Section 2: Instruments
Anchor: #i1001353GPS Receiver
The receivers used for network surveys should record the full-wavelength carrier phase and signal strength of both the L1 L2 frequencies, and track at least eight satellites simultaneously on parallel channels. L1 only receivers are acceptable only for baselines less than 10 km. Ties to CORS sites should be made with dual-frequency instruments if base lines are longer than 10 km. Receivers should have sufficient memory and battery power to record 6-hours of data at 5-second epochs. Receivers should contain the latest manufacturer’s firmware upgrades.
Anchor: #i1001367GPS Antenna
The antennas should have stable phase centers and be designed to minimize multipath interference. All antenna models used should undergo antenna calibration by the National Geodetic Survey (NGS). Users should consult user’s manual for other specifications.
NGS Geodetic Services Division maintains a GPS Antenna Calibration Web site for calibrating a variety of antennas.
When processing GPS baselines, the user must apply the appropriate GPS antenna phase center offsets. Inappropriate phase center offsets can introduce up to 10 cm of error in the baseline.
GPS antenna ground planes should be utilized according to manufacturer specifications. Ground planes must be utilized for all stations when performing TxDOT Level 1 and Level 2 surveys. For other surveys, a ground plane must be used at the base station and should be utilized in areas where there might be significant multipath. Many new antenna models have built in ground planes.
Anchor: #i1001401GPS-RTK Rover Rod
A fixed height rover rod should be used and if possible, it should be the same height as any fixed height tripods on the project – usually 2-meters. Make a physical measurement in the field notes to verify it has been checked. Also, check the level bubble on the rod before and after each project.
Anchor: #i1001411Tripods
The tripods must facilitate precise offset measurements between the mark datum point and the antenna reference point (ARP). Fixed-height rods or fixed height tripods are preferable and required for certain surveys due to the decreased potential for antenna centering and height measurement errors. All tripods should be examined for stability with each use. Ensure that hinges, clamps, and feet are secure and in good repair. Test the fixed-height tripods for stability, plumb alignment, and height verification at the start and end of each project.
Anchor: #i1001425Tribrachs
Tribrachs and rod levels should be field calibrated before use on each project and should be checked at the end of the project. Any data not bracketed by a successful calibration check are suspect. Professional Tribrach calibration, usually scheduled once a year with regular use is a reasonable interval for maintaining the accuracy of the instrument.
Anchor: #i1001435Instrumentation Requirements
Summarized below are instrumentation and data collection requirements:
|
Level of Survey * |
Level 0 |
Level 1 |
Level 2 |
Level 3 |
Level 4 |
|---|---|---|---|---|---|
|
GPS Receivers |
|
|
|
|
|
|
Dual Frequency |
Required |
Required |
Required |
Recommended |
Recommended |
|
Single Frequency |
Not Acceptable |
Not Acceptable |
Not Acceptable |
Acceptable |
Acceptable |
|
GPS antennas |
Ground plane Required |
Ground plane Required |
Ground plane Required |
Ground plane Optional |
Ground plane Optional |
|
Tripods |
Fixed H.I. Required |
Fixed H.I. or Variable |
Fixed H.I. or Variable |
Fixed H.I. or Variable |
Fixed H.I. or Variable |
|
3d ant. H.I. and centering pos. tolerance |
0.2 cm |
0.2 cm |
0.3 cm |
0.4 cm |
0.4 cm |
|
Minimum # of GPS receivers** |
3 |
3 |
2 |
2 |
2 |
|
RTK acceptable |
No |
No |
No |
No*** |
Yes |
* This chart does not apply to mapping level surveys Levels 5, 6 and 7.
** Minimum # of receivers simultaneously logging data during a session not including CORS.
*** Acceptable if points are located from more than one control point at different times.
Anchor: #i1001449Personnel
All field personnel should be trained in the avoidance of systematic errors during field operations. Field personnel often work alone and must be prepared to make wise, on-the-spot decisions regarding mark identification and stability, equipment use and troubleshooting, and antenna setup. Office personnel should be familiar with geodetic concepts and least squares adjustments. Personnel should participate in any available certification and training activities.
All boundary control survey projects performed for TxDOT will be performed under charge of a Texas Registered Professional Land Surveyor (RPLS). Personnel requirements for various types of surveys may vary from one TxDOT district to another. The use of certified survey technicians (CST’s) is encouraged not to fulfill any requirements, but to aid in the efficiency of operations with the use of goal-oriented employees.