The MidStates VRS solution or network RTK solution is an important tool for ag, construction, survey and asset mapping. This FAQ highlights VRS technology and MidStates VRS’s recommended solution for network RTK corrections.
What is VRS?
VRS is a Trimble trademark; it stands for Virtual Reference Station. A VRS network is an integrated system and consists of:
- GPS/GNSS reference stations spread out over a large area, typically 30-45 miles apart.
- A central server creates a correction model for the region covered by the network. GPS rovers communicate using a cell modem with this VRS server and receive RTK type corrections.
The data from the reference stations is used to model errors throughout your region. The model is used to create a network of “virtual” reference stations near your current location, which then provide a “localized” set of standard format correction messages for your roving receiver.
Since the error models are updated every second, all rovers receive an optimal correction model after connecting to the network. This ensures a high quality correction, and accuracy.
I currently use RTK corrections from Rural Tower Network (RTN); should I switch to VRS?
This depends. RTN is for agricultural equipment only and cannot send RTK corrections to construction or land survey equipment. A VRS network like MidStates will support many brands and almost all RTK applications, including construction and surveying, in addition to ag.
Also, it is very important that the correct RTN tower is always used – if not, fields may not match up due to the varying distance from tower to tower. VRS eliminates this problem as the connection is to the MidStates servers, not a GPS tower.
Lastly, if you need to send and receive data between the field and office, continuously back up your field data, or require remote assistance, cellular modems are currently the best tool available. These modems are currently the only way to receive VRS corrections. RTN does not support cellular modems.
What is the difference between CORS and VRS?
Continuously operated reference stations (CORS) are most commonly attributed to the National Geodetic Survey (NGS) in the USA. These GNSS receivers are often permanently installed. End users that access a CORS receive a single base correction, so the position accuracy decreases as the users move further away from the base. This is like using a single base solution with a cellular modem instead of a radio.
All reference stations used in a VRS system can be interpreted as a network CORS. The differentiating factor is that an end user who accesses a VRS network gets a network solution. The position accuracy is maintained even if the user moves away from the single base but is still within the network.
Nearly all of the CORS used by MidStates VRS will be certified by the NGS.
What are the benefits of using cell modems?
All of the rules of RTK radios change with cell modems. Distance limitations of 900Mhz go away, and there are no licensing issues with 450Mhz radios. VRS corrections are most frequently transmitted via cellular.
Cell modems also allow for significantly more data transmission to the field. This opens up additional data services that are impossible to access via 900Mhz and 450Mhz radios. These tools may be data transfer, fleet management, remote support, and cloud backups.
Additionally, VRS networks may offer better signal coverage in rough terrain if the local cellular network is robust and provides good data coverage. VRS networks via cell modems also offer an alternative correction source for agricultural applications with better accuracy than SBAS (or WAAS, EGNOS, or OmniSTAR) corrections.
What does MidStates VRS recommend?
The VRS system is a valuable technology and can cover more miles in harsher terrain without the hassle of setting up base stations. However, performance is dependent on cell phone signal reliability.
If you are in an area with mature and robust cell phone coverage:
- MidStates is confident that using VRS in machine control applications will be satisfying.
If you are in an area where cell phone signal reliability is not consistent, we recommend the following:
- Consider the VRS system as a complementary solution, not a replacement for traditional RTK.
- Understand that the viability of the VRS system will vary by location.
How do I find out if VRS is right for me?
If you are in agriculture and considering RTK quality guidance for row crops or water management, VRS will help tremendously.
Contractors and land surveyors who push the limits of their radio networks or find they are limited by how many base stations they own will also benefit greatly from VRS.
Can I get a more accurate solution by just using a cell modem and a single base solution?
No. The correction message you receive from a single base solution does not change if you use either a traditional 900/450 MHz radio or cell modem. In many cases, you may get a better range with a cell modem but the accuracy will be comparable to that achieved with a 900 MHz or 450 MHz radio. Additionally, the farther away a rover or machine is from a base station, the less accurate the correction. VRS corrections are uniform in the coverage area.
How far from a tower can I be?
If you’re inside the network, you will have coverage no matter where the towers are. If you’re on the edge of the network, MSVRS will provide you RTK accuracy 6 miles from the network border.
How can you have your bases so far apart?
VRS technology has been pushing the envelope on how far you can go without compromising accuracy. A complex filter uses the information from all the base reference stations and computes a solution tailored to your location. MidStates VRS base reference stations average about 45 miles distance.
Are there outages? If so, how long do they last?
MidStates VRS endeavors to keep all outages to an absolute minimum and to notify all users of planned maintenance.
There are a few ways outages could occur:
- On the VRS system side, an outage could be due to maintenance work. Additionally, there could be damage from severe weather. These types of outages could last a few seconds to a few hours. There are backup systems in place to reduce these risks. MidStates VRS will inform you on any planned outages for maintenance or upgrades.
- Cell phone network outages are related to “dropping” a cell phone signal. You may drive out of a coverage area and lose the cell phone link with the virtual reference station. There is no typical characterization on how long this could be. If XFill technology is available for your GPS receiver, these outages will be no problem if limited to less than 5 minutes.
- On the receiver side, we have observed that under certain conditions, certain receivers may momentarily drop out of RTK before re-engaging. The drop should be short, under 15 seconds. This should not be a regular occurrence; please call if any RTK drops are more frequent than once in a 4 hour period or last longer than 15 seconds.
How far is my VRS base ID from my current location, when I am in network?
Typically, when you send your location to the VRS server, the VRS server produces a reference location (base ID) very close to your current location. This base ID is your own personal virtual base station. A new base ID may also be created if you are dropped from a current session and you reconnect to the VRS server.
Will I see a line shift because of this base ID change?
No. Our tests have shown that there are no line shifts or positional drifts when a new base ID is used.
What accuracy do I get if I am in network? How is this related to range?
Typically, VRS system performance is relatively the same as traditional RTK without the limitation of baseline lengths as long as you are in the network footprint.
What about vertical accuracy?
Vertical accuracy is generally twice the horizontal accuracy.
What are the initialization times in a VRS network?
About 30 seconds.
Will VRS work with TopCon GPS equipment / machine control?
Yes. MSVRS supports Trimble and TopCon on construction equipment. We also support Ag Leader, Trimble, and Raven Ag systems.
Do I need to switch towers (Ag) or change channels on my radio (construction)?
No to both. We have a continuous, NGS-certified network. You will get the same accuracy in Williston that you will in Sioux Falls without changing anything.
What accuracy do I get if I am out of network?
Once you are outside of the MidStates network, you may or may not receive a correction. If you do receive corrections, this may be a single base solution and will not provide you with the accuracy that you would have obtained inside the network. The single base solution error increases as you move further away from that base location (like a traditional single base RTK solution).
Can I get VRS system corrections whenever I want?
You can receive VRS system corrections 24/7.
Who do I contact for help?
Email email@example.com or call 844.830.3974 for assistance.