Saturday, March 12, 2011

Countering Doppler Shift during AO-51 satellite pass


Further to the previous post on VO-52, this post the method for countering  the doppler shift for Satellite AO-51 (ECHO) is described below.

I have made many successful DX contacts on this satellite in last 8 weeks using this methodology as explained by OM VU2GUR. 
 
The success of bird tacking depends on how effectively you tackle the doppler shift while satellite is approaching you and while it is moving away from you.

Below given is the table which explains you can tackle the doppler shift for AO-51 (ECHO)

                                                    Uplink(Mhz)         Downlink(Mhz)
 
When Sat is approaching you-----    145.915             435.310
                                                                                435.305

When Sat is near you --------------  145.920              435.300

When Sat is goes away from you-   145.925              435.295
                                                                                435.290



As you get used to these shifting during the advanced stages you can shift both uplink and downlink simultaneously.


Happy Bird Tracking


73s
VU2TQC
+91 9821159097

Tuesday, March 8, 2011

Doppler Shift and VO-52 Satellite


One of the critical aspects of Bird Tracking is handling of Doppler shift during the pass of satellite in your area. 

1. What is Doppler shift/Doppler Effect?

The Doppler effect (or Doppler shift), named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer. The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession.


2. Why does it happen?
 
The relative increase in frequency can be explained as follows. When the source of the waves is moving toward the observer, each successive wave crest is emitted from a position closer to the observer than the previous wave. Therefore each wave takes slightly less time to reach the observer than the previous wave. Therefore the time between the arrival of successive wave crests at the observer is reduced, causing an increase in the frequency. While they are traveling, the distance between successive wavefronts is reduced; so the waves "bunch together". Conversely, if the source of waves is moving away from the observer, each wave is emitted from a position farther from the observer than the previous wave, so the arrival time between successive waves is increased, reducing the frequency. The distance between successive wavefronts is increased, so the waves "spread out".

3. How to effectively handle the doppler shift for bird VO-52 (HAMSAT)?

The success of bird tacking depends on how effectively you tackle the doppler shift while satellite is approaching you and while it is moving away from you.

Below given is the table which explains you can tackle the doppler shift for VO-52 (HAMSAT) while up-linking (transmitting) on UHF 435.250 Mhz, Keep the downlink fixed at 145.900 Mhz. 

                                                    Uplink(Mhz)         Downlink(Mhz)
 
When Sat is approaching you-----    435.245

When Sat is nearer to you --------   435.250              145.900

When Sat is goes away from you-   435.255



The other way could be to keep the Uplinking frequency fixed at 435.250 and  shift the frequencies while as satellite approaches you as described below:-


                         Uplink(Mhz)         Downlink(Mhz)
 
                                                      145.905----When Sat is approaching you

                            435.250              145.900----When Sat is nearer to you

                                                      145.895 --- When Sat is goes away from you


As you get used to these shifting during the advanced stages you can shift both uplink and downlink simultaneously.

Happy Bird Tracking


73s


Next post i shall give the frequency shifts for AO-51


Friday, July 30, 2010

Five Steps to OSCAR Tracking

Article 2. OSCAR/VU2TQC/02/300710


Five Steps to receiving Satellite Signals (HAMSAT; VO-52)

Step 1. Note down the pass timings of VO-52 for the day , and focus on the passes having maximum elevation above 30 degrees (you may refer this blog or download satscape a free software from http://www.amsat.org), ensure that your are prepare and keep yourself ready atleast 15 minutes before the start of the satellite pass.

Step 2. Store the following frequencies in your VHF Rig (read your respective operating manuals)

145.890 MHZ
145.895 MHZ
145.900 MHZ
145.905 MHZ
145.910 MHZ
145.915 MHZ

You are storing these frequencies at a gap of 5khz to adjust the Doppler effect ( shall discuss this subject separately in another posting, but this will not hamper your current goal of satellite tracking)
Note: The downlink frequency band for VO-52 is ( 145.870 MHZ to 145.930 MHZ)

Step 3. Trigger your rig to check if your antenna (any fixed antenna which you have installed for regular VHF QSO) is triggering the repeater in your area, just to ensure your antenna feed-line is connected to your rig (at times due to excitement in the entire process we may miss out the connection).

Step 4. Start your monitoring of frequencies as mentioned in step 2 from 145.915 MHZ, and keep coming down by 5khz to 145.900 MHZ (be patient here you may not copy anything till the 2 to 3 minutes from the start timing of the satellite pass).

Step 5. Note down the call signs you hear ( you will hear the QSOs as clearly as your regular line of sight VHF QSOs ; the only difference is you may not see any pixels on the bar graph of your rig).
Note: Please make sure do not transmit (uplink) on these frequencies as it would not reach any where and jam the satellite band.

Thursday, July 29, 2010

Satellite communciations for the beginers



OSCAR/VU2TQC/01/270710

It is a common perception that tracking the satellite requires sophisticated equipment and large circularly polarised antenna arrays to work amateur satellites. While this may be true for using some of the high altitude ‘birds’ or on the higher bands such as 23cm, it isn’t the case for all satellites. There are several low Earth orbiting satellites which can be worked with relatively simple transceivers and antennas. Our focus shall be to track the LEO (Low Earth Orbit) Satellites.

But before we being lets understand some of the terminologies commonly used in Satellite communication.

OSCAR – Orbiting Satellite Carrying Amateur Radio.

LEO – Low Earth Orbit ( 500km to 1500 km from earth)

Uplink Frequency- The frequencies on which you can transmit

Downlink frequency - The frequencies on which you shall receive the signal

To successfully work an amateur satellite, you need to have transceivers suitable for the satellites you wish to work. All of the FM satellites use 2m and 70cm, with one of these bands being used for the uplink, the other for the downlink. There are a wider variety of frequencies in use by linear transponder satellites.

For antennas, VHF/UHF omnidirectional antennas will work in a pinch. The typical VHF/UHF collinears typically have a low angle of radiation, and better results may be obtained with a simple ¼ wave groundplane, or for the more serious, a turnstile antenna.

Regardless of the rig you use, it has to be capable of tuning in 5 kHz or smaller steps, to enable you to follow the Doppler shift as the satellite passes overhead.

Also important to know is the amount of Doppler shift that will be present on the uplink and downlink frequency. Doppler shift is a phenomenon that all of us will recognise in a different situation. Imagine you're waiting at a railway crossing. A train passes at high speed, blowing its horn. As the train passes you, the pitch of the horn appears lower than when it was approaching. That apparent shift in frequency is Doppler shifting caused by the relative speed of the train to you shortening, then later lengthening the wavelength of the sound as seen by the observer.

As the satellite approaches, you should be listening to the downlink frequency, with the uplink ready to transmit when needed. Remember to allow for any Doppler shift (for FM, it will only be significant on 70cm – around 5-10 kHz). If the uplink is on 70cm (usually the case for HAMSAT), tune 5-10 kHz below the nominal uplink frequency (the Doppler shift will make it arrive at the satellite on the correct frequency). If the downlink is on 70cm, you’ll have to tune the 70cm receiver 5-10 kHz above the nominal frequency.

While calling, pay attention to your signal as heard on the downlink. Too much noise may indicate a need to move the uplink antenna, increase power or adjust frequency to compensate for Doppler shift. If you can’t hear the downlink at all, don’t attempt to transmit, as you may interfere with someone else. Also, keep things short while using the satellite. Only one person can use the transponder at a time and the satellite is usually only accessible for about 10 minutes. Others will appreciate your efficiency and courtesy. Most FM satellite contacts are usually an exchange of callsigns, grid position, signal reports and occasionally a comment about the weather.

IMPORTANT:- For the beginners , start copying the HAMS AT (VO-52) satellite signals through your current VHF setup.

I shall upload the steps for receiving HAMSAT signals in the next article.

Best Wishes

VU2TQC


Friday, July 16, 2010

Ham population around the world

It is interesting to know the population of HAMs (Amateur Radio Operators) for different countries. While surfing the net for satellite CO-66 details, i came across a site which the census of HAM populations countrywise. Although the data seemed little outdated but it gives a good comparison at the scroll of the mouse. The link for the site is

http://www.n0hr.com/ham_radio_population.htm

Visit the site and enjoy.

73

VU2TQC

Thursday, July 15, 2010

Shifting of Shack


21st January 2008 was the day when i received my amateur radio license from Wireless Planning Commission (Government of India). It has been more than two years since i came on air using my VHF rig ICOM 2200. With time added the HF equipments (IC 718 along with the antenna tuner AT-180).

My radio shack was established in the ground floor of my 2 storied house. However almost all through the year (except monsoon), my rag chewing on VHF had been happening from the open balcony on the first floor. It had been a make shift arrangement kind of operating condition. My HF communications were happening through my shack in the ground floor shack.

After initial showers we received in the month of June 2010 at Navi Mumbai (my home
QTH), when i had to rush my some of the rigs inside to protect it from rain. This was the time i firmed up my thoughts of shifting the entire radio shack to the first floor. This was easier said than done, it required convincing my wife to be allowed to shift it to the first floor room adjacent to our bedroom. Surprisingly concurrence came easily.

That was half battle won. Then came shifting the cables running from my roof tops to the first floor. A friendly help from OM VU2AF (Adolf), by lending his drill machine and drill bits.


Shifted my shack table and the rigs. Fixed a soft-board panel and fired the rigs one by one. All the signals tested, SWR under control.
It was a great feeling after the task was accomplished. During this period even HF band condition also improved. I was back on band with a bang.

Antenna Setup for the rigs are

1. VHF (2 meter) - 5/8 , mast height 15 feet.














2. Antenna for Satellite Communications

2.1 UHF - Di-pole the elements facing east west (mast height -15 ft)
2.2 UHF - double 5/8
2.3 UHF Slimjim

3. HF communications

Inverted di-pole (20 m & 40 m) (mast height- 20 ft)




Monday, July 12, 2010

Eye ball with VU2GUR

Last week of June 2010 was an interesting milestone in my satellite communication journey. I had the chance of having an eye ball with OM VU2GUR at Mumbai.

I am sure most of my HAM friends must be aware of the achievements of OM VU2GUR, from Bangalore. He has been the pioneer in Satellite communications using HAM radio in India , you can google and find out more about him. But i remember him for doing maximum satellite QSO's using simple slimjim and di-pole sets of antennas. Many times through the rubber duckies .


It was kind enough of Mr. Guru to have informed me when he arrived and took out some precious moments from his purpose of visit to Mumbai. We had fixed up our meeting at 18:30 hrs which got little delayed by an hour, due to my pre-occupation with the audit getting stretched. I rushed down the moment my morning QTH activity got over.

Like all genius, he was humble and forthcoming on all the topics of bird watch. The discussions started from the activities on satellite communication to the visit to ISRO (the Indian Space Research Organization), Bangalore, which unfortunately i could not visit (due to my morning QTH pressure).

We occupied a table in the lobby of the hotel and his lessons started. During the initial course of our discussions he could judge that i won't be able to remember all the info he was providing me. He took my diary and started scribbling from page to page. The information flowed seamlessly from the master to the disciple. I was thrilled with wonderful moments and information, there were finer aspects of transponders, inverting transponders, the difference between them, the bandwidth, SSB, SSTV, CW , in satcom, functioning of new satellite SO-67. The time flew by and i saw that the time was past 21:00 hrs, i remembered he had to attend a wedding reception. Till the time of drop to the venue we kept chatting on the subject.

It was a brilliant day for me. Promised OM Guru to have our next meeting at Bangalore in few weeks time.