GNSS-SDR-en http://gnss-sdr.ru/gnsssdrenglish.php en-us Nucleus CMS v3.51 © Weblog http://backend.userland.com/rss http://gnss-sdr.ru/gnsssdrenglish.php/nucleus/nucleus2.gif GNSS-SDR-en http://gnss-sdr.ru/gnsssdrenglish.php BeiDou-2 (COMPASS) preliminary results xml-rss2.php?itemid=89 ublox, NovAtel, Septentrio, javad and others.
In this note preliminary results of BeiDou signal processing are described. On the figure 1 acquisition process output is shown. Up to 5 non-geostationary satellites can be seen from my location.

BeiDou acquisition
Figure 1 Acquisition output for non-geostationary satellites

I would like to mention that BeiDou-2 signals are similar to GPS L1 C/A with one significant difference. Each navigation symbol that lasts 20 ms is additionally modulated by 20-symbol Neimann-Hoffman code (0, 0, 0, 0, 0, 1, 0, 0, 1, 1, 0, 1, 0, 1, 0, 0, 1, 1, 1, 0) (details are on the figure 2 taken from ICD).

BeiDou secondary code
Figure 2 Secondary code details

This means that during acquisition of relatively weak signals we can not use directly coherent integration for more than 1 ms (up to 10 ms) like we can do for GPS L1 C/A signals. Straightforward attitude to overcoming this problem can be the following. (At first I want to mention that more than 10 ms of integration is not used as it requires too much time.)
One well-known method of acquisition signals which can change sign every ranging code period is zero-padding. The idea of this method is very well described in "Development and Testing of an L1 Combined GPS-Galileo Software Receiver" doctor philosophy thesis by Florence Macchi (PLAN, Calgary, 2010). If we take one period of ranging code and padd it with zeros (of the same length as ranging code) and make fast correlation with 2 periods of incomming signal then we'll have at least one peak that correspond to what we are looking for. This method fits well for GALILEO E1 BOC(1,1) signal or for GLONASS L3 data-channel, but for BeiDou-2 we have problem: this way we can integrate only 1 ms or 20 ms (or N*20ms). 1ms is usually not enough to acquire all satellites visible in open sky and 20ms is too long and requires too much time.
What can be done in order to use integration periods between 1ms and 20 ms? One of the easiest solutions is to repeat several times acquisition process for different parts of incomming signal to cover all 20 symbols of NH code. For example, if we want to use 5ms integration time then we have to take 25ms of incomming signals, divide them into 4 parts (0..10ms, 5..15ms, 10..20ms, 15..25ms) and correlate each part with the first part of NH-code (0,0,0,0,0 in our case) padded with 5ms zeros in the end. And in the end to find peak of correlations. This is what realized in my current version of SciLab software post-processing receiver available as usual on googlecode. In initSettings.sci file three options are available for acquisition (1ms/3ms/5ms integration time).
Tracking functions also were changed. I prefer to use combined FLL/PLL carrier tracking loop. And during experiments with COMPASS signals one bug was revealed which had not been seen before. Frequency discriminator used in carrier tracking loop worked incorrectly when change of sign in incomming signal occured. For signals like GPS C/A or GLONASS ST this bug was not visible because change of sign occures relatively infrequently. But for signals like GLONASS L3 CDMA or BeiDou-2 which have additional modulation of ranging code this bug can be frequently seen. The results of wrong tracking are shown on figure 3.

BeiDou wrong tracking
Figure 3 Wrong tracking with incorrect frequency discriminator

After correcting frequency discriminator tracking works fine. The results can be seen on figure 4 for the same satellite.

BeiDou correct tracking
Figure 4 Correct results of COMPASS signal tracking

I also tried to get PVT solution but the results are not so good (figure 5). Still looking for the bugs.

BeiDou pvt wrong
Figure 5 PVT solution for COMPASS signals
]]> General xml-rss2.php?itemid=89 Mon, 28 Jan 2013 17:37:33 +0300 Accuracy of FPGA receiver xml-rss2.php?itemid=85
PVT_FPGA_variance.PNG

Variation of coordinates across each axis is about ±80 meters that approximately corresponds to software GPS receiver SoftGNSS.]]> General xml-rss2.php?itemid=85 Tue, 3 Jul 2012 19:54:22 +0400 FPGA + ARM = new results with hardware receiver xml-rss2.php?itemid=81 From software receiver to hardware receiver or what can be achieved from a bundle of OSGPS and NAMURU was used. Now the software part of the project is running under real time operating system TNKernel. The project is based on correlator Namuru and software gpl-gps/namuru-gpl.

PVT_FPGA.png

For the moment the program works not stable. Sometimes incorrect synchronization for some channels happens and hence pseudoranges are calculated incorrectly. As a result wrong coordinates are calculated.]]> General xml-rss2.php?itemid=81 Mon, 2 Jul 2012 18:35:17 +0400 Improving the precision of the software receiver GLONASS L1 xml-rss2.php?itemid=77
PVT solution of GLONASS L1 Scilab receiver
Figure 1 PVT solution of GLONASS L1 Scilab receiver

After improving several weak parts of the code the difference between mean value and max/min values of coordinates lowered to ±6 m, i.e. almost in 6 times. Results from the updated version of the receiver is on the figure 1. The same signal record as in the post GLONASS - frist experiments was used.After analysis of the original GNSS-SDR source code several weak places were found. Their improvement could increase receiver's precision. Here is the list of these weak places:
1. High variance of DLL-discriminator data;
2. Pseudorange calculations are based on round-off measurements;
After reducing correlator's arms spacing from 0.5 to 0.05 chips and after lowering DLL loop filter bandwidth from 2.0 to 0.5 Hz with additional adding assistance from PLL-assisted-FLL variation of DLL discriminator data was lowered. This fact with additional refuse from round-off during pseudorange calculations allowed to receive results that are shown on figure 1.]]> General xml-rss2.php?itemid=77 Fri, 23 Mar 2012 18:35:50 +0300 Successful usage cases of published projects xml-rss2.php?itemid=71 https://dspace.ist.utl.pt/...

master_thesis.jpgThis is the second successful copy of front-end project published on my blog that I know about. In this work author didn't just copy the design but also tried to improve it and to simplify it. I should notice that it's pleasant to know that someone is interested in my project. It's even more pleasant to see how someone else improved it.

PS I hope that in the future this note will grow bigger and bigger. :)]]> General xml-rss2.php?itemid=71 Wed, 21 Mar 2012 23:32:15 +0300 GLONASS L2: NEW RESULTS xml-rss2.php?itemid=53
PVT Solution based on GLONASS L2 signals.
Figure 1 PVT solution for L2.
On the figure 2 PVT solution for L1 is presented for comparision.

PVT Solution based on GLONASS L1 signals.
Figure 2 PVT solution for L1.


The source code is available through google code.]]> General xml-rss2.php?itemid=53 Tue, 7 Feb 2012 19:45:09 +0300 Source code for acquisition and tracking algorithms of GLONASS L3 signals xml-rss2.php?itemid=48 source code is available for download now. It's a reworked version of GLONASS L1 software receiver for scilab.Details about new GLONASS L3 signals can be found here. The main differences of the new signals are:
1) Ranging code - trancated Kasami sequence (in L1 and L2 bands - Ì-sequence);
2) Ranging code bit rate: 10.23 MHz (in L1 and L2 bands - 0,511 MHz);
3) Ranging coded is repeated every 1 ms as it was previously, but ranging code is additionaly modulated with 10-digits Hamming code ("0000110101"). Each digit of Hamming code lasts for 1 ms;
4) New signals are CDMA (in L1 and L2 bands FDMA is used);
5) Digital data is coded with convolutional coder;
6) Superframes, frames and strings length is changed.

Acquisition and tracking algorithms are made for now. The next step is to make viterbi decoder.

File with the new signal record from L3 band can be downloaded.]]> General xml-rss2.php?itemid=48 Sun, 9 Oct 2011 22:05:50 +0400 All source code is moving to Google Code xml-rss2.php?itemid=40 Google Code now.

Current project list includes:
1) GNSS-SDR front-end project wich include:
    a) PCB designed in KiCAD;
    b) CPLD-project made in Xilinx ISE WebPack;
    c) Firmware for USB-bridge cy7c68013a;
2) Real-time GPS receiver GPS-SDR addition to work with gnss-sdr front-end;
3) None real-time GLONASS receiver for SCILAB;
4) Console MS Windows program for streaming data to HDD from gnss-sdr front-end;
5) Hardware project wich include:
    a) Namuru based correlator ported to work with Wishbone bus;
    b) OSGPS based single channel GPS receiver (example of how acquisition and tracking is made in hardware receiver). This projects works in conjuction with Namuru based correlator; ]]> General xml-rss2.php?itemid=40 Sat, 24 Sep 2011 12:26:16 +0400 From software receiver to hardware receiver or what can be achieved from a bundle of OSGPS and NAMURU xml-rss2.php?itemid=38 Namuru (correlator for FPGA) and osgps. Also gpl-gps project was used (reworked version of OSGPS for ARM7 mcu). On the following photo a device model is shown.

Hardware GNSS receiver

This model includes gnss-sdr front-end and digital signal processing board. Digital processing board is SK-LPC2478-S3E board of starterkit.ru. It consists of two main parts: lpc2478 mcu with ARM7 core and FPGA Spartan3e500.
Tracking of one satellite is realized now. On the figure below outputs from 6 correlators during 1 second (after acquisition is done) is shown.

correlator outputs during tracking

It can be seen that satellite signal is stably tracking. But the result is not ideal. Delay locked loop is working not good (There should be one maximum-level signal – I_prompt output from correlator and two weaker signals of approximately the same level – I_late and I_early correlator outputs).]]> General xml-rss2.php?itemid=38 Wed, 21 Sep 2011 23:57:11 +0400 GLONASS L3 - new CDMA signal xml-rss2.php?itemid=35
GLONASS L3

On the top right graph bits of navigation message for both pilot and data channels are shown. On the rest graphs only pilot-channel results are shown.
To make this record an external frequency convertor was used with the previously developed front-end. External frequency convertor is the same as one that was used for GLONASS L2 experiments. But as the bandwidth of the new signal is 20.46 MHz an external reference frequency source (24 MHz) was connected to front-end.
]]> General xml-rss2.php?itemid=35 Fri, 15 Apr 2011 02:39:45 +0400