Ashtech Z-sensor User Manual download pdf

Presented at ION-GPS 97, Kansas City, MO. ÒNew Product DescriptionsÓ Session.

GPS and GPS+GLONASS RTK

Dr. Frank van Di gge l e n

Ashtech Inc. Sunnyvale, California, USA

ION-GPS, September 1997 Ò New Product DescriptionsÓ

ABSTRACT

This paper presents not only two new RTK

products, but also a major breakthrough in global

positioning technology, taking RTK where it has

never gone before.

The first new product is the Z-Sensor, a single board

implementation of AshtechÕs reknowned Z12

receiver.

The second product is the GG-RTK receiver, the

worldÕs first GPS+GLONASS RTK product.

RTK has represented the peak of GPS performance

for several years, but there have been severe

limitations - with fewer than 5 satellites in view

RTK does not work at all, or works so slowly as to

be almost no better than DGPS in many

applications. Now, by combining GPS and GLONASS

in an RTK product, you can do RTK, for the first

time ever, in places such as open pit mines, urban

canyon, river valleys, etc., where GPS-only RTK

simply will not work.

PAPER ORGANIZATION

The paper is organized as follows:

Background & RTK

Introduces RTK. Readers familiar with the

concepts may want to skip directly to The

Products.

The Products

The most significant product specifications.

RTK Performance

Presents the parameters by which we will judge

RTK performance.

Field Test Results

Presents real-world results for both products.

Availability

How much benefit do we get from the current,

partial, GLONASS constellation? More than

youÕd think.

Reliability of GPS+GLONASS

Addresses the ever popular topic of GLONASS

reliability, and shows how the GPS+GLONASS

receiver has been architectured so that, no

matter how GLONASS system performance

compares to GPS system performance, the GG-

RTK receiver always delivers better performance

and reliability than a GPS-only receiver.

BACKGROUND

To obtain precise position from a GPS receiver, we use

techniques called ÒDifferential GPSÓ. This involves

two GPS receivers. One is stationary, at a known

point, we call this the ÒBaseÓ receiver. The Base

receiver transmits data over a radio link. A second GPS

receiver, possibly moving, at an unknown point,

calculates precise position by using the signals it

receives from the satellites, and the data it receives via

radio from the Base. Differential GPS usually gives

about one meter accuracy. RTK is a special form of

Different ial GP S that gives about one-hundred t imes

greater accuracy.

RTK

T h e GP S sy st em uses a coded signal from which a

receiver derives distance and thus position. The code is

a string of bits, like the ones and zeros in a computer.

The receiver sees this code as tick-marks on a giant

tape-measure, every transition from one to zero or

back appears as a tick on the Òtape measureÓ. The C/A

code has ticks at about 300 meter spacing. The

encoded informat ion gives t he equivalent of t he

numbers on a tape-measure, and the receiver uses these

codes to measure position to meter-level accuracy.

The military, encrypted, P-code gives even better

accuracy, roughly twice as good as the best C/A code

receiver. But the microwave carrier, which is there

ostensibly only to carry the coded signals, provides the

best Òtape-measureÓ of all, with tick-marks at about

20cm spacing. The receiver can measure these signals

to centimeter precision. The trouble is, the carrier

provides t he equivalent of a very precisely graduat ed

tape-measure with no numbers on it. If the receiver

soft ware could use t he code t o derive t he numbers on

this carrier Òtape-measureÓ, it would provide GPS

accuracy of centimeters. This is exactly what RTK

does.

1 2 3 4 5 6

Summary of Contents

Page 1 - GPS and GPS+GLONASS RTK

Presented at ION-GPS 97, Kansas City, MO. ÒNew Product DescriptionsÓ Session.GPS and GPS+GLONASS RTKDr. Frank van Di gge l e nAshtech Inc. Sunnyvale,

Page 2 - 1,5401,540

Presented at ION-GPS 97, Kansas City, MO. ÒNew Product DescriptionsÓ Session.GPS - a tape measureGPS - a tape measurefrom spacefrom spaceCodeCarrier01

Page 3 - RTK PERFORMANCE

Presented at ION-GPS 97, Kansas City, MO. ÒNew Product DescriptionsÓ Session.RT K st ands for Ò Real T ime KinematicÓ, but it meansreal-time-centime

Page 4 - FIELD TESTS

Presented at ION-GPS 97, Kansas City, MO. ÒNew Product DescriptionsÓ Session.1. Typical accuracy is one hundred times as good asDGPS.2. There is an

Page 5

Presented at ION-GPS 97, Kansas City, MO. ÒNew Product DescriptionsÓ Session.40%50%60%70%80%90%100%1 10 100 1000Time since reset (seconds)PercGG-RTK R

Page 6 - CONCLUSION

Presented at ION-GPS 97, Kansas City, MO. ÒNew Product DescriptionsÓ Session.AVAILABILITYThe current satellite constellations provide 25healthy GPS sa

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