GNSS Library

A. The WAAS supplies two different sets of corrections: 1) corrected GPS parameters (position, clock, etc) and 2) Ionospheric parameters. The first set of corrections is user position independent - they apply to all users located within the WAAS service area. The second set of corrections is area specific. WAAS supplies correction parameters for a number of points (organized in a grid pattern) across the WAAS service area. The user receiver computes ionospheric corrections for the received GPS signals based on algorithms which use the appropriate grid points for where the user is located. Further, the appropriate grid points may differ for each GPS satellite received and process by the user receiver as the GPS satellites are located at various positions in the sky relative to the user. The combination of the two sets of corrections allows for significantly increased user position accuracy and confidence anywhere in the WAAS service area.  

When the WAAS signal is fully stabilized, and the FAA accepts the system, we believe actually performance will exceed system specifications. Preliminary tests using WAAS software indicate that this is the case.   

A. The FAA remains committed to the implementation of WAAS because of its safety benefits for the aviation community and the flying public, and because it is central to our overall efforts to modernize the NAS.   

In addition, ionospheric data is collected and archived by the NSTB for analysis of scintillation and range delay effects by experts in the ionospheric field.   

A. This answer is highly dependent upon your specific equipment and installation. Please check the Aeronautical Information Manual, Table 1-1-5 and compare this with the documentation provided with your GPS and installation. This table provides information on TSO-C129 units. LPV approaches require WAAS Class 3 equipment built to TSO-C145a or TSO-C146a and installed IAW AC 20-138A.

The short answer is that you cannot use any hand-held receiver for anything other than situational awareness. Older GPS receivers may be for VFR-only use, or may be sufficient to fly non-precision GPS approaches. LNAV/VNAV and LPV approaches require additional equipment beyond the basic GPS receiver.

A.

• TSO-C129 is “Airborne Supplemental Navigation Equipment Using the GPS.”
• TSO-C145a is “Airborne Navigation Sensors Using the GPS Augmented by the Wide Area Augmentation System.”
• TSO-C146a is “Stand-Alone Airborne Navigation Equipment Using the GPS Augmented by the Wide Area Augmentation System.”
• AC 20-130a is “Airworthiness Approval of Navigation or Flight Management Systems Integrating Multiple Navigation Sensors.”
• AC 20-138a is “Airworthiness Approval of GPS Navigation Equipment for Use as a VFR and IFR Navigation System.”
• AC 90-94 is “Guidelines for Using GPS Equipment.”

A. First, the basic information for GPS. To use GPS for navigation, the equipment must be certified in accordance with TSO-C129 and the installation must be done in accordance with AC 20-138 or AC 20-130A. TSO-C115a does not meet the requirements of TSO-C129.

For WAAS, you must use either TSO-C145a or TSO-C146a. Most General Aviation WAAS receivers would comply with TSO-C146a, which applies to panel-mounted navigation equipment (as opposed to sensors that provide data to a flight management system).

A. No, the WAAS message is broadcast on the same frequency as the GPS signal. You will just need a WAAS-capable receiver provided your existing antenna is compatible with the WAAS equipment. Your current system may be upgradeable. Please contact your manufacturer directly for information on availability, installation and price.

A. To fly an LNAV or LNAV/VNAV approach, you must have either (1) WAAS avionics approved for LNAV/VNAV approaches, (2) a certified Baro-VNAV approach system with IFR-approved GPS, (3) a certified Baro-VNAV approach system with an IFR-approved WAAS, or (4) an RNP-0.3 certified system.

A. RNP stands for “Required Navigational Performance.” It is a metric of system navigational capability. The FAA is moving toward a performance-based national airspace system. In the future, your ability to fly in certain areas may be governed by your ability to achieve precise navigational performance within specific tolerances. RNP-0.3 will be used for approaches, and it refers to .3 nautical mile accuracy. This accuracy may be achieved through various means (GPS, WAAS, flight management system using automatic DME updates), but your aircraft will be certified to a particular RNP. There are other requirements beyond accuracy that will be defined for each RNP operation before it is implemented.

A. LPV is a new category of approach that uses the WAAS signal and provides vertical guidance. You can fly LPV with WAAS avionics approved for LPV approaches. WAAS avionics do not automatically mean that you can fly the LPV minima line. WAAS approaches do not require any special ground equipment at the destination airport—no localizer or glide slope transmitters are necessary. To fly an LPV approach, you must have WAAS Class 3 avionics certified to TSO-C145a or TSO-C146A, and installed IAW AC 20-138A.

A.
Integrity refers to usability of the satellite signal, and means that the signal has not been corrupted. Integrity is the ability of a system to provide timely warnings to users when the system should not be used for navigation as a result of errors or failures in the system. WAAS improves upon the integrity of the basic GPS signal and detects much smaller errors more quickly.

Availability refers to the percentage of time in a given period that the signal is expected to be received and usable.

Continuity differs from availability, in that it refers to the continuous reception of the signal. A signal could have high availability but numerous short outages.

Ephemeris is the term for the exact position of a GPS satellite in space at a given time, and it is necessary for GPS calculations.

A. The FAA has authorized GPS as the primary means of navigation in certain areas (oceanic and remote). The FAA requires that the aircraft have the appropriate equipment necessary for the route of flight. Furthermore, a GPS approach cannot be used for the alternate if it is also used for the destination when using TSO-C129 equipment. Therefore, it may be legal to fly a GPS-only equipped aircraft in IMC, but the circumstances are quite rigorous. In Alaska , IFR-approved and installed WAAS avionics are legal as the only navigation equipment on board if the route and destination are identified for GPS/WAAS.

A. Your system updates using ranging from multiple DMEs. Optimal DME geometry of multiple DME facilities and validation is required to achieve this performance. At this time, RNP-0.3 approaches using multiple DMEs are not common. Therefore, there is no way you can fly this approach without having GPS.

A. Receiver Autonomous Integrity Monitor (RAIM) is a form of integrity monitoring performed within the avionics themselves. It ensures available satellite signals meet the integrity requirements for a given phase of flight. By comparing the pseudorange measurements of a number of satellites, the RAIM function can identify a satellite failure and issue an alert to the pilot. Many VFR GPS receivers and most hand-held receivers do not have RAIM capability. Without RAIM capability, the pilot has no assurance of the accuracy of the GPS position. A minimum of five satellites is required to detect a bad satellite; at least six satellites are required to detect and exclude a bad satellite from the navigation solution if your receiver has a fault detection and exclusion (FDE) RAIM algorithm. The GPS receiver should also tell you when its RAIM function is unavailable, at both present time/position and at any selected future time/position. You can get information on satellite outages through the NOTAM system, however the effect of an outage on the intended operation cannot be determined unless the pilot has a RAIM availability prediction program which allows excluding a satellite which is predicted to be out of service.

If you are using GPS to fly an approach and you receive a RAIM annunciation prior to the final approach waypoint, you may not have sufficient accuracy to complete the approach.

You can get information on satellite outages through the NOTAM system, however the effect of an outage on the intended operation cannot be determined unless the pilot has a RAIM availability prediction program which allows excluding a satellite which is predicted to be out of service.

A. Aircraft operating by IFR-approved GPS are considered to be area navigation (RNAV) aircraft, and should file the appropriate equipment suffix.

A. No. Overlay approaches can use GPS instead of the primary designated navigational aid, but the approach must be designated for GPS and be in the current aircraft database. For example, it must say “VOR or GPS RWY 16.” You cannot just use GPS in lieu of VOR, Automatic Direction Finder (ADF) or other navigational source naming the approach. You can, however, use GPS to determine waypoints during the approach.

A. En route, full-scale Course Deviation Indicator (CDI) deflection is typically 5 miles with an accuracy of +/- 2 miles. Within 30 miles of the arrival and departure airport, GPS CDI sensitivity typically transitions to one mile. When flying an approach (and the approach mode is armed), GPS CDI sensitivity transitions from 1 mile to 0.3 miles approximately 2 miles from the FAWP.

A. If your system is a TSO-C129, any required alternate must have an approved instrument procedure other than GPS, and your aircraft must have the appropriate equipment to fly the approach. If you have approved WAAS avionics, you may plan to use any instrument approach authorized for use with WAAS avionics at a required alternate. You must use the LNAV minima line for planning purposes in case vertical guidance is not available.

A. You can file using the GPS, however you must file an alternate. You can fly the approach—even though listed as “Unreliable.” The term UNRELIABLE is an advisory to pilots indicating the expected level of service may not be available. You should use RAIM and all other sources to confirm the suitability of the GPS signal upon arrival. Unreliable and Unavailable have different meanings, and you must be certain of each. Unavailable indicates a loss or malfunction of the GPS signal in the specified area during the specified time period.

A. TSO is a Technical Standard Order, and it describes the minimum performance standard for a system or component. An STC is a Supplemental Type Certificate. An STC is a document issued by the Federal Aviation Administration approving a product (aircraft, engine, or propeller) modification. The STC defines the product design change, states how the modification affects the existing type design, and lists serial numbers of the component affected by the change. Safe flight using GPS equipment depends on airworthiness (does it have a TSO?), installation (was it installed per AC 20-138 or AC 20-130a), and signoff (was it done properly per TC/STC or Form 337).

A. The FAA uses metrics in the process of determining the safety of operations. Two nines would equate to 99%, or one occurrence in 100. Five nines would be .99999, or one occurrence in 100,000.

A. Altimeters are susceptible to atmospheric-related error, and when the temperature is colder than standard, this error can become significant, with your altimeter reading higher than your actual altitude. Some charts list minimum temperatures for certain minimums, and the reason is that your actual altitude is below the altitude that would be displayed on your altimeter. Sometimes there are limitations associated with the availability of a local altimeter as well. The pilot is expected to account for this error in extremely cold conditions (as discussed in the AIM).

A. An MMR is a multi-mode receiver. It would receive the basic GPS signal and the WAAS signal. It may also include the LAAS signal at a future date. It may also receive VHF, UHF, VOR or other signals.

A. These designations were used in the engineering standards for WAAS equipment, but they are not used operationally. WAAS equipment that provides PVT (position, velocity, time) into a flight management system, where the flight management system provides the actual navigation function is marked with a TSO-C145a. Panel-mount navigation systems that include a sensor and a navigation unit are TSO-C146a receivers. A special class of TSO-C146a equipment is an MMR that includes the WAAS capability to conduct LPV approach procedures. It does not support en route navigation other than providing PVT into an FMC.

A. You need to contact the manufacturer of your system. Many systems can be easily and inexpensively upgraded by an avionics technician or the manufacturer.

A. No, although they may share similar accuracies. LNAV/VNAV and LPV approaches are RNAV approaches. RNP approach design involves linear obstacle assessments, while LNAV/VNAV and LPV approaches use angular obstacle assessments.

A. DME signal reception is limited to line-of-sight, and for approach operations, the aircraft typically cannot receive very many DME signals. As a consequence, DME/DME performance can be poor, and is frequently inadequate. The FAA is currently working with industry to develop criteria to implement some DME-based capability for approach procedures using the RNP concept.

A.. WAAS provides better accuracy that GPS alone. Non-aviation WAAS receivers in the aviation WAAS coverage area (http://www.nstb.tc.faa.gov/RT_VerticalProtectionLevel.htm) will get far better accuracies due to the WAAS corrections they are receiving. However, non-aviation users outside of the WAAS aviation coverage area can also benefit from WAAS corrections. Please refer to the Horizontal Protection Limit website (http://www.nstb.tc.faa.gov/RT_NPACoverage.htm) which shows the areas WAAS can be used for non-aviation and what types of accuracies you may expect.

A. WAAS is extremely useful for vehicle navigation, as it increases accuracy from 10-12 meters with GPS alone, to 1-2 meters horizontal. This increase in accuracy can mean knowing which side of the highway a vehicle is on, lane determination,specific vehicle location, or where an exit is early enough so that you can make the turn before the exit is passed.

Because of this, WAAS is becoming more and more popular for use in vehicles. In fact, public safety departments including police, fire, rescue, and state transportation authorities are already using WAAS. CALTRANS has incorporated WAAS into the receivers used to layout construction and improvements of highways. Also, a number of vehicular navigation services are investigating WAAS for the future. OnStar will be incorporating WAAS into 2008 model year GM vehicles. OnStar provides notification of vehicle location to a given GPS location within 15 seconds of airbag deployment. The addition of WAAS provides greater accuracy in the location determination and contributes to this life saving benefit. It has provided vehicle tracking for stolen as well as carjacked vehicles and led to the recovery of infants in cars that have been carjacked. Also, DaimlerChrysler has tested WAAS as a part of an intelligent transportation system in development, and found it to be the best option to provide the accuracy needed for their utilization.