Operations FAQs

• Big misclosure with NADCON?

  Q - I held one horizontal and vertical point fixed in my network adjustment but my checkpoint closures exceed expected tolerances. I transformed the local grid coordinates from the network adjustment module to NAD83 geographics using NADCON. Is this the problem?'

  If NADCON or the CNT (Canadian National Transform) are used to transform points from NAD27 to NAD83, excessively high checkpoint misclosures could occur during fixed integer RTK surveys. This is because both of these datum transfomation methods model the highly irregular differences between these terrestrial datums. The high precision NAD27 coordinates of the network points will be distorted via the datum shift. The solution here would be to determine a local three parameter datum shift to model the ellipsoid differences and the geocentric offsets and use this datum shift throughout the survey. This datum shift should be determined at the horizontal control point which was held fixed in the adjustment.'

  This does not totally exclude NADCON or CNT from use, however. If a region demonstrates very small datum transformation differences, it is possible that a long series of connected 3-D prospects could very well benefit from their use. Rather than greatly extend the use of a three parameter shift, we might in this instance think of using a model to provide a rather seamless geodetic connection from one prospect to the next.'

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• Are higher antennas better?

  A1 - Since the effective radio coverage increases by about 40% if one doubles the transmit antenna height relative to the average terrain elevation, you definitely want to put the base station at a high point in the project. Note that the effort and expense to transport and erect a 15 meter tower when the base elevation is already 300 meters higher than the project is definitely not warranted. Since the hill is serving to elevate the antenna relative to the project area, you only need to raise the antenna the necessary distance to minimize ground effects (absorbtion of the radio signal into the ground). A recommended minimum elevation for a VHF or UHF transmit antenna in order to minimize ground effects is 2 wavelenghts. That would equate to 4 meters for VHF (150 MHz) or 1.3 meters for UHF (450 MHz). Also since these frequencies are effectively line-of-sight, you want to raise the antenna above any nearby physical obstructions, like trees, sheds, etc.'

  Q2 - I strapped my antenna to a grounded metal pole and I'm not getting the coverage I need. Any suggestions?

  A2 - A significant portion of the radio signal is being conducted by the pole and sent directly into the earth. Properly mount the antenna at its base and keep the rest of the unit unobstructed.'

  Q3 - I'm convinced I'm installing my reference station correctly but I'm simply not getting the radio coverage I'd expect. How can I troubleshoot the output power?

  A3 - Every real-time GPS survey crew should have an RF watt meter which can measure both forward and reflected power. These units cost as little as $100 U.S. but can be invaluable in the field. The forward power operation can test the power output of the radio, amplifier, and can measure the power loss in the antenna cable. The reflected power mode can determine if the energy is being properly radiated out of the antenna, or, if the antenna is broken, reflected back into the radio.'

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• Reference station power?

  Q1 - What special concerns are there with regard to power for my reference station?

  A1 - Few things can shut you down faster than reversing polarity to the base station power supply. If you're lucky, you just have to replace a few fuses. If you're not, than you may be looking at extensive repairs or replacing any or all of the GPS receiver, radio, modem, and amplifier. The best (safest) installations incorporate polarized connectors on the cabling to the battery and to the base equipment. Another common problem is to connect the base station directly to a generator. The generator may provide inconsistent power and an occasional surge. A better alternative is to have the generator provide power to a "smart" battery charger and to power the base from the battery. The battery should provide steady power and buffer any surges from the generator.'

  Q2 - Do I have to take any special precautions with my base or rover equipment in order to protect it?

  A2 - A typical GPS base station consists of a GPS receiver, GPS antenna, groundplane, transmit radio, modem, amplifier, radio antenna, cabling, tripod, tribrach, a power supply, and some sort of transport case. Some of the equipment may not be waterproof or cannot function in severe heat or cold and will therefore need to be protected from the weather. If high winds are a problem, then everything must be properly secured to avoid being blown away. In certain areas, theft may be a problem. A number of GPS operators can tell stories about their equipment "walking away."'

  Rover equipment is backpacked, mounted on quads or 'skidoos', or operated from within trucks or other vehicles. Antennas and cabling can be struck by limbs as the rover moves around an area, and shear vibration can often loosen connectors and set screws. In one case we know of, a rover drove his quad a mile through heavy brush in order to reach the location at which he had ended the survey the previous day. After several minutes of failing to track satellites much less initialize, he finally looked at the back of the quad to discover that his GPS and RF antennas had disappeared. Retracing his path, he failed to find the RF antenna and shattered the GPS antenna when he accidentally ran over it.'

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• Controller azimuths

  Q - I want to use the displayed azimuth in my controller in order to offset points. Is this a grid or geodetic azinuth?

  A - The question is a good one. For later versions of one survey controller firmware, with projection information defined, it is, but in earlier versions of the same equipment, it is not. The best suggestion is to do a test to be sure. In the office, compute both the grid and geodetic inverses between two widely separated points along a line. In the field, target and occupy a point, and then target a point on the same line a considerable distance away. If the convergence in the area is significant, it should be easy to see which type of azimuth is being displayed. The key to using the azimuth for offsets would now be not to mix apples with oranges. For example if the grid azimuth between points was 90 degrees and the controller was displaying grid azimuth, then the 0 and 180 degree azimuths would be the ones to steer to stake out offsets. If the geodetic azimuth was 91 degrees and the controller displayed geodetic azimuths, 1 and 181 degree azimuths are the offset azimuths. Its when one type of azimuth is used as the line azimuth, yet a different type of azimuth is used to steer by that offsets begin incorporating an unwanted inline component.

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Operations FAQs

• After the base is started

  Q - After I start my base, how do I check that I've done everything correctly?

  A - In a real-time GPS survey, all points surveyed by the rover(s) are relative to the base station. If the base station coordinates are wrong, all surveyed locations will be in error by at least that amount. The sources of survey errors tend to fall into two categories - the first are often referred to as blunders, the second involves errors in the GPS derived vector. HI errors fall into the prior category. Since the GPS solution uses observations between the phase centers of the base and rover antennae, and we're primarily interested in ground based positions, accurate HI measurements are critical at both ends. The most common error encountered in measuring the HI is measuring and/or entering the HI in the wrong units, ie., feet or meters. The best way to confirm that blunders like this have not been made is by visiting checkpoints. This should be done by all rovers prior to surveying the first point of the day. It is also good practice to visit a checkpoint at the end of the day as well. All checkpoint visits should be logged by the data collector.'

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• Accuracy vs tolerance

  Q1 - I have been given specs that indicate a layout tolerance and an accuracy tolerance. What's the difference?

  A - Unlike conventional or optical surveying, a GPS roving unit is completely independent. There's no instrument man guiding you to the target and recording data referenced to your prisim. Given real-time GPS solutions and a data collector providing navigation instructions, the roving unit can navigate and record on its own. A critical specification in any real-time GPS stake-out survey is what is commonly referred to as the layout tolerance. This is defined as the acceptable limit between the location of the survey evidence, typically a pinflag, and the preplot coordinates. This should not be confused with the accuracy tolerance which is defined as the required accuracy of the coordinates that are assigned to the evidence. One could spend five minutes per station or more fine tuning their location to within centimeters of the target, or, with a reasonable layout tolerance of say 2 meters, very quickly establish and record the points. Seismic data can generally tolerate fairly large perterbations in the preplot geometry as long as the coordinates that are assigned to the sources and receivers are correct. Marine seismic data is a perfect example where cable feather regularly perturbs the preplot geometry. Depending on factors including but not limited to near surface geology and seismic bandwidth, coordinate errors as little as 2 meters in the vertical and 5 meters in the horizontal can degrade the quality of the seismic data. Therefore, it is extremely important to ensure that the survey evidence is placed at the location of the survey measurement (directly underneath the prisim or GPS antenna) and that the sources and receivers are centered around this evidence. So, if the GPS data collector tells you that the target location is 2 meters to the East, well, you get the point...'

  I have an RTK system. I spend a lot of time trying to place the pin flag within several centimeters of the preplot point. If I don't, wouldn't I be accused of inaccurate surveying?

  Building on the previous point, it is far more important to provide accurate coordinates for the survey evidence, and to ensure that the sources and geophones are placed at that evidence, than to precisely place the evidence at the preplot location. For typical seismic work, layout tolerances (not to be confused with the accuracy tolerance) rarely, if ever, have to be less than one meter, and in fact, two meters is generally quite acceptable. Be prepared though, that if you establish a seismic line with a two meter layout tolerance and, say, a sixty meter station spacing, you may see one ugly line when looking along it. Optical surveyors and party managers may cringe at this sight, but rest assured, that if you collected accurate coordinates for the evidence, and the instrument crew was diligent during layout, drilling and/or vib setup, then the positioning objectives were met.'

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• Good DOP bad DOP

  Q1 - What is a good general DOP guideline for differential surveying?

  A1 - Pseudo-range differential and uninitialized "float" mode kinematic surveys are significantly more susceptible to PDOP (Position Dilution Of Precision) and multipath effects than fixed integer kinematic surveys. As a general rule, differential and/or uninitialized kinematic surveys should not be carried out when the PDOP exceeds 5.0 or when tracking less than 5 satellites.'

  Q2 - What is a good general DOP guideline for kinematic surveying?

  A2 - Since fixed integer kinematic surveys are generally quite robust in maintaining precise positions under "poor" geometry conditions, surveying should proceed if you are initialized, tracking 4 or more satellites, and the PDOP is less than 8. While this seems high consider that the precision of our position is a function of DOP and range noise. If the latter is extremely small (e.g., RTK ranging is approximately 2 cm), then we can afford a larger DOP while maintaining precise positioning.'

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• Initialization woes

  Q1 - I am having difficulties initializing. The distance to the reference is less than 10K but the elevation differs greatly. Is this an issue?

  A1 - For carrier phase work, the tropospheric effects may be significant enough to prevent a successful initialization. Some manufacturers recommend a maximum elevation difference of 100 meters between base and rover for RTK operations. Experience has indicated that this number is somewhat conservative. This factor needs to be considered for each working area.'

  Q2 - To make it convenient, I have located my RTK base in our camp. The rovers are having difficulty initializing. What could be wrong?

  A2 - Having the base station within the base camp is great with regards to power, access, security, and overall convenience. However, most seismic base camps consist of numerous metal trailers which serve as prolific sources of multipath. The base station GPS antenna must be placed in a clear location, preferably at least 50 meters from any sources of multipath.'

  Q3 - I am having problems initializing and I have been told to raise my elevation masks. Shouldn't I lower it to track more satellites.

  A3 - Low elevation satellites are often a problem for two reasons: 1) unmodeled tropospheric refraction and 2) the greater possibility of multipath. Static processing has always used a 15 degree mask in processing in order to avoid these problems. Since Real Time Kinematic also relies on phase tracking and resolving cycle ambiguity it makes perfect sense to use a higher elevation mask on the rover to mask the unwanted satellites.'

  The user should also consider using an elevation mask on the reference that is several degrees less than the rover. In this way the reference has the opportunity to lock on to all satellites and compute stable corrections before they transmitted to the rover.

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