Frequently Asked Questions
- Does the system provide both real time GPS corrections and the ability to access Rinex data for post processing?
- Where do I find the Rinex data?
- Do I need to pay a subscription fee to use the system?
- How do I access the system for real time corrections?
- We have two GPS rovers in our office. Can we use both rovers with a single login?
- Do I need a specific brand of GPS equipment to access the system?
- Do I need a dual frequency GPS receiver to access the system?
- What vertical datum and what horizontal datum and adjustment does the Mn/DOT CORS Network use?
- Will the real time coordinate values delivered from the CORS match the values of the HARN stations in my area?
- Is a calibration necessary?
- My rover is able to use Glonass satellites. Does the Mn/DOT CORS Network support the Glonass constellation?
- Are there plans for the CORS to eventually cover the entire state?
- How far can I work outside of the existing network of GPS base stations?
- Should I expect downtime due to network failures?
- Which cellular provider is the best?
- Does Mn/DOT own and maintain all of the GPS equipment/sites in the network?
- We don't have Mn/DOT CORS Network coverage in our area now. What can we do to facilitate the expansion?
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?
Subscribers can click 'Reference Data Shop' on the Mn/DOT CORS - GPS Data Access / User Management page or download directly 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. How do I access the system for real time corrections?
You will need to subscribe to the service on the Mn/DOT CORS GPS Network website. Click 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.
If you have not received a response from a Mn/CORS Administrator within 3 working days please contact Don Seitz.
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5. 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 be 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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6. 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 GPS data is delivered in 4 different formats: RTCM 2.3, RTCM 3.1, CMR+, and CMRx. Because the newer message types are more compact, using the most recent version that your equipment supports will increase the likelihood of an uninterrupted data stream. For more specifics about your GPS equipment please contact your local dealer.
7. 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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8. What vertical datum and what horizontal datum and adjustment does the Mn/DOT 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 10 for calibration recommendations). For the NAD83(1996) adjustment, the recommended model is Geoid03. For NAD83(NSRS2007), Geoid09 should be used. Current evidence suggests that the combination of NAD83(NSRS2007) and Geoid09 will provide better agreement with NAVD88 elevations than NAD83(1996) and Geoid03.
The horizontal datum is NAD83(CORS1996) or NAD83(NSRS 2007). See this NGS Page for more information.
The datum and GPS correction message type used in your application will be determined by the mount point you choose. At this time there are 8 available mount points:
CMR_Plus_NAD83(1996)
CMRx_NAD83(1996)
RTCM_31_NAD83(1996)
RTCM_23_NAD83(1996)
CMR_Plus_NAD83(NSRS2007)
CMRx_NAD83(NSRS2007)
RTCM_31_NAD83(NSRS2007)
RTCM_23_NAD83(NSRS2007)
9. Will the real time coordinate values delivered from the CORS match the values of the HARN stations in my area?
Horizontally - Probably, the horizontal component of the HARN monuments and the horizontal component of the MN/CORS were both derived from GPS measurements adjusted to the National CORS.
Vertically - Maybe, the vertical component of the HARN monuments is derived from traditional Geodetic leveling methods and Mn/DOT CORS Network monuments are tied vertically to the National CORS using GPS measurements.
Calibrating to your local control will resolve any discrepancies between the two.
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10. 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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11. My rover is able to use Glonass satellites. Does the Mn/DOT CORS Network support the Glonass constellation?
Yes, but not the entire state at this time. The green triangles on the CORS map indicate GNSS coverage (see Mn/DOT CORS GPS Network). As money becomes available we will be replacing existing GPS-only receivers with GNSS-capable receivers.
Your equipment and location in the state will determine which satellites are used in your solution. The software includes all satellites that are common to the rover and the nearby CORS sites for computations.
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12. Are there plans for the Mn/DOT CORS Network to eventually cover the entire state?
Yes, that is our plan, but of course this is entirely dependent upon funding. As funding becomes available we will continue upgrading and expanding the network.
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13. 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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14. 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 Mn/DOT 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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15. 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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16. Does Mn/DOT 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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17. We don't have Mn/DOT CORS Network coverage in our area now. What can we do to facilitate the expansion?
If you would like to participate in the expansion effort please contact Don Seitz or Roy Graff.
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