GPS

Question 1

What is Global Positioning System (GPS)?

Answer 1

A US-developed space based radio positioning system capable of very high accuracy!

Question 2

What are some attributes of GPS?

Answer 2

• Jam Resistant
• Continuous Coverage in all weather conditions
• Passive system - infinite users are possible at the same time.
• One of several worldwide: GLONASS (Russia), GALILEO (EU)

Question 3

What are the two primary uses of GPS?

Answer 3

1. Navigation - to navigate from where you are to where you want to be.
2. Mapping - to record and map detailed routes with data about those
   locations stored as attributes (Oderwald & Boucher, 1997

Question 4

What are the three components of GPS?

Answer 4

1. User Segment - GPS units that receive ephemeris and Pseudo Random Noise signals
2. Space Segment - satellites transmit ephemeris and PRN data
3. Control Segment - ground stations that precisely track satellites and correct the ephemeris and time

Question 5

What does ephemeris mean?

Answer 5

Precise orbital data of each satellite

Question 6

How many satellites are there? How many orbital planes?

Answer 6

24 satellites

6 orbital planes

Question 7

What are NAVSTAR Satellites?

Answer 7

NAVigation System using Timing And Ranging

First satellites launched in 1979-1985.
Improved in 1989 with 28 more satellites, 24 total 3 are working spares, 4 additional spares to be launched if needed.
Current constellation is 32 satellites with a mix of technologies, with upgrades scheduled into the 2025 period.

Question 8

For NAVSTAR Satellites, what are control stations and what are master control stations?

Answer 8

1. Control stations: monitor clocks and monitor orbits

2. Master Control Stations: uploads orbit corrections and generates clock corrections

Question 9

Where are the five control stations in the world?

Answer 9

1. Hawaii
2. Colorado Springs
3. Ascension
4. Diego Garcia
5. Kwajalein

Question 10

How does GPS work?

Answer 10

1. GPS calculates position using resection from multiple satellites
2. GPS resects an unknown location by using time to measure the distance from the satellite to the receiver
3. To measure travel time, GPS needs very accurate synchronized timing between satellite and receiver clocks, receivers synch based on data from the satellites.
4. The precise location of the satellite is known from the ephemeris/orbit data
5. For greatest accuracy, the signal must be corrected for any delays it experiences as it travels through the atmosphere

Question 11

What is resection?

Answer 11

2D and 3D positioning by ranging using three measurements that put us at one or two points.

Question 12

How does GPS calculate position?

Answer 12

GPS uses a difference in time (about 0.06 secs when satellite is directly overhead) to measure the distance from each satellite to the receiver

Question 13

What is the distance from each satellite to the receiver called? What is the calculation?

Answer 13

The distance is known as a pseudorange because it is an estimate of the true distance

distance = rate x time
rate = speed of light (186,000 miles per second)

Question 14

For a 2D (2 Dimensional) Point - receiver’s location is determined by using pseudoranges to _____ satellites to solve ________.

Answer 14

Three

Satellites to solve for X, Y, and time error, Z must be assumed

Question 15

3D (3 Dimensional) Point - receiver’s location is determined by using pseudoranges to ______satellites ______________.

Answer 15

four

to solve for X, Y, Z, and time error – no assumptions needed and Z is computed as well

Question 16

What is the Standard Positioning Service? (SPS)

Answer 16

Carrier Phase Frequency
L1 = 1575.4 MHz
 - carries the C/A code and modulation with the navigation message
Coarse Acquisition (C/A) Code
  - civilian PRN code at 1.023 MHz

Selective Ability (S/A) - introduction of error by DoD (REMOVED 5/2000)

SPS 95% accuracy (what you did expect)
100 meter horizontal accuracy
156 meter vertical accuracy
340 nanoseconds time accuracy

Since 5/2000 autonomous accuracy improved to about 22 meters horizontal, 33 meters vertical!!

Question 17

When does Dilution of Precision (DOP) occur?

Answer 17

Dilution Of Precision (DOP) occurs if the satellites are not well distributed around your measurement point. This concern can be reduced by adding more satellites to view except under less than ideal conditions.

Question 18

What affects the DOP?

Answer 18

Clearly the position of the satellites in the sky is not under your control (but you can plan for it), and the coverage of satellites is pretty good (many GPS now do GLONASS as well as GPS satellites). Way more important are vertical surroundings in which the sky is not open in all directions.
Never a problem at sea or in flight, but often on land in urban areas and forests.
Urban areas also cause bouncing of signals (multi-path) off of buildings disrupting timings of the signals and overestimating ranges
Signals that cannot be seen, cannot be used so the DOP goes up in these areas.

Question 19

What does the almanac provide?

Answer 19

Up to date orbit (ephemeris) information for the GPS satellite constellation.

Question 20

How is GPS improvements improved?

Answer 20

Error removal occurs in many ways and is a better strategy than trying to make the system more accurate (it is already very accurate). There are a few methods:

• Real Time Differential correction
• Post-Processing Differential Correction
• Carrier Phase
• Augmentation

Question 21

What is differential correction?

Answer 21

1) Set up a “Base” station at a precisely known point.
2) Have the base station compute the error for every possible GPS computation (permutations of 3-4 satellites).
3) Record data simultaneously from the same satellites at both the “Base” and “Rover” stations – can be done successfully up to about 150 miles.
4) Match the rover data with the base data corrections to improve their accuracy.
5) Results are generally considerably better than underthe autonomous collectionusing a rover alone.

Slide 32

Question 22

What is real time differential?

Answer 22

1) If we are in the field and need to know a precise location in real-time (highly precise navigation for instance) we can elect to use real-time differential corrections.
2) If we add a radio broadcast to the base station and a receiver for that broadcast to the rover, we can do correction in real time.

• The base station computes corrections for all combinations of satellites visible each second, and then broadcasts those out to the rover.
• The rover applies the correction that matched the satellites it used for the calculation and updates the current position.

3) This system is more costly as it requires the use of an extra radio system at both ends.

Question 23

What is post processed differential?

Answer 23

If we are in the field and need to know a precise location in real-time (highly precise navigation for instance) we can elect to use real-time differential corrections.
If we add a radio broadcast to the base station and a receiver for that broadcast to the rover, we can do correction in real time.

• The base station computes corrections for all combinations of satellites visible each second, and then broadcasts those out to the rover.
• The rover applies the correction that matched the satellites it used for the calculation and updates the current position.
• This system is more costly as it requires the use of an extra radio system at both ends.

Additional time available in post-processing often provides marginally better results than real-time

Question 24

How do real-time and postprocessed DGPS’ compare?

Answer 24

Slide 35

Question 25

What are carrier phase GPS?

Answer 25

All previous methods of positioning have relied on the Pseudo-Random Code generated atop the carrier frequency wave sent from the satellite. This code provides the “data” like a radio wave carrier frequency (89.1 Mhz) carries the Car Talk content.

Carrier Phase looks at the constant wavelength of the carrier wave (known frequency ≈ 1575 Mhz = 0.19m = 7.5” wave length)

The codes are matched up between the receiver and the satellite in the PRN method. When they “match”, we assume the slide time to get that match (time difference) is precise and gives us the time thus the distance.
In fact, that is not quite true because the codes are 1.023 microseconds (or more) in duration, thus reducing precision.

They can still be off a little and match when they are sampled. This error can be up to 1.023 microseconds which at the speed of light is 300 meters. Good receivers have this limited to about 3-6 meters

Question 26

What is the carrier?

Answer 26

The carrier is at 1575 mHz. so its wavelength is far shorter (7.5”) than the PRN data spacing (11811”). The carrier allows us to use the distance between the peaks which are 1500 times shorter than the PRN switching.

-Now we find distance errors of about 3-4 millimeters (as opposed to 762 mm. for PRN).

Carrier phase effectively counts the number of wavelengths between the satellite and receiver (including fractions of the last one).

Question 27

What are some issues with the carrier?

Answer 27

The pseudo random code is intentionally complex to make it easier to know which cycle you’re looking at to match it up, but the carrier frequency is hard to match because it’s so uniform. Every cycle looks like every other, so it is very difficult to match the carrier by the receiver (these are special receivers).

• Today, receivers that can do this use the PRN first to get close to the right match, then hone in from there on the carrier to finish the job. The PRN is easy to match, and sits on the carrier so the carrier phase is identified as to its phase.
• Carrier phase GPS was originally very slow (would take an hour to get a point), so it was used for setting benchmarks in the open, not for general work. Could not lose lock on a satellite (remember they are moving). Time has been reduced a great deal to < 10 minutes now, but still only possible in the open.

Question 28

What is an augmented GPS?

Answer 28

The FAA has been looking at adding satellites to assist in landing aircraft under poor conditions. They call this system the Wide Area Augmentation System.

A set of 3 geostationary satellites (they appear to be over the same place on Earth all the time as they orbit and the same speed as the earth) and 38 precisely located ground stations cover the US and provide accuracy in real-time down to 1 meter horizontal, and 1.5 meters vertical at US airports.