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India’s space agency ISRO launches GPS-type satellite

India’s space agency ISRO today successfully used its GSLV rocket to place the first of a new constellation of that country’s second generation GPS-type satellites into geosynchronous orbit, lifting off from its Sriharikota east coast spaceport.

This was India’s fourth successful launch in 2023, matching its entire total last year. It appears that country has finally recovered from its panic during COVID.

The leader board for the 2023 launch race remains the same:

35 SpaceX
19 China
8 Russia
5 Rocket Lab

American private enterprise still leads China 40 to 19 in the national rankings, and the entire world combined 40 to 35. SpaceX alone is now tied with the rest of the world combined 35 to 35, but trails the entire world including American companies 35 to 40.

Genesis cover

On Christmas Eve 1968 three Americans became the first humans to visit another world. What they did to celebrate was unexpected and profound, and will be remembered throughout all human history. Genesis: the Story of Apollo 8, Robert Zimmerman's classic history of humanity's first journey to another world, tells that story, and it is now available as both an ebook and an audiobook, both with a foreword by Valerie Anders and a new introduction by Robert Zimmerman.

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  • LocalFluff

    Last time I checked, India (like Japan) has their own GPS type satellites for navigation. But that they do not enter geosynchronous orbit. Instead they enter a so called “Molnya” orbit that is highly eccentric with its periapsus above India (and Japan respectively). So that the satellites spend the vast majority of their time in line of sight of their region to serve, then quickly dive around the Earth’s horizons and return in their home skies. Such satellites serve to complement GPS (and Glonass, and Galielei and the Chinese global navigational satellite systems) and improve the coverage and precision of navigation in India (and Japan) and their surroundings.

    But it was a few years ago, and if this is a new generation, perhaps they are going global too? Seems a bit wasteful to my intuition. Why would India help navigation on the Atlantic, for example?

  • LocalFluff: According to the web stream, I gathered that these new GPS-type satellites are focused only over India and its neighbors, which suggests also they are in Molnya-type orbits. At the same time, they referred to them as geo-stationary.

    I’d love to hear from our space experts on this.

  • Edward

    Robert Zimmerman wrote: “I’d love to hear from our space experts on this.

    Although I am not an expert on GPS-style navigation constellations and systems, I will play one here on BTB.

    Scott Manley says that India’s navigation satellites are in geostationary orbit with a modification. Rather than hover over one spot on the equator, the “stationary” part of geostationary, their orbits have an inclination to the equator that causes them to move north and south of the geostationary satellites, with the orbital period the same as the Earth’s rotation. (watch 1 minute)

    This means that they are in geosynchronous orbits, tracing the same ground path each day. A regular Molnya-type orbit is also geosynchronous, as it orbits twice daily and traces the same ground path. Molnya orbits have an eccentricity with a low perigee and a fairly high apogee, which gives it a long “hang time” over a region of the Earth. The Soviets used these types of orbits, which orbit twice daily, to use for communication satellites, broadcasting for the half-day that they hovered over the Soviet Union but listening for the half-day that they were over the United States. Another variation is the Tundra orbit, which is similar but orbits only once a day, so it has even more hang time, so much that the target broadcast region rotates into and out of the field of view during the hover period.

    The geostationary satellites have an advantage that an antenna can be pointed at the satellite and never have to be moved. However, navigation receivers don’t have to point at the satellites, so it does not matter that the satellites keep moving north and south as the day progresses.

    For India, having navigation satellites that are only over their own country may be good enough, but worldwide coverage is necessary for the U.S. military GPS system, because we are asked to help the U.N. with their police actions all over the world. We also need to protect our ships from pirates and other adversaries anywhere and everywhere in the world. U.S. GPS satellites are in a different kind of orbit, at 12,500 miles they are lower than geostationary but higher than the Van Allen radiation belts, and with inclinations to the equator that takes them fairly far north and south for worldwide coverage. Different orbits are useful for different purposes. India’s NavIC system apparently has seven or nine navigation satellites, which works for local coverage. The U.S. has many more, 32 active satellites, because they have to cover the whole planet.

    Many or most people think that once you are in space then you are in space. Movies like Gravity don’t help, because they don’t present the distance, altitude difference, and relative velocity difference between orbits.

    The number of navigation satellites for the constellation also depends upon what it takes to reliably get the minimum number of satellites in view at any given time. To work, every point on Earth that you want covered must always view at least the minimum number of satellites, so the constellation must contain enough satellites to meet this condition for all places that are covered.

    The basic concept is that knowing the distance from each satellite gives a spherical solution for where you may be in relation to that satellite. Your position is located at the intersection of all the spheres around all the satellites.

    If all you see is a single satellite, the spherical distance from that satellite intersects a spherical Earth as a circular ring on the Earth, although the accuracy of this solution is complicated by the planet’s non-spherical shape, so the more your instruments know the topography of the Earth (e.g. the bulge of the equator) the more accurate the solution to that ring. You still don’t know where you are, but at least you are narrowed down a bit. This is why the lost Malaysian airliner, a decade ago, is known to have gone down in the Indian Ocean; its engines occasionally checked-in with the manufacturer, and they were able to use those pings to determine the distance from the relaying satellite (eventually some debris from the airliner washed up on an Indian Ocean beach).

    If you see two satellites then the solution gives two locations on the planet, three intersecting spheres gives two points.

    Three satellites solve for one location, unless you are not on the planet, in which case a fourth satellite can solve for altitude. It is my understanding, however, that the U.S. GPS system does not solve for altitude quite as accurately as it solves for latitude and longitude.

    Separation between the satellites increases the accuracy of the solution. For India, having its satellites alternating north and south of the equator can improve the distance between its satellites, and having some to the east and some to the west helps even more. Differential GPS gives much more accurate solutions, so accurate that farmers can use it to plow their fields. Differential GPS compares your receiver with a stationary receiver at a known location, such as the corner of a farmer’s field or the end of an airport’s runway.

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