Real-Time Network Surveying

Real-Time Network Surveying

 

   Real-Time Network (RTN) surveying is becoming an efficient and common method of surveying that eliminates the need for establishing base control.

    RTN surveying is similar in concept to Real-Time Kinematic (RTK) surveying in that corrections are sent from a base station to improve rover positional accuracy in real-time (figure1).

    The primary difference is that unlike RTK surveying, where the reference station is physically located at a permanent or semi-permanent location, RTN surveying computes a “virtual” reference station (VRS) about 1 meter (m) from the rover.

    RTN is capable of operating over inter-receiver distances up to many miles with performance equivalent to a current single-base RTK system operating over much shorter baselines.

    With RTN surveying, a permanent network of reference stations (Virtual Reference Network or VRN) is established with typical spacing between stations of 10 to 50 miles 

    The reference station network continuously streams data (using LAN, Internet, or radio links) to a central location (server) established for each network. The server stores the transmitted RINEX data, performs quality assurance checks on the data, accomplishes network modeling and estimation of systematic errors, calculates and converts correction data to a user format of either Radio Technical Commission for Maritime Services (RTCM) or Compact Measurement Record (CMR+), and communicates the data to the users. When performing a survey, the dataflow is as follows:

1. User’s rover connects to the VRN using LAN, Internet, radio links, or a cellular modem.

2. Rover sends a NMEA1 string (GGA2) to the RTN server.

3. Server uses NMEA data to establish a VRS position ±1 m from the user’s rover position.

4. Using input from the closest surrounding reference stations, the RTN server computes and sends corrections back to the rover as if a real base station were broadcasting from the location of the VRS.

5. Rover receives and applies the corrections in real-time and functions like a normal RTK survey, but essentially receives data as if coming from the VRS, thereby eliminating PPM error that normally occurs. The accuracy of RTN surveying depends on many factors, including the reference station distances, equipment and its settings, survey procedures, and the survey

environment.

 Accuracy typically is in the range of and, in some cases, can exceed that of traditional RTK surveying

Benefits and Limitations of RTN Surveying

1. “The need for a user to establish a permanent/semi-permanent base station is eliminated. This eliminates the time for initial site selection and (daily) setup, any issues of security and power supply, and the possibility of setup errors.”

2. “The RTN can monitor its own integrity and can detect if there is a problem with a particular reference station. With a single-base RTK setup it can be difficult to tell if a problem exists or occurs with a base station while conducting a survey.”

3. “Since the reference stations are part of a network, a loss of one station does not result in failure of the entire network or the resulting survey. Whereas the loss of a reference station with single-base RTK setup results in the end of data collection, with RTN surveying the system accuracy degrades gradually.”

4. “A sufficiently dense reference station network can result in shorter baselines. As with any other style of GPS surveying, shorter baselines result in improved accuracy because of reduced effects of atmospheric interference.”

5. “The RTN reference stations allow for network atmospheric modeling resulting in improved accuracy. With RTK, atmospheric effects are computed using (usually) one location.”

6. Common reference coordinate frame.

Limitations of RTN surveying include the following items as noted by USGS

1. High cost to establish, maintain, and manage an RTN.

2. Use and accuracy of an RTN can be limited by cellular phone coverage, network extent, and network density.

Connection Options From Rover to RTN

A cellular connection is required to accomplish an RTN survey. Many survey equipment setups are now sold with an internal modem included. The Trimble TSC2 and TSC3 devices can be installed with SIM cards. However, they are only compatible with AT&T at this time. Thus, the user should investigate cellular phone coverage at the worksite to determine which carrier has coverage.

Alternatives to an internal SIM card include:

1. MiFi device (e.g., Verizon jetpack)

2. Wi-Fi from a work vehicle

3. Cellular phone that is Bluetooth compatible

4. Commercial RTK bridge or repeater

5. RTK bridge system created with a laptop that has Internet access

Planning for Virtual Reference Network Surveying

The following steps can be followed to plan for VRN survey:

1. Ensure RTK methods are applicable to site.

2. Select VRN.

a. Check VRN coverage for survey area.

• Will the survey be inside or within 5 miles of boundary?

• If only part of the survey area is covered by VRN, can it be used to establish the base control network?

b. Verify quality assurance and control of VRN.

• Who runs it?

• How long has it been up and running?

• How often is VRN updated?

• What kind of technical support is provided?

• How does VRN communicate outages and check if any are planned during field work?

c. Sign up for an account.

• Sometimes VRN will give discounts or free accounts to government/research efforts.

• Note user name, password, IP address, and port.

• Test account is active by logging into Web site.

d. Document contact for VRN to take in the field.

e. Look for support information on the VRN Web site that may help with setup in the field.

f. Determine broadcast format; RTCM3 normally works well.

3. Check for cellular phone coverage relative to the provider option setup in the equipment.

a. Be careful with carrier Web posting maps and talk to locals.

b. Data transfer needs are less than voice transfer, but still need decent signal.

4. Select cellular connection device.

a. Choose reliability over speed; data stream is small so speed is not as important as reliability.

b. Consider spare batteries for cellular device as VRN is draining.

c. Bring waterproof case if using an external modem.

5. Survey schedule.

a. Talk with local operator of network about the application and ensure system will be running during the survey; check that no planned maintenance (downtime) of base stations is scheduled.

b. Ensure that the base stations that encompass the survey area will be running.

c. Plan for times of day with best satellite coverage, particularly if establishing a base station 

d. Check the space weather forecast and use caution when working during increased ionospheric activity.

6. Determine what projection the selected RTN broadcasts in and if there is a need to post-process to a different projection.

7. Determine if RTN will be used for base control only, or the entire survey.

ü  Limited cellular coverage may warrant using VRN only for base setup.

8. Check into which base stations (single vs. network) will be selected when establishing connection.

ü  Some VRNs have multiple networks to represent specific geographic regions; for example, Washington State has five “VRNs” to choose from.

9. Document nearby published benchmarks that can be used as checks in the field.

ü  May need to convert coordinates to those used by RTN.

In the Field

   The basic scenario for RTN surveying is a follows:

1. Due to difficulty in testing prior to survey, have some contingency time at beginning of survey trip to get connections set up and tested.

2. Consider a field reconnaissance along boundaries and within survey extent to test cellular coverage. This can be done during reconnaissance for establishing base control and access points.

3. Set up cellular connection and VRN survey style.

4. Verify quality assurance and control.

a. Stay within the RTN.

b. If a base station is set up, collect at least 2 hours of data to allow comparison to OPUS.

c. Check accuracy of field survey results against known control points (benchmarks) that surround the work area(s) utilizing similar reference frames.

d. Perform in-field calibration of survey job if necessary, and take redundant, independent readings of a selected number of survey points.

e. Perform normal quality assurance and control steps such as checking multipath and distance from base.

f. Recheck same point at beginning and end of day.

g. For important survey points, conduct two separate time-windowed observations of at least 1 minute with unique initializations.

5. Whenever the user moves a set distance (3 to 5 miles) within a set of reference stations, or moves into a new area surrounded by a different set of reference stations, a new VRS and corrections are computed. This ensures that small baseline distances are maintained. The user will notice in the output files that a new VRN base is listed. Typically, only the closest base station is listed in the output file from the controller, even when surveying in a network.

Post-Processing

1. Download job file like normal RTK survey and pull into Trimble Business Center software.

2. If a base station is set up, download the “.T02” file and also pull it into Trimble Business Center software.

3. Check solution at base station against OPUS solution (may need to convert OPUS solution to same projection and datum as VRN). Expect a small amount of discrepancy from published benchmarks or OPUS; are you within published tolerance/accuracy?

4. Review RTN Web site for any noted issues or downtimes during the survey.

5. Download RINEX files from VRS Web site and import and compare against topo solution for base point or a series of ground shots, control points, etc.

Example Applications

   Elwha River, Washington

   Reclamation has utilized the Washington State VRN (WSRN) to survey the Elwha River since 2012, and has an established research account that is free. Reclamation staff used the VRN with a Verizon network to establish base points for RTK surveys, including topo points and bathymetric surveys in continuous topo mode without a base control setup.

   Reclamation was able to Bluetooth from TSC2 and TSC3 controllers simultaneously to a depth sounder and the cellular device and R8 receiver

Siletz River, Oregon

Reclamation accomplished a survey of the Siletz River using the Oregon VRN to establish base control. Reclamation staff used TSC2 controllers linked with R8 receivers and a Verizon cellular connection, and collected in the native broadcast projection of NAD83(2011) Epoch 2010. When Reclamation used the Oregon RTN in 2013, it only broadcast GPS without GLONASS capability,

although this may have been updated