Back in November last year, I wrote a post on my first experiments with a dual frequency u-blox F9P based receiver. At the time it was quite difficult for those without good connections to u-blox to get a hold of the F9P and even now, nearly three months later, it still is not readily available. Ardusimple, the lowest price provider of F9P receivers still has all their receivers on back order till next month and low cost dual frequency antennas are even harder to get. Hopefully all that will change fairly soon though.
Meanwhile, thanks to “clive1” and “cynfab” from the u-blox forum, I have been lucky enough to have been given a prototype receiver based on the dual frequency u-blox F9T, the next product from u-blox in the Generation 9 series. Like the previous generation M8T, this is intended for timing uses and does not include an internal RTK engine. Otherwise I believe the F9T hardware is nearly identical to the F9P. In theory it should be less expensive than the F9P, just as the M8T is less expensive than the M8P but meaningful pricing is not yet available.
In many of my posts, I have focused on post-processing short baseline data sets using a local base station and identical receivers for base and rover. For this particular combination, I have shown that the differences between a single frequency solution and a dual frequency solution are typically fairly small. This assumes that the single frequency solution includes Galileo and possibly SBAS while the dual-frequency solution includes only GPS and Glonass. This makes the total number of observations fairly similar between the two cases. At least until very recently this has been a reasonable assumption given that most existing CORS or other reference base stations and reasonably priced dual frequency receivers offered only GPS and Glonass. It’s also true that time to first fix is longer in the single frequency solutions but post-processing with a combined solution generally eliminates the need for a fast fix.
However there are many other cases where there are definite advantages to using a dual frequency solution. In particular the most important advantages occur for:
- Longer baselines where linear combinations of L1 and L2 can cancel ionospheric errors
- Use of an existing CORS or other reference base station which typically has only GPS and Glonass and hence is not an ideal match-up with a single frequency receiver using additional constellations
- Real-time solutions where time to first fix is more critical
- PPP (Precise Point Positioning) solutions for the same reasons as the long baseline cases.
So for my initial experiments with the F9T I focused on including some of these conditions. In particular I ran two experiments, the first a real-time RTK solution with an existing UNAVCO reference base (P041) located 17 km away. For the second experiment I compared an online PPP solution from the Canadian Spatial Reference System (CSRS) with an RTKLIB SSR based PPP solution.
For the first experiment, I connected the F9T receiver to the dual frequency antenna on my roof and ran a quick five minute RTKLIB real-time solution against the UNAVCO base station using the demo5 b31 RTKLIB code. Other than changing the frequency mode from L1 to L1+L2 I used the exact same configuration file I normally use for the u-blox M8T single frequency receiver. Even though the rover was stationary in this case, I ran the solution as kinematic for better visibility to any variation in the solution. Here’s the result.
Overall the solution looked excellent. First fix occurred within a few seconds, fix rate was 100% after first fix, horizontal variation was roughly +/-0.5 cm and vertical variation was roughly +/-1 cm.
The solution residuals, both pseudorange and carrier-phase also looked very clean.
I only made a brief look at the raw observations but did not see anything unusual there either. At only five minutes of data, it is not much more than a quick sanity check, but so far, so good.
For the second experiment I collected four hours of raw observations, again with the F9T receiver and my rooftop antenna, a ComNav AT330. I then submitted this data to CSRS for their online PPP solution as well as running an RTKLIB SSR solution as I described in this post. Below are the results for both solutions. The plots are all relative to my best estimate of the location of the rooftop antenna based on previous PPP solutions with Swift and ComNav receivers as well as RTK solutions from nearby CORS stations. The left plots shows the first hour of solution with a +/-0.25 meter vertical scale. The right plot shows the second through fourth hours with a +/-0.06 meter vertical scale.
Both solutions get to below 6 cm of error in each axis after 1 hour and below 3 cm of error after four hours. The CSRS solution gets down to almost zero error in all three axes after four hours but I don’t believe my reference is this accurate so I think this was partially luck. The reported accuracies (95%) for the CSRS solution were 1 cm, 4 cm, and 5 cm for latitude, longitude, and height respectively. My previous experience running RTKLIB SSR PPP solutions with other low cost dual frequency receivers is that after running many solutions, they generally all fall within +/-6 cm accuracies in all axes after four hours. Both solutions include only GPS and Glonass observations because both the SSR correction stream I used from the CLK93 source, and the CSRS online PPP algorithm use only GPS and Glonass.
Being able to run accurate PPP static solutions can be a big advantage since it can make it much simpler to precisely locate a base station for RTK solutions with a dynamic rover, especially in more remote areas where there may not be any nearby CORS or other reference stations to run an RTK solution against.
As always, this post is intended to be just a quick snapshot and not an extended analysis of any type, but so far I have been very impressed with both the F9P and F9T and with their compatibility with RTKLIB.