DGPS Overview GPS Positioning Modes

Question 1

Differential GPS (DGPS)

Answer 1

• Developed by the U.S. Coast Guard and Managed by Coast Guard Navigation Center (NAVCEN)
• Enhances the SPS and refines the C/A code precision

Question 2

Differential GPS (DGPS)

Answer 2

 Using a known location:
1.Identifies positioning errors
2.Applies a correction factor to another receiver in the same area
tracking the same satellite
 Augmentation systems: WAAS vs. LAAS
3.Measure pseudo-range errors

Question 3

Differential GPS (DGPS)

Answer 3

 Basic GPS is an accurate radio-based navigation system.
 For many applications, it is quite accurate.
 But it is human nature to want more!

Question 4

Differential GPS

Answer 4

 Differential GPS can yield measurements to a couple of meters in moving applications and even better in stationary situations.
 The improved accuracy has a profound effect on the importance of GPS as a resource.
 With DGPS, 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.

Question 5

DGPS Two Receivers

Answer 5

 Differential GPS
 Reference Receiver: Base,
stationary
 Roving Receiver: Mobile, remote
 The stationary receiver anchors
satellite measurements to a local
reference.

Question 6

DGPS Implementation

Answer 6

In real-time, a link sends data from the base GPS receiver to the
rover.

Question 7

DGPS Implementation

Answer 7

 RTCM (Radio Technical Commission for Maritime)
 Broadcasts real-time DGPS corrections in RTCM
 A beacon receiver is required to utilize these corrections

Question 8

DGPS Applications

Answer 8

 Not all DGPS applications are the same.
 Some don’t need an immediate radio link for precision.
 Example:
 Precise method: A drill bit over a specific spot
 Less-precise method: The track of a new road for inclusion on a map

Question 9

DGPS Applications

Answer 9

 No radio is needed for real-time systems.
 If a reference receiver isn’t available, local correction alternatives might exist.

Question 10

Differential Correction

Answer 10

To correct the GPS rover’s location for latitude,
longitude, and altitude:
1) Calculate the difference:
Difference = Known fix – GPS fix
2) Add the difference to GPS rover’s location:
Corrected location = GPS rover + difference

Question 11

GPS Positioning Mode

Answer 11

 GPS Standalone Positioning vs. GPS Relative Positioning
 What is the major difference between the two?

Question 12

GPS Standalone Positioning

Answer 12

Standalone (or autonomous) Point Positioning (SPP)
- Utilizes one GPS receiver
 Needs four ranges to four satellites.
 Provides satellite coordinates
 Uses either C/A code, or P(Y)-code
 Contains 4 errors: 3 for receiver coordinates, and one for clock
 Suitable for recreation and low-accuracy navigation

Question 13

GPS Standalone Positioning

Answer 13

Precise Point Positing (PPP)
 Utilizes ionosphere-free linear combinations of carrier-phase and pseudo-range measurement
 Uses dual-frequency data, precise ephemeris, and clock
products
 Uses two receivers
 Requires a static receiver, making it unsuitable for real-time
applications

Question 14

GPS Relative Positioning

Answer 14

 Differential Positioning
 Utilizes two GPS receivers.
 A base with known coordinates.
 A rover (remote) with unknown coordinates
 Needs four ranges to four satellites
 Uses both carrier-phase measurements and pseudo-range (code-phase)
measurements
 Achieves high accuracy by eliminating most range errors.

Question 15

GPS Relative Positioning

Answer 15

 Requires more time and incurs greater expenses.
 Regarding the distance between the two receivers:
 The closer the two receivers are, the more their errors align.
 Differencing their measurements can reduce these errors

Question 16

GPS Relative Positioning Techniques

Answer 16

1. Static GPS Surveying
2. Fast Static Surveying
3. Stop-and-Go GPS Surveying
4. RTK GPS Surveying
5. Real-Time Differential GPS Surveying

Question 17

Static GPS Surveying

Answer 17

 Uses multiple stationary receivers:
 Including a fixed base and a remote receiver
 Relies on carrier-phase measurements
 Represents the most accurate positioning technique
 Commonly used for surveying

Question 18

Static GPS Surveying

Answer 18

Accuracy
 1.5 cm for a 10 km baseline
 Sub-centimeter for 100 km
 Centimeter for 1,000 km
Observation times
 20 minutes to several hours, based on visible satellites
Downside
 Needs long observation times

Question 19

Fast (Rapid) Static Surveying

Answer 19

 Carrier-phase based relative positioning technique
 A Fixed base receiver (known) and a moving rover receiver
(unknown), which can be off during motion.
 For measuring baselines and positions
 Suitable for surveys with unknown points near (up to 20 km) a known one, suggesting shorter baselines

Question 20

Fast (Rapid) Static Surveying

Answer 20

 Software Data Processing: Aims to resolve ambiguities quickly
during a short observation period
 Accuracy: Centimeter-level, depending on baseline length and
satellite visibility
 Hardware: Dual-frequency receivers recommended
 Observation Time: 2-10 minutes, shorter than Static Surveying

Question 21

Stop and Go GPS Surveying

Answer 21

 Carrier-phase based relative positioning technique
 Allows the rover receiver to function while moving
 Semi-kinematic: Occurs during motion with about 30-second
stops, collecting data for 1-2 minutes
 Stop-and-go

Question 22

Stop and Go GPS Surveying

Answer 22

 No satellite lock loss is allowed; if lost, re-initialization is required.
 Kinematic GPS, which continues without stops and tolerates lock loss.
 Ideal for multiple unknown points within 15-20 km of a known one, suggesting shorter baselines

Question 23

Stop and Go GPS Surveying

Answer 23

 Kinematic surveying steps:
1. Receiver initialization
 Initialize the receiver in static mode to determine initial integer ambiguity
 Achieve centimeter-level positioning accuracy
2. Reoccupation
 After establishing ambiguities, start the survey. Continuously track
satellites with the antenna
3. Processing with a stationary receiver
 Coordinates of the roving receiver are determined in relation to the static
reference using double-differenced “carrier-range” data.

Question 24

Real - time Kinematic (RTK) GPS Surveying

Answer 24

 Carrier-phase-based relative positioning technique
 An extended stop-and-go technique
 Ambiguities must be resolved before surveying.
 If a cycle slip happens, re-initialization is required.

Question 25

Real - time Kinematic (RTK) GPS Surveying

Answer 25

 Suitable for:
 Surveys with multiple unknown points within 15-20 km of a
known point
 Real-time coordinate needs for unknown points.
 Area with clear line of sight or propagation paths

Question 26

Real - time Kinematic (RTK) GPS Surveying

Answer 26

 On-the-fly (OTF) ambiguity resolution
 Integer ambiguity determined during receiver movement
 Achieves 2-5 cm accuracy with a moving antenna
 No post-processing needed.
 Downside: Data latency due to formatting, transmitting, and
decoding based data.

Question 27

Real - time Kinematic (RTK) GPS Surveying

Answer 27

 Factors affecting accuracy
* Shorter base-rover distance (+)
* Higher RTCM DGPS correction transmission rate (+)
* Smoother performance of C/A-code receivers (+)
 Downside:
* High data volume needs

Question 27

Real - time Kinematic (RTK) GPS Surveying

Answer 27

 A CODE-BASED relative positioning technique
 Ideal for real-time meter-level accuracy
 Assumes GPS errors in pseudoranges are similar for both base and rover within a few hundred kilometers

Question 28

Post - Processing

Answer 28

 Combines base and rover data
 Computes differential corrections
 Applies them post-data collection

Question 29

Real - Time vs Post-processing DGPS

Answer 29

Advantages of real-time mode:
 Instant field results
 Faster data processing
 Increased productivity
 No post-processing needed
Advantages of post-processing mode:
 Higher accuracy
 Data editing flexibility
 No communication link issues

Question 30

Real - Time vs Post-processing DGPS

Answer 30

Differential code real-time correction
 1-10 meters precision
 Receivers must be within 100 km
Post-processed static carrier-phase surveying
 1-5 cm accuracy within 30 km of reference
 Requires two receivers tracking carrier signals simultaneously

Question 31

Communication (Radio) Link

Answer 31

Communication link is vital for:
 RTK (Real-Time Kinematic)
 Real-time DGPS
Obstacles (O) vs. Helpers (H)
 O: buildings, terrain, ground reflection
 H: power amplifier, high-quality coaxial cables, antenna
height, repeater stations