Re: QUESTION - Moon Orbiter "captures" ?
From: Scott Tilley
Date: Sun, 16 Jun 2024 19:37:39 CEST
[Edited Message Follows] Dear Amateur-DSN list
Here I'd like to share some of the methods I use to identify missions in space and be sure of those identifications with a focus on objects around the Moon. Other techniques can come into play for objects in different orbits so I will focus on the Moon as that's the topic of this thread.
A few key things are needed to do this:
- A up to date satellite tracking program with the complete Space-Track.org catalogue loaded (Heavensat, Gpredict etc etc)
- Waterfall display software that you can record long periods of time on to visualize the Doppler effect.
- Frequency lists of the area of spectrum you are interested in. The ITU database and various other internet references.
- An accurately calibrated mount. Calibrating a mount or confirming it's calibration can be done by using geostationary satellites that emit in the band of interest.
- Confirmation of frequency accuracy of your receiving system.
Now go hunting for the mission. Once you have found a signal you suspect is the mission you are looking for, look at the satellite tracking program and take notice of any missions in Earth orbit that may be in the beam of your antenna (or even outside the main beam and in the side lobes). Rock your antenna back and forth and see if the signal disappears based on what your beam width expectations are. If it does then continue to monitor, plot the Doppler over a period of time. Objects orbiting the Moon will always have Doppler associated with them. In the plot below one can see a typical Doppler curve for a few orbits around the Moon of a lunar orbiter.
As you can see the object disappears when it goes behind the Moon and then reappears as it comes back into view and the direction of the Doppler can be rising or falling depending on when you happen to catch the signal. But sometimes you'll see a complete sine curve as the plane of the orbit could be entirely in view (rare but can happen). The amplitude of the Doppler shift can vary considerably depending on the inclination of the object's orbital plane around the Moon. Below is a full lunar day of LRO's Doppler as I recorded it to demonstrate the effect of how we see the orbital plane from Earth as our perspective of it changes and the effect on the Doppler we observe, this effect is particularly prominent in polar lunar orbits.
Another case is your object could be in an eccentric and higher altitude lunar orbit where the Doppler will be somewhat more complex and bizarre looking. Below is a few weeks of Doppler data and predicted Doppler from the DSLWP-B mission. As you can see the Doppler can be different each day and range wildly.
Therefore you should gather some Doppler to confirm it's actually effected by the Doppler effect and determine to what extent it is, no Doppler then it's not in space (even GEO satellites have Doppler). The next important consideration for lunar observing and correctly identifying signals from the Moon and vicinity is to also confirm the signals are not coming from objects closer to Earth or even from another lunar mission! The Moon spends a good part of it's orbit drifting through parts of the sky that transit the geostationary satellite belt for example. Those geostationary satellites are not all in fixed locations. Some are in highly inclined orbits so are drifting to new locations... Other missions in medium Earth orbit and even high Earth orbit can also become Doppelgängers for signals at the Moon. To weed these out one uses a recently update satellite prediction program and a knowledge of frequency usage on the band you are monitoring on (ITU and other frequency lists). You can also use the Doppler and tracking data. You'll typically find these objects will fade out over a period of time as the coincidental nature of their being in or close to you antenna beam won't last forever, but if you are not patient and monitor to confirm long enough you can (will) be fooled. Also be attentive that most signals you find from satellites are not just consisting of a central carrier they can have sub-carriers above and below the main spread out over many MHz.
Here's a thread where AMSAT-DL came across COSMOS 2518 emitting a signal that looked almost identical to the IM-1 lander on the Moon. This signal was detected during a phase of the lander's mission that would have been very significant detection, so careful study of what we were observing was needed. Notice the dialogue of the parties involved. A gentlemen's exchange during a time of idea sharing.
https://x.com/coastal8049/status/1770556129442938922
If you use Twitter, I post about every lunar mission that is ongoing. You can also search Twitter for plots and info that I have posted in the past which may help you. My Twitter handle is @coastal8049
To search my Twitter profile enter my handle and the name of the mission you may be interested in i.e. '@coastal8049 KPLO' you'll get a number of options to refine your search of all the tweets I've ever posted on the topic of KPLO. I tend to use the same keywords in all my tweets to allow this to work well.
I respond to all questions and queries there so feel free to engage in real-time if you have questions about your lunar observing.
In any pursuit people will have different ideas and it is healthy to debate them. However, in the debate it should never turn from debating the ideas to attacking the person(s) with them.
I recently replied to this thread and provided a list of known active missions around the Moon using S-band. I then received a long unsolicited private email from a list member asking for more information about operations around the moon and discrepancies in frequencies etc. I responded with some information from what I know and a few probing technical questions and some ideas to help me understand why some dependencies existed in their observations.
The response I received was an ad hominem attack and a tirade about how I somehow owed this person my time and knowledge to be provided to them in a manner of their pleasing. They didn't attempt to challenge my ideas, they just launched into a personal attack and when I responded with an offer for them to reflect and restart the dialogue it devolved further...
Rather than let this individual's behaviour negatively affect my enjoyment of this hobby, I decided to share the above and also remind those few members that may forget from time to time this is a place of civil dialogue and that no member here owes anyone else anything other than behaving in a civil manner whether on the list or off of it if they are using the list to facilitate private communications. I would prefer if you feel the need to respond to something I post here you respond here for the benefit of the entire group from the exchange and debate of the ideas.
Thank-you for your consideration.
Regards,
Scott Tilley |
