Equipment of the AutoTracker satellite system - what's under the hood? GLONASS / GPS Receivers

    In a previous post, I talked about what satellite systems for monitoring transport in general are and how they appeared. Now we will begin to gradually get acquainted with the composition and components of the system. And we will begin our acquaintance with the hardware basis of any monitoring system - the on-board navigation terminal (or the subscriber terminal or on-board unit, each manufacturer usually has its own designation). The on-board unit is an electronic device that includes a satellite module (GPS or GLONASS / GPS), a data transmission module (GSM modem), a set of interfaces for communicating with external devices and sensors (most often it is RS-232/485, CAN, 1- Wire, USB) and the microcontroller that controls all this. There is also a certain amount of flash memory on board, sometimes a GSM antenna is built into the board, but these are already design features, which we will talk about later. Today I wanted to talk about one of the main components of the airborne unit - the GPS / GLONASS satellite receiver.


    Typical GPS / GSM layout of the airborne unit

    Actually, the development of consumer GPS navigation equipment began in 2000, when the decree of the President of the United States abolished the coarsening of the accuracy of GPS-receivers in the civilian range, and it became possible to determine the location of an object with an error of 10-15 m by ordinary , “Household” GPS-receiver. Electronic giants, such as Trimble, Tyco, uBlox, immediately enter the electronic components market.
    At that time, these were relatively bulky devices, the first GPS modules were the size of a matchbox (hereinafter, the navigation receiver refers to the electronic component, the module responsible for navigation, and not the finished device, such as a GPS navigator or GPS tracker) . For comparison, modern GPS chips used in AutoTracker blocks and representing a system on a chip (System-on-a-Chip, SoC) have a size of about 0.25 cm² and even smaller. Moreover, he has several times higher sensitivity and more channels. Until 2007, in fact, there was no satellite electronic components market in Russia; the choice was very meager. For example, delivery times through an official distributor of a batch of 1000 GPS modules could be six months or more. One of the first companies to enter the Russian market, was an American corporation Trimble. They immediately offered many interesting technical solutions, from electronic components to finished systems. Moreover, their products were of high quality at a low price. This was the reason that the Trimble GPS receivers were in the first models of AutoTracker airborne units.

    The 12-channel Trimble 46240-25 GPS receiver of the 2005 model has dimensions of 26 * 26 * 6 mm, which is not bad for its time. But the latest developments of GPS chips are 500-700 times smaller than it!

    But the market for electronic components has developed rapidly and after 2007 new players came to Russia, including Chinese manufacturers. The competition has intensified significantly, the race of technology and prices has begun, as a result, now 2005-2006 receivers look like giants.

    The latest models of 56-channel GPS / GLONASS modules can be easily integrated into any mobile device

    But in addition to the mass and dimensions of the receivers, all performance characteristics have significantly improved. In particular, the number of channels. A modern receiver contains 50-60 channels, versus 12 in 2006. What does this affect? To determine two coordinates (longitude and latitude), the GPS receiver needs to have a signal from three satellites, and to determine three coordinates (longitude, latitude and altitude) from four satellites. But for reliable operation, the receiver must be able to work with a large number of satellites. For example, if a signal is lost from one satellite, the GPS receiver will be able to use another available channel. We can say that an 8-channel receiver in most cases is enough for normal operation. With a further increase in the number of channels, GPS has the opportunity to receive several signals from one satellite (main and reflected).
    Another very important parameter of the navigation receiver is sensitivity. The higher it is, the weaker the signal from the satellite can be received, the more likely the receiver to accurately determine the coordinates. The sensitivity of the GPS receiver is determined, among other things, by the features of processing and analysis of signals received from satellites. The main parameter of the received signal is its rangefinder delay, which can be calculated using a set of correlators. To achieve high sensitivity, the GPS receiver must simultaneously analyze a sufficient number of signal delays. The presence of a large number of correlators significantly increases receiver performance, which leads to improved sensitivity and reduced time to first location determination (Time To First Fix, TTFF). So, in GPS modules, introduced in 2008 (mainly on the SiRF Star III chip), the total number of parallel correlators is 200,000, which allows the modules to achieve a sensitivity of –159 dBm. In conditions of good reception, this means that signal capture occurs almost instantly. Until that moment, the typical number of correlators in the GPS receiver was 8-10 thousand (for example, uBlox ANTARIS 4). All these parameters must be taken into account when selecting components and developing on-board equipment of AutoTracker. uBlox ANTARIS 4). All these parameters must be taken into account when selecting components and developing on-board equipment of AutoTracker. uBlox ANTARIS 4). All these parameters must be taken into account when selecting components and developing on-board equipment of AutoTracker.
    A separate topic is the GLONASS receivers. Their active production began relatively recently. This was facilitated, including by political factors - on February 18, 2009, the Federal Law “On Navigation Activities” came into force, and later a number of legislative acts obliging public transport, transporting passengers, dangerous goods, etc. to equip a navigation system with GLONASS support . Such enforcement measures pushed the development of GLONASS consumer navigation equipment. At first, there were no offers from developers and manufacturers of receivers at all. Those single and small-scale copies of Russian receivers did not withstand any criticism and could not compete in performance with GPS equipment. At the same time, they cost fabulous money, their cost reached $ 250-300.

    One of the first GLONASS modules. There

    were few who wanted to buy such devices, but a compromise solution was found. The on-board AutoTracker unit remained unchanged with the standard GPS, and in case of urgent need an external GLONASS module was connected to it. This made it possible not to carry out a complete replacement of automobile blocks, but to carry out a kind of modernization of them.
    Later, after about a year, Russian enterprises launched the production of small batches of GLONASS receivers of more or less acceptable quality and characteristics. It was still far from GPS, but they worked pretty well, reminiscent of the dimensions of the same Trimble sample of 2005. And besides, all of them were (and are) two-system - GLONASS / GPS, which made it possible to refine the data using signals from an alternative system.

    AutoTracker AT-10 on-board microcircuit with GLONASS module manufactured by KB NAVIS

    Prices by this time have also dropped significantly. But about another year passed, and foreign developments of GLONASS receivers appeared. They were compact, technologically advanced and very close in specifications and price to GPS. A momentous moment was the appearance of the iPhone 4S smartphone with GLONASS support. Perhaps this served as an impetus for international electronic companies to produce GLONASS equipment. Now GLONASS versions of navigation receivers are available at most large foreign manufacturers - Trimble, uBlox, SiRF, Telit, etc.

    GLONASS / GPS / Gallileo module of Telit company with dimensions of 16 * 12.2 * 2.4 mm is a perfectly acceptable alternative to the traditional GPS module

    And we continue to closely monitor this market, regularly analyze and test samples of new receivers in order to implement the best developments in the airborne unit. We will introduce you to the methodology and results of such tests in one of the following posts.

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