Early PNT/GPS Research
The 1990’s and early 2000’s were an extremely productive period for innovative PNT research at Stanford. Stanford's landmark Gravity Probe B Mission (GP-B) that ran from 1963-2011 pioneered the use of GPS technology for precision spacecraft attitude and translation control. Many spinoff uses of GPS technology emerged from GP-B, including:
- Autonomous Aircraft Control
- Airplane Navigation—Takeoffs and Landings
- Precision Farming/Agriculture
- Enhanced guidance & situational awareness for aircraft
View Paper showing Stanford doctoral student leadership in GPS research: Professional Publishing Trends of Recent GPS Doctoral Students by Leo Mallette, ION Conference Presentation, 2006.
Below is a list of Stanford PNT research projects that either led to the formation of SCPNT or that were sponsored in the early years of SCPNT. Click the More Information link or corresponding sub-menu item to the left to view details about a project.
GPS for Spacecraft Attitude & Translation Control
A commercial GPS Receiver was used for the first time to help determine the attitude (i.e. orientation or flight dynamics) of a spacecraft on the Stanford-led Gravity Probe B (GP-B) Mission. In the late 1980s, a special GPS receiver was developed that could accommodate a rolling spacecraft (i.e. @ 1 rpm) in Low Earth Orbit. The spacecraft had extreme pointing requirements and needed to maintain its’ orbit to within +/- 25 meters. The spacecraft’s attitude control system performed flawlessly on-orbit during the 18-month mission.
GPS for Airplane Navigation, Takeoffs and Landings
A commercial GPS Receiver and multiple antennas were mounted on a small Cessna airplane to perform local flight tests near Stanford in the early 1990’s. The tests showed that GPS could provide aviation attitude control for en route and precision approaches at the Palo Alto airport. The tests were broadened to include local terrestrial pseudolites transmitting a GPS signal, which allowed for successful automatic landing of the Cessna.
Stanford first demonstrated precision Agriculture in the mid 1990’s. The practice of precision agriculture was enabled by the advent of GPS and GNSS ability to locate precise position in a field for the creation of maps of the spatial variability of as many variables as can be measured. The practice of precision agriculture allows the farmer to proactively manage crop performance in several ways, including: crop yield, terrain features/topography, organic matter content, moisture levels, nitrogen levels, pH, EC, Mg, K, etc.
Receiver autonomous integrity monitoring (RAIM) is a technology developed to assess the integrity of GPS signals in a GPS receiver system. It is of special importance in safety-critical GPS applications, such as in aviation or marine navigation. RAIM detects faults with redundant GPS pseudorange measurements. That is, when more satellites are available than needed to produce a position fix, the extra pseudoranges should all be consistent with the computed position. A pseudorange that differs significantly from the expected value (i.e., an outlier) may indicate a fault of the associated satellite or another signal integrity problem (e.g., ionospheric dispersion).
The Joint Precision Approach and Landing System (JPALS) is a military, all-weather landing system based on real-time, dual-frequency (L1 and L2) local-area differential corrections for GPS signals, augmented with integrity information, and transmitted to the user via secure means. JPALS development started from the civilian LAAS/GBAS system (see above) and split into two separate systems. One of these is known as Local Differential GPS (LDGPS), which is used for airfields on land and includes a variant that can be set up quickly.
eLoran is a low frequency terrestrial navigation system based on a number of transmission stations, which emit precisely timed and shaped radio pulses centred at 100 kHz radio frequency. LORAN, short for LOng-RAnge Navigation, was a hyperbolic radio navigation system developed in the United States during World War II. It was first used for ship convoys crossing the Atlantic Ocean, and then by long-range patrol aircraft. The US Navy began development of Loran-B, which offered accuracy on the order of a few tens of feet, but ran into significant technical problems. The US Air Force had worked on a different concept, which the Navy picked up as Loran-C. Loran-C offered longer range than LORAN and accuracy of hundreds of feet. The US Coast Guard took over operations of both systems in 1958.
GPS Tunnel in the Sky
Three-dimensional flight displays can increase the efficiency of remote sensing flight operations by providing enhanced guidance and situational awareness on straight and curved flight paths. Such a display depicts an “out-the-window” perspective view of the world along with a tunnel through which the pilot flies the aircraft. Using differential GPS, inexpensive graphics hardware, and a flat-panel display, a prototype system was built and flight tested to demonstrate significant advantages over conventional instruments.
Arctic Navigation and ION GNSS
The once inaccessible Arctic Ocean has gained economic attention as a result of the recession of the Arctic sea ice. This has triggered the expansion of many industries in the Arctic, some prospective and others very real and rapidly expanding. This growing activity, along with the harsh environment and remote reaches of the Arctic, necessitates the highest levels of safety, using a multi-tiered approach.