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MnCORS GNSS Network Frequently Asked Questions

 

  1. Does the system provide both real time GPS corrections and the ability to access Rinex data for post processing?
  2. Where do I find the Rinex data?
  3. Do I need to pay a subscription fee to use the system?
  4. Do I need a dual frequency GPS receiver to access the system?
  5. How do I access the system for real time corrections?
  6. We have two GPS rovers in our office.  Can we use both rovers with a single login?
  7. My company has multiple customers/rovers.  What is the easiest way to register them?
  8. Do I need a specific brand of GPS equipment to access the system?
  9. What vertical datum and what horizontal datum and adjustment does the MnDOT CORS Network use?
  10. Will the real time coordinate values delivered from the CORS match the values of the HARN stations in my area?
  11. Is a calibration necessary?
  12. My rover is able to use Glonass satellites.  Does the MnDOT CORS Network support the Glonass constellation?
  13. My rover is tracking GLONASS satellites, but they are not being used to correct my position.  Is this a problem with my equipment or MnDOT’s system?
  14. Are there any plans to add stations in the areas of northern Minnesota that do not appear to have good local coverage?
  15. How far can I work outside of the existing network of GPS base stations?
  16. Should I expect downtime due to network failures?
  17. Which cellular provider is the best?
  18. Does MnDOT own and maintain all of the GPS equipment/sites in the network?

 


FAQ last updated on 4/13/2017

 

1. Does the system provide both real time GPS corrections and the ability to access Rinex data for post processing?


Yes.

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2. Where do I find the Rinex data?


15 second - 24 hour RINEX or 5 second - 1 hour T02 files can be download from the CORS FTP site.  If you have a specialized application that requires data to be collected at a rate other than at our standard defaults, contact: Blaine McKeever.

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3. Do I need to pay a subscription fee to use the system?


No.

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4. Do I need a dual frequency GPS receiver to access the system?


Not necessarily.  If your survey demands accuracy in the range of 2-3 cm, you will need a dual frequency receiver.  However, if your accuracy requirements will allow up to 1 meter of error, a single frequency receiver will suffice.  Please contact your local dealer to check the compatibility of your particular equipment with the system.

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5. How do I access the system for real time corrections?


You will need to subscribe to the service on the MnDOT CORS GPS Network website.  Choose the 'MnCORS Data Access and User Management Website' link and then click on Register. After the registration has been completed and submitted a confirmation message that includes your login information will be sent to you by e-mail. Your subscription will remain active as long as you use it at least a few seconds a year. Accounts not accessed for a one year period will be deleted.

If you have not received a response from a MnCORS Administrator within 3 working days, please contact Nathan Anderson.

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6. We have two GPS rovers in our office.  Can we use both rovers with a single login?


Not at the same time.  The username/password you were issued will allow access to only one rover at any given time.  It is possible to use the same username/password combination with more than one rover if used during different time periods, but it is easiest just to request a login for each rover that will use the system.

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7. My company has multiple customers/rovers.  What is the easiest way to register them?


You can:


A.  Register all rovers individually.  This requires that each user has a unique e-mail address.  Each username will consist of the first name and last initial.  All users who register in this manner will be notified by e-mail about any upcoming changes to the system or any expected outages.  This registration also allows MnCORS web access.  (Most people register using this method.)


Or...


B.  You can send a list of preferred usernames and passwords for your rovers to a MnCORS Administrator at CORSVRS.DOT@state.mn.us.  If you do it like this, at least one person in the organization will need to register as suggested in example "A" above.  All additional registrations would reside under that "primary user."  If you like, the passwords can be the same.  None of these users will have web access or receive network informational messages.  Those functions would be the responsibility of the primary user.


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8. Do I need a specific brand of GPS equipment to access the system?


No, the software will work with most, if not all GPS receivers.  At this time the GPS network delivers GPS data in 4 different formats: RTCM 2.3 (older), RTCM 3.1 (newer), CMR+ (older), and CMRx (newer).  Keep in mind that newer GNSS equipment will be using more satellites and because newer message types are more compact, using the most recent version that your equipment supports will increase the likelihood of an uninterrupted data stream.  It is always best to ask your equipment dealer which correction message is the most efficient for your equipment.

The Radio Technical Commission for Maritime Services (RTCM) is an international standards organization.  This message type is understood by most if not all GPS receivers and is generally recommended by equipment dealers other than Trimble.

Compact Measurement Record (CMR) is a correction protocol proprietary to Trimble™.  This message type is generally recommended by Trimble dealers as the best choice for Trimble equipment.


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9. What vertical datum and what horizontal datum and adjustment does the MnDOT CORS Network use?


Strictly speaking, the MnDOT CORS network is not constrained to a vertical datum.  It only facilitates the accurate determination of ellipsoid heights.  To produce an orthometric height, an appropriate Geoid separation must be calculated by software within the roving GPS receiver and subsequently applied to the ellipsoid height.  This separation can be determined either by performing a calibration to local control, or by using a standard Geoid model.  See Question 11 for calibration recommendations).

The datum and GPS correction message type used in your application will be determined by the mount point you choose.  *A discussion of datums and adjustments follows the mountpoint table.  At this time there are 15 available mount points:

 

Mountpoints

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*A discussion of datums and adjustments.

The NAD83(2011) mountpoints will provide corrections consistent with the NGS's recently released NAD83(2011) (Epoch 2010.00) adjustment.  This nationwide adjustment was constrained to active NGS certified GNSS CORS stations, 55 of which are a part of the MnCORS network.  See http://www.ngs.noaa.gov/web/surveys/NA2011 for further details.  It is highly recommended that anyone interested in determining NAVD88 elevations from NAD83(2011) ellipsoid heights, should utilize the latest geoid model, Geoid12A http://www.ngs.noaa.gov/GEOID/GEOID12A.  Initial testing has indicated that the combination of NAD83(2011) and Geoid12A could potentially produce results more consistent with published NAVD88 values than previous adjustment/geoid model combinations.

The AGXXXX entries are NAD83(2011) mountpoints that were recently added in an effort to accommodate users with equipment, most commonly tailored to the agricultural industry, that does not provide a means to enter special characters. The corrections provided should be identical to that of the corresponding standard 2011 mountpoints.

With the NAD83(1996) mountpoints, we are attempting to provide corrections that will more closely agree with the "HARN" values published for passive control marks throughout the state.  Within the Trimble Pivot software that runs our systems, all stations must be positioned in a single NAD83 adjustment; in our case NAD83(2011).  In order to produce 1996 and 2007 corrections, a local transformation is applied to shift the "Virtual Reference Station" location established for each RTK session by approximately the difference between the published NAD83(2011) value and the published value for the select adjustment at the nearest CORS station.

The primary limitation of this translation model is that the differences between the three adjustments at the CORS stations are generally smaller than what is observed at the passive control marks.  This is probably to be anticipated since, to varying degrees, multiple CORS stations were used as (nearly) fixed control in all of the adjustments.  In an effort to provide a better real-world model, we have created a state-wide 8,000 station grid using coordinates extracted from the MnDOT Geodetic database from stations that have a published 1996, 2007 and 2011 coordinate value.  The Pivot software applies a Delaunay triangulation to translate the 2011 position to either 1996 or 2007 based on the values in this grid.

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10. Will the real-time coordinate values delivered from the CORS match the values of the HARN stations in my area?


Horizontally - Over the last several years, the MnDOT Geodetic Unit has been routinely collecting NAD83(1996), NAD83(2007) and more recently and NAD83(2011) RTK positions on survey monuments throughout the state as part of their recovery and monumentation process. Over approximately 15,000 measurements, both adjustments have produced results that agree with published values within 4cm about 90% of the time. We would expect better results when using the 2011 mountpoints. One distinction however, is that there are a lot more outliers when comparing real-time NAD83(1996) results with published values. We are hopeful that the newly implemented NAD83(1996) mountpoints will provide slightly more accurate and consistent results.

Vertically - For ellipsoidal heights, NAD83(2007) values display a consistent bias of approximately -2cm when compared to the values produced in static GPS surveys. For NAD83(1996), the bias is about -4cm. NAVD88 elevations computed from ellipsoid heights tend to display this same bias.

If matching with local control is important for your project, calibration (see Question 11) can help resolve the discrepancies between the real-time and published values.

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11. Is a calibration necessary?


Usually it is best if you want your survey to fit the local control as close as possible.  Although it is difficult to make a blanket statement that covers the entire state, we will say "generally" a one-point vertical calibration is sufficient.  Please remember that this can only be determined to any degree of certainty by actually measuring trusted control monuments in your area and examining the differences.

If you think about it, a calibration is exactly what you do every time you set up a portable base station.  By default, when you set up the base station over a new monument you are in effect localizing the coordinates.

Note: Trimble Recommendations:
    With Pre-Defined Coordinate Systems (ie. County Coordinates):
        Horizontal calibration should be one point only.  More could alter the system.
        Vertical calibration should be either one point or more than three points with the geoid model.
    Without A Pre-Defined Coordinate System:
        Horizontal Calibration should use three or more points.
        Vertical calibration should be either one point or more than three points with the geoid model.

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12. My rover is able to use Glonass satellites.  Does the MnDOT CORS Network support the Glonass constellation?


Yes.

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13. My rover is tracking GLONASS satellites, but they are not being used to correct my position.  Is this a problem with my equipment or MnDOT’s system?

 

In order to produce accurate network corrections, the MnDOT CORS Network must not only be tracking all visible satellites, it also requires access to current and valid orbit information for each satellite.  While the GPS constellation has proven extremely reliable over time, the GLONASS system has had a history of intermittent technical issues.

Two types of satellite ephemeris are used to produce the network corrections, predicted and broadcast.  The predicted orbits are provided by the University of Bern, Switzerland.  In the event that they are experiencing any manner of technical issue, orbit files from the Jet Propulsion Laboratory serve as a backup.  JPL does not provide GLONASS ephemeris, therefore those satellites are excluded until the technical issues at UB have been resolved.

The broadcast orbits are received from the GLONASS satellites themselves.  If the MnDOT system receives a corrupt file or timely orbit information is not being properly transmitted, the satellites begin to drop out of the network solution.  Once this situation is identified, it usually takes between 12 and 24 hours before new, valid orbits become available and the satellites can again be utilized in the field solution.

If you should notice that an event of this nature has occurred it may be in your best interest to update your almanac within your rover to avoid any additional problems even after the event has occurred.  These events are sporadic and intermittent affecting only certain areas within MN, so on any given day users in northern MN may experience this event while users in southern MN do not.  These events are not contained only to MN, but is a problem for all regions of the world while trying to utilize the GLONASS signal.

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14. Are there any plans to add stations in the areas of northern Minnesota that do not appear to have good local coverage?


We need a reliable internet connection in order to move data from a CORS site to our GPS processors in St. Paul. Finding such sites in these areas has proven difficult, if not impossible. As internet connectivity improves and we are able to identify locations for new base stations, we will continue to improve coverage in these areas. If you are aware of locations that would fit our needs, please send us a note at CORSVRS.DOT@state.mn.us.

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15. How far can I work outside of the existing network of GPS base stations?


As a rule of thumb we say you are pretty safe up to around 10 miles outside of the network.  You will receive corrections considerably further out than that, but common sense says you should be more diligent in checking control in order to stay within the tolerances your survey requires.

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16. Should I expect downtime due to network failures?


Although we make every effort to keep the network running 24/7, it is possible you may experience some infrequent outages.  You can check the status by clicking on the Status Messages link on the MnDOT CORS - GPS Data Access / User Management page.  You may also want to think about signing up to receive web feeds.  On the same page, you can click on Current Status Messages RSS Feed to use this service.

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17. Which cellular provider is the best?


That is hard to say.  Essentially you should look for a plan that will allow you to use your phone as a modem and in turn move data to and from the RTK server over the internet.  In the metro area most if not all carriers provide this service.  Out state you may encounter a more limited number of choices.  Your best bet is to contact your local GPS equipment dealer for recommendations about plans and phones compatible with your equipment and have coverage in your area.

Remember that GPS coverage in your area does not necessarily equal cell phone coverage in your area.  You need both for this system to operate as it is intended.

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18. Does MnDOT own and maintain all of the GPS equipment/sites in the network?


No, we have a number of partners from both the public and private sector who participate in the network by either providing equipment or hosting sites with the necessary communications.

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