Monthly Archives: January 2015

Occupation Times vs. Expected Accuracy for Post-Processed Jobs

Today I received this question:

Hi Mark,

I have a question regarding recommended occupation times for a MM120 system (using internal antenna) with post-processing.  I watched your videos on ‘best GPS practices’, your field collection and the follow-on post-processed example.  I wanted to

a) figure out if your results (i.e. a few cm variation with hand-held unit) were typical, and
b) to experiment with occupation times to figure out the time-vs-precision tradeoff 

I ran this experiment yesterday with a MM 120 mounted on a tripod, and acquired a number of points for 60, 120, 300, 600 and even 1200 sec, all in the same location.  I did not move the tripod in between collections.  I post-processed things this morning and they are *all in the same location*.  I am skeptical of anything with zero uncertainty, so can you explain what is going on?  Is there a different method I should use to run this experiment?  Do you have any general guidelines for optimal occupation times with this system?

Thanks for the videos and support,

This is a great question that is asked often. I wrote this (what I think is a pretty good, albeit snarky) reply and I think others will benefit from my answer:

Dear Kind and Most Valued Customer:

Yup, All The Same Position.

The mistake that you made was you should have closed the file and opened a new file… Or, typically what I will do is have an external antenna on a quick connect and shuttle it in a figure-8 pattern between the two points. (You don’t want to move along a linear line between the points as the receivers are smart enough to do some funny business and hot-wire the position knowing the velocity and direction.)

If you don’t move the receiver, the post-processing software treats the entire occupation as a SINGLE occupation (which it clearly is) and computes a solution for the entire occupation and then assigns that single computed value to all of the points that contributed to the seemingly extremely long occupation. Would you have it otherwise? J

 So, that said, let’s make a list of  things that affect the solution:

O Time-On-Point

O Total length of carrier phase tracking in the file before (and after) the occupation (We can talk about this for hours)

O Distance to CORS, or in the case of a Virtual Base the effective distance to CORS?

O Geometry of surrounding CORS?

O Dual Frequency or Single Frequency?

O How many constellations are you tracking? (GPS, GLONASS, BeiDou, Galileo)

O Which SV’s are in common between the base (CORS) and rover?

O SBAS, and then WAAS (twice as good as) or ENGOS?

O Canopy: none, moderate, heavy, forest or urban?

O Is the receiver’s antenna hand held (and moving slightly) or is the receiver securely mounted on a tripod?

O Multipath: hard surfaces (like steel buildings and roofs) nearby?

O Internal Antenna at waist level vs. External Antenna on a pole above your head and shoulders 

Since there are so many variables, I can’t answer your question with a simple answer. In fact, if I am in the field, I often will over and under estimate the results that I will obtain. I can make this prediction:

If you collect GPS only, L1/L2 data and process in OPUS a 4-hour occupation in open or light canopy, low multi-path conditions will converge to 4-cm almost anywhere in the world.

The solution is of course to purchase an RTK Base / Rover pair. Then you can evaluate the accuracy of the baseline from the base to the rover in real time. In real-time you would know the precision of the result. Typically you would usually have enough base data that an OPUS solution would nail down the base position well enough.

However, if you are working under heavy canopy I would also like to make these comments:

O when you move, carrier phase tracking is most certainly fully lost between points. Each point is a new, stand-alone occupation.

O the occupation data may be so trashed by SV’s moving behind tree limbs that post-processed data will be even worse than just WAAS corrected real time data.

O And RTK, even with 4-constellation tracking, won’t ever produce a fixed solution!

In this case, you won’t ever get the precision you seek using GPS measurements.


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Powering My Mi-Fi with Pocket Juice

Not too high tech here, but this is one of the best designed things I have purchased, it works great AND it is only $10:


Run’s my hot spot for an extra 6-hours. I got mine in the checkout line at Wal-Mart before XMas.

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Connecting Data Collector by Bluetooth to PC ‘Windows Mobile Device Center’

It is easy to plug your data collector into your PC with the USB connector and use Windows Mobile Device Center (under Windows XP the sync center is called ‘ActiveSync’).

But it is also easy to connect to your data collector with Bluetooth.

Click [ ConnectingDCbyBlueToothToCPU ] to continue reading…

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Supreme Court of the United States Makes My Life Much Easier

Over the past decade I have wasted thousands and thousands of hours worrying about the reference and time dependent frame differences between realizations of NAD83 and current EPOCH WGS84.

Well, the Supreme Court of the United States (SCOTUS) made my life much easier on December 15th! No longer will I need to screw around with NGS’s HTDP program. No need to carefully consider the realizations of WGS84. In the ending paragraphs of the “UNITED STATES OF AMERICA, PLAINTIFF v. STATE OF CALIFORNIA” we find this gem:

“Plane coordinates refer to the Universal Transverse Mercator (UTM). All coordinates are referenced to the North American Datum 1983 (NAD83), which is equivalent to the World Geodetic System 1984 (WGS 84).”

If anyone else had made this proclamation I would have just brushed it off as the ramblings of a complete idiot. But considering the source, and the huge time savings I am going to enjoy, I gladly accept this statement by the highest court in our great land as the gospel truth.

Praise the Supreme Court! Hallelujah! If only all of our geodetic conundrums were so easily solved.

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Recent FCC Warning: Survey Dealers are Responsible for their Radio Programming Actions

If you want to print this blog entry out, this pdf [ SurveyEquipmentDealerFCCLiability ] might produce a better looking document.

I have written before about FCC regulations concerning the data radios used by the Survey industry before:

[ Pirate Surveyors ]

I have written summaries of FCC rules that concern manufacturers of Survey equipment.

Now it is time to write about the responsibility of ‘Survey Equipment Dealers.’

Let Us Start with an Example

A recent FCC action reinforces the responsibilities of equipment dealers and manufacturers who provide and service radio equipment. I am specifically talking about GPS equipment dealers who deliver and configure equipment for customers.

Section 90.427(b) of The Rules states that:

“[e]xcept for frequencies used in accordance with § 90.417, no person shall program into a transmitter frequencies for which the licensee using the transmitter is not authorized.”

This directive seems pretty clear to me, but let me put a Very-Fine-Point on this with a Very-Clear-Example:

Your local survey dealer sells a surveyor ‘Pirate Joe’ a RTK GPS pair with a UHF transmitter radio (internal or external: it matters not) for the base and a Rover with an internal UHF receiver.

The customer, a surveyor, has decided to not obtain a FCC License because they feel they will not get caught.

In order to deliver and commission the equipment the dealer puts the 4 standard itinerant frequencies into the transmitter radio. No FCC Call Sign is entered into the transmitter as there is not one to enter.

The end customer is happy, accepts the equipment and goes to work.

What might be the consequences for the dealer as a result of this action you ask?

The FCC can impose a monetary forfeiture of $16,000 per day, $122,500 for a single act. In addition, the equipment can be seized and forfeited and the party responsible for the errant programming can be criminally sanctioned and ultimately imprisoned.

This leaves me wondering “how many survey dealers in the United States are in a position to flush $16,000 per day down the drain?”

Not many I expect. (But if you happen to be in that position, I would love to work for you. Trust me, I know how to flush toilets and I can burn money like nobody else!)

But, I happen to know that this happens EVERY single day, all over the country, by plenty of well meaning otherwise law abiding survey dealers.

Mark, you are OVER-REACTING!

When I wrote the ‘Pirate Surveyor’ article a storm of criticism was released. I heard from 4 to 10 people per day for several months claiming that I was up in the night.

In this case, I would like to put an even finer point on my claims by summarizing a recent FCC action.

CAM Electronics Distribution (, a company that sells intercoms, pagers, wireless microphones, security equipment in Elmsford New York programmed a customer’s security system with the frequency 455.500 MHz. They got caught. They got cited. Luckily for them, they got their hands slapped.

You can read the FCC Citation (and I strongly encourage you to do so) here:

[ C201432380001 ]

What can Dealers Do?

As a dealer, how can you protect your customers and your company from FCC action?

  1. Establish a file folder for sold devices.
  2. Print out a copy of the frequency, bandwidth, call sign settings when you configure and deliver the radio.
  3. Print out a copy of the customer’s FCC license (you can search for and download a PDF copy of the license from the FCC ULS Search.)
  4. Staple the License to the Configuration report and put it in the folder.
  5. Have a written policy that spells out certain employment termination for employee violators, provide a copy to every employee and have them sign a list showing they received a copy.

You are going to get pressure from 80% of you customers to program radios with unlicensed frequencies. I know this. It would be reasonable to have your customer read this document and them ask them to provide you with a $300,000; 20-year indemnity bond, just in case you get caught.

Our Survey Supply Company won’t get Caught

Sorry, but the word that comes to my mind is ‘Bullshit’.

You won’t get caught. You will get turned in by (take your choice):

A disgruntled employee
A disgruntled customer
A competitor
Your customer’s competitor

Just like most of the recipients of FCC actions, someone will turn you in.

I suppose that if you are positive that you don’t currently have and will never get any enemies, have no dissatisfied customers, have no unhappy employees then you are safe.

Good luck with that, brother!

It really would be easier to just require your customers to get licensed and only program licensed frequencies into their radios.

As always, good surveying and mapping to you!

ps:     Let me just emphasize that I am not providing legal advice and I strongly encourage you (the reader) to seek counsel from a lawyer specializing in FCC enforcement.

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Connect to Every Wi-Fi Access Point, NOT!

One of the annoying features of Windows CE is the automatic suggestion that you connect to WiFi hotspots.

Even if you are connected to your own Wi-Fi hotspot, the data collector by default will continuously suggest that you connect to new ones as you pass them.

Not only is it annoying, but it can sometimes interfere with a network correction source.

Luckily there is a simple solution…

Continue reading: [ Connect2EveryWiFi_NOT ]

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