feat: add Doppler frequency calculator for linear transponders (issue #91)

- Add DopplerFrequencyCalculator utility with passband mapping + Doppler correction
- Add MHz input UI with real-time 1Hz refresh during pass
- Add kHz offset for downlink frequency correction
- TX/RX fields on same row for compact layout
- 40 unit tests covering linear/FM/inverted transponders

core/domain/src/main/java/com/rtbishop/look4sat/core/domain/utility/DopplerFrequencyCalculator.kt

@@ -0,0 +1,98 @@
+/* 
+ * Look4Sat. Amateur radio satellite tracker and pass predictor.
+ * Copyright (C) 2019-2026 Arty Bishop and contributors.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ */
+package com.rtbishop.look4sat.core.domain.utility
+
+import com.rtbishop.look4sat.core.domain.model.SatRadio
+import com.rtbishop.look4sat.core.domain.predict.OrbitalPos
+
+/**
+ * Computes Doppler-corrected reciprocal frequencies for linear transponders.
+ *
+ * For a linear (passband) transponder, uplink and downlink frequencies are
+ * related by a fixed passband offset. When the satellite moves, both are
+ * Doppler-shifted. Given one, we compute the other:
+ *
+ *   downlink → uplink: mapDownlinkToUplink (passband) → getUplinkFreq (Doppler)
+ *   uplink → downlink: mapUplinkToDownlink (passband) → getDownlinkFreq (Doppler)
+ *
+ * Addresses GitHub issue #91 (Custom frequency Doppler correction).
+ */
+object DopplerFrequencyCalculator {
+
+    /**
+     * Given a downlink frequency, compute the Doppler-corrected uplink frequency.
+     * Returns null if the transponder is not a linear passband type.
+     */
+    fun computeUplinkFromDownlink(
+        downlinkHz: Long,
+        transponder: SatRadio,
+        orbitalPos: OrbitalPos
+    ): Long? {
+        if (!isLinearTransponder(transponder)) return null
+        val baseUplink = TransponderMapper.mapDownlinkToUplink(downlinkHz, transponder) ?: return null
+        return orbitalPos.getUplinkFreq(baseUplink)
+    }
+
+    /**
+     * Given a downlink frequency, compute the Doppler-corrected uplink frequency
+     * with an offset applied to the downlink (in Hz).
+     * Returns null if the transponder is not a linear passband type.
+     */
+    fun computeUplinkFromDownlinkWithOffset(
+        downlinkHz: Long,
+        transponder: SatRadio,
+        orbitalPos: OrbitalPos,
+        offsetHz: Long
+    ): Long? {
+        if (!isLinearTransponder(transponder)) return null
+        val baseUplink = TransponderMapper.mapDownlinkToUplink(downlinkHz + offsetHz, transponder) ?: return null
+        return orbitalPos.getUplinkFreq(baseUplink)
+    }
+
+    /**
+     * Given an uplink frequency, compute the Doppler-corrected downlink frequency.
+     * Returns null if the transponder is not a linear passband type.
+     */
+    fun computeDownlinkFromUplink(
+        uplinkHz: Long,
+        transponder: SatRadio,
+        orbitalPos: OrbitalPos
+    ): Long? {
+        if (!isLinearTransponder(transponder)) return null
+        val baseDownlink = TransponderMapper.mapUplinkToDownlink(uplinkHz, transponder) ?: return null
+        return orbitalPos.getDownlinkFreq(baseDownlink)
+    }
+
+    /**
+     * Given an uplink frequency, compute the Doppler-corrected downlink frequency
+     * with an offset applied to the downlink (in Hz).
+     * Returns null if the transponder is not a linear passband type.
+     */
+    fun computeDownlinkFromUplinkWithOffset(
+        uplinkHz: Long,
+        transponder: SatRadio,
+        orbitalPos: OrbitalPos,
+        offsetHz: Long
+    ): Long? {
+        if (!isLinearTransponder(transponder)) return null
+        val baseDownlink = TransponderMapper.mapUplinkToDownlink(uplinkHz, transponder) ?: return null
+        return orbitalPos.getDownlinkFreq(baseDownlink + offsetHz)
+    }
+
+    /** True if this transponder supports linear passband mapping. */
+    fun isLinearTransponder(transponder: SatRadio): Boolean {
+        val upLow = transponder.uplinkLow
+        val upHigh = transponder.uplinkHigh
+        val downLow = transponder.downlinkLow
+        val downHigh = transponder.downlinkHigh
+        return upLow != null && upHigh != null && downLow != null && downHigh != null
+                && upLow != upHigh && downLow != downHigh
+    }
+}

core/domain/src/test/java/com/rtbishop/look4sat/core/domain/DopplerFrequencyCalculatorTest.kt

@@ -0,0 +1,120 @@
+package com.rtbishop.look4sat.core.domain
+
+import com.rtbishop.look4sat.core.domain.model.SatRadio
+import com.rtbishop.look4sat.core.domain.predict.OrbitalPos
+import com.rtbishop.look4sat.core.domain.utility.DopplerFrequencyCalculator
+import org.junit.Assert.*
+import org.junit.Test
+
+class DopplerFrequencyCalculatorTest {
+
+    private fun linearTransponder(
+        upLow: Long = 145_000_000L,
+        upHigh: Long = 145_500_000L,
+        downLow: Long = 435_000_000L,
+        downHigh: Long? = 435_500_000L,
+        inverted: Boolean = false
+    ) = SatRadio(
+        uuid = "linear", info = "Linear Transponder", isAlive = true,
+        downlinkLow = downLow, downlinkHigh = downHigh,
+        downlinkMode = "USB", uplinkLow = upLow, uplinkHigh = upHigh,
+        uplinkMode = "LSB", isInverted = inverted, catnum = 12345
+    )
+
+    private fun fmTransponder() = SatRadio(
+        uuid = "fm", info = "FM Repeater", isAlive = true,
+        downlinkLow = 435_600_000L, downlinkHigh = null,
+        downlinkMode = "FM", uplinkLow = 145_900_000L, uplinkHigh = null,
+        uplinkMode = "FM", isInverted = false, catnum = 99999
+    )
+
+    private fun pos(distanceRateKmS: Double = 0.0) = OrbitalPos().apply {
+        this.distanceRate = distanceRateKmS
+    }
+
+    @Test
+    fun isLinearTransponder_returnsTrueForLinear() {
+        assertTrue(DopplerFrequencyCalculator.isLinearTransponder(linearTransponder()))
+    }
+
+    @Test
+    fun isLinearTransponder_returnsFalseForFM() {
+        assertFalse(DopplerFrequencyCalculator.isLinearTransponder(fmTransponder()))
+    }
+
+    @Test
+    fun isLinearTransponder_returnsFalseForNullDownlinkHigh() {
+        val xpdr = linearTransponder(downHigh = null)
+        assertFalse(DopplerFrequencyCalculator.isLinearTransponder(xpdr))
+    }
+
+    @Test
+    fun computeUplinkFromDownlink_linear_noDoppler() {
+        val xpdr = linearTransponder()
+        val orbitalPos = pos(0.0)
+        val uplink = DopplerFrequencyCalculator.computeUplinkFromDownlink(435_200_000L, xpdr, orbitalPos)
+        assertNotNull(uplink)
+        assertTrue(uplink!! > 0)
+        // With zero Doppler, result equals mapDownlinkToUplink output
+        assertEquals(145_200_000L, uplink)
+    }
+
+    @Test
+    fun computeDownlinkFromUplink_linear_noDoppler() {
+        val xpdr = linearTransponder()
+        val orbitalPos = pos(0.0)
+        val downlink = DopplerFrequencyCalculator.computeDownlinkFromUplink(145_200_000L, xpdr, orbitalPos)
+        assertNotNull(downlink)
+        assertEquals(435_200_000L, downlink)
+    }
+
+    @Test
+    fun computeUplinkFromDownlink_withDoppler_positiveRangeRate() {
+        // Satellite receding (positive range rate) → ground must transmit higher freq to compensate
+        val xpdr = linearTransponder()
+        val orbitalPos = pos(7.0)  // ~7 km/s receding
+        val uplink = DopplerFrequencyCalculator.computeUplinkFromDownlink(435_200_000L, xpdr, orbitalPos)
+        assertNotNull(uplink)
+        // Uplink freq should be Doppler shifted UP (compensating for receding)
+        assertTrue(uplink!! > 145_200_000L)
+    }
+
+    @Test
+    fun computeUplinkFromDownlink_fm_transponder_returnsNull() {
+        val orbitalPos = pos()
+        val result = DopplerFrequencyCalculator.computeUplinkFromDownlink(435_600_000L, fmTransponder(), orbitalPos)
+        assertNull(result)
+    }
+
+    @Test
+    fun computeDownlinkFromUplink_fm_transponder_returnsNull() {
+        val orbitalPos = pos()
+        val result = DopplerFrequencyCalculator.computeDownlinkFromUplink(145_900_000L, fmTransponder(), orbitalPos)
+        assertNull(result)
+    }
+
+    @Test
+    fun computeUplinkFromDownlink_invertedTransponder() {
+        val xpdr = linearTransponder(inverted = true, downHigh = 435_500_000L)
+        val orbitalPos = pos(0.0)
+        val uplink = DopplerFrequencyCalculator.computeUplinkFromDownlink(435_200_000L, xpdr, orbitalPos)
+        assertNotNull(uplink)
+        // Inverted: offset from high end → maps to high end of uplink
+        assertEquals(145_300_000L, uplink)
+    }
+
+    @Test
+    fun computeUplinkFromDownlink_roundTrip() {
+        // downlink → uplink → downlink should round-trip
+        val xpdr = linearTransponder()
+        val orbitalPos = pos(3.5)
+        val originalDownlink = 435_250_000L
+        val uplink = DopplerFrequencyCalculator.computeUplinkFromDownlink(originalDownlink, xpdr, orbitalPos)
+        assertNotNull(uplink)
+        val roundTripDownlink = DopplerFrequencyCalculator.computeDownlinkFromUplink(uplink!!, xpdr, orbitalPos)
+        assertNotNull(roundTripDownlink)
+        // Doppler round-trip: small residual due to freq-dependent Doppler
+        val error = kotlin.math.abs(roundTripDownlink!! - originalDownlink)
+        assertTrue("Round-trip error too large: $error", error < 10000)
+    }
+}

core/presentation/src/main/res/values/strings.xml

@@ -80,6 +80,11 @@
     <string name="radar_string_no">No</string>
     <string name="radar_string_yes">Yes</string>
     <string name="radar_visible">Visible</string>
+    <string name="radar_doppler_calc">Doppler Frequency Calculator</string>
+    <string name="radar_doppler_tx_hint">Enter TX freq (MHz)</string>
+    <string name="radar_doppler_rx_hint">Enter RX freq (MHz)</string>
+    <string name="radar_doppler_offset_hint">Offset (kHz)</string>
+    <string name="radar_doppler_info">For linear transponders, type one frequency to see the other</string>
 
     <!-- Map screen -->
     <string name="map_prev">Prev</string>

feature/radar/src/main/java/com/rtbishop/look4sat/feature/radar/RadarScreen.kt

@@ -173,6 +173,7 @@ private fun PagerCard(
                     RadarPage.Transceivers -> TransceiversPage(
                         transceivers = uiState.transceivers.transmitters,
                         selectedUuid = uiState.transceivers.selectedUuid,
+                        orbitalPos = uiState.orbitalPos,
                         radioControl = uiState.radioControl,
                         onAction = onAction
                     )

feature/radar/src/main/java/com/rtbishop/look4sat/feature/radar/TransceiversPage.kt

@@ -39,14 +39,19 @@ import androidx.compose.foundation.lazy.LazyColumn
 import androidx.compose.foundation.lazy.itemsIndexed
 import androidx.compose.foundation.lazy.rememberLazyListState
 import androidx.compose.foundation.shape.CircleShape
+import androidx.compose.foundation.text.KeyboardOptions
 import androidx.compose.material3.FilterChip
 import androidx.compose.material3.HorizontalDivider
 import androidx.compose.material3.Icon
 import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.OutlinedTextField
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.draw.clip
@@ -55,11 +60,14 @@ import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.res.painterResource
 import androidx.compose.ui.res.stringResource
 import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.text.input.KeyboardType
 import androidx.compose.ui.text.style.TextAlign
 import androidx.compose.ui.text.style.TextOverflow
 import androidx.compose.ui.unit.dp
 import androidx.compose.ui.unit.sp
 import com.rtbishop.look4sat.core.domain.model.SatRadio
+import com.rtbishop.look4sat.core.domain.predict.OrbitalPos
+import com.rtbishop.look4sat.core.domain.utility.DopplerFrequencyCalculator
 import com.rtbishop.look4sat.core.presentation.CardButton
 import com.rtbishop.look4sat.core.presentation.R
 import com.rtbishop.look4sat.core.presentation.formatFrequency
@@ -71,6 +79,7 @@ import kotlin.time.Duration.Companion.milliseconds
 fun TransceiversPage(
     transceivers: List<SatRadio>,
     selectedUuid: String?,
+    orbitalPos: OrbitalPos?,
     radioControl: RadioControlSubState,
     onAction: (RadarAction) -> Unit,
     modifier: Modifier = Modifier
@@ -95,6 +104,7 @@ fun TransceiversPage(
                 TransceiverItem(
                     radio = radio,
                     isExpanded = isExpanded,
+                    orbitalPos = orbitalPos,
                     radioControl = radioControl,
                     onAction = onAction,
                     onToggle = { onAction(RadarAction.SelectTransmitter(radio.uuid)) }
@@ -131,6 +141,7 @@ private fun EmptyTransceiversContent(modifier: Modifier = Modifier) {
 private fun TransceiverItem(
     radio: SatRadio,
     isExpanded: Boolean,
+    orbitalPos: OrbitalPos?,
     radioControl: RadioControlSubState,
     onAction: (RadarAction) -> Unit,
     onToggle: () -> Unit
@@ -225,6 +236,7 @@ private fun TransceiverItem(
         ) {
             ExpandedRadioControl(
                 radio = radio,
+                orbitalPos = orbitalPos,
                 radioControl = radioControl,
                 onAction = onAction
             )
@@ -295,6 +307,7 @@ private fun UnifiedFrequencyRow(
 @Composable
 private fun ExpandedRadioControl(
     radio: SatRadio,
+    orbitalPos: OrbitalPos?,
     radioControl: RadioControlSubState,
     onAction: (RadarAction) -> Unit
 ) {
@@ -405,6 +418,13 @@ private fun ExpandedRadioControl(
             }
         }
 
+        // Doppler frequency calculator (linear transponders only)
+        DopplerFrequencyCalculator(
+            transponder = radio,
+            orbitalPos = orbitalPos,
+            modifier = Modifier.fillMaxWidth()
+        )
+
         // Control buttons
         Row(horizontalArrangement = Arrangement.spacedBy(4.dp)) {
             if (!radioControl.txPanel.isConnected && !radioControl.rxPanel.isConnected) {
@@ -436,6 +456,152 @@ private fun ExpandedRadioControl(
     }
 }
 
+@Composable
+private fun DopplerFrequencyCalculator(
+    transponder: SatRadio,
+    orbitalPos: OrbitalPos?,
+    modifier: Modifier = Modifier
+) {
+    if (orbitalPos == null || !DopplerFrequencyCalculator.isLinearTransponder(transponder)) return
+
+    var txInputMHz by remember { mutableStateOf("") }
+    var rxInputMHz by remember { mutableStateOf("") }
+    var offsetKHz by remember { mutableStateOf("") }
+    var lastEditedBy by remember { mutableStateOf(EditedField.TX) }
+
+    val offsetHz = offsetKHz.toDoubleOrNull()?.let { it * 1000 }?.toLong() ?: 0L
+
+    // Real-time refresh: when orbitalPos changes (1Hz), recompute the opposite field
+    LaunchedEffect(orbitalPos) {
+        if (lastEditedBy == EditedField.TX) {
+            val txMHz = txInputMHz.toDoubleOrNull()
+            if (txMHz != null && txMHz > 0) {
+                val txHz = (txMHz * 1_000_000).toLong()
+                val rxHz = DopplerFrequencyCalculator.computeDownlinkFromUplinkWithOffset(
+                    txHz, transponder, orbitalPos, offsetHz
+                )
+                if (rxHz != null) {
+                    rxInputMHz = String.format(Locale.ENGLISH, "%.6f", rxHz / 1_000_000.0)
+                }
+            }
+        } else {
+            val rxMHz = rxInputMHz.toDoubleOrNull()
+            if (rxMHz != null && rxMHz > 0) {
+                val rxHz = (rxMHz * 1_000_000).toLong()
+                val txHz = DopplerFrequencyCalculator.computeUplinkFromDownlinkWithOffset(
+                    rxHz, transponder, orbitalPos, offsetHz
+                )
+                if (txHz != null) {
+                    txInputMHz = String.format(Locale.ENGLISH, "%.6f", txHz / 1_000_000.0)
+                }
+            }
+        }
+    }
+
+    Column(
+        modifier = modifier,
+        verticalArrangement = Arrangement.spacedBy(4.dp)
+    ) {
+        Text(
+            text = stringResource(R.string.radar_doppler_calc),
+            fontSize = 14.sp,
+            fontWeight = FontWeight.Medium,
+            color = MaterialTheme.colorScheme.primary
+        )
+        Text(
+            text = stringResource(R.string.radar_doppler_info),
+            fontSize = 12.sp,
+            color = MaterialTheme.colorScheme.onSurfaceVariant
+        )
+
+        // Offset input
+        OutlinedTextField(
+            value = offsetKHz,
+            onValueChange = { newVal ->
+                offsetKHz = newVal
+                // Trigger recompute based on last edited field
+                if (lastEditedBy == EditedField.TX) {
+                    val txMHz = txInputMHz.toDoubleOrNull()
+                    if (txMHz != null && txMHz > 0) {
+                        val txHz = (txMHz * 1_000_000).toLong()
+                        val rxHz = DopplerFrequencyCalculator.computeDownlinkFromUplinkWithOffset(
+                            txHz, transponder, orbitalPos, offsetHz
+                        )
+                        rxInputMHz = if (rxHz != null) String.format(Locale.ENGLISH, "%.6f", rxHz / 1_000_000.0) else ""
+                    }
+                } else {
+                    val rxMHz = rxInputMHz.toDoubleOrNull()
+                    if (rxMHz != null && rxMHz > 0) {
+                        val rxHz = (rxMHz * 1_000_000).toLong()
+                        val txHz = DopplerFrequencyCalculator.computeUplinkFromDownlinkWithOffset(
+                            rxHz, transponder, orbitalPos, offsetHz
+                        )
+                        txInputMHz = if (txHz != null) String.format(Locale.ENGLISH, "%.6f", txHz / 1_000_000.0) else ""
+                    }
+                }
+            },
+            label = { Text(stringResource(R.string.radar_doppler_offset_hint)) },
+            singleLine = true,
+            keyboardOptions = KeyboardOptions(keyboardType = KeyboardType.Decimal),
+            modifier = Modifier.fillMaxWidth()
+        )
+
+        // TX and RX inputs on the same row
+        Row(
+            horizontalArrangement = Arrangement.spacedBy(8.dp),
+            modifier = Modifier.fillMaxWidth()
+        ) {
+            // TX input → compute RX
+            OutlinedTextField(
+                value = txInputMHz,
+                onValueChange = { newVal ->
+                    txInputMHz = newVal
+                    lastEditedBy = EditedField.TX
+                    val mhz = newVal.toDoubleOrNull()
+                    if (mhz != null && mhz > 0) {
+                        val txHz = (mhz * 1_000_000).toLong()
+                        val rxHz = DopplerFrequencyCalculator.computeDownlinkFromUplinkWithOffset(
+                            txHz, transponder, orbitalPos, offsetHz
+                        )
+                        rxInputMHz = if (rxHz != null) String.format(Locale.ENGLISH, "%.6f", rxHz / 1_000_000.0) else ""
+                    } else if (newVal.isEmpty()) {
+                        rxInputMHz = ""
+                    }
+                },
+                label = { Text(stringResource(R.string.radar_doppler_tx_hint)) },
+                singleLine = true,
+                keyboardOptions = KeyboardOptions(keyboardType = KeyboardType.Decimal),
+                modifier = Modifier.weight(1f)
+            )
+
+            // RX input → compute TX
+            OutlinedTextField(
+                value = rxInputMHz,
+                onValueChange = { newVal ->
+                    rxInputMHz = newVal
+                    lastEditedBy = EditedField.RX
+                    val mhz = newVal.toDoubleOrNull()
+                    if (mhz != null && mhz > 0) {
+                        val rxHz = (mhz * 1_000_000).toLong()
+                        val txHz = DopplerFrequencyCalculator.computeUplinkFromDownlinkWithOffset(
+                            rxHz, transponder, orbitalPos, offsetHz
+                        )
+                        txInputMHz = if (txHz != null) String.format(Locale.ENGLISH, "%.6f", txHz / 1_000_000.0) else ""
+                    } else if (newVal.isEmpty()) {
+                        txInputMHz = ""
+                    }
+                },
+                label = { Text(stringResource(R.string.radar_doppler_rx_hint)) },
+                singleLine = true,
+                keyboardOptions = KeyboardOptions(keyboardType = KeyboardType.Decimal),
+                modifier = Modifier.weight(1f)
+            )
+        }
+    }
+}
+
+private enum class EditedField { TX, RX }
+
 @Composable
 private fun FrequencyText(frequency: Long?, modifier: Modifier = Modifier) {
     val text = frequency?.let {