Angosat-1 restores communications with ground control
Despite the unending political and military conflict between Russia and Ukraine, rocket teams from the two former Soviet republics worked side by side to launch the Angosat-1 communications satellite for the government of Angola from Baikonur Cosmodrome in Kazakhstan. The liftoff of the Zenit rocket took place as scheduled on Dec. 26, 2017, marking the first mission for the Ukrainian-built launcher in two years. However, ground control lost communications with the satellite shortly after making its first contact. It took more than 24 hours for controllers to hear from the satellite again...
Zenit lifts off with the Angosat-1 satellite on Dec. 26, 2017.
The Zenit launch with Angosat-1 satellite at a glance:
How Angosat-1 was launched
The liftoff process began with the switch of the rocket's gyroscopic system to flight mode followed by the ignition of the RD-171 engine 0.13 seconds later. Liftoff thrust was achieved 3.43 seconds later and the rocket left the pad at 22:00:03.435 Moscow Time. Within a second, the first stage was firing at full thrust.
After 11.34 seconds in vertical flight, the rocket began its preprogrammed tilt toward an azimuth of 64°11'38" to align its ascent trajectory with an orbital plane of 51.35 degrees toward the Equator. At the one-minute point into the flight, the vehicle experienced maximum dynamic pressure, as it was leaving the dense atmosphere. At 115.18 seconds into the flight, Zenit's first stage began throttling its RD-171 engine in preparation for the completion of firing. A preliminary command to cut off the engine was issued at 131.99 seconds into the flight and the engine entered its final thrust mode exactly two seconds later. While it was still firing, the ignition command was issued to the second-stage propulsion system at 144.25 seconds and its vernier thrusters fired through a lattice structure connecting the two booster stages.
The command to cut off the first-stage engine was issued at 146.59 seconds and two seconds later, the first stage separated. The spent booster was projected to fall in the Karaganda Region of Kazakhstan, likely within Drop Zone No. 226 around 884 kilometers from the launch site. Another 3.67 seconds later the main engine of the second stage joined the vernier thrusters. After around nine seconds, the main engine of the second stage developed its full thrust.
The payload fairing protecting the satellite separated five minutes 22 seconds after launch and its two segments were expected to impact in the Altai Region of Southern Russia, where Drop Zone No. 326 is located 1,924 kilometers downrange from Baikonur.
At T+409.89 seconds into flight, the main engine of the second stage began throttling down in preparation for shutdown. The main engine was cut off first at T+429.89 seconds in flight and the rest of the propulsion system was turned off less than a minute later, after fine-tuning the parameters of the initial parking orbit.
The second stage then separated from the Fregat upper stage eight minutes and 37 seconds after the liftoff. The planned parameters of the initial orbit were 167 by 566 kilometers in altitude with an inclination 51.35 degrees toward the Equator.
Planned timeline for the initial powered ascent of the Angosat-1 satellite on Dec. 26, 2017:
After its separation from the second stage of the Zenit, the Fregat made half a revolution around our planet before initiating its first maneuver to further boost the apogee (the highest point of its orbit) and enter an intermediate elliptical orbit. The space tug first fired its secondary thrusters, SOZ, giving initial acceleration to the stage in order to ensure a reliable supply of propellant to the main engine. The first engine firing lasted almost eight minutes, expending propellant in the external tank strapped to the bottom of the stage. The nearly empty tank separated 37 seconds after the completion of the first maneuver, which inserted the Angosat/Fregat stack into a 282 by 4,087-kilometer orbit and reduced its inclination to 50.45 degrees toward the Equator.
Soon after the maneuver, the telescope of the Russian Academy of Sciences in Mongolia tracked the stack in close-to-nominal trajectory and then the separated external tank was also seen from the town of Gorno Taezhny in the Ussuriisk Region of the Russian Far East, which is the home of the NIP-15 ground station.
For the next two hours, the Fregat/Angosat stack climbed passively along its intermediate orbit, before the SOZ thrusters came back to life to get the main engine ready for the second maneuver. This time, the main engine fired for more than 11 minutes to form a 341 by 36,061-kilometer geostationary transfer orbit, whose apogee was near the geostationary altitude above the Earth's surface. The maneuver also further reduced the mission's orbital inclination from 50.45 to 48.62 degrees toward the Equator.
It took more than five hours for the vehicle to reach the apogee of the orbit, where Fregat conducted its final maneuver to deliver the Angosat satellite. The third maneuver also did most of the job of tilting the orbital plane of the mission to match the Equatorial plane. The Angosat-1 spacecraft separated two minutes after an almost nine-minute firing of the main engine, completing the mission nearly nine hours after the launch.
The Fregat was then expected to maneuver itself into a burial orbit, where it would not present a collision hazard for its former passenger.
The satellite's deployment trajectory was designed to provide its drift toward its operational position over the Equator, where it could use its own propulsion system to fine-tune its orbital parameters ensuring reliable communications coverage across the African continent.
Summary of orbital maneuvers during the Angosat-1 launch on Dec. 26, 2017:
Just minutes after the planned release of the Angosat-1 from the Fregat upper stage at 06:54 Moscow Time on December 27, Roskosmos announced that the satellite had successfully separated. However soon thereafter, semi-official Russian press sources reported that although the spacecraft had first established communications with ground control, the contact had soon been lost. The Interfax news agency quoted industry sources as saying that after the separation everything looked good: the spacecraft had activated its attitude control system, however the telemetry had stopped coming during the deployment of the vehicle's solar panels.
Ground control specialists reportedly continued efforts to restore communications with the spacecraft, Interfax said. The agency then quoted a source claiming that contact with the satellite could be lost due to a computer failure or problems with its power batteries.
According to NORAD radar, all orbital parameters of the Angosat mission seemed normal. According to the nominal flight program, the Angosat-1 satellite was supposed to be released at a point 56.6 degrees East longitude over the Equator into an orbit with a radius of 42,314 by 42,404 kilometers. The resulting trajectory would be around 157 kilometers higher than the geostationary orbit, which would give the satellite a westerly drift relative to the Earth's surface at a rate of around 2.5 degrees per day. As a result, it would appear over its final position in the geostationary orbit in 17 days, industry sources said.
In the evening Moscow Time of December 27, RKK Energia, the satellite developer, posted a press release confirming that "some time" after establishing communications with ground control, telemetry had stopped coming from the spacecraft. According to the company, its specialists were analyzing available telemetry received from the spacecraft and were working on re-establishing communications to resolve the problem.
According to the Izvestiya daily, quoting RKK Energia, the first and so far only communications session with Angosat-1 had lasted 42 minutes, which was enough to confirm that the satellite had successfully maintained its attitude in space and deployed its solar panels and antennas. The battery aboard the satellite was expected to operate from 10 to 16 hours, but specialists at RKK Energia believed that the contact with Angosat-1 could still be reestablished after that period because the deployed solar panels had the ability to recharge the batteries, the newspaper said.
By the end of the day on December 27, Interfax reported that a short circuit in the power supply system of the Angosat satellite had been considered as the most likely culprit in the failure, according to preliminary data.
On December 28, RIA Novosti reported that all efforts to restore communications with Angosat-1 during the night of December 27 had proven fruitless.
Optical observations of Angosat-1 by Russian tracking means indicated that the spacecraft had deployed its solar arrays because it appeared brighter than it would have looked like with folded panels. Also, the absence of variation in brightness of the satellite over time showed that it had remained in stable position. The spacecraft was also performing a westerly drift, as expected, but that movement would take it out of view of the Russian mission control in less than a month, Russian industry sources said.
During most of the day on December 28, efforts to contact Angosat-1 continued and in the evening (Moscow Time), the official TASS news agency reported that flight controllers had received telemetry from the spacecraft. This fact was also confirmed by the Angolan government. Soon thereafter, TASS quoted industry sources as saying that communications could initially have been disrupted due to the draining of the spacecraft's batteries, but, fortunately, Angosat-1 had already deployed its solar panels, which allowed it to restore its power supply. Obviously, such a scenario provides no explanation about how the spacecraft ended up with drained batteries and why such an obvious problem could not be avoided.
On December 29, RKK Energia, the satellite manufacturer, also confirmed that communications with the Angosat-1 had been restored, adding that the telemetry received from the spacecraft showed all the parameters of onboard systems were normal. Around the same time, industry sources said that the spacecraft was receiving commands and sending telemetry back, thus maintaining two-way communications with ground control.
On December 30, Interfax reported that the problems with Angosat-1 had stemmed from incompatibility between standards used by Russian and French-built hardware aboard the satellite. The issue was resolved remotely by specialists at RKK Energia, Interfax said, quoting industry sources.
Despite a brush with death shortly after launch due to breakdown in communications, the Angosat satellite appeared functioning well on its way to an operation position in the geostationary orbit. On Jan. 5, 2018, Central-African Information Agency quoted Russian ambassador to Angola Vladimir Tararov as saying that all systems aboard the satellite had been operating as planned.
In the days after the miraculous revival of Angosat-1, the satellite continued drifting westward, according to NORAD data. Initially, it was assumed to be a planned movement, however independent observers became alarmed, when on January 9, 2018, the Angosat had passed over its operational point without any visible attempt to slow down and stop its drift. Only on January 15, RKK Energia issued a press-release disclosing that the telemetry from the satellite had revealed a problem in the Angosat's power supply system. The company admitted that in the near future, the spacecraft would leave the communications range of the mission control center in Korolev.
"Due to very high criticality of the maneuvers, the decision was made not to perform active influence on the spacecraft until its return to the communications range. The resumption of work associated with the flight testing of the Angosat satellite is planned for the middle of April 2018," RKK Energia said.
In the meantime, the second stage of the Zenit rocket, which delivered Angosat into orbit, was expected to reenter the Earth's atmosphere around January 26. The latest forecast roughly matched the original prognosis for the decay of the second-stage orbit around 28 days after its launch with a possible error of around eight days either way.
Sequence of Angosat-1 activation after the separation from Fregat:
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The upper composite with the Angosat satellite and Fregat upper stage is being integrated with the Zenit rocket on Dec. 19, 2017. Click to enlarge. Credit: RKK Energia
Zenit in preparation for fueling on the day of the launch. Click to enlarge. Credit: Roskosmos
Zenit minutes before launch on Dec. 26, 2017. Click to enlarge. Credit: Roskosmos
Click to enlarge. Credit: Roskosmos
Zenit lifts off on Dec. 26, 2017. Credit: Roskosmos
Fregat-SB maneuvers to deliver Angosat-1 satellite. Click to enlarge. Copyright © 2017 Anatoly Zak