MSPS 54 th Annual Meeting October 15, 2011 Tom Bryant, PLS

MSPS 54 th Annual Meeting October 15, 2011 Tom Bryant, PLS

Tom Bryant PLS Seiler Instrument Company St. Louis MO Technical Support and Training Manager Tom Seiler Seiler Instrument Company Vice President Survey and Microscope Divisions

History of GPS GPS General Concepts Who uses it Classical Real Time Kinematic (RTK) Equipment Best Practices Best Performance

Real Time Networks Equipment Best Practices Best Performance MoDOT VRS Quality Control and Assurance Redundancy, Redundancy and more Redundancy

A system capable of providing position information anywhere on earth Global Positioning System A constellation of orbiting satellites Various orbits around the earth NAVSTAR GPS User receivers acquire signal and determines its position AHT/EVC 10/19/2011 5

Global Positioning System Developed by DOD Cost $10 billion Trilateration-based AHT/EVC 10/19/2011 6

BLOCK IIF SATELLITE CHARACTERISTICS Weight (in orbit): 3758 pounds Orbit altitude: 10,988 nautical miles Power source: solar panels generating up to 2900 watts Dimensions: 8 ft x 6.47 ft (stowed) 70.42 ft (deployed 4 panel solar arrays) x 12 ft Design life: 15 years AHT/EVC 10/19/2011 7

Mathematically perfect orbit Orbits twice per day Large viewable area AHT/EVC 10/19/2011 8

Everyone! Merchant, Navy, Coast Guard vessels Forget about the sextant, Loran, etc. Commercial Airliners, Civil Pilots Surveyors Has completely revolutionized surveying Commercial Truckers Hikers, Mountain Climbers, Backpackers Cars! Cell phones!! Communications and Imaging Satellites Space-to-Space Navigation Any system requiring accurate timing

AAA (who can resist it!) All weather operation Always available (24/7 operation) Anywhere available Economical Increased Productivity Improved Customer service Accuracy (3-D data, Velocity and timing) AHT/EVC 10/19/2011 10

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SV s 11,000 miles up Signals you can t see Go into a receiver that you can t see inside of Comes out of receiver and goes into a data collector using another wireless invisible method (Bluetooth) Spits out positions you are supposed to believe?

Faith? I *know* it is going to work Because the salesman told me it would..and I paid big $ s for it!

Hope? I would rather be lucky than good anyday How s that working for you?

Proof? Test against known values Redundancy

Accuracy and precision may be improved: If we follow directions If we stay within the operating limits of the equipment If we use the equipment properly If we use the right equipment for the job If we use care and preplanning If we build redundancies into the measurement If we can trust the people who are using the equipment! So nothing is new here! Hi-tech or not, we still need to use caution. 10/19/2011 35

1 - dual frequency GPS + GLONASS GNSS base receiver 1- dual frequency GPS + GLONASS GNSS high quality antenna 1- GNSS antenna cable 1- fixed height tripod, weights for the legs on long occupations 1- lead-acid battery with power lead to receiver. (Note: typical power input level on GNSS receivers is in the range of 10.5 volts 28 volts. Users frequently use a 12 volt lawn tractor battery)

25 watt- 35 watt base radio (FCC) licensed (required with severe noncompliance penalties) 2-4 channels (10 or more recommended) Lead acid battery power cable, antenna mast, antenna tripod or mount for base tripod, data cable

Range is typically 5 km - 8 km (3 miles -5 miles) A full-size whip antenna option will enhance communications. Regardless of the type of external battery used, it should supply at least 12 volts and should be fully charged. An underpowered battery can severely limit communication range.

The base broadcast radio antenna should be raised to the maximum height possible. Studies have shown that an increase in antenna height from 5 to 20 will increase the broadcast range from 5 miles to 11 miles. The study shows a doubling in antenna height will increase the range by 40%. However, any height over 25 should use a low-loss cable.

1-dual frequency GPS + GLONASS GNSS integrated receiver/antenna, internal batteries 1- carbon fiber rover pole (two sections fixed height), circular level vial Or Adjustable rover pole The condition of the rover pole should be straight and not warped or bent in any manner.

1 rover pole bipod or tripod with quick release legs 1- data collector, internal battery and pole mount bracket 1- datalink between Receiver and Data Collector Cable Bluetooth wireless connection

Adjust the base and rover circular level vial before every campaign As a good practice or if the circular level vial is not adjusted, it is still possible to eliminate the possible plumbing error by taking two locations on a point with the rover pole rotated 180 between each location

A rover pole with an adjusted standard 40 minute vial located about midpoint of the length should introduce a maximum leveling error of no more than 2.5 mm (less than 0.01 feet). It should be noted that 10 minute vials are available.

Even with RTK at the rover Especially for control points

The greatest contributor to error in GPS measurement is human error. Misreading antenna height measurements Transposing numbers entered electronically and/or on the GPS observation log Rushing observations Poor centering and leveling over points Observing the wrong survey point for example, observing a reference mark instead of the actual mark itself Incorrect equipment configuration settings

A measure of the geometry of the visible satellite constellation. Ideal orientation of four or more satellites Spaced equally around the receiver Including one above and one below Not physically possible due to us being on the earth s surface A low numeric Dilution of Precision value represents a good satellite configuration, whereas a higher value represents a poor satellite configuration.

Precision Dilution of Precision Most common Shows dilution in three dimensions Values A DOP value of less than 2 is considered excellent About as good as it gets, does not happen often, requires a clear view of sky to the horizon DOP values of 2 to 3 are considered very good DOP values of 4 to 5 are acceptable for most other than very precise work DOP values greater than 6 stop working.

It is helpful to partially mitigate the worst effects of atmospheric delay and refraction by setting an elevation mask (cut off angle) of 10-15 to block the lower satellites signals which have the longest run through the atmosphere. A 10 mask is recommended.

Urban canyons Tall buildings block the sky Glass and metal covered reflect GNSS signals very well The receiver s multi-path rejection capability may actually be overloaded. Overhead power lines The higher the power, the worse for GNSS Look at easement width on power lines Electromagnetic radiation surrounding power lines Can interfere Hard or impossible to model this error

Forest canopy? Dense canopy cover can obscure the sky and interfere with the incoming satellite signal. The problem is even worse if the vegetation is wet since the liquid water itself can also interfere with the signal. User s experience using GNSS under canopy? Western Oklahoma version of clear sky

The control of a classical RT positioning survey is always in the hands of the rover. Need to have an awareness of all the important conditions and variables in order to get good RT results. In RT positioning, It Depends is the answer to most questions.

Proven control monumentation with a high degree of integrity Data precision is monitored as the work proceeds Points with known values are checked Before During After Redundant locations are taken on each important point.

THIS NOT THIS

It is possible to perform an accurate RT session from an autonomous-positioned base station point if the correct position can be introduced to the project in the data collector or in the office software later. In fact, it is much better to establish a new, completely open sky view site for the base than it is to try to occupy an existing reliable, well known monument with a somewhat obscured sky view.

Areas with probable multipath conditions should not be used for RT positioned control sites -especially not for a base station position

These sites include locations under or very near tree canopy structures within 30 m that are over the height of the antenna nearby vehicles and nearby metal objects abutting large water bodies nearby signs.

Set the base at a wide open site Set rover elevation mask between 10 & 15 The more satellites the better The lower the PDOP the better The more redundancy the better Beware multipath Beware long initialization times

Beware antenna height blunders Survey with fixed solutions only Always check known points before, during and after new location sessions Keep equipment adjusted for highest accuracy Communication should be continuous while locating a point Precision displayed in the data collector is usually at the 68 percent level (or 1σ), which is only about half the error spread to get 95 percent confidence Have back up batteries & cables RT does not like tree canopy or tall buildings

RTK give coordinates on positions Other information is needed for records datum/adjustment/epoch of the base station Field conditions Temperature, wind, precipitation, storms Equipment used, especially the antennas Firmware in Receivers and Collectors Redundancy used Calibration or localization information Date, time and field technicians names

RT positioning of important data points can not be done reliably without some form of redundancy Redundancy is critical for important point positions using RT The external battery should supply at least 12 volts and should be fully charged. An underpowered battery can severely limit communication range

The base broadcast radio antenna should be raised to the maximum height possible. Make sure that the base and rover circular level vials are in adjustment. Set the base in as open a position as possible Data link should be continuous during measurements

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

0.16 0.14 0.12 Error (m) 0.1 0.08 0.06 1ppm @ 67% 2ppm @ 67% 1ppm @ 95% 2ppm @ 95% 0.04 0.02 0 0 5 10 15 20 25 30 35 40 Distance (km)

Continuously Operating Reference Station

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)

Components of a CORS site Monumentation Hardware Software Communications

Network of Single RTK Stations Accuracy, Reliability, Availability Good Bad

RTK Reference Stations Network Accuracy, Reliability, Availability Good Bad

Reference Station data streams back to server through LAN or Internet

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

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

Eliminate the need for local base stations Only GPS rover receivers are needed Less initial GPS expense because it doubles the number of GPS systems you have now No surveyor required to watch the base station Consistent known datum and coordinate system

Cell phone. Bluetooth or old school cable Internal modem in receiver Internal modem in data collector Other modem like Airlink Ravens Intiucom RTK Bridge WiFi Hotspots MiFI devices Phones Other WiFi s Without the communication link VRS is not possible

Pick whoever has the best coverage in your area Data coverage not just voice In newer systems, such as 3G and EDGE, voice no longer takes precedence over data Only way to really know which carrier has best coverage is to test it yourself Maps are VERY generalized

Here is approximately would you could expect using CMR+ with 14 SVs for 8hrs straight: 1 sec: 310bytes 1 min: (310bytes x 60) = 18.6KB 1 hour: (18.6KB x 60) = 1.11MB 8 hrs: (1.11MB x 8) = 8.93MB 5GB (typical plan amount): 24/7 for 30 days

Total number: 424 (these are August 2011 counts) Agriculture: 64 Surveyor: 306 Construction: 27 Other (GIS) : 26 (which includes: city & county organizations; public water, universities, etc.) Of that Total Number above: around 120 are Out of State signups around 70 are MoDOT employees

Check Equipment, Data Collector Parameters & Site information Measure the actual height of the antenna reference point (ARP) on the rover pole Ensure that all necessary and correct projection parameters are in the data collector Ensure that all project data are in the data collector Adjust the rover pole bubble before every campaign

Check Equipment continued Test wireless data communications (cell/cdma/sim card/etc.) for Internet connectivity at the project site. Make sure the GNSS unit and the communication device batteries are fully charged and that there are backups. For orthometric heights, be sure to preload the current geoid model supplied by the NGS

Conditions Use mission planning Check Space Weather Always be aware of multipath conditions. Be aware of possible electrical interference from sources such as high tension transmission lines or broadcast antennas

Coordinates Know what datum, adjustment and epoch is needed for the coordinate data produced Know what datum, adjustment and epoch coordinates are supplied by the RTN Grid or Ground?

Communication Robust communication is the key to an effective RTN Wireless internet from a variety of sources lets you roam anywhere within an RTN Many options The GNSS solution at a point of interest should become fixed in a normal amount of time and should remain fixed for the duration of the actual data collection

Communication Save all communication configuration information for hardware, firmware, user names, passwords, serial numbers, and Bluetooth connections When no cellular coverage Break out your base and radio Use OPUS or OPUS RS Benefit of having your RTN tied to NSRS

Constraining to passive monuments (a.k.a. Calibrations or Localizations) Orthometric Elevations Use four Benchmarks surrounding site PLUS the geoid model Use one Benchmark, use geoid model, check to another Benchmark Just use what comes out of the RTN Probably should check at least one Benchmark if possible Why are my elevations off by 100 feet?

Constraining to passive monuments (a.k.a. Calibrations or Localizations) Horizontal accuracy of RTN is essentially the same as survey grade RTK Calibrate for Checking coordinate consistancy Tying to project coordinates Sanity check when you are given coordinates and no metadata For best vertical results, calibrate to local, known, trusted, high quality Benchmarks

Collection Check a known coordinate point before, during and at the end of data collection Provides check on Antenna height blunders Incorrect projection parameters Faulty calibrations Initialization Set an elevation cut-off or mask of between 10 and 15.

Confidence Redundancy is the king of RT GNSS positioning Redundancy gives confidence and refines the precision of the data Robust wireless Internet connectivity Coordinate accuracy will suffer if Latencies rise above 2 seconds Communication is intermittent during data capture Checks on known points Before, During, After Obvious Multipath avoid it

Summary Four basic elements to achieve reliability Communication, Checks, Redundancy, Multipath Good GNSS gear, good field conditions and good field procedures will yield good Real Time positions

What is an RTK shot? How can you add redundancy to your work?

Need to measure it more than once to know for sure that it is right How do we achieve redundancy using RTK? Measure it more than once Measurements separated by time Change in the satellite constellation

Classical RTK Move the base, observe points again Store data at each base location Submit to OPUS If setting two points for control Measuring between them with a total station is an excellent check You can store more than one observation, and post process and adjust in office software.

Classically.four hours How big is your budget on the job? Studies in Great Britain have shown For precise work where the height component is important, observe for three minutes and then another three minutes. A separation of 20 minutes gives a 10-20% improvement in coordinate accuracy 45 minute separation gives a 15-30% improvement over a single epoch solution.

Measure your point Dump the antenna Let it reintialize Measure again Store as same number and look at differences Store as a different number and inverse More of a warm and fuzzy check than anything But better than nothing

For practical reasons, most users cannot wait four hours between RTK observations due to cost and logistics. A better alternative for a check shot than moving the base Record your first point measurement Force receiver to lose initialization Raise elevation mask to 90 degrees Change antenna height by 0.3 meters (0.98 feet) Longer than one wavelength of the GPS signal

Change antenna mask back to 10 degrees Do a Known Point initialization on the point you stored If it succeeds, might put more faith in that than moving the base If it does not succeed, the first shot probably wasn t any good.

MEASURED THE 2 ND TIME Selected Average Computed average shown

YOU CAN AVERAGE AS MANY TIMES AS YOU WANT

REVIEWING DETAILS ALL THE SHOTS

FINAL AVERAGE FINAL AVERAGE

Takes longer to get to most points than it does to shoot them Longer observation is better Redundancy is better than longer observation I would rather see two 60 second shots than one 180 second shot NO single shots on important points Just too many variables