GPS Reference Networks

One of the mistakes we see companies make is they forget to be Boy Scouts (always be prepared).  In an effort to save a little money we often do not get back ups such as extra batteries, cables, etc…  This cost cutting usually ends up costing you more than you saved when everything goes down.

The same thing applies to working on GPS Reference Networks.  We assume that this simple solution that saves us thousands of dollars is a perfect end all answer and while it is great it is not the total solution.   We forget the variables involved such as the range of the receiver from the base, cellular connections, and let’s face it the base is being controlled by someone else.

When the company I worked for sent a GPS crew out to survey they had a back up.  When a standard crew went out, they had a back up.  I remember working in Louisville, KY one winter after a nice snow had come through and the temp was well below freezing.  We took 2 extra batteries for the total station which was a good thing because the old batteries did not hold up so well in the cold.  With about an hour of surveying left to do we had gone through the first two batteries and the third was starting to show signs of failing.  Several hours drive back to the office and a client wanting their job done that day did not leave us in a good spot.  My old friend and the Survey Manager (who was much smarter than me) chuckled when I started to hop in the truck to head back to the office.  He came around the truck with a level, a rod, and a tape.  We finished the job that day.

It can be expensive to have these extra things, but when you need them the savings is almost unmeasurable.  Reference networks are no exception.  The idea of being able to work miles from a base (over 20 miles in some cases) without having to ever set the base up or figure out the position is huge and something I would encourage more people to look in to.  The problem is that we get too relaxed and buy a single rover and have no back up plan.  When we reach the limits of the range of the network, the end of cellular coverage, or the network goes down you are essentially shut down and that is not good.

What do you do?  Go to plan “B”.   With so much new technology coming out right now we are finding great deals on used older technology GPS receivers.  We are also seeing products like the Altus APS-3 that can operate as a network rover, or as a base/rover.  For anywhere from $5,000-$12,000 you can have a back up plan that over time can save yourself down time, lost production, and most importantly money.

So do yourself a favor and be a Boy Scout….always be prepared!

Fixed Height Tripods for GPS

With a total station the height of the instrument is usually measured to the height of the “eye” or the center of the optic which makes it relatively simple.  With GPS there is no optic, only an imaginary plane that is used to determine elevation by figuring the distance from the ground to this plane.  Ok, maybe it is not imaginary, but the phase center measurements are neither physical positions nor stable points.

The phase center is variable on any GPS receiver based on directional signal information from satellites. These variations affect the antenna offsets used to connect the measurement to the position being recorded.  To handle this, the National Geodetic Survey calibrates each receiver model creating a measurement between the receiver phase center and it’s antenna reference point (ARP).  When we plug in the height of the receiver to our data collector we key in the measurement from our point to the ARP.

Some receivers have “measure points” on the receiver other than the ARP that are easier to get to for measuring.  You can then either do the simple math to reduce the height from the measure point to the ARP, or in some cases the data collection software already has that equation built in so you only enter the distance from the point to your measure mark.  Some units such as the Leica receivers have a tripod hook that has been calibrated to their ARP.  This allows the user to affix the “hook” to the base of the receiver, pull the tape to the point and know the number read includes the distance from the end of the tape to the ARP.

The simple answer here is to do the same thing you do for your rover.  Use a fixed height base.  There are a few options available such as the Fixed-Height GPS Antenna Tripod with Collapsible Center Staff from SECO Manufacturing (SECO PN: 5119-00).  This particular tripod features a collapsible center staff with twist locking mechanism that rotates 360° to check calibration of plumbing with a 10-minute vial, and quick release & twist locking mechanisms on the legs. It is dual graduated and adjusts to three heights: 2 m, 1.8 m, and 1.5 m. It collapses to 50 inches (127 cm).

I highly recommend these for use with the Altus receiver and have a few clients using them with their Sokkia and TopCon GPS units as well.  For more details on how to use this tripod visit SECO’s website http://www.surveying.com/tech_tips/details.asp?techTipNo=13.