Ashtech Z-sensor Manuel d'utilisateur télécharger pdf (Page 6)

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

AVAILABILITY

The current satellite constellations provide 25

healthy GPS satellites, and 40 healthy GPS+GLONASS

satellites. For RTK initialization, 5 satellites are

required. This is no problem if the whole sky is visible,

and, as already shown, dual frequency GPS-only RTK

performance is similar to single-frequency GG-RTK

when the whole sky is visible. However, there are

occassions when there is no comparison between a

GPS-only system, and a GPS+GLONASS system, and

this is when large parts of the sky are blocked, such as

in an open-pit mine, urban canyon, or river valley. To

demonstrate this we did a simulation with a 30° mask

angle (this is typical for an open pit mine). In this

environment, 5 or more GPS sats are available only 6

hours per day, and the 6 hours is fragmented throug the

day, making RT K a practical impossibilit y in t hese

environments - if you only have GPS. But, with the

combined GPS+GLONASS constellation the availability

of 5 or more satellites improves by 300% to 18 hours

per day.

The following plot shows todayÕs satellite availability,

comparing what you get with 25 satellites, to what you

get with 40 satellites. The plot shows what percentage

of the time the indicated number of satellites are

visible. This plot was generated by doing an 8-day

simulation, with a 10 degree mask angle.

100

GPS

# Satellites

Availability

100%

46%

72%

89%

95%

99%

100

100%

32%

72%

96%

Satellite availability, Kansas City, September

1997

S A TEL L I TE R ELI A B I LI TY

What about the performance of the satellites

themselves? There have been documented cases, for

both GPS and GLONASS, of the satellite clocks

generating errors of thousands of kilometers in

stand-alone positions for many minutes before the

respective system control set the satellite to

ÒunhealthyÓ.

For GG-RTK we automatically enable RAIM

(standard) whenever you set up a base station. If the

base station sees errors on the order of 100m, then

RTK (or Differential) , simply removes the errors as

part of the normal operation. If the base station sees

an unexpected error (e.g. worse than 1km) on any

satellite, then it knows something is wrong, and it

immediately removes that satellite from the set of

broadcast data, and then the remote station stops

using the satellite too.

So, what many think is the BIG issue for

GPS+GLONASS, satellite reliability, is a non-issue

for RTK or Differential, if youÕre using an Ashtech

base station.

The remote unit operates its own RAIM algorithm

to detect and repair cycle slips. The remote receiver

also weights measurements appropriately, so that the

combined GPS+GLONASS position is always at least

as accurate than either system alone.

COST

T his paper ends wit h t he bottom line - cost.

Both new receivers are available at significant cost

reduct ion over compet ing product s. T he Z receiver has

been cost reduced by int egrating what used t o be on 5

circuit boards into one single board. T he GG-RT K

receiver is even less costly for a surprising reason -

adding GLONASS to RTK reduces cost. HereÕs how:

The second GPS frequency is encrypted. This means

that dual frequency GPS systems for civilian use have

to perform significant extra processing to extract the

observables from the encrypted signal. T his makes the

receivers more complex and therfor more costly. By

using GLONASS instead of the second GPS frequency

to provide the extra observables required for RTK, the

cost of RT K systems has been significantly reduced.

CONCLUSION

There are now two options for RTK: dual-

frequency-single-system and single-frequency-dual-

system. Dual frequency systems have advantages on

longer baselines, and present users with a cost-

performance trade-off for anything but short baselines.

The trade-off becomes noticeable at baselines of

around 5km. For shorter baselines there is no trade-off,

single-frequency-dual-systems not only costs less, but

perform better than dual-frequency GPS-only RTK.

Ashtech is a registered trademark, and GPS+GLONASS,

and GG-RT K are trademarks of Ashtech Inc.

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