feat: add NR NTN access detection

Detect Non-Terrestrial Network access by checking for the
NRNTN-TAI-Information protocol extension (ID 287 per 3GPP
TS 38.413) in UserLocationInformationNR received via NGAP,
and track the result in a new NtnAccessInfo struct on AmfUe
and RanUe.

This is detection-only groundwork. No protocol behavior
changes: nothing reads the flag yet. Future patches can
extend NtnAccessInfo with satellite backhaul category,
GEO satellite ID, and NTN TAI list as those fields become
available in omec-project/openapi and omec-project/ngap.

Signed-off-by: Vinod Patmanathan <vinod.patmanathan@forsway.com>

Author: Vinod Patmanathan

context/amf_ue.go

@@ -101,6 +101,7 @@ type AmfUe struct {
 	EventSubscriptionsInfo map[string]*AmfUeEventSubscription `json:"eventSubscriptionInfo,omitempty"`
 	/* User Location*/
 	RatType                  models.RatType      `json:"ratType,omitempty"`
+	NtnAccess                *NtnAccessInfo      `json:"ntnAccess,omitempty"`
 	Location                 models.UserLocation `json:"location,omitempty"`
 	Tai                      models.Tai          `json:"tai,omitempty"`
 	LocationChanged          bool                `json:"locationChanged,omitempty"`

context/ntn.go

@@ -0,0 +1,52 @@
+// SPDX-FileCopyrightText: 2026 Forsway Scandinavia AB
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package context
+
+// NtnAccessInfo captures Non-Terrestrial Network access properties for a
+// UE, as derived from NGAP signaling. Only NTN-access presence is tracked
+// today; future 3GPP-defined fields (satelliteBackhaulCategory,
+// geoSatelliteId, NTN TAI list) can be added without reshaping AmfUe or
+// RanUe.
+type NtnAccessInfo struct {
+	// Detected is set true the first time the NRNTN-TAI-Information
+	// protocol extension is observed in a UserLocationInformationNR for
+	// this UE. It is sticky: once set, it is not cleared while the UE
+	// context lives.
+	//
+	// The extension is PRESENCE optional per 3GPP TS 38.413, and gNBs
+	// include it at their discretion — typically on context-establishing
+	// events (initial UE message, handover) but often not on routine
+	// uplink/location reports. Treating an absent extension as evidence
+	// of "not NTN" would cause IsNtn() to flap on routine NGAP traffic
+	// for a UE that genuinely is on NTN; sticky-on avoids that.
+	Detected bool `json:"detected"`
+}
+
+// IsNtn reports whether NTN access has been observed for this UE during
+// the current UE context. The flag is sticky for the session — see
+// NtnAccessInfo.Detected for the rationale.
+func (ue *AmfUe) IsNtn() bool {
+	return ue.NtnAccess != nil && ue.NtnAccess.Detected
+}
+
+// updateNtnAccess records observed NTN access on this RanUe.
+//
+// detected==false is a no-op: the NRNTN-TAI-Information extension is
+// PRESENCE optional per 3GPP TS 38.413, so an absent extension does not
+// imply the UE has left NTN access. The flag is sticky for the lifetime
+// of the UE context.
+func (ranUe *RanUe) updateNtnAccess(detected bool) {
+	if !detected {
+		return
+	}
+	if ranUe.NtnAccess != nil && ranUe.NtnAccess.Detected {
+		return
+	}
+	if ranUe.NtnAccess == nil {
+		ranUe.NtnAccess = &NtnAccessInfo{}
+	}
+	ranUe.NtnAccess.Detected = true
+	ranUe.Log.Debugf("NR NTN access detected (NRNTN-TAI-Information extension present)")
+}

context/ran_ue.go

@@ -15,6 +15,7 @@ import (
 
 	"github.com/mohae/deepcopy"
 	"github.com/omec-project/amf/logger"
+	"github.com/omec-project/ngap"
 	"github.com/omec-project/ngap/ngapConvert"
 	"github.com/omec-project/ngap/ngapType"
 	"github.com/omec-project/openapi/models"
@@ -46,8 +47,9 @@ type RanUe struct {
 	TargetUe            *RanUe  `json:"-"`
 
 	/* UserLocation*/
-	Tai      models.Tai
-	Location models.UserLocation
+	Tai       models.Tai
+	Location  models.UserLocation
+	NtnAccess *NtnAccessInfo `json:"-"`
 	/* context about udm */
 	SupportVoPSn3gpp  bool       `json:"-"`
 	SupportVoPS       bool       `json:"-"`
@@ -244,12 +246,21 @@ func (ranUe *RanUe) UpdateLocation(userLocationInformation *ngapType.UserLocatio
 		if locationInfoNR.TimeStamp != nil {
 			ranUe.Location.NrLocation.AgeOfLocationInformation = ngapConvert.TimeStampToInt32(locationInfoNR.TimeStamp.Value)
 		}
+
+		// NTN detection runs only after the NR location has parsed cleanly,
+		// so a malformed message (early return above) does not desync NTN
+		// state from the stored location/Tai.
+		ranUe.updateNtnAccess(ngap.HasUserLocationInformationNRExtension(locationInfoNR, ngapType.ProtocolIEIDNRNTNTAIInformation))
+
 		if ranUe.AmfUe != nil {
 			if ranUe.AmfUe.Tai != ranUe.Tai {
 				ranUe.AmfUe.LocationChanged = true
 			}
 			ranUe.AmfUe.Location = deepcopy.Copy(ranUe.Location).(models.UserLocation)
 			ranUe.AmfUe.Tai = deepcopy.Copy(*ranUe.AmfUe.Location.NrLocation.Tai).(models.Tai)
+			if ranUe.NtnAccess != nil {
+				ranUe.AmfUe.NtnAccess = deepcopy.Copy(ranUe.NtnAccess).(*NtnAccessInfo)
+			}
 		}
 	case ngapType.UserLocationInformationPresentUserLocationInformationN3IWF:
 		locationInfoN3IWF := userLocationInformation.UserLocationInformationN3IWF

context/ran_ue_test.go

@@ -0,0 +1,85 @@
+// SPDX-FileCopyrightText: 2026 Forsway Scandinavia AB
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package context
+
+import (
+	"testing"
+
+	"go.uber.org/zap"
+)
+
+func TestRanUeUpdateNtnAccess(t *testing.T) {
+	tests := []struct {
+		name         string
+		initial      *NtnAccessInfo
+		detected     bool
+		wantPresent  bool
+		wantDetected bool
+	}{
+		{
+			name:        "nil + not-detected stays nil",
+			initial:     nil,
+			detected:    false,
+			wantPresent: false,
+		},
+		{
+			name:         "nil + detected allocates true",
+			initial:      nil,
+			detected:     true,
+			wantPresent:  true,
+			wantDetected: true,
+		},
+		{
+			name:         "detected + detected stays true",
+			initial:      &NtnAccessInfo{Detected: true},
+			detected:     true,
+			wantPresent:  true,
+			wantDetected: true,
+		},
+		{
+			name:         "detected + not-detected stays true (sticky)",
+			initial:      &NtnAccessInfo{Detected: true},
+			detected:     false,
+			wantPresent:  true,
+			wantDetected: true,
+		},
+		{
+			name:         "not-detected + detected sets true",
+			initial:      &NtnAccessInfo{Detected: false},
+			detected:     true,
+			wantPresent:  true,
+			wantDetected: true,
+		},
+		{
+			// Unreachable in production (we never allocate with Detected=false),
+			// but documents the function's contract: sticky means a !detected
+			// signal never modifies state.
+			name:         "not-detected + not-detected stays false",
+			initial:      &NtnAccessInfo{Detected: false},
+			detected:     false,
+			wantPresent:  true,
+			wantDetected: false,
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			ranUe := &RanUe{
+				NtnAccess: tc.initial,
+				Log:       zap.NewNop().Sugar(),
+			}
+			ranUe.updateNtnAccess(tc.detected)
+
+			if got := ranUe.NtnAccess != nil; got != tc.wantPresent {
+				t.Fatalf("NtnAccess presence: got %v, want %v", got, tc.wantPresent)
+			}
+			if !tc.wantPresent {
+				return
+			}
+			if ranUe.NtnAccess.Detected != tc.wantDetected {
+				t.Errorf("Detected = %v, want %v", ranUe.NtnAccess.Detected, tc.wantDetected)
+			}
+		})
+	}
+}