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Local Area Augmentation System (LAAS)

What is Local Area Augmentation?

LAAS, or the Local Area Augmentation System, is the FAA version of the of the Ground Based Augmentation System, or GBAS, that has been defined by the International Civil Aviation Organization (ICAO). LAAS is based on a single GPS reference station facility located on the property of the airport being serviced. This facility has three or more (redundant) reference receivers that independently measure GPS satellite pseudorange and carrier phase and generate differential carrier-smoothed-code corrections that are eventually broadcast to user via a 31.5-kbps VHF data broadcast (in the 108 - 118 MHz band) that also includes safety and approach-geometry information. This information allows users within 45 km of the LAAS ground station to perform GPS-based position fixes with 0.5-meter (95%) accuracy and to perform all civil flight operations up to non-precision approach. Aircraft landing at a LAAS-equipped airport will be able to perform precision approach operations up to at least Category I weather minima. The pseudolites shown in the diagram below are optional means of improving user ranging geometries with ground-based GPS-like transmitters but are not likely to be needed in the foreseeable future.

LAAS

While computing and broadcasting differential GPS corrections is now straightforward, the largest challenge in designing and fielding LAAS is the need to verify aircraft safety (in terms of not exceeding a safe error bound known as the alert limit) to a probability of two in ten million (2e-7) per approach for Category I and one in one billion (1e-9) per approach for Category III. In addition, the probability that approaches must be aborted due to detected failures or false alarms must be below one in one hundred thousand (1e-5) per 15 seconds. The LAAS Ground Facility, or LGF, meets these requirements by detecting and excluding anomalous reference receiver measurements before differential corrections are broadcast. The corrections that are broadcast come with bounding standard-deviation values ("sigmas") on errors in the corrections that allow users to compute position error bounds (known as "protection levels") in real time and to compare them to the alert limits for their current operation to verify that the operation remains safe to conduct.

Stanford LAAS Research History

Prototype at Moffett Field For over a decade, Stanford University has been supported by the FAA Satellite Navigation Program Office (FAA AND-700) to perform research on the system architectures, technologies, and algorithms needed for LAAS. Stanford's contributions began in the early 1990's with the development and flight-test demonstration of the pseudolite and carrier-phase based Integrity Beacon Landing System (IBLS) architecture which demonstrated that Category III automatic landings were possible using LAAS. In the mid-1990's, Stanford developed and flight tested the "Intrack Airport Pseudolite (APL)" architecture in which pseudolites were located at both ends of the runway to be serviced. This arrangement ensured that the pseudolites could be sited within airport property, and it provided a significant vertical performance improvement at the end of each approach, where it is needed most.

Since 1998, the focus of Stanford research has been the development and testing of several generations of the Stanford Integrity Monitor Testbed (IMT) prototype of the LAAS Ground Facility. The IMT includes all of the monitor algorithms and logic needed to detect and isolate measurement failures before users are affected. Testing of the IMT under nominal and simulated-failure conditions has demonstrated that its algorithms can meet the requirements of LAAS use for Category I precision approach.

Current Research Areas

The focus of current Stanford research on LAAS is in understanding the threats to LAAS user safety and optimizing detection and mitigation methods for them. These threats include:

  1. Ionosphere Spatial Decorrelation (very sharp gradients in ionosphere delay that may occur during severe ionosphere storms);
  2. Satellite Signal Deformation (anomalies in generation of ranging signal codes that create differential errors between ground and user receivers);
  3. Large Satellite Ephemeris Errors (errors in the satellite locations reported in GPS navigation data that are large enough to cause significant differential error between ground and user receivers);
  4. Satellite "Clock" Anomalies (anomalies in satellite frequency standards that create large apparent deviations in pseudorange and carrier-phase measurements);
  5. Reference Receiver Anomalies (cycle slips, anomalous multipath, and other failure modes.)

LAASIn addition, Stanford research focuses on the logic required to translate monitor detections into measurement exclusions and fault diagnoses (collectively known as "Executive Monitoring") and quantitative means to generate bounding error sigmas for measurements that are not flagged as unhealthy. Longer-term research is aimed at exploring applications of LAAS beyond those than now exist as well as transforming the existing single-frequency LAAS architecture into a more-robust "end state" version that uses the future L5 (and possibly L2) civil signals as well as the future Galileo satellite constellation. Both L5 civil signals and Galileo satellites should become available in numbers during the early part of the next decade.

Link to H. Konno 2006 Technical Note

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