U-blox GPS-PS1 OEM Receiver Tested
The Swiss firm u-blox AG donated (!!!)
two of their GPS-PS1 OEM receivers
for evaluation. These receivers are identical to their extremely small
(3cm x 3cm) GPS-MS1 in terms
of components, schematics and RF section. The receivers are based on the
promising SiRF chipset.
In this page the results of some tests are reported:
In a so-called zero baseline setup the multipath- and noise of the raw C/A code and carrier were determined (see my multipath and noise pages for an explanation of the method).
Two short baseline tests, one in a multipath benign environment and one in a multipath heavy environment, allowed me to gain more insight into the receiver behaviour under favourable conditions and under realistic conditions.
Finally, the receiver tracking performance results in the urban canyon and in the woods are reported.
In the first experiment, I used Tri-M's Big Brother antenna on a small groundplane (25 cm dia) on top of my roof, with an unobstructed view over the horizon. The antenna was connected to a tri-way splitter with a DC coupling to my reference receiver, and the antenna inputs of the two PS-1's AC coupled via a 100pF capacitor. (the three receivers in a 'zero-baseline' setup). The PS-1's were set-up to generate raw measurement data in SiRF's binary format. The data was downloaded to the serial ports of my PC. About three hours of data were recorded and processed.
1.1. C/A Code Multipath
The graph above shows the C/A code multipath magnitude of both receivers,
connected to the Big Brother. No significant differences can be seen between
receiver 1 and 2.
The relation between the multipath (1 sigma value) and the satellite elevation is clear, but not very strong. The maximum value less than 5 m, the average value for elevations between 40 and 90 deg about 1 m, going up to about 3 m at low elevations.
1.2. C/A Code Noise
The graph above gives the C/A code noise with the Big Brother antenna. Obviously,
the noise is independent of elevation, and has an average value of about
2 m (95% value) or about 1 m 1 sigma.
Most probably the receiver noise is the larger contribution, and the antenna (plus pre-amp) noise the smaller contribution.
Comparing this graph with the multipath graph (section 1.1) leads to the interesting conclusion that at elevations above 40 deg the multipath magnitude (1 m 1 sigma) equals the noise value (1 sigma value also 1 m). In other words : the multipath graph actually shows the combined multipath and receiver noise, above 40 deg elevation the (elevation independent) receiver noise dominates, and below 40 deg elevation the multipath component increases in magnitude with decreasing elevation.
To see how noise (and residual multipath) affects the calculated position, I processed the data sets with the Big Brother in the single difference mode, using one receiver as the reference and the other as the moving receiver. Since both receivers share the same antenna, the position difference should be zero, deviations are the result of the receiver (and antenna) noise. The two figures below give a plot of height versus time and latitude versus longitude. Satellites below 10 deg elevation have been removed from the position solution, and position solutions with a PDOP > 6 have been removed from the figures.
The 1 sigma values of the deviations from the 'true' position (zero) are:
|1.6 m||1.6 m||2.3 m|
Smoothing the raw data improves the results, see the two figures below.
In this case the 1 sigma values reduce to:
|1.3 m||1.4 m||1.6 m|
Smoothing the data does provide a clear, but no significant improvement,
probably because the component which is best mitigated by smoothing, multipath,
has already effectively been removed by forming the single differences.
Moreover, the scatter plots certainly 'look' better, and clearly show the effect of a new, not yet smoothed, satellite entering the position solution at appr. the point [-13,-13]. This effect illustrates the following WARNING : even with a high confidence level of about 1.5 m 1 sigma or 3 m 95%, the deviation from the true position can be SIX times as large. And this warning applies to any GPS receiver!
1.3. C/A Carrier Noise
Since the time relation of the code observation and the carrier observation
are not understood (yet) I have not been able to produce an absolute figure
for the carrier noise. However, I have a few observations:
Forming double differences of the carrier phase observations (see the theory page) in a zero baseline test should result in values which remain constant in time (as long as no breaks in the receivers carrier tracking loops occur) and which average to integer values, the deviations from the integer being (twice) the carrier noise contribution.
Because of the above mentioned lack of knowledge the interpolation of the carrier data from receiver 2 to the measurement moment of receiver 1 only occasionally resulted in values which remained constant in time. From this limited data I estimate the carrier noise to be elevation independent, with an 1 sigma value better than 0.5 cm.
Furthermore, where the double differences remained constant in time, they did NOT average to an integer number of cycles.
Sufficient knowledge of the timing relation may lead to values which DO remain constant all the time, and which DO average to integers. With the low noise contribution this would enable the highly wanted feature of measuring with these receiverts short baselines with cm accuracy !!
Next sections to be completed.
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