GPS – Space Segment

Nowadays, you know, the tool that has made the difference in the modern conduct of Ships and Boats is the GPS – Global Positiong System and similar (see Satellite Navigation for in-depth).

You are reading the first of three articles dedicated to the system as the goal will be to go and treat each of its components specifically without limiting themselves to the general definitions and descriptions reported on many sites.

Being the first article in the series we start from a brief introduction.

The system was born following the testing of the U.S. TRANSIT system, which began testing in the early 1960s. This system (obsolete since 1996) allowed the calculation of the position of the receiver through the doppler effect.

We assume that GPS, as well as the previous TRANSIT, was developed and is currently operated by the USDOD – United States Department of Defence and this strongly characterizes the system (we will see as in this article).

Unlike TRANSIT, the basic concept that has decreed the success of GPS is associated with the fundamentals of astronomical navigation: calculate the position of the receiver, note that of the satellites that this (the receiver) receives at the same time. In astronomy, the position of the observer is calculated, known the coordinates of the stars observed with the sextant.

The system is divided into three segments, each with well-defined peculiarities:

  • Space Segment.
  • Control Segment.
  • User Segment.

The space segment consists of a constellation of artificial satellites that form the so-called “Bird Cage”.

iCaptain, segmento spaziale del gps
Image of the GPS Space Segment, note the characteristic “Bird Cage” conformation

At its inception, the system provided for global coverage made with the use of 21/3 satellites (3 satellites were to be used exclusively as a backup system).

Each satellite has an altitude of about 20,200 km from the Earth’s surface and is positioned on orbits with an equatorial inclination of 55 degrees, allowing a minimum annual orbit decay (i.e. the decrease in the Earth-Satellite distance).

To compose the “Bird Cage” 4 satellites are distributed per orbit, each satellite spaced away from the next 90 degrees, each orbit far from the next 60 degrees of celestial longitude.

Within 24 hours, each satellite travels through orbit twice at a speed of about 10,570km/h.

From an engineering point of view, each satellite has a system of photovoltaic panels and a charging regulator intended to charge the lithium-ion on-board batteries necessary to power electronic devices even when it is in areas of shadow.

The peculiar components are: an atomic clock (one of the most accurate clocks in the world, the calculation of time is determined by the resonance frequency of an atom), a bank of oscillators, antennas for the production and transmission of the signal, antennas receiving communicate with the Control Segment and of course a series of computers (similar to PCs but with very “solid” operating systems and components).

The satellites are then covered by a shielding system in order to protect internal electronics from space radiation, in particular from solar storms.

As mentioned at the beginning of the article, the system was built and managed by the USDOD, this led from the beginning to operate in a dual way with different precisions: the civilian and the military.

Each satellite, regardless of the entry into service, transmits the following types of signals:

  • Coarse Acquisition (C/A) queues on the L1 frequency at 1575.42MHz for civil use (this code is the one that can encode our onboard receivers)
  • Precise P(Y) queues on L1 frequencies at 1575.42MHz and L2 at 1227.6MHz.
  • 2nd L2 (L2C) additional signal to improve accuracy for civilian use (only on Block IIR-M satellites launched from 2005 to 2009)
  • 3rd on the L5 frequency to improve accuracy for civilian use by adding an additional frequency (only on Block IIF satellites launched from 2010 to 2016)
  • On Block III and IIIF being tested, there will be an additional civil signal on the L1 frequency denom4th, starting from this constellation on the cicili frequencies should no longer be present the Selective Availability system used by the DOD to system accuracy in civil use.

This definition above is the official definition of the DOD.

iCaptai, satellite of the IIR-M block
IIR-M block satellite launched from 2005 to 2009

To date, the system has the possibility of using Selective Availability, a disruption of the signal for civilian use intended for national security reasons in order not to allow those who use the system the same accuracy as military use.

Selective Availability has been deactivated at the moment (starting in 2000 at the behest of US President Bill Clinton), but could be reactivated at will by the current President for the aforementioned national security reasons although the development of the next generation of Block III/IIIF satellites no longer provides for its presence on board.

The current Space Segment has more functioning satellites than the 24 initially expected, which allows for greater accuracy in the calculation of position by receivers.

The segment is mixed, that is, composed of satellites of the latest generation but also of old generation such as those of the block IIA or IIR that had to go into disuse since 1997 the first and since 2011 the second but that still functioning perfectly remained operational slowing down the putting into orbit of new blocks, saving economic resources.

The life expectancy of satellites set at 7.5 years is currently estimated at least 15 years.

There are currently 31 operational satellites deployed as shown below.

iCaptain, slant chart - posizionamento satelliti gps
Slant Chart ( Gps satellite placement)

To continue the GPS in-depth study, switch to GPS – User Segment.

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