The ability to determine one’s location anywhere on Earth is truly a technological marvel of our modern age. It has enabled countless location based services, which have become so pervasive in our daily lives, that the underlying technology is rarely given any thought. So, let’s take a minute to understand the history of global navigation satellite systems, as a basis for a global reference network.
While not the first, the most recognizable global navigation satellite system (GNSS) is the Global Positioning System (GPS), a project established in 1973 by the United States Department of Defense. This network of satellites (32 today) orbit the Earth while broadcasting navigation messages toward the planet. These navigation messages are comprised of the following:
- The clock status of the satellite, as well as its individual health/status.
- Precise orbital information of the satellite, called the ephemeris.
- Coarse orbit and status information of all satellites in the constellation, called the almanac.
Despite the slow transmission rate by today’s standards (50 bit/s), these messages travel at the speed of light on the radio spectrum, reaching the surface of the planet in under 7/100ths of a second. From the ground, a receiver with line-of-sight to the satellites, performs calculations based on the reported locations of the visible satellites and determines its location via a process of trilateration and clock validation. While the applications that this technology has enabled are virtually limitless, from missile guidance to the future of self-driving cars, there are limitations to underlying technology itself.
As unimpeded line-of-sight to the overhead satellites is required in order to receive the navigation messages, getting a location fix in obstructed areas like skyscraper laden city streets can be time consuming and frustrating to the end user. To supplement traditional GNSS, a range of assistance protocols were created to improve the time to first fix (TTFF) and enhance the ongoing location experience; this is where a global reference network comes in to play.
So what is a global reference network?
- global /ˈɡləʊb(ə)l/ Relating to the whole world; worldwide.
- reference /ˈrɛf(ə)r(ə)ns/ The use of a source of information in order to ascertain something.
- network /ˈnɛtwəːk/ A group or system of interconnected people or things.
With a little bit of help from our friends at the Oxford English Dictionary, a global reference network is a group of interconnected GNSS ground stations, distributed around the globe which are used to ascertain the location of GNSS satellites. In contrast to an end-user device, which uses the navigation messages to determine its own location in relation to the satellites, a ground station’s primary function is to determine the location of each satellite that it can see overhead. Given enough globally distributed ground stations, all satellites in a GNSS constellation can be observed in their orbits in real-time.
Each one of the GRN sites are capable of relaying the satellite navigation messages to centralized data centers which can be distributed to customers across a range of verticals, including chipset manufacturers, OEM’s, telecommunications providers and location based service suppliers. This data can be structured in multiple different ways, from a real-time broadcast ephemeris stream, to processed predictions known as extended ephemeris, which calculate the location of the satellites in space and time for weeks into the future.
Given that mobile devices are capable of receiving information via the data network at hundreds of thousands of times the satellite transmission rate of 50 bit/s, this assistance data can be continually updated in the background to enhance the core GNSS layer. This results in a significantly faster TTFF compared to traditional GNSS, as well as the ability to receive a location fix in obstructed environments where non-assisted devices might never be able to determine their location.
The Rx Networks global reference network is capable of supporting a range of global and regional constellations, including:
- GPS – United States
- GLONASS – Russia
- BeiDou / BeiDou-2 – China
- Galileo – European Union
- QZSS – Japan
With the data derived from the global reference network, Rx Networks offers two distinct product lines, under the GPStream banner, GRN and PGPS. GPStream GRN is offered as a real-time stream or request based broadcast ephemeris service, which is used extensively in the E-911 space. As such, the service takes advantage of multiple levels of redundancy and is backed by a carrier grade, 99.999% SLA uptime guarantee. GPStream PGPS is Rx Networks’ prediction product, which enables mobile devices to generate high accuracy extended ephemeris for weeks into the future, without the need for continual access to a data network. This empowers devices to receive significantly faster and more accurate fixes in obstructed locations while roaming or in areas without cellular coverage.
In the next issue of Points of Interest, I will be discussing the logistics of designing and deploying a global reference network, starting with the base set of requirements:
- Political stability – Is it safe to make an investment in the country which will be hosting the site?
- Power/network stability – Will the site be highly available? Can near real-time data be assured?
- Unobstructed views of the horizon – Will the site be able to see all available satellites in the sky?