How To Gain The Best Price Possible
When Retail Shopping For GPS
Navigation Products.


GPS navigation equipment is still a misunderstood technology based retail product, when in fact it really is one of the easiest technologies to grasp, once you just know the basics.

Most customers in general do not have great knowledge of the different brand types on the market or of the features each GPS product may or may not have. So it is absolutely essential that you research all the information available on this product, so that you will simply find the one that best suits your own needs perfectly. This can be a competitive product area in retail stores,
(profit margins of generally between 5% - 15%). However by just doing your shopping around you can still gain the price you want to pay, because I have found that just about every store has different marked prices on exactly the same product items.

So which type of GPS do you need?

There are 3 main types of GPS units that are of a real interest to consumers; the car navigation system, the portable outdoors unit and the marine system.





Here is a link to a consumer GPS buying guide on CNET. It is a great informative introduction to GPS and I could not have said it better myself.



If you are still hungry for more even information here is another very helpful GPS link.



* And if you want to find out just everything you will ever want to know about GPS, simply keep reading my ebook located below.

* Did You Know These Absolute Facts?



So Do You Want
The Best Value Online GPS/Navigation Deals?

If you are in The USA, Australia, Canada, The UK, France, Germany or in just about any country in the entire world and you just want the best technology prices online then I recommend you visit TigerDirect.




The Definitive Guide To GPS


Introduction

Take a few moments if you will and think about how difficult it must have been for our ancestors to get from place to place.  Back in the days before street signs could guide them and streets were pretty much not even there.  Think back to the time when they couldn’t call AAA for trip planning and they weren’t even always sure which way to go in order to get to a destination.

How do you suppose they got from point A to point B?  They learned in many ways.  They used the stars to guide their way, they erected landmarks to keep track of where they had been, they drew maps and often just wandered about until they got where they thought they wanted to be.

Today, we have it much easier.  Not only do we have the advantage of detailed maps, street names and such, but we also now have sophisticated navigation devices that can help us along our way in the form of GPS.  Global Positioning Systems have revolutionized the way we travel.

At one time, the family vacation meant Dad driving the car and Mom reading a paper map all the while arguing about the best route to take.  God forbid you get lost, because Dad would rather take a beating than stop and ask for directions. 

Technology has come a long way and GPS certainly has made a difference in how and even where we travel.  It was originally developed by the United States Department of Defense for military purposes but soon, enterprising companies realized that this technology could have some truly great applications for the everyday person and they convinced the government to allow the technology to be released for distribution to the general public.

Now, ordinary people, with the aid of a GPS receiver, are able to do so much with a GPS system including navigating their way on trips and even tracking vehicles.  The GPS technology can do so many things that it can be mind boggling and it can help to know how a GPS works and how it can help you.

The applications are changing almost on a daily basis as the technology evolves and grows.  In this book, we will attempt to explain GPS to you, how it works and how it can work for you.  Stick with us through the technical stuff and you may be surprised at what you find out!  Let’s start with the inception of GPS technology.


How GPS Works

When people talk about "a GPS," they usually mean a GPS receiver. The Global Positioning System (GPS) is actually a constellation of 27 Earth-orbiting satellites (24 in operation and three extras in case one fails). As we said in the section above, the U.S. military developed and implemented this satellite network as a military navigation system, but soon opened it up to everybody else.

Each of these 3,000- to 4,000-pound solar-powered satellites circles the globe at about 12,000 miles (19,300 km), making two complete rotations every day. The orbits are arranged so that at any time, anywhere on Earth, there are at least four satellites "visible" in the sky.

A GPS receiver's job is to locate four or more of these satellites, figure out the distance to each, and use this information to deduce its own location. This operation is based on a simple mathematical principle called trilateration. Trilateration in three-dimensional space can be a little tricky, so we'll start with an explanation of simple two-dimensional trilateration.

Imagine you are somewhere in the United States and you are TOTALLY lost -- for whatever reason, you have absolutely no clue where you are. You find a friendly local and ask, "Where am I?" He says, "You are 625 miles from Boise, Idaho."   This is a nice, hard fact, but it is not particularly useful by itself. You could be anywhere on a circle around Boise that has a radius of 625 miles.

You ask somebody else where you are, and she says, "You are 690 miles from Minneapolis, Minnesota.”  Now you're getting somewhere. If you combine this information with the Boise information, you have two circles that intersect. You now know that you must be at one of these two intersection points, if you are 625 miles from Boise and 690 miles from Minneapolis.

If a third person tells you that you are 615 miles from Tucson, Arizona, you can eliminate one of the possibilities, because the third circle will only intersect with one of these points. You now know exactly where you are – Denver, Colorado. 

This same concept works in three-dimensional space, as well, but you're dealing with spheres instead of circles.  Fundamentally, three-dimensional trilateration isn't much different from two-dimensional trilateration, but it's a little trickier to visualize. Imagine the radii from the previous examples going off in all directions. So instead of a series of circles, you get a series of spheres.

If you know you are 10 miles from satellite A in the sky, you could be anywhere on the surface of a huge, imaginary sphere with a 10-mile radius. If you also know you are 15 miles from satellite B, you can overlap the first sphere with another, larger sphere. The spheres intersect in a perfect circle. If you know the distance to a third satellite, you get a third sphere, which intersects with this circle at two points.

The Earth itself can act as a fourth sphere -- only one of the two possible points will actually be on the surface of the planet, so you can eliminate the one in space. Receivers generally look to four or more satellites, however, to improve accuracy and provide precise altitude information.

In order to make this simple calculation, then, the GPS receiver has to know two things:


The GPS receiver figures both of these things out by analyzing high-frequency, low-power radio signals from the GPS satellites. Better units have multiple receivers, so they can pick up signals from several satellites simultaneously.

Radio waves are electromagnetic energy, which means they travel at the speed of light (about 186,000 miles per second, 300,000 km per second in a vacuum). The receiver can figure out how far the signal has traveled by timing how long it took the signal to arrive.

A GPS receiver calculates the distance to GPS satellites by timing a signal's journey from satellite to receiver. As it turns out, this is a fairly elaborate process.

At a particular time (let's say midnight), the satellite begins transmitting a long, digital pattern called a pseudo-random code. The receiver begins running the same digital pattern also exactly at midnight. When the satellite's signal reaches the receiver, its transmission of the pattern will lag a bit behind the receiver's playing of the pattern.

The length of the delay is equal to the signal's travel time. The receiver multiplies this time by the speed of light to determine how far the signal traveled. Assuming the signal traveled in a straight line, this is the distance from receiver to satellite.

In order to make this measurement, the receiver and satellite both need clocks that can be synchronized down to the nanosecond. To make a satellite positioning system using only synchronized clocks, you would need to have atomic clocks not only on all the satellites, but also in the receiver itself. But atomic clocks cost somewhere between $50,000 and $100,000, which makes them a just a bit too expensive for everyday consumer use.

The Global Positioning System has a clever, effective solution to this problem. Every satellite contains an expensive atomic clock, but the receiver itself uses an ordinary quartz clock, which it constantly resets.

In a nutshell, the receiver looks at incoming signals from four or more satellites and gauges its own inaccuracy. In other words, there is only one value for the "current time" that the receiver can use. The correct time value will cause all of the signals that the receiver is receiving to align at a single point in space.

That time value is the time value held by the atomic clocks in all of the satellites. So the receiver sets its clock to that time value, and it then has the same time value that all the atomic clocks in all of the satellites have. The GPS receiver gets atomic clock accuracy "for free."

When you measure the distance to four located satellites, you can draw four spheres that all intersect at one point. Three spheres will intersect even if your numbers are way off, but four spheres will not intersect at one point if you've measured incorrectly. Since the receiver makes all its distance measurements using its own built-in clock, the distances will all be proportionally incorrect.

The receiver can easily calculate the necessary adjustment that will cause the four spheres to intersect at one point. Based on this, it resets its clock to be in sync with the satellite's atomic clock. The receiver does this constantly whenever it's on, which means it is nearly as accurate as the expensive atomic clocks in the satellites.

In order for the distance information to be of any use, the receiver also has to know where the satellites actually are. This isn't particularly difficult because the satellites travel in very high and predictable orbits.

The GPS receiver simply stores an almanac that tells it where every satellite should be at any given time. Things like the pull of the moon and the sun do change the satellites' orbits very slightly, but the Department of Defense constantly monitors their exact positions and transmits any adjustments to all GPS receivers as part of the satellites' signals.

Of course, that is the simplified version – believe it or not.  There is much more involved.  Now is when we get into the technical part of the whole GPS system.  The current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US).

The space segment is composed of the orbiting GPS satellites or space vehicles in GPS parlance.  The GPS design calls for 24 space vehicles to be distributed equally among six circular orbital planes.  These orbital planes are center on Earth and not rotating with respect to the distant stars.  These six planes have a 55 degree tilt relative to the Earth’s equator.  They are separated by a 60 degree angle along the equator from a reference point to the orbit’s intersection.

The satellites orbit at 12,600 miles above the Earth and each one makes two complete orbits each day so it passes over the same location on the Earth once each day.  The orbits are arranged so that at least six satellites are always within a line of sight from almost everywhere on Earth’s surface.

As of April, 2007, there are 30 actively broadcasting satellites in the GPS constellation.  The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. 

With the increased number of satellites, the constellation was changed to a non-uniform arrangement.  This type of arrangement was shown to improve reliability and availability of the system.  This has worked much better than a uniform system especially when multiple satellites fail.

Then there is the control segment.  The flight paths of the satellites are tracked by US Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia and Colorado Springs.  There are also monitor stations operated by the National Geospatial-Intelligence Agency.

The tracking information is sent to the Air Force Space Command’s master control station at Schriever Air Force Base in Colorado Springs which is operated by the 2d Space Operations Squadron of the United States air Force.  The squadron contacts each satellite regularly with a navigational update using the ground antennas at Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs.

These updates synchronize the atomic clocks on board the satellites to within one microsecond and adjust the ephemeris of each satellite’s internal orbital model.  These updates are created by a Kalman Filter which uses inputs from the ground monitoring stations, space weather information, and other various inputs.

Finally, there is the user segment of the GPS system.  The user’s GPS receiver is the user segment.  In general, GPS receivers are composed of an antenna tuned to the frequencies transmitted by the satellites, receiver processors and a highly stable clock.  They may also include a display for providing location and speed information to the user.

A receiver is often described by its number of channels that signifies how many satellites it can monitor simultaneously.  Originally, this number was limited to four or five but it has progressively increased over the years so that now receivers typically have between twelve and twenty channels.

A typical GPS receiver module is based on the SiRF Star 3 chipset and measures 12 x 15 millimeters.  The receivers are basically small, but they are powerful tools that are used on a daily basis by many people.

GPS receivers may include an input for differential corrections using the RTCM SC-104 format which is typically in the form of a RS-232 port at a speed of 4,800 bits per second. 

Data is actually sent at a much lower rate which limits the accuracy of the signal sent using RTCM.  Receivers with internal DGPS receivers can out perform those using external RTCM data.  Even low-cost units commonly include Wide Area Augmentation System receivers.

Many GPS receivers can relay position data to a PC or other device using a NMEA 0183 protocol.  There is a newer and less widely adopted protocol called the NMEA 2000 that has been developed as well.  Both protocols are proprietary and are controlled by the US-based National Marine Electronics Association.

References to the NMEA protocols have been compiled from public records allowing open source tools like gpsd to read the protocol without violating intellectual property laws.  Other proprietary protocols exist as well such as the u-blox, SiRF, and MTK protocols.  Receivers can interface with other devices using methods including a serial connection, USB, or Bluetooth.

GPS receivers come in a variety of formats, from devices integrated into cars, phones, and watches, to dedicated devices from manufacturers Trimble, Garmin and Leica.  The devices are portable handheld or are mounted onto a car’s dashboard or windshield using an easy suction grip mount.

The most essential function of a GPS receiver is to pick up the transmissions of at least four satellites and combine the information in those transmissions with information in an electronic almanac, all in order to figure out the receiver's position on Earth.

Once the receiver makes this calculation, it can tell you the latitude, longitude and altitude (or some similar measurement) of its current position. To make the navigation more user-friendly, most receivers plug this raw data into map files stored in memory.
 
Most people use GPS systems as a way to find their way from place to place.  They are very handy navigational aids that can really help out when you are in an unfamiliar place and don’t want to mess with paper maps.  But how does the GPS receiver achieve this?  How can it know where you are at and then help you get to where you need to go?

Each GPS satellite continuously broadcasts a navigation message at 50 bits per second giving the time, and almanac, and an ephemeris.  The almanac consists of orbit and status information for each satellite in the constellation.  A complete almanac transmission takes 12.5 minutes and is responsible for the long initial acquisition process when a new receiver is first turned on.

The ephemeris gives the satellite’s own precise orbit and is transmitted every 30 seconds.  The almanac assists in the acquisition of other satellites while an ephemeris from each satellite is needed to compute position fixes using that satellite. 

The ephemeris is updated every two hours and is valid for four hours.  The time needed to acquire is a significant element of the delay to first position fix when a receiver is switched on after having been off for several hours.

Each satellite transmits its navigation message with at least two distinct spread spectrum codes.  The first is the Coarse Acquisition code which is freely available to the public.  The second is the precise code which is usually encrypted and reserved for military applications.

The Coarse Acquisition code is a 1,023 chip pseudo-random PRN code at 1,023 million chips per second so that it repeats every millisecond.  Each satellite has its own Coarse Acquisition code so that it can be uniquely identified and received separately from the other satellites transmitting on the same frequency.

The Precise code is a 10.23 mega-chip per second PRM code that repeats only every week.  When the “anti-spoofing” mode is on as it is in normal operation, the Precise code is encrypted by the Y-code to produce a P(Y) code which can only be decrypted by units with a valid decryption key.  Both the Coarse Acquisition code and the P(Y) codes impart the precise time of day to the user.

Once the GPS receiver is switched on, the signal is sent out from it and the satellites lock in its exact position.  Then the user enters in the location they want to go to and it will calculate the best way to get there.

The coordinates are calculated according to the World Geodetic System.  To calculate its position, a receiver needs to know the precise time.  The satellites are equipped with extremely accurate atomic clocks, and the receiver uses an internal crystal oscillator-based clock that is continually updated using the signals from the satellites.

The receiver identifies each satellite’s signal by its distinct Coarse Acquisition code pattern and then measures the time delay for each satellite.  To do this, the receiver produces an identical Coarse Acquisition sequence using the same “seed number” as the satellite. 

By lining up the two sequences, the receiver can measure the delay and calculate the distance to the satellite called the pseudorange.  Overlapping pseudoranges are represented as curves and are modified to yield the probable position.

The orbital position data from the Navigation Message is then used to calculate the satellite’s precise position.  Knowing the position and the distance of a satellite indicates that the receiver is located somewhere on the surface of an imaginary sphere centered on that satellite whose radius is the distance to it.

When four satellites are measured simultaneously, the intersection of the four imaginary spheres reveals the location of the receiver.  Receivers known to be near sea level can substitute the sphere of the planet for one satellite using their altitude. 

Often, these spheres will overlap slightly instead of meeting at one point, so the receiver will yield a mathematically most-probable position and often indicate the uncertainty.

Calculating a position with the P(Y) signal is generally similar in concept assuming a person can decrypt it.  The encryption is essentially a safety mechanism.  That means if a signal can be successfully decrypted, it is reasonable to assume it is a real signal being sent by a GPS satellite.

In comparison, civil receivers are highly vulnerable to spoofing since correctly formatted Coarse Acquisition signals can be generated using readily available signal generators.  RAIM features do not protect against spoofing, since RAIM only checks the signals from a navigational perspective.

Of course, as with any piece of electronic equipment, there are bound to be glitches and problems arises from time to time.  Nothing is exact, and although the GPS satellite system is accurate for the most part, there are times when things can interfere with the signal.

The position calculated by a GPS receiver requires the current time, the position of the satellite, and the measured delay of the received signal.  The position accuracy is primarily dependent on the satellite position and the signal delay.

To measure the delay, the receiver compares the bit sequence received from the satellite with an internally generated version.  By comparing the rising and trailing edges of the bit transitions, modern electronics can measure signal offset with within about one percent of a bit time or approximately ten nanoseconds for the Coarse Acquisition code. 

Since GPS signals propagate nearly at the speed of light, this represents and error of about 3 miles.  This is the minimum error possible using only the GPS Coarse Acquisition signal.

Of course, position accuracy can be improved by using the higher speed P(Y) signal.  Assuming the same one percent bit time accuracy, the high frequency P(Y) signal results in an accuracy rate of about 30 inches.

Just as with any electronic device, there are bound to be some problems.  With those problems, must come for fixes.


Problems And Solutions

Electronics errors are one of several accuracy-degrading effects.  They include ionospheric effects, ephemeris errors, satellite clock errors, multipath distortion, tropospheric effects, and numerical errors.

Inconsistencies of atmospheric conditions affect the speed of the GPS signals as they pass through the Earth’s atmosphere and ionosphere.  Correcting these errors is a significant challenge to improving GPS position accuracy. 

These effects are smallest when the satellite is directly overhead and become greater for satellites nearer the horizon since the signal is affected for a longer time.  Once the receiver’s approximate location is known, a mathematical model can be used to estimate and compensate for these errors.

Because ionospheric delay affects the speed of microwave signals differently based on frequency – a characteristic known as dispersion – both frequency bands can be used to help reduce this error.  Some military and expensive survey-grade civilian receivers compare the different delay in the frequencies to measure atmosphere dispersion and apply a more precise correction.

This can be done in civilian GPS receivers without decrypting the P(Y) signal carried on L2 by tracking the carrier wave instead of the modulated code.  To do this on lower cost receivers, a new civilian code signal on L2 called L2C was added to the satellites.  This new signal allows a direct comparison of the L1 and L2 signals using the coded signal instead of the carrier wave.

The effects of the ionosphere generally change slowly and can be averaged over time.  The effects for any particular geographical area can be easily calculated by comparing the GPS-measured position to a known surveyed location.  This correction is also valid for other receivers in the same general location.

Several systems send this information over radio or other links to allow L1 only receivers to make corrections.  The date is transmitted via satellite system and transmits it on the GPS frequency using a special pseudo-random number so only one antenna and receiver is required.

Humidity also causes a variable delay resulting in errors similar to ionospheric delay but occurring in the troposphere.  This effect is more localized and changes more quickly than ionospheric effects and is not frequency dependent.  These traits make it much more difficult to make precise measurement and compensation for humidity errors than with the ionospheric effects.

Changes in altitude also change the amount of delay due to the signal passing through less of the atmosphere at higher elevations.  Since the GPS receiver computes its approximate altitude, this error is relatively simple to correct.

GPS signals can also be affected by multi-path issues where the radio signals reflect off of surrounding terrain such as buildings, canyon walls, and hard ground.  These delayed signals can cause inaccuracy as a well.

To correct these errors, many techniques have been developed, most notably one called narrow correlator spacing.  For long delay multi-path, the receiver itself can recognize the wayward signal and get rid of it. 

To address shorter delay multi-path from the signal reflecting off the ground, specialized antennas can be used.  Short delay reflections are harder to filter out since they are only slightly delayed.  The effects are almost indistinguishable from routine fluctuations in atmospheric delay.

Multi-path effects are much less severe in moving vehicles.  When the GPS antenna is moving, the false solutions using reflected signals quickly fail to converge, and only the direct signals result in stable solutions.

Another problem we find with satellite signals has to do with clock and ephemeris errors.  The navigation message from a satellite is sent out only every 12.5 minutes.  In reality, the data contained in these messages tend to be out of date by an even larger amount. 

When a GPS satellite is boosted back into a proper orbit, for some time following this movement, the receiver’s calculation of the satellite’s position will be incorrect until it receives another ephemeris update. 

The onboard clocks are extremely accurate, but they do suffer from some clock drift.  This problem tends to be very small but may add up to six feet of inaccuracy.  This class of error is more stable than ionospheric problems and tends to change over days or weeks rather than minutes.  This makes correction fairly simple by sending out a more accurate almanac on a separate channel.

The GPS system includes a feature called Selective Availability that introduces intentional, slowly changing random errors of up to 328 feet into the publicly available navigation signals to confound, for example, guiding long range missiles to precise targets.  Additional accuracy was available in the signal, but in encrypted form that was only available to the United States military, its allies, and a few others government users.

Selective availability typically added signal errors of about 32 feet horizontally and 98 feet vertically.  The inaccuracy of the civilian signal was deliberately encoded so as not to change very quickly. 

As an example, the entire Eastern United States might read 98 feet off but 90 feet off everywhere else and in the same direction.  To improve the usefulness of GPS for civilian navigation, differential GPS was used by many civilian GPS receivers to greatly improve accuracy.

During the Gulf War, the shortage of military GPS units and the wide availability of civilian units among personnel resulted in a decision to disable Selective Availability which was ironic as the concept had been introduced specifically for these situations allowing friendly troops to use the signal for accurate navigation while at the same time denying it to the enemy.

Since selective availability was also denying the same accuracy to thousands of friendly troops, turning it off or setting it to an error of zero – which is effectively the same thing – presented a clear benefit.

In the 1990’s, the FAA started pressuring the military to turn off selective availability permanently.  This would save the FAA millions of dollars every year in maintenance of their own radio navigation systems.  The military resisted for most of the 1990’s, but selective ability was eventually discontinued.  This came after President Bill Clinton announced that users would have access to the error-free L1 signal.

Per the directive, the induced error of selective availability was changed to add no error to the public signals.  Selective availability is still a system capability of GPS and error could, in theory, be reintroduced at any time. 

In practice, in view of the hazards and costs this would induce for US and foreign shipping, it is unlikely to be reintroduced, however.  Various government agencies, including the FAA have state that it is not intended to be reintroduced.

The US military has developed the ability to locally deny GPS and other navigation services to hostile forces in a specific area of crisis without affecting the rest of the world or its own military systems.

With the selective availability hardware, one of the side effects is the capability of it to correct the frequency of the GPS clocks to about one in five trillion.  This is a significant improvement over the raw accuracy of the clocks.

According to the theory of relativity, due to their constant movement and height relative to the Earth-centered inertial reference of frame, the clocks on the satellites are affected by their speed (special relativity) as well as their gravitational potential (general relativity). 

For the GPS satellites, general relativity predicts that the atomic clocks at GPS orbital altitudes will tick more rapidly because they are in a weaker gravitational field than the atomic clocks on the Earth’s surface.  On the other hand, special relativity predicts that atomic clocks moving at GPS orbital speeds will tick more slowly than stationary ground clocks. 

When combined, the discrepancy is 38 microseconds per day.  To account for this, the frequency of the clock on board each satellite is given a rate offset prior to launch so that it will run slightly slower than the desired frequency on Earth.

GPS observation processing must also compensate for another relativistic effect called the Sagnac effect.  The GPS time scale is defined in an inertial system, but observations are processed in Earth centered and Earth fixed system which is co-rotating and simultaneity is not uniquely defined.

The Lorentz transformation between the two systems modifies the signal run time – a correction having opposite algebraic signs for satellites in the Eastern and Western celestial hemispheres.  Ignoring this effect will produce an east-west error on the order of hundreds of nanoseconds – or tens of meters in position.

The atomic clocks on board the GPS satellites are precisely tuned.  This makes the system a practical engineering application of the scientific theory of relativity in a real-world system

Another possible problem for GPS systems has to do with interference and jamming.  There are tons and tons of GPS receivers out there these days, so interference is probably going to come into play.  Plus, jamming can be a problem in overloading the system as well.

Since GPS signals at terrestrial receivers tend to be relatively weak, it’s easy for other sources of electromagnetic radiation to desensitize the receiver.  This makes acquiring and tracking the satellite signals difficult or impossible. 

One of the sources of interference is a naturally occurring emission is called solar flares and they have the potential to degrade GPS reception.  Their impact can affect reception over the half of the Earth facing the sun.  GPS signals can also be interfered with by naturally occurring geomagnetic store that are mostly found near the poles of the Earth’s magnetic field.

Man-made interference can also disrupt or jam GPS signals.  There was one documented case where an entire harbor was unable to receive GPS signals due to unintentional jamming caused by a malfunctioning television antenna.  Intentional jamming is also possible.

Generally stronger signals can interfere with GPS receivers when they are within radio range or line of sight.  Jamming a GPS signal can be done even by the layman.  In fact, a 2002 article that appeared in the online magazine Phrack gave a detailed description on how to build a short range jammer.

The US government believes that jammers such as these were used occasionally during the war in Afghanistan.  The US military also claimed to have destroyed a jammer with a GPS-guided bomb during the Iraq War.  Such a jammer is relatively easy to detect and locate making it an attractive target for anti-radiation missiles.

Because of the potential for natural and man-made noise that interferes with GPS signals, there are many techniques being developed to deal with the interference.  One obvious technique is to not rely on GPS as a sole source.  There should be a fallback plan that should be in place in the event of a GPS malfunction.

In many receivers, there is a feature included called Receiver Autonomous Integrity Monitoring (RAIM).  This is designed to provide a warning to the user if jamming or another problem is detected. 

The US government has also deployed their Selective Availability Anti-Spoofing Module in their Defense Advanced GPS Receiver.  This device is supposed to be able to detect jamming and maintains its lock on the encrypted GPS signals during interference which causes civilian receivers to lose a lock on the signal.

So what is being done to solve some of the problems that can occur with GPS signals both man-made and natural occurrences?  Actually, there are a lot of things being done to help with problems like these.

GPS manufacturers are using augmentation methods to improve accuracy of GPS systems.  These systems rely on external information being integrated into the calculation process.  There are many such systems in place already and they are name or described based on how the GPS sensor receives the information.

Some systems transmit additional information about sources of error like clock drift, ephemeris, or ionospheric delay.  Others give direct measurements of how much the signal was off in the past.  A third group provides additional navigational or vehicle information that is integrated into the calculation process.

The accuracy of a calculation can also be improved through precise monitoring and measuring of the existing GPS signals in additional or alternate ways. 

The first is called dual frequency monitoring.  This method refers to systems that can compare two or more signals like the L1 frequency versus the L2 frequency.  Since these are two different frequencies, they are affected in different yet predictable ways by the atmosphere and objects around the receiver.  After monitoring these signals, it’s possible to calculate and fix the error.

Receivers that have the correct decryption key can decode the P(Y) code relatively easily.  This code is transmitted on both the L1 and L2 to measure the error.  Receivers that do not possess the key can still use a processor called “codeless” to compare the encrypted information on L1 and L2 to gain much of the same error information.

The downside is that this technique is currently limited to specialized surveying equipment.  Developers hope that in the future, additional civilian codes are expected to be transmitted on the L2 and L5 frequencies.  When these become operational, all users will be able to make the same comparison and directly measure some of the errors.

Another form of precise monitoring is called Carrier Phase Enhancement.  The error that this program fixes arises because the pulse transition of the PRN is not instantaneous which makes the satellite-receiver sequence matching operation imperfect. 

This approach utilizes the L1 carrier wave which has a period a thousand times smaller than that of the C/A bit period to act as an additional clock signal and resolve the uncertainty

The phase difference error in the normal GPS amounts to between 6 and 10 feet of ambiguity.  The Carrier Phase Enhancement monitoring works to within one percent of perfect transition reduces this error to one inch of ambiguity.  By eliminating this source of error, Carrier Phase Enhancement coupled with DGPS normally realizes between 8 and 12 inches of absolute accuracy.

Finally, there is another approach for a precise GPS-based positioning system.  This is called Relative Kinematic Positioning.  In this approach, determination of range signal can be resolved to an accuracy of less than four inches.  This is done by resolving the number of cycles in which the signal is transmitted and received by the receiver.

This can be accomplished by using a combination of differential GPS correction data, transmitting GPS signal information and ambiguity resolution techniques via statistical tests.  This is actually possibly able to be conducted with processing in real-time as well.

As we’ve said, most people use their GPS system as a navigational aid.  That means that, depending on where you are going, you will need to have a map of the place you are visiting.  If you are a big traveler, you are going to need a lot of maps then, so let’s take a look at the maps you can get for your GPS receiver.


Take A Left At The Red Barn

Almost all GPS receivers come equipped with pre-loaded maps.  However, these maps are not always what you need and/or want.  That is why you will want to check out resources that will give you additional maps.  And there are plenty of ways to get additional maps for your GPS.

The most essential function of a GPS receiver is to pick up the transmissions of at least four satellites and combine the information in those transmissions with information in an electronic almanac, all in order to figure out the receiver's position on Earth.

Once the receiver makes this calculation, it can tell you the latitude, longitude and altitude (or some similar measurement) of its current position. To make the navigation more user-friendly, most receivers plug this raw data into map files stored in memory. 

You can use maps stored in the receiver's memory, connect the receiver to a computer that can hold more detailed maps in its memory, or simply buy a detailed map of your area and find your way using the receiver's latitude and longitude readouts. Some receivers let you download detailed maps into memory or supply detailed maps with plug-in map cartridges.

A standard GPS receiver will not only place you on a map at any particular location, but will also trace your path across a map as you move. If you leave your receiver on, it can stay in constant communication with GPS satellites to see how your location is changing. With this information and its built-in clock, the receiver can give you several pieces of valuable information:


That’s why having plenty of maps at your disposal is such an important tool of having your GPS receiver do what you are wanting it to do.  When you have a GPS at your disposal, you can be assured that you will be able to get from point A to point B with little problem.  Most of the modern GPS systems have voice capabilities that will provide you with verbal directions that allow you to concentrate on your driving.

So where do you go when you want to find maps for your GPS receiver?  You actually have a lot of options when it comes to this question.  You don’t even have to buy anything if you don’t want to as there are a lot of websites that offer you the option of downloading free maps for your GPS system.

To begin with, there are many different software programs available for purchase that contains complete maps of almost anyplace in the United States.  These software programs are installed on the hard drive of your computer.  Then you use a specific cord that is usually included with your receiver to connect to your computer and upload the maps to your GPS receiver.

One of the best selling software packages is made by – of course – Microsoft – and is called Streets and Trips 2007.  The best part about this product is that it comes with a GPS receiver so you don’t have to buy a separate unit from the software.

This GPS receiver isn’t a cheap one either.  It is stylish and compact with new and improved SiRF star III technology that is 10 times more sensitive than previous models.  It allows you to find your location faster and holds a signal longer even in a building or a crowded city. 

The Streets and Trips software combined with the GPS receiver gives you a tour guide who’s ready to go virtually anywhere you want to go in the U.S. and Canada. It will monitor your progress from the sky and help you stay on course.

Another great software package for your GPS system is made by Delorme – a GPS manufacturer – and is called Street Atlas USA.  Street Atlas USA gives you the most updated maps of the United States, Canada, and even Mexico with the most recent version.  You get a powerful GPS navigation tool with spoken voice directions to guide you along the way, and it also has over four million points of interest.

We should talk a little bit here about points of interest.  Good GPS software is always going to include points of interest (POI) for anyplace that you travel.  POI includes restaurants, hotels, motels, gas stations, and other attractions for the places that you may be interested in visiting.  If you are going to a place you have never been before, the POI will prompt you when you are nearing an attraction.

For example, say you are traveling South in the area of Louisiana.  You have just passed Baton Rouge and your GPS system tells you that you are approaching a POI called The Myrtle’s Plantation.  This is one of the most haunted houses in the country and is a bed and breakfast with some of the most deep Southern history you will ever find in the country.

Now if you had not done any research prior to your trip, you might not have known about The Myrtle’s.&#