diff --git a/server/handlers/api/addspot.py b/server/handlers/api/addspot.py
index f939607..79ec8d5 100644
--- a/server/handlers/api/addspot.py
+++ b/server/handlers/api/addspot.py
@@ -180,9 +180,40 @@ class APISpotHandler(tornado.web.RequestHandler):
                 self.set_header("Content-Type", "application/json")
                 return
 
+            # Validate upstream submission requirements
+            if submit_upstream and upstream_provider_name:
+                if not spot.sig:
+                    self.set_status(422)
+                    self.write(json.dumps("Error - a SIG must be selected to submit upstream.",
+                                          default=serialize_everything))
+                    self.set_header("Cache-Control", "no-store")
+                    self.set_header("Content-Type", "application/json")
+                    return
+                if not spot.sig_refs and upstream_provider_name != "Tiles":
+                    self.set_status(422)
+                    self.write(json.dumps("Error - a SIG reference is required to submit upstream.",
+                                          default=serialize_everything))
+                    self.set_header("Cache-Control", "no-store")
+                    self.set_header("Content-Type", "application/json")
+                    return
+                if not spot.dx_grid and upstream_provider_name == "Tiles":
+                    self.set_status(422)
+                    self.write(json.dumps("Error - a grid reference is required to submit upstream to Tiles on the Air.",
+                                          default=serialize_everything))
+                    self.set_header("Cache-Control", "no-store")
+                    self.set_header("Content-Type", "application/json")
+                    return
+                if not spot.mode and upstream_provider_name == "Tiles":
+                    self.set_status(422)
+                    self.write(json.dumps("Error - a mode is required to submit upstream to Tiles on the Air.",
+                                          default=serialize_everything))
+                    self.set_header("Cache-Control", "no-store")
+                    self.set_header("Content-Type", "application/json")
+                    return
+
             # Submit upstream if requested
             upstream_warning = None
-            if submit_upstream and upstream_provider_name and spot.sig:
+            if submit_upstream and upstream_provider_name:
                 provider = self._find_provider(upstream_provider_name, spot.sig)
                 if provider:
                     try:
diff --git a/templates/add_spot.html b/templates/add_spot.html
index 80b1c80..9d2f007 100644
--- a/templates/add_spot.html
+++ b/templates/add_spot.html
@@ -24,14 +24,14 @@
             </div>
         </div>
         <div class="card-body">
-            <form class="row g-3">
+            <form class="row g-3" onsubmit="return addSpot();">
                 <div class="col-auto">
                     <label for="dx-call" class="form-label">DX Call *</label>
-                    <input type="text" class="form-control input-narrow" id="dx-call" placeholder="N0CALL">
+                    <input type="text" class="form-control input-narrow" id="dx-call" placeholder="N0CALL" required>
                 </div>
                 <div class="col-auto">
                     <label for="freq" class="form-label">Frequency (kHz) *</label>
-                    <input type="text" class="form-control input-narrow" id="freq" placeholder="e.g. 14100">
+                    <input type="text" class="form-control input-narrow" id="freq" placeholder="e.g. 14100" required>
                 </div>
                 <div class="col-auto">
                     <label for="mode" class="form-label">Mode</label>
@@ -60,10 +60,10 @@
                 <div class="col-auto">
                     <label for="de-call" class="form-label">Your Call *</label>
                     <input type="text" class="form-control storeable-text input-narrow" id="de-call"
-                           placeholder="N0CALL">
+                           placeholder="N0CALL" required>
                 </div>
                 <div class="col-auto">
-                    <button type="button" class="btn btn-primary mt-2em" onclick="addSpot();">Spot</button>
+                    <button type="submit" class="btn btn-primary mt-2em">Spot</button>
                 </div>
             </form>
 
diff --git a/webassets/js/add-spot.js b/webassets/js/add-spot.js
index 847afaa..5741d5f 100644
--- a/webassets/js/add-spot.js
+++ b/webassets/js/add-spot.js
@@ -205,40 +205,14 @@ function addSpot() {
 
         // Prepare the spot object for the server
         const spot = {};
-        if (dx !== "") {
-            spot["dx_call"] = dx;
-        } else {
-            // todo maybe for neatness just make all these error/rejections server side rather than having logic in two places
-            showAddSpotError("A DX callsign is required in order to spot.");
-            return;
-        }
-        if (freqStr !== "") {
-            spot["freq"] = parseFloat(freqStr) * 1000;
-        } else {
-            showAddSpotError("A frequency is required in order to spot.");
-            return;
-        }
-        if (mode !== "") {
-            spot["mode"] = mode;
-        }
-        if (sig !== "") {
-            spot["sig"] = sig;
-        }
-        if (sigRef !== "") {
-            spot["sig_refs"] = [{id: sigRef}];
-        }
-        if (dxGrid !== "") {
-            spot["dx_grid"] = dxGrid;
-        }
-        if (comment !== "") {
-            spot["comment"] = comment;
-        }
-        if (de !== "") {
-            spot["de_call"] = de;
-        } else {
-            showAddSpotError("A spotter callsign is required in order to spot.");
-            return;
-        }
+        spot["dx_call"] = dx;
+        spot["freq"] = parseFloat(freqStr) * 1000;
+        if (mode !== "") spot["mode"] = mode;
+        if (sig !== "") spot["sig"] = sig;
+        if (sigRef !== "") spot["sig_refs"] = [{id: sigRef}];
+        if (dxGrid !== "") spot["dx_grid"] = dxGrid;
+        if (comment !== "") spot["comment"] = comment;
+        spot["de_call"] = de;
         spot["time"] = moment.utc().valueOf() / 1000.0;
 
         // Prepare "handling" structure to tell the server what to do with this spot
@@ -246,35 +220,13 @@ function addSpot() {
 
         // Add CAPTCHA token if reCAPTCHA is loaded
         if (window._recaptchaWidgetId !== undefined) {
-            const token = grecaptcha.getResponse(window._recaptchaWidgetId);
-            if (!token) {
-                showAddSpotError("Please complete the CAPTCHA to submit upstream.");
-                return;
-            }
-            handling["captcha_token"] = token;
+            handling["captcha_token"] = grecaptcha.getResponse(window._recaptchaWidgetId);
         }
 
         // Upstream submission
         const submitUpstream = $("#submit-upstream").is(":checked");
         const upstreamProviderName = getSelectedUpstreamProvider();
         if (submitUpstream && upstreamProviderName) {
-            if (!sig) {
-                showAddSpotError("A SIG must be selected to submit upstream.");
-                return;
-            }
-            if (!sigRef && upstreamProviderName !== "Tiles") {
-                showAddSpotError("A SIG reference is required to submit upstream.");
-                return;
-            }
-            if (!dxGrid && upstreamProviderName === "Tiles") {
-                showAddSpotError("A grid reference is required to submit upstream to Tiles on the Air.");
-                return;
-            }
-            if (!mode && upstreamProviderName === "Tiles") {
-                showAddSpotError("A mode is required to submit upstream to Tiles on the Air.");
-                return;
-            }
-
             handling["submit_upstream"] = true;
             handling["upstream_provider"] = upstreamProviderName;
             handling["upstream_credentials"] = loadCredentials(upstreamProviderName);