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Sunday, September 20, 2015

ASTROSAT: India's Space Observatory ... set for launch end Sept, 2015.

Come 28th September 2015, India's workhorse rocket PSLV will carry 7 satellites to space.

 PSLV (XL) C30 to be launched from 2nd Launchpad at Sriharikota will carry:
- AstroSat-1,
- LAPAN-A2 (Indonesia)
- NLS (Canada)   and
- Lemur (UK) - 4 sats.

The orbits of these satellites are very diverse: Inclinations of Lemur,Lapan and NLC are between 97 and 98 degrees while ASTROSAT has an inclination of 6 degrees.

So this is an interesting launch to watch  specially how the vehicle will be maneuvered to release these satellites.

Added 27Sep/1715 IST

So the PSLV C30  launch will be Equatorial...  as can be deducted from the NOTAM issued to Navy

We have plotted the path based on NOTAM issued by Indian Navy.

PSLV C30 path drawn using the Indian Navy's NOTAM

Added 29 Sep 2015

The PSLV C30 launched ALL the satellites in near equatorial orbits of about 6 deg inclination.

Top figure shows the satellites as they are launched on 28th. Note how all are together in one bunch.

Next part shows the position of the satellites a day after launch and the same view is zoomed in the lowest portion of image showing how the satellites have started spreading ..  but still in the launch orbit .

In a few days individual sat operators will start maneuvers to push the satellites in their final designated polar orbits with 97 deg inclination

Watch this place for further updates/commentary  regarding how the orbits will be changed..

Current parameters for objects a to h are as follows:

Object Period ( min ) Inclination Apogee Perigee
Sat A                97.55               5.99     650                633     
Sat B                97.54               6.00     650                632     
Sat C                97.52              6.01      650                630     
Sat D                97.51             6.00       650                630     
Sat E                97.56              6.01      650                 634     
Sat F                97.57              6.00       650                635     
Sat G                97.58             6.00       650                635  
Sat Housing 97.28               5.96      649                608     

Exact satellites have still to be identified officially.

Unlike the other missions ISRO has not given sat release maneuvers this time.

We only know that 1st sat to be released is ASTROSAT , to be released about 35 seconds after 4th stage closure at 650 kms altitude.

Then LAPAN, NLS and 4 LEMURs will be released.

Coming to ASTROSAT satellite, it is one of the very few satellites for Astronomy applications ( after NASA [ CHANDRA ], ESA [ XMM/NEWTON ].
Of course their specs are way ahead of India's ASTROSAT   but still it is India's maiden attempt realized after several years of toil by the designers going through several cycles of revisions.

( To know about this toil read this article. )

The goals of ASTROSAT are :
a. Understand high energy processes in binary systems
b. Search for black hole sources in the Galaxy.
c. Measure magnetic fields of neutron stars.
d. Study high energy processes in extragalactic systems.
e. Detect new transient X-ray sources.
f. Limited high-angular resolution deep field survey in UV

Towards achieving these goals Astrosat carries five experimental payloads:

(i) Large Area X-Ray Proportional Counters (LAXPCs)
(ii) the Soft X-Ray Telescope (SXT)
(iii) Ultraviolet Imaging Telescopes (UVITs) -
(iv)  Cadmium-Zinc-Telluride Imager (CZTI)
(v) the Scanning Sky Monitor (SSM)

Out of the 5, LAXPC is a counter while all the remaining instruments are imaging instruments.

A brief introduction to each of these instruments follows:

(i) Large Area X-Ray Proportional Counters (LAXPCs) for X-ray timing studies ( 3 nos. )

This is the only 'non-imaging' instrument aboard ASTROSAT, all others being imaging type, It has a Field of View ( FOV ) of a square of 1deg*1deg area.
It is interesting  to know that the 3 LAXPCs together, have the largest area proportional counters ever, and with an unprecedented 10 microsecond timing resolution and high photon counting rate.


(ii) the Soft X-Ray Telescope (SXT)  having a X Ray CCD at focal plane.   It covers an area ( FOV )  of 0.35 deg circle.It can capture X Ray images from 10 keV right down to 0.3 keV, a very wide range.

(iii) Ultraviolet Imaging Telescopes (UVITs) - 2 nos.
One telescope is used  for visible and Near UV band observations while the second one is for Far UV region. It has a tube of 3 meters length.
It has Field of View ( FOV ) of 0.5 degrees and a very high angular resolution of 1.8 arc-seconds in UV bands.

UVIT Optics
UVIT   Physical appearance 


(iv)  Cadmium-Zinc-Telluride ( CdZnTe ) Imager (CZTI) using Hard X-ray imaging detector
This detector has the widest FOV: 6deg*6deg below 100 keV and 17deg*17 deg above that.


(v) the Scanning Sky Monitor (SSM)
It monitors and detects bright objects, particularly transients, up to 10 keV X-ray energy.

In addition to these 5 payloads, the satellite also carries a complementary charged particle monitor to provide data on particle flux in realtime.

All thes instruments are so co-aligned that Fields of View (FOVs) of all the instruments is within a few arc seconds.
The pointing accuracy of the satellite is 0.03 degrees in all the axes and the pointing stability is 5x10 deg/sec.
These specs provide simultaneous observation in both X-ray and UV astronomy.

One of the complexity faced during design of the satellite is maintaining thermal balance of the satellite.
When the instrument pointing is changed  from one source to another, the solar radiation and earth’s albedo conditions change and so maintenance of thermal balance is challenging.

For both northern and the southern sky astronomical visibility an equatorial orbit of 650-km height with 6 degrees inclination is chosen.

Both these numbers have special significances : 650 kms ensures that the spacecraft is above atmospheric drag ( which is a worry for orbits upto 500 kms ) increasing the useful life of satellite.

The 6 degrees inclination ensures that the satellite is not entering the South Atlantic Anomaly (SAA). SAA is an area above South America where  high energy charged particles are found upto an altitude of 800 km, which cause onborad instruments to malfunction.

(  To be completed )

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