5.5.6 Modern GPS Surveying: Field Procedures

ISSUES RELATING TO MODERN GPS SURVEYING

Each of these modern GPS surveying techniques has its strengths and weaknesses (see below), however all are less accurate than the conventional GPS surveying technique. This should not be too great a drawback as it is often not necessary that relative accuracies of a few parts per million be insisted upon. Often a combination of conventional static and the other techniques makes for an ideal solution to a surveying problem.

Conventional versus Modern GPS Surveying Techniques

CONVENTIONAL (STATIC) GPS SURVEYING: Advantages Highest accuracy Robust technique Ambiguity resolution not critical Minor impact of orbit error and multipath Undemanding of hardware and software Disadvantages Long observation sessions Inappropriate for engineering and cadastral applications   MODERN GPS SURVEYING: Advantages Higher accuracy than pseudo-range solutions Appropriate for many survey applications High productivity Similar procedures to modern terrestrial surveying Disadvantages Special hardware and software Susceptible to orbit, atmospheric multi-path disturbances Higher capital costs Ambiguity-fixed or continuous lock required

Two negative characteristics of these modern GPS techniques are:

• They are susceptible to multipath disturbance to an axtent (affecting the receiver signals during both the kinematic and static stages of the tracking) than the conventional static technique. Multipath during the ambiguity resolution period is especially dangerous, as wrong ambiguities may result.
  
• The results from short observation sessions are more sensitive to bad satellite geometry (large GDOP) than the conventional static technique.

An example of the possible combination of conventional static GPS surveying and modern GPS techniques is illustrated in Figure 1. In this case conventional GPS surveying provides the control for lower order densification or topographic mapping surveys.

Figure 1. Combining conventional GPS and modern GPS survey techniques.

Applications and Productivity

There is no doubt that the main attraction of "rapid static", "reoccupation" and "stop & go" techniques is the decrease in field time required to collect the data. Hence they are well suited for surveys where 10 parts per million (or lower) accuracy is adequate, and the speed of survey makes GPS an attractive alternative to EDM/theodolite, and other such techniques. The increased productivity of modern techniques compared with the conventional GPS surveying technique is illustrated by an example in the following Table.

A comparison of survey productivity between conventional static
GPS surveying and the "stop & go" technique.

Observation 1010-1111 45 min Move from 1010 to 2222(setup) 13 min Observation 1111-2222 45 min Move from 1111 to 3333(setup) 16 min Observation 2222 to 3333 45 min Move from 2222 to 4444(setup) 14 min Observation 3333-4444 45 min Move from 3333 to 1010(setup) 16 min Observation 1010-4444 45 min 284 min Total time: 4 h 44 minutes	Observation 1010,1111,1011 45 min Move time and setup 23 min Observation 1111,2222,3333 45 min Move time and setup 16 min Observation 4444,3333,1011 45 min Move time and setup 22 min Observation 1010,4444,2222 45 min 241 min Total time: 4 h 01 minutes

Initialize survey (antenna swap @ station 1010: 4 satelites) 13 min Move to 1111(setup) 11 min Observation 1010-1111 1 min Move to 2222(setup) 12 min Observation 1010-2222 1 min Move to 3333(setup) 11 min Observation 1010-3333 1 min Move to 4444(setup) 9 min Observation 1010-4444 1 min Move to 1111(setup) 18 min Reobserve 1111(as a check) 1 min 79 min	This method takes less time than the first configuration: 79 minutes -19 minutes (last reobservation of station 1111) =   68 minutes   Checks: Repeatability. Allows for Free Least Square Adjustment.

There are a variety of combinations of several modern GPS surveying techniques which are possible. The greatest challenge likely to be faced by the GPS surveyor is to use the best combination of techniques for the terrain and logistical constraints that he/she faces. These are illustrated in Figure 2 below. In reality, the constraints are likely to include those in Figure 3 as well, that is, many trees which can "break" the signal reception.

Figure 2. Examples of combinations of several GPS surveying techniques.

Figure 3. A variety of reinitialisation techniques for "stop & go" or "kinematic" surveys.

The development of "on-the-fly" ambiguity resolution algorithms is a dramatic step forward because static ambiguity reinitialisation (as in Figure 3) is no longer necessary. The ambiguities will be resolved while the antenna is moving to the next stationary survey point. If a point X has been surveyed (that is, a few minutes of "carrier-range" tracking data has been collected), and as the antenna is moved from point X to point Y, an obstruction blocks the signals and causes cycle slips to occur, then the antenna does not have to go back to point X (nor need any of the procedures illustrated in Figure 3 be used). New ambiguities can be resolved "on-the-fly" as the antenna moves from X to Y. However, there must be a sufficient period of uninterrupted tracking for this to take place. Although this varies from receiver to receiver (and is influenced by the baseline length, satellite geometry, and several other factors), it may be of the order of several minutes. In an extremely unfavourable scenario, there may be so many signal obstructions that there is insufficient time for the "on-the-fly" (OTF) ambiguity resolution algorithm to work properly during the very short periods of uninterrupted tracking, and hence the survey is not possible using the "stop & go" technique, even when aided by an OTF capability.

Planning and QC Considerations

Being able to determine baselines faster than using conventional GPS surveying does not, in its self, mean that the network planning and design guidelines discussed in section 5.2.1 must be changed. However the following issues should nevertheless be addressed:

As the accuracies attainable are lower than for conventional GPS surveying, the GPS survey "standards & specifications" may be relaxed.

The high speed of survey would suggest that the most appropriate mode of receiver deployment is the "base station" or "radiation" mode (Figure 2 above). However, this provides no redundancy because every point is fixed by a "no check" vector.

It is possible to ensure redundancy by deploying two base stations (Figure 4 below). Each roving receiver point is connected by two vectors (useful if one base receiver malfunctions!). But because the rover site is occupied only once (except when the "reoccupation" technique is being used), then the vectors are still of the "no check" variety as there is no way of knowing if the height of antenna has been measured correctly, or even if the correct station was occupied!

Productivity improves as more GPS receivers are deployed, but the logistics also become more complex. An example of a "hybrid" scenario involving two base receivers, and two roving receivers is illustrated in Figure 4.

It has been found that even though the resolved ambiguities (for example, using OTF techniques) are NOT correct, the relative positions between the surveyed points may be correct (though the position relative to the base receiver is INCORRECT).

Because the modern GPS surveying techniques are likely to be used for land applications which were not addressed by conventional static GPS surveying (for example, cadastral surveys), new recommended specifications may have to be developed for fixed control placement, redundancy, ties to control, calibration, heighting procedures, and other network considerations.

Figure 4. A multi-receiver deployment scenario for modern GPS surveying.

REMEMBER: The network adjustment issue is the same whether the baselines were observed with 30 second data sessions, 3 minute data sessions, or 60 minute data sessions.

Standards & Specifications

This is an unenviable task. The speed with which GPS technology is developing is such that these new GPS surveying techniques were not accessible to the average GPS surveyor when Australia and the U.S. were developing their "standards & specifications" for GPS surveys. New Australian specifications have been developed (section 10.2.5), but some states and countries have already produced "guidelines" for cadastral surveys using "high productivity" GPS techniques.

Future Trends

The following as some likely trends relevant to modern GPS surveying practice:

• Increasing use of combined pseudo-range and phase data in algorithms able to perform "on-the-fly" ambiguity resolution with a single epoch of data -- a "super" kinematic technique.
• Replacement of "rapid static" and "reoccupation" techniques by the "super" kinematic technique referred to above, able to operate in both "static" and "kinematic" positioning mode without the need for clumsy static initialisation procedures.
• Significant receiver development to overcome the effects of AS (section 4.2.3).
• Increasing importance of real-time positioning -- taking the "gamble" out of O-T-F ambiguity resolution.
• Increasing applications for GPS in cadastral, engineering, topographical and detail surveying.

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© Chris Rizos, SNAP-UNSW, 1999