validation of a SpaceX Falcon 9 which ultimately launched the satellite on 23 December 2018. On 22 August 2019, the second GPS III satellite was launched aboard a Delta IV. On 21 September 2016, the U.S. Air Force exercised a US$395 million contract option with Lockheed Martin for the ninth and tenth Block III space vehicles, expected to be available for launch by 2022. Launch history 6 of 10 GPS Block III satellites have been launched. 6 are currently operational, with 0 undergoing testing. New navigation signals Civilian L2 (L2C) One of the first announcements was the addition of a new civilian-use signal to be transmitted on a frequency other than the L1 frequency used for the existing GPS Coarse Acquisition (C/A) signal. Ultimately, this became known as the L2C signal because it is broadcast on the L2 frequency (1227.6 MHz). It can be transmitted by all block IIR-M and later design satellites. The original plan stated that until the new OCX (Block 1) system is in place, the signal would consist of a default message ("Type 0") that contains no navigational data. OCX Block 1 with the L2C navigation data was scheduled to enter service in February 2016, but has been delayed until 2022 or later. As a result of OCX delays, the L2C signal was decoupled from the OCX deployment schedule. All satellites capable of transmitting the L2C signal (all GPS satellites launched since 2005) began broadcasting pre-operational civil navigation (CNAV) messages in April 2014, and in December 2014 the Air Force started transmitting CNAV uploads on a daily basis. The L2C signal will be considered fully operational after it is being broadcast by at least 24 space vehicles, currently projected to happen in 2023. As of October 2017, L2C was being broadcast from 19 satellites. The L2C signal is tasked with providing improved accuracy of navigation, providing an easy-to-track signal, and acting as a redundant signal in case of localized interference. The immediate effect of having two civilian frequencies being transmitted from one satellite is the ability to directly measure, and therefore remove, the ionospheric delay error for that satellite. Without such a measurement, a GPS receiver must use a generic model or receive ionospheric corrections from another source (such as a Satellite Based Augmentation System). Advances in technology for both the GPS satellites and the GPS receivers have made ionospheric delay the largest source of error