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Radar presence: the door checks that a real person is standing there

Every GateStride door node includes a microwave radar presence sensor. It tells the node that a person is physically at the door and counts people passing through. It takes no pictures and cannot tell who anyone is, but it lets the node refuse an unlock when nobody is there.

By Tom Jin · Updated October 2026 · Early access

A phone that answers correctly is not the same as a person at the door

Phone-based and keyless entry systems share a weakness that car owners already know about. A keyless car checks that the right key fob answers. It does not check that the owner is standing next to the car. Thieves have used that gap with relay attacks: one person stands near the house where the fob sits on a hallway table, another stands by the car, and a pair of radios forwards the conversation between them. The car hears a valid answer and opens.

An office door that unlocks for a nearby phone has the same question to answer. Picture an employee eating lunch at a cafe down the street with the phone in a jacket. If someone could carry the radio conversation from that phone to the office door, a system that checks the phone alone would see a valid answer. We did not want a door that trusts a radio signal by itself. We wanted it to also know that a human being is standing at it.

What the radar sensor is

Each door node has Bluetooth receivers and a microwave radar presence sensor. The radar reads reflected microwave signals to tell whether a person is physically at the door, and it counts people as they pass through. It is a sensor, not a camera.

The radar produces no images. It cannot tell a manager from a visitor, read a face or follow someone around the building. It is non-identifying by design. The identity part of every unlock comes from the phone's cryptographic answer. The radar adds one fact: someone is here, right now.

Why an unlock needs both a phone and a person

When an enrolled phone comes near, the door node sends it a one-time challenge over Bluetooth. The phone signs that challenge with a private key created in its secure element (StrongBox on Android, Secure Enclave on iPhone). The key never leaves the phone, and each challenge is used once, so a recorded answer cannot be replayed later.

A relay attack is different from replay. It forwards the live conversation in real time, so the signature can be valid. That is where radar matters. The door node unlocks when two things are true at the same moment: a valid answer from an allowed phone, and a person present at the door. A forwarded radio signal cannot open a door with nobody at it. Every unlock is written to the hash-chained, tamper-evident audit log together with the presence reading, so an investigation can see what the node saw.

Honest about what it stops and what it does not

The presence check raises the bar. An attacker can no longer stand at a distance and open an empty door by forwarding radio signals. No security control removes every attack, and we will not claim that this one does. If an attacker is standing at the door while a partner relays the signal of an employee's phone from somewhere else, the radar sees a person and the phone answer is valid. Other layers still apply: the person must be allowed through that door at that time, the unlock is logged with its presence reading, and your existing cameras and procedures still matter. Removing a lost phone in the center reaches every door node in about a second (see instant revoke).

For more on the cryptography and the on-premises design, read the security page.

Inside or outside: extra Bluetooth receivers for exits

A door has two sides, and a phone near the door could be on either one. Each node's Bluetooth receivers help it judge where a phone is, and extra receivers improve accuracy in telling which side of the door a phone is on. That is what makes exit identification reliable: when an enrolled phone passes from inside to outside through a free-egress door, the node can record who left without anyone tapping anything.

Free egress stays with your certified system. GateStride never requires anything to exit, and exit records are a convenience on top of it. See know who left for live who-is-inside, attendance export and roll call.

Privacy at the door

Because radar is non-identifying, the default GateStride setup records device credentials plus a presence reading, not pictures of people. An optional door camera records like ordinary CCTV and stays on site, and facial recognition is optional and off by default. All data stays on your premises. Some states require employers to tell staff about electronic monitoring, so check your state's notice rules before rollout. The privacy by design page covers the details.

How the presence check fits into an unlock

1. An enrolled phone comes near a covered door. The door node's Bluetooth receivers pick it up.
2. The node sends a one-time challenge, and the phone signs it with the key in its secure element.
3. The radar presence sensor checks whether a person is physically at the door at that moment.
4. If the signature is valid, the person is allowed through that door at that time, and someone is present, the node unlocks through your existing access system. If nobody is present, it does not unlock.
5. The decision is written to the tamper-evident audit log with the presence reading.

What it does not do

• It raises the bar against relay attacks. It does not rule them out, for example when an attacker stands at the door while an accomplice relays a real phone.
• Radar does not identify anyone. It knows a person is there, not who the person is.
• It produces no images and is not a camera. If you want video, an optional door camera records like ordinary CCTV, kept on site.
• It is not a life-safety, alarm or emergency-notification system, and it never controls free egress.

Questions

Does the radar take pictures or video?

No. It produces no images and cannot identify who a person is. It tells the door node that someone is at the door and counts people passing through.

What is a relay attack, in plain terms?

Two people with radios forward the live signal between a phone or key fob and the door, so the door thinks the device is nearby when it is not. Car thieves have used this against keyless cars.

Can a relay attack open a GateStride door?

Forwarding a phone's signal to an empty door will not open it, because an unlock also needs a person present. An attacker standing at the door with a working relay is a harder case, so we describe the presence check as raising the bar, not as a complete defense.

Where are the presence readings stored?

Every unlock is logged with its presence reading in the tamper-evident audit log on your premises. There is no GateStride cloud.

How does the node know which side of the door my phone is on?

The node uses its Bluetooth receivers, and extra receivers improve accuracy in telling the two sides apart. That is what makes exit identification reliable.

Do I need to tell employees about the radar?

Some states require employers to notify staff about electronic monitoring. Check your state's notice rules. We do not provide legal advice.

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