The Global Positioning System

Early Satellite Systems• Satellite Surveying started more than 30 years ago.

Now, High accuracy could be achieved in real time.• Doppler Systems:

– Measure frequency shift of a radio signal transmitted from a satellite as it passes by a receiver.

– Frequency shift is a function of the distance between the satellite and the receiver, the distance then can be calculated.

– Knowing the location of satellites at time of observation, point location is computed.

– Point Positioning: Data is collected from multiple satellites, point location is obtained in a coordinate system relative to the satellite position, then transformed to a traditional global system. Accuracy about 1m, takes 2 to 8 days.

– Translocation Method: Two receivers one on a known point used to correct the other. Still needed lengthy session and of low accuracy

Early Satellite Systems

GPS• A navigation and a positioning system, more

accurate.• NAVASTAR (NAVigation Satellitw Timing And Ranging)

satellites

• Based on observations of signals from satellites whose orbits are precisely known.

• precise satellite to receiver distances are determined from timing and signal information.

• Both the satellites and the receivers must have precise clocks for time determination, is that always the case?

Satellite Constellation• Satellite are in near-circular orbits

at altitude of 20,200km, about 20 times higher than Doppler satellites, why?

• 24 satellites in 6 orbital planes, 4 satellites per orbit + 3 spare

• Orbital planes are 60o apart, all of them are 55o to the equatorial plane.

• 21 satellites are necessary for coverage of almost all the earth at all times. At least 4 satellites can be observed any time nearly at any point on the earth.

• A satellite will travel around the earth twice every sidereal day, and is visible for a maximum of 10 hours per day at a point.

• Orbital information: managed by “GPS Operational Control System”–Broadcast ephemeris: precise, near future

predictions of satellite orbits–Precise ephemeris: precise, post orbital

information after tracking the satellites

Signals transmitted by GPS Satellites• Two carrier frequencies (radio):

– L1 of frequency = 1575.42 MHz, 19 cm– L2 of frequency = 1227.6 MHz, 24 cm

• Pseudo Random Noise (PRN) codes: binary, random like codes. Two codes:– Coarse/Acquisition code(C/A): of frequency 1.023MHZ,available

to the public, on L1 only– P-code: of frequency 10.23 MHz, for more precise positioning, not

available to the public ? on both waves

• Additional info such as: orbital data (broadcast and precise ephemeris), almanac of all other operating satellites.

• What is: Selective Availability (SA) and Antispoofing (AS)

Principles of GPS position Determination

• The general idea: solve a resection problem where the control point (satellite) locations are given and the distances are measured. Three spheres will define two points !

1- By Pseudoranging

• Of lower accuracy > 3m??, suitable for navigation not for geodetic applications.

• Based on using the C/A code for civilians. The receiver generate identical codes simultaneously. By matching the code it receives with the one it generated, it can determine the time difference and compute the distance to a satellite.

• Knowing distances to three satellites, the location (coordinates) of the point can be computed.

R2 = (X2 + Y2 +Z2)

• If a fourth satellite is observed, solve four equations

in four unknowns to eliminate clock bias error r.Usually, more than four are observed.

R2 = (X2 + Y2 +Z2) + r• Differential Pseudoranging: better accuracy. By

placing a receiver on a control point and the other

on unknown point, measure the difference in

Coordinates, corrections improve the accuracy.

2- By Carrier Phase Measurement• By measuring the phase shift of a carrier wave(s) and

traveling time, distance is obtained.• Knowing exact traveling time, the number of full waves

( cycle ambiguity ) may be obtained• The distance is R = Exact time cannot be measured because of inaccurate

clocks in the receiver, error is mathematically corrected for

• Attention must be paid to clock synchronization. Observing more than one satellite simultaneously by one receiver allows for the correction of the receiver “ clock bias ”

GPS Field Procedures• For surveying applications, all procedures are differential, with at

least two receivers, by phase measurement and employs relative positioning techniques.

1- Static:– The most accurate method. Two receivers, one at a control station,

then a “leap frog” technique is applied to close at a control point.– Using a software, coordinate differences are computed and

adjusted. Precise ephemeris could be used.– Accuracy is a bout (5 mm + 1ppm)

2- Rapid Static– The same thing, but sessions are shorter and a receiver is left at

the control point and the rover is moved between the unknown points

– Accuracy is a bout (10 mm + 1ppm)

Kinematic Methods3- Kinematic:

– Two receivers at two known points for about 3 to 15 minutes to solve for the initial ambiguities.

– Maintaining “lock” with at least four satellites for both receivers, the rover is moved between the unknown stations, session time is 1 to 10 seconds.

4- Pseudokinematic:– Short sessions with two receivers ( 5 minutes), but the

session is repeated after an hour.– Eliminates the need to maintain constant lock on the

satellites

5- Real-time Kinematic–Point positions are obtained instantaneously–The base receiver on a control point

broadcasting raw GPS observations to the roving unit(s) via UHF/VHF radio or Celluar communication.

–An onboard computer in the roving unit process the data from both receivers to produce an immediate determination of it’s location.

Coordinates and Advances

• Xs, Ys, Zs: origin is at the earth’s center.• Universal then to Geodetic then to whatever

system

• GPS is changing with the next generations of GPS satellites block III , C/A code on L1, L2,new code on L5. 1 to 3 m with single navigation receiver.

• There are other satellite positioning systems such as the Russian GLONASS or the EU Galileo, no signal encryption (Global Navigation Satellite System GNSS)

• Some receivers can mix the US and the Russian satellites

• Systems that enables users to rove with one unit and get corrections from other sources such as:– CORS: Continuously Operating Reference Stations: GPS

receivers on known points all over the US.

CORS COVEAGE IN THE US

“To provide measurements at this level of accuracy, NGS developed the Continuously Operating Reference Stations (CORS) network. CORS is a network of hundreds of stationary, permanently operating GPS receivers throughout the United States. Working 24 hours a day, seven days a week, CORS stations continuously receive GPS radio signals and integrate their positional data into the National Spatial Reference System. This data is then distributed over the Internet. After logging onto the CORS Web site, users can determine the accuracy of their coordinates to the centimeter. This system has been especially useful in assessing the integrity of buildings and bridges in areas that are geologically active or have been impacted by natural disasters such as hurricanes or floods”

Source is NOAA at: http://oceanservice.noaa.gov/education/kits/geodesy/geo10_cors.html

Real-time Augmentation Systems available in several places in the world today, North America and Europe for example.

1- WAAS: Wide Area Augmentation System: North America, Mexico, and parts of South America:

WAAS is a system of satellites and ground stations that provide GPS signal corrections, giving you even better position accuracy, an average of up to five times better. A WAAS-capable receiver can give you a position accuracy of better than three meters 95 percent of the time. No need for additional receiving equipment or pay service fees to utilize WAAS.

• WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal.

The Federal Aviation Administration (FAA) and the Department of Transportation (DOT) are developing the WAAS program for use in precision flight approaches. Currently, GPS alone does not meet the FAA's navigation requirements for accuracy, integrity, and availability. WAAS corrects for GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors, and it provides vital integrity information regarding the health of each GPS satellite.

2- Virtual Reference Stations• Trimble's VRS™ (Virtual Reference Station) system

uses the RTK solutions from the Trimble® RTKNet software and provides high-accuracy, real-time kinematic (RTK) GPS positioning for wider areas. The fixed VRS network is available at any time without setting up a base station and provides common control wherever you are in the network.

• The VRS system is made up of the latest in GPS hardware, modeling and networking software, plus communications interfacing. Once set up, RTK roving receivers in the field have access to real-time network modeled corrections. In the field, you also have the reassurance of the built-in integrity monitoring system that warns if there are any problems with the data.

GPS Errors• Satellite Ephemeris: Uncertainties in the orbits• Atmospheric Effect:

• Receiver Errors: clock, matching the transmitted signal, slant hight masurmnt, ..

• Multipath:

• Centering Error.• Elevation Errors

Planning GPS Projects

• Satellites are not observed if lower than 15o

• Which satellite will be observed from which point at what time?

• Minimum requirements: Four satellites for enough time, higher than 15o , Position Dilution of Precision (PDOP) not more 6.

• Use a computer to produce a table and a polar map, if needed.