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Understanding GPS: The Nuts and Bolts

April 10, 2017
To improve the country’s navigational capabilities, the U.S. deployed what has become a 32-satellite scheme called the Global Positioning System (GPS). This space-based, all-weather technology allows for positional calculations through the interpretation of time and location data continuously broadcasted by the system’s orbiting satellites. A GPS navigation device (GPS receiver/tracker) collects satellite information and then works out an updated time value as well as a three-dimensional positional assessment. Although primarily a military project, GPS was subsequently customized into a publicly accessible technology to assist civilian navigation.


While every GPS satellite has been assigned a unique encoding signature, the Code Division Multiple Access (CDMA) standard is used across the board to unify the scheme. Because CDMA allows for the concurrent transmission of data over a single channel, multiple satellites can send out information to different users on the same bandwidth at the same time. To limit interference, each satellite signal is sent across a widened spectrum.

CDMA for GPS is split into two coding schemes–one is used by the public, while the other is reserved for U.S. military applications. Public GPS allows for low-resolution navigation, while military GPS makes use of encryption technology to secure the exchange of information.


GPS as it stands now is composed of three segments: the Space Segment (SS), Control Segment (CS), and User Segment (US).

The system’s satellites make up the Space Segment. The SS is currently composed of 32 satellites set on six orbital planes in Medium Earth Orbit (MEO). Only four satellites are needed to render a position, although present redundant provisions place at least nine satellites at the user’s disposal at any given time. The U.S. Air Force handles all SS operations.

The Control Segment, in turn, is made up of a Master Control Station (MCS), an alternate MCS, six monitor stations, and four ground antennas. The system’s MCS is presently located at Schriever Air Force Base in El Paso County, Colorado.

The User Segment is composed of the GPS devices employed by both civilian and military users. The exclusive GPS Precise Positioning Service is reserved for U.S. and allied military applications, while the inclusive GPS Standard Positioning Service is available to anyone with a GPS device (subject to restrictions as defined by the U.S. government).


The GPS experience is delivered to the user via a GPS device that can be broken down into an antenna, a clock, a computer composed of specialized processors, an output display (typically a small electronic screen), and various input provisions. Four-channel GPS devices, originally the system’s only available end-user equipment, have since been supplanted by trackers that can tune in to up to 20 satellites at any given time.

Modern GPS gadgets also typically feature interactive map displays, text-to-speech software, and resistive touchscreens.


Network-independent GPS employing standalone/self-ruling GPS devices calls for antennas with an unobstructed view of the sky. For instance, a receiver located inside a building with no outdoor aerial (and thus with no direct view of at least four of the orbiting satellites) will naturally obtain no information from the system. The presence of metal can also affect the performance of standalone GPS devices.

Moreover, downloading orbital information without the help of ground-based servers takes time. Network-independent GPS units also have to start from scratch each time a satellite stream is lost (because most self-ruling GPS devices lack caching/memory resources, data already downloaded are simply discarded in cases of signal interruptions).


As in the case of most technologies, GPS receivers started out as bulky, cumbersome boxes that were practically unsuitable for field work. These early units also had analog-only processors as well as poor power provisions. Portability and improved device functionalities were achieved in the early 1990s when the U.S. military introduced considerably lighter receivers (units weighing close to 50 pounds were gradually phased out in favor of receivers weighing less than three pounds).

Advancements in technology made GPS receivers not only smaller but also cheaper to manufacture. The preponderance of mobility, meanwhile, pushed GPS into the realm of personal computing and public telecommunications. Today, many users take advantage of the system via GPS-enabled mobile phones.

Telecommunications companies and technology providers deliver GPS to mobile phone users in different ways. Online and offline apps that provide on-demand map information are popular among smartphone users, for instance. Assisted GPS (A-GPS), where location and time data are collated, saved, and sent through to subscribers via an Internet connection, is also available to users who prefer the stability and relative accuracy afforded by established phone networks.


A-GPS, sometimes called indirect GPS, is GPS leveraged on the network resources of a subscriber’s phone or Internet service provider. Where standalone/self-ruling GPS devices download data directly from the system’s satellites, units using A-GPS take information from dedicated A-GPS servers maintained by specific network operators. These network providers typically do all the work; orbital information are downloaded, stored, updated, and, for a fee, made available to subscribers via a data connection.

A-GPS is faster than direct GPS, and users pay more to access the service (downloading data directly from GPS satellites is free).


Network-dependent GPS presently reaches subscribers mainly through two mobility standards: GSM (Groupe Spécial Mobile or Global System for Mobile Communications) and CDMA. Although network operators have agreed on, provided for, and put into service new cellular protocols, the infrastructure currently allocated to GSM and CDMA still encompasses, on the whole, a considerable subscriber and territorial base. Thus, A-GPS as we know it now continues to be largely dependent on these two standards.

Because GSM, a collection of second generation (2G) mobility protocols, is inherently optimized for voice telephony only, old GSM GPS trackers “call” (that is, connect to) the nearest available Access Point Name (APN) to get information. The dialed APN then connects to the Internet in behalf of the user to obtain data from the network’s A-GPS servers. The requested information is then sent through to the subscriber’s unit using the Short Messaging Service (SMS) standard.

The introduction over time of General Packet Radio Services (GPRS) and Enhanced Data Rates for GSM Evolution (EDGE or EGPRS) gave GSM networks not only data capabilities but also relative efficiency and flexibility.

CDMA GPS trackers, on the other hand, are inherently data-capable, so these units access A-GPS servers directly via an Internet connection to acquire time and location values. Because this method of getting satellite data is wholly dependent on a data connection, users of CDMA GPS trackers are generally charged based on each request (or “ping” in GPS parlance).


Other nations are rushing to deploy their own versions of GPS. The European Union’s Galileo positioning system, for example, is expected to be fully operational by 2020. China, meanwhile, is expanding its existing BeiDou Navigation Satellite System into the 35-satellite BeiDou-2 (COMPASS). Both systems aim to compete with GPS and Russia’s Global Navigation Satellite System (GLONASS).

Where the end-user is concerned, the future of GPS is invariably tied to the future of mobility. The arrival of faster networks, for instance, allows for high-speed information exchanges across data lines. At the same time, developments in consumer electronics are pushing the system into more gadgets. Coupled with the emergence of new platforms, this shift toward multifaceted devices promises to make GPS even more ubiquitous.