Indian Space Program: News & Discussion - Sept 2016

[Singha]

russian launchpad uses a recessed design....very ordinary looking with unmowed grass etc.

but the clustered 4 engines must have plenty of meat on them. make going to space like boarding a rattling BMTC bus, but the math and procedures are sound in the usual low-key russian way.
https://twitter.com/RusConsulGen/status ... 9217990657

[SriKumar]

Thanks for some interesting posts. Glad that people found it useful. Some quick comments as I think they are important for basic understanding. Sorry for brevity as I may comment on a few critical parts only, sorry if it seems nit-picking. (I hope it brings some clarity and adds to discussion).

Excellent posts Amber G and SudeepJ . ...
...
So, let me summarise what I understand from AmberG and Sudeepj's posts and my own digging around and thinking to answer the questions I had in reverse order .
..
3. Why should the GPS time need to sync with "ground based true time" ?
It doesn't . As Sudeepj pointed out, what is needed is for the GPS cluster to have an internally consistent time. That means as long as the clocks in the GPS cluster are synched, you can forget about synching with earth (where time is adjusted every now and then ) , as long as you are interested only in location . However if you want GPS also to provide "time service" (i.e. you are in the poles in winter where it is dark 6 months in a yearand lost and you as wtf is the time, GPS time will be a great help). In that case, yeah, you will synch with earth time. Otherwise, you can forget about it and all the relativity and other effects on the satellite clocks.

This is not correct, that is you need GPS cluster to have to sync with TAI (earth's standard time). To locate your position, one needs exact location of the satellite at the time the signal (pulse) was sent. The data from GPS sat contains, among other things, ephemeris of the orbit so that one can calculate the position of sat (wrt to a fixed reference -- say center of the earth ) at the moment when the pulse was sent.

(I other words, the orbit elements are known, so one can calculate (X,Y,Z) coordinates at time T
(Another way to deduct that you need 4 sats)!
(** if your coordinates are (x,y,z ) and your (car's gps clock is offset by t, while the pulse originate at X,Y,Z at time T,
the time delay (T-t) = sqrt(X-x)^2+(Y-y)^2+(Z-z)^2)/c .. 4 unknowns, you need 4 equations! **)

Glad you clarified this. I was thinking that the absolute position of a satellite (however it is defined) is needed in order to compute distances. The absolute location of the focii of the hyperbola have to be known.

And your posts are definitely useful and generate useful discussions.

[shiv]

Had It been a two way communication this could be dealt with. But one way communication means R needs to assume when Sat A and Sat B both say 1:00:00 PM, they are both sending that signal exactly at the same instance. So you have to sync the satellite clocks as well.
Google search tells me they do it every 24hrs using the ground station network.

Ok thanks. So there is no "dialog" between each sat and receiver - it is just one shot one way each - receiver says "beep" and sat says "yeah beep +1" etc

[JayS]

Consider four satellites and a receiver. The local clock has an error term e. As the ranging measurements are made from the four satellites, they are time stamped with the local time at the receiver, with the error term e. All four measurements are made at different time instances, but they all have the common error term e. (We ignore the local oscillator drift at the moment).

This error term is common to all the ranging measurements made from the satellites. This common error term gets eliminated as we solve the non linear system of four equations.

Excellent so can you elaborate when someone gives gyan about military grade versus civilian grade GPS signal is it correct to assume that the only difference is between the 'e' of the local clock in question i.e. in case of civilian use case our mobile phone might be getting a feed via NTP which might be only accurate to milliseconds while the Military chaps have receivers that refer to a local time source which has a far greater resolution ? How the GPS satellite interacts or to be be more specific serves a mobile phone versus say a GPS guided munition is exactly the same , it is the low value of e in case of a military GPS receiver kit that allows it to get more accurate coordinates .

Mil grade signal is 10.x MHz while civilian signal is 1MHz. Thus the signal that Mil receiver gets is every 0.1microsecond while for civilian its 1microsecond. I think this is the biggest factor in enhanced accuracy for Mil use. I don't think the receiver clock accuracy itself really matters that much.

Also I read, since there is change in speed of Light through atmosphere, the GPS satellite will transmit some atmospheric data as well (they must be getting it from ground stations) periodically, which is used to correct speed of light through atmosphere to calculate more accurate distance between receiver and satellite. And since Mil and Civil signals are on different frequencies, this correction is also different for these two signals.

[JayS]

Had It been a two way communication this could be dealt with. But one way communication means R needs to assume when Sat A and Sat B both say 1:00:00 PM, they are both sending that signal exactly at the same instance. So you have to sync the satellite clocks as well.
Google search tells me they do it every 24hrs using the ground station network.

Ok thanks. So there is no "dialog" between each sat and receiver - it is just one shot one way each - receiver says "beep" and sat says "yeah beep +1" etc

Its like FM Radio transmission. Receiver can only listen. GPS sats keep sending a signal which has time data, sat id etc. You start receiver and listen which all satellite signals you are able to receive. And use 4 or more to fix location.

BTW, when I tried getting GPS fix in a flight I could not get a fix. As a matter of fact the wifi service that some airlines now-a-days provide also faced this signal catching issue IIRC due to speed of the jets.

[Zynda]

Totally OT, but one of my friend works in Qualcomm San Diego in field of wireless/mobile signal processing and he told me that QC doesn't guarantee fidelity of cell service beyond some 100mph or so. This was like 7 years ago and probably tech has moved forward enough to work most of the times at airline speeds, but as JayS mentions, the issues with signals may not be completely resolved yet.

[LokeshC]

AFAIK the tech has not moved much in that particular area. The comm. companies (from what I hear) are more into end user experience, bandwidth efficiency and increasing user density (for the operators) than trying to fix issues with Doppler shift and multipath effects at airplane speeds.

The real tech solution as usual will come from something the US (and other) militaries are doing right now. CDMA was more or less a military tech that got adopted to cellphones I think. Not sure though.

[ldev]

BTW, when I tried getting GPS fix in a flight I could not get a fix. As a matter of fact the wifi service that some airlines now-a-days provide also faced this signal catching issue IIRC due to speed of the jets.

In about the 2003-2004 time period, before car navigation devices by Garmin etc. became popular/cheap, I had jerry rigged a solution. I bought a Garmin GPS receiver from a specialty electronics shop and hooked it up to a laptop on which I had downloaded Microsoft's Mappoint (which subsequently became Streets & Trips) and is now discontinued. That GPS receiver did not have any map data on it and had only a monochrome display, plus it had a huge appetite for its portable battery and so had to be plugged in most of the time. Once when flying, my laptop was stowed away, but I started up the GPS receiver when the plane was at it's flight level. The GPS showed the speed of the plane ~530-540 mph and it showed the moving position as a Latitude/Longitude coordinate. It had a function for storing the highest speed recorded and I remember for a long time I did not clear the memory showing that device had reached a speed of ~540 mph.

So I don't know the difference between that GPS receiver which I have since thrown away and the regular GPS navigation devices which one gets nowadays which cannot get a fix at more than 100 mph as you say.

[LokeshC]

I can easily get a fix on an airplane on the latest devices. Usually only works when I am near windows seat (for obvious reasons)

[sudeepj]

Consider four satellites and a receiver. The local clock has an error term e. As the ranging measurements are made from the four satellites, they are time stamped with the local time at the receiver, with the error term e. All four measurements are made at different time instances, but they all have the common error term e. (We ignore the local oscillator drift at the moment).

This error term is common to all the ranging measurements made from the satellites. This common error term gets eliminated as we solve the non linear system of four equations.

Excellent so can you elaborate when someone gives gyan about military grade versus civilian grade GPS signal is it correct to assume that the only difference is between the 'e' of the local clock in question i.e. in case of civilian use case our mobile phone might be getting a feed via NTP which might be only accurate to milliseconds while the Military chaps have receivers that refer to a local time source which has a far greater resolution ? How the GPS satellite interacts or to be be more specific serves a mobile phone versus say a GPS guided munition is exactly the same , it is the low value of e in case of a military GPS receiver kit that allows it to get more accurate coordinates .

No. The Milgrade signal is encrypted, and is at a higher carrier frequency than the civil grade. 1 Mhz vs 10MHz. The 1MHz vs 10 MHz can be thought of as markers on a measuring tape.. the mil grade signal has a marking every mm compared to the civil grade signal which has a marking every cm. The 10MHz signal also has the result of spreading the energy of the signal over a much wider spectrum, it will be very hard to do a brute force jamming of the mil grade signal. The local clock can be any shitty clock, as long as it does not jump around too much, it should be OK.

So jam resistant, denied to hostile users, and finer grain measurements.

[Amber G.]

Few comments:

Also I read, since there is change in speed of Light through atmosphere, the GPS satellite will transmit some atmospheric data as well (they must be getting it from ground stations) periodically, which is used to correct speed of light through atmosphere to calculate more accurate distance between receiver and satellite. And since Mil and Civil signals are on different frequencies, this correction is also different for these two signals.

Among other things, one difference between Mil vs Civil is one can (and does) introduce some random uncertainty in transmitted signal and encrypt the signal such that unless you know the decryption keys you get less accuracy. This is one big reason that many countries want to use their own GPS cluster. Historically it was a very big and useful event when US stopped putting really big intentional inaccuracy in GPS signals (I think it was Bill Clinton who allowed that even US mil opposed it) and GPS in cars started having 10-20 m accuracy.

[Indranil]

Great posts, Amber G and SudeepJ. Joy to read and learn from. Goes to show that when you know the subject, the answers are short and to the point. No handwaiving, chest thumping or dissing if other posters required.

[SriKumar]

Thanks for some interesting posts. Glad that people found it useful. Some quick comments as I think they are important for basic understanding. Sorry for brevity as I may comment on a few critical parts only, sorry if it seems nit-picking. (I hope it brings some clarity and adds to discussion).

So to compute a position by GPS the absolute position of each and every (4) satellite needs to be known. How does the satellite compute its own position? (triangulation from ground stations?). And what is the reference point? (center of the earth?). And I suppose it must also compute its own velocity to update is position every nanosecond or less (from ground stations again?).

[SBajwa]

[disha]

On NaviC (IRNSS) (there was a separate thread on IRNSS started long back., but it went dormant).,

The following sats are in space (from wiki)

IRNSS-1A 1 July 2013 PSLV-C22 Geosynchronous / 55°E, 29° inclined orbit Redundant <-- Atomic clocks failed.[31][32]
IRNSS-1B 4 April 2014 PSLV-C24 Geosynchronous / 55°E, 29° inclined orbit Operational
IRNSS-1C 15 October 2014 PSLV-C26 Geostationary / 83°E, 5° inclined orbit Operational
IRNSS-1D 28 March 2015 PSLV-C27 Geosynchronous / 111.75°E, 31° inclined orbit Operational
IRNSS-1E 20 January 2016 PSLV-C31 Geosynchronous / 111.75°E, 29° inclined orbit Operational
IRNSS-1F 10 March 2016 PSLV-C32 Geostationary / 32.5°E, 5° inclined orbit Operational
IRNSS-1G 28 April 2016 PSLV-C33 Geostationary / 129.5°E, 5.1° inclined orbit Operational

Since only 4 sats are required (to determine just lat/longitude - only 3 are required)., there is no immediate problem for NaviC.

Issue is with clocks of IRNSS-1A only. And replacing IRNSS using PSLV is not a major issue.

Coming to "atomic" clocks., how are the sats sync'ed? Do they communicate with each other and agree to a time. Or are they sync'ed with IST on ground and then launched?

I think it is the later., that is 3 clocks on each sat is sync'ed and launched. Time corrections are then uploaded to the sats periodically.

[disha]

So to compute a position by GPS the absolute position of each and every (4) satellite needs to be known. How does the satellite compute its own position? (triangulation from ground stations?). And what is the reference point? (center of the earth?). And I suppose it must also compute its own velocity to update is position every nanosecond or less (from ground stations again?).

ILRS is the international body carrying out Laser Ranging for all sats with reflectors. This is NASA's ILRS pdf https://ilrs.cddis.eosdis.nasa.gov/docs/slrover.pdf

I think ILRS gives sats its positions w.r.t various ground stations. And that will allow the sat to calculate its own position. In this case everything is referential to each other.

Added later: Check out BLITS (and how it went defunct!) https://en.wikipedia.org/wiki/BLITS

[SBajwa]

[Amber G.]

Nice question, hope the answer brings out clarity.. (short answers are given inside the quote)

Thanks for some interesting posts. Glad that people found it useful. Some quick comments as I think they are important for basic understanding. Sorry for brevity as I may comment on a few critical parts only, sorry if it seems nit-picking. (I hope it brings some clarity and adds to discussion).

So to compute a position by GPS the absolute position of each and every (4) satellite needs to be known. { short answer YES}. How does the satellite compute its own position? (triangulation from ground stations?). {see answer below} And what is the reference point? (center of the earth?). And I suppose it must also compute its own velocity to update is position every nanosecond or less (from ground stations again?).

Satellite does not (so to speak) "calculate" it's position in real time.

We know the sat is in a standard orbit, and knowing its parameters, sat's position, (and velocity) can be calculated (calculations are done by GPS unit in the car) with high precision at any given time... so once we get the sat data pulse with time-stamp we know (or calculate) its position too.

IOW sat does not transmit it's position every second, it does transmit the orbital parameters, along with sat-id (set of 6 values which defines the orbit). Note that in ideal condition these parameters do not change. Over time due to fluctuations orbit may change a little so the accurate values - determined by ground stations - is uploaded from time to time. Point is these value remain practically constant over period.
. It is easy to calculate the position (and velocity) of sat knowing these values. .. (Older Brf posts even have formulas about how to do that )

One easy/convenient reference frame is fixed with center of the earth (and assume it is not spinning) and TAI is standard time... but with fast computers other ways are/may be used.

There are a few other things one has to take into account -- Since accuracy required is so high generally people may not realize that one needs to worry about these -- things like relativistic effects.

[disha]

Thanks AmberG, you have answered one of my questions - the sats are sync'ed from ground stations time to time.

I believe NaviC is within 2 nanoseconds error margin of the international standard time.

[SriKumar]

Nice question, hope the answer brings out clarity.. (short answers are given inside the quote)
IOW sat does not transmit it's position every second, it does transmit the orbital parameters, along with sat-id (set of 6 values which defines the orbit). Note that in ideal condition these parameters do not change. Over time due to fluctuations orbit may change a little so the accurate values - determined by ground stations - is uploaded from time to time. Point is these value remain practically constant over period.
. It is easy to calculate the position (and velocity) of sat knowing these values. .. (Older Brf posts even have formulas about how to do that )

There are a few other things one has to take into account -- Since accuracy required is so high generally people may not realize that one needs to worry about these -- things like relativistic effects.

Thanks for the explanations....and I had not realized until your earlier posts that relativistic effects played a role in satellite-based tracking. In fact, if the precision needed is so high, one wonders if satellite vibration after launch would tend to skew the accuracy.

[JayS]

Thanks for some interesting posts. Glad that people found it useful. Some quick comments as I think they are important for basic understanding. Sorry for brevity as I may comment on a few critical parts only, sorry if it seems nit-picking. (I hope it brings some clarity and adds to discussion).

So to compute a position by GPS the absolute position of each and every (4) satellite needs to be known. How does the satellite compute its own position? (triangulation from ground stations?). And what is the reference point? (center of the earth?). And I suppose it must also compute its own velocity to update is position every nanosecond or less (from ground stations again?).

To add to AmberG's post, the data related to orbital data of the GPS constellation is called almanac data. While the satellites transmit some almanac data in their transmission, the GPS devices also have almanac data stored from which they can calculate satellite positions quite accurately, since the orbits are maintained to accurately known and predictable parameter values through station keeping.

[Amber G.]

Thanks for the explanations....and I had not realized until your earlier posts that relativistic effects played a role in satellite-based tracking. In fact, if the precision needed is so high, one wonders if satellite vibration after launch would tend to skew the accuracy.

The relativistic effects are quite significant. For NaVic (or US) type sats it is about 40,000 nano seconds - approx - per day drift on the sat clocks... so if your GPS unit does not take that into account while doing the calculations it is a big deal. If the clock is synched now so that your position is accurate.. in about 2 minutes or so error will not be acceptable. (rough calculation - the error will be about 10 Km or so with elapse of a day if Einstein equations are not programmed in the unit!!).

[vina]

..
...
So, let me summarise what I understand from AmberG and Sudeepj's posts and my own digging around and thinking to answer the questions I had in reverse order .
..
3. Why should the GPS time need to sync with "ground based true time" ?
It doesn't . As Sudeepj pointed out, what is needed is for the GPS cluster to have an internally consistent time. That means as long as the clocks in the GPS cluster are synched, you can forget about synching with earth (where time is adjusted every now and then ) , as long as you are interested only in location . However if you want GPS also to provide "time service" (i.e. you are in the poles in winter where it is dark 6 months in a yearand lost and you as wtf is the time, GPS time will be a great help). In that case, yeah, you will synch with earth time. Otherwise, you can forget about it and all the relativity and other effects on the satellite clocks.

This is not correct, that is you need GPS cluster to have to sync with TAI (earth's standard time). To locate your position, one needs exact location of the satellite at the time the signal (pulse) was sent. The data from GPS sat contains, among other things, ephemeris of the orbit so that one can calculate the position of sat (wrt to a fixed reference -- say center of the earth ) at the moment when the pulse was sent.

(I other words, the orbit elements are known, so one can calculate (X,Y,Z) coordinates at time T
(Another way to deduct that you need 4 sats)!
(** if your coordinates are (x,y,z ) and your (car's gps clock is offset by t, while the pulse originate at X,Y,Z at time T,
the time delay (T-t) = sqrt(X-x)^2+(Y-y)^2+(Z-z)^2)/c .. 4 unknowns, you need 4 equations! **)

So let me see if I understood you correctly.

Yes. You need the time calculation for GPS, because of the way it is architected and designed. It is not "inherent" in it, and indeed, if architected differently, you wouldn't need the time at all for finding location. You could have used the MLAT kind of scheme which I stumbled up and measured just the phase lags of the signals and found the position.

You explained it yourself , in a later post, which I am quoting below

IOW sat does not transmit it's position every second, it does transmit the orbital parameters, along with sat-id (set of 6 values which defines the orbit). Note that in ideal condition these parameters do not change. Over time due to fluctuations orbit may change a little so the accurate values - determined by ground stations - is uploaded from time to time. Point is these value remain practically constant over period.
. It is easy to calculate the position (and velocity) of sat knowing these values. .. (Older Brf posts even have formulas about how to do that )

So the ONLY reason you need time to find location in a GPS is because , GPS doesn't transmit position , BUT prefers to have the receivers do the the math and calculate the satellite position from orbital parameters (some sort of almanac for want of a better term) that is downloaded by the receiver periodically. The GPS could have indeed been designed to send the position signal as well (it will be a few bytes extra at max) and if they had done that, you actually DONT need a clock time at ALL, in your receiver to find out the position .

Theoretically they could have done the orbit position calculations on the satellite itself and encoded that result in the signal and maybe your older Garmin set could have tracked the signal in a plane without much problems. Indeed, if I look at it, the receiver is actually doing some pretty complex calculations as a result of this, it is not just solving your position ie. x,y,z &Tt from the 4 readings from the satellites (the signal has T ,timestamped into it, otherwise, there is no way you can find the "Date"), but the handheld is simultaneously resolving X, Y & Z (the satellite positions) from the "almanac" downloaded which is based on "T", and it is no wonder that the CPU needs some pretty strong juice in the receiver.

(** if your coordinates are (x,y,z ) and your (car's gps clock is offset by t, while the pulse originate at X,Y,Z at time T,
the time delay (T-t) = sqrt(X-x)^2+(Y-y)^2+(Z-z)^2)/c .. 4 unknowns, you need 4 equations! **)

Now this is WHERE I am thoroughly confused. Maybe I am missing something very basic and you can explain. Okay , I solve these and get X,Y,Z &T x,y,z & t. But why should T correspond to Earth Atomic Time - (Corrected) (called UTC/TAI) and NOT the absolute atomic clock time that it was synced with on the ground at launch? There is no need for that! The "almanacs" that are in the handhelds can work perfectly with the time on the GPS cluster to calculate the position ! Indeed, if you keep syncing the time, you need to keep updating the almanacs as well for time adjustment in addition to tracking updates the ground station upload to the satellite as position error in orbit.

So as far as I can see, you really, you DON'T need to sync the GPS time with Earth time , for location finding. Yes, if you do sync it with earth time, you get a nice "time service" in addition. This was my basic point.

Also, I would be VERY INTERESTED to see how the later than GPS , i.e., the Galielo, the Russian, Chinese and Indian clusters are architected and implemented. For someone architecting a system like the GPS today (which ISRO and others have done), I wonder if the GPS design choices are the "best" ones. In this day and age, with computing so cheap and easy, it would be FAR easier to do the position computation of the satellite on the satellite itself and encode it in the signal. Back in GPS days, I guess, it would be clunky and power hungry to do it on the satellite on a continuous basis and you can kick it down to handhelds. This would also make it far less computationally on the receiver and you can get all the services you get on the GPS (location, speed and time) on that as well.


***

I know what you mean but I think I am correct...(speaking about ordinary networks)

Again for concurrency or time-stamp time problems you need that all clocks in the cluster are in phase with each other.. you do not need it synched with TAI .. (unless you are doing astronomical or GPS type work).

..
Of course, Rb clocks are not that expensive these days (~$1K) and it can attach to your server with truly great accuracy if you need one... And as you already mentioned in another post GPS unit is easily available which can give similar accuracy.

Yes. You are right. The local cluster distributed within a "small" area probably doesn't care too much at TAI and even if the synch is off , it couldn't care. And if it is far flung and if it really matters, they could all synch at the local node level with GPS/atomic clocks.

[SSSalvi]

What data does GPS give?
All the hardwork of using the satellite ephemeris to compute GPS ( or antenna to be precise ) position is done by the receiver and it just provides the user needed information.
GPS provides data in NMEA protocol.
It Is a simple text communication protocol ( used to be the good old RS232 in those days .. now USB port ) between GPS and User Computer ( or other device ).

Either user sends a querry : Similar to : Which satellites are visible to you? What is the current time? What is my position.. Lat/Long/Height etc.
And GPS responds with the appropriate data.

Alternatively user can just listen to the autonomous data being transmitted continuously by GPS.

[vina]

What data does GPS give?
All the hardwork of using the satellite ephemeris to compute GPS ( or antenna to be precise ) position and it just provides the user needed information.
GPS provides data in NMEA protocol.
It Is a simple text communication protocol between GPS and User Computer ( or other device ).

Either user sends a querry : Similar to : Which satellites are visible to you? What is the current time? What is my position.. Lat/Long/Height etc.
And GPS responds with the appropriate data.

Alternatively user can just listen to the autonomous data being transmitted continuously by GPS.

Okay. We are discussing the continuously transmitted listening thing. But if you are building a brand new GPS like system, why should you use the NMEA protocol as is ? You can modify it / create another one to reflect the technology and architecture choices you would make today. There is really no business case for making the same receiver talk to both GPS AND Galileo/IRNSS/Glonass/Baidu at the same time. You can have two different chipsets, in case you want to do that and do something like a differential GPS.

[SSSalvi]

Sorry to say this .. How long will ISRO provide IRNSS support?

Will the user shift to new system for a temporary phenomenon?

Will common Indian user switch to IRNSS ( And why? ). ( Strategic usage is understandable )

What harm in using NMEA protocol? It is not a slavery .. it is convenience that you can use all currently available GPS connected hardware directly.

[JayS]

So the ONLY reason you need time to find location in a GPS is because , GPS doesn't transmit position , BUT prefers to have the receivers do the the math and calculate the satellite position from orbital parameters (some sort of almanac for want of a better term) that is downloaded by the receiver periodically. The GPS could have indeed been designed to send the position signal as well (it will be a few bytes extra at max) and if they had done that, you actually DONT need a clock time at ALL, in your receiver to find out the position .

Theoretically they could have done the orbit position calculations on the satellite itself and encoded that result in the signal and maybe your older Garmin set could have tracked the signal in a plane without much problems. Indeed, if I look at it, the receiver is actually doing some pretty complex calculations as a result of this, it is not just solving your position ie. x,y,z &Tt from the 4 readings from the satellites (the signal has T ,timestamped into it, otherwise, there is no way you can find the "Date"), but the handheld is simultaneously resolving X, Y & Z (the satellite positions) from the "almanac" downloaded which is based on "T", and it is no wonder that the CPU needs some pretty strong juice in the receiver.

Even if GPS satellites transmit their accurate positions, how will the receiver calculate distance that it is at current moment from each of the visible satellites..? For trilateration to work, you need distance of Receiver from minimum 3 satellites and deltaT is needed to do this ranging. Satellites send fixed time stamps, and Receiver measures deltaT that took for the signal to reach from particular satellite to it, multiplies to it by speed of light, make some corrections and then get distance to that particular satellite. Do it for 3 sats and it can get approximate position on Earth's surface. Use 4th Sat to sync time with GPS constellation to improve accuracy. Once this is done there is only one point in Time-Space continuum where the Receive could exist when its is at given L1, L2, L3 distances from those particular satellites (in fact the receiver and those 4 Sats will be co-existing at a single point in time-space continuum). (Of coarse due to real life effects the position still will be within some error, which can be further improved by using more than 4 satellites and solve for over constrained system of equation.)

(** if your coordinates are (x,y,z ) and your (car's gps clock is offset by t, while the pulse originate at X,Y,Z at time T,
the time delay (T-t) = sqrt(X-x)^2+(Y-y)^2+(Z-z)^2)/c .. 4 unknowns, you need 4 equations! **)

Now this is WHERE I am thoroughly confused. Maybe I am missing something very basic and you can explain. Okay , I solve these and get X,Y,Z &T x,y,z & t. But why should T correspond to Earth Atomic Time - (Corrected) (called UTC/TAI) and NOT the absolute atomic clock time that it was synced with on the ground at launch? There is no need for that! The "almanacs" that are in the handhelds can work perfectly with the time on the GPS cluster to calculate the position ! Indeed, if you keep syncing the time, you need to keep updating the almanacs as well for time adjustment in addition to tracking updates the ground station upload to the satellite as position error in orbit.

So as far as I can see, you really, you DON'T need to sync the GPS time with Earth time , for location finding. Yes, if you do sync it with earth time, you get a nice "time service" in addition. This was my basic point.

I think you are correct when you say its not absolutely needed that the GPS constellation be synched with Earth time always. But if it is not, then to calculate deltaT more complicated equations will have to be solved which take care of relativistic effects and the time differences in GPS constellation and Earth time directly, including time drift that happens over the time. You cannot anymore do deltaT = (Sat time stamp) - (time at the time of receiving at the receiver). Because you do not know anymore when GPS time stamp says this message was transmitted at 1:00:00:00 PM, then what exact instance was that with respect to the Earth's time. A separate mechanism would be needed to keep tract of drift in Earth time and GPS time will be needed. Also additional efforts to keep entire GPS constellation to be in time sync by making satellites to talk to each other. IMO, Its much simpler to simply keep each GPS Sat synched with Earth time always. Had it been the case that there would be absolutely no drift in atomic clocks once the Sats are launched then a lot of trouble would be saved. But I am not too sure that's the case in real life.


From this link here:
http://www.astronomy.ohio-state.edu/~po ... tml#note03
This below is already discussed here

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.

But also note this -

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.
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[3] - While the primary general relativistic correction is taken care of on-board by the design clock frequency before launch and does not need to be computed by an individual receiver, the special relativistic corrections that require knowledge of the orbital parameters of the specific GPS satellites whose signals are being measured are not. As described in the GPS Interface Control Document ICD-GPS-200C (10 Oct 1993), applying these corrections is the responsibility of the user's equipment (Section 20.3.3.3.3.1, "User Algorithm for SV Clock Correction"). The calculations are relatively straightforward and require very little beyond basic arithmetic, and make use of information transmitted in the data packets that come down from each spacecraft.

PS: I collect some data from google search - GPS sat atomic clocks could drift 10ns per day if left to themselves. The almanacs for GPS have accuracy of the order of 200-300mtr for satellite positions. Given such real life errors, without actively keeping track of satellite clocks, the system might become useless sooner or later, since even few meters of inaccuracy could be disastrous in applications like autonomous cars or make a PGM hit wrong building and so on.

[vina]

Sorry to say this .. How long will ISRO provide IRNSS support?

That is the crucial problem , I commented in the economy thread with all Indian Govt run/pushed programs. They are not ecosystem enablers, don't open things out fully to everyone and do their own "paanch pyaares" (usually bumbling govt baboons driven and incompetent psu orgs) and that eventually crashes and burns. Even the UPI /Payment Gatways suffer the same flaws. Despite recommendations from the creators and those who built it, the UPI gateway and RBI payment system is not thrown open to all and sundry including tech companies ,but is controlled by a close group of few large banks (including HDFC,ICICI, SBI and others) who would be immediately threatened if it actually takes off. It is open only to "banks" and the RBI comes up with some ridiculous "payment banks", the like of which don't exist anywhere else and with a whole list of restrictions and nonsense which only a Baboon who has never done anything in real life other than being a bureaucrat could have come up with.. net result.. basically unviable, limits competition, and probably never will take off.

It is like asking the fox to guard the chicken coop. It's idiocy, but it simply wont change. This is true in industry after industry and a fundamentally fatal mistake carried over from the "command and control" and "commanding heights days" . But I digress.

Will the user shift to new system for a temporary phenomenon?

Will common Indian user switch to IRNSS ( And why? ). ( Strategic usage is understandable )

I think a govt mandate is in the work to enable NavIC chipsets to be made mandatory in consumer devices. It is the only way to do it. China would have ruthlessly done it (once Baidu is fully operational) and banned any further selling of GPS or any other GNSS enabled devices.

What harm in using NMEA protocol? It is not a slavery .. it is convenience that you can use all currently available GPS connected hardware directly.

1. It is dated.
2. It is not an "convenience" . The GPS and NAVIC and others today operate in different frequencies. You can't use GPS hardware directly. The chipsets are also different. Per this Quora Link where an ISRO engineer has also commented, the future "modern" gps clusters will have same band of NAVIC, so yes, the antennas and some stuff can be common. But you will need a different chipsets anyways for NavIC.

If you are taking the trouble of developing all this, why not a different "modernised" protocol as well ?
PS: I looked up this NMEA protocol. It is NOT a "GPS" protocol (i.e. between sat and receiver or anything), but a protocol like or TCP/IP. So you can put any GNSS chipset in front of it and have the output in NMEA and all the existing devices will be fine (I think).

[sanjaykumar]

In vina veritas.

[Amber G.]

Vina - Few BASIC points.
1 - It is NOT necessary to have clocks or even satellites..one can have known fixed points on earth and a tape. This is essentially how surveying was done for ages and people found their position (lang, lat etc).

2 - It so happens that, relatively speaking, clocks now are very accurate, convenient and inexpensive. This is why even when a surveyor wants to measure a simple distance, instead of using old fashioned standard rod uses a laser range-finder.

3. - Even the definition of meter, now depends on "standard second" (defined by oscillations of Cs atom etc).

One can have alternate system (eg sats sending their X,Y,Z position every nano-second and what not) you have to look at the WHOLE picture. As it is for GPS cluster what drove the idea is:
A. Gravity is well studied and motions of sats are precise, stable, and easy to predict...
B. Atomic oscillations are well studied and one can make very precise atomic clocks ...

So for a GPS cluster of navigational system, one needs a few sats in stable orbit, with little more than a good atomic clock and necessary electronics to send signals.

All one needs for a receiver is just able to receive data (not even a two way communication - or complicated protocol) with a half decent clock (quartz clock will do) and computing power. Rest is just understanding Einstein (to understand gravity) and Max Plank (for quantum/atomic physics).

Think about it - On GPS sat, all you need is an accurate atomic clock - No computing power - No fancy instruments to locate it's position, thermometers to measure temp (like in old days, one has to worry about if your measuring rod expanded due to temp).. ityadi ityadi ...

So if you are trying to suggest, an alternative design like "send 'position data' and then there will be no need to sysnc" ..you need to think about how will you get that 'position data' ??? (Just sayin that it is only x bytes long or can be sent by y protocol is not that important)..

Hope this helps.
(I may make some comments later on a few specific points if that helps to clarify some points)

[Rishi Verma]

Totally OT, but one of my friend works in Qualcomm San Diego in field of wireless/mobile signal processing and he told me that QC doesn't guarantee fidelity of cell service beyond some 100mph or so. This was like 7 years ago and probably tech has moved forward enough to work most of the times at airline speeds, but as JayS mentions, the issues with signals may not be completely resolved yet.

When the receiver is traveling at speed, there are couple things (interference and sync lock) that can go wrong. If snr is decent then tv reception can work but cell reception may get interrupted if it has to switch carriers.

GPS does not require sync lock, it just receives beeps, and there is no speed or altitude limitation. Why else can Brahmos be GPS guided.. Its not going to stop to receive GPS before entering the rat hole in Murdike.

[disha]

Some are precocious kids who want to learn things difficult way. Since what is the point of having intelligence when it is not put to hard exercise - particularly when it is unnecessary?
---

Here is an excellent article on PSLV https://swarajyamag.com/technology/indi ... r-lift-off., of course it uses halo of GSLV-Mk III as a backdrop., but article is all about PSLV.

Quoting from the above article:

There is still a major barrier before Antrix can properly exploit ISRO’s launch capabilities: as a national space agency, the priority for ISRO is not a business, but national missions and commercial launches are accommodated only when some spare capacity opens up.

“There has always been a huge gap in national needs for strategic or civilian use,” says Kiran Kumar, “and we have worked to bridge that gap. But the gap is still there; we need double the number of satellites that we already have, so commercial activity cannot be a priority.”

This year, for example, Antrix has not had much to do after the February launch of PSLV because of the lack of spare capacity.

Basically., PSLV/GSLV-MkII & GSLV-MkIII will be utilized all the way to 2030. Maybe couple of polar tests on GSLV-MkII with multi-sat launch and PSLV will be hived off to private industry.

The Vikas engine built by Godrej Aerospace for PSLV has been a work in progress for years, and it is only in the last ten flights, says Vaidya, that its accuracy has reached 99 per cent on all parameters.

Yes., it takes years of testing and continuous improvement to even build a "simple" engine like Vikas. And here., it is the private industry which is already providing the engine!

And here is another quote:

“There is growing demand for satellites for weather forecasting, earth observation, remote sensing, broadband, and everyone from Google to Facebook want to launch more satellites,” says Babu. “In another seven years, you will have a lot of space entrepreneurs like me.”

All the it-vity munnas can now move into sat making business.

[disha]

It seems that it is the Godrej/ISRO partnership to build out the Semi-Cryo stage. A good thing.

For ISRO., the next goal should be to launch a mission to a near earth asteroid and mine some material and bring it back to earth. Further., to launch such a spacecraft., its booster should target 2 mega Newton (or 2000 kN) liquid methane/oxygen. Having a powerful liquid methane/oxygen engine is a must for ISRO to get into inter-planetary travel for resources.

[sudeepj]

1. Pretty much every country has replicated the GPS architecture with some minor changes on the margins. This is a really good cost effective system, with some limitations. There really isnt much to be optimized here.
2. These systems are implemented for military purposes. Civilian use is a byproduct, though it can classify the system as dual Civil/Mil use, therefore making its targetting by a hostile power more difficult. Military origin is reflected in system design.. For instance, GPS performance over the poles deteriorates because of satellite visibility. But Glonass provides equally good position on the poles as it does on the equator because Russian SSBNs prowl those areas.
3. Its interesting how Chinese have placed their Satellites in highly elliptical orbits, and these satellites are hardly visible over American landmass. They spend most of their time over China and their areas of interest. Perhaps having precise positioning capability over a specific landmass is considered militarily provocative?
4. In war time, its fully expected that at least some satellites will be 'taken down'. The US has a system of 'pseudo-lites', 747s or such transmitting the same signal. One can think of other such implementations integrated into AWACs/AEW planes, drones or even ground base beacons, or systems that rely on satellites of third countries.
5. South Korea and Japan also have satellite positioning systems or plans to implement them. India should proactively offer to help build their systems in an integrated manner that can provide 100%, redundant coverage over 'areas of interest'. In particular, the South Korean system, if integrated with the Indian system will provide all the coverage we need for perhaps, the next 30 years. We should have Indian ground control centers in SK and SK ground control centers in India.

[Gagan]

Disha ji
Godrej does most of ISRO's fabrication. The Vikas engine, tanks etc are all done by Godrej.
ISRO does R&D, builds prototypes with help of Godrej

[Gagan]

HAL makes satellite components (satellite body etc) in Bangalore

[Amber G.]

Okay for fun some questions about NaVic vs US GPS clocks ... basically questions will help the understanding of logic (fundamental physics - or theory of Relativity applicable to sats) behind!

Look up the answers (or derive the answers, math is "relatively" (:) ) easy) .Questions are about Relativistic effects. - answer may (or may not) surprise you. I will put basic math (and my answers) sometime later, meanwhile feel free to comment and discuss as there is lot of physics here which may be useful in real life.

Background: In US GPS cluster, sats are in orbit with 26,600 Km radius, For Indian Navic these are about 42,200 Km radius, (Look up correct values if you need to have better values - but we are looking for approx answers only). They are basically in circular orbits.

Question:

Assume there are identical atomic clocks, one at Ground station on earth (called A) , other at one US GPS sat (Called B ), and third at India's Navik (IRNSS) sat (called C).

Observed from ground station and compared with ground clock A which clock (if any) will run "slower" or "faster" and by how much (approx) ?

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?

****
Another Q to think about, If I had an observer at sat B, what would that result be. (from point of view of observer at the sat whose thinks Clock B is the master clock)

[vina]

1. Pretty much every country has replicated the GPS architecture with some minor changes on the margins. This is a really good cost effective system, with some limitations. There really isnt much to be optimized here.

Have to agree with you. Nearly all these systems are nearly similar and like you said, the difference is at the margins. GPS, Galileo & Navic & probably BieDou are largely similar while the a bit of an "outlier" is the Glonass.

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 . In addition Glonass uses a "Russian" "earth model" and for signalling it uses FDMA, while the others use CDMA . Together the these make Glonass a bit different. But on a broad architectural level, like you said, they are largely the same, with variations in implementation details.

Note.. Glonass updates it's ephemeris data in 30 minute slots, but it too like GPS and others doesn't calculate the coordinates in real time and encode it in the signal (which it well could) and leaves it to the receivers to calculate the actual instantaneous satellite position by extrapolating from the previous update of the satellite ephemeris data to the receiver

2. These systems are implemented for military purposes. Civilian use is a byproduct, though it can classify the system as dual Civil/Mil use, therefore making its targetting by a hostile power more difficult. Military origin is reflected in system design.. ...
4. In war time, its fully expected that at least some satellites will be 'taken down'. The US has a system of 'pseudo-lites', 747s or such transmitting the same signal.

Not just that. In fact, the Block II and Block III GPS "Satellite Vehicles" i.e. SV have an "autonomous" mode, so that if the Control Segment is knocked out , they are able to function independently for 60 days. The SVs do peer to peer ranging to generate their own orbital parameters and also the time correction offsets of their individual clocks from the GPS System Time.

Notice how GPS & Other times actually differ. The TAI is ahead of GPS by 19s (and differ from UTC) etc and GPS system time is maintained that way. Hypothetically if they let the TAI and GPS time drift apart , but correcting ONLY TAI, GPS would still continue to work absolutely fine, as you said, all that GPS needs is for it's system time to be "internally consistent"

Think about it - On GPS sat, all you need is an accurate atomic clock - No computing power - No fancy instruments to locate it's position, thermometers to measure temp (like in old days, one has to worry about if your measuring rod expanded due to temp).. ityadi ityadi ...

So if you are trying to suggest, an alternative design like "send 'position data' and then there will be no need to sysnc" ..you need to think about how will you get that 'position data' ??? (Just sayin that it is only x bytes long or can be sent by y protocol is not that important)..

Not denying anything you wrote or making any comment on the design choices of GPS as it was architected and implemented back then. However, do note that GPS too has modernised and like I pointed out, the Block II (later versions) and Block III have significant compute and other capabilities that are needed for autonomous mode.
The only thing I am saying is that since these satellites anyway can compute their ephemeris data (in normal course that data will be uploaded by the ground stations), a newer GPS like system, could, given the advances in computing, anyway go ahead and compute the instantaneous position and encode it in the signal and that too with a Glonass like Earth Centric cartesian system and computation in receiver becomes less onerous.All the others stuff like "how will you get position data" remains exactly the same as it is now. That is not a "problem" now ,nor will it be.

[disha]

Background: In US GPS cluster, sats are in orbit with 26,600 Km radius, For Indian Navic these are about 42,200 Km radius, (Look up correct values if you need to have better values - but we are looking for approx answers only). They are basically in circular orbits.

Question:

Assume there are identical atomic clocks, one at Ground station on earth (called A) , other at one US GPS sat (Called B ), and third at India's Navik (IRNSS) sat (called C).

Observed from ground station and compared with ground clock A which clock (if any) will run "slower" or "faster" and by how much (approx) ?

Clock of Sat C will run faster than Sat B which will be faster than Sat A. Gravity trumps time.

If you are at Sat B., time at point A (on ground) will be slower and point C (higher up above than point will be faster.

Again all of this is referential., since one has to escape both the Earth's gravity well and the Solar gravity well and the gravity well of the black hole which is in the center of this galaxy to get a time which is constant for the local galaxy cluster. In other words., time is never a constant.

[disha]

The only thing I am saying is that since these satellites anyway can compute their ephemeris data (in normal course that data will be uploaded by the ground stations), a newer GPS like system, could, given the advances in computing, anyway go ahead and compute the instantaneous position and encode it in the signal

What are the security implications *for* a GPS sat which encodes its instantaneous position and relays it?

In sudh hindi., it is like stating "Aaa Bail Mujhe Maar" every few seconds.