Indian Space Program: News & Discussion - Sept 2016

Singha · Post by **Singha** » 17 Jun 2017 21:51

Cheen has a aslv type asat missile for high value ducks up there. Iirc its cslled kt4

SriKumar · Post by **SriKumar** » 17 Jun 2017 22:20

Amber G. wrote:To be specific, suppose today at noon exactly they are in sync -- by my ground clock (A). IOW I get time signal from B and C and can figure out that they are all in sync. (Corrected for delay in time, as time signal which travels at the speed of light, will some time to arrive at earth but we can correct for that). Now one day later again at noon I want to see if the clocks are still in sync. If I do that will I find:
1) Clock B (or Clock C) ahead or behind of clock A (or no difference) ?
2) By How much?

Well....I am going to ignore the effect of gravity and focus purely on effect of velocity of clock. There's an eqn that shows the time speeding up due to gravity but now we have two mechanisms acting on time. I'm making it simple by starting with just the effect of speed. At 26,600 km (B) and 42,000 km (C), the orbital velocities are 3.8 km/sec and 3.0 km/sec. A clock A on surface of earth (at equator) will have a linear velocity of about 0.45 km/sec. Qualitatively speaking, Sat B clock will run more slowly than ground clock. Sat C clock will be faster than Sat B clock and closer to ground clock (but still slower than ground clock). I am basing it on the eqn: time dilation is given by 'T' = T0/sqrt(1- v^2/c^2) where T is the time in a clock moving at velocity 'v'. c=speed of light. T0 is the time in the local reference frame.
By how much: c=3e8 m/s, v= 3800 (or 3000) m/s. T0= 1s. Substituting and multiplying by 86400 seconds (in a day) will give the time lag.
For an observer on Sat B, ground clock A will appear to be the fastest, and Sat C clock will be faster (but slower than clock A).

vasu raya · Post by **vasu raya** » 17 Jun 2017 23:08

Ramana, regarding the recovery of lower rocket stages, with ISRO already having performed the HEX, adding wings is still an incremental step. Maybe they get to do more experiments and less simulations instead of just letting the stages free fall and lose them as they do today.

Gagan · Post by **Gagan** » 18 Jun 2017 18:00

They can possibly recover stages in the A&N islands

vasu raya · Post by **vasu raya** » 18 Jun 2017 19:52

that can work as the footprint diagram of the spent stages that disha posted, the higher stages were towards A&N islands while the lower stages are closer to the Indian coast. Maybe they can (ab)use the GSLV Mk2 for this purpose as PSLV is the current workhorse while GSLV Mk3 is sort of the future.

For PSLV to reach the launch frequency of today, it was a long journey, using Mk2 as the learning step, maybe they could increase the frequency of the Mk3 launches in a shorter timescale

Prithvi with its altitude reach can help with the re-entry and landing studies too

chilarai · Post by **chilarai** » 18 Jun 2017 23:33

Regarding the atomic clocks failing , if we have multiple clocks for redundancy shouldn't they be from different sources or companies ?

prasannasimha · Post by **prasannasimha** » 19 Jun 2017 12:47

Not many manufacturers for the same

jayasimha · Post by **jayasimha** » 19 Jun 2017 13:57

In the years ahead, ISRO scientists have a hectic schedule as a series of satellite launches are in the works. The next major project is India’s second exploration mission to the moon, Chandrayaan 2—an indigenous initiative comprising an orbiter, lander and rover, which are expected to perform mineralogical and elemental studies of the lunar surface. It is slated for lift-off in the first quarter of 2018, ten years after the success of Chandrayaan 1.

ISRO’s next grand project is the scientific mission to the Sun for observing the solar corona (with a Coronagraph--a telescope), photosphere, chromosphere (Sun’ three main outer layers) and solar wind. To be launched by 2020 by the Polar Satellite Launch Vehicle (PSLV-XL) from Sriharikota, the Aditya-L1 satellite will study the Sun from an orbit around the Sun-earth Lagrangian point (L-1), which is about 1.5 million km from earth.

The Aditya-L1 mission will probe why solar flares and solar winds disturb the communication network and electronics on earth. ISRO plans to use the data from the satellite to better protect its satellites from being damaged by hot winds and flares ejected out of the corona.

Very soon, India will gallantly call on Venus for the first time and return to the Red Planet with a second Mars Orbiter Mission (MOM), probably during 2021-2022. It is planned to put a robot on Mars’ surface. India's first interplanetary mission, Mangalyaan, in November 2013, has been orbiting Mars since its arrival at the Red Planet's orbit on September 24, 2014. It is a technology transfer project for designing, planning, management, and operations. It enhanced India’s reputation as a reliable low-cost option for space exploration.
http://pib.nic.in/newsite/printrelease. ... lid=165729

nirav · Post by **nirav** » 19 Jun 2017 16:29

An important milestone.

Mars orbiter mission completed a 1000 days in martian orbit today.

https://www.google.co.in/amp/wap.busine ... 587_1.html

Rishi Verma · Post by **Rishi Verma** » 19 Jun 2017 17:18

Gagan wrote:They can possibly recover stages in the A&N islands

How would recovered and refurbished pieces be cheaper than brand new rocket casings? Especially if there are limited number of launches. ..recovery with coast guard ships and crew isnt cheap.

Its only worth reusing if the rocket lands back like the spacex falcon..which isro may be thinking about, now that the khan's have proven it can be done

Varoon Shekhar · Post by **Varoon Shekhar** » 19 Jun 2017 17:46

http://www.isro.gov.in/launcher/pslv-c3 ... -satellite

PSLV C-38 to be launched this coming Friday, June 23rd. Cartosat 2E plus 29 small satellites to be launched. Some of the countries are 'firsts'. Original number of 37 sats on board reduced a bit. Maybe some simply didn't arrive, like the "3 diamonds" series from Australia. Anyway, 30 satellites is pretty good!

abhik · Post by **abhik** » 19 Jun 2017 18:37

SpaceX, I think spent a couple of years experimenting landing on sea before actual recovery attempts started. Not sure if ISRO can do the same with it's current 3/4 stage rockets with the lower stages mostly being solid and middle stage velocity being too high to simulate a TSTO lower stage.

Kakarat · Post by **Kakarat** » 19 Jun 2017 19:58

Varoon Shekhar wrote:http://www.isro.gov.in/launcher/pslv-c3 ... -satellite

India's Polar Satellite Launch Vehicle, in its 40th flight (PSLV-C38), will launch the 712 kg Cartosat-2 series satellite for earth observation and 30 co-passenger satellites together weighing about 243 kg at lift-off into a 505 km polar Sun Synchronous Orbit (SSO). PSLV-C38 will be launched from the First Launch Pad (FLP) of Satish Dhawan Space Centre (SDSC) SHAR, Sriharikota. This will be the seventeenth flight of PSLV in 'XL' configuration (with the use of solid strap-on motors).

The co-passenger satellites comprise 29 Nano satellites from 14 countries namely, Austria, Belgium, Chile, Czech Republic, Finland, France, Germany, Italy, Japan, Latvia, Lithuania, Slovakia, United Kingdom, and United States of America as well as one Nano satellite from India. The total weight of all these satellites carried on-board PSLV-C38 is about 955 kg.

Why are the using a PSLV-XL which can carry more than 1800kg while PSLV-CA itself can carry more than 1000kg?

vasu raya · Post by **vasu raya** » 19 Jun 2017 21:19

abhik wrote:SpaceX, I think spent a couple of years experimenting landing on sea before actual recovery attempts started. Not sure if ISRO can do the same with it's current 3/4 stage rockets with the lower stages mostly being solid and middle stage velocity being too high to simulate a TSTO lower stage.

They say they kept the weight of the propellant needed to land back at less than 1/6th of the overall weight of the stage or propellent needed and they may be using composite stages as well.

whether a wings based approach can maintain that weight ratio for housekeeping purposes is not known. Playing catch up is not always advised...NASA's budget doesn't go down due to SpaceX

prasannasimha · Post by **prasannasimha** » 19 Jun 2017 22:19

http://www.newsnation.in/science-news/i ... 74573.html

Mars Orbiter Mission (MOM), the Indian Space Research Organisation’s (ISRO) maiden interplanetary mission, successfully completed 1000 earth days in its orbit around the red planet on Monday. ISRO’s mission has completed the 1000 earth days well beyond its designed mission life of six months.

On November 5, 2013, India’s low-cost Mars mission spacecraft was launched on board the PSLV-C25, an expandable launch system. On September 24, 2014, the MOM touched the orbit of the Mars in the very first attempt and broke into an elite club.

“MOM completes 1,000 earth days in its orbit, today (June 19, 2017) well beyond its designed mission life of six months. 1,000 earth days corresponds to 973.24 Mars Sols (Martian Solar day) and MOM completed 388 orbits,” ISRO said.

ISRO further said that the satellite is doing fine and continuous to work as expected. It also said that the scientific analysis of the data sent by the Mars Orbiter spacecraft is in progress.

ISROCiting surplus fuel as the reason, ISRO in March, 2015 had first announced that the spacecraft’s life was extended for another six months. Later in June, 2015 its Chairman AS Kiran Kumar had said it has enough fuel for it to last “many years”.

About Mars Orbiter Mission:

The Rs 450-crore MOM mission is aimed at studying the surface of Mars and mineral composition and to scan its atmosphere for methane, an indicator of life on the red planet.

The MOM has five scientific instruments – Lyman Alpha Photometer (LAP), Methane Sensor for Mars (MSM), Mars Exospheric Neutral Composition Analyser (MENCA), Mars Colour Camera (MCC) and Thermal Infrared Imaging Spectrometer (TIS).

ISRO said that one of the scientific payloads onboard the MOM called the Mars Colour Camera has taken 715 pictures so far.From June 2, 2015 to July 2, 2015, MOM went through a communication ‘blackout’ during its journey as a result of solar conjunction.

During May 18 to May 30, 2016, it experienced the ‘whiteout’ geometry (when the Earth is between the Sun and Mars and too much solar radiation may make it impossible to communicate with Earth).

ALSO READ | ISRO now needs to focus on human space flight, reusable rocket, says Ex-ISRO chief

In order to avoid the impending long eclipse duration for the satellite, an orbital manoeuvre was also performed on MOM spacecraft. MOM Announcement of Opportunity (AO) programmes for researchers in the country to use MOM data for R&D was also launched by ISRO.

ISRO is seeking scientific proposals for Mars Orbiter Mission-2 to expand inter-planetary research, the government said in November last year.

Watch Video | ISRO's GSLV Mk-III rocket successfully places GSAT-19 satellite into orbit; proud moment for India

First Published: Monday, June 19, 2017 07:01 PM

Amber G. · Post by **Amber G.** » 20 Jun 2017 03:40

chilarai wrote:Regarding the atomic clocks failing , if we have multiple clocks for redundancy shouldn't they be from different sources or companies ?

Yes, at present do what makes sense, most reliable, and proven and multiple clocks (for redundancy).

But as Prasannasimha said we may not have much choice at present.

Most now a days uses Rubidium (stable, accurate enough, useful and relatively much much cheaper). NaviC has 3 Rb. clocks on each sat. All three failed on one of the sat

. Older used Cs (Old -- standard).. some uses more than one technology (eg Galileo) and uses H maser (more stable than Rb but much more expensive) in addition to Rb clocks.

May be in future optical clocks which are much more accurate (but are little delicate and expensive) becomes more practical..

I would like to see additional but different tech such as quasar based clocks (which reaches accuracy comparative to atomic clocks ) for more confidence and internal checkup.

Austin · Post by **Austin** » 20 Jun 2017 16:08

Indian Mars probe celebrates 1,000 days in orbit

Kailash · Post by **Kailash** » 20 Jun 2017 16:13

Shouldn't they lower the orbit and take a closer look? who will fault them if anything goes wrong after a 1000 days in orbit.

They should specify how much of the missions objectives were achieved so far, what are the findings from each of the payloads etc.

JTull · Post by **JTull** » 20 Jun 2017 21:24

Isro to launch earth observation satellite Cartosat-2E and 30 nano satellites on Friday

CHENNAI: After successfully launching the maiden developmental flight of its heavy rocket GSLV Mk-III earlier this month, Indian Space Research Organisation (Isro) will launch 31 satellites, including foreign nano satellites, onboard its workhorse PSLV on Friday.

The rocket, in its 40th flight, will lift off with Cartosat-2E, an earth observation satellite, and 30 other nano satellites at 9.29am from the first launch pad at Sriharikota spaceport. The satellites will be placed in a 505km polar sun synchronous orbit.

The nano satellites will include 29 from 14 different countries -- Austria, Belgium, Chile, the Czech Republic, Finland, France, Germany, Italy, Japan, Latvia, Lithuania, Slovakia, the UK and the US. There will be one nano satellite from India made by students of Noorul Islam University in Kanyakumari district of Tamil Nadu.

The 712kg weighing Cartosat-2E is the sixth satellite in the Cartosat-2 series. It will provide regular remote sensing services using its panchromatic and multispectral cameras.

The images sent by the satellite will be useful for cartographic applications, urban and rural applications, coastal land use and regulation, utility management like road network monitoring, water distribution, creation of land use maps, change detection to bring out geographical and man-made features and various other land information system (LIS) as well as geographical information system (GIS) applications.

Varoon Shekhar · Post by **Varoon Shekhar** » 20 Jun 2017 23:23

^
Nice that they are emphasising the civilian and scientific uses of the satellite, Cartosat 2E. Because that will be the major purpose of it. Other reports will hype up the surveillance capabilities of the satellite, and downplay or ignore the civilian applications.

"'Original number of 37 sats on board reduced a bit. Maybe some simply didn't arrive, like the "3 diamonds" series from Australia. Anyway, 31 satellites is pretty good!

"

Not earth shattering, but the 3 diamonds series of satellites will indeed be one of the 31 odd spacecraft being launched on the mission. Before, the satellites were described as Australian, but they are under British control, with an Australian connection.

Amber G. · Post by **Amber G.** » 21 Jun 2017 00:06

Kailash wrote:Shouldn't they lower the orbit and take a closer look? ...

Lowering (or any change) in the orbit uses fuel..

disha · Post by **disha** » 21 Jun 2017 00:35

Kailash wrote:Shouldn't they lower the orbit and take a closer look? who will fault them if anything goes wrong after a 1000 days in orbit.

They should specify how much of the missions objectives were achieved so far, what are the findings from each of the payloads etc.

And still somebody has to go up there to check it.

It is cheaper to send a new sat., with the same Rb clocks and again from the same company.

===

Making atomic clock should be taken up by the Universities within India. I am sure that 2-3 different approaches can be targeted for research by some 15 universities and they can collaborate on those approaches in groups of 3-5.

Sridhar · Post by **Sridhar** » 21 Jun 2017 03:54

disha wrote:
Kailash wrote:Shouldn't they lower the orbit and take a closer look? who will fault them if anything goes wrong after a 1000 days in orbit.

They should specify how much of the missions objectives were achieved so far, what are the findings from each of the payloads etc.
And still somebody has to go up there to check it.

It is cheaper to send a new sat., with the same Rb clocks and again from the same company.

===

Making atomic clock should be taken up by the Universities within India. I am sure that 2-3 different approaches can be targeted for research by some 15 universities and they can collaborate on those approaches in groups of 3-5.

I think Kailash is talking about the Mars Observer Mission, not the IRNSS satellite.

Amber G. · Post by **Amber G.** » 21 Jun 2017 04:58

SriKumar wrote:
Amber G. wrote:To be specific, suppose today at noon exactly they are in sync -- by my ground clock (A). IOW I get time signal from B and C and can figure out that they are all in sync. (Corrected for delay in time, as time signal which travels at the speed of light, will some time to arrive at earth but we can correct for that). Now one day later again at noon I want to see if the clocks are still in sync. If I do that will I find:
1) Clock B (or Clock C) ahead or behind of clock A (or no difference) ?
2) By How much?
Well....I am going to ignore the effect of gravity and focus purely on effect of velocity of clock. There's an eqn that shows the time speeding up due to gravity but now we have two mechanisms acting on time. I'm making it simple by starting with just the effect of speed. ... I am basing it on the eqn: time dilation is given by 'T' = T0/sqrt(1- v^2/c^2) where T is the time in a clock moving at velocity 'v'. c=speed of light. T0 is the time in the local reference frame. ....

The formula you used ('T' = T0/sqrt(1- v^2/c^2) ) for time dial is nice...(and will give correct results assuming the things you say you are assuming)..
(But) the effect of gravity, in this context, is not negligible.

Also one has to be very careful (and clear) to choose a suitable reference frame to calculate (so calculations are easier), and then convert these value (if necessary) to other frame.

There are some numerical values given in Jay'S messages (and also a link given by prasannasimha), and may be others, so one can check the values to check..

I will put my thoughts here in a few days..meanwhile when I tried to google and look for some popular sites and physics forms, I found that many places have quite a sloppy treatment . (they really don't understand the fundamentals) . some have correct results (but by wrong reasoning).. while others even have wrong values/conclusions...I did see one popular article in Physics Today but it was too technical for common people.

Amber G. · Post by **Amber G.** » 21 Jun 2017 05:35

Vinaji - With your background, I think, if you read up good references it will help to clear up many things -- which some times are difficult to debate here. I am going to make some comments. I will request that please read them carefully (and recalibrate with good references to fill in the detail if I am brief or not very clear).. I will comment on some critical points only and may not touch all the points..

vina wrote:

Glonass transmits cartesian coordinates (Earth Centered & Earth Focused) for the satellites orbital parameters (ephemeris) while the others (GPS & Galileo, I would guess Navic & Compass as well) transmit Kepler parameters . ..

I think you are missing a big point here with respect to Glonass. (Vs GPS and NaviC)..

(Disclaimer/Warning): The following is some what technical and may be beyond the scope/interest for common audience here. You do need understanding of general and special theory of relativity ( much more than a typical UG course in physics if you are doing calculations or programming the GPS systems).. I am not going to put all the rational - you have to take my word or check with a good text book or reputable source

First it is not a case of "orbital parameters" for Glonass in conventional sense. The big difference is Glonass orbit is much more elliptical than GPS/Navic etc. This causes speed variations and distance from earth variation (thus gravity potential well). IOW in an elliptical orbit this happens to a much greater extent compared to a circular orbit. . This in turn causes variation relativistic correction of the clocks. The clocks (on sats - to give consistent time among all sat clocks) speed-up/slow-down due to Gravity of earth as well as speed of sat. This is the data which Glonass computes (because it depends on its position in the orbit) locally and transmit to the receiver. This is helpful for the receiver to do the rest of the calculations.

For GPS, (I think JayS message does talk about it.. how the sats do it to get consistency ) the "correction" term is rather static - or predictable - (since the orbit is near circular).. delta from average correction, if needed, is transmitted , but additional calculation - since it is rather simple is done by the receiver.

For some background - for GPS in earlier days - since even a few Km change in altitude of sat will change the "relativistic correction" there was a lot of data gathering to fix these values and make models for calculations..(It was remarkable that the actual correction observed was very close to what was predicted by theory).

ArjunPandit · Post by **ArjunPandit** » 21 Jun 2017 06:30

^^informative post, never thought even the changes in microgravity in elliptical orbits lead to additional corrections

Mort Walker · Post by **Mort Walker** » 21 Jun 2017 07:36

Sometimes clock errors are deliberately induced. GPS and other satellites will use cesium or rubidium oscillator clocks for timing. Rubidium clocks are cheap, under $5K for terrestrial use. See Stanford Instruments.

What hasn't been discussed much here is that these navigation signals are very weak in power, some 4-12 weaker than your mobile phone signal, and is very susceptible to jamming.

Amber G. · Post by **Amber G.** » 21 Jun 2017 09:55

ArjunPandit wrote:^^informative post, never thought even the changes in microgravity in elliptical orbits lead to additional corrections

Yes, this may come as a surprise if one hasn't thought of it before. Now a days the optical clocks are so accurate that one has measured the slowing down of a clock when clock has moved of the order of 1 meter "higher" against gravity.
(Fortunately we do not require that much accuracy for GPS systems).

For ordinary GPS, relativistic corrections, as many have pointed out, are quite large. As Stephen Hawkins famously said that without understanding of relativity theory GPS systems would be useless.

ArjunPandit · Post by **ArjunPandit** » 21 Jun 2017 10:43

Let's take this further over here The New FeeGICS Thread on BRF

Rishi Verma · Post by **Rishi Verma** » 21 Jun 2017 11:27

Amber G. wrote: SNIP

I think you are missing a big point here with respect to Glonass. (Vs GPS and NaviC)..

(Disclaimer/Warning): The following is some what technical and may be beyond the scope/interest for common audience here. You do need understanding of general and special theory of relativity ( much more than a typical UG course in physics if you are doing calculations or programming the GPS systems)..

I am not going to put all the rational - you have to take my word or check with a good text book or reputable source

First it is not a case of "orbital parameters" for Glonass in conventional sense. The big difference is Glonass orbit is much more elliptical than GPS/Navic etc.

Factually Incorrect: First of ALL, Glonass is nearly as perfectly circular orbit at 19,100 km , so the rest of the GENERIC write up is incorrect, see below reference

Reference for Glonass Orbit

(The orbit has higher inclination of about 66 degrees as compared to 55 degrees for GPS)

SNIP... TOO Generic

For some background - for GPS in earlier days - since even a few Km change in altitude of sat will change the "relativistic correction" there was a lot of data gathering to fix these values and make models for calculations..(It was remarkable that the actual correction observed was very close to what was predicted by theory).

The post does not explain anything, except empty words, i challenge you to come up with data such as clock drift of Glonass satellte, clock arrival time corruption (actual numbers) due to gravity,

Here is someone who has done calculations Modelling of GPS Tracking Signals

The below post is how to actually explain a technical subject - such as GPS
http://www.math.tamu.edu/~dallen/physics/gps/gps.htm

disha · Post by **disha** » 21 Jun 2017 11:51

Sridhar wrote:I think Kailash is talking about the Mars Observer Mission, not the IRNSS satellite.

Ummm... so sorry Kailash'ji - since everybody are seized up by GPS.

MOM was more of a TD., the goal was to reach mars and prove technology going to Mars. The secondary mission is to actually take photos of Mars from a distance (TIS & MCC in conjunction with MSM). MOM is the only craft there which covers a significant section & entire Mars disc during its "apogee". This in itself gives significant information on the mars atmosphere (and further MSM/TIS are dependent on MCC).

Hence going down in orbit to get a "closer" look may actually be counterproductive.

Austin · Post by **Austin** » 21 Jun 2017 11:54

This might help as it has something to say on all GPS , GLONASS and BEIDOU

https://www.glonass-iac.ru/en/GLONASS/

Austin · Post by **Austin** » 21 Jun 2017 11:55

Russia may help India to train astronauts, deputy PM says

More:
http://tass.com/science/952522[quote]

NOVOSIBIRSK, June 21. /TASS/. Russia may help India to train its astronauts in the future, Russian Deputy Prime Minister Dmitry Rogozin said on Wednesday.

"In prospect, it is possible to train Indian astronauts on the basis of our center of Roscosmos," Rogozin told the meeting of a bilateral high-level commission for cooperation in advanced technologies for military and civilian purposes.

Rogozin also said he informed India’s Finance Minister, Minister of Defense and Minister of Corporate Affairs Arun Jaitley about the prospects of Russia’s manned spacecraft and the development of the International Space Station (ISS).

"But we should see the prospect after 2024," Rogozin said. "I believe we could discuss cooperation between Russia and India in this area as part of the commission." He explained that it is also possible to gain profit due to creating new generation spacecraft for the Earth’s remote sensing, navigation, communications and researches of the far space.

The deputy prime minister noted that India "has made serious headway in this area" and Russia is carefully watching the success of its Indian partners. "We believe it’s time for a more large-scale cooperation in this area," he stressed.[/quote]

JayS · Post by **JayS** » 21 Jun 2017 12:15

Amber G. wrote:
ArjunPandit wrote:^^informative post, never thought even the changes in microgravity in elliptical orbits lead to additional corrections
Yes, this may come as a surprise if one hasn't thought of it before. Now a days the optical clocks are so accurate that one has measured the slowing down of a clock when clock has moved of the order of 1 meter "higher" against gravity.
(Fortunately we do not require that much accuracy for GPS systems).

For ordinary GPS, relativistic corrections, as many have pointed out, are quite large. As Stephen Hawkins famously said that without understanding of relativity theory GPS systems would be useless.

OT here, but I read there is some experimental setup using optical clocks which is so precise it can measure relativistic difference between two consecutive floors of a building. Those clocks will be used to map Earth's gravitational field in future which will be very useful as we start space travel/exploration on larger scale.

AmberG, I am having little trouble understanding how the slowing down of atomic clock helps match it with atomic clock on Earth, as is done for the GPS Satellite clocks (I posted a reference regarding this earlier). Maybe you can help here. I understand that since overall the Earth atomic clock runs slower by 38microsec as compared to the clock on GPS satellite, the time between two consecutive "ticks" of the atomic clocks of same type would be slightly higher on the earth as compared to that on the Satellite, as observed by someone on the Earth. But why would one slow down the ticking on Satellite clock in order to match it with the Earth clock..? What is it that they are trying to match here..? Number of ticks per second so that earth clock and Satellite clock ticks the same number of times in 1 sec of Earth time as viewed by an observer on the Earth..?? How does that help the GPS system..?

One possible answer I am thinking is that after such synching of number of ticks per second, Earth Observer would receive a number of signals per second from the satellite which can be directly mapped with the earth atomic clock ticks with 1-to-1 relation (say if the signal is passed every 10 nanosec). If such sync is not done however, the Earth observer would receive more number of signals per earth sec than the number of signals per satellite seconds that satellite sent and there would be frequency shift (blue shift in this case). I hope I am clearly expressing my thoughts. Basically synching atomic clock tick rate/sec helps simplify the overall calculations, is what I am thinking. And this is where they would have taken care of General Relativistic effect. While the Special Relativity correction is left to the Receiver which is easier to deal with.

Varoon Shekhar · Post by **Varoon Shekhar** » 21 Jun 2017 18:20

http://www.frontline.in/science-and-tec ... epage=true

http://www.frontline.in/science-and-tec ... sliderNews

Two nice write ups on the GSLV Mark 3 by T.S.S in the latest "Frontline". Good detail, but one wishes for a little more on the core liquid stage and the cryogenic stage. What were the challenges and breakthroughs in building the L-110? Why no mention of regenerative cooling for the L-110, isn't that a pretty significant development? The mass of the cryogenic propellants is given as over 28 tonnes! Also would have been nice to learn of the effort/challenges in developing the turbo pumps and their bearings/seals for the C-25, without divulging any sensitive information, of course. Virtually nothing about the avionics of the Mark 3. And the larger propellant tanks for the liquid and cryogenic stages.

Indranil · Post by **Indranil** » 21 Jun 2017 21:42

Rishi Verma wrote:
Amber G. wrote: SNIP

I think you are missing a big point here with respect to Glonass. (Vs GPS and NaviC)..

(Disclaimer/Warning): The following is some what technical and may be beyond the scope/interest for common audience here. You do need understanding of general and special theory of relativity ( much more than a typical UG course in physics if you are doing calculations or programming the GPS systems)..

I am not going to put all the rational - you have to take my word or check with a good text book or reputable source

First it is not a case of "orbital parameters" for Glonass in conventional sense. The big difference is Glonass orbit is much more elliptical than GPS/Navic etc.

Factually Incorrect: First of ALL, Glonass is nearly as perfectly circular orbit at 19,100 km , so the rest of the GENERIC write up is incorrect, see below reference

Reference for Glonass Orbit

(The orbit has higher inclination of about 66 degrees as compared to 55 degrees for GPS)

SNIP... TOO Generic

For some background - for GPS in earlier days - since even a few Km change in altitude of sat will change the "relativistic correction" there was a lot of data gathering to fix these values and make models for calculations..(It was remarkable that the actual correction observed was very close to what was predicted by theory).
The post does not explain anything, except empty words, i challenge you to come up with data such as clock drift of Glonass satellte, clock arrival time corruption (actual numbers) due to gravity,

Here is someone who has done calculations Modelling of GPS Tracking Signals

The below post is how to actually explain a technical subject - such as GPS
http://www.math.tamu.edu/~dallen/physics/gps/gps.htm

Is it too much to expect civility from you? I am beginning to wonder that this is how you actually speak like this is real life. You are a megalomaniac who speaks condescendingly with an air of the all knowing. Here's a civil version of your post.

Amber G., the premise of your post is that GLONASS uses elliptical orbits, when it uses perfectly circular orbits of 19,100 km radius.

Here are some very good links about GPS and its working.
Modelling of GPS Tracking Signals

http://www.math.tamu.edu/~dallen/physics/gps/gps.htm

If being civil is too difficult for you, consider leaving before I ban you. I have reached the limit of my patience.

sudeepj · Post by **sudeepj** » 21 Jun 2017 21:54

Mort Walker wrote:Sometimes clock errors are deliberately induced. GPS and other satellites will use cesium or rubidium oscillator clocks for timing. Rubidium clocks are cheap, under $5K for terrestrial use. See Stanford Instruments.

What hasn't been discussed much here is that these navigation signals are very weak in power, some 4-12 weaker than your mobile phone signal, and is very susceptible to jamming.

Civil GPS signal is unencrypted, so you know the some of the data that the satellite is transmitting before hand and all of the data if you can decode the civil signal yourself. Its a kind of pseudo random noise sequence, called Gold codes + navigation data. This makes it very easy to jam. Basically, if you can predict the next change in the signal, its easy to jam.

Mil GPS signal is encrypted so you dont actually know what the satellite is transmitting. The mil counter part of the Gold code is also a very long sequence, and is transmitted at ten times the data rate of the civil signal. So it spreads the signal energy across a much wider spectrum. To brute force jam this mil signal, you will have to radiate a *lot* of power across a very wide spectrum. This make the jammer itself visible to Electronic Warfare equipment.

Finally, while the raw signal is really weak, below thermal noise! Once its gone through some signal processing, the SnR of the DATA (as opposed to the signal that its being transmitted on) that is being transmitted is REALLY high.

shiv · Post by **shiv** » 21 Jun 2017 21:56

Rishi Verma wrote:
The below post is how to actually explain a technical subject - such as GPS
http://www.math.tamu.edu/~dallen/physics/gps/gps.htm

Here is another similar "GPS for dummies" link just like the one above..

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!

disha · Post by **disha** » 22 Jun 2017 01:37

Varoon Shekhar wrote: http://www.frontline.in/science-and-tec ... epage=true
http://www.frontline.in/science-and-tec ... sliderNews

What were the challenges and breakthroughs in building the L-110? Why no mention of regenerative cooling for the L-110, isn't that a pretty significant development?

The mass of the cryogenic propellants is given as over 28 tonnes! Also would have been nice to learn of the effort/challenges in developing the turbo pumps and their bearings/seals for the C-25, without divulging any sensitive information, of course. Virtually nothing about the avionics of the Mark 3. And the larger propellant tanks for the liquid and cryogenic stages.

Many thanks for posting the article., just 3-4 days back I was checking the frontline to see if they have an update on GSLV-Mk III.

Varoon'ji., you are expecting the moon., when one of the #mediapimp covering the launch had this to say:

... the relaxed atmosphere at the MCC on June 5 was not surprising. The mission was so flawless that a journalist covering it could not help commenting, rather inappropriately, that “there was no thrill” in reporting it.

Here the cheapskate of a journo wants a "thrill" in reporting GSLV-MkIII in terms of failures., when several of us here would have given our time in heartbeat just to get the thrill of reporting live such an epochal event.

15 years of dedicated effort (yes., some of them would have seen their baby touch the sky from their post-retired ISRO employee section) and the sheer amount of scientific and engineering knowhow is derided! That happens when we think that the effort is trivial and hence worthy of derision.

Importantly:

Locational constraint

An area of major concern was the launch constraint imposed by the location of Sriharikota, India’s space port. The launch had to take place eastward from the island to put a communication satellite into the GTO. This did not offer “full freedom” because after the vehicle cleared the Bay of Bengal, the Indonesian land mass appeared on the scene. The launch vehicle debris—from the jettisoned stages—should not be allowed to fall over Indonesia.

Sivan said: “We had seen that when the vehicle reached a velocity of more than 5 km a second, the Indonesian land mass came in. So we had a requirement of designing a launch vehicle that will have a capacity of reaching [a velocity of] 5 km a second. But it is the lower stages that should produce that velocity of 5 km a second. We then needed one more stage which will produce another 5 km a second of velocity. There cannot, however, be an intermediary stage. [A total of 10.2 km a second velocity is required to put a four-tonne satellite into the GTO.] After the vehicle crosses the land mass, its stages should not come down. They should continuously burn and go into orbit. That means we should have a stage that should give another 5 km a second after the vehicle crosses the land mass. So we had to necessarily go in for a cryogenic stage that will give 5 km a second at a stretch and carry the four-tonne satellite into orbit. To put a 2.2-tonne satellite into orbit, we had a cryo stage with 12 tonnes of liquid oxygen and liquid hydrogen. But to put a four-tonne satellite into orbit, we needed a cryogenic stage which will use 25 tonnes of propellants. That is how the C-25 stage came into the picture.”

In a nutshell., the launch parameters drove the stage design. Further:

Since ISRO wanted to build a vehicle with a minimum number of stages and minimum complexity, “we prepared a configuration with two S-200 strap-on motors around the core liquid stage which uses 110 tonnes of liquid propellants and a third, cryogenic upper stage which uses 25 tonnes of propellants. This configuration can carry a four-tonne satellite into GTO,” Sivan said. In fact, the core liquid stage had two Vikas engines.

(In comparison, the GSLV-MkII had seven propulsion motors: four liquid strap-on motors around the core solid stage, then the liquid stage, followed by the cryogenic upper stage to put a 2.2-tonne satellite into orbit.)

Sivan added: “Thus, we configured a simple system with the general requirement of reducing cost, increasing reliability, using the systems already developed and taking less development time. All these combined together, we arrived at this configuration.”

Of course read both the article in full!

Amber G. · Post by **Amber G.** » 22 Jun 2017 02:37

Indranil wrote: . Here's a civil version of <some one's> post.

Amber G., the premise of your post is that GLONASS uses elliptical orbits, when it uses perfectly circular orbits of 19,100 km radius.

<snip>
.

If <some one> bothered to read his own refs -- The orbits are NOT "perfectly" circular. (No orbits in real life do - they can be "nearly" circular). Here even the eccentricity of 0.01 will give (rough calculations) periodic amplitude of about 20 ns time variation (in each revolution) due to relativistic correction.If you are technically interested - please see a little more context and details in my next post as a response to Vina.