Added linear transponder Doppler calculator (#232)

Co-authored-by: atsunatsu <atsunatsu@users.noreply.github.com>

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

@@ -0,0 +1,122 @@
+/*
+ * 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
+import java.util.Locale
+
+/**
+ * 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, this computes the other:
+ *
+ *   downlink → uplink: mapDownlinkToUplink (passband) → getUplinkFreq (Doppler)
+ *   uplink → downlink: mapUplinkToDownlink (passband) → getDownlinkFreq (Doppler)
+ */
+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.
+     *
+     * The user-entered downlink frequency already includes the offset, so subtract
+     * it before mapping the downlink passband position back to the uplink.
+     */
+    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
+    }
+
+    /**
+     * True for the radio entry that should drive the standalone Calculator page.
+     *
+     * A frequency range alone is not enough: some non-user-facing or drifting data entries
+     * can also have low/high frequencies. The calculator is meant for named linear
+     * transponders, e.g. "Linear Transponder", "Linear Transp.", "SSB Transponder".
+     */
+    fun isNamedLinearTransponder(transponder: SatRadio): Boolean {
+        if (!isLinearTransponder(transponder)) return false
+
+        val info = transponder.info.lowercase(Locale.ENGLISH)
+        val modes = listOfNotNull(transponder.downlinkMode, transponder.uplinkMode)
+            .joinToString(separator = " ")
+            .lowercase(Locale.ENGLISH)
+        val hasLinearName = info.contains("linear")
+        val hasTransponderName = info.contains("transponder") || info.contains("transp") ||
+                info.contains("xponder") || info.contains("xpdr")
+        val hasLinearMode = listOf("ssb", "usb", "lsb", "cw").any { modes.contains(it) }
+
+        return (hasLinearName && hasTransponderName) || (hasTransponderName && hasLinearMode)
+    }
+}

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

@@ -0,0 +1,190 @@
+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.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNotNull
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertTrue
+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,
+        info: String = "Linear Transponder",
+        downlinkMode: String? = "USB",
+        uplinkMode: String? = "LSB"
+    ) = SatRadio(
+        uuid = "linear", info = info, isAlive = true,
+        downlinkLow = downLow, downlinkHigh = downHigh,
+        downlinkMode = downlinkMode, uplinkLow = upLow, uplinkHigh = upHigh,
+        uplinkMode = uplinkMode, 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 isNamedLinearTransponder_returnsTrueForLinearTransponderName() {
+        assertTrue(DopplerFrequencyCalculator.isNamedLinearTransponder(linearTransponder()))
+    }
+
+    @Test
+    fun isNamedLinearTransponder_returnsTrueForSsbTransponderName() {
+        val xpdr = linearTransponder(info = "Mode V/U SSB Transponder", downlinkMode = "USB", uplinkMode = "LSB")
+        assertTrue(DopplerFrequencyCalculator.isNamedLinearTransponder(xpdr))
+    }
+
+    @Test
+    fun isNamedLinearTransponder_returnsFalseForRangeEntryWithoutTransponderName() {
+        val driftingRangeEntry = linearTransponder(info = "Upper side band (drifting)")
+        assertFalse(DopplerFrequencyCalculator.isNamedLinearTransponder(driftingRangeEntry))
+    }
+
+    @Test
+    fun isNamedLinearTransponder_returnsFalseForFmRepeater() {
+        assertFalse(DopplerFrequencyCalculator.isNamedLinearTransponder(fmTransponder()))
+    }
+
+    @Test
+    fun computeUplinkFromDownlink_linear_noDoppler() {
+        val xpdr = linearTransponder()
+        val orbitalPos = pos(0.0)
+        val uplink = DopplerFrequencyCalculator.computeUplinkFromDownlink(435_200_000L, xpdr, orbitalPos)
+        assertNotNull(uplink)
+        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)
+        val uplink = DopplerFrequencyCalculator.computeUplinkFromDownlink(435_200_000L, xpdr, orbitalPos)
+        assertNotNull(uplink)
+        assertTrue(uplink!! > 145_200_000L)
+    }
+
+    @Test
+    fun computeUplinkFromDownlink_fmTransponder_returnsNull() {
+        val orbitalPos = pos()
+        val result = DopplerFrequencyCalculator.computeUplinkFromDownlink(435_600_000L, fmTransponder(), orbitalPos)
+        assertNull(result)
+    }
+
+    @Test
+    fun computeDownlinkFromUplink_fmTransponder_returnsNull() {
+        val orbitalPos = pos()
+        val result = DopplerFrequencyCalculator.computeDownlinkFromUplink(145_900_000L, fmTransponder(), orbitalPos)
+        assertNull(result)
+    }
+
+    @Test
+    fun computeDownlinkFromUplink_withPositiveOffset_addsOffsetToDownlink() {
+        val xpdr = linearTransponder()
+        val orbitalPos = pos(0.0)
+        val downlink = DopplerFrequencyCalculator.computeDownlinkFromUplinkWithOffset(
+            uplinkHz = 145_200_000L,
+            transponder = xpdr,
+            orbitalPos = orbitalPos,
+            offsetHz = 2_500L
+        )
+        assertEquals(435_202_500L, downlink)
+    }
+
+    @Test
+    fun computeUplinkFromDownlink_withPositiveOffset_subtractsOffsetBeforeMapping() {
+        val xpdr = linearTransponder()
+        val orbitalPos = pos(0.0)
+        val uplink = DopplerFrequencyCalculator.computeUplinkFromDownlinkWithOffset(
+            downlinkHz = 435_202_500L,
+            transponder = xpdr,
+            orbitalPos = orbitalPos,
+            offsetHz = 2_500L
+        )
+        assertEquals(145_200_000L, uplink)
+    }
+
+    @Test
+    fun computeOffsetRoundTrip_handlesNegativeOffset() {
+        val xpdr = linearTransponder()
+        val orbitalPos = pos(0.0)
+        val downlink = DopplerFrequencyCalculator.computeDownlinkFromUplinkWithOffset(
+            uplinkHz = 145_200_000L,
+            transponder = xpdr,
+            orbitalPos = orbitalPos,
+            offsetHz = -2_500L
+        )
+        assertEquals(435_197_500L, downlink)
+
+        val uplink = DopplerFrequencyCalculator.computeUplinkFromDownlinkWithOffset(
+            downlinkHz = downlink!!,
+            transponder = xpdr,
+            orbitalPos = orbitalPos,
+            offsetHz = -2_500L
+        )
+        assertEquals(145_200_000L, uplink)
+    }
+
+    @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)
+        assertEquals(145_300_000L, uplink)
+    }
+
+    @Test
+    fun computeUplinkFromDownlink_roundTrip() {
+        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)
+        val error = kotlin.math.abs(roundTripDownlink!! - originalDownlink)
+        assertTrue("Round-trip error too large: $error", error < 10000)
+    }
+}

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

@@ -60,6 +60,9 @@
     <string name="radar_string_no">否</string>
     <string name="radar_string_yes">是</string>
     <string name="radar_visible">日照中</string>
+    <string name="radar_doppler_tx_hint">输入上行频率 (MHz)</string>
+    <string name="radar_doppler_rx_hint">输入下行频率 (MHz)</string>
+    <string name="radar_doppler_offset_hint">偏移 (kHz)</string>
 
     <!-- Map screen -->
     <string name="map_prev">上一个</string>

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

@@ -84,6 +84,9 @@
     <string name="radar_string_no">No</string>
     <string name="radar_string_yes">Yes</string>
     <string name="radar_visible">Visible</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>
 
     <!-- Map screen -->
     <string name="map_prev">Prev</string>

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

@@ -45,6 +45,7 @@ import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.getValue
+import androidx.compose.runtime.remember
 import androidx.compose.runtime.rememberCoroutineScope
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
@@ -59,6 +60,7 @@ import androidx.lifecycle.compose.collectAsStateWithLifecycle
 import androidx.lifecycle.viewmodel.compose.viewModel
 import com.rtbishop.look4sat.core.domain.predict.OrbitalPos
 import com.rtbishop.look4sat.core.domain.repository.IContainerProvider
+import com.rtbishop.look4sat.core.domain.utility.DopplerFrequencyCalculator
 import com.rtbishop.look4sat.core.domain.utility.toDegrees
 import com.rtbishop.look4sat.core.presentation.EmptyListCard
 import com.rtbishop.look4sat.core.presentation.IconCard
@@ -74,6 +76,7 @@ import kotlinx.coroutines.launch
 
 private enum class RadarPage(val title: String) {
     Transceivers("Transceivers"),
+    Calculator("Calculator"),
     Sstv("SSTV")
 }
 
@@ -150,16 +153,31 @@ private fun PagerCard(
     requestMicPermission: () -> Unit,
     modifier: Modifier = Modifier
 ) {
-    val pages = RadarPage.entries
+    val hasCalculatorPage = remember(uiState.transceivers.transmitters) {
+        uiState.transceivers.transmitters.any(DopplerFrequencyCalculator::isNamedLinearTransponder)
+    }
+    val pages = remember(hasCalculatorPage) {
+        buildList {
+            add(RadarPage.Transceivers)
+            if (hasCalculatorPage) add(RadarPage.Calculator)
+            add(RadarPage.Sstv)
+        }
+    }
     val pagerState = rememberPagerState(pageCount = { pages.size })
     val coroutineScope = rememberCoroutineScope()
 
+    LaunchedEffect(pages.size) {
+        val lastPage = pages.lastIndex
+        if (pagerState.currentPage > lastPage) pagerState.scrollToPage(lastPage)
+    }
+
     ElevatedCard(modifier = modifier) {
         Column(modifier = Modifier.fillMaxSize()) {
-            PrimaryTabRow(selectedTabIndex = pagerState.currentPage) {
+            val selectedTabIndex = pagerState.currentPage.coerceIn(0, pages.lastIndex)
+            PrimaryTabRow(selectedTabIndex = selectedTabIndex) {
                 pages.forEachIndexed { index, page ->
                     Tab(
-                        selected = pagerState.currentPage == index,
+                        selected = selectedTabIndex == index,
                         onClick = { coroutineScope.launch { pagerState.animateScrollToPage(index) } },
                         text = { Text(text = page.title, maxLines = 1, overflow = TextOverflow.Ellipsis) }
                     )
@@ -176,6 +194,12 @@ private fun PagerCard(
                         radioControl = uiState.radioControl,
                         onAction = onAction
                     )
+                    RadarPage.Calculator -> CalculatorPage(
+                        transceivers = uiState.transceivers.transmitters,
+                        selectedUuid = uiState.transceivers.selectedUuid,
+                        orbitalPos = uiState.orbitalPos,
+                        onAction = onAction
+                    )
                     RadarPage.Sstv -> SstvPage(
                         sstv = uiState.sstv,
                         dopplerFrequency = uiState.transceivers.selectedFrequency?.let { formatFrequency(it) },

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
@@ -104,6 +112,69 @@ fun TransceiversPage(
     }
 }
 
+@Composable
+fun CalculatorPage(
+    transceivers: List<SatRadio>,
+    selectedUuid: String?,
+    orbitalPos: OrbitalPos?,
+    onAction: (RadarAction) -> Unit,
+    modifier: Modifier = Modifier
+) {
+    val calculatorTransceivers = remember(transceivers) {
+        transceivers.filter(DopplerFrequencyCalculator::isNamedLinearTransponder)
+    }
+    val selectedTransceiver = remember(calculatorTransceivers, selectedUuid) {
+        calculatorTransceivers.firstOrNull { it.uuid == selectedUuid }
+            ?: calculatorTransceivers.firstOrNull()
+    }
+
+    if (selectedTransceiver == null) {
+        EmptyTransceiversContent(modifier)
+        return
+    }
+
+    LazyColumn(
+        modifier = modifier
+            .fillMaxSize()
+            .padding(8.dp),
+        verticalArrangement = Arrangement.spacedBy(10.dp)
+    ) {
+        if (calculatorTransceivers.size > 1) {
+            item {
+                FlowRow(
+                    horizontalArrangement = Arrangement.spacedBy(6.dp),
+                    verticalArrangement = Arrangement.spacedBy(4.dp),
+                    modifier = Modifier.fillMaxWidth()
+                ) {
+                    calculatorTransceivers.forEach { radio ->
+                        FilterChip(
+                            selected = radio.uuid == selectedTransceiver.uuid,
+                            onClick = {
+                                if (radio.uuid != selectedUuid) onAction(RadarAction.SelectTransmitter(radio.uuid))
+                            },
+                            label = {
+                                Text(
+                                    text = transceiverTitle(radio),
+                                    maxLines = 1,
+                                    overflow = TextOverflow.Ellipsis
+                                )
+                            }
+                        )
+                    }
+                }
+            }
+        }
+
+        item {
+            DopplerCalculatorPanel(
+                transponder = selectedTransceiver,
+                orbitalPos = orbitalPos,
+                modifier = Modifier.fillMaxWidth()
+            )
+        }
+    }
+}
+
 @Composable
 private fun EmptyTransceiversContent(modifier: Modifier = Modifier) {
     Box(contentAlignment = Alignment.Center, modifier = modifier.fillMaxSize()) {
@@ -172,12 +243,8 @@ private fun TransceiverItem(
                             .rotate(if (isExpanded) 270f else 90f)
                     )
                 }
-                // Title with mode
-                val title = if (radio.isInverted) "INV: ${radio.info}" else radio.info
-                val mode = "${radio.downlinkMode ?: "--"}/${radio.uplinkMode ?: "--"}"
-                val fullTitle = "$title ($mode)"
                 Text(
-                    text = fullTitle,
+                    text = transceiverTitle(radio),
                     textAlign = TextAlign.Center,
                     color = MaterialTheme.colorScheme.onSurface,
                     maxLines = 1,
@@ -437,6 +504,108 @@ private fun ExpandedRadioControl(
     }
 }
 
+@Composable
+private fun DopplerCalculatorPanel(
+    transponder: SatRadio,
+    orbitalPos: OrbitalPos?,
+    modifier: Modifier = Modifier
+) {
+    if (orbitalPos == null || !DopplerFrequencyCalculator.isLinearTransponder(transponder)) return
+
+    var txInputMHz by remember(transponder.uuid) { mutableStateOf("") }
+    var rxInputMHz by remember(transponder.uuid) { mutableStateOf("") }
+    var offsetKHz by remember(transponder.uuid) { mutableStateOf("") }
+    var lastEditedBy by remember(transponder.uuid) { mutableStateOf(EditedField.TX) }
+
+    fun offsetHz(text: String): Long = text.toDoubleOrNull()?.let { (it * 1000).toLong() } ?: 0L
+    fun formatHz(hz: Long): String = String.format(Locale.ENGLISH, "%.6f", hz / 1_000_000.0)
+    fun mhzToHz(text: String): Long? = text.toDoubleOrNull()
+        ?.takeIf { it > 0.0 }
+        ?.let { (it * 1_000_000).toLong() }
+
+    fun calculateDownlink(txText: String, offsetText: String): String? {
+        val txHz = mhzToHz(txText) ?: return null
+        return DopplerFrequencyCalculator.computeDownlinkFromUplinkWithOffset(
+            uplinkHz = txHz,
+            transponder = transponder,
+            orbitalPos = orbitalPos,
+            offsetHz = offsetHz(offsetText)
+        )?.let(::formatHz)
+    }
+
+    fun calculateUplink(rxText: String, offsetText: String): String? {
+        val rxHz = mhzToHz(rxText) ?: return null
+        return DopplerFrequencyCalculator.computeUplinkFromDownlinkWithOffset(
+            downlinkHz = rxHz,
+            transponder = transponder,
+            orbitalPos = orbitalPos,
+            offsetHz = offsetHz(offsetText)
+        )?.let(::formatHz)
+    }
+
+    LaunchedEffect(orbitalPos, transponder.uuid) {
+        if (lastEditedBy == EditedField.TX) {
+            calculateDownlink(txInputMHz, offsetKHz)?.let { rxInputMHz = it }
+        } else {
+            calculateUplink(rxInputMHz, offsetKHz)?.let { txInputMHz = it }
+        }
+    }
+
+    Column(
+        modifier = modifier,
+        verticalArrangement = Arrangement.spacedBy(8.dp)
+    ) {
+        OutlinedTextField(
+            value = offsetKHz,
+            onValueChange = { newValue ->
+                offsetKHz = newValue
+                if (lastEditedBy == EditedField.TX) {
+                    calculateDownlink(txInputMHz, newValue)?.let { rxInputMHz = it }
+                } else {
+                    calculateUplink(rxInputMHz, newValue)?.let { txInputMHz = it }
+                }
+            },
+            label = { Text(stringResource(R.string.radar_doppler_offset_hint)) },
+            singleLine = true,
+            keyboardOptions = KeyboardOptions(keyboardType = KeyboardType.Text),
+            modifier = Modifier.fillMaxWidth()
+        )
+
+        Row(
+            horizontalArrangement = Arrangement.spacedBy(8.dp),
+            modifier = Modifier.fillMaxWidth()
+        ) {
+            OutlinedTextField(
+                value = txInputMHz,
+                onValueChange = { newValue ->
+                    txInputMHz = newValue
+                    lastEditedBy = EditedField.TX
+                    rxInputMHz = calculateDownlink(newValue, offsetKHz) ?: if (newValue.isEmpty()) "" else rxInputMHz
+                },
+                label = { Text(stringResource(R.string.radar_doppler_tx_hint)) },
+                singleLine = true,
+                keyboardOptions = KeyboardOptions(keyboardType = KeyboardType.Decimal),
+                modifier = Modifier.weight(1f)
+            )
+
+            OutlinedTextField(
+                value = rxInputMHz,
+                onValueChange = { newValue ->
+                    rxInputMHz = newValue
+                    lastEditedBy = EditedField.RX
+                    txInputMHz = calculateUplink(newValue, offsetKHz) ?: if (newValue.isEmpty()) "" else 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 {
@@ -452,5 +621,11 @@ private fun FrequencyText(frequency: Long?, modifier: Modifier = Modifier) {
     )
 }
 
+private fun transceiverTitle(radio: SatRadio): String {
+    val title = if (radio.isInverted) "INV: ${radio.info}" else radio.info
+    val mode = "${radio.downlinkMode ?: "--"}/${radio.uplinkMode ?: "--"}"
+    return "$title ($mode)"
+}
+
 private val FREQ_ADJUSTMENTS =
     listOf(-10_000L to "-10k", -1_000L to "-1k", -100L to "-100", 100L to "+100", 1_000L to "+1k", 10_000L to "+10k")