Tuesday, April 10, 2012

Spring news in the GNSS and SDR domain

I have been following in the last few days interesting developments in the GNSS and SDR domain.

1. NV08C-CSM and dual constellation RAW measurements

It's recent news that NVS has lifted the constraints on the firmware with RAW data for GPS and Glonass on L1. In my opinion this is one of those rare times that the rules of the game are changed by one of its players.
Essentially the NV08C-CSM provides real-time kinematic capability at 10Hz, with a price point as low as 35EUR/piece in small quantities. It is already possible to download an unofficial version of RTKNavi.exe and RTKConv.exe here, but RTKLIB will officially support the receiver from next version anyway.
Below are some pics of what Denga10 and the navXperience 3G+C could do with the good old 2.4.1 "Static Precise Point Positioning" over three hours.. bringing down the error to less than 20cm in complete standalone mode (by using only broadcast products).

Rtknavi (NVS mod) doing static PPP with 14 sats

Rtknavi (NVS mod) available satellite close-up

Glonass broadcast navigation data

GPS broadcast navigation data

GPS/Glonass observations.. 21!

Satellites used in the fix

From cold start, first three hours, ground track
From cold start, first three hours, position

From cold start, third hour, ground track

From cold start, third hour, position

The likes of Novatel, Trimble, Hemisphere, etc are not going to like it I suppose. On the other hand, users have perhaps a cheaper option to enter the high-precision domain.


For once, the group of innovators is European.
These guys of osmocom are smart and -what's even better- their work with the open source team. RTL-SDR is, I believe, one of the freshest finds in the Software Defined Radio domain. The Fun Cube Dongle, developed in UK, is already a very clever tool. But finding the super-tuner Elonics E4000 (note, again a UK Company) in 25$ USB DVB-T dongles and being able to grab data in a 3MHz bandwidth between 64MHz and 1.7GHz... that is really cool.

I had to buy one! ...actually two before I found the E4000..
A Terratec Cinergy T Stick Black rev.1, with the FC0012 which is not suitable for GPS
A Newsky TV28T, with the E4000.

The Realtek RTL2832U demodulator uses a 28.8MHz crystal by default.. which has too poor accuracy for a GPS receiver.. but a SDR acquisition algorithm can easily handle 100+kHz of apparent Doppler shift in the frequency search :)
So for a nominal sky plot as the one below:

And a signal looking like as follows:

Power spectrum around GPS L1, FS=2.728MHz
The ADC is saturated.. the gain I set was too high.

I could acquire the following birds:

SV 9: Doppler +112000.0 CodeShift:     99 xcorr: 553566.6
SV12: Doppler +113500.0 CodeShift:    502 xcorr: 301126.3
SV15: Doppler +110000.0 CodeShift:   1847 xcorr: 871305.9
SV17: Doppler +110500.0 CodeShift:   2225 xcorr: 401100.6
SV18: Doppler +110000.0 CodeShift:     38 xcorr: 368766.1
SV26: Doppler +107000.0 CodeShift:   1779 xcorr: 511844.4
SV27: Doppler +110500.0 CodeShift:    264 xcorr: 650033.9
SV28: Doppler +107500.0 CodeShift:    203 xcorr: 320405.6

The next step is to have my new version of GPS-SDR processing the data in real-time.. won't be long!
I managed to upload the binary file which is long enough for everyone with a SDR receiver to calculate the position. I used FS=2.048MHz and a nominal IF of 0.0Hz (but because of the crystal inaccuracy the IF actually falls at about 110kHz). The data type is I&Q interleaved int8_t.


3. Open source FPGA receivers

I waited too long for the code from the University of Tampere: they were promising to licence the TUTGNSS and never happened.
Finally the Open Source community is bringing to the reality an Open Source FPGA implementation of a GPS receiver based on the Namuru / Zarlink GP2021 correlator structure and soft CPU cores (the LatticeMico32 Milkymist and the Altera NiosIIe). As also scientists at the University of Tokio (remember RTKLIB?) are involved, this time I know it is going to happen.
Some useful links:

Stay tuned, GNSS is evolving rapidly!