Atlantis undocks and departs ISS – TriDAR tests continue with flyaround

by Chris Gebhardt

Space Shuttle orbiter Atlantis has undocked from the ISS for the final time, ahead of carrying out the unique and stunning flyaround. And while the moment resulted in dazzling images of a graceful orbital ballet, as well as a multitude of emotions, Atlantis once again lent a helping hand to future spacecraft as she performed one more TriDAR detailed test objective.

Undocking and Modified ISS Flyaround:

Following a flawless execution of all MPLM Raffaello transfer operations to and from the International Space Station, Atlantis’s crew successfully berthed the Raffaello into Atlantis’s payload bay yesterday morning for the return journey to Earth.

With Raffaello secure, the ISS and Shuttle crews held the traditional farewell ceremony outside of PMA-2 (Pressurized Mating Adaptor -2) in the Node-2/Harmony module.

While the farewell speeches contained the traditional “thank yous” from the Shuttle and Station crews, the event marked the emotional and historic close of yet another chapter in the life of the Space Shuttle fleet: the final crew handshakes and hugs between a visiting Shuttle crew and the crew of the ISS.

Following a good night’s rest, Atlantis’s crew rose at 2159 EDT Monday evening to begin final preparations for Atlantis’s undocking from the ISS – which saw the departure take place at 0228 EDT Tuesday morning.

However, this undocking was not be like the others that have come before it. While Atlantis still conducted a flyaround of the premiere orbital outpost, the ISS maneuvered into a different orientation following Atlantis’s undocking.

As reported during the MOD FRR (Mission Operations Directorate Flight Readiness Review), “If propulsive consumables permit, the ISS Program desires an imagery survey of the ISS port and starboard exterior surfaces by means of a Shuttle flyaround at a distance of 600 ft,” notes the MOD Flight Dynamics presentation, available for download on L2.

In essence, the ISS Program utilized the Shuttle even after undocking to gain “engineering-quality photos of portions of ISS not normally seen (±Y views).”

To accomplish this, Atlantis’s undocking time was carefully chosen to provide “adequate” lighting for photographic documentation of the ISS. Specifically, the undocking window opened at orbital sunrise -59 minutes (resulting in a sunrise at the start of the half-lap flyaround) and closed at sunset -87minutes (resulting in a sunset 5 minutes after the conclusion of the half-lap flyaround).

Following undocking, the ISS maneuvered into a +/-Y orientation in the velocity vector (YVV) attitude after Atlantis backed away from the Station to a distance of 600 – 700 feet along the +V bar.

Moreover, Atlantis’s rate of separation from the ISS once she reached a distance of 30 feet was 3 feet per second instead of the traditional 2 feet per second because of timeline constraints.

After these steps are complete, Atlantis initiated a TORF – Twice Orbital Rate Flyaround – maneuver of the ISS.

The entire flyaround was not the standard complete flyaround, but rather a half-lap flyaround in order to “minimize timeline impacts of new separation profile.”

STS-135 Specific Articles:

Once on the opposite side of the ISS, along the -V bar, Atlantis initiated a separation maneuver to bring herself further out “in front” of the ISS – as viewed from the vehicles’ direction of travel.

Had for whatever the reason, the ISS is not able to support the modified flyaround, all timelines and traditional vehicle attitudes would have been maintained and a standard 1 lap flyaround performed.

With the events all being performed flawlessly, Atlantis completed her two sep burns and departed from the Station one final time, slinking away into a dramatic light show, before the view faded to black.

The crew then unberthed the Orbiter Boom Sensor System (OBSS) and completed the Late Inspections of Atlantis’ Thermal Protection System (TPS), the concluding tasks of Flight Day 12.

TriDAR on Atlantis – A breakthrough orbital docking sensor technology:

Making its third trip to the International Space Station aboard the Space Shuttle (but only its first flight aboard the Atlantis), the TriDAR (triangulation + LIDAR) system has made two previous flights aboard the orbiter Discovery, during her STS-128 (Aug/Sept. 2009) and STS-131 (April 2010) flights.

Unlike the TriDAR hardware elements flown on those two Discovery missions, the TriDAR hardware aboard Atlantis has been slightly modified.

According to a Neptec (the company responsible for TriDAR) presentation following the successful docking of Atlantis to the ISS last week, “For STS-135 the flight hardware was modified to improve thermal stability of optics and add a calibration shutter to the thermal imager.”

In its basic form, TriDAR is designed to “Demonstrate Rendezvous and Docking (AR&D) sensor technology that does not require cooperative targets (i.e. reflectors),” notes the Neptec presentation.

Providing distance, relative position, and attitude information for orbiting (and docking) spacecraft, TriDAR uses absolutely no reflectors and instead relies on 3D data from a LIDAR unit and compares that data to the known configuration of a spacecraft – in this case, the International Space Station.

Aisde from automatically recovering tracking of the International Space Station following completion of the R-bar Pitch Maneuver, TriDAR also provides real-time attitude positioning data instead of “assuming” a set ISS Local Vertical Local Horizontal (LVLV) reference point.

Furthermore, all tracking data is provided real-time to the crew, target acquisition is accomplished automatically by the unit, and a “Thermal imager is used to provide guidance data for long range operations.”

STS-135 Flight Day 3 TriDAR Status Report:

On Flight Day 3 of the STS-135/Atlantis mission, nominal TriDAR activation by the crew was disrupted due to a communication issue with an Ethernet cable/PGSC port.

Atlantis’s crew quickly worked the issue and got TriDAR up and running for docking operations. For undocking, the crew has been fully briefed on the communication issue, and the issue is not expected to affect operations this morning.

During rendezvous operations on FD-3, TriDAR “generated ‘star tracker’ type bearing data day and night” and began acquiring images of the International Space Station as soon as it was activated.

Longer range data was not gathered because of the communication issue and subsequent troubleshooting efforts, but “Good image data [was] acquired from initialization at ~34km to dock” through docking.

This resulted in the collection of 2,640 thermal images of the ISS, as seen in a stunning video of Atlantis’ approach and docking with the ISS via TriDAR (L2).

“Range/bearing and Rdot [data] displayed to crew starting at ~2400 ft,” and Commander Chris Ferguson confirmed data compatibility between TriDAR and the regular TCS rates.

A subsequent, post-docking review of all TriDAR data revealed the DTO (Detailed Test Objective) achieved all of its primary and secondary mission objectives during docking operations.

During undocking this morning, TriDAR will be run in “imaging mode” to collect 3D and thermal images of the ISS during the flyaround maneuver performed by Atlantis.

Background on TriDAR:

With the ever increasing and changing nature of space exploration, the ability to perform unmanned dockings with orbital installations is coming into higher demand.

These unmanned AR&Ds (Approach, Rendezvous, and Dockings) will be vital to the continued operation of the International Space Station once the Space Shuttle following the retirement of the Space Shuttle fleet in just a few days’ time.

Moreover, unmanned AR&Ds will also be a crucial aspect of future robotic exploration missions of the solar system, as well as potential satellite repairs in Low Earth Orbit.

As such, a new kind of AR&D program was/is needed to fill this growing demand.

Enter Neptec’s TriDAR system. According the Neptec’s white paper document from 2009, “TriDAR is a relative navigation vision system . that provides critical guidance information that can be used to guide an unmanned vehicle during rendezvous and docking operations in space.”

The system is designed to automatically acquire and track an object “using only knowledge about its shape.”

Beginning life eight years ago, Neptec’s TriDAR system was developed as part of the effort to “conduct an automated mission to the Hubble Space Telescope and for the Canadian Space Agency (CSA) and Canadian defense,” said TriDAR Program Manager Stephane Ruel in an interview with in August 2009.

“We actually developed the software before we developed the hardware. So we developed all these algorithms and then the hardware was developed for NASA for the Hubble robotics vehicle – which was supposed to be an automated vehicle to rendezvous and dock with the Hubble Space Telescope and conduct repairs.”

The Hubble robotics vehicle was later cancelled when NASA decided to fly Atlantis on the STS-125/SM4 mission in May 2009. 

For Neptec, that meant redefining their project’s goal. “We ended up being able to develop the hardware that would have been used for [the Hubble robotics mission] into a prototype stage that [became] TriDAR.

“Later on, CSA and NASA [expressed interest] in flying the technology on the Shuttle as a demonstration for our test flight. That’s sort of the short answer to the long process of how we ended up on STS-128.

Specifically, the path toward TriDAR’s eventual three flights aboard the Shuttle began in 2006.

For the three missions, TriDAR has been mounted on the Orbiter Docking System (ODS) right next to the TCS (Trajectory Control System). According to Stephane Ruel, “TriDAR operates completely autonomously. One of the key features of TriDAR is that it doesn’t need anything special on the Space Station. So we don’t need a docking target or reflectors.

“That’s really one of the true innovations about the TriDAR system: all we use as a reference is knowledge of what the Space Station looks like. Because TriDAR is a 3D sensor, essentially, we will get 3D point clouds that we can then line up to the shape of the Station.

“When we line up those two data points we can find the Station and calculate its position relative to the Shuttle. Since TriDAR will be scanning continuously, we can take the 3D data gathered by the system to create real-time data about the position of the two vehicles relative to one another.”

But it’s not just the Shuttle and Station that are giving Neptec and TriDAR some attention. Companies involved the COTS program have also expressed interest in seeing TriDAR on their future vehicles – as has the Canadian Space Agency and companies in Europe.

As mentioned by Mr. Ruel in 2009, “We are being considered for the COTS program. We are also working with CSA and looking at other programs in Europe that this system could be used for, like a Mars Sample Return flight.

“We’re also looking at some potential satellite repair missions – which is kind of what TriDAR is ideal for in the sense that current on-orbit satellites don’t have reflectors on them so you really need a system like TriDAR that can operate autonomously and make use of the existing structures on these satellites for unmanned dockings.”

*NEWSFLASH: And on Monday, Neptec announced that TriDAR has been selected to fly as the rendezvous sensor on the Orbital Sciences Cygnus spacecraft – an article will follow*

But the possible uses for TriDAR extend far beyond the confines of Earth and the space immediately surrounding our celestial home.

“Think of Mars Sample Return where you could take the sensor to a planet, use it as a landing system, and then – once you’ve landed – use the system to navigate a rover.

“Then, you could use the LCS (Laser Camera System) capability to take really short range scans of areas that you’d potentially want to drill in and conduct science experiments on or collect samples from. Then, if you launch that rover back to Earth, you could use the system once again to docking with a Station in Earth orbit.

“There really are a multitude of things that TriDAR can be used for. It’s an exciting system!”

Final thoughts from the TriDAR team:

While only originally set to fly with Discovery on STS-128, TriDAR has had the honor of flying three highly successful missions with the Space Shuttle – and an added honor of flying aboard the historic final mission of the Shuttle fleet.

“Thank you from the TriDAR team! It has been a privilege and an honor to fly three missions on the Space Shuttle program. We appreciate all of the help and support over the years. Your contributions have helped make TriDAR a success!”

To read about Atlantis and her sisters – from birth, processing, every single mission, through to retirement, click here for the links:

(Images: Via L2 content and Further articles on STS-135 will be produced during and after her mission, driven by L2′s STS-135 Special Section which is following the mission at MMT/MER level, surrounded by a wealth of FRR/PRCB/MER/MMT and SSP documentation/pressentations, videos, images and more.

(As with all recent missions, L2 is providing full exclusive level mission coverage, available no where else on the internet. To join L2, click here:

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