Flight Planning and Navigation

Flight Planning and Navigation

GPS Navigation

What you should know at the end of the powerpoint. Read these to preview your learning.

1. How many GPS satellites are operating to make the system work?

2. How many satellites can a GPS unit ‘see’ from any location?

3. If you ‘see’ only 2 satellites, there are many possible locations. What shape do all these locations make?

4. If you ‘see’ 3 satellites, there are two possible locations. Why is one not a possibility?

5. What is an ephemeris?

6. Why is a fourth satellite needed to make the GPS system work?

7. What happened on May 1, 2000 that allowed all of us to use GPS?

8. How has the WAAS system allowed pilots to use GPS?

9. Give three benefits of the WAAS system.

10. Why is Geometric Dilution of Position (GDAP) a problem for GPS?

11. What is an advantage of the National Differential GPS system?

Global Positioning System (GPS)

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Global Positioning System (GPS)A global positioning system is based on multiple satellites that are orbiting the Earth. The

triangulation of the satellites is based on extremely precise timing of radio waves that are received from the satellites. Since the satellites are not stationary, but moving through space at thousands of miles per hour, the radio waves are slow, bend, and bounce their way from satellite to receiver.

With the challenges that arise from the satellites orbiting the earth it is easier if you look at how the GPS works in smaller bits. How does GPS really work, how is the accuracy challenged and maintained, and how are technical and engineering techniques used to overcome those challenges to make GPS the most readily available, accurate, and truly global navigational system available.

• Cloud of 24 GPS satellites orbit the Earth• Satellite positions are accurately known• GPS device receives satellite signal with ‘time-sent’ information• Device calculates distance to satellite• Intersection point of multiple satellites defines device location

GPS Satellites

Orbital Configuration

Altitude: 12500 miles, speed: 8700 miles/hr, 4-12 visible at any place

Satellite Precision

One satellitelimits possible GPS receiver location to a spherical location

Two satelliteslimits possible GPS receiver location to a circular location

A GPS system calculates the distance from every satellite that a receiver can see above its local horizon. With one distance you know that you have to be somewhere on a tremendously large sphere that is centered on the satellite. When a second satellite is available and the distance is known, you have a second sphere that intersects with the first sphere. Since the GPS receiver has to be on a sphere, and not inside or outside of it, the intersection of the two spheres is a huge circle in space. This circle can be huge, with a diameter much larger than the entire planet Earth.

Satellite Precision

Simplifieddiagram

One location is impossible due to location and speed

Three satelliteslimits possible GPS receiver location to two locations

If a third satellite is provided, the intersection of all three is reduced to two locations in space. One location is illogical as it is in outer space somewhere or it is moving at a ridiculously high speed. The GPS receiver can be programmed to ignore this point and assume that the other one is where you are located. Of course, you could just as easily use a fourth satellite measurement to confirm this decision.

GPS Orbital Configuration

Tracks Across the Earth

Satellites orbit the earth in a uniform path; however, the earth moves underneath of the path. This is the reason why the track cross the ground is always changing as shown in the image. This is why the blue lines below don’t stay in the same place with each orbit.


Tracks Across the Earth

Unfortunately, knowing just the distances from the satellites doesn’t really help us find our location. The satellites are very far away and moving… so we are XX miles away from where? In order for GPS to work, we need to know where the satellites are much more accurately than we want to know our actual location.

GPS Orbital ConfigurationGPS satellites orbit very high above the Earth. This makes their flight path very predictable into the future. So predictable that it is easy to calculate an “ephemeris” for the satellite. An ephemeris is a look up table of future positions of an object. The ephemeris data for every GPS satellite is confirmed and updated by using very powerful and accurate ground based radar measurements of the satellites current position and velocity in space. The ephemeris is constantly changing due to gravitational pulls of objects (Sun and Moon) as well as drag caused by the solar wind. The Department of Defense broadcasts this information up to the satellite and it sends its ephemeris data out along with its unique Psuedo-Random Code.

Measuring the distance to an orbiting satellite is a daunting task using physical means. However, using radio broadcasts of a satellite makes this a simple task. After all, radio waves travel at a known speed; the speed of light.

Imagine seeing a marathon runner during a race. The runner always runs at 10 mph. When you stop the runner and check his or her wrist-stopwatch, you find that the runner has been running for 2 hours. If you use the distance formula (i.e., Distance = Rate * Time) you can determine that they must be 20 miles into the race.Rate (10 mph) x Time (2 hrs) = Distance (20 miles)

For a radio signal, the rate is about 186,000 miles per second. At speeds this high the calculated distance is only as accurate as your ability to measure time very accurately. If your stopwatch is off by one second, your distance is off by 186,000 miles.Challenge: How accurate must the stopwatch for light have to be in order to achieve an accuracy of less than 52.8 feet (1/100th of a mile)? How about 5.28 feet?


Back to the runner. You wanted to know how far the runner had run and you were able to determine this by checking the stopwatch to determine how long he or she had been running. It makes sense that you must check the stopwatch to find out how far the runner had traveled, but who started the stopwatch in the first place? If you looked in the newspaper, you could look up the fact that the marathon was supposed to start at noon. Using your own watch you could measure the time of arrival and figure out the travel time of the runner. The problem is that you can’t know if the race really started on time, and you need to have a very precise time measuring tool on your wrist.

A GPS system gets around this problem by having a satellite broadcast the same short phrase over and over again; remember the message does not need to be very long as the speed of light is very fast; the message arrives at the receiver very quickly. The short phrase that is used by the satellite is called the Pseudo-Random Code (PRC) and it is an engineering breakthrough even though it is really just a pattern of digital 1s and 0s, or “on” and “off” pulses. The pulses are very complex, almost random noise, to guarantee that the receiver can ID (identify) the satellite that is broadcasting the signal.

GPS Orbital ConfigurationThe complexity of the process allows the system to use the same frequency for all satellites and makes it really hard to jam the system to degrade its accuracy. This is an important concern considering the system was originally developed by and for the military.

The other reason the PRC is an engineering breakthrough is it’s ability to allow the satellites to use very weak signals and the receivers to use very small antennas. This makes the system very affordable and portable. The miracle is in the noise, the real noise—real noise IS random and thus if you compare the 1s and 0s in noise with the PRC’s 1s and 0s they will match only half the time. If you were to declare a +1 every time the two signals match and a -1 every time they mismatch, then add up all the signals you will wind up with a net result of a zero no matter how long you compare the signals.

If you were to compare the PRC to another signal that has lots of noise (i.e., weak signal and small receiving antenna) and the same PRC signal embedded in it. The longer you compare the signals, the more the sum drifts positive. The weaker the signal or the longer you collect the signal, the outcome is the same. You can amplify the signal and confirm the match. All of this is important in order for the receiver to compare its internal repeating phrase to that from the satellite. The two signals start out in sync but the satellite’s phrase is delayed by exactly the travel time to the receiver. If you measure that time delay and multiply it by the speed of light and the result is the distance to the satellite.


On the GPS satellites the timing for the start of the PRC broadcast is synced to a tremendously precise onboard atomic clock that cost tens of thousands each. You can’t afford this same technique for timing on the receiver side. How do you start the internal PRC phrase of the receiver to work at the right time? It is safe to assume that the receiver’s cheap internal clock is not in sync with atomic clock accuracy. However, the inaccuracy creates a predictable error. Let’s assume that the receiver clock started its PRC phrase a little late. Using a simple example, assume the receiver was a second late and remember that real travel times for waves are on the scale of 0.1 second or less. Since the receiver doesn’t know its clock is late it would just assume that every time delay it measured was 1 second shorter than it really is. This shortens the distance to each satellite by 1 second or 186,000 miles, which drastically shifts the predicted location of where the signals from three satellites signals could intersect. The receiver doesn’t know that it has a problem. However, add in a fourth satellite and they can’t possibly overlap at the same place.


This is where computers really shine. The receiver’s computer chip can simply try different time corrections for all four-satellite signals until it finds one that makes them all pass through a single point in space. If it then actually applies this correction to its internal clock,—the clock is now synced to the atomic clocks on the satellites; for dollars rather than tens of thousands of dollars. The secret is the fourth satellite signal and thus most GPS units have at least a four-channel receiver so that they can process the signals simultaneously.

GPS Accuracy

• Within 100 meters (328 ft)– Original GPS

• Within 15 m (49 ft)

Selective Availability removedBefore May 1, 2000 the government purposely degraded the

timing data of the satellites clock by adding noise to the signal for military purposes. They may also have introduced slight inaccuracies to the ephemeris data. Military GPS receivers made use of a decryption key to obtain the full accuracy information. On May 1, 2000, this Selective Availability (SA) was disabled thus improving the accuracy of GPS positions by a factor of 10.

GPS Accuracy

• 3-5 m (10-16 ft)

– Differential position (GDPS)

• < 3 m (10 ft)

– Wide Area Augmentation System (WAAS)

The Wide Area Augmentation System (WAAS) is based on using a geostationary satellite that broadcasts on the GPS frequencies to transmit alerts that a particular satellite has errors and should be ignored and, more importantly, the differential correction data from about 25 stations scattered all across the country. WAAS enabled GPS units have allowed aircraft to commit to “category 1” landings in which they can use the unit to navigate very close to the runway before they must obtain visual references for the final landing.

WAAS - Wide Area Augmentation System• Operated by FAA (U.S. Federal Aviation

Administration)

• Aircraft navigation for all phases of flight

Benefits of WAAS

• Primary means of navigation

• More direct routes

• Approach with vertical guidance

• Decommission older equipment

• Simplify onboard equipment

• Increased capacity

GDOP - Geometric Dilution of Precision GPS accuracy is

influenced by the visibility and wide angles to the satellites.

GPS accuracy is reduced when objects block the signals or the satellite angles are to small (satellites are too close together).

NDGPS – Nationwide Differential GPS System• Accurately surveyed locations used for

reference

• Corrects GPS for increased accuracy for users on land and water

• Developing system for 10-15 cm (4-6 in) accuracy

Quick Learning Check: answer each question in your mind then click to see if you are correct. Go to the slide for more information if you don’t remember the answer.1. How many GPS satellites are operating to make the system work?

21 with 3 spares (slide 6)


4 – 12 (slide 6)

3. If you ‘see’ only 2 satellites, there are many possible locations. What shape do all these locations

make?

A circle (slide 7)


The position is in space or moving ridiculously fast. (slide 8)

5. What is an ephemeris? Why is it important to GPS?

An ephemeris is a table of future satellite positions which allows the GPS satellite to broadcast its position and allow the GPS to calculate your location. (slide 11-14)


The fourth satellite’s signal allows the computer to find the one possible location for all 4 satellites.


Selective availability (SA) was disabled. SA introduced error into the ephemeris data in case our enemies tried to use our satellites against us.


The WAAS system provided a location within 10 feet of actual location and allows for landing using GPS. (slide 17)


For answers, see slide 19.


GPS accuracy is reduced when objects block the signals or the satellite angles are to small (satellites are too close together). (see slide 20)


Can locate you within 10-15 cm (4 – 6 in) of your actual position.

GPS Powerpoint notes

1. How many GPS satellites are operating to make the system work?

21 with 3 spares (slide 6)


4 – 12 (slide 6)

3. If you ‘see’ only 2 satellites, there are many possible locations. What shape do all these locations

make?

A circle (slide 7)


The position is in space or moving ridiculously fast. (slide 8)

5. What is an ephemeris? Why is it important to GPS?

An ephemeris is a table of future satellite positions which allows the GPS satellite to broadcast its position and allow the GPS to calculate your location. (slide 11-14)


The fourth satellite’s signal allows the computer to find the one possible location for all 4 satellites.


Selective availability (SA) was disabled. SA introduced error into the ephemeris data in case our enemies tried to use our satellites against us.


The WAAS system provided a location within 10 feet of actual location and allows for landing using GPS. (slide 17)


For answers, see slide 19.


GPS accuracy is reduced when objects block the signals or the satellite angles are to small (satellites are too close together). (see slide 20)


Can locate you within 10-15 cm (4 – 6 in) of your actual position.

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Flight Planning and Navigation