Hybrid GPS Constellations to Provide High Availability of Cat I Precision Approach, RAIM, and Continuity

Problem: Previous studies have shown that constellations designed for availability of Category I precision approach are best served with 3 plane constellations while RAIM (Receiver Autonomous Integrity Monitoring) and Continuity is best obtained with 6 or more planes. Solution: A study of constellations, which have 6 planes where 3 of the planes have more satellites than the other 3 was conducted. This “hybrid” constellation provides features of both 6 and 3 plane constellations. These constellations were analyzed to determine how well Cat I precision approach, RAIM, and Continuity could be simultaneously achieved. With a UERE of 1 meter and a mask angle of 5 degrees a 24 satellite constellation in 3 planes can meet the Category I precision approach accuracy requirement 99% of the time worldwide and provide a minimum of 6 satellites in view. Adding 12 satellites in 3 planes between the first 3 provides almost 10 in view. This provides RAIM/Continuity availability nearly equivalent to a 6 plane 36 satellite constellation while providing a higher availability of Cat I precision approach than the 6 plane constellation could. This advantage is even greater when the mask angle is increased from 5 degrees. The analysis of the trade-off of availability of Category I and RAIM/Continuity was performed for hybrid constellations ranging from 30 to 36 satellites. In each case availability of Cat I was not as good as the equivalent 3 plane constellation alone but the RAIM/Continuity was as nearly as good as the equivalent 6 plane constellation.