5 ms·
Indeed, but there are other important factors besides size. DSN have way bigger requirements in terms of signal to noise ratios, since they need to demodulate
by myth2018 6y ago
Indeed, but there are other important factors besides size.
DSN have way bigger requirements in terms of signal to noise ratios, since they need to demodulate received signals.
Scott Tilley, on the other hand, seems to be mainly interested on detecting the presence of signals, not necessarily extracting information from it. Then, he could build a bigger dish, but he doesn't actually need one to achieve his goal.
I notice that another guy commented here about that. His comment is dead now, but he had a point. I found it informative indeed, I've always asked myself why spacecrafts "wasted" energy with carriers and never realized they make the signal easer to find.
- schoen 6y agoThanks for that clarification (to you and the author of the dead sibling comment)!
- chokeartist 6y ago> I've always asked myself why spacecrafts "wasted" energy with carriers and never realized they make the signal easer to find. Not exactly. You need a carrier wave / signal to modulate information onto. Can't modulate info onto nothing.
- myth2018 6y agoYes, you can. There are a number of supressed-carrier modes, e.g. single side band.
- chokeartist 6y agoYou still have a reference carrier, doofus.
- madengr 6y agoI'm the shadow banned commenter, due to comments that fall in the right half of the plane; disagreeable to HN mods. But not to digress, it's likely 120 degree BPSK (i.e. QPSK with a DC level for the Q). I'm not in the deep space field (but do RF/Microwave professionally), but the DSN (deep space network) on Earth sends out a carrier with 120 BPSK modulation with PN (pseudo noise) code. The spacecraft transverts and sends that back (a bent pipe). Velocity is determined via Doppler shift of the carrier. Range from the delay in the PN code. So knowing velocity and range (and bearing using antenna pointing or interferometry), you can track the spacecraft with great precision. Even without a bent pipe, you need carrier recovery first, before you can demodulate, and the leakage helps with that. You can also do geolocation with carrier leakage (e.g. ARGOS satellites) and via isodops (contours of constant Doppler shift) like a reverse GPS uses isochrones (contours of constant time delay). So in short, there are benefits of wasting carrier energy. The ham guy can detect it as he can do lots and lots of coherent averaging (i.e. long FFTs or very narrow resolution bandwidths) to find that carrier. Though to recover any modulation will take a much bigger (or lower noise temp) antenna.
- deleted 6y ago[deleted]
- myth2018 6y agoThank you so much for the information. In case you have any links to share about how DSN works, I'd appreciate. And thank you also for mentioning coherent averaging. I'm more or less familiar with the concept but I've been trying to find out the name of the thing so that I could google on how to implement it (I've been collecting some information for assessing the feasibility of a project involving high-loss antennas). Besides, I've never wondered how they tracked spacecrafts, especially those ones as far as new horizons and voyagers.
- madengr 6y agoUplink Downlink is a good, technical history: https://www.amazon.com/Uplink-Downlink-History-1957-1997-033-000-01241-1/dp/016066599X/ref=sr_1_1?dchild=1&keywords=uplink+downlink&qid=1613855940&s=books&sr=1-1 https://www.amazon.com/Uplink-Downlink-History-1957-1997-033...
- tyldum 6y agoAnd smaller dish has larger beam width, making it really difficult to track the signal.