Chandrayan-2 Mission

arshyam · Post by **arshyam** » 10 Sep 2019 09:42

^^ What's the significance of 90 mins? Moon-set over CA?

suryag · Post by **suryag** » 10 Sep 2019 09:42

Sir what 90 minutes you have got me confused and hopeful

Mort Walker · Post by **Mort Walker** » 10 Sep 2019 09:46

NRao wrote:What does all this mean?

It means the Goldstone DSN is transmitting and receiving a signal at 2.27 GHz which may be the carrier signal coming from Vikram. However, I am sceptical as this has happened intermittently before and seems to correspond with a satellite transmitting at the same frequency which happens to come overhead in the Goldstone DSN's antenna transmit/receive path. One would think DSN can resolve range, but I'm not so sure.

suryag · Post by **suryag** » 10 Sep 2019 09:52

Idiosyncrasies

In off-nominal scenarios when a project may be attempting to recover a spacecraft that is in safe mode or experiencing other operational challenges, an antenna may wrongly report that is receiving data from the spacecraft in question. While the ground station is searching for a signal, it may ‘lock on’ to a signal from a different spacecraft and wrongly identify it as the spacecraft being searched for. This is particularly common with spacecraft at Mars as multiple spacecraft are within the field of view of a single DSN antenna. For example, attempts to recover the Opportunity Rover (MERB) may appear successful when the antenna has actually locked on to a signal from one of the orbiters around Mars such as MAVEN or MRO. When this occurs, engineers ask the antenna to ‘drop lock’ and the hunt for the spacecraft continues.

Engineers occasionally need to conduct system tests with an antenna or its subsystems. These may trigger a flow of data suggesting the antenna is preparing to talk to a non-existent spacecraft such as DOUG or SHAN. These are names the engineers apply to the test they’re conducting so data can be tracked through the subsystems that support DSN operations.

In some instances, testing of hardware that was customized for a long-finished mission may cause a ‘phantom’ spacecraft to appear on DSN Now, such as Cassini.

Amber G. · Post by **Amber G.** » 10 Sep 2019 09:55

Folks: There are some "official sounding" twitter handles, including from one K. Sivan - which are fake, so please be careful in believing what you see without confirming.

Dilbu · Post by **Dilbu** » 10 Sep 2019 10:10

Gurulog, I have gone through last couple of pages with heart in my mouth but I am not a tech savvy person. Do we have some hope now?

suryag · Post by **suryag** » 10 Sep 2019 10:14

Very little hope sir let’s see what isro telemetry engineers find

Vayutuvan · Post by **Vayutuvan** » 10 Sep 2019 10:18

ramana wrote:Stabilization is six axis during ...

do you mean six DOF? x, y, z, rxy,ryz, and rzx? there are only three axes in 3D euclidian space.

NRao · Post by **NRao** » 10 Sep 2019 10:20

Mort Walker wrote:
NRao wrote:What does all this mean?
It means the Goldstone DSN is transmitting and receiving a signal at 2.27 GHz which may be the carrier signal coming from Vikram. However, I am sceptical as this has happened intermittently before and seems to correspond with a satellite transmitting at the same frequency which happens to come overhead in the Goldstone DSN's antenna transmit/receive path. One would think DSN can resolve range, but I'm not so sure.

Thanks!!!!!

Insha Ganesha.

arvin · Post by **arvin** » 10 Sep 2019 10:21

From the papers, it is evident ISRO took nothing for granted and had multiple redunduncies at every stage. Very robust hardware design wise I think.
What they lacked was data to tune the software which will be available only by doing such landings multiple times. If vikram wakes up hope they get the data for next attempt.

gashish · Post by **gashish** » 10 Sep 2019 10:39

First official update from ISRO twitter handle:

#VikramLander has been located by the orbiter of #Chandrayaan2, but no communication with it yet.
All possible efforts are being made to establish communication with lander.
#ISRO

Haridas · Post by **Haridas** » 10 Sep 2019 10:42

UlanBatori wrote:Paper says that CY-1 (not CY-2) was forced to raise orbit from 100km to 200km to reduce heating. Is this radiant heat from lunar day? Wow! I thought it only reached 200C which should cause radiant heating only like an average day in Sriharikota summer, hain? Doesn't the orbiter rotate to distribute heating from one side to the other?

Yes, reflected Flux from moon surface.

Vayutuvan · Post by **Vayutuvan** » 10 Sep 2019 10:48

Mort Walker wrote: antilog(-141/10) = 7.94E-15 mW or 7.94E-18 Watts.

antilog(x) is e^x. that said ~1.0e-18 is noise. double calculations in a 64 bit computer consider numerical zero to be 1e-16 or less unless the computations are done in double double, I.e. 128 but.

Haridas · Post by **Haridas** » 10 Sep 2019 11:00

UlanBatori wrote:UBCN diagnosis (posted b4) of "Too Much Thrust" (on one thruster was the quote) means combustion instability. Liquid fuel is metered through a choked nozzle with sufficient pressure ratio to prevent feedback from the combustor to the fuel source. So it is probably not because of too much fuel flow.
Combustion intability is amplitude of oscillation rising, sometimes without bound until the thing explodes. But it gives lots of thrust as the liners, housing all burn.
Average thrust may be high, but vibration level may be extreme. Enough to knock comm. electronics out, maybe? Though a single solder joint breaking should not bring down a mission of this sort!

Avionics are tested to much harder vibration specs, that are encountered during GSLV launch. The 800 N thruster vibration is a piece of cake, no matter what combustion instability.

Haridas · Post by **Haridas** » 10 Sep 2019 11:28

UlanBatori wrote:
I have been ignoring Ulan Batori's posts

Someone could perhaps show this postor how to use the "ignore" key and how to operate it so that she does not ever again sully a friendly discussion with her sneering. Some of us are humble yak-herders, we didn't memorize high-school textbooks.. but once in a while, as Haridasji kindly pointed out once.. (never mind).

You referring to receding altitude during cryo burn?

Madrasa math or no madrasa math.. when "sun comes over the horizon" (IOW morning/noon/evening etc) has NOTHING with when is the full moon!

Yes, elegent & simple.

I did not say it did (perhaps someone could help with reading). What I was trying to figure out is whether the landing site is already at peak sunlight, in which case there is no hope, or it is still early morning, in which case there IS hope. Requires a bit of imagination and optimism...

So, not having Sears-Roebuck memorized, I approximate.

Since the place can be seen from Earth, full Moon is a good indication of when the place is in the middle of the lunar day. Not to 16 decimals precision and relativistic string theory perhaps, but good enough for an Internet Forum discussion. Given the latitude and longitude, I hope the day is not off by more than 1, but it is too much trouble for me to go check with the correct Lat/Long, precession rate, 10-body dynamics etc. It is also unnecessary if one has a slight amount of imagination, not limited to mugging textbooks.

Brutal

Let's see....

The lander was supposed to land on .. Sep. 7. If one were designing a mission where the expected lifetime is the 2-week Lunar Day, when would one land? At the fag end of the Lunar Day? At the middle? Or at the start?

I would land at the start, if I were on the equator.
But at the poles the sun does not come up very high.
At this place, per someone's post, the sun only rises to 10 degrees above the horizon (think Antarctic summer day). So there is no light even by the equivalent of say, 8AM (earth-day equivalent, one might need a SLIGHT bit of imagination to imagine that..). So I would land at the equivalent of 9AM (equatorial mid-morning on Earth) so that my solar panels would get SOME power as I started to roll out the rover.

So it is reasonable to guess that ISRO, being at least as smart as I PLUS having read the kindergarten textbooks, did exactly that, to within plus or minus one day.

So I checked the lunar phase. Sure enough, ISRO is smart: it IS close to the equivalent of 9AM at the landing site. We expect to find out in the next week whether Vikram can be powered up.

Lander has batteries for initial lead up to morning sun.
BTW solar panel power generation has no bearing on lander being on lunar equator or near poles. As long as the side walls of truncated pyramid see the sun at near normal (perpendicular ) angle. That is benifit of no lunar atmosphere. Also ideal for lander spacecraft packaging.

Haridas · Post by **Haridas** » 10 Sep 2019 11:54

UlanBatori wrote:All these say that it is recoverable when the sun is sufficiently up (over the next week!)
But if the comms are destroyed, then it cannot be instructed to correct itself. So the original comm failure is still the big catch. There HAD to be one more error after the fully automatic system took over, as I explained a few pages ago. Maybe something else got shaken loose by the same thing that killed the comms.

Comm on dual gimbled high gain antenna. It's maximum slew rate is way slower than vehicles slewing to recover from transient engine thrust over performance OR excessive control loop gain.

So why anyone be surprised of comm failure while the lander flight control was dancing to avoid transient induced bullet, many many seconds before touching lunar surface?

After touchdown IMHO the dual gimbal of the Hi-gain antenna failed due to mechanism failure.

Haridas · Post by **Haridas** » 10 Sep 2019 12:01

ramana wrote:Need a NG&C expert here.

The AIAA paper says in 2nd para page 5, that
The INS after updating the state vector is used for the first burn. During the long coast phase also, the attitude and gyro drifts are updated using star tracker. The accelerometer bias is also updated during the long coast phase. The INS state vector is used during the second burn. During vertical descent...."

Basically the INS and the other two instruments have to be in sync before vertical descent...

What if the attitude and gyro drifts did not get updated during the long coast from the star tracker and errors accumulate leading to the extra thrust being commanded to make up for the shortfall?
This would show up as extra thrust being felt during the switch over to fine burn.

Maybe this is the start of the violent movement during the switch over?

Even if MING inertial platform is used (not ring laser based IMU) it would be useful for autopilot for upto an hour with less than 1 degree drift without star-fixing. Mind you I am talking about autopilot based vehicle control, not navigation accuracy.

Page 5 &6 most useful. LIRAP (Laser gyro based Inertial Reference Unit and
Accelerometer Package) seems brother of MINS.

Comprising of ISRO laser gyroscope (ILG), mini advanced inertial navigation system (AINS) and rate gyro electronic package device (RGPD).

Haridas · Post by **Haridas** » 10 Sep 2019 12:24

chetak wrote:would anyone know if the stabilization system is 3 axes or 6 axes

All such flight control systems have to be at least 6 axis stablized.

SSSalvi · Post by **SSSalvi** » 10 Sep 2019 12:31

What is 6 axis?

Haridas · Post by **Haridas** » 10 Sep 2019 12:36

Mort Walker wrote:
UlanBatori wrote:Pingreji pls: what is an up signal and what does -141 dB mean? Compared to 1 watt? And is that the source (because receiver power should depend on solid angle of capture), hain? Like if I got this much power with this much area, and I know it came from this source, then the omni-directional source should have a power of... ??
-141 dBm is power in decibels per miliwatt reference. It is the power at the receiver of the ground station after passing through filters, low-noise amplifiers and down converters.

antilog(-141/10) = 7.94E-15 mW or 7.94E-18 Watts.

No sir.
It corresponds to power inputted into the radio receiver input by the the receiver's high gain dish antenna.

Haridas · Post by **Haridas** » 10 Sep 2019 12:37

SSSalvi wrote:What is 6 axis?

abelity to measure 3 orthogonal linear axis and 3 orthogonal rotational axises.

Haridas · Post by **Haridas** » 10 Sep 2019 12:54

Mort Walker wrote:
suryag wrote:Mort sir they have to look at the received tone characteristics before they request data
Yes, I guess they're just looking for a carrier. No data is being sent up.

In extreamly poor Signal to Noise ratio (SNR) signal (often due to link budget exhaustion) one can reduce the receiver bandwidth to few Hz only, thereby maximizing the SNR to be just above noise floor. It just tells you there is definitely a transmitter operating at the said carrier. It's modulated signal however can't be recovered because SNR for the modulated signal bandwidth is below noise floor (assuming the signalling does not provide for processing gain (DSSS has processing gain).

Such narrow band detection of carrier has SNR of just 3 dB, while for data recovery the sideband power SNR at much greater bandwidth should be at least 6dB.

prasannasimha · Post by **prasannasimha** » 10 Sep 2019 13:32

Dasari · Post by **Dasari** » 10 Sep 2019 13:34

Since it is off by 500m, it certainly has horizontal velocity when it hit the moon surface. So is it possible that it landed on its 4 legs but the momentum pushed it towards its side?

chetak · Post by **chetak** » 10 Sep 2019 13:52

Dasari wrote:Since it is off by 500m, it certainly has horizontal velocity when it hit the moon surface. So is it possible that it landed on its 4 legs but the momentum pushed it towards its side?

either way, it would have raised a lot of dust.

Is it possible that some of this dust may have settled on the solar panels thus reducing its output resulting in much reduced power generation apart from all the other on going problems

Singha · Post by **Singha** » 10 Sep 2019 14:08

it seems ESA had a polar lander mission to moon for 2018 FCS that was scrapped for lack of funds.
in their background study they had identified a lot of hazards in the lunar pole as a hostile env

https://timesofindia.indiatimes.com/ind ... 058787.cms

SSSalvi · Post by **SSSalvi** » 10 Sep 2019 14:09

Haridas wrote:
Mort Walker wrote:
Yes, I guess they're just looking for a carrier. No data is being sent up.
In extreamly poor Signal to Noise ratio (SNR) signal (often due to link budget exhaustion) one can reduce the receiver bandwidth to few Hz only, thereby maximizing the SNR to be just above noise floor. It just tells you there is definitely a transmitter operating at the said carrier. It's modulated signal however can't be recovered because SNR for the modulated signal bandwidth is below noise floor (assuming the signalling does not provide for processing gain (DSSS has processing gain).

Such narrow band detection of carrier has SNR of just 3 dB, while for data recovery the sideband power SNR at much greater bandwidth should be at least 6dB.

1. There is no transponder onboard .. It is TC RX and TM TX .. two independent entities.
So the process is .. you send a command and await response on parameters from s/c.
You can't receive back your signal translated at a different frequency.

2. With a 30 footer , the monn's radiation ( Sun Noise reflected by whole Moon disc ) is in the range of -106 to -108 dbm on Full Moon day. So what was seen as -114dbm etc is the radiation noise ... not signal. It is about halfway between Full Moon and Amavasya.

3. Presently El for Goldstone is 17 deg ... so soon it will go mute and next station will be Canberra .. but presently it is working/assigned to other sats.. so as soon as those mission measurements are over .. if ISRO wants .. CH2 can be attempted.

prasannasimha · Post by **prasannasimha** » 10 Sep 2019 14:24

^ 6 degrees of freedom. Pitch yaw and roll are the other 3. 3 DOF is only for a point object.

ArjunPandit · Post by **ArjunPandit** » 10 Sep 2019 14:44

^^anything further to the faint signals from goldstone DSN. Was this false alarm?

arshyam · Post by **arshyam** » 10 Sep 2019 16:09

It's not visible to Goldstone anymore, or they are communicating with other payloads, so the orbiter and lander are not showing up on the website anymore. It had been quiet for a few hours before that, fwiw.

SSSalvi · Post by **SSSalvi** » 10 Sep 2019 16:31

Presently ( 1600 IST ) it is visible to Canberra @ 70deg El
Will be visible till India midnight.
For Madrid it will rise at around 11PM IST
Note : Sat may be visible but station may or may not track depending on operations schedule.
================

^
^

There was a reference that we require 6 degrees of movement .. that is not correct.

Pitch, Roll and Yaw are not degrees of freedom.

They are related LOCALLY to the reference body describing its state ( Orientation ) of movement .. not to global frame.
They can be resolved in xyz and xdot,ydot,zdot of local body and when you fix the local body in Universal XYZ reference frame then one can be expressed in terms of other.

Ashokk · Post by **Ashokk** » 10 Sep 2019 17:11

Chandrayaan-2: Isro has located lander Vikram and is trying to make contact. But how?
How is Isro trying to contact Vikram?
Isro, which knows the frequency in which Vikram was supposed to communicate, has been sending different commands every day, hoping to hear back from the lander. So far, there has been no response. It has been using the 32-metre antenna installed in Byalalu, its deep space network centre near Bengaluru. It also tried another route: Trying to get the orbiter to talk with Vikram, but to no avail.

How can Vikram respond?
Vikram is equipped with three transponders and a phased array antenna — the dome type structure on top of it. The lander will have to use these to receive signals, decipher it and talk back. But it has been unable to do so more than 72 hours after it lost contact with the ground stations here. So far Isro has not officially communicated if these systems are in good condition, or, if they have been damaged.
Also, these systems will need power to operate, which takes us to the next question.

Does Vikram have power/energy?
The solar panel on Vikram is on the outside of its body and needed no manoeuvre for deployment. Had it landed as planned it would have picked up the Sun's energy and generated power. Besides, Vikram also has a battery system. But it is unclear if the lander is generating power at this point. Isro has not confirmed this yet. A hard landing could have damaged some of these systems, but Isro is still analysing the data, its chairman says.

For how long can Isro try?
As per Isro's pre-launch estimates, the lander was to get clear sunlight only for one lunar day, which is 14 Earth days. So Isro can keep trying until then, or stop before if it has ascertained that the systems have been damaged. After 14 Earth days, there will be a long cold night, which the systems were unlikely to survive even if the landing module had achieved a controlled, soft landing.

Katare · Post by **Katare** » 10 Sep 2019 17:31

Why have they not released a picture? Did they located it by some other means?

Mort Walker · Post by **Mort Walker** » 10 Sep 2019 17:33

Haridas wrote:
Mort Walker wrote:
-141 dBm is power in decibels per miliwatt reference. It is the power at the receiver of the ground station after passing through filters, low-noise amplifiers and down converters.

antilog(-141/10) = 7.94E-15 mW or 7.94E-18 Watts.
No sir.
It corresponds to power inputted into the radio receiver input by the the receiver's high gain dish antenna.

It depends if you measure prior to down conversion, as you said, or the IF power. It's not clear from DSN.

UlanBatori · Post by **UlanBatori** » 10 Sep 2019 18:03

Thx very much. No, I was referring to Abdul Bin Kabul's course on air combat at Srinagar madarssa.

Refined model from yesterday: The best-chance time depends on which way the lander toppled (assuming it was a gentle topple pivoting about the first touchdown leg due to sideways momentum), with respect to the directions. The lander is a truncated pyramid. Imagine it lying at 90 degrees (axis is horizontal). "Nearly upside down" is much less hopeful, let's ignore that because I don't see how to rock it back right-side up.
If its head is pointed West and sun comes up in the East, then the solar panel is inclined 10 degrees the wrong way. So until mid-afternoon it may not see sunlight (remember there is no diffused light except for tiny amount from the dust haze). Maybe some star light, that's it. Maybe with extreme luck some radiation from a nearby hillock.

But by mid-afternoon, it IS seeing sunlight, and I think that is enough to power up the transmitter/receiver.

(BTW, I have nooooo idea whether the sun rises in the east or west on the moon and have no plans to do such hard thinking at this hour in Ulan Bator. I do think north and south point at distant galaxies, no sun there. Translate the same reasoning to whichever direction the sun rises).

But suppose it fell down with head pointed East. Now it can catch sunlight earlier than mid-morning (within the next Earth day or so!).

Suppose it came down with its head pointed mostly to north or south?

Now the problem/opportunity: Near the poles as I understand from Earth, the sun goes around the horizon through a Polar Day. So you should at some point get at least some light, unless you are very unlucky. I think same occurs near the lunar pole through a lunar day but again, too difficult to think through that while typing.

This is why it would help if they were to publish a photo of which way the lander is pointed in its present sleep. With north-south reference. One could refine the Best-Chance prediction.

I think they should blast the place with as much radio signal as they can muster (8.41 GHz is microwave, hain?) and then listen with the highest gain possible, all through that best-chance window and some time after (batteries might get some charge during the best-chance window).

No reason to be despondent yet. The Astrologer has confirmed UBCN prediction.

BTW, Experts: 8.41 GHz is pretty close to 10 GHz, the Big Bad Water Vibrational Energy Absorption frequency. Is it far enough away that atmospheric absorption of the signal is acceptably small? Mobile Phones stick to 5.4 to avoid coming closer to this, after all. Never heard of a frequency beyond the next multiple of 2.7 and below 10, until I saw this. I thought you had to jump to 33 or so to steer clear of the 10GHz. Is this because ISRO was too poor to be able to buy a clearer frequency band?

SSSalvi · Post by **SSSalvi** » 10 Sep 2019 18:35

8 GHz is standard allotted freq for satcom and desh had been using it for ages .. for low altitude satellites

Ashokk · Post by **Ashokk** » 10 Sep 2019 18:54

The Dual Frequency Radio Science Experiment (DFRS) which is planned from the Orbiter uses two coherent signals at X (8496 MHz), and S (2240 MHz) to be received using the DSN. It could be that they were testing the transmitters yesterday.

UlanBatori · Post by **UlanBatori** » 10 Sep 2019 19:34

There must be some very narrow sub-window to identify a specific source, because whatever screen posted by our intrepid postors was identifying the signal as coming from CY2 "LANDER". Also these big observatories are pretty high-gain antennae, so they must be focused to a narrow beam width. (I can't BELIEVE I am writing this, as I maintain that 99% of grand "discoveries" at astronomical observatories come from whisky spills on the electronics or bugs on the lens.) Anyway, small chance that they could confuse with anything else unless it was right in the same beam cone, maybe 10km diameter at the lunar surface? (Just wild guess since they can't resolve the lander optically, and they can resolve craters).

SSSalvi · Post by **SSSalvi** » 10 Sep 2019 20:16

Objects which are nearby operate on different freq.
Let's see when DSN starts next operation with orbiter

We have noted lander freq above

Mort Walker · Post by **Mort Walker** » 10 Sep 2019 20:21

The DSN is a digital receiver and works much like a real-time spectrum analyzer. You can look up white papers on operation and theory from Rohde-Schwarz, Keysight and Tektronix as they have lots on RTSAs. Vikram is transmitting at around 2.27 GHz using some sort of PSK modulation at a given 3 dB bandwidth (I'm not an expert on space probe comms). The DSN should be range gated in time to look at a window at a range of approximately 4.0E8 m. The frequency, modulation, and distance are known. If you set a resolution bandwidth that is narrow, say at 1 KHz, the noise floor is -144 dBm. If Vikram is actually transmitting, then there should be cumulative power in time and frequency, you should be able to resolve a signal well below the noise floor.

It may be that Vikram's transmitter and antenna are operational, but other systems are not which upconvert sensor data to the transmitter. It is also possible that other satellites at the same frequency have transmissions which are being range folded into the DSN receiver.