Indian Space Program: News & Discussion - Sept 2016

[JTull]

PSLV to launch Cartosat by end of month June.

http://timesofindia.indiatimes.com/indi ... 044388.cms

The 550kg satellite, the fourth of the Cartosat-2 series, is to be launched by the PSLV-C38 rocket.

Surely they must have secondary payloads than just 550kg satellite.

[sudhan]

^^ Or it could be a PSLV-CA (Core alone config)

[kapio]

Looks ariane5ish now.

But more like Ariane 4 in terms of payload capacity (4 tons to GTO), yes ?

Ariane 5 pushes 7-11 tons into GTO.

I think PSLV is equivalent to Ariane 1, whereas GSLV Mk I and II are similar to Ariane 2/3.

We catchin' up!

[shaun]

Surely they must have secondary payloads than just 550kg satellite.

The Cartosat-2 Series Satellite along with co-passenger satellites from abroad is planned to be launched by PSLV-C38 into a nominal altitude of 500 km.

[SSridhar]

Cartosat is the primary payload. There are a dozen other satellites as co-passenger in C-38.

[Singha]

Heard from my bro, he trained with someone who reported to Nambi Sir, Nambi Sir's teams were the ParaSF equivalents of IA during the 90s and he used to be top notch tech manager(on the lines of korolev) himself and this during the mid 90s when 6th pay commission wasnt there and SC.Engr D used to make 7000Rs PM. No one believed when the spy ring news broke out and the team took about 2 years to recover and start picking up pace. There was a general feeling that if this could happen to Nambi Sir who is next

1991 to 1996 was on PVNR watch. the false case was rigged in 1994 , declared void by CBI in 1996 and nullified by SC in 1998.

1998 was either ABV or IK Gujral(upto march)...but by then it in hands of SC.

it was a failure on the part of PVNR PMO to crush the people involved.

I feel in the chaotic years of VP Singh (starting 1989) -> chandra sekhar -> weak center under PVNR -> deve gowda -> ik gujral (ending 1998) this decade a lot of foreign agencies got their claws deep into the woodwork and set themselves up to shape things. local proxy networks were setup, institutions subverted and investments made to aid the rise of friendly elements in all arms of the republic ... what was existing under the sheets in IG and RG era took off.

some of these roaches have been outed now, but not really baygon sprayed out of the woodwork.

[Mollick.R]

Several Key Informations in the Article, posting in full.....

Navigation satellite clocks ticking; system to be expanded: ISRO
BY IANS | UPDATED: JUN 10, 2017, 03.02 PM IST

CHENNAI: "The clocks are ticking." Every morning this announcement brings relief and
not tension to the Indian space agency team that is managing the navigation satellite
system NavIC with only one rubidium atomic clock switched on instead of two in the six
satellites.
The phrase signals that the atomic clocks that
provide locational data in the six navigation satellites are functioning normally.
Three atomic clocks in the first navigation satellite IRNSS1A
have already failed.
"The clocks are ticking well. It's not possible to share the technical details of mission
management for important reasons. ISRO is adopting various strategies so that best
results are obtained from its satellite systems," A.S. Kiran Kumar, Chairman, Indian Space
Research Organisation (ISRO) told IANS on Saturday over phone.

Sources close to ISRO, on the condition of anonymity, told IANS that two more atomic
clocks in the satellite system started showing abnormalities thereby taking the total number of failed clocks to five.
"Hence as a precaution and also to extend the operational life of satellites, the ISRO is running the NavIC system with one clock switched on instead of two. If the running clock fails then the standby clocks will be switched on," sources said.
The initial plan was to keep two clocks in the satellite on while keeping the third one on standby.

Simply put, the Indian Regional Navigation Satellite System (IRNSS) is similar to the GPS (Global Positioning System) of the US, Glonass of Russia, Galileo of Europe and China's Beidou.
Each satellite has three clocks and a total of 27 clocks for the navigation satellite system (including the standby satellites) were supplied by the same vendor.
The clocks are important to provide precise locational data.
"The clocks are working well. The signals are good. The replacement satellite for IRNSS1A
will be sent up this year. Already our system
is giving precise data even in areas populated with dense buildings and forest areas," said Tapan Misra, Director, Space Applications
Centre, ISRO.
According to him, space sector is an unforgiving business and hence mission management is important.
Misra said it is not only the atomic clocks in the Indian satellite navigation systems that have failed, the clocks in the European system Galileo too have failed as per reports.
The Indian space agency has signed up with several universities in the country to measure the performance of NavIC system, said Misra. The Rs 1,420 crore Indian satellite navigation system NavIC consists of nine satellites seven
in orbit and two as substitutes. "We are already using the NavIC system for several applications. The replacement satellite for IRNSS1A will be launched in July or August. There are also plans to expand the NavIC system by taking the number of satellites to 11 from seven," Kumar said.It is learnt that ISRO has got the atomic clocks replaced in the two standby NavIC satellites.

Starting July, 2013, the Indian space agency has launched seven navigation satellites. The last one was launched on April 28, 2016.
Each satellite has a life span of 10 years.
The NavIC system was performing well till the three clocks in IRNSS1A
the first satellite failed some months back.

http://economictimes.indiatimes.com/new ... 083063.cms

[Mollick.R]

Each satellite has three clocks and a total of 27 clocks for the navigation satellite system (including the standby satellites) were supplied by the same vendor.

If I'm not wrong ISRO bought those Atomic Clocks from some British company.

Provided the deep level of "khujli"of masa & brit$hits, is there any chance of sabotage by deep state ???

[vis]

Each satellite has three clocks and a total of 27 clocks for the navigation satellite system (including the standby satellites) were supplied by the same vendor.

If I'm not wrong ISRO bought those Atomic Clocks from some British company.

Provided the deep level of "khujli"of masa & brit$hits, is there any chance of sabotage by deep state ???

All the clocks were bought from SpectraTime, a Swiss company. Same clocks are also used in the Galileo and also the Chinese constellation.
The brits are not involved.

Edit: See http://www.spectratime.com/about/company/

[nirav]

If I'm not wrong ISRO bought those Atomic Clocks from some British company.

Provided the deep level of "khujli"of masa & brit$hits, is there any chance of sabotage by deep state ???

All the clocks were bought from SpectraTime, a Swiss company. Same clocks are also used in the Galileo and also the Chinese constellation.
The brits are not involved.

Edit: See http://www.spectratime.com/about/company/

Have the europeans and Chinese faced similar failure issues ?

[vis]

All the clocks were bought from SpectraTime, a Swiss company. Same clocks are also used in the Galileo and also the Chinese constellation.
The brits are not involved.

Edit: See http://www.spectratime.com/about/company/

Have the europeans and Chinese faced similar failure issues ?

Yes, Galileo is also facing the clock issues.

There are total of 72 galileo clocks in space. Of these 36 are Rubidium(RAFS) and 36 are Hydrogen(PHM-Passive Hydrogen Maser).

Of these 8 RAFS clocks have failed and 6 PHM clocks have failes. They were able to restart 1 PHM clock.

ESA has delayed most Galileo launches till the clocks in the satellites in the manufacture stage have been refurbished.

ESA and ISRO are cooperating the investigation.

Status of the chinese clocks is unknown.

Edit: See http://www.esa.int/Our_Activities/Navig ... estigation

[nirav]

Thanks for posting the link.

It does mention that inspite of the problems with a few clocks, operational service has not been hit due to quad redundancy. None of their sats have more than 2 clocks hit.

However considering that we have only triple redundancy and that one of our satellites already needs replacing cause of all three clocks having failed, maybe it might make sense to go in for quad redundancy and continue current measures of switching only one clock till the time a permanent fix for this issue is found.

[SaraLax]

Have the europeans and Chinese faced similar failure issues ?

Yes, Galileo is also facing the clock issues.

There are total of 72 galileo clocks in space. Of these 36 are Rubidium(RAFS) and 36 are Hydrogen(PHM-Passive Hydrogen Maser).

Of these 8 RAFS clocks have failed and 6 PHM clocks have failes. They were able to restart 1 PHM clock.

ESA has delayed most Galileo launches till the clocks in the satellites in the manufacture stage have been refurbished.

ESA and ISRO are cooperating the investigation.

Status of the chinese clocks is unknown.

Edit: See http://www.esa.int/Our_Activities/Navig ... estigation

Any chance that this Swiss company is compensating the affected customers via some financial manner (or being hauled to court for causing absolute failure of costly satellites spinning outside of earth) ... more so when this clock vendor from a 'master watch making country' can't go to outer space and rectify the problem on those GPS satellites ?.

[Rishi Verma]

Wonder why a precision vco on board the sat cant be frequency locked to a ground based atomic clock. A slow (ultra narrow bandwidth) control loop should give the needed accuracy and stability.

[shiv]

Wonder why a precision vco on board the sat cant be frequency locked to a ground based atomic clock. A slow (ultra narrow bandwidth) control loop should give the needed accuracy and stability.

Pardon the gap in my teeth but would a ground based clock really serve the purpose? I don't know. If you do please fill the gap

GPS and relativity

More sophisticated techniques, like Differential GPS (DGPS) and Real-Time Kinematic (RTK) methods, deliver centimeter-level positions with a few minutes of measurement. Such methods allow use of GPS and related satellite navigation system data to be used for high-precision surveying, autonomous driving, and other applications requiring greater real-time position accuracy than can be achieved with standard GPS receivers.

To achieve this level of precision, the clock ticks from the GPS satellites must be known to an accuracy of 20-30 nanoseconds. However, because the satellites are constantly moving relative to observers on the Earth, effects predicted by the Special and General theories of Relativity must be taken into account to achieve the desired 20-30 nanosecond accuracy.

Because an observer on the ground sees the satellites in motion relative to them, Special Relativity predicts that we should see their clocks ticking more slowly (see the Special Relativity lecture). Special Relativity predicts that the on-board atomic clocks on the satellites should fall behind clocks on the ground by about 7 microseconds per day because of the slower ticking rate due to the time dilation effect of their relative motion [2].

Further, the satellites are in orbits high above the Earth, where the curvature of spacetime due to the Earth's mass is less than it is at the Earth's surface. A prediction of General Relativity is that clocks closer to a massive object will seem to tick more slowly than those located further away (see the Black Holes lecture). As such, when viewed from the surface of the Earth, the clocks on the satellites appear to be ticking faster than identical clocks on the ground. A calculation using General Relativity predicts that the clocks in each GPS satellite should get ahead of ground-based clocks by 45 microseconds per day.

The combination of these two relativitic effects means that the clocks on-board each satellite should tick faster than identical clocks on the ground by about 38 microseconds per day (45-7=38)! This sounds small, but the high-precision required of the GPS system requires nanosecond accuracy, and 38 microseconds is 38,000 nanoseconds. If these effects were not properly taken into account, a navigational fix based on the GPS constellation would be false after only 2 minutes, and errors in global positions would continue to accumulate at a rate of about 10 kilometers each day! The whole system would be utterly worthless for navigation in a very short time.

The engineers who designed the GPS system included these relativistic effects when they designed and deployed the system. For example, to counteract the General Relativistic effect once on orbit, the onboard clocks were designed to "tick" at a slower frequency than ground reference clocks, so that once they were in their proper orbit stations their clocks would appear to tick at about the correct rate as compared to the reference atomic clocks at the GPS ground stations. Further, each GPS receiver has built into it a microcomputer that, in addition to performing the calculation of position using 3D trilateration, will also compute any additional special relativistic timing calculations required [3], using data provided by the satellites.

Relativity is not just some abstract mathematical theory: understanding it is absolutely essential for our global navigation system to work properly!

[kurup]

Can somebody explain this ?? Why is there such a diff b/w GTO and LEO payloads of HLV ??

Long March 5 : 25T to LEO , 14T to GTO
ISRO HLV : 41T to LEO , 16T to GTO

[SSridhar]

GSLV MkIII has given us a big push: ISRO Chairman - Madhumathi D.S., The Hindu

How important for you has been the maiden success of your new launcher, GSLV MkIII? Could you please explain it beyond its four-tonne-lifting power?

It is indeed a very significant development in the Indian space programme. It gives us a big push. We were short of this capacity for lifting our communication satellites. We had to go outside for our launches; and because of [the launch schedules of foreign space agencies] the pace at which we did our projects was getting affected. Cost was another deterrent for pushing things aggressively.

By achieving MkIII, we will be able to push confidently and launch many of our communication satellites faster and indigenously. We will do one more launch within a year and establish it systematically.

The cost benefit ?

I cannot give you the exact figures as they keep changing. But just to give you the example of last year’s INSAT-3DR, we were able to do both the satellite and launch it here for the cost of an earlier foreign launch.


What would be next in your launcher-related activities? What is their status, schedule and the funds they need?

First we need to consolidate the developments. Our immediate and main task remains how to streamline the realisation of our three launch vehicles PSLV, GSLV-MkII [two-tonne lifter] and the new GSLV-MkIII, sustain them and ensure the number of launchers we need to put the satellites in orbit. For MkII, our target is to do two launches a year. As it is, building its supply chain, managing it and ensuring the required supplies for it are all an effort. To that we will be adding the requirements for MkIII.

On the launch technology side, we will be looking at how to reduce the cost further — by adopting new mechanisms, materials and new capabilities. Including the reusable launch vehicle concept.


You mentioned reusable technology to save costs. Where are we in that?

Last year, we tried out the RLV-TD experiment [Reusable Launch Vehicle Technology Demonstrator]. We got a small, plane-like model to vertically land on water. Next we will look at landing it on the ground with a landing gear system. We are conceiving systems to work on the air breathing propulsion technology that will use atmospheric oxygen. For the present launch vehicles, we will look at recovering [and reusing] some parts.

What is essential today for ISRO?

To build capacities within the organisation and the country to meet the demands.

We have a significant shortage of satellites in space. If we have to roughly double the capacity of the spacecraft, we have to do as many launches and cost effectively. That is the prime driver.

Also, it is essential for a space agency to build new capabilities, constantly get new skills to do complex jobs and to do routine things better.

[Rishi Verma]

Wonder why a precision vco on board the sat cant be frequency locked to a ground based atomic clock. A slow (ultra narrow bandwidth) control loop should give the needed accuracy and stability.

Pardon the gap in my teeth but would a ground based clock really serve the purpose? I don't know. If you do please fill the gap

When the ego is inflated a person loses the ability to read and listen. But since you used the word "please", let me tell you that what i had written was whether ground based atomic clocks can aid the satellites achieve precision clocks. narrow band vco means a temp compensated voltage controlled crystal oscillator that is slaved to a ground based atomic clock.

And you went off on a trolling rant with the help of Wikipedia falsly assuming i was talking of ground based gps...

Engineering knowledge is not attained from Wikipedia articles but by building, testing, marketing, manufacturing actual products - no matter how simple the product may be.

I suggest you seriously try manufacturing and marketing a small batch of gunpowder (podi) then come back and tell us what you learned.

[shiv]

Pardon the gap in my teeth but would a ground based clock really serve the purpose? I don't know. If you do please fill the gap

When the ego is inflated a person loses the ability to read and listen. But since you used the word "please", let me tell you that what i had written was whether ground based atomic clocks can aid the satellites achieve precision clocks. narrow band vco means a temp compensated voltage controlled crystal oscillator that is slaved to a ground based atomic clock.

And you went off on a trolling rant with the help of Wikipedia falsly assuming i was talking of ground based gps...

Engineering knowledge is not attained from Wikipedia articles but by building, testing, marketing, manufacturing actual products - no matter how simple the product may be.

I suggest you seriously try manufacturing and marketing a small batch of gunpowder (podi) then come back and tell us what you learned.

This is an angry response and unsolicited advice. Not an answer. I presume you don't know or are unable to explain how your solution will solve the effects of relativity. I admit I don't know, but I suspect that you only pretend to know, using mumbo-jumbo acronyms, and have evaded the question. Unless of course you have built tested and marketed an atomic clock. I sure would like to hear more about your engineering knowledge in this area. If you please.

[Rishi Verma]

When the ego is inflated a person loses the ability to read and listen. But since you used the word "please", let me tell you that what i had written was whether ground based atomic clocks can aid the satellites achieve precision clocks. narrow band vco means a temp compensated voltage controlled crystal oscillator that is slaved to a ground based atomic clock.

And you went off on a trolling rant with the help of Wikipedia falsly assuming i was talking of ground based gps...

Engineering knowledge is not attained from Wikipedia articles but by building, testing, marketing, manufacturing actual products - no matter how simple the product may be.

I suggest you seriously try manufacturing and marketing a small batch of gunpowder (podi) then come back and tell us what you learned.

This is an angry response and unsolicited advice. Not an answer. I presume you don't know or are unable to explain how your solution will solve the effects of relativity. I admit I don't know, but I suspect that you only pretend to know, using mumbo-jumbo acronyms, and have evaded the question. Unless of course you have built tested and marketed an atomic clock. I sure would like to hear more about your engineering knowledge in this area. If you please.

again major inability to simply read before ranting using misplaced ego. I had given no solution, i was only asking the technical reason why a slave vcxo is not possible with ground-based master oscillator, there must be a good reason but i was asking what that reason was.

But seriously when you misdiagnose a deflating ego as anger, and being a hakim trying to argue with a hard core engineer -about engineering, is like me explaining you the dangers of FGM and why you should not be doing it.. So kindly shoo away, because in engineering you are a child talking to the big daddy.

PS: Bw of a vcxo is narrow.. Which is control system-101. And you think its "mumbo jumbo".. Too funny

[Gagan]

On the subject of alternate/additional launch sites, X-posting fromthe GSLV-3 dhaga

Maldives has an atoll just a few Kms north of the equator with a small airport at Kandudu.
There are small inhabited islands, Thinadhoo, Madaveli, Hoandedhdhoo, Gadhdhoo, close to the airport island, but the rest of the atoll is uninhabited.
There is an uninhabited island called Gan, which is 2.5 x 1.1 Km acrsoss.

This atoll lies 980 Kms from the upcoming Vizhinjam port

Gullalamoda ( 15°49'27.51"N, 80°58'29.35"E ) is the other proposed launch site in AP, which is 230 Kms north of Sriharikota.

India is doing some work in Sri Lanka. India can ask for a space port near the south-western tip of Sri Lanka (Which lies 830 Kms south of SriHarikota). The place is at 6°30' latitude roughly.

[shiv]

. I had given no solution, i was only asking the technical reason why a slave vcxo is not possible with ground-based master oscillator, there must be a good reason but i was asking what that reason was.

There! That is a perfectly good answer shorn of all the other bile, froth and other irrelevant medical jargon (mumbo-jumbo?) from a top class engineer such as you purport to be.

Clearly you are able to respond coherently when you are not angry with my ego. Let us leave the ego part out. My own inflated ego should not trouble you at all unless your own ego detests my irritating (to you) posts. But I don't post to make you or anyone get a warm fuzzy. No apologies. But thanks for a reply that I could understand as easily as I understand Wiki and how to poke around this ego business. It is a game that two can play. It is up to you if you want to keep talking about my inflated ego.

[Kakarat]

For long ISRO has been looking for alternative sites for launching rockets primarily for increasing capacity. The under construction second vehicle assembly building and planned third launchpad has given ISRO more time to select the location. Other than the site in AP another site is being considered is Kulasekarapattinam. The main issue with SHAR is PSLV has to perform 'Dog Tail' maneuver to bypass srilanka thus loosing some payload and for GTO SHAR is better than the proposed site in AP. Some argue that Kulasekarapattinam is good for both SSO and GTO launches. even some TN political Parties are batting for Kulasekarapattinam



2013
Second rocket port depends on satellite size, cost-benefit: ISRO

2016
TN ideal spot for 3rd rocket launch pad: Scientists

June 06, 2017
Study on for rocket launch pad at Kulasekarapattinam

[Gagan]

But the thing to bear in mind is that Arianspace, manufactures engines and most of its important components all over europe and then ships them all the way to French Guiana, 7000 Kms away for launch.
There are definite advantages to an equatorial launch, but in france's or Europe's case, there is no equatorial launch possible from Europe, so they had no choice.

They just pass on the costs of expensive manufacture and transportation in Europe, try to balance it out with an equatorial launch, and to the customer.

[Gagan]

Ariane space launch site is at 5 degree latitude, about 580 odd Km north of the Equator.
SriLanka's south coast is 6 degrees latitude, about 700 odd Km north of the Equator.

Just saying

[Gagan]

Kulasekarapattinam is very ideally located and in very close proximity to all of ISRO rocket facilities in TN and Kerela

[Indranil]

So kindly shoo away, because in engineering you are a child talking to the big daddy.

What nonsense is this? You asked a good question. Stick to it. If you think you are a big daddy of engineering, please keep that megalomania to yourself. I am giving you an informal warning.

Hakeem, let him be. No need to rush him. With time, may be...

Let's return to the question:

Wonder why a precision vco on board the sat cant be frequency locked to a ground based atomic clock. A slow (ultra narrow bandwidth) control loop should give the needed accuracy and stability.

This is an age old question almost as old as the internet and distributed systems. The more basic question is to make two or more distant clocks agree on the "current" time. I studied it in my third year of engineering, and this not being my field, I have forgotten much of it. But IIRC, the basic point is if the variance of the latency of the signal is more than a few multiples of the accuracy of the clock, then they cannot be synchronized. So designers came up with ways to work around this problem.

[LokeshC]

Ultra narrow band control loop. Sounds interesting. What would be the center frequency of such a band? If the center frequency is above high mhz range (microseconds to nanoseconds time period), you will end up having three problems
1) Doppler shift (shifts the center freq upward or downward depending on the direction of the satellite motion)
2) Special rel (shifts the center freq upward)
3) General rel. (shifts the center freq downward)

So I would be very interested in the design of an ultra narrow band to base-band converter (which is a vco kind of does). Since you would need to keep moving your center frequency around in an interesting way. You need to know the sats exact position wrt the earth and then adjust for the center freq shift in an exact manner or you will be missing the signal (since its bandwidth is ultra narrow by assumption). Is this problem solvable?

The other problem is phase noise. If we are talking about distant clocks synced to nanoseconds every single moment of their functioning, a single locking error can cause orders of magnitude error.

That said, IIRC the GPS is re-calibrated from NORAD periodically. It maybe using the same mechanism, but I am not sure how it would work.

Added later: Another problem
4) If China gets to know your center frequency, they can easily fool your sats as and when they want it, by either spoofing or jamming. If your VCO locks on to their control signal, they can slowly make it lose sync with the others, just keep shifting the phase slowly.

[Gagan]

So kindly shoo away, because in engineering you are a child talking to the big daddy.

What nonsense is this? You asked a good question. Stick to it. If you think you are a big daddy of engineering, please keep that megalomania to yourself. I am giving you an informal warning.

Very poor choice of words Rishi
One would think sarcasm and basic civility should have been licked in kindergarden.
The mods have to regulate this too ...
Oh! Well...

[jayasimha]

Space technology and agriculture. Presentation By
Shri. A.S. Kiran Kumar
Chairman ISRO

on Foundation Day and Award Ceremony of UAS Bengaluru on October 09 2015

http://www.isro.gov.in/sites/default/fi ... galuru.pdf

Page 10 talks about 4 web portals of ISRO including Bhuvan.


http://www.mosdac.gov.in
http://nnrms.gov.in
http://india-wris.nrsc.gov.in

[symontk]

PSLV has to perform 'Dog Tail' maneuver to bypass Srilanka

its Dog leg, just nitpicking

[Kakarat]

PSLV has to perform 'Dog Tail' maneuver to bypass Srilanka

its Dog leg, just nitpicking

Thank You for the correction

[arun]

ISRO Chairman A.S. Kiran Kumar to Times of India:

"Three atomic clocks of IRNSS-1A have stopped working. But the rest of satellite components are functioning perfectly. In fact, we are using this satellite for messaging activity. The stopping of these atomic clocks has not affected the overall performance of our navigation system. We are targeting to launch the replacement satellite in July."

Times of India:

Three atomic clocks of desi GPS satellites stop working

[prasannasimha]

viewtopic.php?f=3&t=7346&p=2169694#p2169694

[Amber G.]

Some discussion here reminds me of ..

Long time ago - there was a sundial constructed so that the local tehsil can synchronize its "official" clock. (There was no electricity in the village and there were virtually no radios or other good clocks then. Sundials were often used for that purpose. Actually when I visited that village recently, the sundial still was there and showing accurate time!). Sundial needs sun and works only in daytime. Some one suggested a method to use it at night -- by using a bright lamp which gives light as the sun.. ... but I digress..

There has been some discussion..

Wonder why a precision vco on board the sat cant be frequency locked to a ground based atomic clock. A slow (ultra narrow bandwidth) control loop should give the needed accuracy and stability.

Pardon the gap in my teeth but would a ground based clock really serve the purpose? I don't know. If you do please fill the gap

I don't think ground based clock really will server the purpose....here is why:

First some basic physics - Light travels fast but It takes about a nano-second for light to travel a foot (.3 m). In the preciseness of an atomic clock a distance of even a thousandth (or a millionth) of a millimeter makes a difference.

In other words to synchronize two clocks more than fancy electronics you need to know the distance between these two clocks to a VERY high precession! .. (you need to know the distance between ground clock and sat clock with preciseness of a millimeter (actually much more).

In practice it is the preciseness of the clocks which is the basis to measure the distance. You find the distances by measuring the time difference between two accurate clocks (not the other way around !!). So it is kind of silly to ignore this simple point (and get bamboozled by some fancy electronics engineering) ! Unless one can find a way to accurately find (and keep track) distance of sat's clock from ground clock with high accuracy other points are moot.

(And I am not even talking about refraction of EM waves and other factors which affects the time of travel other than distance)

****

Similarly frequency shifts (doppler shift as some people pointed out) etc depends on relative velocity -- again high accuracy is required and the accuracy in practice is measured by assuming the accuracy of the clocks..



****
Shivji - yes the clocks are precise enough that even the relativistic effects on slowing of clocks needs to be taken into account.

[disha]

My maternal uncle is a space buff. And I do hear tall stories from him. All he nowadays does is fix all his kids/grandkids, nieces and nephews and watch TV and send me running commentary (thanks to whatsapp) on any ISRO launch from his remote town. Of course his fav. time is to watch the INSAT's foto to predict weather (or is he watching the weather woman? never sure) and draw up crop patterns and plan accordingly. June-Sep is his busiest time., watching dookhdarshan weather reports.

In his last bombast about how great GSLV Mk III is (I agree it is great but it does not kick NASA rockets yet., but bringing that point up will result in his hurt ego and how useless his kids/nieces/nephews are ... )., I pointed out that the clocks in ISRO's GPS Sats are not ticking.

He ends the whatsapp message with grandiose and cryptic statement - "Samaay balwaan hain"!

This brings me to AmberG's very valid point -

You find the distances by measuring the time difference between two accurate clocks

., the accurate longitudinal lines on earth were drawn only after the marine chronometer or time piece was invented.

[Gagan]

This doesn't belong here, but just look at the Proton M launched from Baikanor Cosmodrome, launching a sat to Geostationary orbit. See the orbit and the changes they have to do from that latitude !

[vina]

I don't think ground based clock really will server the purpose....here is why:

First some basic physics - Light travels fast but It takes about a nano-second for light to travel a foot (.3 m). In the preciseness of an atomic clock a distance of even a thousandth (or a millionth) of a millimeter makes a difference.

In other words to synchronize two clocks more than fancy electronics you need to know the distance between these two clocks to a VERY high precession! .. (you need to know the distance between ground clock and sat clock with preciseness of a millimeter (actually much more).

Not sure what the accuracy needed for the satellite clocks are and indeed if it is atomic clocks they are carrying there, it probably needs a very high degree of accuracy. However, is that demand for such high degree of accuracy really an "architecture" and engineering problem ?

For instance, clocks sycnhronize over networks (including the clock on your computer and cell phones) to a great degree of accuracy.

To synchronise the clocks, can't a software /protocol/algo based stuff be used ? Something on the lines of the Network Time Protocol (NTP, a daemon will be running for that in your Unix based machines, including Mac) . Notice, in NTP , how even if one or many of the "stratum 0" clocks (the high accuracy clocks) are down, the "stratum 1" clocks , which are the primary time servers you query and synch the time against are fine and the entire network keeps working fine and getting the "correct" time.

So why is it that when 3 "stratum 0" (or atomic clocks) go down, we start losing functionality in the Galileo cluster and also IRNSS ?

The network time doesn't obviously fail if 3 "stratum 0" clocks go down NTP FAQ and How does it work ?

In practice it is the preciseness of the clocks which is the basis to measure the distance. You find the distances by measuring the time difference between two accurate clocks (not the other way around !!).

Yes. However, the problem is is to get accurate clocks here.

So it is kind of silly to ignore this simple point (and get bamboozled by some fancy electronics engineering) ! Unless one can find a way to accurately find (and keep track) distance of sat's clock from ground clock with high accuracy other points are moot.

Like I said , it is being done today over networks with huge latency delays . Wonder if such a synch scheme for all the clocks in the GPS satellites are good enough.

Note, the GPS problem is , as long as all the clocks there are synched and "internally consistent" , i.e.,all clocks in the GPS satellite cluster says time is x.xxxxxxxx to whatever accuracy, while the "actual time" on a ground based "master clock" is x.xxxxxxy (say) , your GPS devices (like say the Garmin handheld, or the GPS chip on the cell phone) that work on time delay of arrival should work fine .. they are after all calculating based on the time as on the satellite, whether that satellite time matches "true ground time" (if such a thing exists , can it , philosophically?) is immaterial and even if it is material, if latency /lag between the two is known, you can actually back calculate the "True" time.

(And I am not even talking about refraction of EM waves and other factors which affects the time of travel other than distance)

***
Similarly frequency shifts (doppler shift as some people pointed out) etc depends on relative velocity -- again high accuracy is required and the accuracy in practice is measured by assuming the accuracy of the clocks..

That is a problem, if you need to synch the clocks in the GPS satellite to a ground based "true time" . It will not matter if all the GPS satellite clocks are synched among themselves (internally consistent) and together have a known time delay /latency to the ground based "true time" (like what happens today in network time).

[Amber G.]

Let me make a few comments here --all basic physics -- yet quite important for fundamental understanding. Hope this helps.

Not sure what the accuracy needed for the satellite clocks ...

For instance, clocks sycnhronize over networks (including the clock on your computer and cell phones) to a great degree of accuracy. ...

The accuracy required (in both stable frequency and latency (wrt to "standard" TAI) is about MILLION times more than what is typical needed (or achieved) on a typical computer or a cell phone. So the NTP daemon running on server can get good enough time value from a reference server (which could be synchronized with an atomic clock). Your computer is okay as long as time drift is not more than (say) a few millisecond.. frequency stabilization of 1 part in 10^9 is much more than one needs). Clocks on sats are (and need to be) synchronized of the order of nano second or better .. with frequency stabilization of values -- typical 1 in 10^15.

(So typical Network time protocols - and time on your computer - does not produce that accuracy).
Typical value of Cs clock frequency is about 9 Ghz with stabilization up to 1 part in 10^14)

(*** side question *** then how does GPS on your cell phone work? if it does not have such an accurate clock ***? (I generally ask this question to test really top students to see if they can reason it out without looking up))

So why is it that when 3 "stratum 0" (or atomic clocks) go down, we start losing functionality in the Galileo cluster and also IRNSS ?...The network time doesn't obviously fail if 3 "stratum 0" clocks go down...

Again, if stratum 0 goes down the local clock on computer is good enough not to drift too much in a day or so .. (or till one makes contact with stratum 0 and adjusts the drift). in Galileo clusters.. you need accuracy *all* the time.. remember you have to broadcast those accurate time-signals all the time)

Wonder if such a synch scheme for all the clocks in the GPS satellites are good enough.

In practice there are about 400 **very** accurate atomic clocks distributed over the world with very sophisticated syntonization and *very* accurately known locations. These are taken as standard atomic time, and other clocks can be synchronized with this system.. ..(This is how atomic clocks on sats calibrate each other.. and this is how they come to know if some clocks are faulty).

In other words, if one GPS sat's clock becomes defective (of fails) one can do something but thats only because there are other sats clocks which are working and one is doing some checking back and forth.

Note, the GPS problem is , as long as all the clocks there are synched and "internally consistent" , i.e.,all clocks in the GPS satellite cluster says time is x.xxxxxxxx to whatever accuracy, while the "actual time" on a ground based "master clock" is x.xxxxxxy (say) , .

This is not exactly true, as not only time but also the position of sats calculated by ephemeris (calculating orbit with very high precision) comes into play. For that one has to know the 'true time' to calculate the orbit.

****

"true ground time" (if such a thing exists , can it , philosophically?) is immaterial and even if it is material, if latency /lag between the two is known, you can actually back calculate the "True" time

As said before , and as you agree, "true ground time" is what people agree as "standard".. in practice there are a few standards

TAI (Atomic time - as said before, this is based on about 400 atomic clocks world wide)
UTC (UT1) - This is what radio stations, your cell phone, (civil time keepers) use-. UT1 is based on earth' spin (and since earth's spin changes a little) it differ a little from TAI - and when difference accumulates to about 1 second a leap second is added etc.... Your cellphone gps etc knows how to convert UTC to TAI etc..
ET (ephemeris time) was standard before Atomic time -- based upon gravity/orbits of earth and planets... it is in sync with TAI (difference < 1 ns).

(And yes the detailed records of latency are kept so one can convert from UTC to TAI)

your GPS devices (like say the Garmin handheld, or the GPS chip on the cell phone) that work on time delay of arrival should work fine .. they are after all calculating based on the time as on the satellite, whether that satellite time matches

Actually your Garmin or GPS on cellphone's clock is not synched with satellite.. in fact there is not even a very precise clock there.. )

So let me post my question again - if you have not heard this question before --- How does GPS on a cell phone can get your position even though it does not have an atomic clock??

[Amber G.]

This doesn't belong here, but just look at the Proton M launched from Baikanor Cosmodrome, launching a sat to Geostationary orbit. See the orbit and the changes they have to do from that latitude !
http://russianspaceweb.com/images/space ... ario_1.jpg

Thanks for interesting post--Did not realize Baikanor is so far from equator. Geostationary orbit from that latitude looks interesting .. have to read up a little for the background why they chose a place so far in north.

As said before, for Geostationary orbit best is launching east as close to equator as possible,