Positioning systems have been in demand for thousands years to aide in all facets of society from the religious practices of the Egyptians to construct large monuments oriented on the cardinal axes and the constructors of Stonehenge celebrating the summer equinox to the scientific practices of land surveying and topographic studies. With the advent of the Global Positioning System (GPS) and other similar systems only now can these questions of space and time be answered.
Some of the first substantial surveying techniques arose in ancient Egypt with their erection of structures along the banks of the Nile River. Through archeology it has been discovered that the Egyptians used control points to replace property corners destroyed by the flooding Nile. These fixed control points were usually mapped to the reoccurring position of celestial bodies in the sky. The periodicity of the Nile flood was reflected by the periodic position of stars in a fixed calendar.1
The more accurate surveying techniques of triangulation, trilateration, and traversing were first developed by the Dutch scientist Willebrord van Roijen Snell when he used interior angles of triangles and backlines to determine the relative position of distant points.2 After further development through the work of French surveyors Picard and Cassini, triangulation became the main means of determining accurate coordinates over continental distances. However the use of surveying techniques for positioning is severely limited by the need for a clear and direct line of sight. To resolve this shortcoming, surveyors climbed to mountain tops and developed special survey towers, fixed by astronomical points, to extend the line of sight if only by small distances.1
Global Positioning Techniques
When the Earth first began being explored, sailors used celestial navigation to determine their position on earth. Using this method, angles were measured using simple devices called sextants to find the position of the sun and moon above the horizon. This angle relates to the bodies' subpoints which correspond with lines of position. The lines of position, or LOP, are then drawn on a map. The place where two celestial bodies' lines of position cross would be a fairly accurate representation of the location of the person on the earth.
Hundreds of years later navigation made a breakthrough with the invention and use of radio navigation. Using this new technology a person was able to tune a common radio to a certain station then use a directional antenna to find the direction (angle) of the source. Using geometry and basic trigonometry, the angles from two separate points can be used to determine the distance of the source.
Actual satellite positioning began in 1957 with the Soviet's launch of Sputnik, the first artificial satellite to orbit the Earth. The first use of satellite positioning employed an optical method system, BC-4, named after the camera used by the satellite that relied on stellar triangulation. However the main limitation of using an optical method was the necessity of a clear sky for accuracy.2 This drawback of optical methods necessitated the development of electromagnetic methods of positioning, most commonly relying on the fundamentals of the Doppler Shift. Using the change in frequency from the radio signal Sputnik transmitted, researchers were able to determine Sputnik's position. Soon after, it was discovered that inversely, if the position of the satellite was known, any thing's position on earth could be determined.34
At the time, all satellite positioning systems were still only dealing in two dimensions, longitude and latitude, which was all that was necessary for their current use within the Navy. The Air-Force required the third dimension, altitude, and developed a system called 621B to fit their needs. The main flaw with this system was that it needed constant signals sent from the ground to function.5
Soon after 621B’s invention, in 1973, a demand for a superior positioning system with qualities from all current positioning systems arose. Combining the clocks from TIMATION, the orbital predicting method from TRANSIT and the signal structure from 621B a new system was made called the Global Positioning System or NAVSTAR. Within five years NAVSTAR’s first launch was made and was accurate within 10 meters. At first, this more advanced system was kept for military use, giving a less precise reading to civilians with an accuracy of only 50 meters. Among the first to take full advantage of the capability of the new system were surveyors.6
The NAVSTAR Global Positioning System (GPS)
Developed to overcome the two major shortcomings of the TRANSIT system of large time gaps between signals and its relative low navigation accuracy the GPS was a result of the work of the Joint Programs Office (JPO) of the United States Air Force.2 Directed in 1973 by the Department of Defense (DoD) JPO created a system that answered the fundamental questions of “what time, what position, and what velocity is it?”
For accurate position determination the GPS proposed at least 3 satellite relays at any given time. To accommodate this requirement initially a constellation of 24 evenly spaced satellites in 6 planes of rotation orbits inclined at 63 degrees to the equatorial plane was proposed. 2 However due to both political changes and scientific study this scheme has been changed many times. As of September 2007 the constellation consisted of 31 orbiting satellites.7
“The Navstar Global Positioning System (GPS) is an all-weather, space-based navigation system developed bu the Department of Defense to satisfy the requirements for the military forces to accurately determine their position, velocity, and time in a common reference frame, anywhere on or near the Earth on a continuous basis.” 8
To meet the above requirements the processing of a GPS receiver consists of complex vector algorithms and vector analysis. These processes measure pseudoranges derived from broadcast satellite signals either by measuring the travel time of the coded signal and multiplying by its velocity (speed of light when considering electromagnetic waves) or by measuring the phase of the signal and comparing it to the transmission frequency. To facilitate this processing and data acquisition the JPO created a network that consists of three segments:
- Space Segment
- Control Segment
- User Segment
We now briefly summarize the function of each segment.
The space segment consists of satellites and data relay. The satellites are used to broadcast signals encoded with the satellites information.
The GPS satellites have nearly circular orbits with an altitude of about 20 200 km and a period of approximately 12 hours.7 As mentioned earlier the number of satellite has steadily evolved from the original plan of 24 to 31 in recent years and from the previous 63 degree planar difference to 55 degrees. This provides greater accuracy through multiple measurements of a single parameter. This means that the minimum of three satellites is always exceeded, at times reaching up to ten simultaneous satellite relays.2
The GPS satellite essentially provide a base for measuring pseudoranges through radio signals, atomic clocks, computers, and various other devices. The satellites employed by JPO are broken up into five class blocks. These are Blocks I, II, IIA, IIR, and IIF. Block II satellites differ from Block I satellites by their 55 degree inclination as opposed to the former 63 degree inclination of Block I. Another main difference is the fact that Block II satellite signals are restricted from civilian use whereas the older Block I constellation is open to civilians.2
The Operational control System (OCS) consists of several master, minor, and ground control stations. The main operational tasks of these stations are time signal synchronization, date relay, satellite orbit tracking, and clock determination. Spread throughout the United States, many of these control stations are located at Air Force Bases.
Due to some restrictions on the use of GPS data, evident in the restriction of Block II satellites, there are two distinct types of GPS users; Military and Civilian users. The NAVSTAR Global Positioning System is a US satellite-based radio-navigation system that provides accurate position, navigation and timing information to users on a continuous, all-weather, worldwide basis. GPS provides two levels of service: the Standard Positioning Service (SPS) and the Precise Positioning Service (PPS). Due to its level of security and precision, SPS is primarily for civilian use while PPS is geared towards military use. 9
The PPS is a highly accurate positioning and timing service and provides the greatest degree of GPS accuracy. Based on US national security considerations the PPS is restricted to US Armed Forces, US Federal Agencies, and selected allied armed forces and governments. Access to the PPS is controlled through the use of Communication Security (COMSEC) cryptography 9. Uses of the PPS system for the military include:
- The US Army has developed a GPS Truth Data Acquisition, Recording, and Display System (TDARDS). It is an easily transportable or mobile GPS-based tracking system that uses up-to-date GPS data, radio data link, and computer technology to provide highly accurate, real-time time-space position information (TSPI) on test objects, such as ground vehicles, helicopters, and fixed-wing aircraft. TDARDS avoids the threat of friendly fire and provides the military with the ability to locate enemy troops. 10 11
- Navigate troops during darkness or sever weather when landmarks are unknown.
- Ability to locate specific enemy targets 10
- Cruise missiles have the ability to hit targets from large standoff distances while using multichannel GPS receivers to accurately determine their location constantly while in flight.
- Multiple Launched Rocket System (MLRS) vehicle uses GPS based inertial guidance to position itself and aim the launch box at the target in a very short time.
- US army has developed a 2000 lb glide bomb, which uses a GPS seeker rather than a Laser for guidance 10
- Provides military planners the ability to create grids of battle zones and up-to-the-hour conditions of landmarks such as highways and bridges 10
- Facility management 10
The SPS is a positioning and timing service that is available to all GPS users. The SPS accuracy is set at levels consistent with US national security interests. The SPS is intended primarily for civil use. Access to the SPS is openly available and does not require the use of cryptography. It is essential for users to understand that SPS systems may not be reliable during conflict as they do not incorporate the same level of security and survivability as military systems.9
Search and Rescue
- Tracking devices can locate the nearest ambulance, patrol car, or fire truck near an accident
- Ability to locate trapped persons in natural catastrophe’s such as earthquakes or mudslides
- Create an up to date map of deadly forest fires
- Survey existing conditions on a site to create a more accurate site plan
- Plot crucial building corners before construction to guarantee building is built according to specifications
- Large scale mining uses GPS to locate where on the grid to find special minerals and extract
- Facilitate miners underground to navigate and excavate the appropriate materials
- Voice activated moving maps allow users to travel from point A to point B
- Real-time maps allow users to plan trips and locate where they are
- Allows bikers, hikers, mariners, pilots, and automobile drivers to navigate in any condition
- Ability to locate migrating birds
- Ability to locate near-extinct animals and learn about animal behaviors
- GPS-equipped balloons monitor holes in the ozone layer and air quality above cities
- Facilitate weather predictions
- Archaeologists can locate remains and excavate at the precise location
The diversity of the uses of GPS listed above is matched by the types of receivers available. This wide range is due to the many measurable quantities of GPS positioning and the coding techniques to relay each measurement (C/A code, P-code, etc.). The following table summarizes the different receiver types.
|C/A-Code Pseudorange Receiver||Measures ranges between satellites. Usually small handheld device. Outputs the three dimensional position in terms of latitude, longitude, and elevation.||Used by hikers and boaters|
|C/A-Code Carrier Receiver||Measures both the psuedorange and the transmission signal phase. Receiver experiences greater accuracy because of clock error calculations.||Used for precision control surveys or geographic mapping|
|P-Code Receivers||Measures satellite signals through single frequency relationship. Outputs precision position and velocity measurements, with an accuracy of 20 cm.||Used for accurate Tracking|
|Y-Code Receivers||Similar to P-Code but allows for dual frequency bandwidths and therefore is of even greater accuracy compared to P-Code receivers.||Restricted to authorized DoD users. Military use only.|
Theory and Principles
The GPS systems throughout the gambit of its applications from driving directions to rocket trajectory all rely on two fundamental physics principles, The Doppler Shift and Kepler’s Laws, to calculate the position, velocity and many other measurable quantities. Also vital is the coordination of the many segments. Therefore GPS systems use specific spatial and time conventions.
The Doppler Shift
Developed in 1842 by Austrian physicist Johann Christian Doppler, the principle proposed that for a moving source a signal transmission frequency will decrease or increase depending on its relative velocity to a receiver.12 An example of this would be the change in frequency of an Ambulance siren as it approaches a stationary onlooker. The implications of this principle are that for a known transmission frequency and travel time, the change in frequency can be used to calculate the source’s relative distance and velocity, expressed in the following equation:
- f' is the change in frequency
- f is the original transmission frequency
- v is signal velocity
- vr is the receiver velocity
- vs is the source velocity
Kepler’s Laws12 The 2nd law describes the rate of passing of an object is constant for a given orbit. Finally, the 3rd law, the one that GPS employs, states that “square of the period of an orbiting object is proportional to the cube of the minor axis of its orbit. Through this the position and velocity a orbiting object, a satellite in the case of a GPS, can be accurately determined thus making other measurements relative to the orbiting object possible.
- M is the mass of a celestial body
- m is the mass of the orbiting object
- r is the radius of orbit
- w is the instantaneous angular velocity of orbit
For global (terrestrial) applications, such as GPS, an equatorial coordinate system is adequate. This system employs the Earth’s rotational axis as its central orbital axis. The other two axes (three axes needed for three dimensional analysis) are designated as the intersection of the equatorial plane with the elliptical plane and the Greenwich meridian planes respectively. The center of the coordinate system or “origin” is specified as the Earth's center of mass.2
Today several time systems are used for different application. Each time system is based on periodic processes such as the Universal time (UT) that relies on the rotation of the Earth. However for precise time calculations and the level of accuracy that GPS needs the atomic time system, based on atomic oscillations, is used. The different time systems are summarized in the following table.
|Periodic Process||Time System|
Even though measuring in a certain time system the GPS receivers are able to convert systems to user preference.
The applications and requirements for a accurate positioning system such as the GPS all require precise positioning measurement of the transmission satellite to function properly. Poor satellite orbital positioning leads directly to receiver error.2To obtain the position of a satellite the control stations mentioned earlier use Kepler’s third law which relates the period of an orbiting satellite to its orbital path. The satellites launched for the GPS program all have fixed orbits therefore through manipulation it is possible to calculate altitude, position, speed and direction of the satellite, the IVP differential equation which describes Keplerian Motion:
This differential equation is simplified through the understanding that an inertial time frame is provided through the synchronization of the GPS clocks and that the satellites are restricted to elliptical orbits, a fact presented in Kepler’s 1st Law. The expression above and its transformation into position and velocity vectors for a satellite is quite complex however many textbooks are dedicated to celestial mechanics that present solutions to problems similar to this.2.
With the satellite’s position and velocity described it is then possible for GPS systems to calculate the position and velocity of a receiver on or near the Earth.
The primary purpose of GPS is to provide the user with their own position on earth, commonly in terms of latitude, longitude, and elevation. To accomplish this GPS receivers measure the distances to several satellites (three minimum). Each satellite distance provides a probability sphere with the signal range as its radius, the intersection of two spheres provides a probability plane and lastly the intersection of three spheres provides probability points. However, for greater accuracy and mathematical redundancy a minimum of four spheres are desired.
Consider a frozen satellite with position vector P_s, relative to the Earth’s center of mass. Now consider a receiver on the the Earth, with position vector P_i, has a clock that is synchronized exactly with the clock on the satellite. Therefore the distance between the satellite and the receiver would simply the time it takes the signal to reach the receiver multiplied by the signal speed. This range would result in a probability sphere or radius S, because any point along the surface of the sphere would have the same distance. Thus three spheres are needed to “close in” on the true position of the receiver. This procedure is summarized in the following equation
GPS receivers, however, apply a slightly different technique, due to the motion of both the receiver and the satellite resulting in synchronization error.2 Because of the offset in clock times between the satellite and the receiver the distance measured above would either be slightly longer or shorter than the “true” range, an error proportional to the difference in clock times. The receiver can overcome this error by increasing the number of satellites used in range calculations. Four would be the next step up however real life GPS systems can use up to eight at any time for greater accuracy. The adjusted range measured is related to the above range through the clock error multiplied by the signal speed:
The second goal of the GPS is to determine the velocity of a receiver on or near the Earth. This is accomplished by using the principles of the Doppler Effect. Summarized above, the Doppler Effect relates the change in frequency for a signal to the relative velocity of the source. For this to be applicable the initial frequency of the source signal must be known, and in GPS satellites this information is actually encoded within the message. Also encoded within the signal is the satellites relative velocity (calculated through the principles of Keplerian orbit) and its position vector. With this information the receiver is able to calculate its velocity. This is just a simple description of the mechanics of signal transformations; however the actual processes involve complex algorithms, vector analysis, and differential equations.
The GPS device used to study the theory presented here was an Earthmate PN-20, provided by the CAEE dept. of Drexel University. We now analyze the components of this device.
Components of a Common GPS Receiver
The Global Positioning Satellite Receiver consist of several vital and fundamental parts as mentioned below:
The antenna receives signals from the satellites above the earth. These signals can come in the from of L1 or L2 bands. The L1 band is centered around 1575.42MHz and is primary for civilian and public use. The L2 bands on the other hand are primarily for military use and are generally centered around 1227.6MHz. 13
Filters are generally one the first component the L1 or L2 band signal will encounter. The purpose of the filter is to eliminate unwanted signals. It absorbs the desired frequency signals that are received via the antenna and rejects the unwanted frequencies. There are several different types of filters such as IF filters and Surface Acoustic Wave filters. The IF filter for examples, is a filter that removes unwanted frequencies of harmonics and such that result from a IF mixer output. 14
Low Noise Amplifiers, or LNA, are also on the first components the band signal from the antenna will encounter. The reason for this so the signal has a low noise factor since it has not been amplified before this by the previous components. The signal strength will be amplified and the noise added will be at a minimum once it passes through this component. 14 15
The purpose of the mixer component in a GPS receiver is to the down-convert the signal. The reason for down-converting is so the signal can be usable and valid for a common GPS receiver. The signals in common receivers are down-converted by the mixer from RF signals to lower frequency IF signals. Mixer components are place in various locations on the GPS receiver circuit. A great example of down-converting in general is when you down-convert 480p content to 480i content on a television set so the signal can be used on prior HDTV sets. 16 14
An Oscillator is another component of a GPS receiver. It is a sine-wave generator where the frequency can be adjusted by varying the feedback circuit. In the sine-wave, you can observe things such as harmonics and phase noise. 14
The purpose of the demodulator in the GPS receiver is the convert the signal from an IF signal to a baseband information signal. The baseband signal is usually the last phase the signal goes through in the process. 14
The microprocessor in the GPS receiver unit is used to compute the atomic time. It uses the data from the atomic clock on the satellite's and the time and distance the wave travels and then calculates time. 17
The purpose of memory storage space in a GPS receiver is to store data such as maps, directions, etc. This is an optional component in the GPS receiver but is handy when using the receiver for navigation or field work. 18
The power supply usually comes in the form of a rechargeable battery in hand held receivers. 18
The Earthmate GPS PN-20 in Practice
The everyday consumer is looking for a personal, hand-held GPS device that is easy to use, yet boasts endless capabilities at an affordable price. The DeLorme Earthmate package we have been equipped with prices at approximately $400. This product boasts being easy, rugged, and high performance.19
Physical Properties of the PN-20
This product boasts being waterproof. Is it? Yes, in fact it is. Upon looking at the unit it seemed that some areas of concern for leakage might arise from where the battery and memory stick encasement is. This was, however, not the case at all. After submerging the unit, it held up quite well. Under further analysis it was found that this encasement was protected by a rubber grommet. All other possible sources of leaking seemed to be protected again by rubber. The buttons seemed to be rubber coated, with the plastic overlaid. The screen protection looked to rely on a silicon sealant.
A unit that is easily held in the hand is the most enjoyable to use. The PN-20 is neither too big nor too small to fit in the palm of a hand without feeling to cumbersome. The screen is of a good enough size so that it does not strain the eyes without being overburdening by displaying too much information. Additionally, the number of buttons and functions available on the unit are well composed. The buttons too are a good size, and able to be cycled through with the use of one thumb, making the unit easily held in one hand. Again, the number of buttons allows the unit to be less cumbersome than it could be.
Interactive Properties of the PN-20
Without a doubt the interactive properties of a product are just as important as the physical properties of a GPS unit. A good product will be easy to use and yet give advanced users the capability to go above and beyond for a more enjoyable experience.
The PN-20 is very simple to work and understand. Primarily, the "Getting Started Guide" is a quick and simple set of instructions to simply plug in your batteries and play. It explains how to copy maps onto the 1 GB SD memory card. It then goes through the steps to use Topo USA 7.0 to install detailed maps onto your device. These are instructions for anyone who simply wants to know where they are or guide them to where they want to be.
By reading the guide provided by the unit, the user can discover and come to understand a lot. Simply reading increases the user's understandability quite a bit.
Due to the physical characteristics of the unit, it is simple to cycle through various pages and pieces of information displayed on the device. DeLorme has provided users with the ability to customize display settings including color schemes, backlight settings, as well as options for units, page order, and sounds so that the user becomes more comfortable with the device and feels more at ease. These options are more psychological than anything, making the use of the unit more enjoyable and fun.
Available Maps and Pages
This device is equipped to show various pages and maps for many different purposes and reasons. They are listed below:
Upon powering up the unit, a satellites page is shown. This shows pictorially available satellites and their approximate positions on the globe. In addition, this page will tell you which satellites you are connected to but are not receiving data from yet, and the ones you are connected to and receiving data from. This page also indicates your relative accuracy when taking a latitude and longitude, as well as how much battery power is left.
The Map Page
This page shows topological maps as per where you are located. Along with the base maps pre-loaded onto this device, high resolution images, colored aerial maps, USGS Quadrilaterals, and NOAA Nautical Charts can be loaded. These are only examples of some of the maps available. Others are available as well. This page is also customizable as far as the information provided such as speed, coordinates, course, elevation, and odometer. On this page the scale can be changed by zooming in and out, and the map can be moved and rotated using the panning buttons and page settings.
The Compass Page
Right from the user manual, the Compass Page is a graphical representation of a traditional floating needle compass. Your heading will always be displayed at the top of the screen. Route tracking is made easy by displaying where trip bearing should be compared to where you are. Again, information on this page can be changed to display various types of information.
The Trip Information Page
The Trip Information Page is not a graphical page at all. In fact it is a quick referential page that can be used to view lots of information quickly at one time. Again, the information presented here as well as what type of information presented here can be customized by the user quite easily. This page would typically be used for traveling on a course or following a direct route, where typical information displayed is trip odometer, trip time, trip speed, time standing still, time moving, and total trip time. All of these values can be reset to start new trip information; or this information can be saved if the trip is to be continued at a later time.
The Find Page
The Find Page would be used to locate and navigate a way to a certain point of interest. Typical points of interest displayed include finding business, addresses, natural features, coordinates, streets, trails, and cities. All of these have to of course be pre-loaded onto the device before they can be located with it.
The Waypoints Page
This page is used to set and keep track of waypoints in terms of coordinates. After a waypoint is put into the system, it can be seen in list form. As more and more waypoints are added, more and more waypoints, will be listed. The characteristics of these waypoints can be edited; characteristics such as elevation and position. Waypoints can then be put together to create a route, or the map can be centered upon a waypoint.
The Routes Page
The Routes Page works in coordination with the Waypoints Page; after creating pages, routes can be drawn from current GPS location to a waypoint, from one waypoint to another, and from current GPS location to another location. After this, route characteristics, length of time traveled, etc., can be simulated. Additionally, directions can be viewed.
The Tracks Page
This page creates a trail using a line of your travels as per GPS location. Tracks can be set to be recorded, tracks can also revisited, and can be used to set a route.
The Sun/Moon Page
Using the Sun/Moon Page can be beneficial for anyone who needs to know essential information such as when the sun rises and sets. When in wilderness or camping, this information is vital for knowing such things as when to set up camp. The times here reflect solar and lunar times, not actual times, so that must be taken into account.
The Tide Page
Many people like to know when high and low tides occur. The Tide Page relays this specific information based on GPS location. Other locations can be panned to as well. This page relays information such as when high and low tides are and the magnitude or amplitude of the tides. In addition, tides can be looked up based on date.
The Hunt/Fish Page
The Hunt/Fish Page is a simple page displaying a prediction for the outcome of the day, as well as good times for hunting and fishing.
A More Hands-on Approach
In this section the steps to disassembling the GPS unit have been cataloged. It is important to remember that all items should be carefully set aside so as to not lose any pieces. When unscrewing screws, it is important not to force them so as to not destroy the heads on them. It may also be advisable to be careful with the circuit board so as not to damage any of the circuitry, capacitors, resistors, wiring, etc.
|Simply start by unscrewing the tabs on the battery encasement on the back of the unit and remove to expose the batteries.|
|As with many electrical products, it is ideal to remove the power source. On this unit, this is done by removing the batteries.|
|Now it is important to remove the memory card to reduce possibility of shorting it and losing all data.|
|It is now important to unscrew the 6 machine screws; this can be done using an eyeglass kit screwdriver.|
|These are the machined screws that are within the unit. Note that some screws have rubber washers around them. These washers are important in order to protect against water penetration.|
|Here the unit has been separated. This images supports the waterproofing theory; the rubber gasket is removable, keep track of it to ensure waterproofing.|
|This is how the unit looks when the power and data connections have been disconnected. Note that the box at the top of the circuit board seems to be the receiver for the unit. This receives the information directly from the satellite.|
|Here the 8-pin ribbon can be seen to relay data. The reverse side can directly be seen to correlate with this. The 2-pin and 3-pin ribbons most likely relay power.|
|After unscrewing to circuit board screws, the circuit board can be disconnected from the casing.|
|This shows the rubber casing protecting the buttons as predicted in the waterproofing section.|
|Here the LCD display screen has been separated from the circuit board. This is done by releasing the plastic clips on the side of the circuit board. There are a total of four. The receiver for the unit is located at the top of the circuit board.|
After reassembling the unit using the same procedure in reverse order to take it apart, the unit seemed to run in working order. The gasket and rubber washers were put back in place so as to restore the waterproof quality.
Use in the Field
The only thing left to do after researching the product, learning about it, as well as disassembling and reassembling the DeLorme Earthmate GPS PN-20 is to put it to use. The package delivered to us included the device, usb data cables, a portable package kit including an adapter and rechargeable battery pack, a 1 GB SD card and usb card reader, and a Travel Power Kit. For our purposes, we did not utilize the travel kit, since the unit was not used in a car. However, the other parts were used.
Upon powering up the unit indoors, no fix could be found. This was reasonable due to the fact that the signals from the satellite could most likely not be read through the exterior structure of the building. Upon walking outside, and going to the Satellite Page, we could watch as the unit communicated with satellites and fixed on them. As more and more satellites were fixed on, the relative accuracy became better and better. It was found that under trees and cloud cover with the device in communication with all satellites, the relative accuracy wained to plus or minus 120 ft. Under a clear sky, the relative accuracy maximized at plus or minus 12 ft. Considering the price of the unit, this seemed to be a very good accuracy.
After reading up on the different available maps and pages, it was very simple to set waypoints, look up locations, and set up routes using said waypoints and found locations. The battery life seems to be very good; after using it for approximately 20 hours, he battery meter dropped only one bar. It was very easy to customize backlight timer settings, color settings, and sound settings. It was intuitive when it came to settings like these, as well as setting up different routes and waypoints. It seems a little less intuitive when it comes to accessing different types of maps under the Map Page. It was very hard to discern how to layer properly. The only type of map accessible without trouble was the Topo USA East Region Map.
Overall, I would rate this unit very well. It is very intuitive. The hardest part might be loading different map types onto the unit. One downside is the turnaround time between relaying information to the user and when the buttons have been pushed. However, this could be improved by better memory within the unit, or a better receiver. Additionally, for the Topo Map, the smallest readable scale I could find was 320 ft; this could be a restriction due to the maps, or a limitation due to loading times for the detailing of the map. Other than these examples, few criticisms can be made. The possibilities for GPS units seem endless.
GNSS and GPS Alternatives
GNSS (Global Navigation Satellite System)20 is a term used to classify any constellation of satellites that offer high-precision positioning data anywhere on the Earth. The United States NAVSTAR GPS (Global Positioning Satellite) is the worlds’ only completely functional GNSS, with Russia’s GLONASS (GLObal Navigation Satellite System) falling just short of complete due to disrepair. 21
- GNSS-1 is composed of all currently active satellite constellations and their augmentation systems. The augmentation systems consist of ground-based monitoring stations and communication satellites. SBAS (Satellite Based Augmentation Systems) is a group of GPS and GLONASS augmentation systems which employ “geostationary satellites to broadcast information to users over a large geographical service,” while GBAS (Ground Based Augmentation System), is the collective of ground based augmentation systems. ABAS (Aircraft Based Augmentation System) is a system integrating GNSS provided data and data collected from devices onboard an aircraft to increase the performance of the satellite positioning systems on the aircraft. Current GNSS are a result of military initiatives and are fully utilized for military applications only, meaning that civilians have limited use of the systems. 20
- GNSS-2 This “second generation” GNSS focuses on the civilian applications of the system. Focus of current GNSS lies with the military, but this new generation will be entirely implemented for the gratification of civilian users. Galileo is a European initiative at this new type of GNSS, which will work with both GPS and GLONASS. 20
Currently Operational GNSS
- GLONASS - The Russian satellite constellation system is the Global'naya Navigatsionnaya Sputnikovaya Sistema, otherwise known as GLObal NAvigation Satellite System. The functional portion of this system is composed of 21 satellites on three orbits, with 3 “on-orbit spares.” The three planes differ by 120 degrees. The satellites that operate on the same orbit differ in position by 45 degrees. The satellites occupy a Medium Earth Orbit (MEO), at approximately 19,100 km, with an inclination of 64.8 degrees. The first GLONASS satellites were launched in 1982. The constellation was not declared complete until late 1995/ early 1996. All of the ground stations for the GLONASS reside inside the former Soviet Union territory. Due to the relatively short life span of the satellites, currently between 3 and 5 years, the constellation is in a state of disrepair. Initiatives have been taken to proceed with the production of new generations of GLONASS satellites. GLONASS-M (2nd generation) and GLONASS-K (3rd generation) are two such projects that resolve to lengthen the lifespan of the satellite and also improve signal characteristics. 21
List procured from 22
China’s current experimental satellite positioning constellation, Compass (BeiDou-1), consists of three satellites, one of which is a spare. This constellation offers navigation and communication coverage over most of East Asia. 24 The more recent Compass (BeiDou-2) is a proposed constellation of 30 satellites in MEO. BeiDou-2 will have accuracy to within 10 meters, 0.2 meters/sec, and 50 nanoseconds. Compass, unlike newer GNSS projects, will have certain features provided for military applications only. 25
The Doppler Orbitography and Radiopositioning Integrated by Satellite 29 system is a French multipurpose satellite positioning system. The DORIS system can measure the position of its satellites to within 2.5 cm. 30 This degree of accuracy can be attributed to the 60 ground stations that the DORIS system is composed of.29 Doris was implemented with 2 purposes in mind:
- determine the orbit of satellites equipped with DORIS receivers with centimeter accuracy using a network of ground stations as reference points on Earth.
- precisely tie points to the International Terrestrial Reference Frame (ITRF).
This European project is a GNSS that can act independently and will also be inter-operable with GPS and GLONASS systems. The system will consist of 30 satellites in MEO at 23,222 km. The constellation will operate in 3 orbits with 10 satellites, 9 primaries and one back up, in each orbit. The angle of inclination for the orbits will be positioned at 56 degrees to the equator. Having access to between 6 and 8 satellites at any one position will give users accuracy to within a few centimeters. Obstacles in cities and problems with fewer satellites shouldn’t be an issue since the Galileo system will be inter-operable with GPS and GLONASS. The first type of Galileo satellite to be launched was the GIOVE-A, a more recent model is named the GIOVE-B. 23
India’s currently implemented satellite system is GAGAN (GPS And Geo Augmented Navigation). This system is specialized for civil aviation. India’s effort to construct a more universal navigation system has led to the development of the Indian Regional Navigational Satellite System. The constellation will consist of 7 satellites, 3 in Geostationary Earth Orbit (GEO) and 4 in Geosynchronous Orbit (GSO). The orbit of the satellites will be at 29 degrees from the Equator. Once Functional, the IRNSS should provide the following: 26
- High accuracy real time position, velocity and time for authorized users on a variety of platforms.
- Good accuracy for a single frequency user with the help of Ionospheric corrections.
- All weather operation on a 24 hour basis.
The Quasi-Zenith Satellite System is Japan’s proposed constellation of a few satellites that will orbit the earth with one almost always positioned near zenith over Japan. This system is to work in conjunction with the US’s GPS system. Since the System will have a satellite located over Japan at nearly all times, obstacles such as buildings will no longer be an issue. The QZSS is to provide communication and positioning signals. 27
GPS Disadvantages and Alternatives
The current GPS system is not without its’ flaws. The system, independent of satellite health, has had issues with gaps in coverage and various forms of inaccuracies. 33 Any of the following can lead to diminished accuracy:
- sub-optimal position of satellites
- obstacles such as buildings, canyons and other geographic anomalies can interfere with a GPS receivers ability to access the satellite signals.31
- orbital errors, an inaccuracy in provided data
- synchronization errors with the satellites atomic clock can produce time inaccuracies32
Satellite based systems are the leading edge in methods of navigation, meaning that the alternatives are methods that have been rendered archaic with the successful implementation of systems like GPS.
- Celestial means
- Radio Waves
1. Virtual Museum of Surveying
2. [Wellenhof, B. H., & Lichtenegger, H. (1997). GPS: Theory and Practice, 4th Ed. New York: Sringer Wien.]
3. The Global Positioning System, Appendix B - History
4. Timation And GPS Satellite History
5. A Brief History Of Satellite Navigation
6. History Of The GPS System
7. Global Positioning System
8. [Wooden, W. H. (1985). Navstar Global Positioning System. ISPP Proceedings, 1(1), 23-32]
9. GPS Global Support Center
10. GPS: A Military Perspective
11. Patriot Missile
12. [Halliday, D., R. Resnick, & J. Walker. (2005). Fundamentals of Physics, 7th Ed. New York: Wiley.]
13. Dallas Semiconductor Maxim
14. GPS Receiver Technology
15. A Fully-Integrated GPS Receiver Front-End with 40mW Power Consumption
16. A 1.57-GHz RF Front-End for Triple Conversion GPS Receiver
17. Fundamentals of Global Positioning System
18. PC Nation
19. Earthmate PN-20 Home Page
20. A Beginner's Guide To GNSS in Europe
22. Global Navigation Satellite System
23. How to build up a constellation of 30 navigation satellites
24. Compass Satellite Navigation Experimental System (BeiDou01)
25. Compass Navigation Satellite System (BeiDou-2)
26. Satellites For Navigation
27. Quasi-Zenith Satellite Accessible Anytime
28. Doris the space surveyor
29. Doris at a glance
30. A multipurpose system
31. Project Construction
32. Disadvantages: Problems that may cause GPS failure
33. Current GPS constellation
Hussain Burhanpurwala - Different Sectors and their use of GPS
Joel Fitzgerald - Advantages/Disadvantages/Alternatives
Liam Hendriken - Practical components of the unit and field testing and formatting
Darshan Mistry - Breakdown of a generic GPS receiver
Ray Panchari - History Of GPS and formatting
Pradeep Parekh - Logic and theory