# GPS and IFR GPS Operations ## Overview The **Global Positioning System (GPS)** is a **space-based radio navigation system** that provides aircraft with accurate information for: - **Position** - **Velocity** - **Time** - **Navigation** GPS uses a constellation of satellites that transmit signals to GPS receivers. These signals allow the receiver to determine the aircraft’s position anywhere in the world, in all weather conditions, as long as the receiver can receive usable satellite signals. GPS was originally developed for military use, but in the 1980s, the United States government made the system available for civilian use. --- # GPS System Segments The GPS system has three main segments: 1. **Space Segment** 2. **Control Segment** 3. **User Segment** --- ## Space Segment The **space segment** consists of GPS satellites orbiting the Earth. These satellites broadcast navigation signals that include: - Satellite identification - Satellite position - Timing information - Navigation data The GPS constellation is designed so that multiple satellites are visible from nearly anywhere on Earth. A minimum of **24 satellites** allows worldwide coverage, but the operational constellation normally includes additional satellites for redundancy and improved reliability. --- ## Control Segment The **control segment** consists of ground-based monitoring and control stations. These stations: - Monitor satellite health - Track satellite positions - Update satellite data - Maintain satellite timing accuracy - Correct errors in satellite orbital data The control segment ensures that the satellites are transmitting accurate information. --- ## User Segment The **user segment** is the equipment that receives and processes GPS signals. In aviation, this includes: - GPS antenna - GPS receiver - Navigation processor - Cockpit display - Integrated avionics systems, such as a Garmin G1000 The aircraft GPS receiver uses satellite signals to calculate the aircraft’s position. --- # How GPS Determines Position Each GPS satellite transmits a signal containing: - The satellite’s position - The exact time the signal was transmitted - Navigation data used by the receiver GPS signals travel at approximately the speed of light. The receiver compares the time the signal was sent with the time the signal was received. By measuring how long the signal took to arrive, the receiver can calculate its distance from each satellite. This process is called **ranging**. --- ## Number of Satellites Required A GPS receiver generally needs: - **At least 3 satellites** for a two-dimensional position - Latitude - Longitude - **At least 4 satellites** for a three-dimensional position - Latitude - Longitude - Altitude Additional satellites improve accuracy, reliability, and integrity monitoring. --- # Mask Angle The GPS receiver only uses satellites that are above the **mask angle**. The **mask angle** is the lowest angle above the horizon at which a GPS receiver will use a satellite signal. Satellites too low on the horizon may produce less reliable signals because the signal has to travel through more atmosphere and is more likely to be affected by terrain, buildings, or other obstructions. --- # IFR Approved GPS Not every GPS is approved for IFR navigation. To be used for IFR operations, a GPS must be: - **Approved for aircraft use** - **Permanently installed in the aircraft** - **Installed in accordance with FAA-approved procedures** - **Tested after installation** - **Documented in the aircraft maintenance records** - **Operated according to the AFM/POH supplement** The FAA does not only approve the GPS unit itself. The **installation** must also be approved. --- ## IFR GPS Installation Requirements The installer must: - Be an appropriately qualified technician - Follow approved installation procedures - Conduct required post-installation checks - Verify proper system operation - Complete aircraft logbook entries - Provide required AFM/POH supplements, if applicable A handheld GPS may be useful for situational awareness, but it is not approved as the primary means of IFR navigation unless it is part of an approved installed system. --- # Current GPS Database For IFR operations, the GPS navigation database is extremely important. The database contains information such as: - Airports - Runways - Waypoints - Fixes - Airways - Instrument approaches - SIDs - STARs - Holding patterns - Navigation facilities --- ## Database Requirements for IFR Approaches To fly an instrument approach using GPS, the navigation database must be: - **Current** - **Appropriate for the operation** - **Loaded from an approved source** - **Used without manual modification of approach waypoints** For instrument approaches, the FAA requires the approach procedure to be retrieved from a current approved database. A pilot should not manually build a GPS approach by entering fixes one at a time. --- ## If the Database Is Expired If the GPS database is not current, the aircraft may still be able to use the IFR-approved GPS for some enroute and terminal operations, but only if the pilot verifies that the data being used is still valid. This verification may include checking: - Fix names - Waypoint locations - Airway structure - Procedure changes - NOTAMs - Chart updates However, for instrument approaches, the database normally must be current. --- ## Checking the Database on the G1000 On a Garmin G1000, the database information can be checked during startup. When the G1000 powers on: 1. Review the splash screen. 2. Check the navigation database effective dates. 3. Confirm the database is current. 4. Confirm the system is approved and available for the intended operation. The navigation database dates should be checked during preflight, especially before IFR operations. --- # Receiver Autonomous Integrity Monitoring ## RAIM **RAIM** stands for **Receiver Autonomous Integrity Monitoring**. RAIM is a system that allows a GPS receiver to monitor the integrity of the satellite signals it is receiving. In simple terms, RAIM asks: **Can I trust the GPS position being displayed?** RAIM is similar in purpose to a flag on a VOR indicator. If the system cannot verify the accuracy or integrity of the signal, the pilot needs to know that the navigation information may not be reliable. --- ## Why RAIM Matters GPS navigation depends on satellite signals. If a satellite signal is corrupted or if the satellite geometry is poor, the GPS position may not meet the accuracy required for the current phase of flight. RAIM helps detect these problems. RAIM is important because IFR GPS navigation requires not only accuracy, but also **integrity**. Integrity means the system can warn the pilot when the navigation information should not be trusted. --- # RAIM Satellite Requirements ## Basic GPS Position A GPS receiver normally needs at least **4 satellites** to determine a three-dimensional position. ## RAIM Fault Detection For basic RAIM fault detection, the receiver generally needs: - **5 satellites**, or - **4 satellites plus barometric aiding** This allows the receiver to detect that a satellite signal may be unreliable. This is called **fault detection**. --- ## RAIM Fault Exclusion For fault detection and exclusion, the receiver generally needs: - **6 satellites**, or - **5 satellites plus barometric aiding** This allows the receiver to: 1. Detect a bad satellite signal 2. Identify which satellite is bad 3. Exclude that satellite from the navigation solution This is called **fault exclusion**. --- # Barometric Aiding **Barometric aiding** uses altitude information from the aircraft’s pressure altimeter system to assist the GPS receiver. Barometric aiding can help the GPS receiver by substituting for one satellite in the RAIM calculation. This improves: - RAIM availability - Navigation reliability - Satellite geometry coverage - Integrity monitoring Barometric aiding does not replace the GPS system. It helps the GPS receiver improve its ability to monitor and validate the navigation solution. --- # RAIM Warning Messages There are two general types of RAIM-related warnings: 1. **RAIM not available** 2. **RAIM position error** --- ## RAIM Not Available A **RAIM not available** message means the receiver does not have enough satellite geometry or signal availability to provide integrity monitoring. This does not necessarily mean the GPS position is wrong. It means the receiver cannot guarantee the integrity of the position for the required phase of flight. The pilot should treat this seriously during IFR operations. --- ## RAIM Position Error A **RAIM position error** message means the receiver has detected a possible error that exceeds the allowable limits for the current phase of flight. This is more serious than simple RAIM unavailability. It means the GPS position may not be reliable for navigation. --- # GPS Protection Limits by Phase of Flight GPS accuracy requirements become more strict as the aircraft gets closer to the airport and begins an approach. Common protection limits include: | Phase of Flight | Protection Limit | |---|---:| | **Oceanic** | 4 NM | | **Enroute** | 2 NM | | **Terminal** | 1 NM | | **Nonprecision Approach** | 0.3 NM | The approach phase requires the tightest accuracy because the aircraft is closest to terrain, obstacles, and the runway environment. --- # RAIM Prediction A **RAIM prediction** is a preflight check used to determine whether RAIM will be available for a planned route, destination, and time of arrival. RAIM prediction considers: - Number of satellites available - Satellite geometry - Aircraft location - Planned route - Planned arrival time - GPS outages - Satellite NOTAMs RAIM prediction helps determine whether there will be enough usable satellites in the right positions to support IFR GPS navigation. --- ## When RAIM Prediction Matters RAIM prediction should be verified as part of IFR GPS preflight planning when required. It is especially important when using a non-WAAS GPS for: - GPS approaches - IFR enroute navigation - Terminal GPS navigation - Flights into areas with known GPS outages - Flights where GPS is the primary navigation source --- # Wide Area Augmentation System ## WAAS **WAAS** stands for **Wide Area Augmentation System**. WAAS improves GPS accuracy, availability, and integrity. A WAAS-capable GPS receiver receives correction information that allows it to improve the GPS position solution. WAAS can correct for certain errors and provide integrity monitoring without requiring traditional RAIM in the same way as older non-WAAS GPS systems. --- ## What WAAS Does WAAS improves GPS by helping correct for: - Satellite clock errors - Satellite orbit errors - Ionospheric delays - Signal errors - Integrity issues WAAS allows properly equipped aircraft to fly certain GPS approaches with vertical guidance. --- # WAAS and Instrument Approaches WAAS allows aircraft to fly GPS-based approaches such as: - **LNAV** - **LNAV/VNAV** - **LP** - **LPV** The type of approach minimums available depends on: - Aircraft equipment - GPS certification - WAAS availability - Approach procedure design - Database currency - NOTAMs - Receiver annunciations --- ## LPV Approaches **LPV** stands for **Localizer Performance with Vertical Guidance**. An LPV approach uses WAAS to provide lateral and vertical guidance. LPV approaches can provide very low minimums, sometimes similar to an ILS, but LPV is not an ILS. LPV guidance is GPS-based, not ground-based localizer and glideslope guidance. --- # GPS NOTAMs GPS has its own NOTAM system. GPS NOTAMs may include: - Satellite outages - GPS interference testing - WAAS outages - Approach procedure outages - Unreliable GPS areas - Service interruptions A GPS NOTAM does not automatically mean the flight cannot be conducted, but the pilot must determine whether the outage affects the planned route, destination, alternate, or approach. --- ## Why GPS NOTAMs Matter A GPS outage may affect: - Enroute navigation - Arrival procedures - RNAV departures - GPS approaches - WAAS approaches - RAIM availability - Alternate airport planning A pilot may not know whether a GPS outage affects the flight unless they check the applicable NOTAMs and, when required, complete a RAIM prediction. --- # IFR GPS Preflight Planning Before using GPS for IFR flight, the pilot should verify: - The GPS is approved for IFR use - The installation is approved - The database is current for the intended operation - The correct approach is loaded from the database - GPS NOTAMs have been reviewed - RAIM or WAAS availability is sufficient - The receiver annunciations are understood - The aircraft AFM/POH supplement has been reviewed - The pilot understands the equipment limitations --- # GPS Approach Loading When flying a GPS approach: - Load the approach from the approved database. - Verify the correct airport. - Verify the correct approach. - Verify the correct runway. - Verify the correct transition or initial approach fix. - Cross-check the GPS flight plan against the published chart. - Do not manually create the approach by entering each fix separately. - Monitor CDI scaling and receiver annunciations. --- # CDI Sensitivity and Scaling GPS CDI sensitivity changes depending on the phase of flight. The GPS receiver may automatically change CDI scaling as the aircraft transitions from enroute to terminal to approach mode. Typical values include: | Mode | CDI Sensitivity | |---|---:| | **Enroute** | 5 NM full-scale deflection | | **Terminal** | 1 NM full-scale deflection | | **Approach** | 0.3 NM full-scale deflection | The pilot must confirm that the GPS has entered the correct mode for the phase of flight. --- # GPS Annunciations GPS annunciations tell the pilot what the GPS is doing and what type of guidance is being provided. Common annunciations may include: - **ENR** - Enroute - **TERM** - Terminal - **APR** - Approach - **LNAV** - Lateral navigation - **LPV** - Localizer Performance with Vertical Guidance - **DR** - Dead reckoning - **LOI** - Loss of integrity The exact annunciations depend on the GPS unit installed in the aircraft. Pilots must understand the annunciations for the specific receiver they are using. --- # Common IFR GPS Errors Common GPS errors include: - Using a non-IFR-approved GPS for IFR navigation - Assuming a handheld GPS is legal for IFR primary navigation - Flying an approach with an expired database - Manually entering approach waypoints instead of loading the procedure - Failing to check GPS NOTAMs - Failing to check RAIM when required - Not understanding GPS annunciations - Not verifying the approach against the chart - Selecting the wrong approach transition - Assuming WAAS makes all GPS limitations irrelevant - Confusing LPV with ILS - Continuing an approach after loss of integrity --- # GPS Failure or Loss of Integrity If GPS integrity is lost during IFR flight, the pilot should: 1. Maintain aircraft control. 2. Use other approved navigation sources if available. 3. Advise ATC if GPS navigation capability is lost. 4. Request vectors or an amended clearance if needed. 5. Do not continue a GPS approach if required integrity is lost. 6. Follow the aircraft checklist and GPS receiver guidance. --- # GPS vs Ground-Based Navigation GPS is space-based, while systems like VOR, DME, LOC, and ILS are ground-based. ## GPS Advantages - Accurate worldwide navigation - Direct routing - RNAV capability - Useful in remote areas - Supports modern instrument procedures - Provides distance, groundspeed, and time estimates ## GPS Limitations - Requires satellite signal reception - Can be affected by outages or interference - Requires approved equipment for IFR use - Requires database management - Requires pilot understanding of receiver operation - May require RAIM or WAAS availability - Can fail or lose integrity --- # Quick Student Summary ## GPS Basics - GPS is a space-based radio navigation system. - GPS provides position, velocity, and time. - The system includes space, control, and user segments. - Aircraft GPS equipment is part of the user segment. ## Satellite Requirements - 3 satellites can provide a two-dimensional position. - 4 satellites can provide a three-dimensional position. - 5 satellites are generally needed for RAIM fault detection. - 6 satellites are generally needed for fault detection and exclusion. ## IFR GPS For IFR use, the GPS must be: - Approved for aviation IFR operations - Permanently installed - Properly tested - Documented in the aircraft records - Operated according to the AFM/POH supplement ## Database - A current database is required for GPS instrument approaches. - Enroute and terminal use with an expired database may be allowed only if the data is verified as current and valid. - Approaches should be loaded from the database, not manually built. ## RAIM - RAIM checks GPS integrity. - RAIM not available means the receiver cannot guarantee position integrity. - RAIM position error means a detected error may exceed allowable limits. - RAIM prediction should be part of IFR GPS preflight planning when required. ## WAAS - WAAS improves GPS accuracy, availability, and integrity. - WAAS allows certain approaches with vertical guidance. - LPV is GPS-based vertical guidance, not an ILS. ## GPS NOTAMs - GPS outages and WAAS outages may be published by NOTAM. - Pilots must check whether GPS outages affect their route, arrival, approach, or alternate planning.