RTK Networks: The Wild, Wild West

RTK Networks: The Wild, Wild West
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Survey & Construction Newsletter, April 2009

Apr 1, 2009
By: Eric Gakstatter
GPS World

What can you say about RTK Networks, except wow! They have popped up ever=
ywhere and=20
continue on a path of rapid growth. In the last five years, I=E2=80=99d s=
ay it=E2=80=99s clear that two=20
GNSS technologies have changed the survey/construction industry more than=
any others;=20
machine control and RTK networks.

As a follow-on to our GNSS Precise Positioning Market Report, Rob Lorimer=
and I have=20
produced another market research report entitled GNSS Augmentation and In=
frastructure. In=20
addition to CORS, SBAS, and other infrastructure, it includes quite a bit=
of information=20
about RTK networks, growth projections, and technology trends. You can do=
wnload an=20
abstract here. RTK networks is a very complex subject. A full discussion =
would much more=20
space than this newsletter can accommodate. In that light, I=E2=80=99m go=
ing to keep it as simple=20
as I can make it while touching on the hot points I=E2=80=99ve heard abou=
t and experienced.

RTK Clusters vs. RTK Networks

RTK clusters are a set of strategically spaced GNSS reference stations se=
t up and operated=20
by an entity within a specific geographic region. They were first conceiv=
ed for the survey=20
engineering industry as a solution to the headache of operating a referen=
ce station. RTK=20
clusters provide single-baseline RTK correctors within that region. It=E2=
=80=99s worth emphasizing=20
that it is a single-baseline solution similar to when a user operates his=
own reference=20
station. By single baseline, I=E2=80=99m referring to the rover receiving=
correctors from the=20
closest reference station in the cluster. If the user moves significantly=
within the=20
cluster region, he must manually select another reference station. RTK pe=
rformance in RTK=20
clusters is the same as traditional base-rover RTK configurations, in tha=
t position=20
accuracy is subject to degradation (=E2=80=9Cppm error=E2=80=9D) as the u=
ser moves further from the=20
reference station being utilized.

RTK networks are also a set of strategically spaced GNSS reference statio=
ns within a=20
specific geographic region. The advantage of an RTK network over an RTK c=
luster is that=20
the RTK network utilizes all of the reference stations, included in the n=
etwork. Unlike=20
RTK clusters, RTK networks are driven by a sophisticated suite of network=
software (such=20
as VRS, SpiderNET/SmartNet, or TopNET). The network software significantl=
y reduces =E2=80=9Cppm=20
error=E2=80=9D that is introduced by the ionosphere, troposphere, and sat=
ellite orbits the further=20
one travels from a reference station. In essence, if you are working with=
in an RTK network=20
coverage area, the distance from the nearest reference station becomes so=
mewhat of a moot=20
point, certainly much less of an issue than when discussing traditional R=
TK and RTK clusters.

The graphic below illustrates a simple RTK network. Data is collected by =
the reference=20
stations and sent to a central processing server where it is compiled, an=
d correctors are=20
sent to all of the rovers that are subscribed to and logged onto the serv=
ice. The number=20
of users using the service at any one time can be several hundred or more=
=2E In an RTK=20
cluster, the graphic would look similar to below but without the central =
processing=20
server. The data link to the user wouldn=E2=80=99t be from a central proc=
essing server but rather=20
directly from one of the reference stations.


Source: Trimble Navigation Ltd.
The National Geodetic Survey published its latest versions of =E2=80=9CUs=
er Guidelines For=20
Classical Real-Time GNSS Positioning=E2=80=9D in September 2008. It=E2=80=
=99s good reading for anyone=20
using RTK and RTK networks. Appendix A of the document discusses RTK and =
RTK network=20
testing done by the Vermont Transportation Department in 2006/2007.

Another notable report that is worthwhile to read was published by The Su=
rvey Association=20
(UK) and University of New Castle. It was conducted in 2008. It contains =
empirical data=20
collected and analysis of RTK network performance. One particular point o=
f interest in the=20
report stated that using GLONASS observations do not improve RTK accuracy=
=2E I=E2=80=99ve always=20
subscribed to the notion of =E2=80=9Cthe more observables, the better=E2=80=
=9D for RTK, because it=20
improves productivity (field work is not shut down from lack of satellite=
s). With respect=20
to the accuracy, I think you have to take the above conclusion with a gra=
in of salt. I=E2=80=99m=20
not claiming GLONASS will improve accuracy, but I think we have to be car=
eful using such a=20
statement categorically. For example, would I rather use a five-satellite=
GPS-only=20
solution up against a tree line vs. a five satellite GPS and three satell=
ite GLONASS=20
solution in the same location? I would choose the latter. Which would far=
e better with=20
respect to accuracy? Well, satellite positioning accuracy is all about co=
nfidence and I=E2=80=99d=20
have much more confidence in an eight-satellite RTK position than a five-=
satellite RTK=20
position=E2=80=A6especially when working up against a tree line.

Evolution

Before RTK networks/clusters were developed, all survey/construction RTK =
users had to=20
manage their own reference stations (setup, manage, protect, etc.). Once =
this became=20
accepted as mainstream technology, survey/construction managers began to =
understand the=20
time investment, potential blunders, and risks associated with each crew =
operating their=20
own reference station. The next logical step was for survey/construction =
managers to=20
establish permanently (or semi-permanently) mounted reference stations in=
offices or=20
temporary trailers with the antennas tied to the desired reference datum =
and a reliable=20
power supply so one could merely =E2=80=9Cflip the switch=E2=80=9D and be=
broadcasting RTK correctors=20
within minutes. Risk of having a reference station stolen and risk of a b=
lunder in the=20
setup was greatly reduced.

Permanently and semi-permanently mounted reference stations managed by sm=
aller=20
organizations for their specific application soon morphed into department=
s of=20
Transportation and other organizations setting up a number of permanently=
mounted=20
reference stations in highly populated areas that covered entire cities. =
These were the=20
first RTK clusters. They broadcast RTK correctors similar to the way that=
traditional=20
base-rover RTK users do=E2=80=A6mostly UHF and VHF data radios which have=
a limited broadcast=20
range. Also, these systems were still subject to =E2=80=9Cppm errors=E2=80=
=9D described above. These two=20
factors meant that the permanently mounted reference stations needed to b=
e located a=20
relatively close distance from each other to ensure full coverage of the =
areas.

Two technology developments enabled the transition from RTK clusters to R=
TK networks.

First of all, mobile phone networks have experienced explosive growth in =
the past five=20
years. This was critical in overcoming the distance limitations of UHF/VH=
F radios. Using a=20
mobile phone network, I can log onto an RTK network 1,000 miles away. Gra=
nted, the=20
positioning would be useless (way outside of the network) but my point is=
that it was a=20
huge step forward in RTK communications technology. It=E2=80=99s true tha=
t mobile phone networks=20
still don=E2=80=99t provide coverage everywhere that survey/construction =
people want to work, but=20
they do cover a significant portion of it and, where they don=E2=80=99t, =
other communication=20
technologies such as RTK bridges are being developed.

Second, manufacturers such as Trimble, Leica, and Topcon began developing=
highly=20
sophisticated RTK network software to optimize accuracy and reliability o=
f positioning=20
within the network coverage area regardless (for the most part) of distan=
ce to the nearest=20
reference station.

Who Runs the Networks an Clusters?

Worldwide there are literally hundreds (maybe more than a thousand) RTK n=
etworks/clusters.=20
The growth rate is astounding.

Today, I would venture to state that all RTK systems setup by survey/engi=
neering-based=20
organizations are RTK networks. For example, departments of Transportatio=
n, survey=20
equipment dealers, cooperatives, and even GNSS manufacturers set up and o=
perate RTK networks.

Here are some examples of RTK networks:

Ordnance Survey (UK)

Can-Net (Canada)

ORGN (USA)

Geotop (Italy)



RTK clusters still exist. In fact, they are proliferating in the precisio=
n agriculture=20
market. There are huge RTK clusters being run by agriculture equipment de=
alers and=20
agricultural cooperatives. Cost is a major issue why RTK networks have ra=
rely been=20
installed for precision agriculture. RTK network systems are significantl=
y more expensive=20
and technically complex to install and manage than RTK clusters. Farmers =
are less apt to=20
pay the higher subscription rates charged by RTK network service provider=
s.

Here are some examples of RTK clusters:

Tri-State RTK (USA)

South Plains Precision Ag (USA)



Largely, precision agriculture and survey engineering/construction RTK sy=
stems are=20
operated separately and independently. It seems odd that given the signif=
icant cost of the=20
infrastructure that this wouldn=E2=80=99t be a shared resource. In many c=
ases, RTK clusters and=20
RTK networks overlap themselves.

In rare cases, the RTK network owner/operator services both the survey=20
engineering/construction and precision agriculture markets. Here is an ex=
ample:

eGPS Solutions (USA)

Subscription Costs

What are the costs of subscriptions to RTK networks and RTK clusters?

The answer to this question varies widely. If the RTK network used public=
funding, many=20
times there is no cost to subscribe to the network. However, the user mus=
t obtain a=20
wireless network (mobile phone) data plan to access the network.

If the RTK network is operated by a survey equipment dealer, there is a s=
ubscription cost=20
that varies with each service provider that can run as much as US $500 pe=
r month per receiver.

Subscription fees to RTK clusters are generally lower than RTK Nnetworks=E2=
=80=A6on the order of=20
US$1,500 per year.

Where Are We Heading?

This technology is developing and deploying rapidly and on a worldwide ba=
sis. Entire=20
countries such as Croatia and Turkey have invested in nationwide RTK netw=
orks.

I think it=E2=80=99s clear that RTK networks are the foundation of real-t=
ime precise positioning=20
in the future. They will replace RTK clusters=E2=80=A6or RTK clusters wil=
l be upgraded to RTK=20
networks. There are just too many benefits for that not to happen.

It will be interesting to see how the subscription rates are settled, as =
well as the=20
competition between public and private networks.

As I wrote in the beginning, this is a complex subject worthy of words wa=
y beyond what is=20
written here. I only hoped to provide a broad view. For those of you who =
are interested,=20
I=E2=80=99m conducting a webinar on the subject later this month, April 2=
1. You can register here.


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