This hypothetical case of sending a large multimedia message instead of 8 bytes for latitude and longitude and 3 more bytes for the letters "SOS" doesn't seem sincere.

minor fyi. Had to look up how to get lat/long on an iphone without internet access. It’s in the default compass app.

Where? Mine has zero options now - just displays the compass, with the level in the middle.

Seems it's not available everywhere, for some reason.

> Note: Your coordinates and elevation may not be available in certain countries or regions.

https://support.apple.com/en-gb/guide/iphone/iph1ac0b663/ios

some reason = GNSS: https://medium.com/@ilyakorogodin/where-is-a-navigation-chip...

Article opens iPhones of several generations since iPhone 3G to discover how they do GNSS positioning.

Thanks, this article pointed me in the right direction. Coordinates were missing for me because locations services were disabled for the Compass app.

Google Maps and Apple Maps show it when you set a marker. You don‘t need internet.

I would be less worried about bytes and more about the general TCP handshake/acks making it through. You can easily add height, heart rate, and all other kinds of information before you come close to the network overhead. You could also gamble UDP+overhead, but I assume you want some kind of assurance that your SOS made it through.

You also need to be able to get a session within the beam. If you're stuck in the middle of a desert you'll be fine, but if you're within range of a minor town you may not be able to get any signal.

Not an expert, but my understanding is that SMS text messages don't rely on TCP or UDP. Instead, they use their own protocol called SMPP with an overhead of less than 100B.

This hypothetical case, on the other hand, only require two bytes of the reply, with letters "OK".

To achieve this kind of speed, you need at least around 600kbps at the barest minimum for TCP ACK packets (~50 bytes), assuming that downstream packets are full-frame 1500 bytes.

The tweet's 17 Mb/s is UDP down from satellite, so no ack replies from the phone, and 15% packet loss.

So it really doesn't say anything about upload speed.

That’s not really true since one ACK packet can acknowledge multiple RXed data packets.

The nuance of this news is that it is a for a standard consumer mobile device. It is not intended to replace coverage in areas where telcos exist, rather to supplement in times of spotty coverage or emergencies.

>@Elon: This service works in partnership with wireless providers, like what @SpaceX and @TMobile announced.

In your scenario you would likely be sending text/sms based communication as a primary priority (ie. coordinates, instructions) with MMS as a secondary luxury.

If you are permanently or frequently in an area without telco service you would want to make use of specialized hardware which is optimized for starlink communication. For example, here is video of Elon using starlink for video conferencing which demonstrates not only high upload speeds, but also quick swapping between satellites without disconnect.

https://youtu.be/tKqJ5-kkUGk?si=RVilCEke7J3zXoQK&t=367

A map for how to get out, or a video for first aid procedures, or a guide on what's safely edible around you, or other survival information until help can get there can make a big difference.

Those all could be unnecessary with advanced planning

Antennas couple the same both ways.

But power doesn’t, right? Happy to be corrected (please, educate me!) but I think this is asymmetric, in practice.

That does not guarantee two-way communication. Pick-up patterns, resonance and filtering, power and local environment makes SNR highly asymmetrical.

Imagine you focus a high gain antenna dish A at an isotropic antenna B. A will almost exclusively hear B, but B will hear things from all directions, making it harder to single out A. Now, A could use their higher gain to scream louder at B - but that's not always a viable option, and could just peak the frontend of B if done wrong.

Different antennas and radio circuits have different selectivity, dictating how they react to nearby noise. A dedicated tranceiver station is often much more selective than consumer devices meant to handle many bands and protocols using a small set of multi-purpose antennas and with space requirements limiting room for radio frontends and filters.

The power rating of a dedicated transceiver station may also be quite a bit higher than that of a portable device, adding to the asymmetry.

There are also differences in atmospheric effects - primarily reflection and refraction - which for these wavelengths happen in the troposphere. With refraction happening closer to the ground (from the perspective of orbit), that makes for another SNR asymmetry.