Since Rosetta’s lander Philae initial woke adult from hibernation and called ‘home’ on 13 June, a teams during a Lander Control Center (LCC – DLR), a Science Operations and Navigation Center (SONC – CNES), a Max-Planck Institute (MPS – Göttingen) and a Institute for Particle and Nuclear Physics (Wigner Research Centre for Physics – Budapest) have been operative with ESA’s Rosetta Mission Operations Centre (RMOC – ESOC) and a Rosetta Science Ground Segment (RSGS – ESAC), and in tighten team-work with a Philae and Rosetta scientists, to settle unchanging and likely contacts with Philae, and to resume systematic measurements.
This blog post has been created by Koen Geurts, Philae technical manager, and Cinzia Fantinati, Philae operations manager (both from a LCC during DLR), and gives a notation discernment into a work being finished by a teams.
Following Philae’s deployment by Rosetta to a aspect of Comet 67P/Churyumov-Gerasimenko on 12 Nov 2014, a lander operated for 2.5 days before descending into hibernation during a final alighting site, Abydos. The elemental emanate was a miss of object to assign Philae’s delegate batteries.
Due to a vast array of unknowns with honour to Philae’s final alighting site, not slightest a lander’s march with honour to a inner topography, it was formidable to precisely envision when Philae competence wake-up again as a comet approached a Sun and a strength of a object increased. Another vital regard was a ability of a hardware to tarry a really low temperatures, good successive a –55°C gift temperature, approaching during a approaching several month prolonged hibernation phase.
Previous posts have described a essential conditions that contingency be met for Philae to arise adult and boot: an inner heat above –45°C and some-more than 5.5W of power. Based on measurements finished before Philae entered hibernation in Nov and models formed on a approaching plcae and a comet’s orbit, it was motionless to embark wake-up campaigns in Mar 2015 during comparison durations where Rosetta’s orbital geometry was enlightened to communication.
No signals were listened during those periods, and during a unchanging assembly of a Rosetta Science Working Team (SWT) in Rome on 11–12 June, a contention was hold on providing serve support to creation hit with Philae. The SWT authorized a 2-week hunt campaign, during that Rosetta’s arena would be optimised for probable contacts with Philae.
The debate was due to start midst July. Little did anyone know that Philae was prepared to speak to us a successive day!
Link investiture procedure
In method to settle communication with Philae, Rosetta’s on-board Electrical Support System Processor Unit (ESS), obliged for a interface between a dual probes, contingency be switched on and configured in “Research Mode”. In this mode, a ESS conductor is switched on and promulgation signals with a specific settlement that are recognized by Philae. In parallel, a ESS receiver is switched on in method to listen out for any probable replies.
When possibly of a receivers on Philae hears a vigilance from Rosetta, a on-board program performs a check to see if there is adequate energy to switch on one of a dual over-abundance transmitters. If so, a conductor is incited on and earnings a vigilance with a specific pattern, signalling a ESS on Rosetta that it is prepared to settle a two-way link. This done, Philae immediately starts uplinking scholarship and housekeeping information that are stored in a on-board mass memory (MM). Once a two-way couple is established, Rosetta can also send new authority sequences for Philae to bucket and execute, initiating systematic measurements, for example.
The favoured operations mode foreseen for Philae during comet operations was to spin on a receivers periodically. However, there were concerns that this competence lead to brief hit opportunities being missed. Thus, shortly before Philae went into hibernation final November, a operations group configured a on-board program to switch on both of a over-abundance receivers whenever a rechargeable delegate battery reached a smallest dungeon voltage of 3.4V or if sufficient over-abundance solar energy was available. The aim was to maximize a chances of hit being finished after Philae came out of hibernation.
First lander contact
At 20:28:11 UTC on 13 Jun 2015, Philae and Rosetta managed to settle their initial two-way couple given November, for a generation of usually 78 seconds. In this time, a sum of 343 packets of housekeeping telemetry (TM) data, including information from a thermal, power, and on-board resource subsystems, were transmitted from Philae to Rosetta. One TM parcel comprises 141 16-bit difference or 2256 bits, and so a sum volume of information transmitted was usually underneath 100 Kbytes. There is a eminence between “real-time” TM, that provides live information on a standing of several systems during a time of a link, and “stored” TM, information from some indicate in a past that has been stored in a on-board MM.
The method in that Philae’s TM is transmitted to Rosetta is related to how it interacts with a MM. Not usually “old” TM is stored in a MM as we would expect, yet real-time TM generated during an persisting communication also goes around a aegis to Philae’s MM. Since a MM operates on a First-In-First-Out (FIFO) principle, a older, stored TM gets transmitted first, before a real-time TM. Completely clearing a full MM takes 40 minutes.
Thus, if a couple is fast yet comparatively short, it is not probable to obtain real-time standing information, as those TM packets sojourn queued in a MM.
However, in a box of a really inconstant and brief (e.g. reduction than 20–30 seconds) link, a poise is different. In this instance, a on-board program routes a real-time TM from a aegis loyal to a communications section for transmitting to Rosetta, totally bypassing a MM.
Decoding when any retard of TM was generated is tricky. Each time Philae has adequate energy to reboot, a on-board time starts from zero, and loyal time synchronisation to UTC can usually start when a couple is finished with Rosetta. In a deficiency of contacts with Rosetta, an choice proceed of gripping lane of a time when a TM is generated is needed. To do so, a on-board program computes a “comet day counter”, that is stored in non-volatile memory, by counting in units of 744 mins (i.e. 12.4 hours, a revolution generation of a comet as specified in a software) and adding this to a predefined starting date of 28 Nov 2014 during 00:00:00. This gives an estimation of a UTC time, yet usually underneath a arrogance that Philae is handling invariably or reboots during any inner sunrise.
This date equivalent was incidentally comparison in a weeks before alighting as a placeholder and was ostensible to be updated in a hours after landing, formed on predictions of a initial wake-up on a comet surface. Due to a astonishing inlet of Philae’s landing, a parameter was never updated.
The information sent to Rosetta during a initial tie on 13 Jun contained stored TM with comet day depends of 2, 19, and 20. In addition, 6 real-time TM packets with count 95 were obtained. If Philae had been handling invariably given a alighting in Nov 2014, afterwards comet day 95 would have occurred on 16 Jan 2015. But given these ‘count 95’ TM packets indeed conform to 13 June, that offers a event to work retrograde and see when Philae indeed initial came behind to life again, even if it was incompetent to promulgate with Rosetta during a time.
The real-time TM packets indicated that Philae rebooted during 20:24:48 UTC on 13 June, reduction than 4 mins before hit was perceived by Rosetta. If that impulse corresponds to count 95, afterwards count 20 contingency have occurred 75 x 744 mins earlier, i.e. 38.75 Earth days earlier, on 6 May. Similarly, count 19 was 12.4 hours progressing on 5 May, and count 2 was on 26 April.
There are some caveats here. First, this arithmetic does not take into comment a probability of a reboot due to a energy train collapse, that a on-board program can't heed from a “real” comet morning. However, by correlating a daily heat boost seen in TM from undeviating days with a comet day count, such “fake” reboots can in element be identified.
Also, nonetheless a MM operates on a FIFO mechanism, it is not transparent because there are gaps in a downlinked TM: for example, a burst from count 2 to 19. While a date count is created to a non-volatile memory immediately after boot, successive housekeeping and scholarship information are stored in flighty RAM and usually eliminated to non-volatile memory once nightfall is rescued around a diminution in incoming solar power.
One probable reason afterwards is that comet nightfall occurred too fast on those days, and a rechargeable battery was not means to yield a compulsory 30–60 second aegis until a saving of TM into a non-volatile memory was complete. Conversely, a burst from 20 to 95 is accepted as being due to a fact a count 95 TM were generated in real-time on a day of a connection, while there was not adequate time to downlink any probable stored TM from comet day depends 21–94.
The downlinked TM for comet days 2 and 19 camber a full generation between inner morning and nightfall as seen by Philae, so giving a proceed denote of a generation of object during that location. The TM from comet day 20 is incomplete, however, as a couple to Rosetta was damaged before it could all be transmitted.
In serve to a reformation of a tangible UTC foot times, a engineering housekeeping information contained in a downlinked TM packets were also evaluated, including a inner and outmost temperatures, a occurrence solar power, a state of a battery, ubiquitous currents and voltages, and a peculiarity and settlement of a communication link. In particular, a latter information came to play an critical purpose in a weeks to come.
Immediate Rosetta arena changes
Obviously, a initial vigilance perceived on 13 Jun triggered a array of teleconferences, discussions, and need for evident preference taking. Together with a Philae consortium, pivotal people from a ESA teams during RMOC and RSGS, a Mission Manager, and a Project Scientist started evaluating and assessing Philae’s status. It was immediately transparent that Rosetta’s arena indispensable to be optimised in method to be concordant with Philae communication, and stairs were concluded to prioritise this activity.
The comet embodiment operation where Philae was suspicion to be was tangible and compelled to be within 0° and +55°N, and processed for doing by a RMOC Flight Dynamics and Flight Control teams. This remarkable change was not yet impact on a already foreseen and scheduled Rosetta scholarship activities. For example, a indicating of Rosetta for systematic observations was strongly compelled during likely communication windows, so a RSGS group had to fast weigh and cgange a baseline formulation in relationship and agreement with a instrument teams. All in all, this compulsory a outrageous bid by everybody involved, with a sum design of restarting Philae’s scholarship measurements.
Subsequent contacts between Philae and Rosetta
The second hit with Philae occurred one Earth day (i.e. 2 comet days) after on 14 Jun during 21:22:47 UTC. This time, a generation between a initial and final hit was 04:04 minutes, nonetheless visit couple interruptions occurred in this period. A sum of usually 26 TM packets were received, all real-time, with comet day count 97.
There was afterwards a longer check until a third hit occurred on 19 Jun during 13:20:33 UTC. This time it was for a significantly longer generation of 18:53 minutes, yet it was again frequently interrupted. A sum of 180 packets were received, comprising a brew of day count 107 (i.e. real-time) information and information with day count 96 generated during a before contact. The real-time information authorised us to settle a settlement of Philae’s communication hardware for a initial time, with conductor TX1 and receiver RX2 in use.
The fourth contact, on 20 Jun during 13:55:25 UTC, had a sum generation of 31:01 mins with many interruptions. This time, a sum of 744 packets were eliminated containing stored TM from comet days 21 to 25 and 97, and real-time information from a stream comet day 109. For a initial time, this authorised a reformation of several consecutive comet days. Again, a couple was determined with TX1 and RX2.
The fifth hit occurred on 21 Jun during 02:32:50 UTC and lasted 11:25 minutes, with a 10:37 notation interruption. A sum of 294 TM packets were eliminated with comet day count 25 to 27; no real-time TM was received.
The sixth hit occurred on 24 Jun during 17:23:48 and lasted 17:11 mins with visit couple interruptions. A sum of 83 real-time TM packets were perceived with comet day count 118. Analysis of a sum real-time information and a meagre stored information relayed behind to Earth during these 6 contacts showed that Philae’s inner temperatures were usually increasing, a battery was being charged, and a comet days were removing longer, all unchanging with augmenting solar enlightenment and anniversary variations en-route towards perihelion on 13 August.
Rosetta’s arena vs Philae contacts
The 6 contacts between 13 and 24 Jun clearly supposing many reduction information than hoped for, and a concentration of a teams was on improving this situation.
During this period, Rosetta was during a stretch of approximately 200 km from Comet 67P/C-G (and Philae), especially as a effect of perplexing to equivocate problems with Rosetta’s star trackers in a increasingly dry coma sourroundings around a comet. Based on a telemetry parameters indicating couple quality, this seemed to be during a really corner of a operational operation for creation good hit with Philae.
Philae’s antennas are aligned with a instruction of a spacecraft’s +Z axis, i.e. indicating plumb adult by a body. The larger a angle between Rosetta and +Z axis, a weaker a signal, and ideally, Rosetta would fly over Philae’s plcae within a 20° half-cone around +Z to safeguard a strongest contact.
However, Philae’s march during a final landing site, Abydos, is such that a +Z pivot is in fact destined into a comet. In practice, this creates it unfit for Rosetta to fly over and see Philae during angles of reduction than 30° from a +Z axis, with transparent consequences for a strength of a communications signal.
Another proceed to urge a vigilance strength is to revoke a stretch between a dual spacecraft: roughly vocalization it should urge with a opposite retard of a distance. Mindful of a concerns per a star trackers and a dry environment, a RMOC group yet pushed Rosetta towards a protected operational limits, stepping down a stretch to a comet any 3–4 days in late Jun and early July. For example, a hit finished on 24 Jun was during a stretch of 180 km, while a successive hit was during 155 km on 9 July, as described in some-more fact after in this post.
In serve to a varying distance, a arena of Rosetta and a revolution of 67P/C-G meant that Rosetta seemed over a segment where Philae is suspicion to be, during opposite comet latitudes any comet day. Over these few weeks, a operation in latitudes between 0° and +55°N was lonesome several times.
As communication between Rosetta and Philae did not start any comet day, it was hypothesised that there was a dependency on latitude. However, contacts did not take place during a same latitudes any time, and so inner topographical facilities competence also hinder a vigilance in some cases, complicating a analysis. In addition, it is not transparent because a contacts – when determined – were so heavily interrupted. For a brief durations during that a couple was established, a vigilance was comparatively strong, again creation it tough to come adult with a elementary explanation.
Overall, it was proof tough for a group to come adult with arguable predictions of when good contacts would be made.
Hardware issues and CONSERT switch-on
The singular information retrieved from Philae’s MM during a meagre contacts supposing a engineers with nonetheless another puzzle. The information contained several gaps and during a hit of 24 June, a real-time TM indicated that one of a MM play was ‘empty’, nonetheless no information were indeed eliminated from it. In addition, a housekeeping information showed a aloft than approaching stream on a MM input. Apparently a MM wiring play were not functioning as reliably as before.
Furthermore, a real-time TM perceived during a 19 Jun and 20 Jun contacts showed that usually one of a dual receivers (RX2) was switched on, even yet a settlement was set to use both receivers. On a other hand, information stored from a May generation and downlinked on 20 Jun showed that both receivers had been powered on during that time, as expected.
Further investigation of a information gave a transparent design of a situation: RX1 had suffered a brief circuit on a submit and was switched off by a hardware overcurrent protection. The disaster endangered a team, yet this is a reason because a lander has over-abundance units available. At that time, there was no denote of additional hardware issues on a receiver side, i.e. all currents and voltages were nominal.
However, after a two-week overpower following a 24 Jun contact, a group had to during slightest perform a probability that maybe RX2 had also failed.
Failure of both receivers would clearly cancel a Philae partial of a mission, as no commands can be sent to it and it can't be educated to do anything. Another probability for a miss of hit in this generation was that a signals being sent from Philae to Rosetta were too weak. The energy of Philae’s conductor is usually half that of a one on-board Rosetta, and it competence have been that Philae’s respond vigilance would be too twisted by a comet sourroundings for Rosetta’s ESS to recognize it.
The group worked tough to find ways of exclusive one or other possibility. For example, maybe Earth-based observations with a really vast radio telescope or one of a radio-sensitive systematic instruments on-board Rosetta competence be means to detect a vigilance from Philae in a suitable magnitude range. After serve study, this incited out not to be possible.
In a end, it was motionless to try regulating a CONSERT instrument, designed to make measurements of a interior of a comet by lucent radio signals behind and onward between Rosetta and Philae and, importantly, regulating separate, eccentric antennas from a communications system.
The problem, of course, was that CONSERT was not switched on. Fortunately, in serve to a customary proceed of substantiating a couple as described progressing in this article, commands can also be sent to Philae “in a blind” regulating a supposed TC Backup Mode (TCBM). In this way, a tiny array of commands are installed into Rosetta’s ESS and transmitted regularly over several hours in a specific settlement that should means them to be recognized and processed by Philae’s on-board software, yet a need to settle a two-way link.
The thought was to use a TCBM to authority CONSERT to spin on and afterwards promote signals from a CONSERT section on Rosetta towards a lander. If a CONSERT signals were perceived on-board Philae and sent behind to and perceived by Rosetta, that would have reliable that a RX2 receiver on Philae was working, differently a authority to spin on CONSERT would never have been perceived and acted upon.
TCBM was designed as a fall-back resolution to promulgate with Philae in box zero else worked, yet it comes with limitations. In particular, in a stream situation, Philae’s program is configured to concentration on battery charging and progressing inner temperatures, not to work a systematic payload. Also, it is not probable to pledge that all commands are received, while conversely, a steady receipt and execution of a same authority competence means problems. Therefore, before an try could be finished to use TCBM to spin on CONSERT, mutated commands (essentially program routines) to equivocate steady execution had to be developed, tested extensively, and certified on a belligerent anxiety indication of Philae.
When Philae receives a authority revelation it to spin on e.g. CONSERT, a on-board program instructs a energy subsystem to switch on CONSERT. In addition, it loads a method of telecommands from an inner memory that contingency be eliminated to CONSERT during a right time and usually once, a supposed timeline. Thus, a elementary telecommand revelation Philae to switch on CONSERT triggers several opposite actions. For this reason, a on-board program and CONSERT get confused if a same authority is perceived some-more than once. This was avoided by formation of a authority in a program routine, that checks if a authority is perceived for a initial time or not.
The initial try to use TCBM to spin on CONSERT was finished on 5 July, yet unsuccessful as no CONSERT vigilance from Philae was detected. A second try was finished on 9 Jul and was followed by a large surprise, as a full two-way couple was finished between Philae and Rosetta during 17:45 UTC. It lasted for a sum generation of 22 mins and with an undeviating generation of approximately 12 mins – a best so far! A sum of 246 packets were received, all from that comet day, both real-time and stored in a MM.
Curiously, however, a CONSERT receiver on Rosetta did not detect a vigilance from Philae during a same time. The TM downloaded during a hit suggested that a commands sent in TCBM had been perceived and CONSERT had indeed switched on, yet that a foot method had stopped after roughly 6 minutes, before CONSERT started transmitting a signal. The section afterwards remained on, yet not transmitting.
As a engineers looked some-more closely during a data, a poser deepened. These showed that TCBM commands were being perceived from Rosetta shortly after Philae booted when unprotected to adequate sunlight, yet it still took 35 mins before a two-way couple was established.
In principle, a on-board program should switch on one of Philae’s dual transmitters as shortly as a vigilance is received, in method to settle a two-way link. However, if a couple is not finished within 3 mins and signals continue to be perceived from Rosetta, a initial conductor section is announced faulty, and a program switches on a other one.
It appears as yet conductor section TX1 was ordered on briefly, yet timed-out, causing a switch to TX2. No information were permitted from TX1 in that brief window to endorse either a section was sketch current, i.e. indeed transmitting. By contrast, once TX2 was ordered on, approximately 10 mins elapsed before it too timed-out, yet information taken in that generation showed that no stream was being drawn, suggesting that TX2 was pang from a brief circuit. This would means a brief spike in a stream after a section was incited on, triggering a limiter and branch a section off again. However, a spike would be too brief to be seen in a stream measurement.
When a switch behind to TX1 occurred, it took a section roughly 17 mins to energy adult and finally start transmitting to Rosetta, substantiating a two-way link. It is not accepted because it took so prolonged to foot and either this poise is reproducible, nonetheless it is famous that a normal 3 notation time-out was over-ridden by a participation of TCBM signals: this resets a time-out, permitting some-more time for a section to boot.
Based on research and a probable disaster of TX2, a engineers motionless to concentration destiny efforts on regulating usually TX1. This meant building and validating a special program patch that would means Philae to name TX1 only, in tandem with a TCBM vigilance to reset a time-out and yield adequate time for a section to boot.
The TCBM would be used to upload a patch, so no two-way couple would be indispensable required. The patch would be stored in a flighty memory of a software, so it would need to be resent during any communication window. This was meant to concede switching behind to a normal settlement yet a need to again reconfigure Philae. Unfortunately, there have not been any serve contacts given 9 July, so it has not been probable to endorse if this patch has been uploaded. It contingency also be remarkable that a stretch between Rosetta and Comet 67P/C-G (and Philae) was augmenting due to a increasingly dry sourroundings as a comet approached perihelion.
Last outing by a enlightened embodiment operation before to going south
As described above, Rosetta had been flown around a comet in a compelled embodiment operation after Philae initial finished hit in June, in method to maximize a chances of serve communication with a lander. However, that meant that other regions of a comet of augmenting systematic interest, many particularly a southern hemisphere, were not accessible.
It was therefore concluded that after 25 July, Rosetta’s arena would change to concede it to fly over regions on a southern hemisphere, permitting a systematic instruments on a orbiter to examine this partial of a comet that had recently changed into inner “summer”. That would however afterwards make it unfit to hit Philae.
Until then, a Philae group motionless to make use of a remaining hit opportunities between a dual booster to continue regulating a TCBM to send commands to a lander in a blind. These commands enclosed a reconfiguration of a on-board program to trigger renewed systematic measurements, regulating a by-then charged delegate battery. This would have a advantage that once a new hit was made, new systematic information would be immediately available.
The downside is that due to a stipulations of TCBM, usually “pre-existing” on-board sequences could be commanded: for example, a 2-hour retard of measurements by ROMAP, SESAME, MUPUS, COSAC, and Ptolemy, with a CIVA scenery image, such as a one achieved on 13 November. More importantly, perhaps, a use of TCBM mode meant that a program settlement was being finished yet any feedback, potentially withdrawal Philae in an uncertain status.
Current standing and destiny outlook
On 25 July, Rosetta was changed towards a southern hemisphere of Comet 67P/C-G as planned. During a following weeks, communication between a dual booster was not approaching to be probable unless there had been a thespian change in Philae’s orientation. Nevertheless, Rosetta’s ESS was kept on, transmitting signals towards a comet, and listening out for probable lapse signals from Philae.
Just before perihelion on 13 August, Rosetta flew behind over a northern hemisphere again, yet this time in a afternoon terminator plane, definition that any probable hit with Philae would take place in a hours before nightfall during a lander’s location, as against to a morning contacts finished in Jun and early July. At that time, no hit was made.
Between mid-August and a commencement of Sep Rosetta flew over a southern hemisphere again, during a stretch of about 400 km – no hit was approaching during that period.
Since a commencement of September, Rosetta is once again drifting over a northern hemisphere during a identical distance. Although a chances for hit during such distances are not really high, Rosetta will yet continue to listen for Philae. On 23 September, Rosetta will skip on a distant outing arena to strech a stretch adult to 1500 km, that is exclusive with Philae contacts. However, a distant outing is scientifically intensely engaging as it will fly in a terminator craft and will be perplexing to detect a comet’s crawl shock. Moreover, it was never approaching that a Philae partial of a idea would continue until after perihelion. (Editor’s note: we’ll provide more sum about a 1500 km outing in a after blog post.)
In parallel, a engineers dedicated a time given a final hit to control in-depth tests with a energy and communication hardware in method to imitate a celebrated behaviour. Together with a still ongoing information analysis, a pieces of a nonplus are being meticulously put together to refurbish a scenarios as they were seen during a contacts. The idea is to conclude a many earnest plan to hit Philae, to be practical after a distant outing when Rosetta is approaching to proceed a comet during distances many some-more enlightened for communication. From a thermal and energy viewpoint Philae should be means to work until a finish of 2015, so a attempts to get a vigilance will continue during slightest until then. The impact of a augmenting comet activity around perihelion is clearly unknown: usually time will tell if and how Philae survived this.
Many people from all of a Philae and Rosetta teams have worked really tough over a past few months to try and lapse Philae to full operational status, and these efforts will positively continue. Current meditative is that a problems being gifted with Philae’s communications hardware are substantially due to a really low temperatures gifted by a lander in a months immediately following a alighting during a dim Abydos location.
But as communications were re-established on 13 Jun and afterwards intermittently on several occasions given then, it is hoped that a constantly changing thermal conditions on Comet 67P/C-G will make it probable for a hardware to lapse to a some-more fast state, to reinstate contact, and to continue Philae’s rare systematic measurements from a aspect of a comet, a pivotal partial of Rosetta’s altogether mission.
Summary of abbreviations used in this article:
ESS: Electrical Support System Processor Unit
TCBM: Tele-Command Back-up Mode
RAM: Random-access memory
UTC: Coordinated Universal Time
LCC: Lander Control Center
SONC: Science Operations and Navigation Center
RMOC: Rosetta Mission Operations Centre
RSGS: Rosetta Science Ground Segment
DLR: Deutsches Zentrum für Luft- und Raumfahrt
CNES: Centre National d’Études Spatiales
ESOC: European Space Operations Centre
ESAC: European Space Astronomy Centre
SWT: Science Working Team
Source: Rosetta blog