Here the speed of the signal is not assumed to be equal to speed of light! This is because the signal experiences slowing down when it pass through ionosphere and troposphere. In addition, when signal enters these two layers, it also undergoes refraction and therefore the signal actually travels longer distances on account of this. The receiver factors in all of these during computation. By knowing the exact location of the satellites and the distance to these satellites, the receivers can then compute its location.
Satellites use highly accurate onboard atomic clocks which is periodically synchronized to more accurate clock in ground station, to compute the distance from satellites whose precise location in space is known. Satellites also continuously send out two set of data – Ephemeris and Almanac. Almanac data is course orbital parameters for all Satellites. Each satellite broadcasts Almanac data for ALL satellites. This Almanac data is not very precise and is considered valid for up to several months. Ephemeris data by comparison is very precise orbital and clock correction for each satellites and is necessary for precise positioning. The ephemeris data is information that enables the precise orbit of the GPS satellite to be calculated. Ephemeris also carries current time. Each satellite broadcasts only its own Ephemeris data. This data is only considered valid for about 30 minutes. The Ephemeris data is broadcast by each satellite every 30 seconds.
There are few additional information that you should know about GPS. GPS receivers are about the best RF receivers out there! That’s because, they receive and decode signals that are much below the noise floor!! The noise floor at the receiver input with 2MHz bandwidth (GPS signal bandwidth) is around -111dBm (computed as kTB, where K is Boltzmann constant, T is temperature and B is bandwidth). The typical signal levels of the received GPS signals are in the vicinity of -127dBm and receivers are designed to receive signals at levels all the way down to -160dBm!! This is about 50dB (100,000 times) lower than the noise floor!! Signal levels are low because they are traveling over long distances (~20,000Kms) and the transmit power at the satellite is restricted because of limitation in available power at the satellite. At such low signal levels, the receivers are hampered by multipath fading (interferences due to GPS signals bouncing off reflecting surfaces like buildings, vehicles, etc) and obstructions like buildings, tree cover, bridges, flyovers, etc…
To conserve power and bandwidth, the signal follows low bit rate and therefore takes long to transfer a bunch of data. Due to low receive signal strength and low bit rate, the receivers take long to get the first fix. TTFF (Time To First Fix) is the time taken for the receiver during turn ON to receive the satellite signals and compute the location. TTFF during “cold start” is the time taken when a receiver starts receiving signal from the satellites afresh (either there are no stored data or the stored data are not valid) and compute the location. TTFF during cold start can last anywhere from 40 seconds to several minutes, depending on how many satellites are visible and the signal strength of the received signals. However, receivers save the location and last received data (pertaining to Ephemeris and Almanac) before turn off and use the same data during their next turn ON. They can then cut down the TTFF, provided all of the stored data are still valid (Ephemeris data is updated once every 2 hours and is valid for upto 4 hours) and the satellites which are visible has not changed much since the last reception. In such cases, typically referred to as “hot start”, the TTF can be significantly low – in the order of few seconds if the location from the last stored value has also not changed much.
Under some other conditions referred to as “warm start”, if part of the stored data (last location and almanac if it is still valid), the receiver has a general idea of which satellites to look for because it knows its last position and the almanac data helps identify which satellites are visible in the sky. In these cases, it takes longer than a hot start but not as long as a cold start.
AGPS or Assisted-GPS is a technique to improve TTFF, the cell phone manufacturers and mobile operators have introduced, which allows the smart phones to download Ephemeris and Almanac data via cellular network and get a rough location information through triangulation of signals from cellular towers. Instead of receiving these data over slow rate GPS signals, relatively higher speed wireless data network can deliver the data quite quickly so that the receiver can compute the location quickly.
Having said all of these, I have to admit that there’s more to understanding GPS than these. These are just the basics covering earlier generation of GPS signals handled by Block II, IIA, IIR and IIR-M series of satellites. Newer generation satellites Block IIF have additional signal L5 at 1176MHz which make it compatible with other GNSS systems and also the signals are transmitted at higher power.
This article is not complete without discussing our own indigenous IRNSS from ISRO. Indian Regional Navigation Satellite System (IRNSS), an independent navigation satellite system consisting of 3 satellites in GEO (Geostationary) orbit and 4 satellites in GSO (Geosynchronous) orbit, approximately 36,000 km above earth surface. The nation saw the last of the 7 satellites successfully launched and parked at the designated spot late April 2016. With this, the nation is self-reliant and does not have to depend on any one else for precision signals and accurate location data. During the Kargil war, when it was denied the GPS data of the locality, India committed to develop an indigenous navigation satellite system and IRNSS is the outcome. With this, we can be proud that India is the 5th nation to have its own (G)NSS after the US, Russia, China and European Union. Although IRNSS is not global in coverage, it does well for the region covering India and its immediate neighboring countries. India will no more be arm-twisted into submission!!
Now that we know what GPS is and how it works, we could now look at various uses of GPS. Most of us can quite easily relate to navigation, which was the first application of GPS – i.e. to help you get to your target location. There are scores of navigation devices – vehicle mounted, hand held, etc… Most smart phone offer navigation help. However, this requires map data – either locally stored data or online data. If you are connected over 3G or 4G data plan on your mobile, you can use online Google map to navigate. In the recent past, Google has provided an option to store google map data of an area that is of interest and of reasonable size, in which case you could do without data connectivity to the google server. There also other free map data available which you can download store on your smart phone, like Here Maps. Your navigation device can also help with “Point of interest” – guide you to the nearest point of interest – a filling station, a restaurant, a hospital, an ATM, etc…
Another application of GPS is Telematics – monitoring and tracking your vehicle (or your goods). If you are a vehicle owner or a fleet owner, you can monitor and track the location of your vehicle/s at any point of time. The vehicle would then have an On-board unit (OBU) with a GPS receiver and GSM/3G/4G transmitter, which will continuously transmit the GPS location of the vehicle to a predetermined server. You can then trace the route on the map the vehicle has taken and you would know if it has deviated from its original plan. With Geo-fencing, you can get an alert soon as the vehicle breaches its intended route or geographical area. E-Call feature will help crash victims by providing timely assistance. The OBU, on sensing a crash will send out the last GPS coordinates of the vehicle to a server, following which either a telephone operator can render support over a return voice call or automatic dispatch of an ambulance to the location of the crash. A vehicle mount or a ship mount container can also be tracked with a battery powered OBU fitted to the container.
In addition to vehicles and goods, it is possible to monitor and track people – old and the weak, senior citizens with dementia, visually impaired, children and even pets through wearable tracking devices having GPS receivers and Wireless transmitters. Wildlife officials can monitor and track movement of animals with radio collars having similar tracking devices. In farming, harvesting machines and planting machines working on huge farms / plantations, on auto-pilot mode use GPS coordinates to guide them through every piece of land for their operations. The much talked about driverless cars would depend on GPS data for their basic operation – getting the car to the target location. Taxis use GPS for locating customers and to get them to their destination.
But of course, we should not forget their use in military, which incidentally was the main motive for developing the GPS– delivering the weapons and warheads to the intended targets precisely. Lastly, GPS has been guiding aircrafts in all weather conditions during flight and difficult terrains (among the mountains and on islands) during landing which benefits hugely from precise GPS location information.
In addition to location, GPS can also be used for maintaining precise time. Timing accuracy in the league of nano-seconds can be achieved with ~95% probability using GPS signals. Communication networks use GPS time for synchronization. For example – Communication networks following TDMA (Time division multiple access) use GPS precise timing information to synchronize nodes in the network. Precise time information is used to synchronize and switch in power into electrical grids. The Train Collision Avoidance System use GPS as one of the inputs for getting accurate time, to compute speed of the train and also location information; to detect if two trains are in collision path and automatically realize braking operation in one or both the trains to prevent collisions and to save scores of lives.
All the usages and applications that I have listed above are only some very obvious ones. GPS receivers are used in many more applications. However, kindly keep in mind that, in this context, I have used GPS to refer to GNSS. In fact, many GPS receivers use other already deployed GNSS signals (GloNASS and or BeiDou) to improve the performance. They seem to impact all parts of our lives – from improved lifestyle to emergency situations. Therefore, GPS or GNSS receivers are no more a luxury, but an essential part of our lives.