PyEphem

Loading GPS orbits from Rinex NAV file?

Asked by mungewell

Hi,
I discovered pyephem when performing some analysis of raw GPS data. My desire was to quickly compute GPS satellite location in Azimuth/Elevation and my previous solution (GPStk's 'wheresat') was proving very slow to compute 1Hz positions.

I successfully used the 'readtle' function to load/compute the orbits, but have the feeling that these will not be as accurate as the ephemeris transmitted by the satellites themselves - which I have in the form of a Rinex NAV file.

Does anyone know of a method to import/convert/read the from a NAV file?
Simon.

Sample NAV below
--
     2.10 N: GPS NAV DATA RINEX VERSION / TYPE
RTKCONV 2.4.1 20111026 152034 UTC PGM / RUN BY / DATE
log: C:\Users\swood\Desktop\20111018_overnight\RAW10111019 COMMENT
format: NovAtel OEM4/V COMMENT
    2.5146E-08 -7.4506E-09 -1.1921E-07 1.1921E-07 ION ALPHA
    1.3107E+05 -1.6384E+04 -1.9661E+05 -6.5536E+04 ION BETA
     .000000000000E+00 .000000000000E+00 503808 1658 DELTA-UTC: A0,A1,T,W
    15 LEAP SECONDS
                                                            END OF HEADER
27 11 8 10 23 59 44.0 .293537508696E-03 .386535248253E-11 .000000000000E+00
     .910000000000E+02 .859062500000E+02 .397980863215E-08 -.204959952042E+01
     .499747693539E-05 .215929013211E-01 .220909714699E-05 .515364279747E+04
     .345584000000E+06 .106170773506E-06 .849556551143E+00 .493600964546E-06
     .981332201865E+00 .346875000000E+03 -.121844958010E+01 -.792675875275E-08
     .311798701953E-09 .100000000000E+01 .164800000000E+04 .000000000000E+00
     .240000000000E+01 .630000000000E+02 -.419095158577E-08 .910000000000E+02
     .338556000000E+06 .400000000000E+01
--

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Revision history for this message
mungewell (simon-mungewell) said : 		#1

Here's a quick comparison between my script and GPStk's 'wheresat'.

--
$ wheresat.exe -p 1 -e ../rinex_gps_glonass/RAW1011102100.nav -f "%4F_%010.3g" --start="10/21/2011 06:00:15" -u "-1635377.1361 -3665372.6408 4941664.1946" | head -n 30
Scanning over prnSet.
Scan complete, size = 1
Scanning over PRNs indices.
Scan complete, size = 1
Set timeFormat to %4F_%010.3g
positionOpt has count = 1
# time, PRN, X(m), Y(m), Z(m), Clock Correction(ms), Azimuth(deg), Elevation(deg), Range(m)
File read by EphReader.
10/21/2011 06:00:15
10/21/2011 18:00:00
1658_453615.000 1 -17243579.062 -3462659.095 19889982.761 0.042121 287.903 45.806 21587470.047
1658_454515.000 1 -18181568.633 -5459328.551 18563831.884 0.042146 278.244 47.502 21481909.842
1658_455415.000 1 -19133873.329 -7261100.358 16917926.110 0.042171 267.898 47.519 21481878.682
1658_456315.000 1 -20061089.121 -8845371.192 14980638.294 0.042195 257.842 45.852 21587679.410
1658_457215.000 1 -20921707.465 -10197677.027 12785358.304 0.042220 248.846 42.720 21795723.659
1658_458115.000 1 -21673746.266 -11312049.795 10369915.426 0.042245 241.233 38.470 22098740.548
1658_459015.000 1 -22276411.910 -12191047.263 7775924.133 0.042270 234.954 33.457 22486278.141
1658_459915.000 1 -22691727.749 -12845455.923 5048064.631 0.042295 229.776 27.980 22945401.375
1658_460815.000 1 -22886065.726 -13293679.148 2233311.130 0.042320 225.428 22.268 23461471.371
1658_461715.000 1 -22831521.297 -13560836.474 -619880.644 0.042345 221.660 16.491 24018907.742
1658_462615.000 1 -22507078.443 -13677606.940 -3462407.437 0.042370 218.259 10.771 24601854.701
1658_463515.000 1 -21899519.728 -13678866.016 -6245365.779 0.042395 215.055 5.203 25194728.900
1658_464415.000 1 -21004044.993 -13602167.080 -8920893.368 0.042420 - - -
1658_465315.000 1 -19824575.465 -13486128.027 -11442991.485 0.042444 - - -
1658_466215.000 1 -18373731.115 -13368785.997 -13768314.060 0.042469 - - -
1658_467115.000 1 -16672481.979 -13285984.436 -15856909.935 0.042494 - - -
1658_468015.000 1 -14749486.726 -13269855.304 -17672905.779 0.042519 - - -
1658_468915.000 1 -12640143.664 -13347455.389 -19185118.261 0.042544 - - -
1658_469815.000 1 -10385390.391 -13539609.646 -20367585.378 0.042569 - - -
1658_470715.000 1 -8030297.770 -13860006.350 -21200008.272 0.042594 - - -

$ python wheresat.py -p 1 -s 1319176800 | head -n 20
1658_453615.000 1 0 0 0 0 287.64385463 45.8833666931 21613000.0
1658_454515.000 1 0 0 0 0 277.964055897 47.5382839629 21507782.0
1658_455415.000 1 0 0 0 0 267.619843682 47.5136372253 21508050.0
1658_456315.000 1 0 0 0 0 257.587279339 45.8078043103 21614092.0
1658_457215.000 1 0 0 0 0 248.627355223 42.6455052717 21822312.0
1658_458115.000 1 0 0 0 0 241.052757396 38.3737512856 22125432.0
1658_459015.000 1 0 0 0 0 234.807778834 33.3464656791 22513000.0
1658_459915.000 1 0 0 0 0 229.659291521 27.8619877001 22972082.0
1658_460815.000 1 0 0 0 0 225.334825549 22.1496665632 23488044.0
1658_461715.000 1 0 0 0 0 221.584737506 16.380357743 24045312.0
1658_462615.000 1 0 0 0 0 218.199176659 10.6831458384 24628038.0
1658_463515.000 1 0 0 0 0 215.005926619 5.18521281361 25220648.0
1658_464415.000 1 0 0 0 0 211.863206317 0.199962939338 25808254.0
1658_465315.000 1 0 0 0 0 - - -
1658_466215.000 1 0 0 0 0 - - -
1658_467115.000 1 0 0 0 0 - - -
1658_468015.000 1 0 0 0 0 - - -
1658_468915.000 1 0 0 0 0 - - -
1658_469815.000 1 0 0 0 0 - - -
1658_470715.000 1 0 0 0 0 - - -

Revision history for this message
Brandon Rhodes (brandon-rhodes) said : 		#2

I myself don't have a library for reading a NAV file — I would suggesting asking your question on Stack Overflow, where it will reach a much wider audience. Good luck — and if you tag your Stack Overflow question “PyEphem,” then I will watch its progress and could pull any good advice you get back into the PyEphem docs.

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Provide an answer of your own, or ask mungewell for more information if necessary.

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