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viswatechynology · 3 years ago

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GPS

GPS?

GPS is one of many GNSS that provides positioning, navigation and timing (PNT) measurements. While operated by the U.S. Space Force, a branch of the U.S. Armed Forces, GPS is available for use by anyone worldwide.

GPS was started in 1973, launching its first satellite in 1978. Satellites are developed and launched in series known as blocks. In total, 10 Block I GPS satellites were launched between 1978 and 1981. The Block II series satellites were launched beginning in 1989 and were capable of broadcasting on two L-Band radio frequencies. GPS’ Block II had several developmental series, including Block IIA, IIR, IIR-M and IIF. Each set of satellites built upon the previous designs and capabilities, culminating in Block III. This third generation of GPS satellites begins with Block IIIA series’ new signals and higher broadcasting power. The first IIIA satellite of 10 was launched in 2018.

What does GPS stand for?

GPS stands for Global Positioning System. It’s also often used to describe the positioning system itself, for example, your vehicle’s built-in GPS.

How does GPS work?

Like many other GNSS constellations, GPS includes three main segments: the space segment, control segment and user segment.

The GPS space segment includes over 30 satellites in orbit operated and maintained by the U.S. Space Force. These satellites broadcast radio signals to control and monitoring stations on Earth and directly to users requiring highly precise satellite positioning.

The U.S. Space Force also oversees the GPS control segment. It includes master control and backup control stations, dedicated ground antennas and several monitor stations located worldwide. These stations work to ensure GPS satellites are healthy, orbiting in the correct locations and have accurate atomic clocks on board. These stations are integral to the overall health and accuracy of the GPS constellation.

The user segment includes everyone relying upon GPS satellites for PNT measurements. From a mobile phone providing directions to autonomous vehicles requiring lane-level positioning accuracy; from a farmer tracking planting and harvesting routes year-over-year to a UAV mapping a rainforest, many applications use GPS for high precision positioning and accuracy around the world.

What are GPS satellite signals?

Satellites are continually broadcasting their orbital position and exact time at that position on radio frequencies. That signal is received by antennas, along with at least three other satellite signals, then processed in a GPS receiver to compute a user’s location.

GPS broadcasts on L1 (1575.42 MHz), L2 (1227.60 MHz) and L5 (1176.45 MHz) civilian frequencies; GPS also broadcasts on L3 (1381.05 MHz) and L4 (1379.913 MHz) for governmental and regional satellite-based augmentation systems (SBAS). Several satellites also broadcast M-code, a military code carried on the L1 and L2 frequencies designed for exclusive use by the U.S. military.

What is M-code?

M-code is a GPS-specific signal broadcast to support the United States Department of Defense. This signal was first broadcast with the launch of the Block IIR-M satellite in 2005. M-code provides a layer of defense against jamming interference through 21 M-code-capable GPS satellites.

M-code broadcasts on the existing GPS L1 and L2 L-Band frequencies but is modulated to not interfere with L1/L2 signals. Military receivers can compute PNT through M-code alone. Further, military applications use M-code to increase power to L1 and L2 signals to build resilience against interference, jamming and spoofing incidents. GPS signals are still susceptible to jamming, but M-code provides a layer of defense against such interference. There are many additional layers of anti-jamming defenses critical to establishing assured PNT on GPS systems.

GPS accuracy

A positioning system is only as good as its processor. A high-precision GPS receiver will be far more accurate than a mobile phone, for example. Potential sources of errors are identified and modeled at monitoring and control stations to optimize accuracy.

Most errors come from clock errors, orbital drift, atmospheric and multipath delays and radio frequency interference. These sources constantly threaten positioning, navigation and timing accuracy by contributing to geometric dilution of precision.

Some technologies help mitigate dilution of precision and these errors, including subscriptions to GNSS/GPS correction services, SBAS and the fusion of additional sensors like inertial navigation systems or radar. More precise GPS receivers also help mitigate errors through different algorithms by computing a position through pseudorange or carrier wave calculations.

We explain more about how to mitigate errors in both episode three and episode four of our Introduction to GNSS webinar series.

GPS vs. GNSS: What is the difference?

GNSS is a way of describing every satellite constellation in orbit; GPS is one of several constellations making up GNSS. From GPS to GLONASS (operated by Roscosmos State Corporation for Space Activities in Russia), many constellations make up GNSS. Positioning technology relies on many different constellations to provide accurate and reliable PNT. Instead of GNSS vs. GPS, a better way to consider these technologies is how GPS compares to other GNSS constellations.

We compare GPS to other constellations like GLONASS, BeiDou and Galileo in our article, What is GNSS.

Applications of GPS

GPS supports applications around the world relying on satellite technology for assured positioning, navigation and timing measurements. These applications differ by industry, but the use of GPS is based on their need for a precise position, reliable and safe navigation, tracking and monitoring an object’s movement, surveying and mapping of an area, or timing within a billionth of a second.

For example, mining applications rely on GPS to survey an area before beginning operations. Companies track potential mineral deposits, identify which areas to avoid to lessen their environmental impact and enable autonomous machinery transporting minerals across the site.

Applications requiring high-precision positioning use GPS alongside other constellations. However, because of its encrypted M-code signal, the U.S. military relies on GPS in a unique way. M-code enables the military to secure continual access to positioning and build resiliency to potential jamming and interference sources.

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viswatech · 3 years ago