# Presentation: 576271

## South Carolina Virtual Reference Station Network - Centimeter Positional Accuracy in Real Time for South Carolina

- South Carolina Geodetic Survey

- DMT ‘07
- May 22, 2007

## Technology 

- Technology

**Notes:**
Technology: What does it give us?
Simplifies the way we do business
Greater Efficiency at Tasks (especialy menial tasks)
Cool to show your friends

## Miniaturization of Components

## The Global Positioning System

- 3.) User Segment

- 1.) Control Segment
- Master Control Station
- Colorado Springs

- Control Segment
- Monitor Stations
- Diego Garcia
- Ascension Is.
- Kwajalein
- Hawaii

- 2.) Space Segment

- Three Segments

**Notes:**

This slide shows the three distinct segments of GPS, the Control Segment, the Space Segment, and the User Segment.  If one of these segments did not operate, you would not be able to use GPS.  Satellite information and orbital corrections are sent UP to the satellites from the control segment, and the satellites provide the necessary information to obtain positions on the earth as well as relative baselines between points

## Space Segment

- 29 Satellites
- 6 orbital planes inclined 55 degrees to equator
- 4 or 5 SVs in each plane
- 20,200 Km high orbits (12,550 Miles)
- Orbit twice daily  (11 hr 58 min)

**Notes:**
The Space Segment is an earth-orbiting constellation of 21 Navstar satellites (plus 3 on-orbit operational spares) in six planes. The nominal circular orbit is 20,200 kilometer (10,900 nautical mile) altitude orbits inclined at an angle of 55 degrees with a 12-hour period. The spacing of satellites in their orbital <Picture>planes are arranged such that a minimum of four satellites will be in view everywhere on and near the surface of the earth at any time. Each Navstar satellite is designed to broadcast a pair of L-band radio frequency (RF) signals, known as Link 1 (L1 = 1575.42 MHz) and Link 2 (L2 = 1227.6 MHz). The L1 signal carries a precision ranging code and coarse / acquisition code, while L2 carries only the precise ranging code. The signals are broadcast using spread spectrum techniques, employing two different spreading functions: a 1.023 MHz coarse / acquisition (C/A) code on L1 only and a 10.23 MHz precision (P) code on both L1 and L2. The minimum signal power for the different signals at a GPS receiver are: L1 C/A = -160 dBW; L1 P = -163 dBW; and for L2 P = -166 dBW. Superimposed on these codes are low-rate navigation message data, including satellite clock and ephemeris parameters, satellite signal health data, and Coordinated Universal Time (UTC) synchronization information. 

## Control Segment

**Schriever AFB**

**Notes:**
The Control Segment (aka Operational Control Segment) consists of one Master Control Station (MCS) at Falcon AFB in Colorado Springs, plus monitor stations at the MCS, Hawaii, Kwajalein, Diego Garcia and Ascension Island. All monitor stations except Hawaii and Falcon are equipped with ground antennas for communications with the GPS satellites. The monitor stations passively track all GPS satellites in view, collecting ranging data from each satellite. This information is passed on to the MCS where the satellite ephemeris and clock parameters are estimated and predicted. The MCS periodically uploads the ephemeris and clock data to each satellite for re-transmission in the navigation message

## Typical Future Geologist At WorkUser Segment

- Typical Future Geologist At WorkUser Segment

## One measurement narrows down our position to the surface of a sphere

- Calculate your position

## Second measurement narrows it down to intersection of two spheres 

- Second measurement narrows it down to intersection of two spheres* *

- Calculate your position, cont’d** **

## Third measurement narrows to just two points 

- Third measurement narrows to just two points

- In practice 3 measurements are enough to determine a position.
- We can usually discard one point because it will be a ridiculous
- answer, either out in space or moving at high speed.

- Calculate your position, cont’d

## Fourth measurement will decide between the two points

- The fourth measurement allows us to solve for the receiver
- clock bias.

- Fourth measurement will only go through one of the two points

- Calculate your position, cont’d

## Integer Ambiguity Resolution Yields Survey Quality Baselines 

****** ****= First Partial Wavelength**

**N**

**N = Integer Ambiguity**

**D**

**Distance D = N******** + ******

- Integer Ambiguity Resolution Yields Survey Quality Baselines

## Initialization  From The Phase Measurements

****

**Continuous Carrier**
**Phase Measurement**

- Initialization  From The Phase Measurements

## Good Satellite Geometry - PDOP

**Notes:**
One major factor in choosing session times is when the satellite configuration is best.  Although only 4 satellites are needed to obtain a position with GPS, it is also important where those satellites are in space.  That is, what is our satellite geometry?  You only want to survey when we have good geometry. [the instructor should now stand in the middle of the room]
The best way to describe good satellite geometry is to imagine a satellite in all four corners of the room. <B> Now imagine a string from each satellite to the top of this pen [or some other object]. <B>  Note how I’m not able to vary the position of this pen very much.

## Multipath

**Notes:**
Mountains, trees, towers, and buildings are just a few examples of possible obstructions. Remeber, the GPS signal is an RF signal that can easily be blocked.  <B> 
If obstructions are present, it’s important that you note the azimuth and elevation above the horizon of these structures<B>
You can then account for these in your planning.  Also, some structures that are obstructions in one direction can also be sources of multipath in another direction.<B>  Beware of other sources of multipath.<B>

## GPS Signals 

- GPS Signals

- Variable signal paths and piercing points

**Notes:**
By adding a second receiver at a fixed location near where you want to make measurements we can measure those delays and errors and then factor that error into any measurements made by the roving receiver.   
This type of GPS is called “Differential GPS” and it’s the basis of all high accuracy GPS.
The fixed receiver, which is also called the “reference” receiver doesn’t have to be particularly close to the roving receiver to get good results.  In fact they can be a few hundred kilometers apart.   Many Coast Guard and other government facilities are starting to broadcast corrections, so  for many applications near one of these facilities great accuracy can be achieved without setting up your own reference receiver.

## GPS Post Processed or RTK Uses Baselines From A Known Position

**Determined**
**33****o**** 23’ 30.195065” N**
**079****o**** 02’ 33.948394” W**
**-12.445 m**

**Known**
**33****o**** 23’ 28.607434” N**
**079****o**** 02’ 41.161474” W**
**-12.637 m**

- GPS Post Processed or RTK Uses Baselines From A Known Position

**Notes:**
From the logged full wavelengths, the processor uses a differencing algorithm to determine the difference in wavelengths between one receiver and the other.

We then start “guessing” at the integer ambiguity and applying it to the differenced amount of wavelength to produce a range from the satellite to the receiver.

This is done for each satellite we are tracking.

## Real Time Kinematic - RTK

**Notes:**
Collecting RTS data:
  At least 2 receivers are needed (one base, one rover).<B>
  Both receivers observe data from at least 4 SVs.<B>
  The Base Antenna sends the observe data to the Base Receiver<B>
  The Base receiver sends the observed data to it’s radio modem <B>
  The Base radio modem sends the observed data to the Roving radio modem <B>
  The Roving receiver processes the observed data from both receivers and computes the baseline solution.<B>

The radio link between the base and rover must be available in order to process data in real-time
One base can supply data to multiple rovers.

## Ambiguity Resolution for RTK

****

**Notes:**
Next, we see if the “guessed” integer ambiguity is correct by seeing where all these ranges intersect.

On this slide we have three satellites that intersect in three difference locations based on the “guessing” our integer ambiguity.

As we look at these intersections over time we will see more error around the two intersections away from the antenna.

Which will eliminate those “guesses” at the integer ambiguity.

## Limited range from single reference station
Potential gross error in establishing reference station
No integrity monitoring
Dependency on single reference station  
Productivity loss
Security
Communications
Power supply 

- Potential gross error in establishing reference station
- No integrity monitoring
- Dependency on single reference station
- Productivity loss
- Security
- Communications
- Power supply

- Limitations of Classical RTK Survey

**Notes:**
Productivity lost through having to establish base station – locate co-ordinates, set up comms and power, provide security (additional person).
Radio frequency and range issues.
Battery charging.

## RTK PPM Error  vs. Baseline Length

## What is a CORS?

## Reference Stations...defined

- Located at a precisely known position
- Records GPS data for later use
- Post Processing
- Generates GPS corrections for immediate use
- Real - time, broadcast or dial - in
- Results degrade with distance from Reference Station
- Useable range can be from 10 km to 500 km (RTK vs DGPS)

**Notes:**
Classical RTK =
Single base station solution
Limited by distance from single base station
Requires dedicated comms

## Monumentation

- Components of a CORS site

- Hardware

- Communications

- Software

- Photo Courtesy UNAVCO

**Notes:**
CORS Sites have 4 main components:
Monumentation:
The actual mounts, structures, conduit and the work involved putting them in.
Hardware:
Receiver, antenna, cabling, surge supression devices
Communications links:
Some overlap with hardware but essentially these are the actual links whether it is phone lines, ISDN, Internet, VSAT, UHF radios
Software:
The means of controlling and interfacing with the hardware and communications equipment

## Progress?

## Virtual Reference Stations

- Infrastructure - The future for your industry

## Component Monitoring: Server

- Server: Central Data Management

## RTKNet VRS  -  Data Flow

- RTKNet VRS  -  Data Flow

- Reference Station data streams back to server through LAN or Internet

## RTKNet VRS  -  Data Flow

- RTKNet VRS  -  Data Flow

- Roving receiver sends an NMEA string back to server using cellular modem - VRS position is established

- NMEA - GGA

- VRS

## RTKNet VRS  -  Data Flow

- Server uses VRS position to create corrected observables and broadcasts to rover

- RTCM or CMR+

- VRS

## RTKNet VRS  -  Data Flow

- Rover surveys as in normal RTK survey but getting data as if coming from the VRS

- VRS

## VRS  -  How does it work?

- Network Processes
- Integrity monitoring
- Geometric correction
- Correction for tropospheric errors
- Correction for ionospheric errors
- Ambiguity resolution
- Consistency check
- Delivery of VRS corrections for each mobile‘s location

**Notes:**
Productivity lost through having to establish base station – locate co-ordinates, set up comms and power, provide security (additional person).
Radio frequency and range issues.
Battery charging.

## Network Multiple Base Station Solution 

- Network Multiple Base Station Solution

- B1

- B2

- B3

- R

- Let B1, B2 B3 be base stations, R be the rover

- The indexes *r* *= 1,2,3* are for bases, *r=0 *stands for the rover

- Four RTK engines run simultaneously:
- (1,2)
- (2,3)
- (3,1)
- (1,0)+(2.0)+(3,0)

## Field Test Results 

- Time (secs)

- Probability of Certainty

## Modeling gradients of the ionosphere

***Reference Stations***

- Changes of satellite geometry lead to changes in mapping functionChanges of piercing coordinates and dynamical changes in ionosphere lead to time dependence of expansion parameters

- Gradients

## Modeling gradients of the ionosphere

- Changes of satellite geometry lead to changes in mapping functionChanges of piercing coordinates and dynamical changes in ionosphere lead to time dependence of expansion parameters

- Gradients

***Ionosphere***

***Reference Stations***

## Kalman filter – part 1: observation equation

## Components of the state vector

## Advantages of VRS

- Eliminates dependency on single reference station
- Graceful degradation should a R.S. fail
- Uses established communications
- Establishes a single coordinate system

## Slide 40

## The South Carolina Virtual Reference StationNetwork

## SC Prototype VRS Network

- ~120km

- ~100km

## Test Results from the SC Original VRS Prototype Network

- 0.150 ft = ~5 cm

## Scatter Gram of DifferencesWith Logical Rejection of 5 Points2 – Five Minute Sessions

- RMSE(2-D) = 0.020m @95%

- Axis Units in Meters

**0.05 ft  = 1.5 cm**

## Computation of Horizontal Positional Accuracy RMSE (2-D) @95%

- Local Network Accuracy (NGS) = 0.020m
- (equivalent relative accuracy = 1:400,000)
- Centering Error = 0.003m
- Contract Specification for VRS = 0.012m
- Combined RMSE = (0.0202 + 0.0032 + 0.0122)1/2 = 0.024m

## Scatter Gram of All Differences2 – One Minute Sessions

- RMSE(2-D) = 0.024m @95%

- Axis Units in Meters

## Quality Control

**RS Data**

**BAD!**

- USERS

**Quality**
**Control**

## SC VRS Network Construction

- Questions?