GPS Satellite Communication, Research Paper Example

The Global Positional System (GPS) refers to a space-based satellite system which is used to provide information about locations and time. This system works under all conditions, either on earth or near it so long as there are four or more unhindered satellites with regard to the line of sight. This system provides vital capabilities to the civil, military, commercial and civilian users across the globe. The GPS system is maintained by the government of the United States of America (Elbert & Bruce, 2007). However, the system is readily available to all people with a GPS receiver. Presently, there are about 30 or so networks satellites that are orbiting the earth. They are orbiting at an altitude of 20,000km. Initially, the US government developed the system for purposes of military navigation. Nonetheless, the system has achieved an extensive application in a number of platforms and now anyone that has a GPS device (e.g. mobile phones, SatNav or Handleheld GPS unit) can be able to access the radio signals broadcasted by the satellites.

Anywhere and anytime on the surface of the earth, there is a minimum of four visible satellites. The signals from the satellites are travelling at the speed of light and each of the satellites conveys information about the current time (at regular intervals) and its position. The GPS receivers intercept the signals from the satellites and calculate the distance from the satellites and the time the message took to arrive on the earth. After gathering enough information about the distance between at least three satellites, the receiver can be able to pin down your location by use of the trilateration process.

Technological Evolution

LORAN and Decca navigator (developed in 1940s) are some of the foundational radio-navigation systems of GPS. The larger part of 1960s saw the U.S Air Force and Navy working on a number of systems which would provide navigational capabilities for various reasons. Nevertheless, many of the systems were incompatible with each other. This called for a move by the Department of defense to demand that all services unify their systems. The resulting system would be called the Navstar Global Positioning System. Atomic clocks carried on the satellites would be the basis of the new system. The concept had been successfully tested early by the TIMATION (a navy program). It was the Air Force that would be responsible for operating this new system. From then on, the system has since come to be simply referred to as the GPS. The new system comprised of three elements namely: the ground stations (controls the system); satellite constellation in earth’s orbit; and receivers which are carried by users.

GPS satellites were first launched in the year 1978. Thereafter, a second generation (Block II) satellites began to be launched starting from 1989. Currently, GPS constellation comprise of a minimum of 24 Block II satellites. The Navstar GPS became exclusively functional in the year 1995. One of the earliest portable GPS units is the PSN-8 (manpack) GPS receivers. It is estimated that 1,400 of these receivers were manufactured during the period of 1988-1993. In the year 1993, the manpack was replaced by the handy Precision Lightweight GPS Receiver (PLGR). This tool was popularly referred to as the plugger. Though similar to the civilian receivers, they can be able to utilize high precision GPS signals. Initially, GPS was designed in such a way that civilian users could not gain same precision as compared to the military even though the civilian applications were also intended.

President Ronald Reagan issued a directive in 1983 that allowed the availability of GPS signals to the entire world with no charge following the downing of a Korean flight. This opened up the commercial market. In 1990, there was a steady deployment of GPS satellites as a result of the increasing number of military and civilian users. The Persian Gulf War of 1991 extensively increased the civilian use of GPS so much such that there were no sufficient receivers for the military. The Department of Defense thus moved in to acquire civilian units to deal with this demand. It also changed the GPS transmissions giving the civilian receivers access to high precision signals meant for the military.

How GPS works

Determining the position

GPS receivers passively receive the signals transmitted by the GPS satellites. These receivers do not transmit any signal and require unbarred view of the sky. The GPS functioning depends on the accuracy of the time reference that is provided by the atomic clocks that are on board the satellites. The signals which move at the speed of light arrive at the receiver at somewhat different time because of the distance between the satellites. Once the receiver has information about how far at least three satellites are from each other, it ascertains the location on the surface of the earth by use of the trilateration process. It does this by establishing the point of overlap of the different imaginary spheres that are centered at each satellite. The point of overlap is the location on the earth. It therefore means that the more the satellites in the line of sight, the more accurate the prediction about the location. Additionally, the GPS receiver is aware of the satellites’ location since the information is contained within the Ephemeris data that is transmitted.

GPS Accuracy

The type of the receiver is the one of the factors that determines the accuracy in prediction of the positions. The normal user GPS units has a + or – 10 meters accuracy. Some receivers use Differential GPS units (DGPS) thus gaining higher accuracy. This requires an extra receiver which is fixed at a nearby location. The stationary receiver makes observations that are used to correct the locations that are recorded by the roaming units. This provides a precision that is higher than that of a single meter.

Timing of signals

The satellites have a number of atomic clocks. The signals are sent in a random sequence repeatedly (each part of the signals is different). The receivers recognize these sequences and repeat them internally (Miller & Michael, 2003). After picking the satellite transmission signals, the receiver compares it with its internal signals. Travel time becomes known after the receiver has compared the degree of lagging of the satellite signal.

Satellite Signals

There are two GPS radio signals i.e. L1 and L2. The civilians use L1 (1575.42) signal frequency in the UHF band. The signals will travel in the line of sight but not through solid objects. The signal has three dissimilar bits of information.

i. Pseudo random code

This is basically an I.D code which identifies the satellite that is transmitting the signal. It is attached to the signal bar.

ii. Almanac data

It describes the orbit pathways of satellites. All satellites broadcasts the almanac data each satellite. The receivers use this information to find out the satellite that it expects to observe in the local sky.

iii. Ephemeris data

This information tells the receiver about the location of the satellites at specific times of the day. All of the satellites broadcast their own ephemeris data showing their orbital information. It has a shorter validity due the fact that it relies on very precise orbital positioning that is continuously changing. It is only considered valid up to four (4) hours.

Factory, Cold and warm Starts

In most cases, manufacturers and reviews will denote to factory, warm and cold start times. They are explained as follows;

a. Factory start

All data (i.e. almanac and ephemeris) are considered invalid

b. Cold start

Ephemeris data has expired or is not current whereas the almanac data is current

c. Warm start

Both the almanac and ephemeris data is current

The position velocity time (PVT) solution by a receiver is achieved by estimating the roughly location of a satellite provided that the almanac is current. It then starts to download the ephemeris data of the satellites it finds. After receiving the data from three satellites, the receiver then starts to calculate the position fix. Re-location from the initial position while trying to find the position using a receiver will initiate a factory start and therefore begin afresh the attempt to download the almanac and ephemeris data.

QuickFix files are part of the solution for GPS chips. They contain specially prepared ephemeris data valid for a period of 7 days. Therefore the GPS chip uses this data instead of downloading it from the satellite. A valid QuickFix allows a device to calculate your position in about 5-15 seconds as compared to the minute it would have taken if downloading of the data was to be involved. It also lowers the signal strength demand of calculating the location to 15 dHBHz (from 28 dBHz). However, you have to be stationary.

Application of GPS

The most common application that uses GPS is Google Earth. It helps the public to get access to the maps that tell them about various locations all around the world. People use GPS for a number of reasons. Recent research has identified the use of GPS in the following areas: military (1%); vessel voyage (2%); manufacturing (7%); survey (7%); GIS (8%); tracking (10%); hand held (26%); and car navigation (37%).

Navigation is the most common use of GPS. Various software applications such as the DeLorme Street Atlas 2010 (used for trip planning, mapping and navigation) and Gramin nRoute (used by laptop owners to make work easy with the use of GPS maps) are some of the examples. Tracking is also another use of GPS (Elbert & Bruce, 2007). It has quite a number of uses including car tracking. For instance the Locimobile applications available to iPhone users are used for tracking people. The Auto Pilot application is another software that allows a vehicle to travel automatically using the GPS maps. Engineers and scientists are also using GPS receiver applications to monitor things such as tremors and earthquakes as well as in scientific experiments. At present, there are some devices that have been strategically to help track climate change.

Advantages and Disadvantages of GPS

Advantages

I. GPS is able to work in all weather

II. Compared to other navigation systems, GPS relatively cost saving

III. It has a 100% coverage of the whole planet

IV. Accuracy varies from millimeters to a number of meters based on the device used

V. Its low cost makes integration into other technologies (e.g. phones) easier

VI. It is regularly maintained as compared to other systems such as the GLONASS

Disadvantages

i. The signals are weak making them not to work well in indoors under trees or underwater as compared to other signals such as those of cellular phones

ii. Because high accuracy demands a line of sight to the satellite, usage in urban areas becomes problematic.

iii. The U.S Department of Defense can decide to deny the user access to the GPS at any time by either degrading or shutting down the satellites. 

Competing Systems

1. Russia’s GLONASS

Russia is trying to develop its navigation system which is known as GLONASS. The system is already working but within the country alone. They aim to expand the system to the all regions of the world after the completion of the remaining two satellites (already 22 out of the required 24 have been built). Russia intends to unveil better satellites that will double the precision of GLONASS.

2. European Union’s Galileo

This navigation system is specifically for public use unlike the GPS which was initially meant for military use. It has four satellites already in orbit Plans are underway to have 30 operational satellites in orbit by 2014.

3. India’s GAGAN

This is a regional navigation system. It was only built for aircraft navigation.

4. China’s COMPASS

This global positioning system that was initially restricted to the Chinese government and its military has finally been made available for commercial use. It functions like the GPS but the receiver chips for this system are a lot higher than that of the GPS. The system can locate the user’s position to about 10 meters, their speed to within 20cm/s and is bale to synchronize the space and earth clocks to in the interior of 50 nanoseconds (Miller & Michael, 2003).

The future of the GPS application

Because the GPS application has been widely available, there is a possibility of a number of conceivable further implementations (Elbert & Bruce, 2007). For instance, GPS can provide a solution to the issue of deforestation. This is due to the fact that the density of the green plants in specific areas cab be estimated or precisely calculated. This can be useful in calculating the deforestation rate (by comparison and analysis). As a result the forest officials can focus on areas that are greatly affected. Another field that might benefit is farming. GPS can help with the exploration of idle land and therefore let the people identify empty land that can be used for cultivation. This will improve farm production (Elbert & Bruce, 2007).

Currently, the operations of GPS have not been fully developed. Therefore, there is room for further developments of advanced features. For instance, development of cameras that are more sensitive can assist in identifying illegal plantations of marijuana. The developers of the GPS systems can have a much more advanced API for better outcomes.

 Reference

Gerard Maral,Michel Bousquet, “Satellite Communication System: Systems, Techniques &Technology”, John Wiley & Sons, Incorporated, 2003

Wood, James, Satellite communications and DBS systems. Boston: Focal Press, 2002.

Elbert, Bruce R, The satellite communication applications handbook, Boston, MA: Artech House, 2007.

Miller, Michael J., Vucetic, Branka., Satellite communications : mobile and fixed services, Boston: Kluwer Academic Publishers, 2003