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This answer to the question How often do reaction wheels require desaturation, normally? says:
One practical example I know of is for a 6U in GEO that is always sun pointed, there's a thruster desaturation once a week. (not enough magnetic field to use magnetorquers)
I can't tell if this is a simulation or real data from an actual cubesat in GEO. So I thought I'd start by simply asking Have there ever been cubesats in GEO?
Edited update:
Have there ever been cubesats in GEO?
Yes, 5 in the period 2021-2023.
As of 2023, there are two operational cubesats in GEO, and three that were operational in GEO:
Ascent, launched 2021 completed its mission in October 2022.
Three cubesats: Lockheed Martin’s In-space Upgrade Satellite System, aka LINUSS-A1 and LINUSS-A2, and Millenium Space Systems TETRA-1, launched November 1 2022, deployed January-February 2023. Both LINUSS cubesats completed their mission April 2023.
TETRA-1 remains in operation.
GS-1, launched June 2023, and its payloads (Nusantara H-1A, OrbitGuard-1) are still operational.
Future:
6 cubesats of the SunRISE mission will launch to GEO at end of 2024.
8 more lined up for sometime up to 2025.
TL;DR:
https://space.skyrocket.de/doc_sdat/ascent.htm
Ascent is a mission to demonstrate various CubeSat operations in geostationary orbit (GEO).
AFRL awarded a second contract for BCT to build, test and deliver a 12U-CubeSat bus for the Ascent mission. AFRL provided and integrated the Ascent payload with BCT’s 12U spacecraft bus.
Ascent’s mission has been to evaluate the performance of COTS technology, in the GEO space environment, where satellites match the Earth’s rotation.
Ascent completed its mission in October 2022.
https://www.nanosats.eu/sat/ascent
(From memory I noted that with the ASCENT launch it is possible that some other military cubesats or small satellites were also launched that had less info on them)
https://twitter.com/SpaceflightNow/status/1652903606159392768
The 55-pound (25-kilogram) Gravity Space GS-1 satellite separated from the Falcon Heavy. It's the first commercial CubeSat-based craft to fly in geostationary orbit.
GS-1 will secure orbital slot reservations and perform a rendezvous/docking experiment.
https://space.skyrocket.de/doc_sdat/g-space-1.htm
This is also providing hosting services for the payloads Nusantara H-1A, OrbitGuard-1.
Technology demonstrators in GEO for Rendezvous and Proximity Operations
The pair of satellites were initially 750 kilometers apart and by Feb. 28 had maneuvered within 400 meters of each other, proving out the algorithms that will allow for on-orbit upgrades and servicing
The two satellites are identical and for the demonstration one is acting as the servicing vehicle and the other as the resident space object,
The flight software aboard the satellites is being updated from the ground as lessons are learned
The intent is to show how small satellites can be used to upgrade constellations or provide life-extension services like refueling.
The mission will seek to “validate essential maneuvering capabilities for Lockheed Martin’s future space upgrade and servicing missions, as well as to showcase miniaturized space domain awareness capabilities,”
https://terranorbital.com/missions/linuss/
https://news.lockheedmartin.com/linuss-small-sats-mission
https://everydayastronaut.com/ussf-44-falcon-heavy/
https://www.teslarati.com/spacex-falcon-heavy-rocket-aces-geostationary-launch-dual-landing/
https://www.nanosats.eu/sat/linuss
Built for the United States Space Force, will test on-orbit maneuverability, as well as offer a training scenario for both new and experienced satellite operators, demonstrating small satellite capabilities with payloads for communications, reconnaissance, and space networking.
Use of small sats in a layered architecture spread across low-Earth orbit, medium-Earth orbit, and GEO will let the DoD quickly reconstitute capabilities should certain satellites become inoperative. For instance, loss of one SBIRS satellite in its five-satellite constellation could leave a major gap in missile-warning coverage. Conversely, loss of a few satellites in a larger constellation of dozens of satellites will only incrementally degrade capabilities and can be replaced in a faster fashion.
“Distributing the space architecture will help us build resiliency over time so that there’s not a single point of failure anywhere,” ... “We’re focused on making our systems and architectures more resilient and robust so if something happens, the Space Force can still perform the mission without fail.”
https://space.skyrocket.de/doc_sdat/tetra-1.htm
https://breakingdefense.com/2022/11/u-s-space-force-tetra-1-launches-to-geo/
https://space.skyrocket.de/doc_sdat/alpine.htm
So currently it looks like there are two operational cubesats in GEO.
Air Force Research Laboratory Technology Demonstration Orbiters used for testing orbital debris tracking technologies, through optical calibration and satellite laser ranging. (Subsequent Technology Demonstration Orbiters 3 and 4, launched in 2021 went to MEO)
AFRL TDO, launched August 2019, re-entry 2022.
AFRL TDO 2, launched March 2020, re-entry 2022.
https://space.skyrocket.de/doc_sdat/tdo.htm
https://adaptivelaunch.com/2020/03/20/launch-of-tdo-2-aehf-6-primary-satellite/
https://spaceflightnow.com/2020/03/25/space-forces-first-launch-scheduled-for-thursday/
https://science.nasa.gov/resource/nasa-facts-sunrise-six-small-satellites-to-study-the-sun/
USUVL and WL2XOU went to super synchronous orbits, these rideshared the LINUSS cubesat launch in 2022.
Along with TDO 3 and 4, Trisat-R (2022) went to MEO.
https://fcc.report/IBFS/SES-STA-20211130-01849/13754719.pdf
https://forum.nasaspaceflight.com/index.php?topic=47474.msg2315959#msg2315959
https://space.skyrocket.de/doc_sdat/trisat-r.htm
https://forum.nasaspaceflight.com/index.php?topic=47474.msg2438074#msg2438074
tl;dr As of June 2019, there are zero CubeSats in GEO
The Union of Concerned Scientists has a great database of satellites orbiting the Earth, the smallest satellite that they have in GEO orbit is the S5 smallsat launched by the ARFL, it has a launch weight of 60kg definitely in the smallsat range but much bigger than the largest CubeSats.
Bonus! Here are some histograms of the mass of satellites in different orbits:
