II. GPS SURVEY OBSERVATIONS
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DRAFT - DRAFT - DRAFT. Comments?
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Joe Evjen
TABLE OF CONTENTS:
A. Planning
B. Procedures
C. Observer Checklist
The following surveys are described below:
1. Reconnaissance Surveys
2. High Accuracy Reference Network (HARN) Surveys
3. User Densification Network (UDN) Surveys
4. GPS Orthometric Height Surveys
5. Airport Surveys
6. Validation Surveys
Proper field reconnaissance is a prerequisite for any control survey. Reconnaissance
should include a review of existing control networks, mark recovery and
maintenance, station selection, setting of new monuments as necessary,
and compilation of updated station descriptions
into standard FGCS bluebook format.
See the NGS ANA website for helpful
information about reconnaissance, station selection, marksetting, etc.
| A-2. High Accuracy
Reference Network (HARN) Surveys: |
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High Accuracy Reference Networks (HARNs)
are statewide GPS survey networks which form the highest order of monumented
control for the National Spatial Reference System (NSRS).
Comprised of an NGS-maintained Federal Base Network (FBN)
at 100 km station spacing and a volunteer-densified Cooperative Base Network
(CBN) at 25-50 km spacing, HARN stations serve as control for regional
and local surveys. Contemporary HARN surveys are conducted in accordance
with the following specifications:
Station Requirements:
HARN Stations: The survey shall include FBN stations
established at 100 km nominal spacing and additional CBN stations if desired.
Refer to "FBN
Station Selection Guidelines" for HARN monumentation and siting criteria.
Hub Stations: The survey shall include at least 1 hub station
located within 300 km of each HARN station. Hub stations should include
all Continuously Operating Reference Stations (CORS)
in the project area, and any HARN or bench mark stations located in secure
areas.
Bench Marks: The survey shall include at least 1 each, first-
or second-order, stability class -A or class-B bench mark located within
200 km of each HARN station.
Observation Requirements:
HARN Stations: All HARN stations shall be observed in
at least 3 sessions on 3 different days. Each observation shall be continuous
for at least 5 ½ hours, and simultaneous with all other stations
in the observing session. At least 1 of the 3 sessions shall be observed
under a unique satellite configuration, offset 4 or more sidereal hours
from other observations. Session times shall be selected to minimize Positional
Dilution of Precision (PDOP) throughout the observation.
Each HARN station shall be co-observed with a hub station and adjacent
stations. At least half of the station pair (baseline) observations shall
be repeated. Data shall be recorded at 15 second epochs and 10 degree elevation
masks.
Hub Stations: Each hub station shall be observed with the nearest
National CORS station for 1 session of at least 72 continuous hours with
30 second epochs.
These guidelines for HARN surveys are summarized in Table 4.1 below:
| Table 4.1 Network Design Guidelines for HARN Surveys |
| Minimum Number of Stations |
All National CORS in or near the project area |
| Maximum Station Spacing |
Between HARN stations < 100 km (nominal spacing)
Between HARN and hub stations < 300 km
Between HARN and bench mark < 200 km |
| Required Baselines |
For HARN stations:
To a hub station in each session
To adjacent stations
Repeat at least 50% of all baselines
For hub stations:
To the nearest National CORS
|
| Observations per Baseline |
For HARN stations:
3 each, 5½ hour observations on 3 different days
For hub stations:
1 each, 72 hour observation, 30 second epochs
|
| Sidereal Time Offset Between Repeated Observations |
Repeated observations are conducted on different days
At least 1 observation shall be offset by ± 4 hours |
| Fixed-height Tripods Required? |
Required for all FBN stations hub stations and bench marks |
| Acquire Meteorological Data? |
At all stations at the beginning, middle, and end of each observation |
| Data Acquisition Parameters |
15 second epochs, 10 degree elevation masks |
| Data Processing Parameters |
30 second epochs, 15 degree elevation masks, precise ephemerides |
| A-3. User Densification
Network (UDN) Surveys: |
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[home] |
User
Densification Network (UDN) surveys allow regional densification of the
HARN networks. UDN surveys are conducted in accordance with the following
specifications:
Station Requirements:
UDN Stations: The survey shall include UDN stations
established at 25 km spacing or less.
HARN Stations: The survey shall include at least 2 HARN stations.
Observation Requirements:
UDN Stations: Each UDN station shall be observed in
at least two 30 minute sessions. Each UDN station shall be co-observed
with adjacent stations. Data shall be recorded at 15 second epochs and
10 degree elevation masks.
These guidelines for UDN surveys are summarized in Table 4.2 below:
| Table 4.2 Network Design Guidelines for UDN Surveys |
| Minimum Number of Stations |
At least 2 HARN stations |
| Maximum Station Spacing |
< 25 km between UDN stations |
| Required Baselines |
To adjacent stations |
| Observations per Baseline |
2 each, 30 minute observations |
| Sidereal Time Offset Between Repeated Observations |
No time offset required |
| Fixed-height Tripods Required? |
No fixed-height tripods required |
| Acquire Meteorological Data? |
At representative stations in the middle of each observation |
| Data Acquisition Parameters |
30 second epochs, 15 degree elevation masks |
| Data Processing Parameters |
30 second epochs, 15 degree elevation masks, precise or rapid ephemerides |
NOTE: For published GPS Orthometric Height survey requirements, see
NOAA Technical Memorandum NOS-NGS-58; "Guidelines
for Establishing GPS-derived Ellipsoid Heights".
Recent studies have shown that with sufficient orthometric height control,
high accuracy vertical control projects can be efficiently performed using
GPS. Orthometric height surveys are conducted in accordance with the following
specifications:
Station Requirements:
Local Stations: The height stations established in this
survey are referred to as local stations. See Table 4.3 for maximum station
spacings.
Control Stations: The survey shall include 3 or more primary
control stations distributed throughout the project area. All control stations
shall be National CORS or vertical first-order, stability class-A or class-B
HARN stations.
Hub Stations: The survey shall include 3 or more subordinate
control stations, called hub stations, distributed throughout the project
and located within 75 km from the control stations. See Table 4.3 for maximum
station spacings. Although hub stations can be newly established in this
project, the use of existing HARN stations and stable bench marks is preferred.
Bench marks: The survey shall include at least 4 each, vertical
first-order, stability class-A or class-B bench marks spaced less than
20 km apart and distributed both horizontally and vertically throughout
the project area. Use additional bench marks whenever practical.
Observation Requirements:
Local Stations: All local stations shall be connected to the
2 nearest adjacent stations of any type by observing for 30 minutes on
each of 2 or more days, at different times of day, and shall be connected
to 2 hub or control stations by independent paths. For these observations,
please note the following exceptions:
For baselines longer than 10 km, increase the 30 minute observations
to 1 hour.
For baselines longer than 15 km, increase the 30 minute observations
to 2 hours.
For the 2 cm horizontal, 5 cm orthometric height local accuracy level,
there is no minimum time requirement. The baselines shall be observed long
enough to ensure that all integers are fixed and the Root Mean Square Error
(RMSE) for the baseline solution does not exceed 1.5 cm.
Hub Stations: Each hub station shall be directly connected to
the nearest control station and 2 other hub stations, and shall be connected
to a second control station by an independent path. Each of these baselines
shall be observed for 5 hours on each of 3 or more days. Hub stations shall
be connected to the 2 nearest hub stations, and the 2 nearest adjacent
stations of any type, by observing for at least 30 minutes on each of 2
or more days, at different times of day.
Independence of Observations:
In an attempt to provide independent atmospheric, satellite,
and tidal conditions, subsequent observations of all repeated baselines
shall occur on different days, with as long a time interval between observations
as is practical. Because the GPS satellite geometry repeats every 12 hours*,
an additional time shift of ± 3 to 9 hours is required to observe
unique satellite constellations. Refer to the following table for examples:
| First Session |
Repeated Session (a.m. or p.m.) |
|
First Session |
Repeated Session (a.m. or p.m.) |
| Began at 1 |
Observe between 4 and 10 |
|
Began at 7 |
Observe between 10 and 4 |
| Began at 2 |
Observe between 5 and 11 |
|
Began at 8 |
Observe between 11 and 5 |
| Began at 3 |
Observe between 6 and 12 |
|
Began at 9 |
Observe between 12 and 6 |
| Began at 4 |
Observe between 7 and 1 |
|
Began at 10 |
Observe between 1 and 7 |
| Began at 5 |
Observe between 8 and 2 |
|
Began at 11 |
Observe between 2 and 8 |
| Began at 6 |
Observe between 9 and 3 |
|
Began at 12 |
Observe between 3 and 9 |
* Note that the actual orbit period is approximately 11 hours and 58
minutes, precessing 4 minutes per day. For repeated baselines observed
more than 1 week apart, this daily 4 minute change should be taken into
account when scheduling the repeated sessions to meet the satellite geometry
requirement.
These guidelines for GPS orthometric height surveys are summarized in
Table 4.3 below:
| Table 4.3 Network Design Guidelines for GPS Orthometric
Height Surveys |
| Station Type: |
Control Stations |
Hub Stations |
Local Stations |
| Local Accuracy: |
1 cm Horizontal
2 cm Orthometric |
2 cm Horizontal
5 cm Orthometric |
1 cm Horizontal
2 cm Orthometric |
2 cm Horizontal
5 cm Orthometric |
1 cm Horizontal
2 cm Orthometric |
2 cm Horizontal
5 cm Orthometric |
| Min. # stations |
3 or more control stations |
3 or more hub stations |
No minimum number |
| Max. Station Spacing |
Between control and hub stations
75 km spacing |
40 km spacing |
50 km spacing |
10 km spacing
average 7 km |
20 km spacing
average 10 km |
| Required Baseline Ties |
Control stations must be CORS
or existing A-or B- order NSRS stations with 3-D ITRF coordinates |
5 hour observations: To nearest control, 2 other hub stations,
and independent path to second control
30 minute obs: To 2 nearest hub stations and 2 nearest adjacent stations. |
Independent paths to 2 hub or control stations; To 2 nearest
adjacent stations. |
| Observations per Base line |
5 hour observations on 3 days |
5 hour observations on 3 days
30 minute observations on 2 days |
30 minute obs
on 2 days |
No minimum time obs. on 2 days |
| Sidereal Time Between Obs. |
Repeat observations on different
days ±3 to 9 hours |
Repeat observations on different
days ± 3 to 9 hours |
Repeat observations on different
days ± 3 to 9 hours |
| Fixed Height Tripod Rqd. |
Yes |
Yes |
No |
Yes |
No |
| Acquire Met. Data |
At the beginning middle and end of session |
At the beginning and end of session |
No |
| Data Acq. Parameters |
15 seconds 10 degrees
VDOP < 6 for 90% of session |
15 seconds 10 degrees
VDOP < 6 for 90% of session |
15 sec 10 degrees VDOP<6 for 90% |
5 sec 10 degrees
VDOP<6 for 100% |
| Data Proc. Parameters |
30 seconds 15 degrees
precise ephemerides |
30 seconds 15 degrees
precise ephemerides |
30 sec 5 degrees
precise ephemerides |
5 sec 15 degrees
precise ephemerides |
1. For baselines longer than 10 km, increase the 30 minute observations
to 1 hour. For baselines longer than 15 km, increase the 30 minute observations
to 2 hours. While there is no minimum observation time for the 2 cm horizontal,
5 cm orthometric height local accuracy observations at local stations,
each baseline shall be observed long enough in each session to ensure that
all integers are fixed and the RMSE for the baseline solution does not
exceed 1.5 cm.
See the NGS Aeronautical Survey website for
information on ANA and other airport surveys.
Validation survey requirements include the following:
Under development. For more information, see Canadian
GPS Validation Networks.
The following activities are described below:
1. Receiver Operation
2. Tripod Setup
3. Antenna Height Measurement
4. Tribrach Operation
5. Weather Observations
6. Personnel Operation
Receivers should be configured to acquire data in compressed mode, with
elevation-masking-angle and epoch-interval (recording rate) settings as
specified in the project instructions.
Fixed height tripods are preferred over slip-leg tripods, as they reduce
the potential for antenna height measurement errors.
If a slip-leg tripod is used, a low tripod setup is preferred to minimize
eccentricities, though the antenna should be set high enough to avoid obstructions.
Eccentric setups (antenna out of plumb
from the station datum point) are to be avoided. Note any eccentricities
on the observation log.
Tripod legs should be well set and weighted with sandbags, chain, or
spikes to minimize movement.
Plumbing bubbles must be shaded for at least 3 minutes before use to
minimize convective currents in the bubble fluid. On tripods with rotating
center poles, the bubble must be rotated and checked level throughout a
180-degree arc.
Antennas should be oriented towards true north, as closely as can be
accomplished with a hand compass. Note the magnetic
declination in your local area to convert from magnetic north to true
north.
If possible, take a photograph of the tripod setup with a close-up of
the antenna as viewed from the side. This photograph may be useful in identifying
any variations in antenna configuration (type of radome or ground plane
used.)
Link - Setup
Instructions for SECO Precise GPS Antenna Tripods
The proper recordation of antenna height is critical. The
Antenna Height used at NGS is the vertical distance between the station
datum point and the Antenna Reference Point (ARP). Observers must
carefully measure and check this height, and record and describe all measurements
and antenna constants. Record all values to 0.0001 meters or 0.001 foot.
All measurement computations must be checked and initialed by another person.
Fixed-height tripods
simplify the measurement of antenna height
(H).
The calibrated tripod height (A) should be checked with a quick
measurement. Ensure that the antenna mates securely with the tripod head,
and that any gap
(B) between the tripod head and ARP is measured
and included. The antenna height can then be computed from the following
equation:
| Antenna Height H= (A + B) |
NOTE: Leica brand antennas use a measuring hook
to determine the vertical distance between the mark and antenna. Record
the measured distance from the mark to the hook as "a", and the offset
from the hook to the ARP as "b".
Slip-leg tripods antenna
height
(H) is usually measured by slant-height
(S),
the distance of the hypotenuse from the station datum point to the bottom
edge of the antenna ground plane (BGP). Measure the slant height to at
least 3 points around the antenna; these measurements should all agree
to within 1 millimeter or the antenna should be replumbed. Independent
measurements of the antenna height above the mark in both metric and Imperial
units must be made before and after each session. From the antenna specification
sheet in your user's manual, determine the radius
(R) of the ground
plane and the offset constant
(C) between the BGP and the ARP. The
antenna height can then be computed from the following Pythagorean equation:
| Antenna Height H= ( sqrt(S
2 - R 2)
- C) |
Compare Metric and Imperial measurements using the following equations:
Meters = Feet × (0.3048) Example: 1.286 Meters = 4.219 Feet
Feet = Meters ÷ (0.3048) Example: 5.345 Feet = 1.629 Meters
Note that the 3-dimensional datum point of a standard survey disk is
located at or above the dimple in the disk's center, on a level with the
highest point of the disk, where the foot of a level rod would rest. If
the point of the fixed-height pole or slant-height measuring rod intrudes
significantly below this level to reach the bottom of the dimple (1 millimeter
or more), make a careful measurement of the vertical separation and note
this on the observation log. |
See your user's manual for operational details.
Link - SECO
Tips-Setting Up the GPS Optical Plummet
Measure and record weather data (temperature, pressure, relative humidity,
and weather codes) at the beginning, middle, and end of each observing
session, and at sufficient intervals during weather events (storm fronts,
wind, snow, or rain, etc.) to describe all conditions observed. For accurate
readings, acclimate the weather equipment for approximately 10 minutes
prior to use. Note that even though these meteorological data may not all
be used during vector processing, they may
be helpful during the analysis of results and in future reprocessing with
more robust software.
The measurements must be made in the vicinity of the GPS antenna, taking
care not to obstruct the GPS signals. Indicate on the observation
log the time and location of the meteorological observations. At stations
in the vicinity of airports (within 5 km and at approximately same elevation),
weather data from the airport (available on the Interactive
Weather Information Network) may be substituted for actual measurements,
if no portable meteorological instruments are available.
| Temperature: Read and record the dry-bulb
and wet-bulb temperatures to the nearest degree Celsius. |
|
Conversion: C = 5/9 (F-32) |
Example: 76oF = 24.4 C |
| Pressure: Read and record the absolute
or station pressure (NOT corrected for sea level) to the nearest millibar. |
|
Conversion: mbar = ( in Hg/0.75
) * 25.4 |
Example: 29.98 in Hg = 1015.32 mbar |
Relative Humidity: Compute the relative humidity
from temperature and pressure readings using NGS program RELHUM or the
NWS
Weather Calculator. Electronic hygrometers are acceptable as long as
the equipment is calibrated and all calibration documentation and offsets
are furnished to the Project Coordinator.
Weather Codes: A 5-digit code is used to summarize
the general weather conditions, as described below:
| WEATHER CODE |
0 |
1 |
2 |
| PROBLEM |
(0 or 1) |
0.... = No problem encountered |
1.... = Problem encountered |
2.... = Not used |
| VISIBILITY |
(0, 1, or 2) |
.0... = Good (over 15 miles) |
.1... = Fair (7 to 15 miles) |
.2... = Poor (under 7 miles) |
| TEMPERATURE |
(0, 1, or 2) |
..0.. = Normal (32oF to 80oF) |
..1.. = Hot (over 80oF) |
..2.. = Cold (under 32oF) |
| CLOUD COVER |
(0, 1, or 2) |
...0. = Clear (under 20%) |
...1. = Cloudy (20% to 70%) |
...2. = Overcast (over 70%) |
| WIND |
(0, 1, or 2) |
....0 = Calm (under 5 mph) |
....1 = Moderate (5 to 15 mph) |
....2 = Strong (over 15 mph) |
Examples:
| Code 00000 = |
0.... (=No problems), |
.0... (=good visibility), |
..0.. (=normal temperature), |
...0. (=clear sky), |
....0 (=calm wind) |
= Code 00000 |
| Code 12121 = |
1.... (=Problems), |
.2... (=poor visibility), |
..1.. (=hot temperature), |
...2. (=overcast), |
....1 (=moderate wind) |
= Code 12121 |
Survey operations shall be conducted with due regard to the safety of personnel
and equipment. Contact with the airport traffic control tower is mandatory
during surveys at any controlled airports.
| ____ |
Determine the scheduled start day and time for the session. Note the
UTC-to-local
time difference (e.g., UTC to Pacific Daylight is - 7 hours). |
| ____ |
Assemble all items from the Equipment
Checklist. Ensure all equipment is in good repair and within
calibration
limits. |
| ____ |
Arrive at the station about 1 hour prior to the scheduled start time. |
| ____ |
Park your vehicle in a safe manner. Avoid hindering the equipment setup
or creating multipath signals near the GPS
antenna. |
| ____ |
Obtain a station rubbing or sketch on a Pencil
Rubbing Form. Compare the station stamping with the Station Description. |
| ____ |
Set the tripod and GPS antenna plumb over the
station. |
| ____ |
Place sandbags, chain, or spikes at all tripod feet. |
| ____ |
Align the antenna orientation indicator (arrow) to True North. |
| ____ |
Attach the GPS receiver to the
antenna and batteries, confirming all connections. |
| ____ |
Ensure that the antenna setup is stable and that the antenna is plumb
over the station. |
| ____ |
Measure the antenna height in meters and feet,
and record your measurements in the "Before" column of the NGS
Observation Log |
| ____ |
Reconfirm that the antenna is plumb. |
| ____ |
Check GPS receiver available memory, power supply, and data
logging parameters |
| ____ |
Begin the session no more than 10 minutes prior to the scheduled start
time. |
| ____ |
Confirm that the GPS receiver is actually recording data. |
| ____ |
If possible, input the following into the GPS digital data file: |
|
Change filename to standard NGS filename
Enter antenna type and antenna height
Do NOT record weather data, event markers, or survey notes in the digital
data
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| ____ |
Monitor the receiver (logging), antenna setup (plumb), and battery
health (good). |
| ____ |
Consult the Troubleshooting Guide
if you encounter problems. |
| ____ |
Acclimate the meteorology equipment;
measure and record weather observations. |
| ____ |
Fill-in the NGS Observation Log with
all pertinent data. |
| ____ |
Check the provided Visibility Obstruction
Diagram and the Station Datasheet
or Station Description; note any changes. |
| ____ |
Photograph the station mark, antenna setup, to-reach view, and visibility
obstructions with a digital camera. |
| ____ |
Acclimate the meteorology equipment; measure and record weather observations
at the session midpoint. |
| ____ |
Monitor the receiver (logging), antenna setup (plumb), and battery
health (good). |
| ____ |
Terminate the session at the scheduled stop time; turn off the receiver. |
| ____ |
Acclimate the meteorology equipment; measure and record weather observations. |
| ____ |
Reconfirm that the antenna is still plumb over the station and remeasure
the antenna height in meters and feet. |
| ____ |
Record the antenna height in the "After" column on the observation
log; check the form for completeness. |
| ____ |
Tear down the equipment; depart the station site and return to the
office. |
| ____ |
Download the data directly from the
receiver into your computer hard-drive twice (to ensure clean copies) then
copy to diskette. |
| ____ |
Ensure sufficient memory is available in the GPS receiver for the next
session; delete older sessions if necessary. |
| ____ |
Recharge batteries for the next session. |
| ____ |
Check and verify all entries on the
NGS Observation
Log, Pencil Rubbing Form,
Visibility
Obstruction Diagram, and Station Description. |
| ____ |
Ship data to the Project Coordinator. |