Observation 6111502: GRIFEX (40379)

Satellites & Observations Observations
1

Regarding Observation 6111502 … Looks like the frequency was shifted a bit

2

Can you please also have a look at your SDR LO, if this is a RTL-SDR look at Kalibrate.

kalibrate v0.4.1-rtl, Copyright (c) 2010, Joshua Lackey
modified for use with rtl-sdr devices, Copyright (c) 2012, Steve Markgraf
Usage:
	GSM Base Station Scan:
		kal <-s band indicator> [options]

	Clock Offset Calculation:
		kal <-f frequency | -c channel> [options]

Where options are:
	-s	band to scan (GSM850, GSM-R, GSM900, EGSM, DCS, PCS)
	-f	frequency of nearby GSM base station
	-c	channel of nearby GSM base station
	-b	band indicator (GSM850, GSM-R, GSM900, EGSM, DCS, PCS)
	-g	gain in dB
	-d	rtl-sdr device index
	-e	initial frequency error in ppm
	-E	manual frequency offset in hz
	-v	verbose
	-D	enable debug messages
	-h	help
3

nice, will check, thanks!

4

I tried this and experimentally I’ve picked up 1.6kHz as a seemingly reasonable offset, at least when scanning the GSM / 900MHz band with kal. I’ve entered it now into the SatNOGS configuration, but that should only make frequency configuration a bit more precise, right? I.e. just place the 0 mark of the waterfall a bit more exactly. Maybe it will improve some data decoding, but hardly significantly? And otherwise it shouldn’t actually improve the general reception quality?

5

Share your satnogs config to see what PPM value you have configured

6

oh, none, I thought I should use that for LO_OFFSET

7

Ok, I read in SatNOGS Client Setup - SatNOGS Wiki that I shouldn’t use LO_OFFSET. Since 1.6kHz was the offset suitable for 900MHz, should I specify a PPM_ERROR value of 2 then?

1 Like
8

hm, but looking at my LUSAT captures, its signal didn’t become closer to 0 after this PPM adjustment, in fact it became a bit further from it, trying to change the sign…

9

kal give you a proposal most of mine PPM values are negative, and you can only use integer values.

As an example: kal ppm proposal is -1.3 then you will configure SATNOGS_PPM_ERROR="-1"
An other example: kal ppm proposal is -1.7 then you will configure SATNOGS_PPM_ERROR="-2"

Here a run I did on one of my systems:

kal -s GSM900 -g 0

Found 1 device(s):
  0:  Generic RTL2832U OEM

Using device 0: Generic RTL2832U OEM
Found Rafael Micro R820T tuner
Exact sample rate is: 270833.002142 Hz
[R82XX] PLL not locked!
kal: Scanning for GSM-900 base stations.
GSM-900:
    chan:    5 (936.0MHz + 1.834kHz)    power:   32080.79
    chan:    8 (936.6MHz + 2.414kHz)    power:   29299.42
    chan:   10 (937.0MHz + 2.587kHz)    power:   28248.30
    chan:   45 (944.0MHz + 2.894kHz)    power:   23174.72
    chan:   46 (944.2MHz + 2.582kHz)    power:   30341.13
    chan:   88 (952.6MHz + 1.963kHz)    power:   39869.48
    chan:  100 (955.0MHz + 2.569kHz)    power:   40559.64
...chan 124

Run multiple times to take temperature influences into account, and at the end use the average.

while true
do
  kal -c 100 -b GSM900 -g 0
  sleep 2
done


Found 1 device(s):
  0:  Generic RTL2832U OEM

Using device 0: Generic RTL2832U OEM
Found Rafael Micro R820T tuner
Exact sample rate is: 270833.002142 Hz
[R82XX] PLL not locked!
kal: Calculating clock frequency offset.
Using GSM-900 channel 100 (955.0MHz)
Tuned to 955.000000MHz (reported tuner error: 0Hz)
average		[min, max]	(range, stddev)
+ 2.495kHz		[2452, 2531]	(79, 19.737494)
overruns: 0
not found: 0
average absolute error: -2.613 ppm

So at the end I use -2.613 and set SATNOGS_PPM_ERROR="-3"

10

nice, thanks! I tried two different channels, it was about 1.3 for one and 1.1 for another, so I’m using 1 for now