ES.1.2 GPS WITHOUT AUGMENTATION
A 24-satellite GPS constellation without augmentation cannot meet oceanic, en route, terminal, and nonprecision approach service requirements of the NAS. The removal of selective availability and/or the addition of a second civil frequency did not alter this finding. The best performance was achieved with a 30-satellite constellation (with selective availability off and a second civil frequency available), and even that configuration met the required levels of service for only oceanic navigation.
ES.1.3 GPS/WAAS
A GPS/WAAS configuration with 24 GPS satellites and 4 GEOS can satisfy all NAS positioning requirements from oceanic through Category I approach. This result did not require any specific improvements to the GPS satellites. Performance is sensitive to the ionospheric correction methods and further analysis is recommended to better optimize the WAAS configuration (i.e., number of GEOS and number of ground stations). It must also be noted that the current GEOS establishment and replacement plan is not yet clearly identified; this plan must be defined to ensure the required capabilities are provided.
ES.1.4 GPS/LAAS
A GPS/LAAS configuration based on a 30-satellite GPS constellation or one with 24 GPS satellites and 4 GEOS can satisfy all precision approach requirements. Some airports will require ground transmitters that act like additional GPS satellites (APLs) and/or improved GPS antennas and extra receivers to achieve the highest availability levels (i.e.,>0.99999). This level of performance will require no GPS satellite improvements.
ES.1.5 PENDING GPS SIGNAL IMPROVEMENTS
Because the current augmentation designs are responsive to the current GPS satellite signal conditions, the removal of selective availability and the addition of a second civil frequency did not have a major impact on the cases analyzed for this study. However, the pending GPS signal improvements are very important to system robustness and to eventual cost savings and/or performance improvements of the final system.
Removal of selective availability greatly reduces the information rate required for the corrections provided by WAAS and LAAS, which reduces the communications burden. More importantly, removal of selective availability could allow the system to maintain acceptable performance even with a brief interruption of communications. With GPS/LAAS, for example, the corrections provided at the start of an approach would be valid throughout the approach.
As announced by Vice President Al Gore in March 1998, the secondary military frequency (1227.6MHz) would have an added signal modulation that could be used for civil applications. However, the second frequency referred to in this report is required to be in portion of the spectrum that is internationally allocated for aeronautical radio-navigation services. A White House press release on 25 January 1999 announced that agreement has now been reached on the addition of a new GPS frequency (1176.45MHz) that will provide the second frequency capability needed to serve the NAS requirements.
