A Nationwide RTK Network: A Great Idea, But…

December 12, 2011  - By 0 Comments

Gavin Schrock, LS, is a licensed surveyor, technology writer, and administrator of the Washington State Reference Network, a regional cooperative GPS network (RTN) in the Pacific Northwest. He has worked in surveying, mapping, data management, and GIS for over three decades in the civil, utility, and mapping disciplines. He has published in these fields and has taught these subjects at local, state, national, and international conferences.


Some folks are proposing that a nationwide RTK Network (RTN) be piggy-backed on the controversial LightSquared communications network. That could be cool, if it can be done. No one is saying that it can’t be done, but there are reservations on whether it would be worth the massive investments needed to pull it off, and that there might be little gain at all over the existing presence of RTN in the U.S.

RTN are arrays of continuously operating GNSS reference stations that can provide correctors for high precision positioning. Centimeter positions instantaneously; imagine what could be done with a capability like that. People have not only imagined such things, but have implemented over 100 of these in the U.S. and over 350 worldwide serving industries such as surveying, mapping, construction, precision agriculture, science, machine control, public safety, precise navigation. If you feel you have heard all of this before, you probably have, and chances are you might have heard this from an RTN junkie like me.

I am a strong supporter, even a rabid supporter and promoter of the expansion of RTN and the many benefits that can be realized where RTN exist. I have bored many people to tears with my idealistic ramblings about RTN, and have seized opportunities to jump on any bandwagon that promotes more widespread or even nationwide RTN (e.g. On-Grid Goal, GPS World 2006). There are many countries that already have nationwide RTN like Japan, Germany, Denmark, Greece, and many others; but under completely different circumstances, and none piggybacked on communication network towers. So why haven’t we seen a nationwide RTN in the U.S.? There are a lot of good practical reasons why this has not happened, and likely won’t. It is not a matter of a single design or business model issue standing in the way, and likewise the solving of a single issue will not bring the entire dream to reality. There are far too many moving parts to an RTN; hurdles that would have to be overcome to realize a nationwide RTN. Examining those hurdles might bring us closer to visualizing the dream, but perhaps instead we should focus on what is realistically possible and provide the best possible amalgam of many well run RTN to provide the same utility.

The Nationwide RTN Carrot. In the course of the past year, and the LightSquared broadband plan interference controversy, RTN have been mentioned in the context of both a reason to oppose the broadband plan in question, and by others as a reason to support the broadband plan. Some have suggested that the LightSquared plan in question would be the catalyst for a nationwide RTN, as it could possible fulfill the crucial communications element of an RTN, and have touted this as a carrot for approval of the entire broadband plan. The idea of piggybacking an RTN on a communications network towers is not a new idea, and it has been studied seriously by many folks, including myself. There have been GNSS manufacturers and mobile phone service providers who have looked at this idea; but none that have acted on the idea; for good reasons.

I would really like to see a nationwide RTN, but this particular carrot is not backed up yet by a credible plan that has been formally proposed and presented for scrutiny, it does look mighty tasty at first glance. Are there too many compound assumptions being made with regards to this particular carrot? Or is there real potential for a grand RTN? The controversial broadband plan asks a lot of people to sacrifice a lot in direct costs and lost productivity during transition; so the various carrots being touted should be scrutinized very carefully. The first glance look at the assertion that a nationwide RTN could be piggybacked on the proposed LightSquared LTE build-out does appear to provide two key RTN elements: secure station sites (perhaps as many as 40,000 to choose from) with power and low-latency communications for both stations and rovers. But are tower sites really suitable? And can it be done with the tower sites alone? Can it be done in a manner that would greatly improve the coverage of RTN and at a dramatically lower cost? Let’s takes a closer look at what it would take to stake a nationwide RTN on an array of wireless communication towers before we jump to any conclusions.

Secure sites with power. Yes, the proposed tower sites are essentially cellular tower sites with fences and reliable AC power. But the assumption that one can simply rely on tower sites only applies to the limited area of the country that will be covered by the terrestrial component, the rest would need new stand-alone CORS sites to be presumably served by the satellite component of the plan (not a good idea and adds more infrastructure costs).

Tower mounts. A communications tower is subject to movement, and therefore not a good candidate for mounting a high-precision GNSS CORS antenna. Even as little as one centimeter of incidental movement (and much more in high winds) is not only not a good practice for an RTN station, it would compromise the relative integrity between RTN stations and the resultant real-time solutions. If you expect your rovers to achieve centimeter positions, the RTN stations must be stable to a few millimeters. But don’t cell towers already have GPS antennas on them? Yes, but these are typically tiny little single frequency units used to time the communications systems where positional precision is not a consideration.

Co-Location at Tower Sites. You will not find very many RTN stations co-located at wireless communications tower sites, and those that are have been placed on stable ground mount far from tower (south side preferred for maximum constellation) to mitigate as much multipath from the tower as possible. Most tower sites are not big enough to accommodate this. It may take a separate lease of a fenced area far away from the tower. This greatly reduces the number of potential sites.

Leases. Wireless communications  tower sites are mostly leased from local land owners, and the towers themselves are often owned by third parties from whom communications companies lease space on the towers. The LightSquared plan is not calling for wholly-owned and leased sites; other parties and leases will be required. For instance, Sprint has been proposed as a LightSquared partner for providing tower infrastructure. Site and tower owners want to make money from their property. Towers = more ongoing costs.

Site Geology. Potential RTN station sites are carefully vetted for sources of incidental geological movement. For example, alluvial fans or slumping slopes are not good candidate sites. An RTN serves as the active control component of a geodetic reference framework; and strict criteria are followed. Tower sites are not necessarily vetted on the same criteria. The potential site list becomes even more narrow.

Interference. While sources of interference from other radio frequency appurtenances on the towers might not be an issue, then there is the question (ironically) of the possible LightSquared interference as these stations would be at ground zero. Assuming that there are solutions for what is referred to as the lower 10MHz plan interference, what of the upper 10Mhz plan? Recent lower 10MHz filtering tests aside, the upper 10 MHz band plan has still not been taken off the table. No one has demonstrated any credible filtering plan (even LightSquared admits this is still theoretical or at least years away) for the upper 10MHz. Would the RTN stations be immune to such interference? Depending on how the upper band issue plays out, this idea (and viability of every other every other RTN, not to mention all high precision GPS in the U.S.) might be dead in the water.

Geometry and Coverage. RTN stations are spaced as close as 30km or as far apart as 100km depending on what type of solution is being sought, terrain and elevation differences, tropospheric trends, redundancy considerations, and site suitability/availability as outlined above. With the LightSquared plan proposing as many as 40,000 possible tower sites it would otherwise  be possible to find enough in densely populated areas of the country to have decent geometry and coverage, but only if all of the other design criteria can be met. The point may be moot as tower sites overall are not good candidate sites and won’t cover the majority of the country without adding satellite communication-served sites.

Geodesy. If the relative positional integrity of an RTN is not maintained, and elements like plate tectonics and ocean tide loading are not taken into account, the resultant solutions suffer. Poor geodesy renders an RTN useless for high precision positioning. There are amazing tools for monitoring, maintaining, and updating the geodesy of an RTN available in some of the commercial RTN operations software suites, but this proposal would be taking on an unprecedented huge and expensive geodetic burden – even if a fraction of the 40,000 sites are included. The National Geodetic Survey maintains system of 1,800 CORS maintained by over 200 different partnering organizations. Even with the most advanced tools and some of the finest geodetic minds in the world, maintaining the geodesy of these sites is straining the NGS resources. The threshold for update on NGS CORS is when its network integrity exceeds two centimeter horizontal by for centimeter vertical; completely unacceptable for the relative integrity that RTN requires. RTN operators maintain registration to the National Spatial Reference System via constraining to a minimum number of CORS, but then have to maintain a further level of relative integrity locally for the RTN to run. A nationwide RTN would need to be run as an array of sub-networks for independent geodetic regions; some RTN have to do this even within a single state to accommodate regions of varied tectonic velocity. A small army of geodesists would be needed to oversee a nationwide RTN resulting in another significant cost.

Ubiquitous Communications. The term “ubiquitous” gets thrown around a lot with regards to the current plan. Go online and look at a population density map and then look at any of your favorite cellular coverage maps. Now look at a terrestrial component deployment map (Source: TMF Associates) for the proposed network from October 2010. It does not cover huge areas of the country; instead the satellite component of the proposed plan would need to be used. RTN CORS do not need a lot of bandwidth, but they do need low latency communications. Satellite communications links are rarely used for RTN. An RTN might get away with a few isolated high-latency satcomm served sites, but too many clustered together in a network solution do not work. Also notice the population map and the coverage map of some common cell/broadband providers look very similar; the profitable areas are targeted. Many companies are steadily deploying LTE broadband (LTE was not invented in the past year). While the plan calls for providing services to an admirable goal of 260 million potential subscribers, the remaining 50 million plus in rural areas will be left out as they have been by other carriers, or simply served by slower satellite communications.

Nationwide does not really mean nationwide in the commercial communications business, and that would be the same for RTN. Communications networks get built where the potential subscriber base can support the investments. The same can be said for RTN. You will find RTN covering the same densely populated areas, or over areas where precision agriculture is being implemented. There are actually RTN and arrays of single-base RTK stations in places that are not covered well by broadband and would not likely be covered by this plan or the others. In these areas radio and satellite-based augmentation systems are the cost effective alternative. Even though the communications component of the plan (that might arguably be more bandwidth and possibly faster or cheaper) will not be much more ubiquitous in terms of RTN functionality than what is available now, there would still be big holes in a “nationwide” RTN.

Wholesale. LightSquared plans to offer wholesale bandwidth. This might equate to any number of retail providers offering the bandwidth through proprietary or open source communications devices. LightSquared is promoting this as “the dumbest of pipes”; essentially a great big pipe of bandwidth, which is a cool idea and prime for a wholesale model. More options for communications through these retailers might arguably be a good thing for RTN users, but not necessarily for any entity trying to put together a nationwide RTN unless there was some kind of exclusive deal attached. Competition can lead to lower costs overall, but subscriptions are typically what the market can bear and that might not be stupendously lower than what we pay now because everyone in between needs to take a cut. One strong point of the model was supposed to be unified communications for RTN, but instead we may be looking at a fractured element. The potential RTN operator would have to deal with as many, if not more, wireless communications providers than currently exist.

But in another potential model, if the RTN provider were also a LightSquared broadband retail “reseller”, that might be a key to streamlining the model. However, if every end user was to buy the same units or brand with built in broadband receivers from one of the preferred retailers (wishful thinking), that would streamline the model even more. There are too many existing RTN (some free or at nominal cost), and too much legacy equipment out there to expect users to accept and rapidly execute dramatic upgrades, replacements, or carrier changes unless the full LightSquared plan is approved and they are forced to upgrade.

The Elastic and the Brittle. I hate to rain on anyone’s parade, but RTN are not the dramatic cash cow one might imagine. The worldwide experience of RTN is very similar in that there is a limited market for network corrections. Even if one was to count on signing up all of the current RTN users in the U.S., plus all of the precision agriculture market (and a mighty hard sell that would be as they have made some huge investments in their own systems), it is still unlikely that there would be enough revenue to fund the initial and ongoing infrastructure investments, and to sustain the ongoing costs of operations, geodesy, leasing, maintenance contracts, and account management. If anyone is entertaining thoughts of consumers paying extra for higher precision on their cell phones and car navigation devices they might be greatly mistaken. The consumer seems quite happy with accuracy on the order of a few meters, and multiple constellations and  modernization will be providing higher fidelity to them soon enough. One wireless service provider even experimented with delivering corrections to mobile phone users from the national RTN where they are based and found consumers in their test group to be indifferent and even thinking it was a silly idea.

Private RTN have spread across areas of the U.S., somewhat organically as opportunities arise, partners are secured, and where the market can support them. Public and cooperative RTN have spread in areas where the sponsoring entities can realize cost-benefits from their investments like a state department of transportation for their own projects. Public RTN have often filled regions where a private network may not have otherwise been cost effective. Together public and private RTN have covered a substantial area of the U.S. The nature of RTN in the U.S. is a healthy elasticity which fits the market and needs. With RTN being narrow-margin enterprises, this is a good thing. Developing a huge single entity RTN on narrow margins leaves the entire enterprise quite brittle. Investors might view areas that have a low or negative return as not worth retaining or even building out in the first place. The cards are really stacked against a ubiquitous nationwide RTN, unless as some assert there were elements of overriding public interest to justify some level of public investment or partnering.

RTN Coverage of the U.S. as a percentage of Total Area

Infrastructure Investment. Typical RTN stations have cost between $10,000 and $50,000 each to establish and sites requiring satellite communications start at a minimum of $20,000. Let’s say for arguments sake that only 10,000 of the tower sites were utilized, with perhaps just as many in satellite communications-served sites also needed. That might not even exceed the coverage of existing RTN. Even so, at $10,000 each, that is $100,000,000 up front; not to mention the satellite communications-served sites on top of that. Some may question those costs, so let’s break them down. A RTN receiver has to be dual-frequency, multi-constellation, geodetic-grade, enable remote operations, and be paired with a geodetic-grade antenna. Sure, used receiver/antenna pairs can be had for as little as $2,000-$6,000. Let’s say for arguments sake a manufacturer was able to build and sell (or essentially give away) a new unit for the unlikely price of $2,000, there is still the cost of a stable ground mount, conduit, enclosures, labor, site selection, engineering, fuel, logistics, and contract management. These would very likely add up to $10,000. But let’s say for arguments sake this could be done for $8,000. It would still cost $80,000,000 up front, and maybe triple that to add enough satellite communications-served sites. One would have to question the robustness and viability of an RTN built so cheaply. Realistically, it would be more like $100,000,000 to $360,000,000 to build out.

Ongoing Costs. Break even operations costs for an RTN average around $1,000-$4,000 per station annually. This includes hardware replacement, software contracts, operations staff, geodesy, training, support, billing, leases, power, communications, data processing, and more. Again, for arguments sake let’s say on a grand scale that cost could be brought down to $1,000 per station per year, that sill represents $8,000,000 to $10,000,000 per year, but more realistically like $15,000,000 to $20,000,000 annually with double or triple to that cost for satellite communications-served sites.

Pricing Model. The carrot has been touted with assertions that the services would be provided at dramatically reduced costs for both communications and corrections. No one involved would be expected to give anything away. A fair price for all elements would be exacted like it would for any other enterprise. For existing RTN, price is not typically what holds back potential customers. The RTN’s in the U.S. charge very reasonable prices, and much lower than some RTN in other countries. The limitation is the existing and potential pool of users as a function of geographic area. To operate an RTN at greatly reduced prices does not work because many public RTN that initially offered free services are exploring at least nominal fees for the future. It does cost money to run an RTN. Even if a new cut-rate nationwide RTN were to assume it could assimilate all current RTN users, plus a substantial segment of agriculture users, it is likely that the revenues would not be able to justify covering more area of the country than existing RTN already do.

What do we make of this carrot?

I completely welcome this idea for consideration, but it needs to be examined seriously before any speculative cost benefits can be added to the value equations folks are presenting as rationale for approving the LightSquared plan. There are a lot of unknowns about what folks have in mind when they tout this piggyback-on-LightSquared-nationwide-RTN carrot.

Too many unknowns encircle this carrot. If a credible plan were offered up for scrutiny and proposed coverage were shown, all of the design and business model issues I’ve outlined were addressed, the FCC approves the LightSquared LTE plan and there were investors who were willing to see modest returns at best, then I would be among the first to jump on the bandwagon, sing praises, and actively promote the idea.

However, in light of the tremendous uncertainty we face not only in considering this carrot, but the fate of the broadband proposal it serves to sweeten, touting of this particular nationwide RTN proposal must be viewed at best with a not insignificant amount of skepticism and perhaps at worst be viewed as somewhat disingenuous. The seed for this carrot has not yet even been sown.

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