (20,200 km). They are arranged in six separate orbital planes, with four satellites in each. There are approximately 32 operational satellites, however, at any given time, one or more may be decommissioned or be out of service temporarily for maintenance.

GPS positioning is based on precise timing. Each satellite has four atomic clocks on board, guaranteeing an accuracy of one billionth of one second, and broadcasts a digital PRN code that is repeated every millisecond. All GPS receivers start generating the same code at the same time. Code matching techniques establish the time of arrival difference between the generation of the signal at the satellite and its arrival at the receiver. The speed of the signal is closely approximated by the speed of light, with variations resulting from ionospheric and atmospheric effects modeled or directly measured and applied. The time of arrival difference is converted to a distance, referred to as a pseudorange, by computing the product of the time of arrival difference and the average speed of the signal. The satellites also broadcast orbit information (ephemeris) to permit receivers to calculate the position of the satellites at any instant in time.

A receiver normally needs four pseudoranges to calculate a three-dimensional position and to resolve the time difference between receiver and satellite clocks. GPS accuracy depends on transit time and signal propagation speed to compute pseudoranges. Therefore, accurate satellite clocks, broadcast orbits, and computation of delays as the signals pass through the ionosphere are critical. The ionosphere, which is a zone of charged particles several hundred kilometres above the Earth, causes signal delays that vary from day to night and by solar activity. Current receivers contain a model of the nominal day/night delay, but this model does not account for variable solar activity. For applications requiring high accuracy, GPS needs an augmentation system to correct the computed transit time to compensate for this delay.

Receiver Autonomous Integrity Monitoring (RAIM) and Fault Detection and Exclusion (FDE) functions in current IFR-certified avionics are considered ABAS. RAIM can provide the integrity for the enroute, terminal, and NPA phases of flight. RAIM uses extra satellites in view to compare solutions and detect problems. It usually takes four satellites to compute a navigation solution, and a minimum of five for RAIM to function. The availability of RAIM is a function of the number of visible satellites and their geometry. It is complicated by the movement of satellites relative to a coverage area and temporary satellite outages resulting from scheduled maintenance or failures.

If the number of satellites in view and their geometry do not support the applicable alert limit (2 NM en route, 1 NM terminal and 0.3 NM NPA), RAIM is unable to guarantee the integrity of the position solution; however, this does not imply a satellite malfunction. In this case, the RAIM function in the avionics will alert the pilot, but will continue providing a navigation solution. Except in cases of emergency, pilots must discontinue using GNSS for IFR navigation when such an alert occurs.

A second type of RAIM alert occurs when the avionics detects a satellite range error (typically caused by a satellite malfunction) that may cause an accuracy degradation that exceeds the alert limit for the current phase of flight. When this occurs, the avionics alerts the pilot and denies navigation guidance by displaying red flags on the HSI or CDI. Continued flight using GNSS is then not possible until the satellite is flagged as unhealthy by the control centre, or normal satellite operation is restored.

Some avionics go beyond basic RAIM by having an FDE feature that allows the avionics to detect which satellite is faulty, and then to exclude it from the navigation solution. FDE requires a minimum of six satellites with good geometry to function. It has the advantage of allowing continued navigation in the presence of a satellite malfunction.

The U.S. FAA’s Wide Area Augmentation System (WAAS) is an example of a Space Based Augmentation System (SBAS) . SBAS uses a network of ground-based reference stations that monitor navigation satellite signals and relay data to master stations, which assess signal validity and compute error corrections. The master stations generate two primary types of messages: integrity, and range corrections. These are broadcast to SBAS-capable GNSS receivers via GEO satellites in fixed orbital positions over the equator. The SBAS GEO satellites also serve as additional sources of navigation ranging signals. The integrity messages provide a direct validation of each navigation satellite’s signal. This function is similar to RAIM, except that the additional satellites required for RAIM are not necessary when SBAS integrity messages are used. The integrity messages are available wherever a GEO satellite signal can be received.

SBAS-capable GNSS receivers can compute the position of the aircraft with the accuracy necessary to support flight operations with vertical guidance. Vertical guidance provides safer stabilized approaches and transition to visual for landing. This represents one of the principal benefits from SBAS service. The other is lower approach minima at certain airports, as a result of greater lateral accuracy.

GBAS, also known as LAAS, sends corrections directly to GBAS-capable receivers from a ground station at an airport. GPS receivers with antennas at surveyed surface locations provide measurements used to generate and broadcast pseudorange corrections. Aircraft receivers use the corrections for increased accuracy, while a monitor function in the ground station assures the integrity of the broadcast. GBAS provides service over a limited area, typically within 30 NM of the ground station. GBAS is not yet available in Canada.

In practice, pilots can use GNSS for guidance most of the time. If an integrity alert occurs while en route, the pilot can then continue by using conventional aids, diverting if necessary from the direct routing, notifying ATS of any changes to the flight and obtaining a new clearance, as required.

When using GNSS to navigate along VHF/UHF or LF/MF airways, ground-based NAVAID reception is not an issue. This means that pilots using GNSS for navigation can file or request an altitude below the MEA, but at or above the MOCA, to avoid icing, optimize cruise altitude, or in an emergency. However, an ATS clearance to fly at a below-MEA altitude could be dependent on issues such as radiocommunication reception and the base of controlled airspace. In the rare case of a RAIM alert while en route below the MEA, and out of range of the NAVAID, pilots should advise ATS and climb to continue the flight using alternate means of navigation.

GNSS-based approaches are charted as “RNAV (GNSS) RWY XX.” The “(GNSS)” before the runway identification indicates that GNSS must be used for guidance. Pilots and controllers shall use the prefix “RNAV” in radio communications (e.g. “CLEARED TO THE VANCOUVER AIRPORT RNAV RUNWAY ZERO FOUR APPROACH”).

GNSS-based RNAV approaches are designed to take full advantage of GNSS capabilities. A series of waypoints in a “T” or “Y” pattern eliminates the need for a procedure turn. The accuracy of GNSS may result in lower minima and increased capacity at the airport. Because GNSS is not dependent on the location of a ground-based aid, straight-in approaches are possible for most runway ends at an airport.

In Canada, RNAV (GNSS) approach charts may depict up to five sets of minima:

• LPV (Approach Procedure with Vertical Guidance)
• LP (Non Precision)
• LNAV/VNAV (Approach Procedure with Vertical Guidance)
• LNAV (Non Precision)
• CIRCLING 

The LP and LNAV minima indicate an NPA, while the LNAV/VNAV and LPV minima refer to APV approaches (RNAV approaches with vertical guidance). However, the actual terms “NPA” and “APV” do not appear on the charts because they are approach categories not related to specific procedure design criteria. In Canada, the depiction of the five sets of minima is similar to the way that an ILS approach may show landing minima for ILS, LOC and CIRCLING.

GNSS overlay approaches are included on certain traditional VOR or NDB based approaches, that have been approved to be flown using the guidance of IFR approach certified GNSS avionics. Because of approach design criteria, LOC-based approaches cannot be overlaid. GNSS overlay approaches are identified in the CAP by including “(GNSS)” after the runway designation (e.g. NDB RWY 04 [GNSS]). When using GNSS guidance, the pilot benefits from improved accuracy and situational awareness through a moving map display (if available) and distance-to-go indication. Unless required by the AFM or AFM supplement, when conducting GNSS overlay approaches, the VOR, DME and/or NDB onboard navigation equipment does not need to be installed and/or functioning and the underlying approach navigation aid(s) do(es) not need to be functioning. Nevertheless, good airmanship dictates that all available sources of information be monitored.

• Operators using TSO-C129/C129a avionics who wish to take advantage of an RNAV (GNSS) approach when specifying a destination or alternate airport must check KGPS NOTAM to verify the status of the constellation.
• When WAAS service is expected not to be available for a duration of more than 15 min, a NOTAM will be issued. This typically implies a WAAS GEO satellite failure.
• Global navigation satellite system (GNSS) avionics used for instrument flight rules (IFR) flight require an electronic database that can be updated, normally on 28- or 56-day cycles. The updating service is usually purchased under subscription from avionics manufacturers or database suppliers.