[5436436347]

Two things:

1. A GPS satellite is in a very high orbit compared to LEO, and so all stages were requested to burn to completion, to ensure the satellite has enough margin to reach that orbit.

2. The second stage ends at a ~800 km orbit, while the satellite itself needs to increase this to ~12,000 km (I'm rounding these numbers). The sat has to carry enough on-board propellant to raise it's orbit that much which necessarily increases it's wet-weight due to the rocket equation.

[the8472]

> while the satellite itself needs to increase this to ~12,000 km (I'm rounding these numbers). The sat has to carry enough on-board propellant to raise it's orbit that much which necessarily increases it's wet-weight due to the rocket equation.

Looks like they use a liquid fuel thruster lift it to its final orbit[0]. Some other missions have used ion engines for orbit raising[1]. I wonder why that wasn't incorporated into the GPS design.

[0] https://en.wikipedia.org/wiki/Lockheed_Martin_A2100 [1] https://space.skyrocket.de/doc_sdat/ses-12.htm

[pilsetnieks]

Ion engines still need propellant, you might be thinking of the so far unproven EM drive.

[elihu]

Ion engines need propellant, but they need a lot less of it to achieve a similar result as a traditional rocket engine.

[_0w8t]

Still SpaceX routinely recovers the first stage after launching geostationary communication satellites. I thought GPS should be lighter as they are closer to Earth and needs to send much less information in their signal, so the power requirements and size of Solar panels should be lower.

[inamberclad]

Looks like GPS-III 3,880 kg at launch. That's fairly hefty, but Falcon 9 has launched other, more massive satellites all the way to GTO (Geostationary Transfer Orbit).

The only information I can find on why this one was so energy intensive has to do with the high inclination the GPS satellite has to be in (55 degrees), which is pretty far off from inclination of the launch pad in Florida.

Wikipedia says this launch was originally planned on a Delta IV. That would make sense because it has higher specific impulse hydrogen engines. The Falcon does well in LEO, but improved Isp really helps as you go farther out.

Here's a video comparing ULA's rockets to SpaceX: https://www.youtube.com/watch?v=QoUtgWQk-Y0

[marvin]

This sattelite was some 8000kgs, vs. 4000-ish kgs for the previous-generation GPS satellites, if I remember correctly.

[Roritharr]

Any idea why their weight increased so much?

[nradov]

The latest generation of GPS satellites has more antennas and more powerful transmitters. This improves signal reception and reduces the impact of jamming.

[walrus01]

GPS as received by a small hockey-puck sized antenna is a VERY weak signal, like -140 to -150dB. It only works because the channel size is so narrow and the modulation is very rudimentary. Having a satellite with much larger photovoltaic arrays and really powerful transmitters is one method to overcome regional jamming.

[tgsovlerkhgsel]

I wouldn't be surprised if the satellites had additional, classified capabilities.

[InclinedPlane]

3. USAF requires the upper stage do a deorbit burn to avoid long lived high altitude space debris.