imilarly, it can receive a spacecraft emergency signal from the spacecraft. If an emergency occurs after a predetermined time late enough in flight that the spacecraft can still complete its mission or perform an abort, an emergency main-engine shutdown is commanded, the payload fairing is jettisoned, and the payload is separated. If a launch vehicle emergency occurs in the first 20 s of flight, instead of commanding engine shutdown, the safety system commands a 6-deg/s pitchover maneuver to move the vehicle downrange as rapidly as possible so that the launch complex will not be destroyed. When used in the three-stage configuration, the two lower stages of "Zenit" are still controlled by their own avionics, with separate systems on the Block DM-SL upper stage. The avionics for the lower stages are similar to "Zenit 2", but with a new, more-redundant guidance computer that has been flight proven on other Soviet launch systems.  Block DM-SL avionics

are installed in a toroidal pressurized compartment at the top of the stage. The Block DM-SL has an updated flight computer, and a new inertial navigation unit that is better able to operate from a floating launch platform. The "Zenit 3SL" telemetry system can transmit to either the tracking antennas on the assembly and command ship, or to TDRSS communications relay satellites. "Zenit" was intended to become one of the primary Soviet launch systems once it became operational, with a launch rate of 20-40 flights per year (this total may include production of the "Energia" strap-on boosters). However, this did not occur. Some planned programs that would have used "Zenit" never materialized, and government funding for the three-stage "Zenit 3" version, which could deliver payloads to GTO, never became available. Following the collapse of the Soviet Union, "Zenit" is in the awkward position of being a primarily Ukrainian launch vehicle, while its primary user has been the Russian government. Russia now prefers to launch most of its spacecraft on domestically built vehicles such as "Soyuz" and "Proton". As a result, Zenit's flight rate has remained low, and its use for launching Russian spacecraft to the International Space Station has apparently been canceled. The planned "Zenit" launch complex at the Plesetsk cosmodrome was never completed. "Zenit" continues to carry occasional government payloads, but its survival depends now on its commercial viability.