Global Positioning System (GPS) GPS Navigation Global Positioning System (GPS)

Global Positioning System (GPS) Cloud of 24 GPS satellites orbit the Earth Satellite positions are accurately known GPS device receives satellite signal with ‘time-sent’ information Device calculates distance to satellite Intersection point of multiple satellites defines device location Image source: National Coordination Office for Space-Based Positioning, Navigation, and Timing (2013). Retrieved from http://www.gps.gov/multimedia/images/constellation.jpg A global positioning system utilizes multiple satellites that orbit the Earth. The triangulation of the satellites is based on extremely precise timing of radio waves that are received from the satellites. Since the satellites are not stationary but moving through space at thousands of miles per hour, the radio waves are slow. They bend and bounce their way from satellite to receiver. With the challenges that arise from the satellites orbiting the Earth, it is easier to examine how the GPS works in smaller bits. How does GPS really work, how is the accuracy challenged and maintained, and how are technical and engineering techniques used to overcome those challenges to make GPS the most readily available, accurate, and truly global navigational system available?

GPS Orbital Configuration 24 satellites 20,000 km (Approximately 12,500 mi) above Earth Orbits take 12 hours Cover entire Earth Image source: National Coordination Office for Space-Based Positioning, Navigation, and Timing (2013). Retrieved from http://www.gps.gov/multimedia/images/constellation.jpg

GPS Accuracy Within 100 meters (328 ft) Original GPS Within 15 m (49 ft) Selective availability removed 3-5 m (10-16 ft) Differential position (GDPS) < 3 m (10 ft) Wide Area Augmentation System (WAAS) Selective Availability was the military degradation of the GPS accuracy for defense purposes. More information is available at the National Executive Committee for Space-Based PNT website: http://pnt.gov/public/sa/ The PRC and ephemeris signals from four satellites should provide an absolutely accurate position to the precision of the atomic clock on the satellite. The key word is “should” as there are many problems associated with timing the signal’s travel time. Remember even a thousandth of a second is a huge error!   Problems include the reduction in the speed of light as it enters the atmosphere (error to greater distance), signals that reflect off of multiple objects and create echoes that arrive at different times, purposeful errors, and even atomic clock errors. We can correct for the atmospheric problems by using more advanced technology. If the receiver can monitor dual frequencies, then it can compare the amount of variation between a low-frequency (slowed more) and a high-frequency (slowed less) signal to deduce the error and correct for it. The GPS system broadcasts on two different carrier frequencies called L1 and L2. Unfortunately, this requires a very sophisticated receiver. Only the military has access to the L2 carrier channel. The other option is to build in atmospheric models so that “typical” corrections can be made to all incoming signals. Receivers can deal with the multi-path errors by employing signal rejection analysis software. The basic principle is that the first signal to arrive will have traveled along the shortest route and thus any signal that arrives later is most likely an echo and should be ignored. Before May 1, 2000, the government purposely degraded the timing data of the satellite’s clock by adding noise to the signal. They may also have introduced slight inaccuracies to the ephemeris data. Military GPS receivers made use of a decryption key to obtain the full accuracy information. This Selective Availability (SA) was disabled, which improved the accuracy of GPS positions by a factor of 10. All of these errors combined introduced errors of about 10 meters. With SA active, this led to errors of hundreds of feet. Without SA the basic GPS receiver is capable of measuring positions to within 30 or 50 feet. This is accurate enough for an aircraft approaching a runway, but unfortunately it isn’t accurate enough to land the aircraft on the centerline of the runway.

GDOP - Geometric Dilution of Precision GPS accuracy is influenced by the visibility and wide angles to the satellites.

GPS Augmentations Systems to increase GPS accuracy Nationwide Differential GPS System (NDGPS) Wide Area Augmentation System (WAAS) Continuously Operating Reference Station (CORS) Global Differential GPS (GDGPS) International GNSS Service (IGS) The key to the improved accuracy of the DGPS system (meters for a moving receiver, better yet for a stationary one) is the presence of a fixed local GPS station. If the fixed station is fairly close to our mobile DGPS unit (within hundreds of miles), it should share the same errors in its signals. If the location of the fixed station is accurately surveyed, the errors in its GPS position can be transmitted to the mobile unit to correct its position. The fixed station measures the errors in all visible satellite data.   These corrections can be broadcast live to the receiver if extreme accuracy is essential. Corrections can also be applied later when the GPS unit returns to base for corrections. Another method is called “inverted differential” GPS where each mobile unit broadcasts its positions back to a base station for immediate correction. All methods improve the accuracy to a few meters. The ultimate accuracy of the system is based on timing. If we could use an even more accurate timing method, the accuracy of the system would be improved. Normal GPS is based on “code phase” by sliding the psuedo-random code from the receiver back until it matches that from the satellite. But the frequency of the code is very low, which means that a 1% error in matching up the timing is a significant meter error. By switching to a “carrier-phase” system, the GPS unit can pay attention to the much higher frequency carrier code in the 1.57 GHz range. The entire wavelength is in the centimeter scale. This makes the phase errors that are possible in the few millimeter range. This is 1000 times more accurate than the traditional GPS system. The Wide Area Augmentation System (WAAS) is based on using a geostationary satellite that broadcasts on the GPS frequencies to transmit alerts that a particular satellite has errors and should be ignored. More importantly, the system broadcasts differential correction data from about 25 stations scattered all across the country. WAAS enabled GPS units have allowed aircraft to commit to “category 1” landings in which they can use the unit to navigate very close to the runway before they must obtain visual references for the final landing. If we place the differential receiver stations on the airport near the runway, this Local Area Augmentation System (LAAS) would be capable of such extreme accuracy as to allow “category 3” landings where the GPS unit is trusted all the way to touchdown in even zero visibility. Challenge: How would you extend this level of accuracy to the entire globe?

NDGPS – Nationwide Differential GPS System Accurately surveyed locations used for reference Corrects GPS for increased accuracy for users on land and water Developing system for 10-15 cm accuracy HA-NDGPS (High Accuracy NDGPS) intended to be accurate within 10-15 cm.

WAAS - Wide Area Augmentation System Operated by FAA (U.S. Federal Aviation Administration) Aircraft navigation for all phases of flight

Benefits of WAAS Primary means of navigation More direct routes Approach with vertical guidance Decommission older equipment Simplify onboard equipment Increased capacity Primary means of navigation (Takeoff, enroute, approach, and landing). More direct routes. Not restricted by location of ground-based navigation equipment. Approach with vertical guidance at any qualified US airport. Decommission older equipment. Reduce maintenance costs of expensive equipment. Simplify onboard equipment. Increased capacity due to reduced separation of aircraft. Improved accuracy allows this.