al Positioning System (GPS), known as NAVSTAR GPS. One of its main goals was to unify several efforts already underway or deployed by various branches of the military. Rockwell International designed and launched the first prototype satellite in 1978, starting the system validation and design qualification process. Launching of operational satellites started in 1989 and concluded in early 1993, with a total of 24 satellites launched, the original number required for a complete constellation. The medium earth or- bit (MEO) satellites are positioned in orbit at about 20,200 Km above the earth. They are arranged into six equally spaced orbital planes in such a way that any GPS receiver on the earth has a clear line of sight to at least four GPS satellites from virtually any point on the earth and at any time. They circle the earth every 12 hours. The system was declared to have reached full operational capability in 1995, and since then, there have been constant, ongoing improvements to it. For example, recently updated blocks of satellites have introduced additional signals (for civilian use) on new frequencies, which will allow more ac- curacy and higher resiliency as these satellites replace older models. Having signals in different fre- quency bands allows the receiver to make better estimates of anomalies in the signal suffered when passing through the upper atmosphere. The early blocks of satellites had design lives of about 5-7.5 years, but many are lasting two to three times longer than that-there are still eight block IIR satellites operational (February 2021) with the youngest being already 16 years old. They are currently launching the third generation of craft, known as block IIIA, which should be completed by 2023. These latest satellites have design lifetimes ranging up to 15 years. There is already planning underway for the next block, known as IIIF, which should start to launch in the second half of the 2020s. In June 2011, the U.S. Air Force completed a GPS constellation expansion by adding three mo