5.4.3 GPS Static Surveying: Field Procedures

ON-SITE PROCEDURES

All procedures for the operation of GPS receivers should strictly follow the manufacturer's instructions in the operator's manual. For example, warmup times for the receiver oscillator, operations as "cold" and "warm" starts, minimum power operation, data storage capacity, etc.

Antenna Setup and Height Measurement

The following are some procedures that should be adhered to in this regard:

• The antenna normally bears a direction indicator that should be oriented in the same direction at all sites using a compass. This ensures that any antenna centre offset (as measured from the mechanical centre to the electrical phase centre) will propagate into the baseline solution (groundmark to groundmark) in a systematic manner.
• The same antenna, receiver and cabling should be maintained together in a "kit".
• Because of the high precision of GPS surveys, the centring of the antennas is important. If centring is poor, the accuracy of the overall survey will suffer hence plumbobs should be avoided. Tribrachs with built-in optical plummets should be regularly calibrated.
• The antenna assembly should therefore be mounted on a standard survey tribrach with an optical plummet, on a good quality survey tripod.
• Setting up on a pillar is, of course, reasonably effortless and to be preferred.
• If the receiver is to remain on-site for two or more observing sessions, the antenna should be re-positioned each time.
• Care must be taken with antenna height measurement.

As the latter is probably the most critical of all antenna setup operations some further comments must be made. The height of the antenna above the station marker, measured from the standard reference point on the antenna housing should be measured to the nearest millimetre, and should be done at the beginning and at the end of each session. As this is a common source of error, the measurement should be checked, for example by independent measurement by another person, or by measurement in imperial units, and cross-checked.

Different antenna types have different recommendations for height measurement (some of the common antenna types are indicated on the attached sample booking sheet at the end of this section). The Figures below illustrates two common antenna height measurement procedures for a tripod-mounted antenna. All antenna height measurements must be carefully noted, preferably with a diagram, as indicated on the sample field sheets.

Methods of measuring the antenna height for tripod setups.

Synchronising Observation Sessions

• All receivers must record data for the same epochs to within a few microseconds, hence:
  • they should start at a well defined instant in time
  they should track at the same data rate
• For example, start on the minute and make observations every 15 seconds: 0, 15, 30, 45, 0, ...

How to coordinate data collection? Modern GPS receivers will independently synchronise to GPS Time, and as long as they are programmed for the same data observation rate, then the receivers will be automatically synchronised to the required accuracy.

Field Log Sheets

A field log should be maintained, in which pertinent information concerning the site being occupied and the data collection process itself is entered. Such a sheet would typically contain the following information:

• Date and time, field crew details, etc.
• Station name and number (including aliases, site codes, etc.).
• Session number, or other campaign indicator.
• Serial numbers of receiver, antenna, data logger, memory card, etc.
• Start and end time of observations (actual and planned).
• Satellites observed during session (actual and planned).
• Antenna height (several measurements), and eccentric station offsets (if used).
• Weather (general remarks), and meteorological observations if requested (such as temperature, pressure and relative humidity).
• Receiver operation parameters such as data recording rate, type of observations being made, elevation mask angle imposed, data format used, etc.
• Any receiver, battery, operator or tracking problems that were noticed.
• Sketch of the site showing all marks, possible obstructions, etc.

Many examples of GPS field log sheets are possible, from the rather elaborate examples used for high precision geodetic GPS surveys, to simple one page, general-purpose booking sheet formats.

On-Site Procedures: A Checklist

The following is a list of some GPS on-site field procedures:

• GPS receiver initialisation procedures.
• Set-up and orientation of antenna.
• Correct cable connection of antenna to receiver, receiver to battery, etc.
• Double (and triple) checking of centring and antenna height measurement.
• Receiver startup procedure, for example entry of site number, height of antenna, etc.
• Start of tracking.
• Survey of eccentric station.
• Temperature, pressure and humidity measurements (if required).
• Monitoring receiver operation and data recording.
• Field log entries.
• Photographs of point occupancy.
• Procedures at completion of session, for example communication, data transfer to P.C.
• Instructions in event of receiver problems, contingency plans, etc.

GPS receivers display a lot of information during a data measurement session. Devise a field routine to periodically check some of the important indicators ...   Don't forget to note in the field sheets if anything unusual is detected!!

Receiver Operation Monitoring Checklist: Battery status Memory capacity left Satellites being tracked Real-time navigation position solution Satellite health (also useful for post-processing) Date and time (UTC or local) Elevation & azimuth of satellites (compare with predictions or skyplot) "Signal-to-noise" ratios Antenna connection indicator Tracking channel status Amount of data being logged

What about collecting met data? FORGET IT!! Why? Commercial software does not accept meteorological data for tropospheric refraction correction. Met data cannot be measured to the required precision (particularly that part critical for the wet component of the troposphere). Field staff have other more important things to do while data are being collected.

Eccentric Station Survey

Unlike conventional surveys, GPS surveying requires a comparatively clear view of the sky above the elevation cutoff angle (15° to 20°). Sometimes the groundmark that is to be surveyed does not satisfy this condition, perhaps because it is a previously monumented mark at an already existing geodetic control station. In such circumstances an eccentric station may be occupied by the GPS antenna. However, to reduce the baseline components to the required groundmark it is necessary to make certain site measurements.

Three mutually intervisible marks are identified: the groundmark A, the antenna site B and a third mark C -- mark C would typically be a "witness mark". Some typical scenarios are:

• (a) Set up a theodolite at A, and orient it by observing a distant trig station; (b) observe the azimuth A to B (and C); (c) measure the distances A to B (and A to C) with a tape; (d) measure the difference in height A to B (and A to C). (Mark C provides a means of checking these operations.)
• (a) If orientation is not possible (no distant trig stations are visible), then set up a theodolite at A and observe the astronomic azimuth A to C (and A to B); (b) measure the distances A to B (and A to C) with a tape; (c) measure the difference in height A to B (and A to C).
• (a) If orientation is not possible (no distant trig stations are visible), then set up a second GPS receiver/antenna at C and observe the baseline B to C; (b) measure the distances A to B (and A to C) with a tape; (c) measure the difference in height A to B (and A to C) using a theodolite.

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© Chris Rizos, SNAP-UNSW, 1999