X9x: Which Tripod?

Which Tripod Should I Use with a Static Receiver?

Which tripod should I use with the a Static GPS/GNSS Receiver?

This is an EXCELLENT question.

Long OPUS-Static occupations will generate very accurate and repeatable horizontal and vertical positions. OPUS-Project occupations will do even better. It is important to hold extremely tight leveling procedures and HI measurements.

It is strongly recommend that you use:

(1.) A fixed height tripod, and if you have more than one receiver then all tripods should be the same height.

(2.) A tripod that allows you to check the bubble in the field, immediately prior to every occupation.

(3.) A tripod that allows you to rotate the X90-OPUS head so the pushbuttons are facing North.

Why?

When you run an OPUS occupation there are only a few things that you can do to mess it up. A bad HI (Instrument Height above the survey mark) is the most common problem. Choosing the wrong antenna type is another, but our download tool fills in the value for you automatically so that won't be an issue.

(1.) If you ONLY use 2-meter fixed height tripods or poles, then you can just set the default HI to 2-meters and never mess with it. There will never be a question of an occupation's height, the answer will ALWAYS be 2.000 meters.

We sell some absolutely fantastic fixed height tripods. The SECO 5119-00 is the top-of-the-line. But they sell for nearly $815 list! That is 1/3 the cost of an X90-OPUS receiver!

For this reason, our favorite inexpensive solution is a 2-meter SECO range pole PN 5125-00 ( pdf ). This pole unscrews into two pieces in the center and has virtually no run-out¹. (A SECO PN 5125-20 snap-loc pole is decent, however they all have 3 to 5 mm of run-out.)

We use a 'Hold-A-Pole' (and we include a 'Hold-A-Pole' in the box with every X90-OPUS receiver):

It has the advantage of fixed height, the ability to quickly check your bubble prior to EVERY shot, allows the receiver and pole to rotate 360 degrees and leverages the weight and availability of your existing tripods.

The combination is relatively cow-proof too!

Rod and Pole Lengths

This is Important: If you have a 2-meter pole or tripod, don't just assume that it really is 2.000 meters! The point could be worn or the pole could be slightly short or long. It is best to take a few moments in your shop to verify exact pole length and shim or file if required. If you are using a split pole, don't interchange top and bottoms from two different poles as they may not be exactly the same length.

One of the best discussions I have seen is Robert Reese's articles in the California Surveyor's publication. See page 26 in both these issues:

http://www.californiasurveyors.org/calsurveyor/CalSurv148.pdf
http://www.californiasurveyors.org/calsurveyor/CalSurv149.pdf

Even with brand new poles and points, it may still be necessary to shim or file a point to make a pole the perfect-length.

Obviously the freedom to pick any pole in your quiver and have the same HI (2.000) every time is worth any effort!

Checking Bubble Adjustment

(2.) Before each use, with a 'Hold-A-Pole', you can move the pole bubble to the right and center the bubble. Now rotate the bubble 180 degrees to the left, the bubble should remain exactly centered, if it is not adjust the bubble 1/2 way back to the center and repeat.

Keeping your poles in a padded case and treating them like an expensive rifle scope will serve you well! Just like a rifle scope, if you drop a pole, you HAVE TO check the bubble before the next shot.

Rotating the Receiver to North

(3.) If the world were perfect, the ground plane and antenna in the X90-OPUS would be EXACTLY centered to a tenth of a millimeter above the center of the rod.

The world is not perfect.

So we do everything we can to make every X90-OPUS exactly the same, then we model the eccentricy of the GPS head by measuring sample heads on a jig.

In addition to phase center eccentricy, there is also a change in apparent antenna phase center with the elevation of satellites above the horizon.

The results can be seen on the NGS website [ here ] and below. The L1 horizontal centering error is highlighted below:

CHCX90D-OPUS    NONE P/N:1190403181, X90 L1/L2/L2C      MMI-> NGS (  2) 13/03/19
       1.1      -0.7      89.3                              
   0.0  -0.1  -0.3  -0.5  -0.8  -1.3  -1.7  -2.1  -2.4  -2.5
  -2.6  -2.6  -2.6  -2.5  -2.2  -1.7  -0.7   0.0   0.0
       0.7      -3.1     101.7                              
   0.0  -0.9  -1.6  -2.2  -2.4  -2.7  -2.9  -3.2  -3.4  -3.6
  -3.8  -3.8  -3.7  -3.6  -3.6  -3.8  -3.8   0.0   0.0

The X90-OPUS has excellent centering, not perfect but on the order of 1-millimeter. (All the measurements above are millimeters.) The NGS OPUS processor knows about this error (if we choose the correct antenna type when submitting a job) and compensates for the centering error assuming that the MMI (Man-Machine-Interface or pushbuttons) are pointed to the North when you make an observation.

If you rotate the GPS head so that the MMI faces south, then the NGS OPUS processor will double the centering error! That's not so good...

Conclusion

By diligently recording HI (or making it the same every single time); checking the bubble on every shot; routinely checking the pole height and always rotating the head to North you can minimize the chance for error and maximize the accuracy of your OPUS solutions.

---

¹Run-out: if the pole is not straight, when you turn the pole in a jig the top will move from side to side; the deviation in one revolution, when held at the bottom point and pole center is the run-out.