how gps works

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How GPS Works

US Army photo taken from http://electronics.howstuffworks.com/gps1.htm

How GPS Works

• Each GPS satellite transmits a microwave radio signal composed of two carrier frequencies– L1: 1575.42 MHz– L2: 1227.60 MHz

• Embedded into each carrier frequency are two codes– Coarse acquisition (C/A) code – Precision (P) code

How GPS Works

• The codes are often referred to as pseudo-range (PRN) codes because they initially appear as random, noise-like signals

• The C/A code is less precise than the P code but available to all users

• The P code is more precise but access to this code is restricted

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GPS Services

• Standard Positioning Service– Utilizes the C/A code– Available to all users worldwide– A feature called Selective Availability (SA) can

potentially limit the accuracy of the service

• Precise Positioning Service– Utilizes the P code– Domain of the Department of Defense – More accurate military positioning– Only available to users authorized by the military– The measuring accuracy is on the order of centimeters

on modern receivers

GPS Services

GPS POSITIONING

• There are two (2) broad categories for GPS positioning:– 1. Pseudo-range measurements

– 2. Carrier-phase measurements

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Pseudo-Range Measurements

• The range from one satellite defines a sphere, where somewhere is a point that represents the receiver

http://electronics.howstuffworks.com/gps1.htm


• If two pseudo-ranges are obtained, both will define radii of spheres and the result will be the intersection of two spheres



• If three pseudo-ranges are obtained, their intersection will define one point that represents the receivers horizontal position

• The reception of four satellite signals allows for the determination of the actual receiver position.


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Determining Position

http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html

Calculating Position

• In order to determine position in two dimensions three satellites are required

• Four satellites are required for three dimensions• Mathematically there are four unknowns: X, Y, Z

and time– Four knowns are required to solve for four unknowns


• Knowns: light travels at 3.0 x 105 (300,000) km/s AND the satellite are in a 20,200 km orbit

• The least amount of time for the radio signal to travel is 20200 km /(3.0 x 105 km/s) = 67.3 milliseconds

• Mathematically we can represent the true range R as being made up of the pseudorange PS and the clock bias, CB

R = (PS - CB) = {(Xs, Xr)2 + (Ys, Yr)2 + (Zs, Zr)2}

Where Xs, Ys, Zs is the position of the satelliteand Xr, Yr, Zr is the position of the receiver

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• Clock bias can be divided into two segments– Satellite error – Receiver error

• The satellite clock error is transmitted for each satellite in the Navigation Message in the form of four coefficients: a0, a1, a2, and a reference time t0

• This yields the equation

CBs = a0 + a1(t - t0) + a2(t - t0)2

for given time, t

• Solving this equation for each satellite will resolve most of the satellite clock error


• Once the original equations are mathematically solved for each satellite, three coordinates of position are obtained

• This data is translated into values referenced to a particular datum

Carrier-Phase Measurements

• Another way of measuring the distance to satellites is with carrier-phases

• The range is determined to be the sum of the total number of carrier cycles plus fractional cycles at the receiver and satellite, multiplied by the carrier wavelength

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• The L1 and/or L2 carrier signals are used in carrier phase surveying• Tracking carrier phase signals provides no time of transmission

information• The carrier signals, while modulated with time tagged binary codes,

carry no time-tags that distinguish one cycle from another• The measurements used in carrier phase tracking are differences

in carrier phase cycles and fractions of cycles over time• At least two receivers track carrier signals at the same time• Receivers usually need to be within about 30 km of each other• Carrier phase is tracked at both receivers and the changes in

tracked phase are recorded over time in both receivers

• The lack of a time-tag results in a potential error source known as the ambiguity bias

• Despite the ambiguity bias, highly accurate measurements can be achieved with carrier-phase measuring techniques if relative positioning is employed.

• A cycle slip is defined as a discontinuity or jump in the carrier-phase measurements due to temporary signal loss– Signal loss can occur due to obstruction from trees, buildings,

or other objects, radio interference, ionospheric disturbance, or receiver malfunction


• Cycle slips may last a couple minutes or much longer and affect more than one satellite signal

• The size of the slip could vary from one cycle to millions

• In order to avoid large errors in position computation, cycle slips must be identified and corrected


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Real-Time vs. Post-Processing

• Real-time data collection means that results of the location computation are displayed almost instantaneously.

• Post-processing means that the measurements are collected in the field and processed at a later time to obtain the results

Advantages of Real-time– Results are obtained immediately– Higher productivity (only a certain amount of data is

collected)– No training on post-processing software required


• Advantages of post-processing– Generally obtain more accurate results due to

flexibility of editing data– No accuracy degradation due to data latency– Communication link problems (line of sight) are

avoided– Input parameters like base station coordinates

or antenna height may contain errors, which will lead to errors in the computed rover coordinates. These errors can be corrected through post-processing but not in real-time


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GPS Positioning Modes

• Point positioning– Employs one GPS receiver to track at least 4 satellites and

measures the PRC to determine the users position– Must acquire satellite coordinates as well as satellite ranges – Satellite coordinates obtained through navigation message,

while the ranges are obtained from the C/A code or the P code• Relative positioning

– Also called differential positioning– Uses two receivers to simultaneously track the same satellites

to determine their relative coordinates– One receiver is selected as a reference that remains stationary

at a site with precisely known coordinates– The other receiver has unknown coordinates– Provides higher accuracy than point positioning

• Static Surveying– Employs two or more stationary receivers simultaneously

tracking the same satellites– One receiver is setup over a point with precisely known

coordinates– Method is based on the two receivers collecting simultaneous

measurements at the base and the rover over a certain period of time

– After processing, the coordinates of the rover position can be extrapolated

– Observation time varies from about 20 minutes to a couple of hours

– After completing the field measurements, the collected data is downloaded to a PC for processing

– Static surveying with carrier-phase measurements is the most accurate positioning technique


• Fast surveying– Based on carrier-phase measurements– Similar to static surveying technique– The base receiver remains stationary over a point during

the entire data collecting session– The rover receiver remains stationary over a point for a

short period of time, then is moved over another point whose coordinates are sought

– The base receiver can support any number of rovers– This method is best used when many point locations are

needed in an area (within 15km of base)


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• Stop and go surveying– Based on carrier-phase measurements– Employs two or more receivers simultaneously tracking

the same satellites– The base receiver remains stationary over a point with

known coordinates– The rover receiver(s) travel between points with unknown

location data– The rover stops at each location for about 30 seconds– This method is best used when a large number of

locations are needed


• Stop and go surveying– This method requires that the observations be made in a

systematic manner– An observation plan, or static network design, should

entail short, closed geometric loops so the rover can travel the shortest distance between adjacent observation points

– The goal in designing the pattern is to connect with as many adjacent points as possible while moving along the pattern

– Visits to each location at least twice are recommended– This allows the GPS constellation to move so different

angles for each pseudo range are taken


• Real-Time kinematic– Similar to Stop and Go– The rover receives correction information from the base

receiver– On-the-fly ambiguity resolution determines most initial

ambiguity parameters so no post-processing is necessary– Best utilized when

• There are many locations to identify (up to 10-15 km)• The coordinates of the locations are required in real-time• The line of sight from rover to base is relatively

unobstructed


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• Real-time differential– Based on code positioning– Employs two or more receivers tracking the same satellites– Used when meter-level accuracy is sufficient– Based on the fact that errors in measured pseudoranges are

essentially the same at the base and the rover– The base receiver remains stationary over a known point– Built-in software in the base receiver uses precisely known base

coordinates as well as the satellite coordinates, derived from the navigation message, to compute the ranges to the satellites in view

– The software takes the difference between the computed ranges and the measured pseudo-code ranges to obtain the pseudorange errors (or DGPS corrections)


• Real-time differential GPS (cont.)– The corrections are transmitted in a format called Radio

Technical Commission for Maritime Service (RTCM) to the rover through a communication link

– The rover then applies the corrections to correct the measured pseudoranges at the rover

– The corrected pseudoranges are used to compute the rover coordinates

– Accuracy ranges from sub-meter to ~5m, depending on base-rover distance, transmission rate of the RTCM corrections, and performance of the C/A code receivers


how gps works

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