All about GPS - Lecture Material - Completely GPS, GIS dan Remote Sensing tutorial - facegis.com
All about GPS

why GPS?

Why GPS?
Trying to figure out where you are and where you're going is probably one of man's oldest pastimes.

Navigation and positioning are crucial to so many activities and yet the process has always been quite cumbersome.

Over the years all kinds of technologies have tried to simplify the task but every one has had some disadvantage. [view other Positioning Systems]

Finally, the U.S. Department of Defense decided that the military had to have a super precise form of worldwide positioning. And fortunately they had the kind of money ($12 billion!) it took to build something really good.
Why Did the Department of Defense Develop GPS?

In the latter days of the arms race the targeting of ICBMs became such a fine art that they could be expected to land right on an enemy's missile silos. Such a direct hit would destroy the silo and any missile in it. The ability to take out your opponent's missiles had a profound effect on the balance of power.

But you could only expect to hit a silo if you knew exactly where you were launching from. That's not hard if your missiles are on land, as most of them were in the Soviet Union. But most of the U.S. nuclear arsenal was at sea on subs. To maintain the balance of power the U.S. had to come up with a way to allow those subs to surface and fix their exact position in a matter of minutes anywhere in the world Hello GPS!

The result is the Global Positioning System, a system that's changed navigation forever.

 

what is GPS?

What is GPS?
The Global Positioning System (GPS) is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations.

GPS uses these "man-made stars" as reference points to calculate positions accurate to a matter of meters. In fact, with advanced forms of GPS you can make measurements to better than a centimeter!

In a sense it's like giving every square meter on the planet a unique address.

GPS receivers have been miniaturized to just a few integrated circuits and so are becoming very economical. And that makes the technology accessible to virtually everyone.

These days GPS is finding its way into cars, boats, planes, construction equipment, movie making gear, farm machinery, even laptop computers.

Soon GPS will become almost as basic as the telephone. Indeed, at Trimble, we think it just may become a universal utility.

 

how GPS?

How GPS works?

Here's how GPS works in five logical steps:

  1. The basis of GPS is "triangulation" from satellites.
    We're using the word "triangulation" very loosely here because it's a word most people can understand, but purists would not call what GPS does "triangulation" because no angles are involved. It's really "trilateration."
    Trilateration is a method of determining the relative positions of objects using the geometry of triangles.
  2. To "triangulate," a GPS receiver measures distance using the travel time of radio signals.
  3. To measure travel time, GPS needs very accurate timing which it achieves with some tricks.
  4. Along with distance, you need to know exactly where the satellites are in space. High orbits and careful monitoring are the secret.
  5. Finally you must correct for any delays the signal experiences as it travels through the atmosphere.

We'll explain each of these points in the next five sections of the tutorial. We recommend you follow the tutorial in order. Remember, science is a step-by-step discipline!

 

how GPS?

Triangulating from Satellites

Improbable as it may seem, the whole idea behind GPS is to use satellites in space as reference points for locations here on earth.

That's right, by very, very accurately measuring our distance from three satellites we can "triangulate" our position anywhere on earth.

Forget for a moment how our receiver measures this distance. We'll get to that later. First consider how distance measurements from three satellites can pinpoint you in space.

The Big Idea Geometrically:

Step One:

Suppose we measure our distance from a satellite and find it to be 11,000 miles.

Knowing that we're 11,000 miles from a particular satellite narrows down all the possible locations we could be in the whole universe to the surface of a sphere that is centered on this satellite and has a radius of 11,000 miles.

how GPS?

Step Two:

Next, say we measure our distance to a second satellite and find out that it's 12,000 miles away.

That tells us that we're not only on the first sphere but we're also on a sphere that's 12,000 miles from the second satellite. Or in other words, we're somewhere on the circle where these two spheres intersect.

how GPS?

Step Three:

If we then make a measurement from a third satellite and find that we're 13,000 miles from that one, that narrows our position down even further, to the two points where the 13,000 mile sphere cuts through the circle that's the intersection of the first two spheres.

So by ranging from three satellites we can narrow our position to just two points in space.

To decide which one is our true location we could make a fourth measurement. But usually one of the two points is a ridiculous answer (either too far from Earth or moving at an impossible velocity) and can be rejected without a measurement.

A fourth measurement does come in very handy for another reason however, but we'll tell you about that later.

Next we'll see how the system measures distances to satellites.

how GPS?

In Review:
  • Position is calculated from distance measurements (ranges) to satellites.
  • Mathematically we need four satellite ranges to determine exact position.
  • Three ranges are enough if we reject ridiculous answers or use other tricks.
  • Another range is required for technical reasons to be discussed later.

 

how GPS?

Measuring distance from a satellite

We saw in the last section that a position is calculated from distance measurements to at least three satellites.

how GPS?

The Big Idea Mathematically:

In a sense, the whole thing boils down to those "velocity times travel time" math problems we did in high school. Remember the old: "If a car goes 60 miles per hour for two hours, how far does it travel?"

Velocity (60 mph) x Time (2 hours) = Distance (120 miles)

In the case of GPS we're measuring a radio signal so the velocity is going to be the speed of light or roughly 186,000 miles per second.

The problem is measuring the travel time.

how GPS?

  • Timing is tricky
  • We need precise clocks to measure travel time
  • The travel time for a satellite right overhead is about 0.06 seconds
  • The difference in sync of the receiver time minus the satellite time is equal to the travel time

The timing problem is tricky. First, the times are going to be awfully short. If a satellite were right overhead the travel time would be something like 0.06 seconds. So we're going to need some really precise clocks. We'll talk about those soon.

But assuming we have precise clocks, how do we measure travel time? To explain it let's use a goofy analogy:

Suppose there was a way to get both the satellite and the receiver to start playing "The Star Spangled Banner" at precisely 12 noon. If sound could reach us from space (which, of course, is ridiculous) then standing at the receiver we'd hear two versions of the Star Spangled Banner, one from our receiver and one from the satellite.

These two versions would be out of sync. The version coming from the satellite would be a little delayed because it had to travel more than 11,000 miles.

If we wanted to see just how delayed the satellite's version was, we could start delaying the receiver's version until they fell into perfect sync.

The amount we have to shift back the receiver's version is equal to the travel time of the satellite's version. So we just multiply that time times the speed of light and BINGO! we've got our distance to the satellite.

That's basically how GPS works.

Only instead of the Star Spangled Banner the satellites and receivers use something called a "Pseudo Random Code" - which is probably easier to sing than the Star Spangled Banner.

how GPS?

In Review:
  1. Distance to a satellite is determined by measuring how long a radio signal takes to reach us from that satellite.
  2. To make the measurement we assume that both the satellite and our receiver are generating the same pseudo-random codes at exactly the same time.
  3. By comparing how late the satellite's pseudo-random code appears compared to our receiver's code, we determine how long it took to reach us.
  4. Multiply that travel time by the speed of light and you've got distance.

 

Timing

Getting perfect timing

If measuring the travel time of a radio signal is the key to GPS, then our stop watches had better be darn good, because if their timing is off by just a thousandth of a second, at the speed of light, that translates into almost 200 miles of error!

On the satellite side, timing is almost perfect because they have incredibly precise atomic clocks on board.

Atomic Clocks

Atomic clocks don't run on atomic energy. They get the name because they use the oscillations of a particular atom as their "metronome." This form of timing is the most stable and accurate reference man has ever developed.

But what about our receivers here on the ground?

Remember that both the satellite and the receiver need to be able to precisely synchronize their pseudo-random codes to make the system work. (to review this point click here)

If our receivers needed atomic clocks (which cost upwards of $50K to $100K) GPS would be a lame duck technology. Nobody could afford it.

Luckily the designers of GPS came up with a brilliant little trick that lets us get by with much less accurate clocks in our receivers. This trick is one of the key elements of GPS and as an added side benefit it means that every GPS receiver is essentially an atomic-accuracy clock.

Timing

Using GPS for Timing

We generally think of GPS as a navigation or positioning resource but the fact that every GPS receiver is synchronized to universal time makes it the most widely available source of precise time.

This opens up a wide range of applications beyond positioning. GPS is being used to synchronize computer networks, calibrate other navigation systems, synchronize motion picture equipment and much more.

The secret to perfect timing is to make an extra satellite measurement.

That's right, if three perfect measurements can locate a point in 3-dimensional space, then four imperfect measurements can do the same thing.

This idea is so fundamental to the working of GPS that we have a separate illustrated section that shows how it works. If you have time, cruise through that.

 

Timing

Getting perfect timing

Extra Measurement Cures Timing Offset

If our receiver's clocks were perfect, then all our satellite ranges would intersect at a single point (which is our position). But with imperfect clocks, a fourth measurement, done as a cross-check, will NOT intersect with the first three.

So the receiver's computer says "Uh-oh! there is a discrepancy in my measurements. I must not be perfectly synced with universal time."

Since any offset from universal time will affect all of our measurements, the receiver looks for a single correction factor that it can subtract from all its timing measurements that would cause them all to intersect at a single point.

That correction brings the receiver's clock back into sync with universal time, and bingo! - you've got atomic accuracy time right in the palm of your hand.

Once it has that correction it applies to all the rest of its measurements and now we've got precise positioning.

One consequence of this principle is that any decent GPS receiver will need to have at least four channels so that it can make the four measurements simultaneously.

With the pseudo-random code as a rock solid timing sync pulse, and this extra measurement trick to get us perfectly synced to universal time, we have got everything we need to measure our distance to a satellite in space.

But for the triangulation to work we not only need to know distance, we also need to know exactly where the satellites are.

Timing

In the next section we'll see how we accomplish that.

In Review:

  1. Accurate timing is the key to measuring distance to satellites.
  2. Satellites are accurate because they have atomic clocks on board.
  3. Receiver clocks don't have to be too accurate because an extra satellite range measurement can remove errors.

 

Positions

Satellite Positions

Knowing where a satellite is in space

In this tutorial we've been assuming that we know where the GPS satellites are so we can use them as reference points.

But how do we know exactly where they are? After all they're floating around 11,000 miles up in space.

A high satellite gathers no moss

That 11,000 mile altitude is actually a benefit in this case, because something that high is well clear of the atmosphere. And that means it will orbit according to very simple mathematics.

The Air Force has injected each GPS satellite into a very precise orbit, according to the GPS master plan.

Positions

GPS Master Plan

The launch of the 24th block II satellite in March of 1994 completed the GPS constellation.

Four additional satellites are in reserve to be launched "on need."

The spacings of the satellites are arranged so that a minimum of five satellites are in view from every point on the globe.

On the ground all GPS receivers have an almanac programmed into their computers that tells them where in the sky each satellite is, moment by moment.

The basic orbits are quite exact but just to make things perfect the GPS satellites are constantly monitored by the Department of Defense.

They use very precise radar to check each satellite's exact altitude, position and speed.

The errors they're checking for are called "ephemeris errors" because they affect the satellite's orbit or "ephemeris." These errors are caused by gravitational pulls from the moon and sun and by the pressure of solar radiation on the satellites.

The errors are usually very slight but if you want great accuracy they must be taken into account.

 

Positions

Satellite Positions

Getting the message out

Once the DoD has measured a satellite's exact position, they relay that information back up to the satellite itself. The satellite then includes this new corrected position information in the timing signals it's broadcasting.

So a GPS signal is more than just pseudo-random code for timing purposes. It also contains a navigation message with ephemeris information as well.

With perfect timing and the satellite's exact position you'd think we'd be ready to make perfect position calculations. But there's trouble afoot. Check out the next section to see what's up.

Positions

In Review:

  • To use the satellites as references for range measurementswe need to know exactly where they are.
  • GPS satellites are so high up their orbits are very predictable.
  • Minor variations in their orbits are measured by the Department of Defense.
  • The error information is sent to the satellites, to be transmitted along with the timing signals.

 

GPS error

Error Correction

Up to now we've been treating the calculations that go into GPS very abstractly, as if the whole thing were happening in a vacuum. But in the real world there are lots of things that can happen to a GPS signal that will make its life less than mathematically perfect.

To get the most out of the system, a good GPS receiver needs to take a wide variety of possible errors into account. Here's what they've got to deal with.

GPS error

First, one of the basic assumptions we've been using throughout this tutorial is not exactly true. We've been saying that you calculate distance to a satellite by multiplying a signal's travel time by the speed of light. But the speed of light is only constant in a vacuum.

As a GPS signal passes through the charged particles of the ionosphere and then through the water vapor in the troposphere it gets slowed down a bit, and this creates the same kind of error as bad clocks.

Ionosphere

The ionosphere is the layer of the atmosphere ranging in altitude from 50 to 500 km.

It consists largely of ionized particles which can exert a perturbing effect on GPS signals.

While much of the error induced by the ionosphere can be removed through mathematical modeling, it is still one of the most significant error sources.

Troposphere

The troposphere is the lower part of the earth's atmosphere that encompasses our weather.

It's full of water vapor and varies in temperature and pressure.

But as messy as it is, it causes relatively little error.

There are a couple of ways to minimize this kind of error. For one thing we can predict what a typical delay might be on a typical day. This is called modeling and it helps but, of course, atmospheric conditions are rarely exactly typical.

Error Modeling

Much of the delay caused by a signal's trip through our atmosphere can be predicted.

Mathematical models of the atmosphere take into account the charged particles in the ionosphere and the varying gaseous content of the troposphere.

On top of that, the satellites constantly transmit updates to the basic ionospheric model.

A GPS receiver must factor in the angle each signal is taking as it enters the atmosphere because that angle determines the length of the trip through the perturbing medium.

Another way to get a handle on these atmosphere-induced errors is to compare the relative speeds of two different signals. This "dual frequency" measurement is very sophisticated and is only possible with advanced receivers.

 

how GPS?

Error Correction

Trouble for the GPS signal doesn't end when it gets down to the ground. The signal may bounce off various local obstructions before it gets to our receiver.

This is called multipath error and is similar to the ghosting you might see on a TV. Good receivers use sophisticated signal rejection techniques to minimize this problem.

Multipath Error

The whole concept of GPS relies on the idea that a GPS signal flies straight from the satellite to the receiver.

Unfortunately, in the real world the signal will also bounce around on just about everything in the local environment and get to the receiver that way too.

The result is a barrage of signals arriving at the receiver: first the direct one, then a bunch of delayed reflected ones. This creates a messy signal.

If the bounced signals are strong enough they can confuse the receiver and cause erroneous measurements.

Sophisticated receivers use a variety of signal processing tricks to make sure that they only consider the earliest arriving signals (which are the direct ones).

how GPS?

Problems at the satellite

Even though the satellites are very sophisticated they do account for some tiny errors in the system.

The atomic clocks they use are very, very precise but they're not perfect. Minute discrepancies can occur, and these translate into travel time measurement errors.

And even though the satellites positions are constantly monitored, they can't be watched every second. So slight position or "ephemeris" errors can sneak in between monitoring times.

how GPS?

Ephemeris Errors

Ephemeris (or orbital) data is constantly being transmitted by the satellites.

Receivers maintain an "almanac" of this data for all satellites and they update these almanacs as new data comes in.

Typically, ephemeris data is updated hourly.

There are a couple of ways to minimize this kind of error. For one thing we can predict what a typical delay might be on a typical day. This is called modeling and it helps but, of course, atmospheric conditions are rarely exactly typical.

Error Modeling

Much of the delay caused by a signal's trip through our atmosphere can be predicted.

Mathematical models of the atmosphere take into account the charged particles in the ionosphere and the varying gaseous content of the troposphere.

On top of that, the satellites constantly transmit updates to the basic ionospheric model.

A GPS receiver must factor in the angle each signal is taking as it enters the atmosphere because that angle determines the length of the trip through the perturbing medium.

how GPS?

Intentional Errors!

As hard as it may be to believe, the same government that spent $12 billion to develop the most accurate navigation system in the world intentionally degraded its accuracy. The policy was called "Selective Availability" or "SA" and the idea behind it was to make sure that no hostile force or terrorist group can use GPS to make accurate weapons.

Basically the DoD introduced some "noise" into the satellite's clock data which, in turn, added noise (or inaccuracy) into position calculations. The DoD may have also been sending slightly erroneous orbital data to the satellites which they transmitted back to receivers on the ground as part of a status message.

Together these factors made SA the biggest single source of inaccuracy in the system. Military receivers used a decryption key to remove the SA errors and so they're much more accurate.

Turning Off Selective Availability

On May 1, 2000 the White House announced a decision to discontinue the intentional degradation of the GPS signals to the public beginning at midnight. Civilian users of GPS are now able to pinpoint locations up to ten times more accurately. As part of the 1996 Presidential Decision Directive goals for GPS, President Clinton committed to discontinuing the use of SA by 2006. The announcement came six years ahead of schedule. The decision to discontinue SA was the latest measure in an on-going effort to make GPS more responsive to civil and commercial users worldwide.

how GPS?

The bottom line

Fortunately all of these inaccuracies still don't add up to much of an error. And a form of GPS called "Differential GPS" can significantly reduce these problems. We'll cover this type of GPS later.

To get an idea of the impact of these errors click here for a typical error budget.

 

DGPS?

Differential GPS

In this section you will see how a simple concept can increase the accuracy of GPS to almost unbelievable limits.

And you will see:

  • Why we need Differential GPS
    • Differential GPS or "DGPS" can yield measurements good to a couple of meters in moving applications and even better in stationary situations.
  • How Differential GPS works
    • Differential GPS involves the cooperation of two receivers, one that's stationary and another that's roving around making position measurements.
  • Where to get Differential Corrections
    • Many new GPS receivers are being designed to accept corrections, and some are even equipped with built-in radio receivers.
  • Other ways to work with Differential GPS
    • Not all DGPS applications are created equal. Some don't need the radio link because they don't need precise positioning immediately.
  • Advanced Concepts
    • Imagine the possibilities. Automatic construction equipment could translate CAD drawings into finished roads without any manual measurements.

 

DGPS?

Why we need Differential GPS?

Basic GPS is the most accurate radio-based navigation system ever developed. And for many applications it's plenty accurate. But it's human nature to want MORE!

So some crafty engineers came up with "Differential GPS," a way to correct the various inaccuracies in the GPS system, pushing its accuracy even farther.

Differential GPS or "DGPS" can yield measurements good to a couple of meters in moving applications and even better in stationary situations.

That improved accuracy has a profound effect on the importance of GPS as a resource. With it, GPS becomes more than just a system for navigating boats and planes around the world. It becomes a universal measurement system capable of positioning things on a very precise scale.

 

DGPS?

How Differential GPS works?

Differential GPS involves the cooperation of two receivers, one that's stationary and another that's roving around making position measurements.

The stationary receiver is the key. It ties all the satellite measurements into a solid local reference.

Here's how it works:

The problem

Remember that GPS receivers use timing signals from at least four satellites to establish a position. Each of those timing signals is going to have some error or delay depending on what sort of perils have befallen it on its trip down to us.

(For a complete discussion of all the errors review the "Correcting Errors" section of the tutorial.)

Since each of the timing signals that go into a position calculation has some error, that calculation is going to be a compounding of those errors.

DGPS?

An extenuating circumstance

Luckily the sheer scale of the GPS system comes to our rescue. The satellites are so far out in space that the little distances we travel here on earth are insignificant.

So if two receivers are fairly close to each other, say within a few hundred kilometers, the signals that reach both of them will have traveled through virtually the same slice of atmosphere, and so will have virtually the same errors.

Common Errors

Differential GPS can eliminate all errors that are common to both the reference receiver and the roving receiver.

These include everything except multipath errors (because they occur right around the receiver) and any receiver errors (because they're unique to the receiver).

DGPS?

That's the idea behind differential GPS: We have one receiver measure the timing errors and then provide correction information to the other receivers that are roving around. That way virtually all errors can be eliminated from the system, even the pesky Selective Availability error that the DoD puts in on purpose.

DGPS?

The idea is simple. Put the reference receiver on a point that's been very accurately surveyed and keep it there.

This reference station receives the same GPS signals as the roving receiver but instead of working like a normal GPS receiver it attacks the equations backwards.

Instead of using timing signals to calculate its position, it uses its known position to calculate timing. It figures out what the travel time of the GPS signals should be, and compares it with what they actually are. The difference is an "error correction" factor.

The receiver then transmits this error information to the roving receiver so it can use it to correct its measurements.

DGPS?

Since the reference receiver has no way of knowing which of the many available satellites a roving receiver might be using to calculate its position, the reference receiver quickly runs through all the visible satellites and computes each of their errors.

Then it encodes this information into a standard format and transmits it to the roving receivers.

Error Code Transmission

GPS receivers don't actually transmit corrections by themselves. They are linked to separate radio transmitters.

The roving receivers get the complete list of errors and apply the corrections for the particular satellites they're using.

 

DGPS?

Where to get Differential Corrections?

In the early days of GPS, reference stations were established by private companies who had big projects demanding high accuracy - groups like surveyors or oil drilling operations. And that is still a very common approach. You buy a reference receiver and set up a communication link with your roving receivers.

But now there are enough public agencies transmitting corrections that you might be able to get them for free!

The United States Coast Guard and other international agencies are establishing reference stations all over the place, especially around popular harbors and waterways.

These stations often transmit on the radio beacons that are already in place for radio direction finding (usually in the 300kHz range).

Anyone in the area can receive these corrections and radically improve the accuracy of their GPS measurements. Most ships already have radios capable of tuning the direction finding beacons, so adding DGPS will be quite easy.

Many new GPS receivers are being designed to accept corrections, and some are even equipped with built-in radio receivers.

 

DGPS?

Other ways to work with Differential GPS

Post Processing DGPS

Not all DGPS applications are created equal. Some don't need the radio link because they don't need precise positioning immediately.

It's one thing if you're trying to position a drill bit over a particular spot on the ocean floor from a pitching boat, but quite another if you just want to record the track of a new road for inclusion on a map.

For applications like the latter, the roving receiver just needs to record all of its measured positions and the exact time it made each measurement.

Then later, this data can be merged with corrections recorded at a reference receiver for a final clean-up of the data. So you don't need the radio link that you have to have in real-time systems.

If you don't have a reference receiver there may be alternative sources for corrections in your area. Some academic institutions are experimenting with the Internet as a way of distributing corrections.

DGPS?

There's another permutation of DGPS, called "inverted DGPS," that can save money in certain tracking applications.

Let's say you've got a fleet of buses and you'd like to pinpoint them on street maps with very high accuracy (maybe so you can see which side of an intersection they're parked on or whatever).

Anyway, you'd like this accuracy but you don't want to buy expensive "differential-ready" receivers for every bus.

With an inverted DGPS system the buses would be equipped with standard GPS receivers and a transmitter and would transmit their standard GPS positions back to the tracking office. Then at the tracking office the corrections would be applied to the received positions.

It requires a computer to do the calculations, a transmitter to transmit the data but it gives you a fleet of very accurate positions for the cost of one reference station, a computer and a lot of standard GPS receivers. Such a deal!

 

DGPS?

Advanced Concepts

If you want to know where DGPS might be headed, take a look at your hand, because soon DGPS may be able to resolve positions that are no farther apart than the width of your little finger.

Imagine the possibilities. Automatic construction equipment could translate CAD drawings into finished roads without any manual measurements. Self-guided cars could take you across town while you quietly read in the back seat.

To understand how this kind of GPS is being developed you need to understand a little about GPS signals. If two receivers are fairly close to each other, say within a few hundred kilometers, the signals that reach both of them will have traveled through virtually the same slice of atmosphere, and so will have virtually the same line.

 

DGPS?

Code-Phase vs Carrier-Phase

The words "Code-Phase" and "Carrier-Phase" may sound like electronic mumbo-jumbo but, in fact, they just refer to the particular signal that we use for timing measurements. Using the GPS carrier frequency can significantly improve the accuracy of GPS.

The concept is simple but to understand it let's review a few basic principles of GPS.

Remember that a GPS receiver determines the travel time of a signal from a satellite by comparing the "pseudo random code" it's generating, with an identical code in the signal from the satellite.

The receiver slides its code later and later in time until it syncs up with the satellite's code. The amount it has to slide the code is equal to the signal's travel time.

The problem is that the bits (or cycles) of the pseudo random code are so wide that even if you do get synced up there's still plenty of slop.

Consider these two signals:

If you compared them logically you'd say they matched. When signal A is a one, signal B is a one. When signal A is a zero, signal B is a zero.

But you can see that while they match they're a little out of phase. Notice that, even though they are the same most of the time, signal A may change state a little before signal B. This is the source of positioning error.

That's the problem with code-phase GPS. It's comparing pseudo random codes that have a cycle width of almost a microsecond. And at the speed of light a microsecond is almost 300 meters of error!

Code-phase GPS isn't really that bad because receiver designers have come up with ways to make sure that the signals are almost perfectly in phase. Good machines get with in a percent or two. But that's still at least 3-6 meters of error.

Survey receivers beat the system by starting with the pseudo random code and then move on to measurements based on the carrier frequency for that code. This carrier frequency is much higher so its pulses are much closer together and therefore more accurate.

If you're rusty on the subject of carrier frequencies consider your car radio. When you tune to 94.7 on the dial you're locking on to a carrier frequency that's 94.7 MHz.

Obviously we can't hear sounds at 94 million cycles a second. The music we hear is a modulation (or change) in this carrier frequency. So when you hear someone sing an "A" note on the radio you're actually hearing the 94.7 MHz carrier frequency being varied at a 440 cycle rate.

GPS works in the same way. The pseudo random code has a bit rate of about 1 MHz but its carrier frequency has a cycle rate of over a GHz (which is 1000 times faster!)

At the speed of light the 1.57 GHz GPS signal has a wavelength of roughly twenty centimeters, so the carrier signal can act as a much more accurate reference than the pseudo random code by itself. And if we can get to within one percent of perfect phase like we do with code-phase receivers we'd have 3 or 4 millimeter accuracy! Yeeow!

 

DGPS?

Code-Phase vs Carrier-Phase

Survey receivers beat the system by starting with the pseudo random code and then move on to measurements based on the carrier frequency for that code. This carrier frequency is much higher so its pulses are much closer together and therefore more accurate.

The concept is simple but to understand it let's review a few basic principles of GPS.

Remember that a GPS receiver determines the travel time of a signal from a satellite by comparing the "pseudo random code" it's generating, with an identical code in the signal from the satellite.

The receiver slides its code later and later in time until it syncs up with the satellite's code. The amount it has to slide the code is equal to the signal's travel time.

The problem is that the bits (or cycles) of the pseudo random code are so wide that even if you do get synced up there's still plenty of slop.

Consider these two signals:

If you compared them logically you'd say they matched. When signal A is a one, signal B is a one. When signal A is a zero, signal B is a zero.

But you can see that while they match they're a little out of phase. Notice that, even though they are the same most of the time, signal A may change state a little before signal B. This is the source of positioning error.

That's the problem with code-phase GPS. It's comparing pseudo random codes that have a cycle width of almost a microsecond. And at the speed of light a microsecond is almost 300 meters of error!

Code-phase GPS isn't really that bad because receiver designers have come up with ways to make sure that the signals are almost perfectly in phase. Good machines get with in a percent or two. But that's still at least 3-6 meters of error.

Survey receivers beat the system by starting with the pseudo random code and then move on to measurements based on the carrier frequency for that code. This carrier frequency is much higher so its pulses are much closer together and therefore more accurate.

If you're rusty on the subject of carrier frequencies consider your car radio. When you tune to 94.7 on the dial you're locking on to a carrier frequency that's 94.7 MHz.

Obviously we can't hear sounds at 94 million cycles a second. The music we hear is a modulation (or change) in this carrier frequency. So when you hear someone sing an "A" note on the radio you're actually hearing the 94.7 MHz carrier frequency being varied at a 440 cycle rate.

GPS works in the same way. The pseudo random code has a bit rate of about 1 MHz but its carrier frequency has a cycle rate of over a GHz (which is 1000 times faster!)

At the speed of light the 1.57 GHz GPS signal has a wavelength of roughly twenty centimeters, so the carrier signal can act as a much more accurate reference than the pseudo random code by itself. And if we can get to within one percent of perfect phase like we do with code-phase receivers we'd have 3 or 4 millimeter accuracy! Yeeow!

 

DGPS?

Code-Phase vs Carrier-Phase

In essence this method is counting the exact number of carrier cycles between the satellite and the receiver.

The problem is that the carrier frequency is hard to count because it's so uniform. Every cycle looks like every other. The pseudo random code on the other hand is intentionally complex to make it easier to know which cycle you're looking at.

So the trick with "carrier-phase GPS" is to use code-phase techniques to get close. If the code measurement can be made accurate to say, a meter, then we only have a few wavelengths of carrier to consider as we try to determine which cycle really marks the edge of our timing pulse.

Resolving this "carrier phase ambiguity" for just a few cycles is a much more tractable problem and as the computers inside the receivers get smarter and smarter it's becoming possible to make this kind of measurement without all the ritual that surveyors go through.

 

DGPS?

Augmented GPS

You've got to hand it to the FAA. They think big!

They realized the great benefits GPS could bring to aviation, but they wanted more. They wanted the accuracy of Differential GPS and they wanted it across the whole continent. Maybe the whole world.

Their plan is called the "Wide Area Augmentation System" or "WAAS," and it's basically a continental DGPS system.

The idea grew out of some very specific requirements that basic GPS just couldn't handle by itself. It began with "system integrity." GPS is very reliable but every once in a while a GPS satellite malfunctions and gives inaccurate data.

The GPS monitoring stations detect this sort of thing and transmit a system status message that tells receivers to disregard the broken satellite until further notice. Unfortunately this process can take many minutes which could be too late for an airplane in the middle of a landing.

So the FAA got the idea that they could set up their own monitoring system that would respond much quicker. In fact, they figured they could park a geosynchronous satellite somewhere over the U.S. that would instantly alert aircraft when there was a problem.

Then they reasoned that they could transmit this information right on a GPS channel so aircraft could receive it on their GPS receivers and wouldn't need any additional radios.

But wait a second! If we've got the geosynchronous satellite already transmitting on the GPS frequency, why not use it for positioning purposes too? Adding another satellite helps with positioning accuracy and it ensures that plenty of satellites are always visible around the country.

But wait another second! Why not use that satellite to relay differential corrections too?

Oh, this is sounding good!

The FAA figured that with about 24 reference receivers scattered across the U.S. they could gather pretty good correction data for most of the country. That data would make GPS accurate enough for "Category 1" landings (i.e. very close to the runway but not zero visibility)

This system is underway. Specifications have been drafted and approved and it's expected that the system could be working as early as 1997.

The ramifications of this go well beyond aviation, because the system guarantees that DGPS corrections will be raining out of the sky for everyone to use.

 

DGPS?

Local Area Augmention

To complete the system the FAA wants to eventually establish "Local Area Augmentation Systems" near runways.

These would work like the WAAS but on a smaller scale. The reference receivers would be near the runways and so would be able to give much more accurate correction data to the incoming planes.

With a LAAS aircraft would be able to use GPS to make Category 3 landings (zero visibility).

 

DGPS?

Putting GPS to work

GPS technology has matured into a resource that goes far beyond its original design goals. These days scientists, sportsmen, farmers, soldiers, pilots, surveyors, hikers, delivery drivers, sailors, dispatchers, lumberjacks, fire-fighters, and people from many other walks of life are using GPS in ways that make their work more productive, safer, and sometimes even easier.

In this section you will see a few examples of real-world applications of GPS. These applications fall into five broad categories.

Click below to learn more about each application:

  • Location - determining a basic position
  • Navigation - getting from one location to another
  • Tracking - monitoring the movement of people and things
  • Mapping - creating maps of the world
  • Timing - bringing precise timing to the world

 

DGPS?

Location

"Where am I?"

The first and most obvious application of GPS is the simple determination of a "position" or location. GPS is the first positioning system to offer highly precise location data for any point on the planet, in any weather. That alone would be enough to qualify it as a major utility, but the accuracy of GPS and the creativity of its users is pushing it into some surprising realms.

Knowing the precise location of something, or someone, is especially critical when the consequences of inaccurate data are measured in human terms. For example, when a stranded motorist was lost in a South Dakota blizzard for 2 days, GPS helped rescuers find her.

GPS is also being applied in Italy to create exact location points for their nationwide geodetic network which will be used for surveying projects. Once in place it will support the first implementation of a nationally created location survey linked to the WGS-84 global grid.

The Italian Grid

Using Trimble SSE GPS receivers, the Italian Military Geographic Institute is creating what is reputed to be the first nationwide geodetic network . This grid is based on the WGS-84 global grid, a mathematically created grid that surrounds the earth. While this global grid is accurate enough for geodetic research and measurements, it lacks the precision for loc