FIGURE 1 Today, GPS has approximately 31 satellites in medium Earth orbit. GPS is augmented by seven satellites in geostationary orbit. Source: Adapted from van Diggelen, 2009. Reprinted courtesy Frank van Diggelen.
For example, every new Boeing or Airbus aircraft carries a GPS receiver for navigation in the enroute and terminal area airspace. GPS is also used to guide aircraft while approaching airports. In some cases, it provides the critical vertical dimension of location down to altitudes of 200 feet.
In addition, most new smart phones or feature phones carry GPS receivers that have a bill of materials around $1. These receivers are used to guide our walking and driving lives. They also provide our location automatically to emergency services when we make such a call. In time, they will provide our location to good Samaritans standing next to automatic electronic defibrillators when someone suffers a heart attack. GPS receivers in cell phones will also serve significantly less uplifting applications that push advertising to our phones based on our location.
Other applications include ship navigation, pointing information for spacecraft, land survey, energy exploration; and time transfer for telecommunications, power delivery, and financial transactions.
Attracted by this utility, Russia is rejuvenating its satellite navigation system, called GLONASS. China is expanding their regional system, BeiDou (also known as Compass), to include global coverage. Europe has launched its first prototype satellites for the Galileo system. Japan and India have launched satellites for the regional systems. Figure 2 depicts a satellite navigation future based on more than 100 satellites from this family of systems. Taken together, we will have a Global Navigation Satellite System (GNSS).
The next section describes the new signals to be delivered by this multiplicity of constellations. It will be followed by a focus on my area of greatest interest: