Why are deep space "beacons" usually UHF, so low (in freq) that DSN can't hear them? And how unusual is it for a Mars lander to have an X-band beacon?

This answer to Is there independent evidence that China landed a robot on Mars in May 2021? and an extensive comment below it indicate that the Zhurong Mars lander had an X-band beacon, information that is cited from https://destevez.net/2021/05/tianwen-1-landing/ which links to this tweet.

Beacons, tones, heartbeats are radio signals that have little to no encoded data, but their existence, amplitude and especially Doppler shift contains a wealth of information about spacecraft health, mission progress and successful execution of orbital maneuvers, and in the past five years have been used to both demonstrate successes and post-mortem failures of landings.

To my knowledge these beacons are at VHF/UHF frequencies, Perseverence's was about 400 MHz for example. From:

This VHF tone is around 400 MHz and was tracked by the Green Bank Observatory earlier. I don't know if it was Green Bank who received it during EDL also or not, but Mars was certainly high in the sky at the time (elev. > 65°), so it seems likely.

Schiaparelli's was 401.6 MHz: ExoMars Schiaparelli Direct‐to‐Earth Observation using GMRT (also here).

Basically, as long as you are well above say 60 MHz you can assume the ionosphere to be fairly transparent. There will be more refraction, scintillation and astronomical seeing effects as you get closer to the ionosphere's plasma frequency, but probably anything above 100 or 150 MHz should be okay.

For deep space links when high data rates or accurate range-rate measurements are required, much higher frequencies are used. This is primarily because the strength of the received signal depends on the ratio of the diameter of the transmitting dish to the wavelength (narrowness of the beam's diffraction pattern as it expands over millions to billions of km). The efficiency of the receiving antenna is not linearly dependent on wavelength in the same way, though a narrow beam can in some cases improve S/N.

You have to be careful because the ratio of each antenna's diameter to the wavelength are used to calculate gains for each as part of a classical link budget calculation, but this is an idiosynchrocy of how link budgets are normally performed and there's a $$\lambda^2$$ present in the "free space path loss" which one would normally think should be only $$1/r^2$$.

Anyway Beacons are necessarily transmitted with omnidirectional antennas because we don't want to have to orient the spacecraft to send them to Earth, so those transmit antennas don't have meaningful diameters, and we just assign them gains close to 0 dBi (decibels relative to a theoretical isotropic radiator).

Questions:

1. So why are beacons traditionally in the UHF in frequency bands so low that the Deep Space Network can't even receive them? It wouldn't make sense to add a UFH receiver to several DSN dishes, but why not raise the frequency of the beacons?
• Is it harder to make an isotropic transmitting antenna when the wavelengths get down to centimeters?
• Are the transmitters heavier, more expensive, less efficient/reliable or otherwise undesirable at say S-band, or the higher X, Ku and Ka bands?
• Is it something else entirely?
1. How unusual is it for a Mars lander like Zhurong to have an X-band beacon?

• I thought I was reading an answer here. Consider cutting out the stuff that isn't a question. May 17 at 5:50
• @Innovine the information serves two functions essential for a Stack Exchange question: it supports the premise of the question and provides evidence of research. If folks are comfortable with both they can immediately skip down to the part labeled Questions: in bold font, but some of these sources provide resources that may be helpful in formulating an answer. Many of my questions are quite short, some are long. This particular one is long because it really requires premise support and evidence of prior research. I have experience asking here and I know what happens when I leave those out
– uhoh
May 17 at 6:58
• It seems to have prevented all answers, so far. May 17 at 10:24
• @Innovine there are several really knowledgable answer-authors who don't even sign in every day, possibly some only on the weekends. I'm confident (based on a few thousand previous questions here) that answers will be forthcoming. Let's just give it some time.
– uhoh
May 17 at 10:39
• Does beacon include ranging and other navigation signal analysis? May 17 at 17:39

I don't think the UHF systems at Mars are intended to act as 'beacons', it's just a consequence of the existing UHF infrastructure at Mars and the operator's desire to squeeze more information out of their spacecraft.

Take some of the examples listed in the question; Schiaparelli and Mars 2020/Perseverance:

The paper [1] on ExoMars Schiaparelli EDM observations from the Giant Metrewave Radio Telescope (GMRT) linked in the question states:

Schiaparelli does not have Direct-to-Earth capability, and it relies on the Mars Orbiters to relay its UHF communications back to Earth.

Additionally, this ESA blog post [2] states:

The signal will, for sure be very weak – Schiaparelli was never designed to transmit all the way to Earth. Therefore GMRT tracking of Schiaparelli is very much an experiment – a ‘nice-to-have’ to allow us to watch the descent in real time. [emphasis mine]

So this is not an intentional 'beacon' as outlined in the question (I think), but rather operators trying to find another source of information about their spacecraft by creating a 'beacon' from the signal. The UHF transmission was sending actual data to the listening orbiting assets, and the GMRT was just trying to catch the much stronger carrier frequency for doppler analysis.

I think it is important to note the distinction between the scenario described above and an explicit, intentional 'beacon' signal/device.

For MSL/Curiosity and Mars 2020/Perseverance I sense the same theme [3]:

the possibility exists to monitor the UHF signal from a large Earth-located ground station, such as the Green Bank radio telescope in West Virginia

[4]:

Will Armentrout, Green Bank Observatory project scientist for Perseverance [...] When asked what he will be doing for the 2021 Perseverance landing, Armentrout replied, “Supporting the GBT controls, praying we see that signal.” [emphasis mine]

Source [3] being the fantastic (and dare I say definitive source) DESCANSO article on MSL/Curiosity's, and by extension Perseverance's, Telecommunications System Design. Interestingly, searching the document for the words 'beacon' and 'heartbeat' return zero results. 'Tone' yields 54 results:

• Goldstone: 15 (DSN station)
• DOR tones: 1 (excluded from scope of question)
• Direct-To-Earth (DTE) "semaphores, the so-called multiple frequency shift keying (MFSK) tones"[3]: 36 (I think excluded from scope of question)

These DTE 'tones' are X-band [3]. They are not a beacon.

Are the transmitters [worse] at [higher frequencies]:

No, they seem very comparable, here is a selection of some I could find data sheets for (good luck comparing cost):

Unit: Frequency Band: Mass: Electrical Power: Size: Source:
T-748 High Data Rate Transmitter X & Ka < 5.4 lbs < 30 W 6.325” L x 7.1” W x 4.0” H [5]
Electra UHF Transceiver UHF 4.9 kg 68 W (Transceive), 18.4W (Receive only) 17.2 cm (W) x 21.9 cm (L) x 14.0 cm (H) [6]
(C/TT-510) Electra-Lite UHF 3 kg / 6.61 lbs (typical) 65 W (Transceive, typical), 15 W standby 6.35” W x 8” D x 4.11” H [7]
C/TT-520 S-Band Multimode Transponder S 6.5 lbs (2.95 kg) w/o diplexer, < 9.0 lb with diplexer < 8 W max (Receive only), < 80 W (Transceive) 8.6” L x 4.1” W x 4.6” H w/o diplexer, 9.5” L x 5.7” W x 4.6” H with diplexer [8]
High Rate Transmitter (HRT150) Ku 5 lbs max 47 W max 8.0”L x 6.6”W x 3.2”H max [9]
S-Band TDRSS / DSN Transponder S 10.7 lbs (4.9 kg) max 13.8 W (Receive only, nominal), 18 W (Transceive, nominal) 7.575”L x 8.92”W x 6.00”H [10]
Space Qualified Multi-Mode Standard Transponder (MST) S < 6.0 lbs < 10 W (Receive only), 44 - 46 W (Tranceive) 9.40” x 7.30” x 2.30” [11]
Small Deep-Space Transponder (SDST) X & Ka 7.0 lbs (3.2 kg) max 12.5 W (Receive only), 19.5 W (Transceive) 7.13”L x 6.55”W x 4.50”H [12]

How unusual is it for a Mars lander like Zhurong to have an X-band beacon:

As posed by the question I would say very unusual. In the dedicated beacon signal/device sense, not the X-band sense. The stated information attainable from a beacon-like signal is equally available from any signal from the spacecraft, so why not get that info in combination with telemetry?

Sources:

• Thank you for your answer! I want to object but it seems I can't :-) I'd like to "believe" in beacons so it will take some time getting used to the idea that they're not as much of a thing as I thought they were. Such a nicely written, carefully researched and thoroughly sourced answer. This gets a +n!
– uhoh
May 17 at 23:05
• @uhoh that okay, I still believe in the planet Pluto! ;) May 17 at 23:59