Antenna: Harxon HX-CSX601A
Receiver: SkyTraq GPS/BD RTK receiver
Baseline: 1.7Km
Max Speed: 81.2Km/hr
Blue point: single solution
Yellow point: float solution
Green point: fixed solution
RTK is well known for land survey and automated precision farming applications, which are low speed or static. With emergence of lower-cost single frequency RTK receivers for less demanding applications (cm-level accuracy yet with shorter baseline not beyond 10Km and longer time of around 2 minutes to reach initial cm-level accuracy solution) we are interested find out how it performs under dynamic situation.
A road test was performed on a car; we see that it could maintain RTK fixed solution (meaning cm-level accuracy) up to 81Km/hr; it’s over speed limit of the road on which we are testing, not the limit of the receiver though.
Currently we are still using survey grade antenna to first understand performance capability of the single frequency RTK receiver, not to be limited by performance of smaller antenna in these early tests. After more testing done understanding the performance capability, we'll try smaller antenna on quadcopter!
More information can be found here:
http://www.navspark.com.tw/blog/1-x1-centimeter-level-accuracy-rtk-module
Comments
We hope to be able to make this single-frequency RTK technology more usable. This baseline vs variance vs time to ambiguity fixed result we got is also interesting:
Very impressive results, also Piksi team tried to manage the FLOAT/FIXED solution algorithm but this is a tough job even for best engineers. In my opinion a hobby priced RTK system will be a game changer. So all eyes and ears on you Mr Oliver.
Optimizing of the internal algorithm is still being performed to make it more robust for commercial application; expect to be available in Q4.
@Oliver, when will this solution be commercially available? We are looking for an inexpensive cm gps solution and I have see a cloud-sourced project for RTK, but no actual 'affordable' solution as far as I can see. My company is researching using drones for making precise RF measurements and we need high position accuracy but not the expense of DGPS.
There are many single frequency carrier phase raw measurement receivers available for RTK post-processing; some are listed in Table 1 of this article: http://gpspp.sakura.ne.jp/paper2005/isgps_2009_rtklib_revA.pdf.
The receiver measurements are synchronized to its 1PPS rising edge. If you are logging carrier phase raw measurement data at 1Hz and have camera taking picture synchronized to the 1PPS rising edge, then they are in sync.
Single frequency RTK can only operate below 10Km between base and rover, larger the distance larger the position variance. With good antenna logging result over 12 hour in Taiwan, we see 97% RTK fixed solution availability using dual satellite of GPS + Beidou, and 73% RTK fixed solution availability using just GPS; these include in-use satellite falling below usable elevation angle causing solution becoming float and taking a minute or more to get fixed solution again, a problem we haven’t time to optimized yet. So if you are using GPS-only receiver, you can use GNSS radar, http://www.taroz.net/GNSS-Radar.html, to find which time period at your location having more satellites, better for your logging operation, to later get more RTK fixed solution.
Being an order of magnitude cheaper than multi-frequency RTK receiver, it’s interesting to find out how far the "less robust" single-frequency receiver performance could be optimized using currently available hardware and what new applications could benefit from it.
The RT in RTK does stand for Real Time. Being able to record the raw data for post processing would almost have to be a given.
Ground speed really doesn't matter. The tracking loops of most any receiver should be able to handle several times 80 km/hr speeds.
What does matter is maintaining a cycle slip free continuous lock on at least 4 sats. When moving near the ground this can be a problem because of trees and buildings. It can also be a problem if the antenna's attitude changes significantly and the signal from lower elevation sats becomes weaker - even if just for an instant.
Another problem especially with smaller rotor craft is electronic interference. Various types of onboard transmitters (including ESCs) may weaken the GPS signal just enough that a lock is lost on some sats. If this happens for just a fraction of a second when doing meter level positioning it's not a big deal as that can resume almost immediately. If this happens when doing (L1) RTK it could take a minute or more to resume cm-level positioning.
RTK, especially L1 RTK, is fragile. That said it does work and has been around for a quarter century now.
Multiple frequency RTK is more robust, but much more expensive. While multiple frequency equipment prices are bound to fall, for the foreseeable future single frequency, multiple GNSS chips like this one are going to be the cheaper option.
I am not looking for realtime rtk processing, since the area where we operate are fairly remote so we may not get proper cellular network. Is post processing possible along with precise image geotagging ? Thanks in advance :)
It's done on the rover in realtime.
how is the rtk being done? post process?