fix: correct linear calculator offset mapping

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

@@ -1,4 +1,4 @@
-/* 
+/*
  * Look4Sat. Amateur radio satellite tracker and pass predictor.
  * Copyright (C) 2019-2026 Arty Bishop and contributors.
  *
@@ -45,6 +45,9 @@ object DopplerFrequencyCalculator {
      * 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,
@@ -53,7 +56,7 @@ object DopplerFrequencyCalculator {
         offsetHz: Long
     ): Long? {
         if (!isLinearTransponder(transponder)) return null
-        val baseUplink = TransponderMapper.mapDownlinkToUplink(downlinkHz + offsetHz, transponder) ?: return null
+        val baseUplink = TransponderMapper.mapDownlinkToUplink(downlinkHz - offsetHz, transponder) ?: return null
         return orbitalPos.getUplinkFreq(baseUplink)
     }
 

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

@@ -118,6 +118,53 @@ class DopplerFrequencyCalculatorTest {
         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)