Radio navigation is the process of determining one’s position by use of radio signals. Initially, this was only of interest to ships (and later, to airplanes). The radio signals would provide:
- Angular position to the transmitter (by signal strength and interferometry)
- Distance to the transmitter (by time required for the radio signal to reach one’s position
- Speed (by ‘Doppler’ -phase- shift of received signal)
The most basic system (and first) was a Radio Direction Finder (RDF). Using the angular direction of two different radio navigation signals, a location could be determined by simple triangulation; determining one’s position by comparing the direction of two known signal sources.
A variation of this technique is VHF Omni-directional Range (VOR), which uses a radio ‘beacon’ modulated with two sub-carriers; one as a Station ID and the other a directional carrier modulated with a 30Hz reference. By comparing phase of the 30Hz signals, angular direction is obtained (and thus heading, on a compass scale). VOR is used to provide aircraft ILS (Instrument Landing System) capability, to aid in radio-directed landing. VOR landing began in the 1940s, and is still used today.
Distance Measuring Equipment (DME) is typically used in conjunction with VOR, and provides a distance reading (in addition to VOR’s angular reading). An aircraft broadcasts a series of pulses, which a ground-based transponder returns (after a delay). Simple time delay measurement results in a fairly accurate distance measurement. VOR/DME systems provide direction (to an airport, for example) via VOR and distance (via DME).
LORAN-C (LOng-Range Aid to Navigation) was used for determining shipboard (and eventually aircraft) location. A series of coastal stations broadcast a low frequency (and thus long-distance) signal, sending pulses modulated with an AM (Amplitude Modulation) sub-carrier. A basic direction and range was obtained by interferometry, and more accurate location by measuring phase differences of the signals. The USCG (United States Coast Guard) managed a network of LORAN-C stations around the world, with other countries adding compatible stations to augment the system. As electronics became cheaper and more compact, most ships began to carry LORAN-C receivers for accurate position information.
About this time, something better came along…
With the advent of the US Navy’s constellation of Global Positioning System (GPS) satellites, truly accurate positioning (almost worldwide) became available. By acquiring the signals of at least four GPS satellites, one could determine position with a high degree of accuracy. The more satellites, the more accurate the position information.
This system was originally reserved for US military only, but eventually became the worldwide standard for navigation, with civilian systems using GPS receivers which demodulated the non-secure component of the GPS signal.
The signal is composed of satellite ephemeris data (showing the satellite position and movement), while another time signal helps derive distance to the satellite. Once a number of satellites are acquired, simple triangulation determines location on the Earth’s surface.
The constellation of GPS satellites (originally 24, currently 32, with 27 active at any given time) ensures this triangulation can occur. The satellites are in a MEO (Medium Earth Orbit) of about 12,500 mile altitude, and rotate around the Earth twice in a sidereal day. This meant that (with the original constellation) at least seven satellites were ‘in view’ of any point on the Earth, at any given time (there could be more). With the new constellation, the minimum satellites in view has increased to 9 (thus increasing accuracy even further).
The GPS satellites and constellation (as well as Earth-based receivers) have undergone continual improvements, with GPS III scheduled for implementation soon (with centimeter position accuracy).
One additional benefit of GPS is the propagation of accurate UTC (Universal Coordinated Time). Each satellite has an on-board Cesium Beam Frequency Standard (an ‘atomic clock’). These clocks are synchronized to the Naval Observatory main clock which is in turn synchronized to the National Bureau of Standards (NBS) main clock in Boulder, Colorado. That clock is synchronized to the Royal Naval Observatory clock, for a stable time hierarchy, compensated for ephemeral and sidereal variations
Each cesium clock is based on the decay of the cesium-32 atom, which just happens to be a multiple of sixty (on which our time is based). As accurate time is required for accurate navigation, this means GPS serves two functions: position location services, and accurate, synchronized UTC time distribution. (See my upcoming “Time and Timing Primer” for more information on UTC distribution and synchronization).
In addition to the GPS constellation, the Russian GLONASS (Global Navigation Satellite System) and two others from the EU (European Union) are modeled on the GPS system and architecture, and serve to augment the GPS signals. (Communist China plans to launch its own fleet this year (2020). Modern timing receivers (small micro-chips) typically receive both GPS and GLONASS, for increased accuracy and reliability (in case one system is ‘down’ or obstructed, or if Selective Availability (SA) functionality makes the GPS timing available only to the US military.
Wikipedia actually has a pretty good entry on GPS. For those who want to know more, look here: https://en.wikipedia.org/wiki/Global_Positioning_System
You might wonder why all this matters, and what it has to do with the ‘drone world’. Well, with modern wireless communications (for example, from your remote controller to your UAV), precise timing is critical. With modern gyro-compasses and inertial management systems (IMS) in aircraft, precise location information is required.
To enable a true 5-G (Fifth Generation) wireless network (and the data rates it promises), precise timing is critical. To allow seamless communications between wireless and land-based wire-line networks (as well as virtual networks like the ‘internet’), precise and synchronized timing is fundamental.
To summarize; if you want your drone to be able to find itself in space (or you to find it if it is lost), you need modern radio navigation (e.g., GPS). If you want it to communicate with your remote controller in an error-free fashion, you need GPS timing. If you want to have a hope in heck of implementing (or communicating over) a 5-G wireless network, or enabling mandated Remote ID and Tracking of UAS, then you need GPS.
That’s why it matters.
A Brave New Drone World 