Uncorrected: Uncorrected, GPS/GNSS is quite imprecise, and can take time to come up with a position. GNSS does ranging using the satellite signals. The simplest and least expensive GNSS receivers, like those in phones, early recreational handheld units (without any augmentation turned on), can get the basic information directly from the GNSS satellites, and yield positions within a few meters.
But there are a lot of sources of error in that equation. Most of the augmentation approaches concentrate on addressing these four key sources of error: clock, orbit, iono, and tropo. And there can be a delay in getting to the best position a receiver could be expected to achieve. This is known as “time to first fix” TTFF. Once a receiver “knows” its rough location, subsequent positions can be derived much more quickly. But if you walk out of the building with the tiny GPS chip in your phone, and if there is no augmentation, you may notice that your position is wildly off for a short while.
Pluses: Free
Negatives: Imprecise, slow TTFF
Expected precision: 1m–20m
Cellular Carrier Augmentations: There is great value in at least having positions for phones being within a few meters, not only for personal navigation, commercial applications, and especially public safety, but also in getting to those precisions rapidly. For example, the Federal Communications Commission (FCC) of the US has mandated that all cell phones sold must be capable of using a simple augmentation that is known as “AGPS” (assisted or augmented GPS). This to overcome the TTFF delay, so that semi-precise locations are available more rapidly for public safety responses. There are similar systems in many countries.
Pluses: May be included with your cellular service, improves TTFF
Negatives: Imprecise, requires a cellular service
Expected precision: 1m–10m
SBAS: Satellite Based Augmentation Systems gather observation data from regional, nationwide, or even continental networks of continuously operating GNSS stations. Together with models of iono and tropo data (depending on the system), the “correction” data is broadcast to end users from dedicated GEO satellites (that will maintain a location above a specific region as the Earth rotates).
For instance, in North America, the Wide Area Augmentation System (WAAS), was chartered by the Federal Aviation Administration (FAA). In Europe, the equivalent is EGNOS, GAGAN in India, MSAS and QZSS in Japan, SDCM of the Russian Federation, SNAS/BDSAS in development in China, SPAN for Australia and New Zealand, and others in development for South America and the Caribbean (SACCSA), KASS (Korea), and an initiative for SBAS for Africa and the Indian Ocean (ASECNA).
Pluses: Civilian SBAS is free
Negatives: SBAS satellites must be in view, hardware must have such systems enabled
Expected precision: 1m–5m (sometimes better depending on the respective system)
Broadcast DGPS: Early applications, like marine navigation, took advantage of GPS for applications like harbor approaches. Having previously relied on sextants, compass, and certain radio beacon solutions, the initial 10-meter precision of early uncorrected GPS was a game changer. Soon after the first differential solutions were delivered from fixed GPS bases and companion dedicated coastal radio towers to ships. Commonly referred to as DGPS, networks of such towers were also sometimes established inland to support river navigation and low-precision terrestrial applications like mapping.
Pluses: Typically, free broadcast services
Negatives: Requires compatible radio receiver. Many DGPS systems being phased out
Expected precision: 1m–3m (sometimes better depending on the respective system)