Why can an IMT message take several minutes?
Series Building with the Iridium 9704 Part 9 of 10
In short
Session setup, the transfer itself and waiting for a satellite; 100 KB takes about 3 minutes.
Where the time goes
| Stage | Roughly how long | Depends on |
|---|---|---|
| Session setup | About 5 seconds on average, commonly 5–12 seconds | Signal strength, how much sky the antenna sees |
| Transfer | Longer for larger messages; about 3 minutes for 100 KB on a rooftop | Message size, signal |
| Waiting for a satellite (downlink MT) | Almost none on a rooftop; about 44 minutes with half the sky in view | View of the sky, whether the device is awake |
How long each size takes
On 25 October 2026, N3D ran a 9704 Launch Pad for 4 hours on a rooftop with the whole sky in view, rotating 1, 30, 50, 70 and 100 KB round trips. 54 of 55 messages came back complete and identical byte for byte:
| Size | Download (MT): average (min–max) | Upload (MO): average (min–max) | Round trip, median |
|---|---|---|---|
| 1 KB | About 2 s | About 4 s (2–4) | 0.6 min |
| 30 KB | 60 s (54–112) | 58 s (55–62) | 2.4 min |
| 50 KB | 91 s (90–95) | 96 s (92–114) | 3.6 min |
| 70 KB | 131 s (124–178) | 137 s (128–171) | 4.8 min |
| 100 KB | 183 s (178–205) | 200 s (180–270) | 6.9 min |
The round-trip median includes platform scheduling and ring time.
Transfer time is proportional to size: about 4.4 kbps down and 4.0 kbps up, with a fixed cost of only about 2 seconds per message. The longer outliers are messages where the signal worsened midway or the 9704 restarted and began again.
Cold starts: idle too long and the session closes
If nothing is sent for about 15 seconds, the satellite session closes and the next message has to set up a new one. When sending several messages, send them together rather than spread out. Using a ring alert to tell the device to collect an MT adds about 2 seconds.
Why downlink can take longer
A downlink message (MT) waits until the device answers the ring and has a satellite in view. In N3D's tests, every rooftop message was collected on the first ring; at a window with half the sky, each message queued at Iridium for about 44 minutes. The delay was in the queue, not the transfer.
How to make it faster
- Place the antenna where it sees the whole sky; this matters more than anything else.
- Send important status as small messages first, photos and logs afterwards.
- Compress before sending; smaller data is faster and cheaper.
- Send several messages together to avoid a cold start for each.
- Track each message's progress on the server instead of treating "not arrived after a few minutes" as a failure and resending. IMT CC updates status from Iridium's reports.
Parameters and sources
- Session setup about 5 s, observed 5–12 s, session closes after about 15 s idle, ring alert adds about 2 s: Ground Control, "RockBLOCK 9704 — IMT" documentation.
- Transfer times by size: N3D rooftop test, 25 Oct 2026, 06:43–11:04, Iridium 9704 Launch Pad (module firmware SX-1.6.0) on battery. Download MT is from the board starting to receive until complete; upload MO is from Iridium's recorded transfer start to end.
- Window queueing: N3D test, 9 Oct 2026, with about half the sky in view.
- Speeds: linear trend over the transfer times and sizes of 54 messages.
How does this compare with SBD?
An SBD message is at most a few hundred bytes, so it never takes minutes to transfer; a 9704 message can be much larger, and large messages naturally take longer.
Can it be used for real-time commands?
Commands work, but allow for seconds to minutes of delay, plus the sleep interval if the device sleeps on a schedule.
All parts in this series
From developer kit to certified product: what an Iridium 9704 product needs, the limits of the developer kit, why certification matters, how to choose hardware, where it fits, and measured timing and power. Ten parts, in order, each readable on its own.
- What do you need to build an Iridium 9704 product?
- What problems does IMT CC actually solve?
- What are the limits of the Iridium 9704 Launch Pad?
- Why do 9704 products need certification, and what does it cover?
- Who is the RockBLOCK 9704 for?
- Launch Pad, N-9704, RockBLOCK 9704 or your own design: which should you choose?
- Where can the IMT 9704 be used? Mountain rescue, maritime and monitoring
- How should power be designed for the 9704?
- Why can an IMT message take several minutes?
- How does the Iridium 9704 compare with Starlink and OneWeb?
Related notes
Related hardware
Related services
Updated 2026-10-27
N3D supplies satellite communication for alpine and industrial use. See services