UHM (You Heard Me)

UHM stands for You Heard Me. It is a way to cut down unnecessary transmissions on crowded packet digipeater satellites — the kind of QRM that makes a busy pass almost unusable for everyone.

If you have ever worked IO-117 (GreenCube), the ISS packet digipeater, or a similar bird, you know the pattern: you call someone, you are not sure they copied you, so you call again. And again. Fifty stations doing that on one pass adds up to hundreds of extra digipeats fighting for a limited transponder.

UHM gives you a simple answer: did the station I called actually hear me? When you know that, you can stop repeating when you do not need to — and everyone on the pass benefits.

The problem UHM solves

Digipeater satellites have a finite amount of airtime. On a popular pass there may be dozens of stations on frequency at once. Between telemetry bursts you might only see one or two digipeats come through.

Without feedback, every operator tends to transmit until something happens. That is rational for one station, but disastrous for the pass as a whole. Each extra transmission you did not need is QRM for someone else trying to complete a real contact.

UHM does not replace operating skill or courtesy. It gives you timely information so you can choose not to add to the pile-up when your signal already got through — or to wait intelligently when it did not.

How UHM works

All real QSO traffic still happens over RF, station to station via the satellite. Nothing in UHM replaces the radio link.

What UHM adds is a confirmation service run by OscarWatch:

Your satellite app decodes a packet that involves you and another station.
The app sends a short hearing report to OscarWatch (who transmitted, who received, which satellite, when — and optionally the decoded message text).
When the other station’s app does the same in the reverse direction, OscarWatch can confirm mutual copy within a short time window (about one minute).
Your app shows you whether the station you called heard you — so you know whether to transmit again.

Some apps can also confirm a specific decoded message (not only that your callsign pair appeared recently). That is optional: clients that omit the message text keep the simpler callsign-based check.

No station-to-station chat happens over the internet. The internet only answers: “Yes, they decoded your packet” or “No, not yet.”

Think of it as something like PSK Reporter for FT8, but aimed at real-time satellite packet work during a pass.

Which satellites?

UHM only makes sense where you are decoding packet traffic through an onboard digipeater.

Works well	Not suitable
IO-117 / GreenCube	SO-50 and other FM voice birds
ISS (packet / APRS digipeater)	Linear SSB/FM voice transponders
Other packet digipeater satellites	Anything without packet frames

Your app should only use UHM when you are on a packet pass, not for every satellite in the catalogue.

What you see in your app

Different programs present UHM differently — see Apps that support UHM below for known clients.

In general, compatible apps use simple cues:

Cue	Meaning
Heard / checkmark	The station you called copied your transmission. You do not need to call them again right away.
Not heard / stop	Your packet was digipeated, but the other station did not copy you (yet). See Using a “not heard” result wisely.
No symbol	The other station is not participating in UHM. You have no confirmation either way — same as before UHM existed.
Unknown on final 73	You sent a closing transmission to a non-UHM station. You cannot confirm they got it through UHM.

Participation is opt in. If you do not want your app to send hearing reports, you can disable that in your client’s settings.

Using a “not heard” result wisely

This is the most important operating habit UHM is trying to teach.

A “not heard” result does not mean “transmit again immediately.”

It usually means the station you called is busy — finishing another QSO, calling someone else, or dealing with their own pile-up. If you keep transmitting on top of them, everyone loses.

When you are not copied:

Pause. Do not instinctively key up again.
Assume they are transmitting to someone else.
Wait until you see their call again in the pass traffic, or wait a sensible interval before one careful retry.

A “not heard” result is valuable information. Used well, it reduces QRM. Used poorly — as a trigger to spam the transponder — it makes things worse.

When you are copied, the opposite applies: stop repeating to that station for a while. If one operator avoids five needless transmissions per pass and sixty stations do the same, that is hundreds fewer digipeats on a single pass.

Who can use UHM?

As an operator, you need:

A satellite app that supports UHM and can report decoded frames to OscarWatch.
An OscarWatch account with your callsign set on your profile.
UHM reporting enabled in the app (and internet access during the pass, if the app requires it).

As a developer building or integrating a client, see the UHM API documentation for authentication, endpoints, and matching rules.

Apps that support UHM

UHM needs a client application that decodes packet frames and reports hearing results to OscarWatch. These apps are known to support UHM today:

GreenCube Terminal (GC-T)

By OZ9AAR. A terminal program for IO-117 / GreenCube with built-in UHM support — checkmarks, stop signs, and the Calling Me window described in the operator PDF.

GreenCube Terminal program — download and setup
UHM in GreenCube Terminal (PDF) — plain-language guide to symbols, behaviour on a busy pass, and rover tips

More clients may be added here as they integrate with the OscarWatch UHM API.

Privacy and what OscarWatch stores

UHM stores hearing reports — which callsigns appeared in which decoded frames, on which satellite, and when — so mutual confirmation can work during the pass. Clients may also send the decoded message text; when they do, that text (and a hash used for matching) is stored with the report. Reports are kept for a short period (about seven days) for operations and audit, then pruned.

OscarWatch does not replace your logbook. A mutual UHM match means both stations’ apps reported decoding each other’s traffic in the match window. It is a real-time operating aid, not a substitute for logging a QSO by your usual rules.