6_GPSGlonassTech_GG24.pdf

GPS+GLONASS Technology

and the GG24© Receiver

_____________

Magellan Corporation

471 El Camino Real, Santa Clara, CA 95050-4300, USA

Tel: +1 408-615-5100 ¤ +1 800-922-2401 ¤ Fax: +1 408-615-5200

Washington D.C. Tel: +1 703-476-2212 ¤ Fax: +1 703-476-2214

Europe, Africa & Middle East Tel: +44 118 9319600 ¤ Fax: +44 118 9319601

Website www.ashtech.com ¤ E-mail sales@ashtech.com

Table of Contents

GPS_________________________________________________________________________3

GPS + GLONASS_____________________________________________________________4

Availability _________________________________________________________________

Integrity ___________________________________________________________________

Accuracy ___________________________________________________________________

HOW GPS & GLONASS WORK________________________________________________9

The Basic Idea Ï Satellite Ranging ______________________________________________

Signal Structure Ï How the Time Delay is Measured ______________________________

Signal Structure Ï Technical Details ___________________________________________

Satellite Orbits Ï Technical Details ____________________________________________

GPS+GLONASS STANDARDS________________________________________________14

RTCM SC-104 _____________________________________________________________

NMEA 0183 _______________________________________________________________

HOW THE ASHTECH GG24 WORKS __________________________________________15

Navigation Modes (Availability and Accuracy) ___________________________________

RAIM (Integrity) ___________________________________________________________

CONCLUSION ______________________________________________________________16

GG24 SPECIFICATIONS _____________________________________________________17

GPS

When the Global Positioning System (GPS)

became operational in 1993, it promised to

provide a new utility as pervasive and as useful

as the telephone. For many GPS users, this

potential has already become a reality. Pilots

now use GPS to locate airports, mariners can

find harbors, hikers can find their way, and

surveyors can measure positions to centimeter

accuracy. New applications allow farmers,

miners and construction workers to guide their

machines using GPS. However, just as the

telephone system had limitations, the GPS

system has limitations that become apparent

in certain applications. Many of these

impediments to full utilization of GPS are

dramatically reduced by the augmentation of

GPS with GLONASS satellites.

cannot meet this very stringent requirement.

So, for the moment, airlines still include a

more expensive, less accurate means of

navigation with their GPS.

ACCURACY

Stand-alone GPS has a demonstrated accuracy

of better than 20 meters, horizontal, 95% of

the time. The imperfect predictions of

satellite orbits, satellite clock behavior and

atmospheric effects on the signals are the

primary causes of error in the basic GPS. GPS

accuracy has been so good that the U.S.

decided to deny the full capability of GPS to

users who are not specifically authorized this

level of accuracy. The denial of accuracy is

called Selective Availability (SA) and is part of

the GPS Standard Positioning Service (SPS).

SPS promises a position within 100 meters of

truth, horizontal, 95% of the time. Authorized

users have access to the GPS Precise

Positioning Service (PPS), which is not

corrupted by SA, and achieve 20 meter

accuracy 1 .

AVAILABILITY

A navigation system is ÐavailableÑ when it

produces valid position fixes. The availability

of a valid and accurate GPS position fix

depends most strongly on the visibility of

enough satellites. A GPS receiver needs to

ÐseeÑ at least four satellites to calculate

latitude, longitude and altitude. For real-time

centimeter accuracy, five or more satellites

are required. This is easy in a perfect

environment. With 24 GPS satellites orbiting

the earth, there are usually seven satellites

visible 10 degrees or more above the horizon.

But if there is a mountain, building or other

obstruction nearby, the number of visible

satellites may fall to four, three, or fewer, with

the possibility that the GPS receiver has too

few satellites to compute a position.

For marine safety, accuracy of 10m or better

is often required. Navigation aids, such as

buoys, are usually positioned to 10m accuracy.

A GPS receiver alone cannot provide this level

of accuracy because of Selective Availability.

Users and governments in many countries

have set up differential reference stations to

minimize the effect of SA errors and the

errors in the less than perfect predictions of

orbit, clock and atmospheric behavior. The

reference stations compute and transmit

corrections to users in the service area

depending on the reference station provider.

Unfortunately, the radios needed to receive

these corrections often cost more than the

GPS receiver itself.

INTEGRITY

A navigation system has ÐintegrityÑ when it

can warn the user that the position fix is in

error. ItÓs even better if the system can

remove the error and provide a correct

solution. A GPS receiver must have five

satellites to be able to detect an integrity

problem. To remove the satellite that is

causing the problem, a sixth satellite must be

visible. The U.S. Department of

Transportation suggests that the capability to

exclude an anomalous satellite be possible

99.999% of the time for a primary-means air

navigation system. Even if all 24 GPS

satellites are operational, GPS-only navigation

The White House has pledged to review the

policy of Selective Availability yearly, starting

in the year 2000. Until then, some SPS GPS

users have a choice of less accuracy or more

1 Unless specifically identified otherwise, the term

ÐaccuracyÑ used in this paper will mean that the

horizontal distance from the true location to the

estimated location is within the accuracy value 95%

of the time. This value is often referred to as

2dRMS.

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GG24 GPS+GLONASS Receiver

expensive differential GPS. In parts of the

world where no differential reference stations

are available, SPS GPS users will typically be

stuck with an accuracy of 100 meters.

additional 24 satellites to the current GPS

constellation with no deliberate degradation of

accuracy and no encryption of the most

accurate signals.

The impact of SA is magnified in the vertical

component. Hikers may spend all day

climbing a mountain, for example, ColoradoÓs

Pikes Peak. Upon reaching the top at 14,110

feet, their GPS receiver tells them they are at

only 13,800 feet. Ten minutes later, the same

receiver may indicate that they are at 14,400

feet. This is a result of Selective Availability,

which not only produces errors, but constantly

changes them. Not very reassuring for hikers

hoping to use GPS in areas where an extra few

hundred feet may mean the difference between

being on the cliff or over the edge.

Now ask yourself another question: ÐWhen

might we expect such a system to be in place

and ready to use?Ñ

The answer is: Now!

Believe it or not, the extra 24 satellites needed

to expand GPS to a true utility are already in

orbit and operational. In January 1996, the

Russians completed their full constellation of

24 operating satellites in the GLO bal

NAv igation S atellite S ystem (GLONASS), a

system almost exactly the same as GPS.

However, GLONASS has two significant

differences from GPS: no deliberate

degradation of accuracy and no encryption of

the most accurate signals. The addition of

GLONASS to GPS enhances the three

important factors: availability, integrity and

accuracy.

Surveyors, miners, farmers and others have

generally solved the accuracy problem by

installing their own differential reference

stations. They can and do achieve position

accuracy of centimeters. However, even here,

the constantly changing errors from Selective

Availability make an impact; the radio

corrections must arrive rapidly and constantly.

A few seconds of lost radio reception results in

rapidly growing errors, even though the GPS

receiver may be tracking several satellites.

Summary:

The extra satellites needed to remove the GPS

limitations are already operational.

The addition of GLONASS to GPS dramatically

improves availability, integrity and accuracy.

Summary:

For many users, GPS is a utility like a

telephone, but the system has limitations.

AVAILABILITY

In areas of signal blockage, there may not be

enough visible satellites to compute a position.

GPS alone cannot guarantee the six or more

satellites needed for integrity determination.

Figure 1

Availability: GPS-only Limited in the

Urban Canyon

GPS accuracy is degraded by the policy of

Selective Availability.

GPS + GLONASS

Ask yourself this question: ÐWhat if we could

add another 24 satellites to the GPS system,

but this time without any deliberate

degradation of accuracy. Would this remove

the limitations on the system?Ñ

The answer is: Yes!

All of the limitations discussed earlier would be

dramatically reduced simply by adding an

September 1996

GG24 GPS+GLONASS Receiver

Compare the number of visible satellites

shown in Figure 1 with the number of satellites

visible in Figure 2. The boat on the water sees

enough satellites to compute a position even

with GPS-only. But the car in the city doesnÓt

have enough GPS satellites to determine

position.

We use tools called Mission Planners to

analyze how many satellites will be visible

from any given location, with any known

obstructions blocking part of the sky. Satellite

visibility changes depending on time and

location. For the purposes of this paper, we

chose an arbitrary point at 37 degrees North,

122 degrees West (this is Sunnyvale,

California, where Ashtech GPS+GLONASS

receivers are built and tested). We constructed

an obstruction 45 degrees above the horizon,

covering the whole western sky, as well as a 10

degree obstruction for the eastern sky.

Examples of this kind of obstruction include

urban canyons, especially when the user is

close to tall buildings, open pit mines and

mountainous terrain. Figure 3 shows this

blockage scenario.

Figure 2

More Availability:

GPS+GLONASS Approaches 100%

Figure 1 clearly shows that environments with

obstructions, such as a cityscape, reduce

satellite visibility often to the point where the

receiver can no longer compute a position.

By adding the 24 GLONASS satellites to the

24 GPS satellites, there is twice the likelihood

that there is a satellite in the part of the sky

that is visible. The availability of a

improved over GPS-only in such situations.

Figures 3 and 4 demonstrate this point.

Figure 4 shows the satellite availability for

only the 24 GPS satellites. The straight lines

and right-side axis show the number of

satellites visible at each time through 24

hours. The broken line and left-side axis shows

a value called PDOP. PDOP is a statistical

measure of the accuracy of the computed

three-dimensional position and is influenced

by how the satellites are spread around the

sky. If PDOP doubles, then the expected

position errors also double. A good range of

values for PDOP is 6 or less. When fewer

than 4 satellites are visible, latitude, longitude

and altitude cannot be calculated. When fewer

than 5 satellites are available, real-time

centimeter accuracy is not possible.

Figure 4

Satellite Availability with GPS-only and

Figure 3

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GG24 GPS+GLONASS Receiver

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