Navigation Center, based in Alexandria, VA. There are currently 85 NDGPS sites in the US network, administered by the U.S. Department of Homeland Security Navigation Center. In 2015, the USCG and the United States Army Corps of Engineers (USACE) sought comments on a planned phasing-out of the U.S. DGPS. In response to the comments received, a subsequent 2016 Federal Register notice announced that 46 stations would remain in service and "available to users in the maritime and coastal regions". In spite of this decision, USACE decommissioned its remaining 7 sites and, in March 2018, the USCG announced that it would decommission its remaining stations by 2020. As of June 2020, all NDGPS service has been discontinued as it is no longer deemed a necessity owing to the removal of selective availability in 2000 and also the introduction of newer generation of GPS satellites. Canadian DGPS The Canadian system is similar to the US system and is primarily for maritime usage covering the Atlantic and Pacific coast as well as the Great Lakes and Saint Lawrence Seaway. It has been discontinued as a service since December 15, 2022. Australia Australia runs three DGPSes: one is mainly for marine navigation, broadcasting its signal on the long-wave band; another is used for land surveys and land navigation, and has corrections broadcast on the Commercial FM radio band. The third at Sydney airport is currently undergoing testing for precision landing of aircraft (2011), as a backup to the Instrument Landing System at least until 2015. It is called the Ground Based Augmentation System. Corrections to aircraft position are broadcast via the aviation VHF band. The marine DGPS service of 16 Ground stations covering the Australian coast was discontinued from 1 July 2020. Improved multichannel GPS capabilities, and signal sources from multiple providers (GPS, GLONASS, Galileo and BeiDou) was cited as providing better navigational accuracy than could be obtained from GPS + DGPS. An Australian Satellite-Based Augmentation System (SBAS) project, led by Geoscience Australia (GA), will offer higher accuracy positioning for GNSS users within the next 2 – 3 years (as of September 2020). Post processing Post-processing is used in Differential GPS to obtain precise positions of unknown points by relating them to known points such as survey markers. The GPS measurements are usually stored in computer memory in the GPS receivers, and are subsequently transferred to a computer running the GPS post-processing software. The