in view. With respect to GPS-hosted payloads, experimental S-band payloads aboard 18 GPS Block IIR and GPS Block IIF satellites, and four payloads aboard GPS Block IIIA satellites are used operationally by the Cospas-Sarsat System. GPS Block IIIF satellites are planned to have dedicated, operational L-band SAR payloads, with launches beginning around 2026. The GPS SAR system is known as the Distress Alerting Satellite System (DASS) by NASA. Additionally, the Galileo component of the MEOSAR system is able to download information back to the distress radio-beacon by encoding "Return Link Service" messages into the Galileo navigation data stream. It can be used to activate an indicator on the beacon to confirm receipt of the distress message. Ground segment As at December 2019 the LEOSAR satellites are tracked and monitored by 60 commissioned LEOLUT (low-altitude Earth-orbit local user terminals) antennas, the GEOSAR satellites by 25 commissioned GEOLUT antennas and the MEOSAR satellites by 24 commissioned MEOLUT stations, each having multiple antennas. The data from these earth stations is transferred to and distributed by 32 MCCs established globally, of which 3 remain under development. (See infobox for the countries and agencies that are ground-segment providers.) Beacons There has been one transmission modulation method used by Cospas-Sarsat 406-MHz digital beacons since their inception more than 30 years ago, binary phase-shift keying (BPSK), with two allowed bit-string lengths: 112 (with 87 bits of message information) and 144 (with 119 bits of message information). Several message protocols are allowed in the available message-bit string to accommodate different kinds of beacons (ELTs, EPIRBs and PLBs), different vessel/aircraft identifiers, and different national requirements. The time length of these transmissions is approximately one-half second. These narrowband transmissions occupy approximately 3 kHz of bandwidth in a channelized scheme across the assigned 406.0 to 406.1 MHz band. Cospas-Sarsat is in the process of specifying a new, additional beacon modulation and message scheme based on spread-spectrum technology with quadrature phase-shift keying (QPSK). Presently beacons that will use this scheme are termed "second generation" beacons. This technology will allow the use of battery-saving lower-power transmissions, improve the accuracy of the determination of the beacon location by the Cospas-Sarsat System, and avoid the need for discrete channelization in the assigned 406.0 to 406.1 MHz band (e.g., avoiding the need for periodic closing and opening of channels by Cospas-Sarsat for use by beacon manufacturers based on narrowband channel loading). Second-generation beacons will have a