The Indian Space Programme

by Gurbir Singh

  In 2006, the Government of India approved the Indian Regional Navigation Satellite System (IRNSS) with the objective of providing independent regional GPS-like services for national applications across mainland India and 1,500 km beyond its coastline.[731] Unlike the well-established American GPS, IRNSS is not a global but a regional system, even though it is frequently lauded in India as India’s GPS. On 28 April 2016, India launched the seventh and final satellite, IRNSS-1G, of the IRNSS constellation. The first satellite, IRNSS-1A, was launched on 1 July 2013. Following the successful launch of IRNSS-1G, the Prime Minister of India, who had watched the launch from his office in New Delhi, renamed the collection of navigational satellites as Navigation Indian Constellation (NavIC).[732] It was a culmination of a project initiated almost a decade earlier.

  In the Union Budget presented to the Parliament on 29 February 2008, the Indian Finance Minister P. Chidambaram had made a budgetary allocation of Rs.270 crore ($67.5 million) for IRNSS. Speaking in early 2014, Kopillil Radhakrishnan (born 1949), Chairman of ISRO, estimated the total cost of the IRNSS programme, consisting of seven satellites in orbit and two on standby on Earth and the associated ground infrastructure, at Rs. 3,260 crore ($527.9 million). He stated that each IRNSS satellite would cost Rs.150 crore ($24.3 million); nine would cost Rs. 1,350 crore ($218.6 million).

  The ground-support infrastructure would cost an additional Rs. 1,000 crore ($161.9 million), and the cost of each launch would be Rs.130 crore ($21 million), totalling Rs.910 crore ($147.4 million) for seven launches.[733] The launch vehicle used for IRNSS was the PSLV-XL. The PSLV is ISRO’s primary launcher of satellites to LEO, SSPO/SSO and GTO and comes in three configurations with the XL being the most powerful. It was used for the first time to launch India’s Moon mission Chandrayaan-1 and since then several times, including the MOM. With its six strap-ons, the PSLV-XL could take the 1,432-kg IRNSS satellite to its eventual slot in GSO.

  Unlike the navigation satellite constellations of the US, China, Russia and ESA, which have a global coverage from medium Earth orbits at altitudes between 19,000 and 23,000 km, all seven IRNSS satellites are located in a combination of GEO and GSO. The PSLV-XL delivered each of the 1.4-tonne satellite initially to a sub-GTO orbit (283 km × 20,630 km) 20 minutes after launch.[734] Once the solar panels were deployed, the engineers at ISRO’s MCF in Hassan used the satellite’s onboard liquid apogee motor (LAM) to deliver it to its designated GSO or GEO at 35,786 km.[735]

  The IRNSS constellation facilitates a variety of applications within the public and private sectors in India. It is configured to provide two levels of service. A Standard Positioning Service (SPS) with a 20-m resolution over the Indian Ocean and less than 10-m resolution over mainland India and a Restricted Service (RS) with an encrypted signal offering an undisclosed level of higher resolution available only to authorised users. [736]

  In addition to the traditional uses of satellite-based navigation, such as navigation aid for drivers in urban areas, hikers and travellers in remote areas, IRNSS is also called upon to assist India’s dairy farmers, commercial aviation industry, its expansive railway network and the Archaeological Survey of India. It will be used for commercial services, such as fleet management, location-based advertising, land surveys, real-time tracking, as well as broader regional and national services, including:

  terrestrial, aerial and marine navigation

  disaster management

  integration with mobile phones

  precision timing

  mapping and geodetic data capture

  NavIC will enable a multitude of business opportunities in India. In time, these businesses and their supply chain will provide a significant boost to the national economy. A recent UK government report determined that the Global Navigation Satellite System makes a daily contribution of $1.263 billon (Rs.7798 crore) to the UK economy. Given the larger economy, NavIC will inevitably have a larger impact in India once the service is imbedded within the national economy.[737]

  IRNSS implements two services typically associated with developed nations, the RS for use by the Indian security services and through GPS Augmented Navigation (GAGAN) navigation support for the civilian aviation sector. These and other disparate uses of space technology continue to drive India’s modernisation plans, pulling it into the 21st century, and reflect how ISRO is fulfilling the ambitious aim of its founders to use space technology for national development.[738]

  The NavIC/IRNSS system has three distinct system segments, space, ground and user. The space segment consists of the seven satellites in orbit. The ground segment consists of ground facilities used to manage the navigation satellites in orbit. The user segment consists of the physical receiver unit that calculates the position on Earth using the timing information embedded in the navigation signal from the orbiting satellites and the applications that use them.

  Space Segment

  The seven satellites in orbit and the two on standby on Earth ready for launch when required make up the space segment. All seven satellites hover over or near the equator with a direct view of mainland India at all times. Three of the satellites are in GEO directly over the equator at 32.5ºE, 83ºE and 129.5ºE and remain stationary in the sky. The other four satellites are in GSO arranged in pairs at longitudes 55°E and 111.75°E inclined at 29° to the equatorial plane and 180° out of phase with each other. One satellite is almost directly over New Delhi at 35,786 km with three each on either side. The number of satellites in orbit and on standby is likely to increase with time.[739] The architecture of all nine satellites is identical with an expected lifetime of around 12 years.

  Unlike the satellites in GEO, those in GSO are not stationary but scribe the figure 8 (a little like the analemma that the sun scribes over a year) in the sky every 24 hours. Between them, the seven satellites have a footprint over a rectangular area between latitudes 30°N and 30°S of the equator and longitudes 32.5°E and 129.5°E. This is the region of India’s regional satellite system.

  IRNSS

  Satellite

  Longitude (E)

  Orbit

  Orbital

  Inclination

  Launch Date

  1A

  55.0º

  GSO

  29º (±2)

  1 July 2013

  1B

  55.0º

  GSO

  29º (±2)

  4 April 2014

  1C*

  83.0º

  GEO

  ± 5º

  15 October 2014

  1D

  111.75º

  GSO

  29º (±2)

  28 March 2015

  1E

  111.75º

  GSO

  29º (±2)

  20 January 2016

  1F

  32.5°

  GEO

  ± 5º

  10 March 2016

  1G

  129.5º

  GEO

  ± 5º

  28 April 2016

  Table 13‑1 IRNSS Orbits. Credit ISRO (*New Delhi is at longitude 77º E

  from where IRNSS-1C is nearly overhead)

  Each satellite has three rubidium clocks, a total of 21 amongst the 7 satellites in orbit. By late 2016, ISRO had started to detect problems with some of the rubidium clocks. By mid-2017, it was reporting that five of the 21 clocks had failed. ISRO has not been the only victim of the failure of rubidium clocks. The European GPS constellation, Galileo has also suffered from a similar clock failure.[740] Although the satellites continue to function, the timing signals of required precision are not available and thus NavIC as a system cannot operate as designed. All three clocks had failed in IRNSS-1A by June 2016 and attempt to find a fix or a workaround had not been successful. ISRO attempted to replace IRNSS-1A with IRNSS-1H launch on 31 August 2017 but the mission was not successful. The payload faring did not detach stranding IRNSS-1H inside the 4th stage of the PSLV. Following a failure analysis report, another launch will be scheduled.
Eventually, an additional three will also join the NavIC constellation bring the total number of satellites in orbit to 11.

  Ground Segment

  The seven satellites in orbit are supported by a ground segment consisting of facilities for ranging (determining the satellites’ position in space), network timing (synchronisation with a master clock), spacecraft control (monitoring and maintaining satellite health and position) and data communications.

  Figure 13‑1 IRNSS Ground Segment. Credit ISRO

  The data communication network consists of 15 sites across India operating 24/7 in real time. ISRO’s 32-m fully steerable antenna at Byalalu, is the primary element in IDSN. On 28 May 2013, Byalalu formally became ISRO Navigation Centre (INC) and the centre of IRNSS’s ground segment. In addition to the INC, the ground segment includes;

  IRNSS Satellite Control Facility: It controls the space segment from the ground through TT&C network. It also uplinks the navigation parameters generated by the INC. Each satellite transmits its navigational signal to the Earth in the L5 band and the timing information in S-band.

  IRNSS Range and Integrity Monitoring Stations: Perform continuous one-way ranging of the IRNSS satellites to determine and help maintain the positional integrity of the IRNSS constellation.

  The ground segment is responsible for maintaining and operating the IRNSS constellation during its designated lifetime. At least 15 sites across India have direct communication links to the orbiting satellites. The ground segment has four key responsibilities: (i) calculating with high precision the location of each satellite in its orbit using two independent mechanisms, radio and optical lasers, each satellite is equipped with corner-cube reflectors; (ii) ensuring that the satellite’s onboard rubidium atomic clocks are synchronised with the master caesium clock located at INC; (iii) maintaining the quality of the radio signals transmitted by each satellite for use by the ground-based receivers and (iv) correcting, when required, the orbit of each satellite on command from the IRNSS Satellite Control Facility.

  New elements of the ground segment have been built, or existing capability expanded, to facilitate IRNSS. An IRNSS Control Centre, along with one 11-m antenna and four 7.2-m antennae, has been added to the Satellite Control Facility at Hassan with the responsibility of controlling IRNSS constellation. An additional 11-m and three 7.2-m antennae are being constructed at the backup Satellite Control Facility in Bhopal.[741]

  User Segment

  The IRNSS user segment is concerned primarily with the design and construction of receivers that will receive the navigation signals from the satellites. The receivers are then embedded within stand-alone devices, including mobile phones, tablet computers, dedicated handheld navigation devices and satnav devices for use in cars, ships and aircraft.

  An IRNSS receiver usually operates on a single frequency (L5 at 1176.45 MHz or S-band at 2492.028 MHz). It can also operate in a dual mode on both frequencies at the same time. Single- and dual-band receivers are capable of receiving both the SPS (accuracy of 20 m) and the RS, but only authorised users have access to decryption keys necessary to access the RS.

  An IRNSS satellite receiver calculates its position with an accuracy of 20 m on the Earth’s surface from the signals it receives from IRNSS satellites in their orbits 35,786 km away. For this to work, the position of the satellite in orbit and the timing signal it transmits must be known with high precision. The rubidium clock at the heart of each satellite is the central component of a navigational satellite. Though not as precise or as expensive as a caesium atomic clock, rubidium atomic clocks are used by high-end data centres, central television transmitter stations, cell phone base stations, as well as navigation satellites.

  Figure 13‑2 IRNSS Architecture. Credit ISRO

  ISRO has designed the IRNSS architecture for interoperability with existing international standards.[742] The specific characteristics of the navigation and timing signals that ISRO has selected ensure that the IRNSS system is interoperable with the US’s GPS and Europe’s Galileo systems developed by the ESA. China and Russia have formally agreed to cooperate with their global navigation systems, BeiDou and GLONASS, respectively.[743]

  Navigation Satellite

  Ultimately, the primary purpose of IRNSS is the broadcasting of navigation and timing signals by each satellite. To maintain operational status, each IRNSS satellite’s position must be known to a high precision, and the onboard atomic clock must be continuously synchronised with the one at INC. For redundancy, the navigation and timing signals are duplicated in the L5 and S bands. These signals received from multiple satellites by a receiver on Earth (for example, a mobile phone) are then used to calculate with precision the receiver’s position, altitude, direction and speed.

  Each satellite has three onboard components to serve this requirement

  three high-precision rubidium atomic clocks for timing

  a corner-cube reflector for ranging

  onboard radio transmitter.

  The primary ground station, INC at Byalalu, has caesium and hydrogen maser clocks that provide the highest accuracy stratum-1-level timing signal. This is used to synchronise the rubidium (stratum-2) clocks on-board each IRNSS satellite and other IRNSS ground stations.[744] While the stationary Earth-based atomic clocks provide consistent, reliable time, the clocks on-board the satellites are subject to two key relativistic effects that must be compensated for to maintain the required precision.

  Figure 13‑3 IRNSS Subsystems Showing the Central Location of

  the Corner Cubes. Credit ISRO

  The speed of the satellite at 3 km/s in orbit slows the onboard clock due to Special Relativity, and the satellite’s distance from the centre of the Earth and the resulting weaker gravitational field speeds it up as described by General Theory; consequently, onboard clocks require daily adjustment. These are tiny variations but are critical to achieving an accuracy of 20 m on the surface of the Earth. The onboard clocks require a regular correction typically in the order of 5 to 10 nanoseconds (nanosecond = thousandth of a millionth of a second). Even though this compensation is largely built in prior to launch, the clocks still require daily monitoring and adjustment.[745] Before a navigational satellite can provide a precise location to a receiver on Earth, the precise location of the satellite in space must be known.

  One of the techniques used to measure the position of a satellite in orbit is to direct pulses of laser light from Earth and detect the reflections from it. The corner cubes attached to the satellite facilitate this independent mechanism (using light, not radio) for determining its position in space. Very short laser pulses are beamed from the Earth to the satellite, and the corner cubes reflect the pulses back to the source. Corner cubes are designed to reflect light back in the direction of the source, irrespective of the direction it comes from. The time between sending the pulse and its arrival back on Earth is a measure of the satellite’s distance. Corner-cube retro-reflectors are used not only on navigation satellites but also on other spacecraft for range determination; they have also been used to determine the Moon-Earth distance.[746]

  Figure 13‑4 IRNSS-1A. Credit ISRO

  Each IRNSS satellite has an array of 40 corner cubes manufactured by ISRO’s Bengaluru-based Laboratory for Electro-Optics Systems. Each 38-mm cube is designed to provide the position of the satellite with a precision of 5 mm over a distance of 35,786 km.[747] A Satellite Laser Ranging (SLR) station, like an astronomical observatory, is usually located on high ground away from populated regions. An SLR station has a high-energy pulsed laser, a high-precision timer and a small telescope connected to a sensitive photon detector. The photon detector detects the reflected laser pulse from the satellite after its journey to and from the satellite. Modern digital systems allow the time-of-flight (interval between the sending and receiving) of the laser light to be measured with high precision in the order of 10 picoseconds (1 picosecond = 1 million million or 10-12). Although not as efficient as at night, the SLR is used to successfully, locate
satellites in orbit during either day or night. The concentration of charged particles in the Earth’s ionosphere, through which the satellite’s signals must travel, is the largest error source for the radio signals.[748] Ranging by use of laser pulses is not affected by the charged particles in the ionosphere.

  Figure 13‑5 Mount Abu Satellite Laser Ranging Station. Credit ISRO

  All SLR lasers currently in use around the world are considered unsafe for the human eye for the entire path of the laser pulse from the ground station to the subsequent reflection. SLR stations are thus established well away from airports and common flight paths. Even so, SLR-operating procedures require that, prior to their use, checks are conducted to verify that no aircraft is operating in the area. Most stations use a traditional ground-control wide-field tracking radar that follows the laser beam around the sky. If an approaching aircraft is detected, the laser is switched off automatically.

  SLR stations are highly computerised, and some are automated to operate entirely without human intervention. Automated SLR stations operate to a predefined schedule, self-calibrate for each session, search for satellites using a telescope and shut down if the weather (cloud/rain/snow/wind) exceeds predefined thresholds. ISRO has established two laser stations. One is located at Mount Abu, between Jodhpur and Ahmedabad in northern India, and the other is 2,000 km away in Ponmudi in Kerala in southern India.[749]