Monday, March 7, 2016

The Gumstix Pi Pre-GO Project Chapter 3: I can't Rely on Google Maps.

There was a nice break in the weather yesterday and I managed to get outside to take some readings.  I set up on an exposed hilltop with my laptop, my Pre-GO-to-go and my smartphone.  I was hoping that I might find some kind of survey marker with exact GPS coordinates at the base of the flag pole, but no such luckI fired up my Pi, gave it a few minutes to warm up, and connected it and my laptop to my phone's WiFi hotspot.

With no guarantee of clear weather, I decided to do a quick test and grab four different data snapshots over a 15 minute window.  I'll formulate some more informative tests for when the weather is better.  For now, I just want to see how accurate it is.

The PPP in Pre-GO PPP stands for Precise Point Positioning.  PPP provides centi- to decimeter-level accuracy so, even if Google doesn't drop its pin directly on the flagpole, my data points should be very close together.  I have to assume that Google Maps co-ordinates are going to be somewhat skewed.  Without an accurate geodetic survey marker, I can't be certain.

When I got back to the office, I dropped some pins on the map and here's what I came up with:

The outlier is my initial result.  The tight grouping south west of it are the three other results I recorded.  They sit about 1.5m away from the outlier but are all within 12cm of each other.

I found this blog post that confirms my assumption about Google's geodetic accuracy.  If you compare my Maps screen shot to his, it looks like the skew is about equivalent.  I trust my results over Google's now.

So I still have not concrete baseline measure of accuracy.  I really want to know if my horizontal co-ordinates, when HDOP (horizontal dilution of precision) is less than 1, are accurate at the decimetre level.

To that end, my colleague sent me a link to, where I found a survey marker nearby.  GPS enthusiasts have, thankfully, taken the time to accurately measure its geographic position.  OK great, but I have been rained out.  I guess the next test is going to have to wait awhile.  While we wait, let's see if the Pre-GO works with the BeagleBone Black and the Gumstix Rover cape.

Next Time:  Pre-GO and the BeagleBone Black Rover Cape

Thursday, March 3, 2016

The Gumstix Pi Pre-GO Project Chapter 2: Pre-GO Packaged to Go

So, while we're debugging the UART header, I thought I may as well get some results from the Pi using the FTDI cable. With the help of a USB hub, a WiFi dongle and some masking tape, I set the Pre-GO up on the window.  I figured if I was going to get any kind of signal anywhere in the office, this would be it.

I SSH'ed in and ran gpsmon. This is a great tool related to GPSD that spits out human-readable GPS data in real time.  It looks something like this:

Yep, my Pi kind of knows where it is... within 10 metres, that is.  Not good enough for a Precise Point Positioning (PPP) GPS module.  I have to get it outside to get accurate geolocation.

Good news! The UART problem was resolved with a single line:


I appended this line to /boot/config.txt and said goodbye to the FTDI converter.  With the bulky USB cable and hub gone I decided to get my gear ready for test.

A circuit board with a delicate makeshift UART connection is not easily transported.  Not to mention the need for a power cord makes taking my rig on the road a bit of a challenge.  No problem. Tape, scissors, and a shipping package, combined with a little time gives you this:



Next Up: Where am I, Precisely?